Physical Review B

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

November 2009

Volume 80, Number 18 , partial issue

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RAPID COMMUNICATIONS

Structure, structural phase transitions, mechanical properties, defects

Inhomogeneous, disordered, and partially ordered systems

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

RAPID COMMUNICATIONS

Structure, structural phase transitions, mechanical properties, defects

Driving-induced crossover in the avalanche criticality of martensitic transitions

Eduard Vives, Daniel Soto-Parra, Lluís Mañosa, Ricardo Romero, and Antoni Planes

Published 17 November 2009 (4 pages)
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An experimental study of the statistical distribution of acoustic emission avalanches in martensitic transitions is presented. The critical exponents characterizing the power-law distributions of avalanches are measured as a function of cycling under soft-driving (stress-driving) and hard-driving (strain-driving) procedures. Results provide experimental support for a recent theory based on a spin model that predicts a crossover from classical criticality in the soft-driving case to self-organized criticality in the hard-driving case.

Inhomogeneous, disordered, and partially ordered systems

Icosahedral ordering in liquid iron studied via x-ray scattering and Monte Carlo simulations

Masanori Inui, Kenji Maruyama, Yukio Kajihara, and Masaru Nakada

Published 5 November 2009 (4 pages)
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X-ray diffraction measurements were carried out for liquid iron near the melting temperature and atomic configurations were constructed from the structure factor S(Q) obtained, by reverse Monte Carlo modeling and Monte Carlo simulation with the effective pair potential deduced by the inverse method. The bond-orientational order parameter 6 calculated from the atomic configurations obtained from both simulations indicates a pronounced icosahedral ordering, and the fraction of nearly icosahedral configurations is estimated to be approximately 14% in liquid iron. These experimentally obtained results seem consistent with recent results of ab initio molecular-dynamics simulation for liquid iron [P. Ganesh and M. Widom, Phys. Rev. B 77, 014205 (2008)].

Phase diagrams and isotopic effects of normal and deuterated water studied via x-ray diffraction up to 4.5 GPa and 500 K

G. Weck, J. Eggert, P. Loubeyre, N. Desbiens, E. Bourasseau, J.-B. Maillet, M. Mezouar, and M. Hanfland

Published 13 November 2009 (4 pages)
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We present synchrotron x-ray measurements in a diamond anvil cell of the molecular structure factor of H2O and D2O fluids up to 4.5 GPa and 500 K. We observe large changes in the structure factor and a dramatic increase in the oxygen coordination number over a 2 GPa pressure range. A P-T diagram of the nearest-neighbor oxygen coordination number, nOO, is disclosed. Also, a counterintuitive isotopic shift of the variation of nOO with pressure is observed.

Dynamics, dynamical systems, lattice effects, quantum solids

Selection-rule blockade and rectification in quantum heat transport

Teemu Ojanen

Published 18 November 2009 (4 pages)
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We present a thermal transport phenomenon, a unidirectional selection-rule blockade, and show how it produces unprecedented rectification of bosonic heat flow through molecular or mesoscopic quantum systems. Rectification arises from the quantization of energy levels of the conduction element and selection rules of reservoir coupling operators. The simplest system exhibiting the selection-rule blockade is an appropriately coupled three-level system, providing a candidate for a high-performance heat diode. We present an analytical treatment of the transport problem and discuss how the phenomenon generalizes to multilevel systems.

Pressure tuning of the thermal conductivity of the layered muscovite crystal

Wen-Pin Hsieh, Bin Chen, Jie Li, Pawel Keblinski, and David G. Cahill

Published 20 November 2009 (4 pages)
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The physics of heat conduction in layered, anisotropic crystals is probed by measurements of the cross-plane elastic constant C33 and thermal conductivity of muscovite mica as a function of hydrostatic pressure. Picosecond interferometry and time-domain thermoreflectance provide high-precision measurements of C33 and , respectively, of micron-sized samples within a diamond-anvil cell; changes from the anomalously low value of 0.46  W m⁻¹ K⁻¹ at ambient pressure to a value more typical of oxides crystals with large unit cells, 6.6  W m⁻¹ K⁻¹, at P=24  GPa. Most of the pressure dependence of can be accounted for by the pressure dependence of the cross-plane sound velocities.

Magnetism

Long-range order in the classical kagome antiferromagnet: Effective Hamiltonian approach

Christopher L. Henley

Published 2 November 2009 (4 pages)
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See Also: Publisher's Note

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Following Huse and Rutenberg [Phys. Rev. B 45, 7536 (1992)], I argue the classical Heisenberg antiferromagnet on the kagome lattice has long-range spin order of the × type in the limit of zero temperature. I start from the effective quartic Hamiltonian for the soft (out of plane) spin-fluctuation modes and treat as a perturbation those terms which depend on the discrete coplanar state. Soft-mode expectations become the coefficients of a discrete effective Hamiltonian, which (after a coarse graining) has the sign favoring a locking transition in the interface representation of the discrete model.

Flavor-twisted boundary condition for simulations of quantum many-body systems

Wei-Guo Yin and Wei Ku

Published 2 November 2009 (4 pages)
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We present an approximative simulation method for quantum many-body systems based on coarse graining the space of the momentum transferred between interacting particles, which leads to effective Hamiltonians of reduced size with the flavor-twisted boundary condition. A rapid, accurate, and fast convergent computation of the ground-state energy is demonstrated on the spin- quantum antiferromagnet of any dimension by employing only two sites. The method is expected to be useful for future simulations and quick estimates on other strongly correlated systems.

Role of the nonmagnetic layer in determining the Landé g-factor in a spin-transfer system

J.-S. Lee, E. Vescovo, C.-C. Kao, J.-M. Beaujour, A. D. Kent, H. Jang, J.-Y. Kim, J.-H. Park, and J. H. Shim

Published 2 November 2009 (4 pages)
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The microscopic origin of the Landé g-factor in two ferromagnetic/nonmagnetic (FM/NM) bilayer systems-Co/Cu and Ni/Pd-has been investigated using x-ray magnetic circular dichroism, resonant magnetic reflectivity, and band calculations. The FM/NM bilayer represents the building block of any complete spin-transfer structure (FM1/NM/FM2). The valence electronic structure is profoundly altered over a finite length across the FM/NM interface. A considerable charge transfer takes place from the NM to the FM material. This results in an enhancement of the orbital-to-spin magnetic moment ratio in the FM layer and an induced magnetic polarization in the NM layer. Both effects turn out to be crucial for a correct understanding of the g-factor in spin-transfer systems.

Exchange splitting of the three surface states of Ni(111) from three-dimensional spin- and angle-resolved photoemission spectroscopy

T. Okuda, J. Lobo-Checa, W. Auwärter, M. Morscher, M. Hoesch, V. N. Petrov, M. Hengsberger, A. Tamai, A. Dolocan, C. Cirelli, M. Corso, M. Muntwiler, M. Klöckner, M. Roos, J. Osterwalder, and T. Greber

Published 4 November 2009 (4 pages)
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The valence-band electronic structure of a clean Ni(111) surface is investigated by spin-resolved photoemission. At room temperature the orientation of the photoelectron spins on the Bloch sphere and the exchange splitting of surface and bulk states along the surface normal () are determined. All investigated states are found to have a sizable exchange splitting >50  meV. Since the splitting is smaller than the intrinsic line width in the spin-integrated spectrum this is only seen with a spin-resolved technique. At room-temperature photoemission reaching above the Fermi level directly shows that the Shockley type surface state S1 has an occupied majority and an unoccupied minority band with a splitting Eex=62±15  meV. The surface states below the Fermi energy show a larger exchange splitting for in-plane hybridization [Eex(S3)=160  meV] than for out-of-plane hybridization [Eex(S2)=55  meV].

Magnetic properties of the spin- XXZ model on the Shastry-Sutherland lattice: Effect of long-range interactions

Takahumi Suzuki, Yusuke Tomita, and Naoki Kawashima

Published 5 November 2009 (4 pages)
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We study magnetic properties of the spin- Ising-like XXZ model on the Shastry-Sutherland lattices with long-range interactions, using the quantum Monte Carlo method. This model shows magnetization plateau phases at one-half and one-third of the saturation magnetization when additional couplings are considered. We investigate the finite temperature transition to one-half and one-third plateau phases. The obtained results suggest that the former case is of the first order and the latter case is of the second order. We also find that the system undergoes two successive transitions with the two-dimensional Ising model universality although there is a single phase transition in the Ising limit case. Finally, we estimate the coupling ratio to explain the magnetization process observed in TmB4.

Canted spiral: An exact ground state of XXZ zigzag spin ladders

C. D. Batista

Published 6 November 2009 (4 pages)
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We derive the exact ground states for a one-dimensional family of S=1/2 XXZ Hamiltonians on the zigzag ladder. These states exhibit true long-range spiral order that spontaneously breaks the U(1) invariance of the Hamiltonian. Besides breaking a continuous symmetry in d=1, this spiral ordering has a ferromagnetic component along the symmetry axis that can take any value between zero and full saturation. In this sense, our canted spiral solutions are a generalization of the SU(2) Heisenberg ferromagnet to nonzero ordering wave vectors of the transverse spin components. We extend this result to the d=2 anisotropic triangular lattice.

Ultrafast demagnetization of ferromagnetic transition metals: The role of the Coulomb interaction

Michael Krauß, Tobias Roth, Sabine Alebrand, Daniel Steil, Mirko Cinchetti, Martin Aeschlimann, and Hans Christian Schneider

Published 9 November 2009 (4 pages)
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The Elliott-Yafet (EY) mechanism is arguably the most promising candidate to explain the light-induced ultrafast demagnetization dynamics in ferromagnetic transition metals on time scales on the order of 100 fs. So far, only electron-phonon (or impurity) scattering has been analyzed as the scattering process needed to account for the demagnetization. We show that an EY-like mechanism based on electron-electron scattering has the potential to explain time-resolved magneto-optical Kerr effect measurements on thin magnetic Co and Ni films, without reference to a “phononic spin bath.”

Anomalous induction of ferroelectric polarization by magnetization reversal in the phase-separated multiferroic manganite Eu_0.8Y_0.2MnO₃

S. Danjoh, J.-S. Jung, H. Nakamura, Y. Wakabayashi, and T. Kimura

Published 10 November 2009 (4 pages)
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Regarding electronic phase separation and its magnetoelectric control in multiferroics, we have investigated magnetic and ferroelectric properties of Eu0.8Y0.2MnO3 located near the boundary between a ferromagnetic phase and a ferroelectric phase. We observed special coexistence of the two phases, which fractions are tuned at an arbitrary level by changing the cooling magnetic-field strength. In addition, repeated magnetization reversals by applying an alternating magnetic field induce anomalous enhancement of ferroelectric polarization, suggesting the development of the ferroelectric phase during the magnetization reversal process.

Large-J approach to strongly coupled spin-orbital systems

Gia-Wei Chern and Natalia Perkins

Published 11 November 2009 (4 pages)
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We present an approach to study the ground-state and elementary excitations in compounds where spins and orbitals are entangled by on-site relativistic spin-orbit interaction. The appropriate degrees of freedom are localized states with an effective angular momentum J. We generalize J to arbitrary large values while maintaining the delicate spin-orbital entanglement. After projecting the intersite exchange interaction to the manifold of effective spins, a systematic 1/J expansion of the effective Hamiltonian is realized using the Holstein-Primakoff transformation. Applications to representative compounds Sr2IrO4 and particularly vanadium spinels AV2O4 are discussed.

Antisymmetric magnetoresistance of the SrTiO₃/LaAlO₃ interface

Snir Seri and Lior Klein

Published 12 November 2009 (4 pages)
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The longitudinal resistance Rxx of the SrTiO3/LaAlO3 interface with magnetic fields applied perpendicular to the interface has an antisymmetric term [namely, Rxx(H)Rxx(−H)] which increases with decreasing temperature and increasing field. We argue that the origin of this phenomenon is a nonhomogeneous Hall effect with clear contribution of an extraordinary Hall effect, suggesting the presence of nonuniform field-induced magnetization.

Long-timescale fluctuations in zero-field magnetic vortex oscillations driven by dc spin-polarized current

V. S. Pribiag, G. Finocchio, B. J. Williams, D. C. Ralph, and R. A. Buhrman

Published 12 November 2009 (4 pages)
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We report time and frequency domain studies of spin-torque-driven vortex self-oscillations at zero magnetic field. We observe two types of abrupt fluctuations in the frequency and amplitude, with very long random mean lifetimes (~10² to ~10⁴ oscillation cycles). First, we observe fluctuations between two center frequencies separated by 10s of MHz that we identify with switching between quasiuniform and vortex states of the magnetic polarizing layer. Second, we resolve much smaller, discrete frequency fluctuations that lead to a fine structure of the oscillations. We find that this fine structure plays a key role in determining the long-time average linewidths and we suggest a possible physical origin.

Spin dimers in the quantum ferrimagnet Cu₂Fe₂Ge₄O₁₃ under staggered and random magnetic fields

T. Masuda, K. Kakurai, and A. Zheludev

Published 12 November 2009 (4 pages)
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We study S=1/2 dimer excitation in a coupled chain and dimer compound Cu2Fe2Ge4O13 by inelastic neutron-scattering technique. The Zeeman split of the dimer triplet by a staggered field is observed at low temperature. With the increase in temperature, the effect of a random field is detected by a drastic broadening of the triplet excitation. Basic dynamics of dimer in the staggered and random fields are experimentally identified in Cu2Fe2Ge4O13.

Incommensurate spin correlation driven by frustration in BiCu₂PO₆

O. Mentré, E. Janod, P. Rabu, M. Hennion, F. Leclercq-Hugeux, J. Kang, C. Lee, M.-H. Whangbo, and S. Petit

Published 13 November 2009 (4 pages)
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The magnetic properties of BiCu2PO6 have been analyzed by means of magnetic-susceptibility and inelastic neutron-scattering measurements on powder samples by evaluating the spin-exchange interactions on the basis of density-functional calculations and by simulating the inelastic neutron scattering in terms of spin-exchange parameters. BiCu2PO6 exhibits magnetic properties described by the two-leg spin ladder with strong spin frustration along each leg chain and has a gapped quantum singlet ground state with excited magnetic states, showing an incommensurate dispersion arising from frustration.

Antiferromagnetic to valence-bond-solid transitions in two-dimensional SU(N) Heisenberg models with multispin interactions

Jie Lou, Anders W. Sandvik, and Naoki Kawashima

Published 16 November 2009 (4 pages)
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We study two-dimensional Heisenberg antiferromagnets with additional multispin interactions which can drive the system into a valence-bond-solid state. For standard SU(2) spins, we consider both four- and six-spin interactions. We find continuous quantum phase transitions with the same critical exponents. Extending the symmetry to SU(N), we also find continuous transitions for N=3 and 4. In addition, we also study quantitatively the crossover of the order-parameter symmetry from Z4 deep inside the valence-bond-solid phase to U(1) as the phase transition is approached.

Ferromagnetic resonance linewidth in ultrathin films with perpendicular magnetic anisotropy

J.-M. Beaujour, D. Ravelosona, I. Tudosa, E. E. Fullerton, and A. D. Kent

Published 17 November 2009 (4 pages)
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Transition-metal ferromagnetic films with perpendicular magnetic anisotropy (PMA) have ferromagnetic resonance (FMR) linewidths that are one order of magnitude larger than soft magnetic materials, such as pure iron (Fe) and Permalloy (NiFe) thin films. A broadband FMR setup has been used to investigate the origin of the enhanced linewidth in a material in which the PMA could be systematically reduced by irradiation with Helium ions: Ni|Co multilayers. The FMR linewidth depends linearly on frequency for perpendicular applied fields and increases significantly when the magnetization is rotated into the film plane. Irradiation of the film with Helium ions decreases the PMA and the distribution of PMA parameters, leading to a large reduction in the FMR linewidth for in-plane magnetization. These results suggest that fluctuations in PMA lead to a large two magnon scattering contribution to the linewidth for in-plane magnetization and establish that the Gilbert damping is enhanced in such materials (0.04, compared to 0.002 for pure Fe).

Theory of electromagnon resonances in the optical response of spiral magnets

A. Cano

Published 17 November 2009 (4 pages)
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The optical response of spiral magnets is studied theoretically, with special attention to its electromagnon features. We show that these features trace back to the resonant magnetoelectric response resulting from the spiral ordering (irrespective of any concomitant ferroelectricity). This response, being magnetoelectric in nature, cannot always be reduced to an effective electric permittivity. We argue that electromagnons in spiral magnets can produce, in addition to the observed peaks in the optical absorption of multiferroics, a (dynamically enhanced) optical rotation, and a negative refractive index behavior.

Competing interactions in ferromagnetic/antiferromagnetic perovskite superlattices

Y. Takamura, F. Yang, N. Kemik, E. Arenholz, M. D. Biegalski, and H. M. Christen

Published 19 November 2009 (4 pages)
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Soft x-ray magnetic dichroism, magnetization, and magnetotransport measurements demonstrate that the competition between different magnetic interactions (exchange coupling, electronic reconstruction, and long-range interactions) in La0.7Sr0.3FeO3(LSFO)/La0.7Sr0.3MnO3(LSMO) perovskite oxide superlattices leads to unexpected functional properties. The antiferromagnetic order parameter in LSFO and ferromagnetic order parameter in LSMO show a dissimilar dependence on sublayer thickness and temperature, illustrating the high degree of tunability in these artificially layered materials.

Finite-temperature spin dynamics and phase transitions in spin-orbital models

C.-C. Chen, B. Moritz, J. van den Brink, T. P. Devereaux, and R. R. P. Singh

Published 19 November 2009 (4 pages)
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We study finite temperature properties of a generic spin-orbital model relevant to transition metal compounds, having coupled quantum Heisenberg-spin and Ising-orbital degrees of freedom. The model system undergoes a phase transition, consistent with that of a two-dimensional Ising model, to an orbitally ordered state at a temperature set by short-range magnetic order. At low temperatures the orbital degrees of freedom freeze out and the model maps onto a quantum Heisenberg model. The onset of orbital excitations causes a rapid scrambling of the spin spectral weight away from coherent spin waves, which leads to a sharp increase in uniform magnetic susceptibility just below the phase transition, reminiscent of the observed behavior in the Fe-pnictide materials.

Two distinct ferroelectric phases in the multiferroic Y-type hexaferrite Ba₂Mg₂Fe₁₂O₂₂

Hajime Sagayama, Kouji Taniguchi, Nobuyuki Abe, Taka-hisa Arima, Yusaku Nishikawa, Shin-ichiro Yano, Yusuke Kousaka, Jun Akimitsu, Masato Matsuura, and Kazuma Hirota

Published 20 November 2009 (4 pages)
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The magnetic phase diagram of the Y-type hexaferrite Ba2Mg2Fe12O22 has been studied using single-crystal neutron diffraction. The result indicates successive phase transitions where the magnetic modulation wave number changed discontinuously when a magnetic field is applied and the temperature is varied. For the low-temperature spin-driven ferroelectric state, we have found a sixfold structure with q=(0  0  1/2) in weak magnetic fields and a twofold structure with q=(0  0  3/2) in strong magnetic fields between which a first-order transition intervenes accompanied by a hysteresis.

Superfluidity and superconductivity

Spontaneous fluxoid formation in superconducting loops

R. Monaco, J. Mygind, R. J. Rivers, and V. P. Koshelets

Published 2 November 2009 (4 pages)
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See accompanying Viewpoint Physics 2, 92 (2009)

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We report on the experimental verification of the Zurek-Kibble scenario in an isolated superconducting ring over a wide parameter range. The probability of creating a single flux quantum spontaneously during the fast normal-superconducting phase transition of a wide Nb loop clearly follows a scaling relation on the quenching time Q, as one would expect if the transition took place as fast as causality permits. However, the observed Zurek-Kibble scaling exponent =0.62±0.15 is two times larger than anticipated for large loops. Assuming Gaussian winding number densities we show that this doubling is well founded for small annuli.

Infrared phonon anomaly in BaFe₂As₂

A. Akrap, J. J. Tu, L. J. Li, G. H. Cao, Z. A. Xu, and C. C. Homes

Published 2 November 2009 (4 pages)
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The detailed optical properties of BaFe2As2 have been determined over a wide frequency range above and below the structural and magnetic transition at TN138  K. A prominent in-plane infrared-active mode is observed at 253  cm⁻¹ (31.4 meV) at 295 K. The frequency of this vibration shifts discontinuously at TN; for T<TN the frequency of this mode displays almost no temperature dependence, yet it nearly doubles in intensity. This anomalous behavior appears to be a consequence of orbital ordering in the Fe-As layers.

Investigating the superconductor-insulator transition in thin films using drag resistance: Theoretical analysis of a proposed experiment

Yue Zou (邹悦), Gil Refael, and Jongsoo Yoon

Published 6 November 2009 (4 pages)
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The magnetically driven superconductor-insulator transition in amorphous thin films (e.g., InO and Ta) exhibits several mysterious phenomena, such as a putative metallic phase and a huge magnetoresistance peak. Unfortunately, several conflicting categories of theories, particularly quantum-vortex condensation, and normal region percolation, explain key observations equally well. We present a experimental setup, an amorphous thin-film bilayer, where a drag resistance measurement would clarify the role quantum vortices play in the transition, and hence decisively point to the correct picture. We provide a thorough analysis of the device, which shows that the vortex paradigm gives rise to a drag with an opposite sign and orders of magnitude larger than the drag measured if competing paradigms apply.

Topological order in paired states of fermions in two dimensions with breaking of parity and time-reversal symmetries

Noah Bray-Ali, Letian Ding, and Stephan Haas

Published 11 November 2009 (4 pages)
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We numerically evaluate the entanglement spectrum (singular value decomposition of the wave function) of paired states of fermions in two dimensions that break parity and time-reversal symmetries, focusing on the spin-polarized px+ipy case. The entanglement spectrum of the weak-pairing (BCS) phase contains a Majorana zero mode, indicating non-Abelian topological order. In contrast, for the strong-pairing (Bose-Einstein condensation) phase, we find no such mode, consistent with Abelian topological order.

Functional renormalization-group study of the doping dependence of pairing symmetry in the iron pnictide superconductors

Ronny Thomale, Christian Platt, Jiangping Hu, Carsten Honerkamp, and B. Andrei Bernevig

Published 13 November 2009 (4 pages)
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We use the functional renormalization group to analyze the phase diagram of a four-band model for the iron pnictides subject to band interactions with certain A1g momentum dependence. We determine the parameter regimes where an extended s-wave pairing instability with and without nodes emerges. For electron doping, the parameter regime in which a nodal gap appears is in correspondence to recent predictions [A. Chubukov et al., arXiv:0903.5547 (unpublished)], however, at very low Tc. Upon hole doping, the s-wave gap never becomes nodal: above a critical strength of the intraband repulsion, the system favors an exotic extended d-wave instability on the enlarged hole pockets. At half filling, we find that a strong momentum dependence of interband pair hopping yields an extended s-wave instability instead of spin-density wave ordering. These results demonstrate that an interaction anisotropy around the Fermi surfaces generally leads to a pronounced sensitivity of the pairing state on the system parameters.

Transverse instabilities of multiple vortex chains in magnetically coupled NbSe₂/permalloy superconductor/ferromagnet bilayers

G. Karapetrov, M. V. Milošević, M. Iavarone, J. Fedor, A. Belkin, V. Novosad, and F. M. Peeters

Published 16 November 2009 (4 pages)
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Using scanning tunneling microscopy and Ginzburg-Landau simulations, we explore vortex configurations in magnetically coupled NbSe2/permalloy superconductor/ferromagnet bilayer. The permalloy film with stripe domain structure induces periodic local magnetic induction in the superconductor, creating a series of pinning-antipinning channels for externally added magnetic flux quanta. Such laterally confined Abrikosov vortices form quasi-one-dimensional arrays (chains). The transitions between multichain states occur through propagation of kinks at the intermediate fields. At high fields we show that the system becomes nonlinear due to a change in both the number of vortices and the confining potential. The longitudinal instabilities of the resulting vortex structures lead to vortices “levitating” in the antipinning channels.

Local density of states and superconducting gap in the iron chalcogenide superconductor Fe₁₊Se_1−xTe_x observed by scanning tunneling spectroscopy

Takuya Kato, Yoshikazu Mizuguchi, Hiroshi Nakamura, Tadashi Machida, Hideaki Sakata, and Yoshihiko Takano

Published 17 November 2009 (4 pages)
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We report on the investigation of the quasiparticle local density of states and superconducting gap in the iron chalcogenide superconductor Fe1+Se1−xTex(Tc~14  K). The surface of a cleaved crystal revealed an atomic square lattice, superimposed on the inhomogeneous background, with a lattice constant of ~3.8  Å without any reconstruction. Tunneling spectra measured at 4.2 K exhibit the superconducting gap, which completely disappears at 18 K, with a magnitude of ~2.3  meV, corresponding to 2/kBTc=3.8. In stark contrast to the cuprate superconductors, the value of the observed superconducting gap is relatively homogeneous, following a sharp distribution with a small standard deviation of 0.23 meV. Conversely, the normal-state local density of states observed above Tc shows spatial variation over a wide energy range of more than 1 eV, probably due to the excess iron present in the crystal.

Theory of small charge solitons in one-dimensional arrays of Josephson junctions

Stephan Rachel and Alexander Shnirman

Published 17 November 2009 (4 pages)
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We identify and investigate the parameter regime of small charge solitons in one-dimensional arrays of Josephson junctions. We obtain the dispersion relation of the soliton and show that it unexpectedly flattens in the outer region of the Brillouin zone. We demonstrate Lorentz contraction of the soliton in the middle of the Brillouin zone as well as broadening of the soliton in the flat band regime.

Scaling of the superfluid density in strongly underdoped YBa₂Cu₃O_6+y: Evidence for a Josephson phase

C. C. Homes

Published 19 November 2009 (4 pages)
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Recent measurements on extremely underdoped YBa2Cu3O6+y [Phys. Rev. Lett. 99, 237003 (2007)] have allowed the critical temperature Tc, superfluid density s0s(TTc), and dc conductivity dc(TTc) to be determined for a series of electronic dopings for Tc3–17  K. The general scaling relation s0/84.4dcTc is observed, extending the validity of both the ab-plane and c-axis scaling an order of magnitude and creating a region of overlap. This suggests that strongly underdoped materials may constitute a Josephson phase; as the electronic doping is increased a more uniform superconducting state emerges.

ARTICLES

Structure, structural phase transitions, mechanical properties, defects

Superhard diamondlike BC₅: A first-principles investigation

Chao Jiang, Zhijun Lin, and Yusheng Zhao

Published 4 November 2009 (6 pages)
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We perform first-principles density-functional calculations to identify the possible crystal structure of a superhard diamondlike BC5 phase, which was recently synthesized under high-pressure and high-temperature conditions. Interestingly, we find only a small total-energy difference between the energetically most favorable ordered configuration and the fully disordered state of BC5 modeled using a 54-atom special quasirandom structure, indicating a weak ordering tendency. It is thus likely that the BC5 phase synthesized under experimental conditions is disordered in nature. Such a conclusion is further corroborated by the fact that the disordered BC5 structure displays volume-per-atom, bulk modulus and its pressure derivative, and simulated x-ray diffraction spectrum in good agreements with experiments.

Basic Co-rich decagonal Al-Co-Ni: Average structure

Angelica Strutz, Akiji Yamamoto, and Walter Steurer

Published 10 November 2009 (9 pages)
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The two-layer average structure of the high-temperature phase basic Co-rich decagonal Al72.5Co18.5Ni9 was determined based on single-crystal x-ray diffraction data. The five-dimensional (5D) structure model was refined in the noncentrosymmetric 5D space group Pm2 (112 parameters, wR=0.123 and R=0.156 for 957 reflections). The close relationship of the model structure with that of W-Al-Co-Ni, a 3/2,2/1 approximant, is shown.

Electronic structure and energetics of the tetragonal distortion for TiH₂, ZrH₂, and HfH₂: A first-principles study

Ramiro Quijano, Romeo de Coss, and David J. Singh

Published 10 November 2009 (8 pages)
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The electronic structure and energetics of the tetragonal distortion for the fluorite-type dihydrides TiH2, ZrH2, and HfH2 are studied by means of highly accurate first-principles total-energy calculations. For HfH2, in addition to the calculations using the scalar relativistic (SR) approximation, calculations including the spin-orbit coupling have also been performed. The results show that TiH2, ZrH2, and HfH2 in the cubic phase are unstable against tetragonal strain. For the three systems, the total energy shows two minima as a function of the c/a ratio with the lowest-energy minimum at c/a<1 in agreement with the experimental observations. The band structure of TiH2, ZrH2, and HfH2 (SR) around the Fermi level shows two common features along the two major symmetry directions of the Brillouin zone, −L and −K, a nearly flat doubly degenerate band, and a van Hove singularity, respectively. In cubic HfH2 the spin-orbit coupling lifts the degeneracy of the partially filled bands in the −L path, while the van Hove singularity in the −K path remains unchanged. The density of states of the three systems in the cubic phase shows a sharp peak at the Fermi level. We found that the tetragonal distortion produces a strong reduction in the density of states at the Fermi level resulting mainly from the splitting of the doubly-degenerate bands in the −L direction and the shift of the van Hove singularity to above the Fermi level. The validity of the Jahn-Teller model in explaining the tetragonal distortion in this group of dihydrides is discussed.

Influence of nanoscale Cu precipitates in -Fe on dislocation core structure and strengthening

Zhengzheng Chen, Nicholas Kioussis, and Nasr Ghoniem

Published 11 November 2009 (6 pages)
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Atomistic simulations of the interaction of a screw dislocation in -Fe with different size bcc Cu precipitates suggest two plausible strengthening mechanisms. For precipitate diameters in the range 1.5  nmd3.3  nm, the dislocation core structure within the Cu precipitate undergoes a polarized to nonpolarized transformation, leading to the dislocation pinning at the precipitate-matrix interface and the bowing out of the dislocation line. The calculated bow-out angle and resolved shear stress required to detach the dislocation from the precipitate are in agreement with recent experiments. The structural transition of larger (d3.3  nm) Cu precipitates under high shear stress is responsible for the loss of slip systems and hence for dislocation pinning.

X-ray diffraction peaks from partially ordered misfit dislocations

Vladimir M. Kaganer and Karl K. Sabelfeld

Published 11 November 2009 (13 pages)
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We calculate the x-ray diffraction peak profiles from distributions of misfit dislocations in the whole range of their positional correlations, from completely random to periodic. Both the spatial integration and the integration over the dislocation ensemble are performed by Monte Carlo techniques. The diffraction peaks from thin relaxed films consisting of a narrow coherent and a broad diffuse component are explained. Correlation functions are calculated analytically for different types of positional correlations between dislocations.

Angular-dependent embedded atom method potential for atomistic simulations of metal-covalent systems

Avinash M. Dongare, Matthew Neurock, and Leonid V. Zhigilei

Published 12 November 2009 (13 pages)
184106 Full Text: PDF (668 kB) | Buy Article

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A computationally efficient interatomic potential is developed for the description of interatomic interactions in multicomponent systems composed of metals, Si and Ge. The potential is based on reformulation of the embedded atom method (EAM) potential for metals and Stillinger-Weber (SW) potential commonly used for Si and Ge in a compatible functional form. The potential incorporates a description of the angular dependence of interatomic interactions into the framework of the EAM potential and, therefore, is dubbed angular-dependent EAM (A-EAM) potential. The A-EAM potential retains the properties of the pure components predicted by the original EAM and SW potentials, thus limiting the scope of potential parameterization to only the cross interactions among the components. The ability of the potential to provide an adequate description of binary systems with mixed type of bonding is illustrated for Au-Si/Ge system, with the parameters for Au-Si and Au-Ge interactions determined based on the results of density-functional theory calculations performed for several representative bulk structures and small clusters. To test the performance of the A-EAM potential at finite temperatures, the values of the enthalpy of mixing of liquid Au-Si and Au-Ge alloys, as well as the equilibrium lines on the Au-Si phase diagram are evaluated and compared with experimental data. The calculation of the phase diagram is based on the values of the excess chemical potential difference between Au and Si, evaluated in a series of semi-grand canonical ensemble Monte Carlo simulations performed for different temperatures and alloy compositions. The potential is shown to provide an adequate semiquantitative description of the thermodynamic properties of the alloy at different temperatures and in the whole range of compositions, thus showing a considerable promise for large-scale atomistic simulations of metal-Si/Ge systems.

Ferroelectric order stability in the Bi_1−xPb_xFeO₃ solid solution

J. Chaigneau, R. Haumont, and J. M. Kiat

Published 16 November 2009 (6 pages)
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Ferroelectric properties of Bi1−xPbxFeO3, through a structural approach, by structural Rietveld refinement and x-ray diffraction versus temperature have been carried out. This study allows us to draw a phase diagram for the system Bi1−xPbxFeO3. Although lead atom gets a lone pair and is commonly understood as having a “ferroelectric” character, we report in this paper that adding of lead atom in the Bi subsystem of BiFeO3, destroys the long-range FE order induced by Bi lone pairs on contrary, and progressively breaks the ferroelectric ordering up to the point where the structure becomes cubic on average. The decreasing of TC with increasing Pb content also confirms that adding of Pb atoms destabilizes the ferroelectric rhombohedral order and stabilizes the cubic phase via a complex texture of mixed phases, occurring at a local scale.

Theory of dynamical scattering in near-edge electron energy loss spectroscopy

C. Witte, S. D. Findlay, M. P. Oxley, J. J. Rehr, and L. J. Allen

Published 17 November 2009 (15 pages)
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Beyond chemical information, the fine structure of an absorption edge gives bonding and electronic information. We provide a synthesis of fine structure and dynamical scattering theory, allowing the exploration of the effects of dynamical scattering on the measured fine structure. We discuss the effects of experimental geometry in the context of site-specific near-edge spectroscopy of NiAl2O4 and find that large detectors serve to localize the inelastic signal and may be preferable to the small off-axis detectors currently used. We then explore the possibility of measuring changes in fine structure within a unit cell using scanning transmission electron microscopy. We demonstrate that, in principle, it is possible to measure a subtle change in the fine structure of the O K edge in SrTiO3 as the probe is scanned across the unit cell. We explore the best experimental conditions to achieve this and find that large probe-forming and detector apertures help to localize the signal to the atomic sites.

Constraints on the phase diagram of nonmolecular CO₂ imposed by infrared spectroscopy

Mario Santoro and Federico A. Gorelli

Published 17 November 2009 (5 pages)
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The investigation of the high-pressure phase diagram of CO2, specifically in the nonmolecular forms, is a challenging experimental issue, due to the large metastability of this material. Here we report on the determination of the pressure threshold for forming nonmolecular CO2 from the molecular crystal, in the P-T range of 10–53 GPa and 296–715 K, respectively. The study is based on a large set of IR spectroscopy measurements, which allows to detect, with a high sensitivity, the onset of the formation of nonmolecular, amorphous carbonia. This metastable form is first formed in the attempts to obtain nonmolecular, crystalline CO2, upon increasing pressure, for kinetic reasons. Therefore, it is identified an upper pressure bound for the real-phase boundary between the molecular solid and the nonmolecular, thermodynamically stable crystal, which results to be remarkably lower than those detected by other techniques. Also, it is shown how the hysteresis of the molecular-to-nonmolecular transformation is reduced upon increasing the temperature. The study adds constraints on the high-pressure, thermodynamic phase diagram of nonmolecular CO2.

Interactions between hydrogen impurities and vacancies in Mg and Al: A comparative analysis based on density functional theory

Lars Ismer, Min Sik Park, Anderson Janotti, and Chris G. Van de Walle

Published 17 November 2009 (10 pages)
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Using first-principles methods we have studied the interactions between hydrogen impurities and vacancies in hcp Mg and fcc Al. We find that single vacancies can, in principle, host up to 9 H atoms in Mg and 10 in Al, not 12 as recently reported in the case of Al. The difference between our results and the results in previous work is attributed to a more appropriate definition of the trapping energy of hydrogen impurities in vacancies. The concentration of hydrogen-vacancy complexes depends on the amount of hydrogen dissolved in the metal, which in turn is dictated by the hydrogen chemical potential µH. We evaluated the concentration of all relevant hydrogen-vacancy complexes as a function of µH, corresponding to different H loading conditions—ranging from low pressures to high pressures of H2 gas, up to hydrogen plasma conditions. Our analysis reveals fundamental differences in the characteristics of the hydrogen-vacancy interaction between Mg and Al. In the case of Al, up to 15% of H atoms are trapped in single vacancies in the form of H-vacancy complexes even for very low values of µH. The trapping effect slows down the diffusion of H atoms in Al by more than an order of magnitude. While interactions between vacancies and single hydrogen atoms are therefore clearly important, interactions with multiple H atoms and related mechanisms (such as hydrogen-induced superabundant vacancy formation) are predicted to occur in Al only at very high values of µH. In the case of Mg, the effects of H trapping in single vacancies are negligible for low values of µH due to the relatively low formation energy of isolated interstitial H. However, vacancies containing multiple H atoms and related mechanisms such as hydrogen-induced superabundant vacancy formation are predicted to occur in Mg at much lower values of µH than in Al. We estimate that, at room temperature, the critical pressure of an H2 gas to induce hydrogen-enhanced (superabundant) vacancy formation is ~1  GPa in Mg and ~10  GPa in Al.

3d transition metal impurities in aluminum: A first-principles study

M. Mantina, S. L. Shang, Y. Wang, L. Q. Chen, and Z. K. Liu

Published 18 November 2009 (7 pages)
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In this work, appropriate description of interactions of 3d transition metals in aluminum (Al-3d) is attained from first-principles using LDA+U potential within density-functional theory. By reproducing diffusion coefficients of 3d transition metals in aluminum in agreement with reliable data from experiments, activation energies, and diffusion prefactors along with different aspects of the Al-3d systems are presented. Al alloy with dilute concentration of 3d solutes Fe, Cr, or Mn is magnetic. The physics underlying the anomalously low diffusivities of 3d solutes in Al is discussed.

Changing the physical and chemical properties of titanium oxynitrides TiN_1−xO_x by changing the composition

Jesús Graciani, Said Hamad, and Javier Fdez. Sanz

Published 18 November 2009 (10 pages)
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The stability and structural properties of titanium oxynitrides, TiN1−xOx, of different compositions are theoretically analyzed by means of first-principles periodic density-functional calculations. We show that at x=0.55–0.6 there is a change in the preferred structure from that of NaCl type to the -TiO arrangement. For the NaCl-type structure the cell volume increases with x while it decreases with x for the -TiO structure. The bulk moduli are always much larger for NaCl-type structures than for -TiO and they decrease as the amount of O increases, moving from 280 GPa for TiN to 226 GPa for TiO (NaCl-type structure) or 197 GPa for -TiO. Changes in the electronic structure with the composition are also considered. In general we found that in the two types of structure (NaCl and -TiO), both the band gap and the ionic character increase with the O concentration.

Expansion-limited aggregation of nanoclusters in a single-pulse laser-produced plume

E. G. Gamaly, N. R. Madsen, D. Golberg, and A. V. Rode

Published 19 November 2009 (12 pages)
184113 Full Text: PDF (397 kB) | Buy Article

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Formation of carbon nanoclusters in a single-laser-pulse created ablation plume was studied both in vacuum and in a noble gas environment at various pressures. The developed theory provides cluster radius dependence on combination of laser parameters, properties of ablated material, and type and pressure of an ambient gas in agreement with experiments. The experiments were performed on carbon nanoclusters formed by laser ablation of graphite targets with 12 picosecond 532 nm laser pulses at MHz-range repetition rate in a broad range of ambient He, Ar, Kr, and Xe gas pressures from 2×10⁻² to 1500 Torr. The experimental results confirmed our theoretical prediction that the average size of the nanoparticles depends weakly on the type of the ambient gas used, and is determined exclusively by the single laser pulse parameters even at the repetition rate as high as 28 MHz with the time gap 36 ns between the pulses. The most important finding relates to the fact that in vacuum the cluster size is mainly determined by hydrodynamic expansion of the plume while in the ambient gas it is controlled by atomic diffusion in the gas. We demonstrate that the ultrashort pulses can be used for production of clusters with the size less than the critical value, which separates the particles with properties drastically different from those of a material in a bulk. The presented results of experiments on formation of carbon nanoclusters are in close agreement with the theoretical scaling. The developed theory is applicable for cluster formation from any monatomic material, such as silicon for example.

Inhomogeneous, disordered, and partially ordered systems

Structural heterogeneity and medium-range order in Zr_xCu_100−x metallic glasses

Maozhi Li, C. Z. Wang, S. G. Hao, M. J. Kramer, and K. M. Ho

Published 2 November 2009 (7 pages)
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Realistic three-dimensional atomistic structures of ZrxCu100−x (x=35,50) bulk metallic glasses are constructed using a combination of x-ray diffraction experiment and computational modeling. A cluster correlation method is developed to analyze the medium-range order in amorphous systems. We show that the glass systems consist of a stringlike backbone network formed by icosahedral clusters and a liquidlike structure filling in the remaining space. These findings are consistent with those obtained from our independent classical molecular-dynamics studies with embedded-atom method potential for ZrCu system. Such a heterogeneous structure provides a fundamental structural perspective of dynamical heterogeneity and glass formation.

Molecular dynamics investigation of mixed-alkali borate glasses: Short-range order structure and alkali-ion environments

A. Vegiri, C.-P. E. Varsamis, and E. I. Kamitsos

Published 2 November 2009 (12 pages)
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Structural properties of mixed-alkali borate glasses, 0.3[(1−x)Li2O-xCs2O]−0.7B2O3 and 0.3[(1−x)Li2O-xNa2O]−0.7B2O3, have been studied by molecular dynamics simulations at T=300  K and for several values of the alkali mixing parameter, x, to explore structural foundations of the mixed-alkali effect (MAE). The short-range order (SRO) structure was found to consist of borate tetrahedra, B, and of neutral, B3, and charged, B2O⁻, triangular units [=bridging oxygen atom]. The abundance of B units was found to decrease from Li to Cs and to exhibit negative deviation from linearity in Li-Cs glasses. However, no appreciable change in SRO structure was detected in mixed Li-Na glasses. Even though alkali metal (M) ions occupy in mixed glasses sites, i.e., coordination environments with O atoms, similar to those formed in single alkali borates, mixing was found to affect the M-O bonding properties of dissimilar alkalis in an opposite manner. Thus, for both systems investigated here the Li ion-coordination environment was found to become better defined and the Li-O interactions to strengthen upon alkali mixing, whereas the Cs-O and Na-O interactions become progressively weakened. The origin of these trends was traced to cationic environments formed around nonbridging oxygen (NBO) atoms in glass; it was found that the dominant cation configurations around NBOs consist of dissimilar cations in mixed-alkali glasses. The formation of dissimilar ion pairs affects by polarization effects the bonding and vibrational properties of metal ions in their oxide sites. This was demonstrated for Li-Cs glasses by both experimental and calculated far infrared spectra, where the metal ion-oxide site vibrations are strongly active. It was discussed that the preference of unlike-alkali ion pairing around NBOs and the consequent drastic reduction in the number of NBOs that sense like-cations could provide a structural explanation for the MAE.

Yield stress in metallic glasses: The jamming-unjamming transition studied through Monte Carlo simulations based on the activation-relaxation technique

David Rodney and Christopher A. Schuh

Published 10 November 2009 (12 pages)
184203 Full Text: PDF (750 kB) | Buy Article

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A Monte Carlo approach allowing for stress control is employed to study the yield stress of a two-dimensional metallic glass in the limit of low temperatures and long (infinite) time scales. The elementary thermally activated events are determined using the activation-relaxation technique (ART). By tracking the minimum-energy state of the glass for various applied stresses, we find a well-defined jamming-unjamming transition at a yield stress about 30% lower than the steady-state flow stress obtained in conventional strain-controlled quasistatic simulations. ART is then used to determine the evolution of the distribution of thermally activated events in the glass microstructure both below and above the yield stress. We show that aging below the yield stress increases the stability of the glass, both thermodynamically (the internal potential energy decreases) and dynamically (the aged glass is surrounded by higher-energy barriers than the initial quenched configuration). In contrast, deformation above the yield stress brings the glass into a high internal potential energy state that is only marginally stable, being surrounded by a high density of low-energy barriers. The strong influence of deformation on the glass state is also evidenced by the microstructure polarization, revealed here through an asymmetry of the distribution of thermally activated inelastic strains in glasses after simple shear deformation.

Ultra short-time dynamics of radiation damage in fcc metals

Marc Hayoun, Gerrit Coddens, and Guillaume Petite

Published 11 November 2009 (7 pages)
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We have performed molecular-dynamics simulations of displacement cascades in copper in order to investigate the nonequilibrium ultra-short-time damage and to evaluate the possibility of observing it experimentally in situ (e.g., in a pump/probe laser experiment). The atomic trajectories have been analyzed by calculating their x-ray diffraction patterns as a function of time. The results show that an integrated x-ray intensity can indeed be used to evidence the irradiation effects. Even though the number of Frenkel defects is large, the main effect of the irradiation showing up in the x-ray intensities at ultrashort times is an important alteration of the lattice vibrations. On the basis of these results, a pump/probe setup is proposed.

Fast-ion conduction and flexibility and rigidity of solid electrolyte glasses

M. Micoulaut, M. Malki, D. I. Novita, and P. Boolchand

Published 16 November 2009 (10 pages)
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Electrical conductivity of dry, slow cooled (AgPO3)1−x(AgI)x glasses is examined as a function of temperature, frequency, and glass composition. From these data compositional trends in activation energy for conductivity EA(x), Coulomb energy Ec(x) for Ag⁺ ion creation, Kohlrausch stretched exponent (x), low-frequency [s(x)] and high-frequency [(x)] permittivity are deduced. All parameters except Ec(x) display two compositional thresholds, one near the stress transition, x=xc(1)=9%, and the other near the rigidity transition, x=xc(2)=38% of the alloyed glass network. These elastic phase transitions were identified in modulated differential scanning calorimetry, IR reflectance, and Raman-scattering experiments earlier. A self-organized ion-hopping model of a parent electrolyte system is developed that self-consistently incorporates mechanical constraints due to chemical bonding with carrier concentrations and mobility. The model predicts the observed compositional variation in (x), including the observation of a steplike jump when glasses enter the intermediate phase (IP) at x>xc(1), and an exponential increase when glasses become flexible at x>xc(2). Since Ec is found to be small compared to network strain energy (Es), we conclude that free carrier concentrations are close to nominal AgI concentrations, and that fast-ion conduction is driven largely by changes in carrier mobility induced by an elastic softening of network structure. Variation in the stretched exponent (x) is square-well like with walls localized near xc(1) and xc(2) that essentially coincide with those of the IP (xc(1)<x<xc(2)), and suggest filamentary (quasi-one-dimensional) conduction in the IP, and conduction with a dimensionality greater than 1 outside the IP.

Dynamics, dynamical systems, lattice effects, quantum solids

Electrode polarization and charge transport at solid interfaces

A. Serghei, M. Tress, J. R. Sangoro, and F. Kremer

Published 5 November 2009 (5 pages)
184301 Full Text: PDF (674 kB) | Buy Article

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A quantitative description is suggested for electrode polarization, an ubiquitous phenomenon which takes place at the interface between a metallic and an ionic conductor and results in an increase by many orders of magnitude in the net dielectric response of the sample cell. Based on the fact that due to coulombic interactions, the mobility of charge carriers is drastically slowed down at the metal/ionic conductor interface, this approach quantitatively reproduces the observed scaling laws and opens perspectives in the physics of charge transport at interfaces.

Phonon density of states and heat capacity of La_3−xTe₄

O. Delaire, A. F. May, M. A. McGuire, W. D. Porter, M. S. Lucas, M. B. Stone, D. L. Abernathy, V. A. Ravi, S. A. Firdosy, and G. J. Snyder

Published 19 November 2009 (9 pages)
184302 Full Text: PDF (353 kB) | Buy Article

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The phonon density of states (DOS) of La3−xTe4 compounds (x=0.0,0.18,0.32) was measured at 300, 520, and 780 K, using inelastic neutron scattering. A significant stiffening of the phonon DOS and a large broadening of features were observed upon introduction of vacancies on La sites (increasing x). Heat-capacity measurements were performed at temperatures 1.85T1200  K and were analyzed to quantify the contributions of phonons and electrons. The Debye temperature and the electronic coefficient of heat capacity determined from these measurements are consistent with the neutron-scattering results, and with previously reported first-principles calculations. Our results indicate that La vacancies in La3−xTe4 strongly scatter phonons and this source of scattering appears to be independent of temperature. The stiffening of the phonon DOS induced by the introduction of vacancies is explained in terms of the electronic structure and the change in bonding character. The temperature dependence of the phonon DOS is captured satisfactorily by the quasiharmonic approximation.

Magnetism

SU(N) Heisenberg model on the square lattice: A continuous-N quantum Monte Carlo study

K. S. D. Beach, Fabien Alet, Matthieu Mambrini, and Sylvain Capponi

Published 2 November 2009 (7 pages)
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A quantum phase transition is typically induced by tuning an external parameter that appears as a coupling constant in the Hamiltonian. Another route is to vary the global symmetry of the system, generalizing, e.g., SU(2) to SU(N). In that case, however, the discrete nature of the control parameter prevents one from identifying and characterizing the transition. We show how this limitation can be overcome for the SU(N) Heisenberg model with the help of a singlet projector algorithm that can treat N continuously. On the square lattice, we find a direct, continuous phase transition between Néel-ordered and crystalline bond-ordered phases at Nc=4.57(5) with critical exponents z=1 and /=0.81(3).

Density dynamics in translationally invariant spin- chains at high temperatures: A current-autocorrelation approach to finite time and length scales

Robin Steinigeweg and Jochen Gemmer

Published 2 November 2009 (10 pages)
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We investigate transport in several translationally invariant spin- chains in the limit of high temperatures. We concretely consider spin transport in the anisotropic Heisenberg chain, the pure Heisenberg chain within an alternating field, and energy transport in an Ising chain which is exposed to a tilted field. Our approach is essentially based on a connection between the evolution of the variance of an inhomogeneous nonequilibrium density and the current-autocorrelation function at finite times. Although this relationship is not restricted to the case of diffusive transport, it allows to extract a quantitative value for the diffusion constant in that case. By means of numerically exact diagonalization we indeed observe diffusive behavior in the considered spin chains for a range of model parameters and confirm the diffusion coefficients which were obtained for these systems from nonequilibrium bath scenarios.

Ferromagnetism in nitrogen-doped MgO: Density-functional calculations

Phivos Mavropoulos, Marjana Ležaić, and Stefan Blügel

Published 2 November 2009 (9 pages)
184403 Full Text: PDF (239 kB) | Buy Article

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The magnetic state of nitrogen-doped MgO, with N substituting O at concentrations between 1% and the concentrated limit, is calculated with density-functional methods. The N atoms are found to be spin polarized with a moment of 1µB per nitrogen atom and to interact ferromagnetically via the double-exchange mechanism in the full concentration range. The long-range magnetic order is established above a finite concentration of about 1.5% when the percolation threshold is reached. The disorder is described within the coherent-potential approximation, with the exchange interactions harvested by the method of infinitesimal rotations. The Curie temperature TC, calculated within the random-phase approximation, increases linearly with the concentration, and is found to be about 30 K for 10% concentration. Besides the substitution of single nitrogen atoms, also interstitial nitrogen atoms, dimers and trimers, and their structural relaxations are discussed with respect to the magnetic state. Possible scenarios of engineering a higher Curie temperature are analyzed, with the conclusion that an increase in TC is difficult to achieve, requiring a particular attention to the choice of chemistry.

Strong-coupling limit of a Kondo spin coupled to a mesoscopic quantum dot: Effective Hamiltonian in the presence of exchange correlations

Stefan Rotter and Y. Alhassid

Published 2 November 2009 (12 pages)
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We consider a Kondo spin that is coupled antiferromagnetically to a large chaotic quantum dot. Such a dot is described by the so-called universal Hamiltonian and its electrons are interacting via a ferromagnetic exchange interaction. We derive an effective Hamiltonian in the limit of strong Kondo coupling, where the screened Kondo spin effectively removes one electron from the dot. We find that the exchange-coupling constant in this reduced dot (with one less electron) is renormalized and that additional interaction terms appear beyond the conventional terms of the strong-coupling limit. The eigenenergies of this effective Hamiltonian are found to be in excellent agreement with exact numerical results of the original model in the limit of strong Kondo coupling.

Geometry-dependent scaling of critical current densities for current-induced domain wall motion and transformations

L. Heyne, J. Rhensius, Y.-J. Cho, D. Bedau, S. Krzyk, C. Dette, H. S. Körner, J. Fischer, M. Laufenberg, D. Backes, L. J. Heyderman, L. Joly, F. Nolting, G. Tatara, H. Kohno, S. Seo, U. Rüdiger, and M. Kläui

Published 2 November 2009 (4 pages)
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In a combined theoretical and experimental study, we investigate the critical current densities for vortex domain walls in magnetic nanowires. We systematically determine the critical current densities for continuous motion of vortex walls as a function of the wire width for different wire thicknesses and we find that the critical current density increases monotonously with decreasing wire width. Theoretically we present a mechanism that predicts a threshold current density based on wall transformations and this leads to a scaling of the critical current density jc1/width. The origin of this scaling is found to be the different dependence of the spin torque energy and the vortex nucleation energy on the wire width and good agreement with the experimental observations is found.

Spin-transfer torque in disordered weak ferromagnets

Yoshisuke Ban and Gen Tatara

Published 3 November 2009 (5 pages)
184406 Full Text: PDF (112 kB) | Buy Article

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We study theoretically the spin-transfer effect on a domain wall in disordered weak ferromagnets. We have identified the adiabatic condition for the disordered case as D, where D and sd are the diffusion constant and the spin splitting energy due to the sd type exchange interaction, respectively, and found out that perfect spin-transfer effect occurs even in weak ferromagnets as long as this condition is satisfied. The effective term arising from the force turns out to govern the wall dynamics, and therefore, the wall motion can be as efficient as in strong ferromagnets even if sd is small.

Low-energy effective-field theories of Sp(4) spin systems

Cenke Xu

Published 6 November 2009 (7 pages)
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We study the classical and quantum phase transitions of Sp(4) spin systems on three-dimensional stacked square and triangular lattices. We present general Ginzburg-Landau field theories for various types of Sp(4) spin orders with different ground-state manifolds such as CP(3), S⁷/Z2, Grassmann manifold G2,5, G2,6, and so on, based on which the nature of the classical phase transitions are studied, and a global phase diagram is presented. The classical phase transitions close to quantum phase transitions toward spin-liquid states are also discussed based on renormalization group flow. Our results can be directly applied to the simplest Sp(4) and SU(4) Heisenberg models which can be realized using spin-3/2 atoms and alkaline-earth atoms trapped in optical lattice.

Strain-induced nonequilibrium magnetoelastic domain structure and spin reorientation of NiO(100)

Suman Mandal, Krishnakumar S. R. Menon, Francesco Maccherozzi, and Rachid Belkhou

Published 9 November 2009 (6 pages)
184408 Full Text: PDF (445 kB) | Buy Article

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We report the observation of strain-induced antiferromagnetic domain structure on cleaved surface of NiO single crystal. This nonequilibrium domain structure undergoes various spin reorientations (from in plane to different in plane, out of plane to in plane) after mild annealing, indicating a direct correlation between the surface strain field and domain morphology. These reorientations are found to be driven by structural modification on the surface generated by cleaving process and buried dislocations, altering the surface magnetic anisotropy and their relaxation through mild annealing. These observations establish that the magnetoelastic effect plays a dominant role in determining antiferromagnetic domain structure.

Magnetic properties of LCoAsO (L=La–Gd)

Hiroto Ohta and Kazuyoshi Yoshimura

Published 9 November 2009 (6 pages)
184409 Full Text: PDF (396 kB) | Buy Article

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We synthesized a series of CoAs based weakly itinerant ferromagnetic compound LCoAsO (L: lanthanoids) and systematically studied a L dependence on magnetic properties. Lattice constants a and c decrease monotonically with the decrease in the ion size of L³⁺. In the cases of L=Nd, Sm, and Gd, a ferromagnetic-antiferromagnetic transition was observed at TN=15, 35, and 75 K, respectively, indicating the existence of unconventional interaction between ferromagnetically ordered itinerant electrons of Co. The Curie temperature TC increases from 55 to 75 K by changing La to Ce while from Ce to Gd the TC does not change so much, being quite similar to the L site dependence of the superconducting transition temperature Tc in FeAs-based high-Tc compound LFeAsO. We discussed the mechanism of ferromagnetic-antiferromagnetic transition and the lanthanoid dependence of TC.

Accuracy of the spin sum rule in XMCD for the transition-metal L edges from manganese to copper

Cinthia Piamonteze, Piter Miedema, and Frank M. F. de Groot

Published 10 November 2009 (12 pages)
184410 Full Text: PDF (573 kB) | Buy Article

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The effective spin sum rule is widely used in the quantitative analysis of x-ray magnetic circular dichroism spectra. Here, this important, though imperfect, sum rule is reviewed with a detailed analysis of the various sources for errors and deviations. The simulations confirm that the final state effects of the core level spin-orbit coupling and the core-valence exchange interactions (multiplet effects) are linearly related with the effective spin sum-rule error. Within the charge transfer multiplet approach, we have analyzed these effects, in combination with the interactions affecting the magnetic ground state, including the crystal field strength, the charge transfer effects, the exchange (magnetic) field, and the 3d spin-orbit coupling. We find that for the late transition-metal systems, the error in the effective spin moment is between 5% and 10%, implying that for covalent and/or metallic systems the effective spin sum rule is precise to within 5–10 %. The error for 3d⁵ systems is ~30% and for 3d⁴ systems, the error is very large, implying that, without further information, the derived effective spin sum-rule values for 3d⁴ systems have no meaning.

Hydrodynamic theory of coupled current and magnetization dynamics in spin-textured ferromagnets

Clement H. Wong and Yaroslav Tserkovnyak

Published 10 November 2009 (13 pages)
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We develop the hydrodynamic theory of collinear spin currents coupled to magnetization dynamics in metallic ferromagnets. The collective spin density couples to the spin current through a U(1) Berry-phase gauge field determined by the local texture and dynamics of the magnetization. We determine phenomenologically the dissipative corrections to the equation of motion for the electronic current, which consist of a dissipative spin-motive force generated by magnetization dynamics and a magnetic texture-dependent resistivity tensor. The reciprocal dissipative, adiabatic spin torque on the magnetic texture follows from the Onsager principle. We investigate the effects of thermal fluctuations and find that electronic dynamics contribute to a nonlocal Gilbert damping tensor in the Landau-Lifshitz-Gilbert equation for the magnetization. Several simple examples, including magnetic vortices, helices, and spirals, are analyzed in detail to demonstrate general principles.

Dynamical signature of a domain phase transition in a perpendicularly magnetized ultrathin film

N. Abu-Libdeh and D. Venus

Published 11 November 2009 (7 pages)
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Domain phases in ultrathin Fe/Ni/W(110) films with perpendicular anisotropy have been studied using the ac magnetic susceptibility. Dynamics on time scales of minutes to hours was probed by quenching the system from high temperature to the stripe phase region, and varying the constant rate of temperature increase as the susceptibility traces were measured. The entire susceptibility peak is observed to relax slowly along the temperature axis, with the peak temperature increasing as the rate of heating is decreased. This is precisely opposite to what would happen if this slow relaxation was driven by changes in the domain density within the stripe phase. The data are instead consistent with a simple model for the removal of a significant density of pattern defects and curvature trapped in the quench from high temperature. A quantitative analysis confirms that the relaxation dynamics are consistent with the mesoscopic rearrangement of domains required to remove pattern defects, and that the experiment constitutes a “dynamical” observation of the phase transition from a high temperature, positionally disordered domain phase to the low temperature, ordered stripe phase.

First order ferromagnetic transition in binary CeIn₂

D. P. Rojas, J. I. Espeso, J. Rodríguez Fernández, J. C. Gómez Sal, J. Sanchez Marcos, and H. Müller

Published 12 November 2009 (6 pages)
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Measurements of the magnetic, thermal, and transport properties of the CeIn2 binary alloy are consistent with a paramagnetic-ferromagnetic transition at TC=22  K. A discontinuity in the magnetic entropy, electrical resistivity and thermal expansion, and a huge anomaly in the specific heat of 113 J/mol K (cmag=103  J/mol K), at the magnetic transition, are observed. In addition, the Arrott plots show negative slope at low fields, the field-cooling and field-warming magnetization present irreversibility, and both the susceptibility and the resistivity evidence a small thermal hysteresis of 0.05 K. Moreover, the values of the entropy change calculated from the magnetization data using the Clausius-Clapeyron equation are in good agreement with those directly obtained from the specific-heat data. The joint analysis of all these results provides evidence for the first order character of this magnetic transition in CeIn2.

Nonlocal properties of a multidomain magnetic configuration

A. Vanhaverbeke, O. Klein, M. Viret, and J. Ben Youssef

Published 12 November 2009 (5 pages)
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We study the nonlocal properties of the magnetic multidomain configuration of a soft ferrimagnetic iron garnet using an original combination of two local probe techniques. The domain pattern of the garnet, which has a strong perpendicular anisotropy, consists of a maze of alternating up and down domains that are several micrometers wide. The magnetic tip of a scanning force microscope is used to locally excite this configuration. The resulting long-range perturbation is measured by a static nickel Hall cross deposited on top of the garnet, which is used as another local (magnetoresistive) sensor. It is found that punctually perturbing a domain-wall affects the magnetic configuration several domains away on distances well over the 5  µm range.

Oscillating spin-density pattern in gold metallocene and phthalocyanine molecules

Diego Carrascal, Lucas Fernández-Seivane, and Jaime Ferrer

Published 13 November 2009 (6 pages)
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We present a theoretical study of the magnetic properties of the spin-1/2 dicyclopentadienyl metallocene (MCp2) and phthalocyanine molecules that contain the transition-metal atoms M=Co, Cu, and Au. We find that the spin-density pattern of gold molecules shows a fully delocalized and oscillating behavior. This spin pattern is to be contrasted with the well-known cases of cobalt or iron molecules, where the magnetic moment is strongly localized at the transition-metal ion.

Magnetostructural transition in Gd₅Sb_0.5Ge_3.5

A. S. Chernyshov, Ya. Mudryk, D. Paudyal, V. K. Pecharsky, K. A. Gschneidner, Jr., D. L. Schlagel, and T. A. Lograsso

Published 17 November 2009 (9 pages)
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Magnetic and crystallographic properties of Gd5Sb0.5Ge3.5 were investigated using dc magnetization, ac magnetic susceptibility, and heat capacity of an oriented single crystal, combined with temperature and magnetic field dependent x-ray powder diffraction. The compound undergoes an unusual magnetostructural transition at 40 K and a nonmagnetic second-order transition around 63 K. The detailed crystallographic study of Gd5Sb0.5Ge3.5 shows that contrary to the R5(SixGe1−x)4 systems (R is a rare-earth metal), the structural transition occurs without shear displacements of the [R5T4] slabs (T=Si, Ge, and Sb), and a substantial volume change (−0.5%) does not lead to a change in crystallographic symmetry. The first-principles electronic structure calculations show higher interslab than intraslab ferromagnetic exchange interaction indicating that Sm5Ge4 type of structure supports a ferromagnetic ground state in Gd5Sb0.5Ge3.5.

Anisotropy of the spin density wave onset for (TMTSF)₂PF₆ in a magnetic field

Ya. A. Gerasimenko, V. A. Prudkoglyad, A. V. Kornilov, V. M. Pudalov, V. N. Zverev, A.-K. Klehe, and J. S. Qualls

Published 18 November 2009 (6 pages)
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In order to study the spin density wave transition temperature (TSDW) in (TMTSF)2PF6 as a function of magnetic field, we measured the magnetoresistance Rzz in fields up to 19 T. Measurements were performed for three field orientations Ba,b and c^* at ambient pressure and at P=5  kbar, that is nearly the critical pressure. For Bc^* orientation we observed quadratic field dependence of TSDW in agreement with theory and with previous experiments. For Bb and Ba orientations we have found no shift in TSDW within 0.05 K, both at P=0 and P=5  kbar. This result is also consistent with theoretical predictions.

Elastic constants of the single crystalline Yb based heavy-fermion compound YbCo₂Zn₂₀

Yoshiki Nakanishi, Takuya Fujino, Kankichi Ito, Mitsuteru Nakamura, Masahito Yoshizawa, Yuta Saiga, Masashi Kosaka, and Yoshiya Uwatoko

Published 18 November 2009 (6 pages)
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Ultrasonic measurements have been carried out on the heavy-fermion compound YbCo2Zn20 to investigate the elastic properties and the 4f electric state of Yb ion. A strong temperature dependence at low temperature was found in elastic constants C11, (C11−C12)/2, and C44 of YbCo2Zn20. Furthermore, the elastic constants are significantly magnetic field dependent at low temperatures. These experimental facts strongly suggest an importance of the highly degenerated 4f low-lying level scheme of Yb ions under a crystalline electric field effect, probably leading to such an extremely large Sommerfeld coefficient of YbCo2Zn20. We argue that the near spherical distribution of neighboring Zn atoms centered at the Yb site is an essential requirement for the formation of such an exotic heavy-fermion system.

Effect of microstructure on magnetic properties and anisotropy distributions in Co/Pd thin films and nanostructures

Justin M. Shaw, Hans T. Nembach, T. J. Silva, Stephen E. Russek, Roy Geiss, Christopher Jones, Noel Clark, Titus Leo, and David J. Smith

Published 19 November 2009 (8 pages)
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The structure of Co/Pd multilayers has a strong effect on the localized anisotropy distribution within a film and on the resulting switching properties of nanostructures fabricated from identical material. By varying the underlying seed layer in sputtered films, the microstructure can be controlled from being highly (111) textured to having a random grain orientation. We find a strong correlation between the lateral homogeneity of grain orientations and the localized anisotropy distribution in the material. X-ray diffraction and reflectivity indicate that the interface is better defined and more uniform in the textured case, consistent with the presence of a strong interfacelike anisotropy.

Spin-density wave in Cr: Nesting versus low-lying thermal excitations

Veerle Vanhoof, Michel Rots, and Stefaan Cottenier

Published 19 November 2009 (10 pages)
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It is well known that present versions of density functional theory do not predict the experimentally observed spin-density wave state to be the ground state of Cr. Recently, a so-called “nodon model” has been proposed as an alternative way to reconcile theory and experiment: the ground state of Cr is truly antiferromagnetic, and the spin-density wave appears due to low-lying thermal excitations (“nodons”). We examine in this paper whether the postulated properties of these nodons are reproduced by ab initio calculations.