Physical Review B

(Condensed Matter and Materials Physics - 1 (I))

November 2009

Volume 80, Number 17 , partial issue

Click on icons for further information

Rapid Communication

     Close

Editors' Suggestion
The editors and referees of PRB find these papers to be of particular interest, importance, or clarity. Please see our Announcement PRB 77, 130001 (2008).

     Close

Physics Viewpoint
Selected for a Viewpoint in Physics. Please visit http://physics.aps.org/.

     Close

Featured in Phys. Rev. Focus
Please visit http://focus.aps.org.

     Close

Free to Read
Marks articles whose full text is available without a subscription. See the FREE TO READ FAQ.

     Close

BRIEF REPORTS

Structure, structural phase transitions, mechanical properties, defects

Dynamics, dynamical systems, lattice effects, quantum solids

Magnetism

Superfluidity and superconductivity

ARTICLES

Structure, structural phase transitions, mechanical properties, defects

Inhomogeneous, disordered, and partially ordered systems

Dynamics, dynamical systems, lattice effects, quantum solids

Magnetism

Superfluidity and superconductivity

ERRATA

BRIEF REPORTS

Structure, structural phase transitions, mechanical properties, defects

Proton-irradiation-induced anomaly in the electrical conductivity of a hydrogen-bonded ferroelastic system

Se-Hun Kim, Kyu Won Lee, Kwang-Sei Lee, and Cheol Eui Lee

Published 3 November 2009 (4 pages)
172101 Full Text: PDF (365 kB) | Buy Article

+

-

Show Abstract

An anomalous abrupt drop in the electrical conductivity has been observed at the ferroelastic phase transition of a proton-irradiated system of hydrogen-bonded TlH2PO4. As a result of the high-resolution ³¹P NMR chemical-shift measurements, distinct changes in the atomic displacements due to the irradiation were identified in the ferroelastic and paraelastic phases. Besides, ¹H NMR spin-spin relaxation measurements revealed a change due to the irradiation in the proton dynamics at the ferroelastic phase transition, apparently accounting for the much-reduced electrical conductivity in the paraelastic phase of the irradiated system.

Relationship between low-temperature boson heat capacity peak and high-temperature shear modulus relaxation in a metallic glass

A. N. Vasiliev, T. N. Voloshok, A. V. Granato, D. M. Joncich, Yu. P. Mitrofanov, and V. A. Khonik

Published 4 November 2009 (4 pages)
172102 Full Text: PDF (131 kB) | Buy Article

+

-

Show Abstract

Low-temperature (2  KT350  K) heat capacity and room-temperature shear modulus measurements (=1.4  MHz) have been performed on bulk Pd41.25Cu41.25P17.5 in the initial glassy, relaxed glassy, and crystallized states. It has been found that the height of the low-temperature Boson heat capacity peak strongly correlates with the changes in the shear modulus upon high-temperature annealing. It is this behavior that was earlier predicted by the interstitialcy theory, according to which dumbbell interstitialcy defects are responsible for a number of thermodynamic and kinetic properties of crystalline, (supercooled) liquid, and solid glassy states.

Magnetoelectric coupling in polycrystalline FeVO₄

Bohdan Kundys, Christine Martin, and Charles Simon

Published 11 November 2009 (4 pages)
172103 Full Text: PDF (676 kB) | Buy Article

+

-

Show Abstract

We report coupling between magnetic and electric orders for antiferromagnetic polycrystalline FeVO4 in which magnetism-induced polarization has been recently found in noncollinear antiferromagnetic state below the second antiferromagnetic phase transition at TN215.7  K. In this low symmetry phase space group P, the magnetic field dependence of electric polarization evidences a clear magnetoelectric coupling in the noncollinear spin-configured antiferromagnetic phase. The discontinuity of magnetodielectric effect observed at the vicinity of the polar to nonpolar transition evidences competition between different magnetodielectric couplings in the two different antiferromagnetic states. The existence of thermal expansion anomaly near TN2 and magnetostriction effect support magnetoelastically mediated scenario of the observed magnetoelectric effect.

High-pressure Raman and x-ray diffraction studies on LaB₆

B. K. Godwal, E. A. Petruska, S. Speziale, J. Yan, S. M. Clark, M. B. Kruger, and R. Jeanloz

Published 19 November 2009 (4 pages)
172104 Full Text: PDF (292 kB) | Buy Article

+

-

Show Abstract

X-ray diffraction measurements and Raman spectroscopy at room temperature document the equation of state and the frequency shifts for Eg, T2g, and A1g vibrational modes of polycrystalline LaB6 under pressure. The data exhibit smooth pressure dependencies, yielding a zero-pressure isothermal bulk modulus K0T=164(±2)  GPa in good accord with independent ultrasonic measurements, and show no evidence of structural or electronic phase transitions up to at least 25 GPa.

Dynamics, dynamical systems, lattice effects, quantum solids

Reversal of thermal rectification in quantum systems

Lifa Zhang, Yonghong Yan, Chang-Qin Wu, Jian-Sheng Wang, and Baowen Li

Published 12 November 2009 (4 pages)
172301 Full Text: PDF (245 kB) | Buy Article

+

-

Show Abstract

We study thermal transport in anisotropic Heisenberg spin chains using the quantum master equation. It is found that thermal rectification changes sign when the external homogeneous magnetic field is varied. This reversal also occurs when the magnetic field becomes inhomogeneous. Moreover, we can tune the reversal of rectification by temperatures of the heat baths, the anisotropy, and size of the spin chains.

Zero-point divacancy concentration in the shadow wave function model for solid ⁴He

R. Pessoa, M. de Koning, and S. A. Vitiello

Published 16 November 2009 (4 pages)
172302 Full Text: PDF (108 kB) | Buy Article

+

-

Show Abstract

We address the issue of interaction between zero-point vacancies in solid ⁴He as described within the shadow wave function model. Applying the reversible-work method and taking into account finite-size effects, we obtain a zero-point monovacancy concentration of (2.03±0.02)×10⁻³, which is slightly higher than the result due to Reatto et al. for the same model. Utilizing the same methodology, we then consider the divacancy, taking into account both the in-plane as well as out-of-plane configurations with respect to the basal plane. We find no significant anisotropy between both conformations. Furthermore, although there is a small binding tendency, the expected divacancy concentration is only ~4–5 times larger than the value expected in the absence of any clustering propensity, 2.5×10⁻⁵. This result suggests that, within the employed model description, no vacancy aggregation leading to phase separation is to be expected in the ground state.

Magnetism

Measuring the magnetization of three monolayer thick Co islands and films by x-ray dichroism

A. Mascaraque, L. Aballe, J. F. Marco, T. O. Mente, F. El Gabaly, C. Klein, A. K. Schmid, K. F. McCarty, A. Locatelli, and J. de la Figuera

Published 6 November 2009 (4 pages)
172401 Full Text: PDF (283 kB) | Buy Article

+

-

Show Abstract

Co islands and films are characterized by x-ray magnetic circular dichroism photoemission electron microscopy. The spatial resolution capabilities of the technique together with atomic growth control permit obtaining perfectly flat triangular islands with a given thickness (3 ML), very close to an abrupt spin-reorientation transition. The magnetic domain configurations are found to depend on island size: while small islands can be magnetized in a single-domain state, larger islands show more complex patterns. Furthermore, the magnetization pattern of the larger islands presents a common chirality. By means of dichroic spectromicroscopy at the Co L absorption edges, an experimental estimate of the ratio of the spin and orbital magnetic moment for three monolayer thick films is obtained.

Origin of electric-field-induced modification of magnetocrystalline anisotropy at Fe(001) surfaces: Mechanism of dipole formation from first principles

Kohji Nakamura, Riki Shimabukuro, Toru Akiyama, Tomonori Ito, and A. J. Freeman

Published 6 November 2009 (4 pages)
172402 Full Text: PDF (357 kB) | Buy Article

+

-

Show Abstract

First-principles full-potential linearized augmented plane-wave studies reveal that a surface magnetocrystalline anisotropy (MCA) modification by an external electric field arises from a dipole formation mechanism. The precise calculations demonstrate that the formation of dipoles on Fe(001) surface atoms, which counteract the electric-field-induced charge in the vacuum region, changes the surface states around the Fermi level in the minority-spin d bands, and yields a modification of the surface MCA. These findings greatly advance our understanding of the electric-field-induced MCA modifications in itinerant ferromagnetic surfaces.

Spontaneous generation of voltage in the magnetocaloric compound La(Fe_0.88Si_0.12)₁₃ and comparison to SmMn₂Ge₂

M. Zou, J. A. Sampaio, V. K. Pecharsky, and K. A. Gschneidner, Jr.

Published 17 November 2009 (4 pages)
172403 Full Text: PDF (235 kB) | Buy Article

+

-

Show Abstract

Relationships among spontaneous generation of voltage (SGV), magnetocaloric effect, temperature induced first-order magnetic phase transformation, and its thermal effect have been studied based on experimental results of La(Fe0.88Si0.12)13 and SmMn2Ge2 compounds. Remarkable differences in magnetocaloric effects and temperature-induced unit-cell volume changes during their first-order magnetic phase transformations lead to dramatic differences in their SGV effects. Both temperature and magnetic field trigger SGV in La(Fe0.88Si0.12)13 compound, but no SGV has been observed in SmMn2Ge2. Our results clarify that it is not the first-order crystallographic or magnetic phase transformation per se, but the strong thermal effects, i.e., latent heat and magnetocaloric effect that play the key role in the SGV mechanism.

Toroidal moment in the molecular magnet V₁₅

A. K. Zvezdin, V. V. Kostyuchenko, A. I. Popov, A. F. Popkov, and A. Ceulemans

Published 19 November 2009 (4 pages)
172404 Full Text: PDF (123 kB) | Buy Article

+

-

Show Abstract

Quantum-mechanical calculations predict the existence of toroidal spin structure in the molecular magnet V15. It is shown that nonzero toroidal moment arises from symmetry violation of the exchange interactions between spins of the base triangle of the V15 molecule due to the Jahn-Teller effect. It is established that the value of the toroidal moment is connected with value of total spin projection of V15 on z axis. It enables to induce the toroidal moment by external magnetic field and (or) to induce the magnetic moment by variable electric field or by current.

Superfluidity and superconductivity

Anisotropic inelastic scattering and its interplay with superconductivity in URu₂Si₂

Zengwei Zhu, Elena Hassinger, Zhu'an Xu, Dai Aoki, Jacques Flouquet, and Kamran Behnia

Published 2 November 2009 (4 pages)
172501 Full Text: PDF (374 kB) | Buy Article

+

-

Show Abstract

In contrast to almost all anisotropic superconductors, the upper critical field of URu2Si2 is larger when the field is oriented along the less conducting direction. We present a study of resistivity and Seebeck coefficient extended down to sub-Kelvin temperature range uncovering a singular case of anisotropy. When the current is injected along the c axis URu2Si2 behaves as a low-density Fermi liquid. When it flows along the a axis, even in presence of a large field, resistivity remains T-linear down to Tc and the Seebeck coefficient undergoes a sign change at very low temperatures. We conclude that the characteristic energy scale is anisotropic and vanishingly small in the basal plane.

Impurity resonance peaks in the vortex core of cuprate superconductors with induced spin density wave order

Wonkee Kim, Yan Chen, and C. S. Ting

Published 3 November 2009 (4 pages)
172502 Full Text: PDF (328 kB) | Buy Article

+

-

Show Abstract

Considering the competing spin density wave (SDW) and d-wave superconductivity interactions, we investigate the effects of a nonmagnetic impurity on the vortex-core state of cuprate superconductors within the Bogoliubov–de Gennes formalism. We illustrate that the local SDW order is induced by the impurity on top of the magnetic field induced SDW order. The local density of states of a pinned vortex core exhibit a resonance peak at negative energy, which is drastically different from the double-peak structure observed in an unpinned vortex core. This resonance peak is insensitive to the impurity-scattering strength. Consequently, the impurity resonance peak may be used to identify the nature of vortex-core state of cuprate superconductors.

Three-band s± Eliashberg theory and the superconducting gaps of iron pnictides

G. A. Ummarino, M. Tortello, D. Daghero, and R. S. Gonnelli

Published 11 November 2009 (4 pages)
172503 Full Text: PDF (446 kB) | Buy Article

+

-

Show Abstract

Superconductivity in iron pnictides has been recently proposed to be due to spin-fluctuation mediated coupling between hole and electron bands with order parameters of opposite sign. BCS multiband models give qualitative predictions but cannot simultaneously reproduce the experimental values of critical temperature and energy gaps. We show, instead, that a three-band strong-coupling Eliashberg model can quantitatively reproduce the gaps and their temperature dependence in both the 1111 and 122 families. We also show that this requires small typical boson energies (in agreement with experiments) and high values of the electron-boson coupling constants.

Observation of a coherent optical phonon in the iron pnictide superconductor Ba(Fe_1−xCo_x)₂As₂ (x=0.06 and 0.08)

B. Mansart, D. Boschetto, A. Savoia, F. Rullier-Albenque, A. Forget, D. Colson, A. Rousse, and M. Marsi

Published 16 November 2009 (4 pages)
172504 Full Text: PDF (110 kB) | Buy Article

+

-

Show Abstract

We report the observation of a coherent lattice oscillation in a pnictide superconductor. A coherent fully symmetric optical phonon was detected in Ba(Fe1−xCox)2As2 (x=0.06 and x=0.08) using time-resolved pump-probe reflectivity with 40 fs time resolution. The analysis of the phonon parameters for various excitation fluences reveals no evident difference below and above the critical temperature, suggesting that the A1g mode is not involved in the superconducting phase transition.

ARTICLES

Structure, structural phase transitions, mechanical properties, defects

Predicted structure and stability of A₄B₃O₁₂ -phase compositions

C. R. Stanek, C. Jiang, B. P. Uberuaga, K. E. Sickafus, A. R. Cleave, and R. W. Grimes

Published 2 November 2009 (11 pages)
174101 Full Text: PDF (501 kB) | Buy Article

+

-

Show Abstract

A combination of atomistic simulation techniques has been employed to predict ordered structures for a series of A4B3O12 -phase compounds, where A is a 3+ cation ranging in size from Sc³⁺ to Ho³⁺ and B is a 4+ cation ranging from Ti⁴⁺ to Zr⁴⁺. Experimentally, a fully ordered cation structure has yet to be resolved for any of these compounds. Monte Carlo energy-minimization calculations using short-range pair potentials identified three low-energy arrangements of A³⁺ and B⁴⁺ cations. The details of these three structures were analyzed with the layer motif method. To quantitatively determine the -phase structure of each composition, the three configurations were reevaluated with density-functional theory. We also used special quasirandom structures to compare the ordered low-energy configurations to cation disorder. For all compositions considered, we find that at least one of the three ordered structures is lower in energy than the disordered structure, suggesting the thermodynamic stability of an ordered phase. Of the three ordered structures identified by this approach, one has not been identified previously in the literature for any composition. In addition, we discuss the stability of -phase compounds with respect to other “ABO4−x” fluorite-derivative compositions and predict the structure of compositions for which none has been reported.

Finding the low-energy structures of Si[001] symmetric tilted grain boundaries with a genetic algorithm

Jian Zhang, Cai-Zhuang Wang, and Kai-Ming Ho

Published 4 November 2009 (6 pages)
174102 Full Text: PDF (2206 kB) | Buy Article

+

-

Show Abstract

We developed a global structure optimization method, genetic algorithm, for a fast and efficient prediction of grain-boundary structures. Using this method we predicted the most stable structures and a number of low-energy metastable structures for Si[001] symmetric tilted grain boundaries with various tilted angles. We show that most of the grain-boundary structures can be described by the structural unit model with the units being the dislocation cores and perfect-crystal fragments. The energies of the grain-boundary structures obtained from the genetic algorithm optimization are evaluated by tight-binding calculations using the environment-dependent Si tight-binding potential developed previously and found to be in very good agreement with the first-principles calculation results.

Damping and decoherence of a nanomechanical resonator due to a few two-level systems

Laura G. Remus, Miles P. Blencowe, and Yukihiro Tanaka

Published 4 November 2009 (14 pages)
174103 Full Text: PDF (2668 kB) | Buy Article
See Also: Publisher's Note

+

-

Show Abstract

We consider a quantum model of a nanomechanical flexing beam resonator interacting with a bath comprising a few damped tunneling two-level systems. In contrast with a resonator interacting bilinearly with an ohmic free oscillator bath (modeling clamping loss, for example), the mechanical resonator damping is amplitude dependent, while the decoherence of quantum superpositions of mechanical position states depends only weakly on their spatial separation.

Structure of self-interstitial atom clusters in iron and copper

Akiyuki Takahashi and Nasr M. Ghoniem

Published 5 November 2009 (10 pages)
174104 Full Text: PDF (1122 kB) | Buy Article

+

-

Show Abstract

The dislocation core structure of self-interstitial atom (SIA) clusters in bcc iron and fcc copper is determined using the hybrid ab initio continuum method of Banerjee et al. [Philos. Mag. 87, 4131 (2007)]. To reduce reliance on empirical potentials and to facilitate predictions of the effects of local chemistry and stress on the structure of defects, we present here a hybrid extension of the Peierls-Nabarro continuum model, with lattice resistance to slip determined separately from ab initio calculations. A method is developed to reconstruct atomic arrangements and geometry of SIA clusters from the hybrid model. The results are shown to compare well with molecular-dynamics simulations. In iron, the core structure does not show dependence on the size of the self-interstitial cluster, and is nearly identical to that of a straight edge dislocation. However, the core structure of SIA clusters in Cu is shown to depend strongly on the cluster size. Small SIA clusters are found to have nondissociated compact dislocation cores, with a strong merging of Shockley partial dislocations and a relatively narrow stacking fault (SF) region. The compact nature of the SIA core in copper is attributed to the strong dependence of the self-energy on the cluster size. As the number of atoms in the SIA cluster increases, Shockley partial dislocations separate and the SF region widens, rendering the SIA core structure to that of an edge dislocation. The separation distance between the two partials widens as the cluster size increases, and tends to the value of a straight edge dislocation for cluster sizes above 400 atoms. The local stress is found to have a significant effect on the atomic arrangements within SIA clusters in copper and the width of the stacking faults. An applied external shear can delocalize the core of an SIA cluster in copper, with positive shear defined to be on the (111) plane along the [2] direction. For an SIA cluster containing 1600 atoms, a positive 1 GPa shear stress delocalizes the cluster and expands the SF to 30b, while a negative shear stress of 2 GPa contracts the core to less than 5b, where b is the Burgers vector magnitude.

Preferential stability of the d-BCT phase in ZnO thin films

Benjamin J. Morgan

Published 10 November 2009 (5 pages)
174105 Full Text: PDF (152 kB) | Buy Article

+

-

Show Abstract

Stoichiometric B4 thin films have formally divergent surface energies, which arise from the intrinsic dipole of the unit cell. Previous density functional theory studies have predicted that below a critical thickness this results in relaxation to the nonpolar planar h-MgO structure. The calculations presented here demonstrate that h-MgO-structured ZnO thin films are themselves unstable with respect to further relaxation to the d-BCT structure, which restores near-tetrahedral local coordination while minimizing the surface dipole. Although the B4h-MgO relaxation is disfavored for slabs thicker than 20 layers, d-BCT is predicted to be the favored polymorph for slabs up to 54 layers. Nudged elastic band calculations and vibrational analysis indicate that the h-MgOd-BCT relaxation is spontaneous at nonzero temperatures.

Quantitative comparisons of contrast in experimental and simulated bright-field scanning transmission electron microscopy images

James M. LeBeau, Adrian J. D'Alfonso, Scott D. Findlay, Susanne Stemmer, and Leslie J. Allen

Published 10 November 2009 (5 pages)
174106 Full Text: PDF (466 kB) | Buy Article

+

-

Show Abstract

Quantitative, atomic resolution bright-field scanning transmission electron microscopy experiments are reported. The image intensities are placed on an absolute scale relative to the incident beam intensity. Features in the experimental images, such as contrast reversals, intensities, and the image contrast, are compared with image simulations that account for elastic scattering and the effect of phonon scattering. Simulations are carried out using both the multislice absorptive and frozen phonon simulation methods. For a SrTiO3 sample with thicknesses between 4 and 25 nm, both models agree within the experimental uncertainty. We demonstrate excellent agreement between the simulated and the experimentally observed image contrast. The implications for the contrast mismatch commonly reported for high-resolution transmission electron microscopy using plane-wave illumination are discussed.

Tricritical Lifshitz point in the temperature-pressure-composition diagram for (Pb_ySn_1−y)₂P₂(Se_xS_1−x)₆ ferroelectrics

O. Andersson, O. Chobal, I. Rizak, and V. Rizak

Published 11 November 2009 (5 pages)
174107 Full Text: PDF (374 kB) | Buy Article

+

-

Show Abstract

The heat capacity of Sn2P2S6 ferroelectric crystals has been measured under quasihydrostatic pressures up to 0.7 GPa. The analysis of the heat-capacity and literature data for the birefringence shows that the tricritical point of Sn2P2S6 is in the 0.20–0.25 GPa range. Moreover, in the approximation of a linear change in the free-energy expansion coefficients, with respect to concentration and pressure, thermodynamic trajectories have been constructed for (PbySn1−y)2P2(SexS1−x)6 solid solutions. We have thereby identified the region of the T-p-y-x diagram for (PbySn1−y)2P2(SexS1−x)6 showing the tricritical Lifshitz point.

Formation mechanism of a faceted interface: In situ observation of the Si(100) crystal-melt interface during crystal growth

M. Tokairin, K. Fujiwara, K. Kutsukake, N. Usami, and K. Nakajima

Published 12 November 2009 (4 pages)
174108 Full Text: PDF (364 kB) | Buy Article

+

-

Show Abstract

We investigated the formation mechanism of a faceted crystal-melt interface by in situ observation. It was directly proved that a wavy perturbation is introduced into a planar crystal-melt interface and the perturbation results in zigzag facets. Such a facet formation process was observed when growth velocity was high, although planar interfaces were maintained at low growth velocities. It was shown by theoretical analysis that the negative temperature gradient generated by the latent heat of crystallization at high growth velocities amplifies the perturbation and leads to the facet formation.

Nuclear magnetic resonance study of Li implanted in a thin film of niobium

T. J. Parolin, J. Shi, Z. Salman, K. H. Chow, P. Dosanjh, H. Saadaoui, Q. Song, M. D. Hossain, R. F. Kiefl, C. D. P. Levy, M. R. Pearson, and W. A. MacFarlane

Published 13 November 2009 (8 pages)
174109 Full Text: PDF (331 kB) | Buy Article

+

-

Show Abstract

We report results of beta-detected NMR of ⁸Li⁺ implanted in Nb at high magnetic field. We identify two distinct sites for ⁸Li in the body-centered lattice. At low temperature, the site is characterized by a well-defined quadrupolar splitting. At about 50 K this site becomes unstable. Close to room temperature, Li occupies the cubic substitutional site. Spin-lattice relaxation measurements are consistent with a site-dependent coupling to the Nb conduction electrons and suggest that the site change proceeds in two steps. We report Knight shifts for the two well-defined sites and perform a Korringa analysis.

Transferred hyperfine interactions for Yb³⁺ ions in CsCaF₃ and Cs₂NaYF₆ single crystals: Experimental and ab initio study

M. L. Falin, O. A. Anikeenok, V. A. Latypov, N. M. Khaidukov, F. Callens, H. Vrielinck, and A. Hoefstaetter

Published 13 November 2009 (11 pages)
174110 Full Text: PDF (442 kB) | Buy Article

+

-

Show Abstract

The results of an electron-nuclear double resonance study of the cubic paramagnetic Yb³⁺ center in Cs2NaYF6 and CsCaF3 single crystals are presented. The values and signs of the transferred hyperfine interaction (THFI) parameters for several neighboring shells are determined. It is found that the relevant parameters for the two studied matrices differ, in spite of the fact that the nearest environment of the rare earth ion is nearly identical. A first-principles theoretical analysis is performed for the THFI parameters of the first coordination shell of F⁻ ions. Several mechanisms of metal ion-ligand coupling are considered and it is found that one of them, ligand polarization, explains the difference observed for the THFI parameters in Cs2NaYF6 and CsCaF3.

Carbon nitride: Ab initio investigation of carbon-rich phases

Judy N. Hart, Frederik Claeyssens, Neil L. Allan, and Paul W. May

Published 17 November 2009 (13 pages)
174111 Full Text: PDF (1353 kB) | Buy Article

+

-

Show Abstract

We have examined the potential energy hypersurfaces for the carbon-rich phases of carbon nitride, CN and C3N, and discovered low-energy structures different from those reported previously. Trends in the preferred local bonding environments have been analyzed as a function of nitrogen content. For each composition, several structures with similar energies were found, but they have very different equilibrium volumes; the structure produced during synthesis will strongly depend on the preparation conditions. When low densities are favored, conjugated planar-ring structures with sp² hybridized carbon are most likely to be formed. These structures are similar to those suggested as potential photocatalytic materials. At high pressures, the preferred structures contain three-coordinate nitrogen and sp³ hybridized carbon, including the -InS structure, which we predict to be the thermodynamically preferred structure for CN under positive hydrostatic pressures. This structure has a moderately high bulk modulus with a lower formation energy than -C3N4 and so should be more easily synthesized.

Picosecond ultrasonic measurements of attenuation of longitudinal acoustic phonons in silicon

B. C. Daly, K. Kang, Y. Wang, and David G. Cahill

Published 19 November 2009 (5 pages)
174112 Full Text: PDF (209 kB) | Buy Article

+

-

Show Abstract

We report ultrafast optical measurements of the attenuation of 50 and 100 GHz longitudinal acoustic-phonon pulses in Si. Picosecond acoustic measurements were made at temperatures 50<T<300  K on thinned (50-µm-thick) wafers. The measured phonon lifetimes at 300 K, 5–7  ns, are an order of magnitude less than expected based on three-phonon scattering rates derived from thermal conductivity data. We find instead that relaxational damping is the dominant mechanism in this frequency and temperature range. This attenuation sets an intrinsic limit on the quality factor of nanomechanical resonators that operate near room temperature.

Nucleation and first-stage growth processes of extrinsic defects in GaAs triggered by self-interstitials

F. Gala and G. Zollo

Published 19 November 2009 (13 pages)
174113 Full Text: PDF (1423 kB) | Buy Article

+

-

Show Abstract

The growth process of small self-interstitial clusters In (n7) in crystalline GaAs has been addressed by semi-empirical tight-binding molecular-dynamics technique. The In ground-state structures have been found among many possible choices of topological properties and stoichiometric compositions. The stable structure have been fully characterised concerning the structural, electronic, energetic, and elastic properties; some remarkable findings emerged concerning, among the others, the stability scenario of the ground-state structures, the possible low-energy reaction paths involved in the growth process, the electrostatic and the elastic capture volumes and the Fermi-level pinning. It is demonstrated that compact geometries are no longer energetically favoured for n5 and that the In growth proceeds via capture processes involving either isolated interstitials or di-interstitials. An extended pentainterstitial (I5) ground-state structure has been identified as the possible core-basic structure of extrinsic linear defects along the {111} direction of the GaAs lattice.

Accurate Hartree-Fock energy of extended systems using large Gaussian basis sets

Joachim Paier, Cristian V. Diaconu, Gustavo E. Scuseria, Manuel Guidon, Joost VandeVondele, and Jürg Hutter

Published 20 November 2009 (8 pages)
174114 Full Text: PDF (126 kB) | Buy Article

+

-

Show Abstract

Calculating highly accurate thermochemical properties of condensed matter via wave-function-based approaches (such as, e.g., Hartree-Fock or hybrid functionals) has recently attracted much interest. We here present two strategies providing accurate Hartree-Fock energies for solid LiH in a large Gaussian basis set and applying periodic boundary conditions. The total energies were obtained using two different approaches, namely, a supercell evaluation of Hartree-Fock exchange using a truncated Coulomb operator and an extrapolation toward the full-range Hartree-Fock limit of a Padé fit to a series of short-range screened Hartree-Fock calculations. These two techniques agreed to significant precision. We also present the Hartree-Fock cohesive energy of LiH (converged to within sub-millielectron volt) at the experimental equilibrium volume as well as the Hartree-Fock equilibrium lattice constant and bulk modulus.

Inhomogeneous, disordered, and partially ordered systems

Randomness-induced redistribution of vibrational frequencies in amorphous solids

Valery Ilyin, Itamar Procaccia, Ido Regev, and Yair Shokef

Published 3 November 2009 (13 pages)
174201 Full Text: PDF (784 kB) | Buy Article

+

-

Show Abstract

Much of the discussion in the literature of the low-frequency part of the density of states of amorphous solids was dominated for years by comparing measured or simulated density of states to the classical Debye model. Since this model is hardly appropriate for the materials at hand, this created some amount of confusion regarding the existence and universality of the so-called “boson peak” which results from such comparisons. We propose that one should pay attention to the different roles played by different aspects of disorder, the first being disorder in the interaction strengths, the second positional disorder, and the third coordination disorder. These have different effects on the low-frequency part of the density of states. We examine the density of states of a number of tractable models in one and two dimensions and reach a clearer picture of the softening and redistribution of frequencies in such materials. We discuss the effects of disorder on the elastic moduli and the relation of the latter to frequency softening, reaching the final conclusion that the boson peak is not universal at all.

Annealing effect on the reduction of Fermi-level density of states in CoTiSb: NMR evidence

C. S. Lue, C. F. Chen, Fu-Kuo Chiang, and M.-W. Chu

Published 5 November 2009 (6 pages)
174202 Full Text: PDF (296 kB) | Buy Article

+

-

Show Abstract

With the aim to examine the variation in the electronic properties of CoTiSb due to heat treatment, a comparative study of the as-cast and annealed samples using ⁵⁹Co nuclear magnetic resonance (NMR) spectroscopy was performed. All NMR observations clearly indicate a significant change in the local electronic characteristics for the annealed sample. The spin-lattice relaxation rate measurements further provide an estimate of Co-d Fermi-level density of states, Nd(EF), indicating a substantial reduction in Nd(EF) for the specimen with heat treatment. This finding gives a microscopic interpretation for the larger electrical resistivity and Seebeck coefficient in the annealed half-Heusler alloys, as the samples with higher electrical resistivity and Seebeck coefficient usually are associated with lower carrier densities in the vicinity of the Fermi level.

Polymerization effects and localized electronic states in condensed-phase eumelanin

L. Sangaletti, P. Borghetti, P. Ghosh, S. Pagliara, P. Vilmercati, C. Castellarin-Cudia, L. Floreano, A. Cossaro, A. Verdini, R. Gebauer, and A. Goldoni

Published 17 November 2009 (6 pages)
174203 Full Text: PDF (346 kB) | Buy Article

+

-

Show Abstract

The electronic structure of eumelanin thin films has been investigated by means of x-ray absorption and photoemission spectroscopies. The main features of the experimental data are interpreted on the basis of density-functional calculations for the isolated monomers participating to the eumelanin macromolecule. In order to single out the polymerization effects, we followed a bottom-up scaling approach to establish the minimum supramolecular level of organization that can provide a consistent spectroscopical picture of an altogether complex and highly disordered system. A tetramer macrocycle, made by three hydroquinones and one indolequinone, is found to reproduce the observed polymerization effects at the N K edge, while preserving the experimental spectral weight among the different monomers. This tetramer is different from that predicted for the synthesis from isolated monomers, providing an experimental evidence of the role of the reaction path on the stabilization of macrocycles in condensed-phase eumelanin.

Inhomogeneity and glass-forming ability in the bulk metallic glass Pd_42.5Ni_7.5Cu₃₀P₂₀ as seen via x-ray spectroscopies

S. Hosokawa, H. Sato, T. Ichitsubo, M. Nakatake, N. Happo, J.-F. Bérar, N. Boudet, T. Usuki, W.-C. Pilgrim, E. Matsubara, and N. Nishiyama

Published 17 November 2009 (6 pages)
174204 Full Text: PDF (179 kB) | Buy Article

+

-

Show Abstract

Core-level photoemission spectroscopy and anomalous x-ray scattering (AXS) measurements were performed for the Pd42.5Ni7.5Cu30P20 (PNCP) excellent metallic glass to investigate the chemical nature and local atomic structure, and the results were compared to those in Pd40Ni40P20 and Pd40Cu40P20. The P 2p core levels clearly separate into two states, indicating that the P atoms have two different chemical sites, which is a strong experimental proof for the existence of an elastic inhomogeneity. From the AXS close to the Pd K edge, a specific Pd-P-Pd atomic configuration was observed, which is related to the stable state in the P 2p core levels. All of the core levels measured in PNCP have the deepest binding energies among these glasses, indicating the most stable electronic states. Local structure around the P atoms is discussed by the AXS data and a metastable crystal appeared in a supercooled metallic alloy close to PNCP.

Localization-delocalization transitions in a two-dimensional quantum percolation model: von Neumann entropy studies

Longyan Gong and Peiqing Tong

Published 20 November 2009 (6 pages)
174205 Full Text: PDF (490 kB) | Buy Article

+

-

Show Abstract

In two-dimensional quantum site-percolation square lattice models, the von Neumann entropy is extensively studied numerically. At a certain eigenenergy, the localization-delocalization transition is reflected by the derivative of von Neumann entropy which is maximal at the quantum percolation threshold pq. The phase diagram of localization-delocalization transitions is deduced in the extrapolation to infinite system sizes. The nonmonotonic eigenenergies dependence of pq and the lowest value pq0.665 are found. At localized-delocalized transition points, the finite scaling analysis for the von Neumann entropy is performed and it is found the critical exponents not to be universal. These studies provide an evidence that the existence of a quantum percolation threshold pq<1 in the two-dimensional quantum percolation problem.

Dynamics, dynamical systems, lattice effects, quantum solids

One-dimensional nanostructure-guided chain reactions: Harmonic and anharmonic interactions

Nitish Nair and Michael S. Strano

Published 13 November 2009 (10 pages)
174301 Full Text: PDF (1329 kB) | Buy Article

+

-

Show Abstract

We have performed a parametric study of self-propagating chain reactions along a one-dimensional bead-spring array. The coupling between beads is modeled using harmonic and anharmonic Fermi-Pasta-Ulam (FPU)- and ⁴ potentials. The parameters that define the system are the activation energy (Ea) of the reactive group and the fraction () of the reaction enthalpy that is converted to the kinetic energies of the reacted products. The mean conversion for a 100-bead lattice was investigated as a function of these handles. Assemblies of pristine chains with reactive groups having Ea<25  kcal/mol are shown to be inherently unstable. At loads of 3–4 energetic molecules/bead (Ea=35  kcal/mol,  =0.7), the FPU and harmonic lattices behaved similarly with reaction velocities ranging between 8 and 8.5 km/sec. The ⁴ lattice exhibited lower conversions along with the formation of a reaction initiation zone where the velocity was at least half of the bulk value at the aforementioned loads. Fourier analyses of the kinetic energy traces of the ⁴ lattice revealed that only high-frequency excitations led to viable wave propagation, which explains the prominence of the start-up zone at lower loadings of the energetic molecules. High velocity reaction waves are only observed in perfect crystal arrays. The presence of defects in the chain, i.e., beads with weaker force constants, hampers the progress of the wave.

Magnetism

Spin excitations in the excitonic spin-density-wave state of the iron pnictides

P. M. R. Brydon and C. Timm

Published 3 November 2009 (16 pages)
174401 Full Text: PDF (907 kB) | Buy Article

+

-

Show Abstract

Motivated by the iron pnictides, we examine the spin excitations in an itinerant antiferromagnet where a spin-density wave (SDW) originates from an excitonic instability of nested electronlike and holelike Fermi pockets. Using the random-phase approximation, we derive the Dyson equation for the transverse susceptibility in the excitonic SDW state. The Dyson equation is solved for two different two-band models, describing an antiferromagnetic insulator and metal, respectively. We determine the collective spin-wave dispersions and also consider the single-particle continua. The results for the excitonic models are compared with each other and also contrasted with the well-known SDW state of the Hubbard model. Despite the qualitatively different SDW states in the two excitonic models, their magnetic response shows many similarities. We conclude with a discussion of the relevance of the excitonic SDW scenario to the iron pnictides.

Magnetic cylindrical nanowires with single modulated diameter

S. Allende, D. Altbir, and K. Nielsch

Published 9 November 2009 (5 pages)
174402 Full Text: PDF (192 kB) | Buy Article

+

-

Show Abstract

Nucleation and propagation of a domain wall along a modulated wire as a function of its geometry has been investigated. In all the cases, nucleation began at the thicker section and propagated toward the thinner section, regardless of the reversal mode. At the interface, the wall was pinned and a higher nucleation field was required to continue the propagation of the wall. This behavior led to the existence of two nucleation fields along each branch of the hysteresis curve.

Quantum phase transition in the four-spin exchange antiferromagnet

Valeri N. Kotov, Dao-Xin Yao, A. H. Castro Neto, and D. K. Campbell

Published 10 November 2009 (6 pages)
174403 Full Text: PDF (154 kB) | Buy Article

+

-

Show Abstract

We study the S=1/2 Heisenberg antiferromagnet on a square lattice with nearest-neighbor and plaquette four-spin exchanges [introduced by A. W. Sandvik, Phys. Rev. Lett. 98, 227202 (2007).] This model undergoes a quantum phase transition from a spontaneously dimerized phase to Néel order at a critical coupling. We show that as the critical point is approached from the dimerized side, the system exhibits strong fluctuations in the dimer background, reflected in the presence of a low-energy singlet mode, with a simultaneous rise in the triplet quasiparticle density. We find that both singlet and triplet modes of high density condense at the transition, signaling restoration of lattice symmetry. In our approach, which goes beyond mean-field theory in terms of the triplet excitations, the transition appears sharp; however since our method breaks down near the critical point, we argue that we cannot make a definite conclusion regarding the order of the transition.

Current-induced dynamics in noncollinear dual spin valves

P. Baláž, M. Gmitra, and J. Barnaś

Published 11 November 2009 (6 pages)
174404 Full Text: PDF (880 kB) | Buy Article

+

-

Show Abstract

Spin-transfer torque and current-induced spin dynamics in spin-valve nanopillars with the free magnetic layer located between two magnetic films of fixed magnetic moments is considered theoretically. The spin-transfer torque in the limit of diffusive spin transport is calculated as a function of magnetic configuration. It is shown that noncollinear magnetic configuration of the outermost magnetic layers has a strong influence on the spin torque and spin dynamics of the central free layer. Employing macrospin simulations we make some predictions on the free layer spin dynamics in spin valves composed of various magnetic layers. We also present a formula for critical current in noncollinear magnetic configurations, which shows that the magnitude of critical current can be several times smaller than that in typical single spin valves.

Spin-torque-induced switching and precession in fully epitaxial Fe/MgO/Fe magnetic tunnel junctions

Rie Matsumoto, Akio Fukushima, Kay Yakushiji, Satoshi Yakata, Taro Nagahama, Hitoshi Kubota, Toshikazu Katayama, Yoshishige Suzuki, Koji Ando, Shinji Yuasa, Benoit Georges, Vincent Cros, Julie Grollier, and Albert Fert

Published 12 November 2009 (8 pages)
174405 Full Text: PDF (547 kB) | Buy Article

+

-

Show Abstract

We experimentally investigated current-driven oscillation in fully epitaxial Fe(001)/MgO(001)/Fe(001) magnetic tunnel junctions (MTJs) to pave the way for a better understanding of why the linewidth (a few hundred MHz) of microwave oscillation in spin-torque nano-oscillators (STNOs) based on textured MTJs is much larger than that (smaller than 10 MHz) in STNOs based on current-perpendicular-to-plane giant-magnetoresistance junctions. The epitaxial Fe/MgO/Fe STNO is a model system for studying the physics of spin-transfer torque because it has a well-defined single-crystal barrier and electrode layers with atomically flat interfaces. In the Fe/MgO/Fe STNOs, clear spin-torque-induced switching and spin-torque-induced precession were observed in epitaxial MTJs. When the initial magnetic alignment was antiparallel and the bias current exceeded the threshold current, a state in which the spin-torque compensates for the damping, the STNOs showed a rapid increase in the peak intensity, a redshift of the peak frequency, and a minimum linewidth, all clear evidence of spin-torque-induced precession above the threshold current. The minimum linewidth of the STNOs was 200 MHz, which is comparable to that of textured CoFeB/MgO/CoFeB MTJs. This indicates that the origin of the large linewidth cannot be attributed to structural inhomogeneity in textured MTJs. When the initial magnetic alignment was parallel, the microwave spectrum showed a single peak, which has rarely been observed in textured MTJs without application of a perpendicular magnetic field. The mechanism of the single-peak oscillation can be explained by taking account of the induced perpendicular magnetic anisotropy in the 3-nm-thick Fe(001) free layer grown on the MgO(001) barrier layer.

Prediction of electrically induced magnetic reconstruction at the manganite/ferroelectric interface

J. D. Burton and E. Y. Tsymbal

Published 13 November 2009 (6 pages)
174406 Full Text: PDF (2754 kB) | Buy Article

+

-

Show Abstract

The control of magnetization via the application of an electric field, known as magnetoelectric coupling, is among the most fascinating and active research areas today. In addition to fundamental scientific interest, magnetoelectric effects may lead to new device concepts for data storage and processing. There are several known mechanisms for magnetoelectric coupling that include intrinsic effects in single-phase materials, strain-induced coupling in two-phase composites, and electronically driven effects at interfaces. Here we explore a different type of magnetoelectric effect at a ferromagnetic-ferroelectric interface: magnetic reconstruction induced by switching of electric polarization. We demonstrate this effect using first-principles calculations of a La1−xAxMnO3/BaTiO3 (001) interface, where A is a divalent cation. By choosing the doping level x to be near a transition between magnetic phases we show that the reversal of the ferroelectric polarization of BaTiO3 leads to a change in the magnetic order at the interface from ferromagnetic to antiferromagnetic. This predicted electrically induced magnetic reconstruction at the interface represents a substantial interfacial magnetoelectric effect.

In-plane spin reorientation transition in a two-dimensional ferromagnetic/antiferromagnetic system studied using Monte Carlo simulations

J. H. Seok, H. Y. Kwon, S. S. Hong, Y. Z. Wu, Z. Q. Qiu, and C. Won

Published 13 November 2009 (6 pages)
174407 Full Text: PDF (383 kB) | Buy Article

+

-

Show Abstract

We investigated the in-plane spin reorientation transition (SRT) in a two-dimensional ferromagnetic/antiferromagnetic system by using Monte Carlo simulation based on the Heisenberg model. The temperature-driven SRT in the ferromagnetic layer is studied for the case of that the antiferromagnetic layer has an uniaxial in-plane or fourfold in-plane anisotropy. For the case of an uniaxial in-plane anisotropy in the antiferromagnetic layer, the ferromagnetic layer spin processes a 90° in-plane SRT from perpendicular to parallel direction of the anisotropy. For the case of a fourfold in-plane anisotropy in the antiferromagnetic layer, the ferromagnetic layer spin processes a 45° in-plane SRT. We also studied the Curie temperature of the ferromagnetic layer and find a significant reduction in the Curie temperature as the Curie temperature is close to the SRT temperature.

Muon spin relaxation studies of the frustrated quasi-two-dimensional square-lattice spin system Cu(Cl,Br)La(Nb,Ta)₂O₇: Evolution from spin-gap to antiferromagnetic state

Y. J. Uemura, A. A. Aczel, Y. Ajiro, J. P. Carlo, T. Goko, D. A. Goldfeld, A. Kitada, G. M. Luke, G. J. MacDougall, I. G. Mihailescu, J. A. Rodriguez, P. L. Russo, Y. Tsujimoto, C. R. Wiebe, T. J. Williams, T. Yamamoto, K. Yoshimura, and H. Kageyama

Published 13 November 2009 (9 pages)
174408 Full Text: PDF (770 kB) | Buy Article

+

-

Show Abstract

We report muon spin relaxation (µSR) and magnetic-susceptibility measurements on Cu(Cl,Br)La(Nb,Ta)2O7, which demonstrate: (a) the absence of static magnetism in (CuCl)LaNb2O7 down to 15 mK confirming a spin-gapped ground state; (b) phase separation between partial volumes with a spin-gap and static magnetism in (CuCl)La(Nb,Ta)2O7; (c) history-dependent magnetization in the (Nb,Ta) and (Cl,Br) substitution systems; (d) a uniform long-range collinear antiferromagnetic state in (CuBr)LaNb2O7; and (e) a decrease in Néel temperature with decreasing Br concentration x in Cu(Cl1−xBrx)LaNb2O7 with no change in the ordered Cu moment size for 0.33x1. Together with several other µSR studies of quantum phase transitions in geometrically frustrated spin systems, the present results reveal that the evolution from a spin-gap to a magnetically ordered state is often associated with phase separation and/or a first-order phase transition.

Quantum phase transition in (CuCl)La(Nb_1−xTa_x)₂O₇

A. Kitada, Y. Tsujimoto, H. Kageyama, Y. Ajiro, M. Nishi, Y. Narumi, K. Kindo, M. Ichihara, Y. Ueda, Y. J. Uemura, and K. Yoshimura

Published 13 November 2009 (5 pages)
174409 Full Text: PDF (311 kB) | Buy Article

+

-

Show Abstract

We demonstrate the synthesis and magnetic properties of a quasi-two-dimensional frustrated quantum spin system (CuCl)La(Nb1−xTax)2O7. We observed persistence of the spin-singlet state in (CuCl)LaNb2O7 up to x~0.4, accompanied by a slight reduction in the spin gap with increasing x. In spite of unaltered cell parameters and a preserved CuCl plane, (CuCl)LaTa2O7 exhibits collinear antiferromagnetic (CAF) order with TN~7  K as observed in (CuBr)LaNb2O7. In the intermediate region (0.4<x<1), we observed CAF order with a significantly reduced magnetic moment but with a nearly constant TN, suggesting that the CAF state coexists with the spin-singlet state in agreement with recent µSR results.

Spontaneous antiferromagnetic long-range order in the two-dimensional hybrid model of localized Ising spins and itinerant electrons

Jozef Strečka, Akinori Tanaka, Lucia Čanová, and Taras Verkholyak

Published 16 November 2009 (8 pages)
174410 Full Text: PDF (207 kB) | Buy Article

+

-

Show Abstract

The generalized decoration-iteration transformation is adopted to treat exactly a hybrid model of doubly decorated two-dimensional lattices, which have localized Ising spins at their nodal lattice sites and itinerant electrons delocalized over pairs of decorating sites. Under the assumption of a half filling of each couple of the decorating sites, the investigated model system exhibits a remarkable spontaneous antiferromagnetic long-range order with an obvious quantum reduction in the staggered magnetization. It is shown that the critical temperature of the spontaneously long-range ordered quantum antiferromagnet displays an outstanding nonmonotonic dependence on a ratio between the kinetic term and the Ising-type exchange interaction.

Electrical resistivity and magnetoresistance of single-crystal Tb₅Si_2.2Ge_1.8

M. Zou, V. K. Pecharsky, K. A. Gschneidner, Jr., Ya. Mudryk, D. L. Schlagel, and T. A. Lograsso

Published 16 November 2009 (7 pages)
174411 Full Text: PDF (362 kB) | Buy Article

+

-

Show Abstract

A positive colossal magnetoresistance (CMR) of 160% has been observed in Tb5Si2.2Ge1.8 with the magnetic field applied parallel to the a axis. When the magnetic field is applied parallel to the b and c axes, the magnetoresistance (MR) is less than 8% and 5%, respectively. The CMR effect originates from intrinsic crystallographic phase coexistence. The anisotropy of the MR effect is due to a unique geometric arrangement of the interphase boundaries and large magnetocrystalline anisotropy of the compound.

Large extraordinary Hall effect and anomalous scaling relations between the Hall and longitudinal conductivities in -Fe₃N nanocrystalline films

Y. H. Cheng, R. K. Zheng, Hui Liu, Yong Tian, and Z. Q. Li

Published 17 November 2009 (5 pages)
174412 Full Text: PDF (486 kB) | Buy Article

+

-

Show Abstract

An enhancement of extraordinary Hall coefficient over two orders of magnitude larger than that of bulk Fe is found in conductive -Fe3N nanocrystalline films with similar and nearly temperature-independent conductivities, but extremely different structural defect content. A scaling exponential of n=1.59 in xy~ between the Hall and longitudinal conductivities is obtained for the well-crystallized sample, which fits well with the recent developed universal scaling theory characterized by n=1.6 in the dirty limit. However, no scaling relation is valid for the sample with a large amount of amorphous parts and the fitting relation of xy between the Hall and longitudinal resistivities at the lower resistivity range gives an unexpected high exponential of n=17.6. The anomalous scaling behavior may be qualitatively explained by the mean free path model due to the temperature-dependent scattering by spin-disordered grain boundaries and amorphous phases. Because of the large Hall coefficient, nearly temperature-independent Hall and longitudinal resistivity, and rather low Ohmic resistivity, the -Fe3N nanocrystalline film might be a promising candidate for low-field Hall sensors.

Low-temperature study of field-induced antiferromagnetic-ferromagnetic transition in Pd-doped Fe-Rh

Pallavi Kushwaha, Archana Lakhani, R. Rawat, and P. Chaddah

Published 17 November 2009 (7 pages)
174413 Full Text: PDF (1318 kB) | Buy Article

+

-

Show Abstract

The first-order antiferromagnetic (AFM) to ferromagnetic (FM) transition in the functional material Fe49(Rh0.93Pd0.07)51 has been studied at low temperatures and high magnetic fields. We have addressed the nonmonotonic variation in lower critical field required for FM to AFM transition. It is shown that critically slow dynamics of the transition dominates below 50 K. At low temperature and high magnetic field, state of the system depends on the measurement history resulting in tunable coexistence of AFM and FM phases. By following cooling and heating in unequal magnetic field protocol it is shown that equilibrium state at 6 T magnetic field is AFM state. Glasslike FM state at 6 T (obtained after cooling in 8 T) shows reentrant transition with increasing temperature; viz., devitrification to AFM state followed by melting to FM state.

Magnetic and electronic properties of Eu_1−xSr_xMnO₃ (0.3<x<0.7) single crystals

Y. Tomioka, R. Kumai, T. Ito, and Y. Tokura

Published 17 November 2009 (8 pages)
174414 Full Text: PDF (2931 kB) | Buy Article

+

-

Show Abstract

Among versatile perovskite manganites showing colossal magnetoresistance (CMR), Eu1−xSrxMnO3 around x~0.4 shows the ferromagnetic metal with the lowest transition temperature (~40  K) due to the narrowest one-electron bandwidth and hence is readily subject to the drastic phase changes with variations in temperature, magnetic field, and hole doping (x). We have investigated the variation in the electronic state with x with use of single crystals. As x increases, the ferromagnetic metal at 0.38x0.47 changes to a spin-glasslike insulator at 0.48x0.5, the layered antiferromagnetic (A-type) state at 0.51x<0.6, and the chained antiferromagnetic (C-type) state at 0.6x, respectively. Due to the increased disorder upon alloying of Eu/Sr, the charge/orbital ordering with the modulation vector (0, q, 0) with q=1/2 in the orthorhombic Pbnm setting (a0~b0~c0/2~2ap, ap being the lattice parameter of the pseudocubic lattice) cannot be formed in a long range but remains short ranged. In the vicinity of x=0.5, an application of an external magnetic field removes such short-range charge/orbital ordering (q=1/2) to cause an insulator to metal transition or CMR. In a broad range of x (0.55x0.59), the critical temperature for the layered antiferromagnetic state is relatively lowered and alternatively the charge/orbital ordering with q~1/3 becomes to be visible. It has also been found that the modulation is temperature dependent; commensurate with q~1/3 upon the transition while incommensurate with q>1/3 at low temperatures.

Temperature, magnetic field, and pressure dependence of the crystal and magnetic structures of the magnetocaloric compound Mn_1.1Fe_0.9(P_0.8Ge_0.2)

D. M. Liu, Q. Z. Huang, M. Yue, J. W. Lynn, L. J. Liu, Y. Chen, Z. H. Wu, and J. X. Zhang

Published 17 November 2009 (8 pages)
174415 Full Text: PDF (617 kB) | Buy Article

+

-

Show Abstract

Neutron powder-diffraction studies of the crystal and magnetic structures of the magnetocaloric compound Mn1.1Fe0.9(P0.8Ge0.2) have been carried out as a function of temperature, applied magnetic field, and pressure. The data reveal that there is only one transition observed over the entire range of variables explored, which is a combined magnetic and structural transformation between the paramagnetic (PM) and ferromagnetic phases (Tc255  K for this composition). The structural part of the transition is associated with an expansion of the hexagonal unit cell in the direction of the a and b axes and a contraction of the c axis as the FM phase is formed, which originates from an increase in the intralayer metal-metal bond distance. The application of pressure is found to have an adverse effect on the formation of the FM phase since pressure opposes the expansion of the lattice and hence decreases Tc. The application of a magnetic field, on the other hand, has the expected effect of enhancing the FM phase and increasing Tc. We find that the substantial range of temperature/field/pressure coexistence of the PM and FM phases observed is due to compositional variations in the sample. In situ high-temperature diffraction measurements were carried out to explore this issue, and reveal a coexisting liquid phase at high temperatures that is the origin of this variation. We show that this range of coexisting phases can be substantially reduced by appropriate heat treatment to improve the sample homogeneity.

Anisotropy barrier reduction in fast-relaxing Mn₁₂ single-molecule magnets

Stephen Hill, Muralee Murugesu, and George Christou

Published 18 November 2009 (14 pages)
174416 Full Text: PDF (1554 kB) | Buy Article

+

-

Show Abstract

An angle-swept high-frequency electron paramagnetic resonance (HFEPR) technique is described that facilitates efficient in situ alignment of single-crystal samples containing low-symmetry magnetic species such as single-molecule magnets (SMMs). This cavity-based technique involves recording HFEPR spectra at fixed frequency and field, while sweeping the applied field orientation. The method is applied to the study of a low-symmetry Jahn-Teller variant of the extensively studied spin S=10 Mn12 SMMs (e.g., Mn12-acetate). The low-symmetry complex also exhibits SMM behavior, but with a significantly reduced effective barrier to magnetization reversal (Ueff43  K) and, hence, faster relaxation at low temperature in comparison with the higher-symmetry species. Mn12 complexes that crystallize in lower symmetry structures exhibit a tendency for one or more of the Jahn-Teller axes associated with the Mn^III atoms to be abnormally oriented, which is believed to be the cause of the faster relaxation. An extensive multi-high-frequency angle-swept and field-swept electron paramagnetic resonance study of [Mn12O12(O2CCH2Bu^t)16(H2O)4]·CH2Cl2·MeNO2 is presented in order to examine the influence of the abnormally oriented Jahn-Teller axis on the effective barrier to magnetization reversal. The reduction in the axial anisotropy, D, is found to be insufficient to account for the nearly 40% reduction in Ueff. However, the reduced symmetry of the Mn12 core gives rise to a very significant second-order transverse (rhombic) zero-field-splitting anisotropy, ED/6. This, in turn, causes a significant mixing of spin projection states well below the top of the classical anisotropy barrier. Thus, magnetic quantum tunneling is the dominant factor contributing to the effective barrier reduction in fast relaxing Mn12 SMMs.

Quantum phase transition of the one-dimensional transverse-field compass model

Ke-Wei Sun and Qing-Hu Chen

Published 20 November 2009 (8 pages)
174417 Full Text: PDF (428 kB) | Buy Article

+

-

Show Abstract

The quantum phase transition (QPT) of the one-dimensional (1D) quantum compass model in a transverse magnetic field is studied in this paper. An exact solution is obtained by using an extended Jordan and Wigner transformation to the pseudospin operators. The fidelity susceptibility, the concurrence, the block-block entanglement entropy, and the pseudospin correlation functions are calculated with antiperiodic boundary conditions. The QPT driven by the transverse-field only emerges at zero field and is of the second order. Several critical exponents obtained by finite-size scaling analysis are the same as those in the 1D transverse-field Ising model, suggesting the same universality class. A logarithmic divergence of the entanglement entropy of a block at the quantum critical point is also observed. From the calculated coefficient connected to the central charge of the conformal field theory, it is suggested that the block entanglement depends crucially on the detailed topological structure of a system.

Transition from spin-density-wave to layered antiferromagnetic state induced by hydrogen as a test for the origin of spin-density waves in chromium

V. M. Uzdin, H. Zabel, A. Remhof, and B. Hjörvarsson

Published 20 November 2009 (7 pages)
174418 Full Text: PDF (171 kB) | Buy Article

+

-

Show Abstract

Neutron scattering experiments of Cr/V(001) superlattices are discussed, which show that the incommensurate spin-density-wave (SDW) in thick Cr layers becomes suppressed when the V spacer layers are loaded with hydrogen. The hydrogen loading triggers a transition from the incommensurate SDW state to a commensurate antiferromagnetic state. Model Hamiltonian calculations are presented, which show that this transition is not connected with the nesting property of the Cr Fermi surface. Instead, the transition is a manifestation of the antiferromagnetic ground state of Cr, which is separated from the incommensurate SDW state by an energy barrier. Hydrogen is identified as an effective trigger for reducing the activation barrier, enabling the system to relax to the ground state.

Mechanism of exchange bias for isolated nanoparticles embedded in an antiferromagnetic matrix

Valera P. Shcherbakov, Karl Fabian, and Suzanne A. McEnroe

Published 20 November 2009 (10 pages)
174419 Full Text: PDF (591 kB) | Buy Article

+

-

Show Abstract

Single magnetic nanodots, exchange coupled to an antiferromagnetic (AF) matrix, can produce large exchange bias, while superparamagnetic behavior of the nanodots is suppressed. The exchange bias originates from the formation of a (quasi)spherical domain wall inside the AF matrix when the particle moment rotates under the influence of an external magnetic field. Micromagnetic calculations show that for isolated nanodots the energy of this domain wall increases nearly quadratically with the deflection angle of the nanodot moment. By introducing the corresponding quadratic energy term in a modified Stoner-Wohlfarth model, a two-parameter family of hysteresis loops is obtained, depending on scaled anisotropy energy and field direction. The loops are represented in a phase diagram with three main regions, containing (1) reversible loops, (2) irreversible loops with a metastable 180° AF domain wall, and (3) loops with metastable AF domain walls with 360° or higher rotation angles. According to this model, isolated nanodots display reversible negatively biased loops for all field directions, if their anisotropy energy is small in comparison to the AF domain-wall energy. For higher anisotropy, irreversible, mostly negatively biased, loops result from switching between the ground state and an higher-energy inverse state with a 180° AF domain wall. At even higher anisotropy energy, the loops can show positive exchange bias after an initial “training branch.” Switching after “training” takes place between states having a 180° and a 360° AF domain wall, respectively. While for thin films, the bias field increases in inverse proportion to thickness, for nanodots it increases in inverse proportion to the square of particle diameter. Therefore, nanodots can show significantly larger exchange bias than thin films of similar dimension. Hysteresis loops, obtained from averaging over directions and sizes using the modified Stoner-Wohlfarth model, were compared to measurements from a natural sample with nanometer-scale ilmenite-exsolution lamellae in a hematite matrix. The shapes of the hysteresis difference, the difference between upper and lower branches, are similar for model and experiment, whereby increasing temperature in the measurement corresponds qualitatively to decreasing the relative anisotropy energy in the model.

Superfluidity and superconductivity

Dynamic many-body theory: Pair fluctuations in bulk ⁴He

C. E. Campbell and E. Krotscheck

Published 2 November 2009 (14 pages)
174501 Full Text: PDF (284 kB) | Buy Article

+

-

Show Abstract

Dynamical excitations in bulk liquid ⁴He are investigated by using a manifestly microscopic theory of excitations that includes multiple-phonon scattering. The wave function of the dynamic system is represented in terms of one- and two-body excitation amplitudes. Equations of motion for the linear response of boson liquids to a scalar external field are then derived from a stationarity principle. For a consistent treatment of long- and short-wavelength properties of the excitation amplitudes we derive and solve three sets of generic “hypernetted chain” equations determining the basic ingredients of the theory. From those ingredients, we calculate a dynamic structure function for ⁴He at saturation density. It is shown that the complete solution of the hypernetted chain equations leads, partly by the cancellation of errors, to an insignificantly improved theoretical prediction for the dynamic structure function compared with approximations introduced by Jackson, Feenberg, and Campbell. The implications of this result and the need for including higher-order multiparticle fluctuations are discussed.

Power-dependent internal loss in Josephson bifurcation amplifiers

Michio Watanabe, Kunihiro Inomata, Tsuyoshi Yamamoto, and Jaw-Shen Tsai

Published 3 November 2009 (8 pages)
174502 Full Text: PDF (477 kB) | Buy Article

+

-

Show Abstract

We have studied nonlinear superconducting resonators: /2 coplanar-waveguide (CPW) resonators with Josephson junctions (JJs) placed in the middle and /4 CPW resonators terminated by JJs, which can be used for the qubit readout as “bifurcation amplifiers.” The nonlinearity of the resonators arises from the Josephson junctions, and because of the nonlinearity, the resonators with appropriate parameters are expected to show a hysteretic response to the frequency sweep, or “bifurcation,” when they are driven with a sufficiently large power. We designed and fabricated resonators whose resonant frequencies were around 10 GHz. We characterized the resonators at low temperatures, T<0.05  K, and confirmed that they indeed exhibited hysteresis. The sizes of the hysteresis, however, are sometimes considerably smaller than the predictions based on the loaded quality factor in the weak drive regime. When the discrepancy appears, it is mostly explained by taking into account the internal loss, which often increases in our resonators with increasing drive power in the relevant power range. As a possible origin of the power-dependent loss, the quasiparticle channel of conductance of the JJs is discussed.

Off-diagonal long-range order and supersolidity in a quantum solid with vacancies

Yu Shi, Yin Yang, and Shao-Ming Fei

Published 3 November 2009 (9 pages)
174503 Full Text: PDF (141 kB) | Buy Article

+

-

Show Abstract

We consider a lattice of bosonic atoms, whose number N may be smaller than the number of lattice sites M. We study the Hartree-Fock wave function built up from localized wave functions w(r) of single atoms, with nearest-neighboring overlap. The zero-momentum particle number is expressed in terms of permanents of matrices. In one dimension, it is analytically calculated to be N(M−N+1)/M, with =|w(r)d|²/[(1+2a)l], where a is the nearest-neighboring overlap and l is the lattice constant. is on the order of 1. The result indicates that the condensate fraction is proportional to and of the same order of magnitude as that of the vacancy concentration, hence there is off-diagonal long-range order or Bose-Einstein condensation of atoms when the number of vacancies M−N is a finite fraction of the number of the lattice sites M.

Negative impurity ions in liquid ⁴He

F. Ancilotto, M. Barranco, and M. Pi

Published 4 November 2009 (8 pages)
174504 Full Text: PDF (1352 kB) | Buy Article

+

-

Show Abstract

We present theoretical results, based on zero-temperature density-functional theory, for the formation and properties of a negative impurity ion in bulk liquid ⁴He. We first consider Ca which, due to its very low electron affinity, does not easily form a negative ion in vacuum. We show that a neutral Ca atom in bulk liquid ⁴He can easily capture a nearby electron bubble leading to the exothermic formation of a Ca⁻ ion trapped inside a spherical cavity of ~15  Å radius. The Ca negative ion in bulk ⁴He turns out to be a metastable state, the lowest-energy configuration being represented instead by a weakly bound Ca⁻ ion floating over the nearly unperturbed free surface of liquid ⁴He. We have computed the threshold negative pressure at which the trapped Ca⁻ ion bubble explodes and we discuss our results in light of recent experimental measurements. We have also considered the possible ion formation in the case of a Ne atom, i.e., an atomic impurity that does not form a negative ion in vacuum. Despite the long-range attraction between the electron bubble and the Ne atom due to polarization forces, in the minimum-energy configuration the electron bubble and the Ne atom rather than merge together remain spatially separated in bulk liquid ⁴He, forming a weakly bound state that has no analog in vacuum.

Origin of the checkerboard pattern in scanning tunneling microscopy maps of underdoped cuprate superconductors

Kai-Yu Yang, Wei-Qiang Chen, T. M. Rice, and Fu-Chun Zhang

Published 6 November 2009 (7 pages)
174505 Full Text: PDF (565 kB) | Buy Article

+

-

Show Abstract

The checkerboard pattern in the differential conductance maps on underdoped cuprates appears when the scanning tunneling microscopy is placed above the O sites in the outermost CuO2 plane. In this position the interference between tunneling paths through the apical ions above the neighboring Cu sites leads to an asymmetric weighting of final states in the two antinodal regions of k space. The form of the asymmetry in the differential conductance spectra in the checkerboard pattern favors asymmetry in the localization length rather than a nematic displacement as the underlying origin.

Evidence for exchange interaction between donor and acceptor layers in -(BEDT-TTF)(TCNQ)

Yoshihiro Eto, Atsushi Kawamoto, Noriaki Matsunaga, Kazushige Nomura, Kaoru Yamamoto, and Kyuya Yakushi

Published 6 November 2009 (5 pages)
174506 Full Text: PDF (463 kB) | Buy Article

+

-

Show Abstract

We assessed the infrared-absorption spectra and ¹³C-NMR measurements in a layered organic salt, -(BEDT-TTF)(TCNQ), which exhibits antiferromagnetic transitions at 20 and 3 K. The former originates from the spin in the bis-(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) layers, while the latter originates from the localized spin in the tetracyanoquinodimethane (TCNQ) layers. Using infrared-absorption spectroscopy, we estimated the degree of charge transfer, , between BEDT-TTF and TCNQ as 0.5. Using ¹³C-NMR spectroscopy, we observed an exchange field at the BEDT-TTF site, which is produced by the localized spins of TCNQ dimers. Using the obtained value of and the molecular arrangement of -(BEDT-TTF)(TCNQ), which is similar to that of the highest Tc organic superconductor, -(BEDT-TTF)2ICl2, we concluded that the absence of the pressure-induced superconductivity in -(BEDT-TTF)(TCNQ) results from the presence of this exchange field. The exchange interaction, J, and the exchange field, Hex, were estimated as −12  K and −19  T/µB on the TCNQ dimer unit, respectively. These findings suggest that superconductivity may arise in -(BEDT-TTF)(TCNQ) by the application of an external field of 19 T under high pressure.

Anisotropic microwave conductivity of cuprate superconductors in the presence of CuO chain-induced impurities

Zhi Wang and Shiping Feng

Published 6 November 2009 (6 pages)
174507 Full Text: PDF (222 kB) | Buy Article

+

-

Show Abstract

The anisotropy in the microwave conductivity of the ortho-II YBa2Cu3O6.50 is studied within the kinetic-energy driven superconducting mechanism. The ortho-II YBa2Cu3O6.50 is characterized by a periodic alternative of filled and empty -axis CuO chains. By considering the CuO chain-induced extended anisotropy impurity scattering, the main features of the anisotropy in the microwave conductivity of the ortho-II YBa2Cu3O6.50 are reproduced based on the nodal approximation of the quasiparticle excitations and scattering processes, including the intensity and line shape of the energy and temperature dependences of the â-axis and -axis microwave conductivities. Our results also confirm that the -axis CuO chain-induced impurity is the main source of the anisotropy.

Nonlocal transport through multiterminal diffusive superconducting nanostructures

F. S. Bergeret and A. Levy Yeyati

Published 9 November 2009 (9 pages)
174508 Full Text: PDF (1326 kB) | Buy Article

+

-

Show Abstract

Motivated by recent experiments on nonlocal transport through multiterminal superconducting hybrid structures, we present self-consistent calculations based on quasiclassical Green's functions for the order parameter, currents, and voltages in a system consisting of a diffusive superconductor connected to two normal and one superconducting electrodes. We investigate nonequilibrium effects for different biasing conditions corresponding to measurements of the nonlocal conductance and of the nonlocal resistance. It is shown that while the nonlocal conductance does not change its sign, this change might be observed in a nonlocal resistance measurement for certain parameter range. The change in sign of the nonlocal signal takes places at a voltage of the order of the self-consistent gap of the superconducting region. We show that this is not related to the nonlocal Andreev processes but rather to nonequilibrium effects. We finally discuss the case of four terminal measurements and demonstrate that a change in sign in the nonlocal resistance appears when the current injected into the superconductor exceeds a critical value. The connection to the existing experiments is discussed.

Charge-carrier localization induced by excess Fe in the superconductor Fe_1+yTe_1−xSe_x

T. J. Liu, X. Ke, B. Qian, J. Hu, D. Fobes, E. K. Vehstedt, H. Pham, J. H. Yang, M. H. Fang, L. Spinu, P. Schiffer, Y. Liu, and Z. Q. Mao

Published 9 November 2009 (6 pages)
174509 Full Text: PDF (1074 kB) | Buy Article

+

-

Show Abstract

We have investigated the effect of Fe nonstoichiometry on properties of the Fe1+y(Te,Se) superconductor system by means of resistivity, Hall coefficient, magnetic susceptibility, and specific-heat measurements. We find that the excess Fe at interstitial sites of the (Te, Se) layers not only suppresses superconductivity but also results in a weakly localized electronic state. We argue that these effects originate from the magnetic coupling between the excess Fe and the adjacent Fe square-planar sheets, which favors a short-range magnetic order.

Unconventional electronic reconstruction in undoped (Ba,Sr)Fe₂As₂ across the spin density wave transition

M. Yi, D. H. Lu, J. G. Analytis, J.-H. Chu, S.-K. Mo, R.-H. He, M. Hashimoto, R. G. Moore, I. I. Mazin, D. J. Singh, Z. Hussain, I. R. Fisher, and Z.-X. Shen

Published 9 November 2009 (10 pages)
174510 Full Text: PDF (7623 kB) | Buy Article

+

-

Show Abstract

Through a systematic high-resolution angle-resolved photoemission study of the iron pnictide compounds (Ba,Sr)Fe2As2, we show that the electronic structures of these compounds are significantly reconstructed across the spin density wave transition, which cannot be described by a simple folding scenario of conventional density wave ordering. Moreover, we find that LDA calculations with an incorporated suppressed magnetic moment of 0.5µB can match well the details in the reconstructed electronic structure, suggesting that the nature of magnetism in the pnictides is more itinerant than local, while the origin of suppressed magnetic moment remains an important issue for future investigations.

Square lattice hard-core bosons within the random phase approximation

T. N. Antsygina, M. I. Poltavskaya, I. I. Poltavsky, and K. A. Chishko

Published 10 November 2009 (9 pages)
174511 Full Text: PDF (590 kB) | Buy Article

+

-

Show Abstract

Using random phase approximation we build finite-temperature phase diagrams and calculate thermodynamic functions of a square lattice hard-core boson model with nearest (nn) and next-nearest-neighbor (nnn) interactions in terms of equivalent to it an anisotropic spin-1/2 XXZ model in a magnetic field. The system undergoes liquid-solid phase transitions that can be either of the first or second order. Depending on the hopping value and ratio between nn and nnn interactions the system displays two types of critical behavior. The line of the first-order transitions terminates in a bicritical end point inside the solid phase or ends in a tricritical point continuously giving way to the second-order phase transition line. The connection of the hopping and the ratio between the nn and nnn interactions with criticality of the system is investigated. The critical line separating the regions of specific critical behavior is built. Reentrant liquid-solid-liquid phase transitions are found and discussed.

Neutron-irradiation effects in polycrystalline LaFeAsO_0.9F_0.1 superconductors

A. E. Karkin, J. Werner, G. Behr, and B. N. Goshchitskii

Published 11 November 2009 (6 pages)
174512 Full Text: PDF (270 kB) | Buy Article

+

-

Show Abstract

The effect of atomic disordering induced by fast neutron irradiation on normal and superconducting state properties of polycrystalline LaFeAsO0.9F0.1 samples was investigated. Fast neutron (E>0.1  MeV) irradiation to a “moderate” fluence =1.6×10¹⁹  cm⁻² at Tirr=50±10 °C leads to a complete suppression of superconductivity which recovers almost completely after annealing at Tann750 °C. It is shown that decrease of Tc under disordering is determined mainly by reduction in electronic relaxation time . Such behavior is qualitatively described by the universal Abrikosov-Gor'kov equation, which testifies to an anomalous type of electron pairing in Fe-based superconductors.

Theory of two-dimensional macroscopic quantum tunneling in YBa₂Cu₃O₇₋ Josephson junctions coupled to an LC circuit

Shiro Kawabata, Thilo Bauch, and Takeo Kato

Published 13 November 2009 (12 pages)
174513 Full Text: PDF (805 kB) | Buy Article

+

-

Show Abstract

We investigate classical thermal activation (TA) and macroscopic quantum tunneling (MQT) for a YBa2Cu3O7− (YBCO) Josephson junction coupled to an LC circuit theoretically. Due to the coupling between the junction and the LC circuit, the macroscopic phase dynamics can be described as the escape process of a fictitious particle with an anisotropic mass moving in a two-dimensional potential. We analytically calculate the escape rate including both the TA and MQT regime by taking into account the peculiar dynamical nature of the system. In addtion to large suppression of the MQT rate at zero temperature, we study details of the temperature dependence of the escape rate across a crossover region. These results are in an excellent agreement with recent experimental data for the MQT and TA rate in a YBCO biepitaxial Josephson junction. Therefore the coupling to the LC circuit is essential in understanding the macroscopic quantum dynamics and the qubit operation based on the YBCO biepitaxial Josephson junctions.

Angular dependence of the upper critical field of Sr₂RuO₄

S. Kittaka, T. Nakamura, Y. Aono, S. Yonezawa, K. Ishida, and Y. Maeno

Published 13 November 2009 (6 pages)
174514 Full Text: PDF (283 kB) | Buy Article

+

-

Show Abstract

One of the remaining issues concerning the spin-triplet superconductivity of Sr2RuO4 is the strong limit of the in-plane upper critical field Hc2 at low temperatures. In this study, we clarified the dependence of Hc2 on the angle between the magnetic field and the ab plane at various temperatures, by precisely and accurately controlling the magnetic field direction. We revealed that, although the temperature dependence of Hc2 for ||5° is well explained by the orbital pair-breaking effect, Hc2(T) for ||<5° is clearly limited at low temperatures. We also revealed that the Hc2 limit for ||<5° is present not only at low temperatures but also at temperatures close to Tc. These features may provide additional hints for clarifying the origin of the Hc2 limit. Interestingly, if the anisotropic ratio in Sr2RuO4 is assumed to depend on temperature, the observed angular dependence of Hc2 is reproduced better at lower temperature with an effective-mass model for an anisotropic three-dimensional superconductor. We discuss the observed behavior of Hc2 based on existing theories.

Measurement of spin memory lengths in PdNi and PdFe ferromagnetic alloys

H. Z. Arham, Trupti S. Khaire, R. Loloee, W. P. Pratt, Jr., and Norman O. Birge

Published 16 November 2009 (7 pages)
174515 Full Text: PDF (350 kB) | Buy Article

+

-

Show Abstract

Weakly ferromagnetic alloys are being used by several groups in the study of superconducting/ferromagnetic hybrid systems. Because spin-flip and spin-orbit scattering in such alloys disrupt the penetration of pair correlations into the ferromagnetic material, it is desirable to have a direct measurement of the spin memory length in such alloys. We have measured the spin memory length at 4.2 K in sputtered Pd0.88Ni0.12 and Pd0.987Fe0.013 alloys using methods based on current-perpendicular-to-plane giant magnetoresistance. The alloys are incorporated into hybrid spin valves of various types, and the spin memory length is determined by fits of the Valet-Fert spin-transport equations to data of magnetoresistance vs alloy thickness. For the case of PdNi alloy, the resulting values of the spin memory length are l=2.8±0.5  nm and 5.4±0.6  nm, depending on whether or not the PdNi is exchange biased by an adjacent Permalloy layer. For PdFe, the spin memory length is somewhat longer, l=9.6±2  nm, consistent with earlier measurements indicating lower spin-orbit scattering in that material. Unfortunately, even the longer spin memory length in PdFe may not be long enough to facilitate observation of spin-triplet superconducting correlations predicted to occur in superconducting/ferromagnetic hybrid systems in the presence of magnetic inhomogeneity.

Critical currents in weakly textured MgB₂: Nonlinear transport in anisotropic heterogeneous media

M. Eisterer, W. Häßler, and P. Kováč

Published 17 November 2009 (4 pages)
174516 Full Text: PDF (273 kB) | Buy Article

+

-

Show Abstract

A model for highly nonlinear transport in heterogeneous media consisting of anisotropic particles with a preferred orientation is proposed and applied to the current transport in weakly textured magnesium diboride, MgB2. It essentially explains why, unlike in conventional superconductors, a significant macroscopic anisotropy of the critical currents can be induced by the preparation of MgB2 tapes. The field and angular dependence of the critical current is calculated for various degrees of texture and compared to experimental data.

Intrinsic pinning on structural domains in underdoped single crystals of Ba(Fe_1−xCo_x)₂As₂

R. Prozorov, M. A. Tanatar, N. Ni, A. Kreyssig, S. Nandi, S. L. Bud'ko, A. I. Goldman, and P. C. Canfield

Published 18 November 2009 (5 pages)
174517 Full Text: PDF (352 kB) | Buy Article

+

-

Show Abstract

Critical current density was studied in single crystals of Ba(Fe1−xCox)2As2 for the values of x spanning the entire doping phase diagram. A noticeable enhancement was found for slightly underdoped crystals with the peak at x=0.058. Using a combination of polarized-light imaging, x-ray diffraction and magnetic measurements we associate this behavior with the intrinsic pinning on structural domains in the orthorhombic phase. Domain walls extend throughout the sample thickness in the direction of vortices and act as extended pinning centers. With the increasing x domain structure becomes more intertwined and fine due to a decrease in the orthorhombic distortion. This results in the energy landscape with mazelike spatial modulations favorable for pinning. This finding shows that iron-based pnictide superconductors, characterized by high values of the transition temperature, high upper critical fields, and low anisotropy may intrinsically have relatively high critical current densities.

Collective cavity mode excitations in arrays of Josephson junctions

I. Ottaviani, M. Cirillo, M. Lucci, V. Merlo, M. Salvato, M. G. Castellano, G. Torrioli, F. Mueller, and T. Weimann

Published 18 November 2009 (5 pages)
174518 Full Text: PDF (597 kB) | Buy Article

+

-

Show Abstract

We report on oscillations of a Josephson-junction system revealing a close analogy with fundamental effects known in laser physics. The experiments are performed on series arrays of junctions whose IV curves show evidence of a mode in which all the junctions oscillate in synchronism on resonances appearing, in zero external magnetic field, at multiples of the first cavity mode. Evidence is provided that the mode is generated by collective oscillations which develop spontaneously because the frequencies of the modes are equal within a 1% uncertainty. Numerical simulations are employed to elucidate the reasons leading to the synchronous collective excitation and to explain why it is characterized by a low emission of electromagnetic radiation.

Devil's staircases and supersolids in a one-dimensional dipolar Bose gas

F. J. Burnell, Meera M. Parish, N. R. Cooper, and S. L. Sondhi

Published 19 November 2009 (7 pages)