Physical Review B

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

January 2006

Volume 73, Number 1 , Articles (01xxxx)

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BRIEF REPORTS

Structure, structural phase transitions, mechanical properties, defects

Inhomogeneous, disordered, and partially ordered systems

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

First-principles calculations of the single-bonded cubic phase of nitrogen

H. L. Yu, G. W. Yang, X. H. Yan, Y. Xiao, Y. L. Mao, Y. R. Yang, and M. X. Cheng

Published 9 January 2006 (4 pages)
012101 Full Text: PDF (327 kB) | Buy Article

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We performed the extensive first-principles simulations for understanding of the stability of complex structures of the nonmolecular nitrogen. We found that the single-bonded cubic phase of nitrogen, so-called cubic gauche structure (cg-N), is the most stable phase among these phases in high pressure, and predicted that a phase transition of cg-N from nonmetal to metal takes place at the pressure of 600  GPa. The bulk modulus, Young modulus, shear modulus, Poisson ratio, and band gap of cg-N were calculated that are quite well consistent with experimental data.

Acoustic emission study of phase transitions and polar nanoregions in relaxor-based systems: Application to the PbZn_1/3Nb_2/3O₃ family of single crystals

E. Dul'kin, M. Roth, P.-E. Janolin, and B. Dkhil

Published 11 January 2006 (4 pages)
012102 Full Text: PDF (65 kB) | Buy Article

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The restored strain energy in PbZn1/3Nb2/3O3 (PZN) and 9%PbTiO3-doped PZN (PZN-9%PT) relaxor crystals has been studied by means of acoustic emission (AE). Two types of AE activity signals have been recorded: (i) related to temperature- or electric-field-induced macroscopic phase transitions and (ii) associated with formation/disappearance of intrinsic polar nanoregions. Monitoring of AE under varying [001] electric fields has allowed a unique in situ observation of a low-field (1  kV/cm) irreversible orthorhombic-to-MC phase transition within the morphotropic phase boundary region of PZN-9%PT.

Cooperative formation of high-spin species in a photoexcited spin-crossover complex

Y. Moritomo, M. Kamiya, A. Nakamura, A. Nakamoto, and N. Kojima

Published 24 January 2006 (4 pages)
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The dynamics of the photoinduced phase transition was investigated in a prototypical spin crossover complex, [Fe(ptz)6](BF4)2  (ptz=1-propyltetrazole), under photoexcitation at 77 K near the transition temperature (Tc=130  K). When the excitation power I exceeds 1  mW/mm², we observed an "acceleration" of the creation rate of the density nHS of the high-spin (HS) species after a characteristic incubation period. The "acceleration" is interpreted in terms of the negative pressure effect; the photocreated HS species with a larger ionic radius expands the volume to cause the structural phase transition.

Inhomogeneous, disordered, and partially ordered systems

Melting and critical superheating

A. B. Belonoshko, N. V. Skorodumova, A. Rosengren, and B. Johansson

Published 4 January 2006 (3 pages)
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Two mechanisms of melting are known, heterogeneous, where melting starts at surfaces, and homogeneous, where the liquid nucleates in the bulk crystal. If melting occurs homogeneously, a crystal can be superheated significantly above its melting temperature (Tm). At present, the physical meaning of the limit of superheating (TLS) is unknown. We demonstrate, by molecular dynamics simulations, that the total energy of a solid at TLS is equal to the total energy of its liquid at Tm at the same volume. In the high pressure limit TLS and Tm are connected by the constant kAB=ln  2/3 via the relation kAB=TLS/Tm–1.

Hybrid cluster expansions for local structural relaxations

H. Y. Geng, M. H. F. Sluiter, and N. X. Chen

Published 4 January 2006 (4 pages)
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A model is constructed in which pair potentials are combined with the cluster expansion method in order to better describe the energetics of structurally relaxed substitutional alloys. The effect of structural relaxations away from the ideal crystal positions and the effect of ordering is described by interatomic-distance-dependent pair potentials, while more subtle configurational aspects associated with correlations of three or more sites are described purely within the cluster expansion formalism. Implementation of such a hybrid expansion in the context of the cluster variation method or Monte Carlo method gives improved ability to model phase stability in alloys from first-principles.

Tunneling and interaction effects in two-level systems in glasses studied by atomistic simulations

Kostya Trachenko and Misha Turlakov

Published 9 January 2006 (4 pages)
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We use molecular dynamics simulations to characterize low-energy two-level systems (TLS) in glasses. We estimate that in silica glass, TLS parameters are broadly distributed in such a way that the tunnel splitting is in the 0.01–1  K range. We also observe simultaneous atomic jumps in different TLS below 30  K and suggest that this is evidence of strong interaction between TLS below this temperature scale.

Ordering of Si atoms on the fivefold Al-Pd-Mn quasicrystal surface

J. Ledieu, P. Unsworth, T. A. Lograsso, A. R. Ross, and R. McGrath

Published 12 January 2006 (4 pages)
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Silicon atoms are found to form an ordered overlayer on the fivefold surface of the icosahedral Al-Pd-Mn quasicrystal. Using scanning tunneling microscopy, the adsorption site is identified as the center of truncated clusters which are building blocks of the bulk structure. Comparison with theoretical models suggests that this site is favored because of bonding to Mn atoms.

Local atomic ordering and nanoscale phase separation in a Pd-Ni-P bulk metallic glass

Yoshihiko Hirotsu, T. G. Nieh, Akihiko Hirata, Tadakatsu Ohkubo, and Nobuo Tanaka

Published 24 January 2006 (4 pages)
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The local structure in a Pd40Ni40P20 bulk metallic glass was examined using a spherical-aberration-corrected high resolution TEM. Fcc-Pd(Ni) type nanoclusters and local compound (phosphide)-like nanoclusters with sizes of 1–2  nm embedded in a dense-randomly-packed amorphous matrix were clearly observed under an appropriate imaging condition. However, three-dimensional atom-probe elemental mapping revealed there is virtually no nanoscale compositional difference between the nanoclusters and amorphous matrix beyond the statistical error range. A very small interfacial energy between the nanophase and the matrix is able to form a metastable amorphous phase with a structural fluctuation.

Magnetism

Incommensurate antiferromagnetism in FeAl₂: Magnetic, Mössbauer, and neutron diffraction measurements

D. Kaptás, E. Sváb, Z. Somogyvári, G. André, L. F. Kiss, J. Balogh, L. Bujdosó, T. Kemény, and I. Vincze

Published 9 January 2006 (4 pages)
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Magnetic, Mössbauer, and neutron diffraction measurements were used to study the anomalous magnetic behavior of FeAl2. The magnetization is almost linear with the applied field up to 14  T at 5  K. The Fe-Al system Mössbauer measurement in the 7  T external magnetic field clearly shows the presence of canted, antiferromagnetically coupled Fe magnetic moments. Neutron diffraction indicates an incommensurate magnetic structure with a periodicity of about 1.1  nm.

Ferromagnetic coupling in Eu/Gd(0001) observed by spin-resolved photoelectron spectroscopy

Yu. S. Dedkov, Th. Kleissner, E. N. Voloshina, S. Danzenbächer, S. L. Molodtsov, and C. Laubschat

Published 10 January 2006 (4 pages)
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We report on a magnetic analysis by means of spin-resolved photoelectron spectroscopy of an atomically flat heteromagnetic rare-earth interface of 1 ML Eu/Gd(0001). The measurements reveal a high net Eu magnetization at low temperatures reflected by a spin polarization ~15% of the Eu 4f state. This magnetic Eu configuration is due to a strong ferromagnetic interlayer exchange coupling across the Eu/Gd interface which overcomes a weak negative intralayer coupling between Eu spins in the hexagonal two-dimensional lattice.

Magnon wave forms in the presence of a soliton in two-dimensional antiferromagnets with a staggered field

M. P. P. Fonseca and A. S. T. Pires

Published 10 January 2006 (4 pages)
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In this paper we study the interactions between magnons and a soliton, in a classical and isotropic two-dimensional Heisenberg antiferromagnet in the presence of a staggered field applied perpendicularly to the xy plane. We obtained the exact solutions to the magnons in the presence of the soliton. As a consequence we obtain the exact phase shifts, which were compared to the ones obtained by Born approximation. The quantum corrections of the energy of the soliton were also encountered. Our results can be applied to study the thermodynamics and generalized for two-dimensional isotropic ferromagnets with an axial magnetic field.

Nonlinear magnetoelastic behavior of the bcc phases of Co and Ni: Importance of third-order contributions for bcc Ni

Matej Komelj and Manfred Fähnle

Published 12 January 2006 (4 pages)
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The first- and second-order magnetoelastic coefficients of the bcc phases Co and Ni are calculated by using a combination of the phenomenological theory of nonlinear magnetoelasticity with the ab initio density functional electron theory. The magnetoelastic behavior of the bcc phases is drastically different from that of the corresponding fcc phases. The recently synthesized bcc phase of Ni appears to be an example of a material for which third-order magnetoelastic effects are essential.

Absence of induced moment in magnetic tunnel junction barriers

M. Bowen, V. Cros, H. Jaffrès, P. Bencok, F. Petroff, and N. B. Brookes

Published 13 January 2006 (3 pages)
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To confirm the theoretical underpinnings of spin-polarized transport, we have performed x-ray magnetic circular dichroism experiments on samples exhibiting a ferromagnet-insulator interface grown using processes which have yielded junctions with appreciable tunneling magnetoresistance. Within experimental sensitivity, no magnetic moment induced due to the proximity of the ferromagnet was detected on any atomic barrier site using this element-specific technique.

Field-induced structural transition and quasistatic susceptibility of one-dimensional magnetic nanoparticle chains

Haiwen Xi, Yiming Shi, Kai-Zhong Gao, and Song Xue

Published 13 January 2006 (4 pages)
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By studying the dipolar interaction and the elastic interaction in the ordered magnetic nanoparticle chains connected by or imbedded in organic medium, we predict a Peierls-like structural phase transition under proper conditions. The phase transition can be induced by applying a transverse magnetic field and manifests itself in the magnetization curve measurements. The magnetic effect of the Peierls phase and of the magnetoelastic coupling is also reflected in the magnetic susceptibility of the magnetic nanoparticles chains. A phase diagram of the nanoparticle chain structure has also been established.

NMR study of ⁵¹V in quasi-one-dimensional integer spin chain compound SrNi₂V₂O₈

B. Pahari, K. Ghoshray, R. Sarkar, B. Bandyopadhyay, and A. Ghoshray

Published 18 January 2006 (4 pages)
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NMR studies of ⁵¹V in SrNi2V2O8, a S=1 quasi-one-dimensional antiferromagnet, reveal no magnetic long-range order down to 3.75  K, in contradiction to the prediction from the inelastic neutron scattering data. The temperature dependence of the NMR frequency shift leads to spin gap, ~25  K, in the low energy excitations. The spin-lattice relaxation rates suggest a temperature dependence of a spin gap with ~38  K in 80K<T<15K and are dominated by the spin diffusion term below 10  K. Thus the ground state of SrNi2V2O8 is more correctly described as a disordered "spin liquid" Haldane state as in isomorphous PbNi2V2O8.

Time dependence of muon spin relaxation spectra and spin correlation functions

C. A. Steer

Published 19 January 2006 (4 pages)
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The existing theory of the microscopic interpretation of the dynamical contribution to zero-field muon depolarization spectra in a longitudinal geometry is developed. The predicted relaxation of the muon depolarization is calculated from two forms of the spin correlation function. First, when the spin correlation function has an exponential form with a single wave-vector-dependent relaxation rate considered, it is shown that this form of the spin correlation function reproduces the slow and fast fluctuation limits of stochastic spin theory regardless of the choice of microscopic spin model. Second, if the spin correlation function is a homogeneous scaling function (such as a power-law decay with time), as suggested by the mode-coupling theory of spin dynamics, this results in a stretched exponential relaxation of the muon spectra. For simple spin diffusion, the muon spectra are shown to be relaxed with a root-exponential form.

Glassy magnetic behavior in the phase-separated perovskite cobaltites

Yan-kun Tang, Young Sun, and Zhao-hua Cheng

Published 19 January 2006 (4 pages)
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In this paper we demonstrate that the origin of the glassy behaviors (memory, aging, etc.) in the phase-separated perovskite cobaltites cannot be simply ascribed to intercluster interactions as the phase-separated manganites can. Instead, our study indicates that both the intercluster interactions and a spin glasslike phase contribute to the glassy behaviors. Thus, this study distinguishes the picture of phase separation between manganites and cobaltites.

Current localization and Joule self-heating effects in Cr-doped Nd_0.5Ca_0.5MnO₃ manganites

A. S. Carneiro, R. F. Jardim, and F. C. Fonseca

Published 24 January 2006 (4 pages)
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The effects of dc excitation current on the current-voltage curves of polycrystalline samples of Nd0.5Ca0.5Mn0.96Cr0.04O3 were investigated. The experimental results show that an abrupt jump of the voltage is concomitant with a huge increase in the temperature of the sample. A simple model and estimates for Joule self-heating effects support the experimental data. Moreover, the data strongly suggest that both the current localization in the metallic paths and local Joule self-heating effects are essential ingredients to understand the current-induced phase transition in phase-separated manganites.

Antiferromagnetic Heisenberg ladders in staggered magnetic field

Jize Zhao, Xiaoqun Wang, Tao Xiang, Zhaobin Su, Lu Yu, Jizhong Lou, and Changfeng Chen

Published 24 January 2006 (4 pages)
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We study the low-energy excitations of the spin- antiferromagnetic Heisenberg chain and N-leg (N=2,3,4) ladders in a staggered magnetic field hs. We show that hs induces gap and midgap states in all the cases and we will examine their field scaling behavior. A modified boundary scheme is devised to extract accurate bulk excitation behavior. The gap values converge rapidly as N increases, leading to a field scaling exponent = for both the longitudinal and transverse gaps of the square lattice (N). The midgap states induced by the boundary edge effects share the bulk gap scaling exponents but their overall scaling behavior in the large-N limit needs further investigation.

Order of sequential magnetic phase transitions in Ni-Mn-Ga alloys as revealed by Arrott plots

Xuezhi Zhou, Wei Li, H. P. Kunkel, and Gwyn Williams

Published 25 January 2006 (4 pages)
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The slope of H/M vs M² isotherms is an accepted criterion used to distinguish first-order–discontinuous magnetic phase transitions from second-order–continuous transitions. This criterion is shown to yield the order of not only simple isolated transitions, but also sequential transitions occurring in the Ni-Mn-Ga system, specifically order-order transitions between two ferromagnetic phases, provided certain constraints are met.

Magnetic characterization of the sawtooth-lattice olivines ZnL₂S₄ (L=Er,Tm,Yb)

G. C. Lau, B. G. Ueland, R. S. Freitas, M. L. Dahlberg, P. Schiffer, and R. J. Cava

Published 25 January 2006 (4 pages)
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We report the magnetic properties of the ZnL2S4 (L=Er,Tm,Yb) olivines, in which the magnetic lanthanide ions are in a potentially frustrated geometry consisting of sawtooth chains of corner-sharing triangles. Fits to the high-temperature magnetic susceptibility yielded Curie-Weiss temperatures of W–4, –13, and –75  K for the Er, Tm, and Yb compounds, respectively. None of the compounds displayed magnetic long-range order above T=1.8  K. The lack of ordering at temperatures near W may be attributed to either the low dimensionality of the structure or the frustrating effect of the triangular geometry.

Spin-filter tunneling magnetoresistance in a magnetic tunnel junction

Dafei Jin, Yuan Ren, Zheng-zhong Li, Ming-wen Xiao, Guojun Jin, and An Hu

Published 30 January 2006 (4 pages)
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A systematic study is carried out on the spin-filter (SF) tunneling magnetoresistance (TMR) occurring in a ferromagnetic metal/ferromagnetic insulator/ferromagnetic metal (FM/FI/FM) tunnel junction. The theoretical investigation gives a unified and compact description on the SF and TMR effects in this structure, and qualitatively explains the relevant experiments in this area. Specifically, due to the strong SF effect, the TMR can be separately controlled by the extended Slonczewski's polarization factors, leading to both the barrier-height and bias-voltage induced sign-change behavior. It is also proved that this structure can provide a positively or negatively large and stable TMR, which does not vary appreciably with increasing the bias. These features are very prominent compared with an FM/I/FM conventional magnetic tunnel junction and are believed to be of practical use in designing spintronic devices.

CeMnNi₄: Impostor half metal

I. I. Mazin

Published 31 January 2006 (4 pages)
012415 Full Text: PDF (686 kB) | Buy Article

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Recent experiments show CeMnNi4 to have a nearly integer magnetic moment and a relatively large transport spin polarization, as probed by Andreev reflection, suggesting that the material is a half metal or close to it. However, the calculations reported here show that it is not a half metal at all, but rather a semimetal of an unusual nature. Phonon properties should also be quite unusual, with rattling low-frequency Mn modes. Nontrivial transport properties, including a large thermoelectric figure of merit ZT, are predicted in the ferromagnetic state of the well-ordered stoichiometric CeMnNi4.

Superfluidity and superconductivity

Optical properties of boron-doped diamond

Dan Wu, Y. C. Ma, Z. L. Wang, Q. Luo, C. Z. Gu, N. L. Wang, C. Y. Li, X. Y. Lu, and Z. S. Jin

Published 6 January 2006 (4 pages)
012501 Full Text: PDF (304 kB) | Buy Article

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We report optical reflectivity study on pure and boron-doped diamond films grown by a hot-filament chemical vapor deposition method. The study reveals the formation of an impurity band close to the top of the valence band upon boron doping. A schematic picture for the evolution of the electronic structure with boron doping was drawn based on the experimental observation. The study also reveals that the boron doping induces local lattice distortion, which brings an infrared-forbidden phonon mode at 1330  cm^–1 activated in the doped sample. The antiresonance characteristic of the mode in conductivity spectrum evidences the very strong coupling between electrons and this phonon mode.

Comparative analysis of specific heat of YNi₂B₂C using nodal and two-gap models

C. L. Huang, J.-Y. Lin, C. P. Sun, T. K. Lee, J. D. Kim, E. M. Choi, S. I. Lee, and H. D. Yang

Published 9 January 2006 (4 pages)
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The magnetic field dependence of low temperature specific heat in YNi2B2C was measured and analyzed using various pairing order parameters. At a zero magnetic field, the two-gap model, which has been successfully applied to MgB2 and the point-node model, appear to describe the superconducting gap function of YNi2B2C better than other models based on the isotropic s-wave, the d-wave line nodes, or the s+g wave. The two energy gaps, L=2.67  meV and S=1.19  meV, are obtained. The observed nonlinear field dependence of the electronic specific heat coefficient, (H)~H^0.47, is quantitatively close to the (H)~H^0.5 expected for nodal superconductivity or that can be qualitatively explained using a two-gap scenario. Furthermore, the positive curvature in Hc2(T) near Tc is qualitatively similar to that in the other two-gap superconductor MgB2.

Charge-inhomogeneity doping relations in YBa₂Cu₃O_y detected by angle-dependent nuclear quadrupole resonance

Rinat Ofer, Shahar Levy, Amit Kanigel, and Amit Keren

Published 9 January 2006 (4 pages)
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The origin of charge inhomogeneity in YBa2Cu3Oy is investigated using an experimental method designed to determine the nuclear quadrupole resonance (NQR) asymmetry parameter for very wide NQR lines at different positions on the line. The method is based on the measurement of the echo intensity as a function of the angle between the radio frequency field H1 and the principal axis of the electric field gradient. Static charge inhomogeneity deduced from >0 is found in this compound, but only in conjunction with oxygen deficiency. This limits considerably the possible forms of charge inhomogeneity in bulk YBa2Cu3Oy.

Penetration depth anisotropy due to the proximity effect in a d-density-wave scenario

Angsula Ghosh

Published 11 January 2006 (4 pages)
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We calculate the penetration depth along the a and b directions in the presence of the pseudogap and superconducting phases for a simple model that incorporates two layers—a CuO2 plane and a CuO chain per unit cell. The CuO chains become superconducting due to the proximity to the planes below the critical temperature. The pseudogap phase has been considered to be a result of the d-density-wave (DDW) phase. The temperature dependence of a is always different from b as it depends on the induced gap in the chains. The DDW phase plays a vital role not only to distinguish the temperature dependence of b from that of a but also to identify the mixed phase from the pure phase of these superconductors.

Decoupled critical dynamics of the TMTSF donor molecules in (TMTSF)₂X organic superconductors

S. H. Kim, K. W. Lee, Cheol Eui Lee, W. Kang, and K. S. Hong

Published 11 January 2006 (3 pages)
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We have studied a high-temperature phase transition in the organic superconductors (TMTSF)2PF6 and (TMTSF)2ClO4 (where TMTSF indicates tetramethyltetraselenafulvalene) by means of ¹H laboratory-frame and rotating-frame nuclear magnetic resonance (NMR) relaxation measurements. The ¹H NMR spin-lattice (T1) and spin-spin relaxation time (T2), representing dynamics of the TMTSF donor molecules, manifested a divergence associated with the structural phase transition at 160  K. As no anomalies were observed in the ¹⁹F NMR T1 measurements representing dynamics of the PF6 anion, the TMTSF donor molecules and the anions are shown to be well decoupled regarding the critical fluctuations accompanying the structural phase transition.

Subharmonic gap structure in superconductor/ferromagnet/superconductor junctions

I. V. Bobkova

Published 12 January 2006 (4 pages)
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The behavior of dc subgap current in magnetic quantum point contact is discussed for the case of low-transparency junction with different tunnel probabilities for spin-up (D) and spin-down (D) electrons. Due to the presence of Andreev bound states ±0 in the system the positions of subgap electric current steps eVn=(±0)/n are split at temperature T0 with respect to the nonmagnetic result eVn=2/n. It is found that under the condition DD the spin current also manifests subgap structure, but only for odd values of n. The split steps corresponding to n=1,2 in subgap electric and spin currents are analytically calculated and the following steps are described qualitatively.

Coverage dependence of the Landau Fermi-liquid parameters F and F for ³He on thin ⁴He films

H. Akimoto, J. D. Cummings, and R. B. Hallock

Published 12 January 2006 (4 pages)
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We present the results from our heat capacity experiments on two-dimensional liquid ³He on thin superfluid ⁴He films adsorbed to Nuclepore. Measurement of the specific heat at low temperatures (40<T<220  mK) over a ³He coverage range 0.02–0.93 atomic layers on 3.14 and 4.33 bulk-density atomic layer ⁴He films enables the determination of the ³He quasiparticle effective mass m^*. Combining these results with previous ³He NMR magnetic susceptibility measurements for the same substrate and ⁴He coverages permits the determination of the two-dimensional Landau Fermi-liquid parameters F and F.

Excess conductivity in high-T_c superconducting thin films: Role of smooth doping disorder

R. Seto, R. Botet, and H. Kuratsuji

Published 12 January 2006 (4 pages)
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While the time-dependent Ginzburg-Landau theory successfully describes a number of experimental results for homogeneous high-Tc superconductors, the proper value of the characteristic depairing electric field was recently found unreasonably small for Bi-Sr-Ca-Cu-O thin films. We present numerical simulations which show that these features can be the consequence of smooth spatial inhomogeneity of the local critical temperature. Because of the interplay of percolation and superconducting transition, even a small disorder affects strongly the nonlinear excess conductivity in the transition region, causing considerable rescaling of the characteristic electric field.

Experimental observation of two-dimensional fluctuation magnetization in the vicinity of T_c for low values of the magnetic field in deoxygenated YBa₂Cu₃O_7–x

S. Salem-Sugui, Jr., A. D. Alvarenga, V. N. Vieira, and O. F. Schilling

Published 12 January 2006 (4 pages)
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We have measured isofield magnetization curves as a function of temperature in two single crystals of deoxygenated YBaCuO with Tc=52 and 41.5  K. Isofield magnetization curves were obtained for fields running from 0.05 to 4  kOe. The reversible region of the magnetization curves was analyzed in terms of a scaling proposed by Prange, but searching for the best exponent . The scaling analysis carried out for each data sample set with =0.669, which corresponds to the three-dimensional-xy exponent, did not produced a collapsing of curves when applied to M vs T curves data obtained for the lowest fields. The resulting analysis for the Y123 crystal with Tc=41.5  K, shows that lower field curves collapse over the entire reversible region following Prange's scaling with =1, suggesting a two-dimensional behavior. It is shown that the same data obeying Prange's scaling with =1 for crystal with Tc=41.5  K, as well as low field data for crystal with Tc=52  K, obey the known two-dimensional scaling law obtained in the lowest-Landau-level approximation.

Shape resonances in the superconducting order parameter of ultrathin nanowires

A. A. Shanenko and M. D. Croitoru

Published 13 January 2006 (4 pages)
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We study the shape-resonance effect associated with the confined transverse superconducting modes of a cylindrical nanowire in the clean limit. Results of numerical investigations of the Bogoliubov–de Gennes equations show significant deviations of the energy gap parameter from the bulk value with a profound effect on the transition temperature. The most striking is that the size of the resonances is found to be by order of magnitude larger than in ultrathin metallic films with the same width.

Interatomic potential for the condensed phases of helium atoms

Sebastian Ujevic and S. A. Vitiello

Published 19 January 2006 (4 pages)
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Results from diffusion Monte Carlo have been used in fits of a damping function of the triple dipole and of the intensity of the exchange three-body interactions. The equations of state obtained considering this three-body potential are in excellent agreement with experiment both at the solid and liquid phases. The calculations show that exchange nonadditivities that contribute less than 0.2% of the two-body potential energy in the solid phase are needed to describe the properties of a system of helium atoms.

Scaling of cross-over currents in current-voltage characteristics of YBa₂Cu₃O_7– films

D. R. Strachan, M. C. Sullivan, and C. J. Lobb

Published 19 January 2006 (4 pages)
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We investigate current-voltage measurements of a superconductor in a magnetic field for a continuous superconducting transition. Existence of such a transition in the mixed state has been the subject of recent controversy due to flexibility in the conventional scaling analysis. To address this, we analyze current-voltage data using scaling forms based on the crossover current. One of these scaling forms, based on the logarithmic derivative of current-voltage isotherms, is a stringent test for a superconducting transition. Applying this derivative scaling test to the data shows marked disagreement with a superconducting transition, which indicates that one does not occur within the mixed state.

Measuring the penetration depth anisotropy in MgB₂ using small-angle neutron scattering

D. Pal, L. DeBeer-Schmitt, T. Bera, R. Cubitt, C. D. Dewhurst, J. Jun, N. D. Zhigadlo, J. Karpinski, V. G. Kogan, and M. R. Eskildsen

Published 26 January 2006 (4 pages)
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Using small-angle neutron scattering, we have measured the misalignment between an applied field of 4  kOe and the flux-line lattice in MgB2, as the field is rotated away from the c axis by an angle . The measurements, performed at 4.9  K, showed the vortices canting towards the c axis for all field orientations. Using a two-band/two-gap model to calculate the magnetization, we are able to fit our results yielding a penetration depth anisotropy, =1.1±0.1.

ARTICLES

Structure, structural phase transitions, mechanical properties, defects

Mechanisms of oxygen ion diffusion in a nanoporous complex oxide 12CaO·7Al₂O₃

Peter V. Sushko, Alexander L. Shluger, Katsuro Hayashi, Masahiro Hirano, and Hideo Hosono

Published 3 January 2006 (10 pages)
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We performed a theoretical analysis of O^2– diffusion mechanisms in a nanoporous complex oxide 12CaO·7Al2O3 (C12A7). This material can be viewed as a positively charged framework, arranged in subnanometer sized cages, hosting extra-framework O^2– ions occupying one in six cages. Using both classical molecular-dynamics simulations and ab initio calculations we demonstrate that the diffusion of O^2– species is dominated by the exchange of framework and extra-framework O^2– ions rather than by an interstitial diffusion mechanism. The results allow us to rationalize the origins of the experimentally observed high oxide ion conductivity of C12A7 and the stability of its lattice under positive ion-beam irradiation.

Diffuse x-ray scattering from misfit and threading dislocations in PbTe/BaF₂/Si(111) thin layers

S. Dani and V. Holý

Published 4 January 2006 (6 pages)
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Diffuse x-ray scattering from epitaxial PbTe layers on Si(111) is analyzed both theoretically and experimentally. Reciprocal-space maps and x-ray diffraction profiles are measured and simulated for symmetrical and asymmetrical diffractions as well. The intensity distribution of diffusively scattered radiation is simulated within the statistical theory of x-ray scattering. Both types of expected defects—misfit and threading dislocations—are discussed. Comparing simulated maps to measured ones we can distinguish between contributions arising from misfit and threading dislocations. In the case of PbTe thin layers, the majority diffuse scattering comes from misfit dislocations.

High-temperature, structural disorder, phase transitions, and piezoelectric properties of GaPO₄

J. Haines, O. Cambon, N. Prudhomme, G. Fraysse, D. A. Keen, L. C. Chapon, and M. G. Tucker

Published 6 January 2006 (10 pages)
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Gallium orthophosphate was studied at high temperature up to 1303  K by total neutron scattering and 1173  K by piezoelectric measurements. Rietveld refinements at 1223  K confirm the stability of the structural distortion in the -quartz-type phase with an average tilt angle =18.8° at this temperature. In contrast, reverse Monte Carlo (RMC) refinements of total neutron scattering data indicate that, whereas the degree of structural disorder initially slowly varies over a very large temperature interval in the -quartz-type phase, an increase in disorder is observed beginning above 1023  K. Piezoelectric measurements indicate that the quality factor (Q) of GaPO4 resonators remains stable up to this temperature above which the piezoelectric properties of the material degrade. This degradation can be correlated to the increase in structural disorder. RMC refinements indicate that the high-temperature -cristobalite-type phase at 1303  K is characterized by significant thermally induced disorder with oxygen atom density forming a continuous ring around the vector joining neighboring gallium and phosphorous atoms. Gallium phosphate may be expected to retain its piezoelectric properties up to within 200  K of the phase transition temperature and as a consequence be used in applications at temperatures slightly above 1000  K.

Thermal properties of CaMoO₄: Lattice dynamics and synchrotron powder diffraction studies

A. Senyshyn, H. Kraus, V. B. Mikhailik, L. Vasylechko, and M. Knapp

Published 11 January 2006 (9 pages)
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The structure of calcium molybdate was studied by means of synchrotron based high-resolution powder diffraction methods in the temperature range 12–300  K. The scheelite structure type was confirmed for CaMoO4 in the temperature region investigated and no structural anomalies were observed. Thermal expansion coefficients extracted from the thermal dependencies of the cell sizes are found to be in good agreement with the predictions from our lattice dynamics calculations that form the background for microscopic interpretation of the experimental data. From the analyses of experimental results and the calculated thermal expansion coefficients, elastic constants, phonon density of states, heat capacities, entropy, and Grüneisen parameters it is concluded that a quasiharmonic lattice dynamics approach provides a good description of these properties for CaMoO4 at temperatures below 800  K.

High-pressure phases in SnO₂ to 117 GPa

Sean R. Shieh, Atsushi Kubo, Thomas S. Duffy, Vitali B. Prakapenka, and Guoyin Shen

Published 13 January 2006 (7 pages)
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X-ray diffraction of SnO2 (cassiterite) at high pressures and temperatures demonstrates the existence of four phase transitions to 117 GPa. The observed sequence of phases for SnO2 is rutile-type (P42/mnm)CaCl2-type(Pnnm)pyrite-type(Pa)ZrO2 orthorhombic phase I (Pbca)cotunnite-type  (Pnam). Our observations of the first three phases are generally in agreement with earlier studies. The orthorhombic phase I and cotunnite-type structure (orthorhombic phase II) were observed in SnO2 for the first time. The Pbca phase is found at 50–74 GPa during room-temperature compression. The cotunnite-type structure was synthesized when SnO2 was compressed to 74 GPa and heated at 1200 K. The cotunnite-type form was observed during compression between 54–117 GPa with additional laser heating carried out at 91 and 111 GPa. Fitting the pressure-volume data for the high-pressure phases to the second-order Birch-Murnaghan equation of state yields a bulk modulus of 259(26) GPa for the Pbca phase and 417(7) GPa for the cotunnite-type phase.

Modeling of hydrogen storage in hydride-forming materials: Statistical thermodynamics

A. Ledovskikh, D. Danilov, W. J. J. Rey, and P. H. L. Notten

Published 13 January 2006 (12 pages)
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A new lattice gas model has been developed, describing the hydrogen storage in hydride-forming materials. This model is based on the mean-field theory and Bragg-Williams approximation. To describe first-order phase transitions and two-phase coexistence regions, a binary alloy approach has been adopted. A complete set of equations describing pressure-composition isotherms and equilibrium electrode potential curves of hydride forming materials in both solid-solution and two-phase coexistence regions has been set up. The proposed model defines both the equilibrium pressure and equilibrium potential as explicit functions of the normalized hydrogen concentration, using eight physically well-defined parameters. Gibbs free energies, entropies, and phase diagrams of both model (LaNiyCu1.0) and commercial, MischMetal-based, AB5-type materials at different compositions and temperatures have been simulated. Good agreement between experimental and theoretical results for the pressure-composition isotherms obtained in the gas phase and the equilibrium potential curves measured in electrochemical environment has been found in all cases.

Monte Carlo simulation of ferroelectric domain growth

B. L. Li, X. P. Liu, F. Fang, J. L. Zhu, and J.-M. Liu

Published 13 January 2006 (7 pages)
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The kinetics of two-dimensional isothermal domain growth in a quenched ferroelectric system is investigated using Monte Carlo simulation based on a realistic Ginzburg-Landau ferroelectric model with cubic-tetragonal (square-rectangle) phase transitions. The evolution of the domain pattern and domain size with annealing time is simulated, and the stability of trijunctions and tetrajunctions of domain walls is analyzed. It is found that in this much realistic model with strong dipole alignment anisotropy and long-range Coulomb interaction, the powerlaw for normal domain growth still stands applicable. Towards the late stage of domain growth, both the average domain area and reciprocal density of domain wall junctions increase linearly with time, and the one-parameter dynamic scaling of the domain growth is demonstrated.

First-principles calculations of impurity diffusion activation energies in Al

Nils Sandberg and Randi Holmestad

Published 20 January 2006 (5 pages)
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We present first-principles density functional theory calculations of the diffusion activation energies of Mg, Si, and the 3d transition metals Sc–Zn in Al. In general, the calculated activation energies are underestimated with respect to experiments by 5–25%. The trend seen in experiments, namely, that impurities around Ti, V, and Cr have high diffusion activation energies leading to "anomalously" slow diffusion, is well reproduced in the present calculations. We provide an explanation in terms of electrostatic screening effects.

Structural properties and cation ordering in layered hexagonal Ca_xCoO₂

H. X. Yang, Y. G. Shi, X. Liu, R. J. Xiao, H. F. Tian, and J. Q. Li

Published 20 January 2006 (6 pages)
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A series of CaxCoO2 (0.15x0.40) materials have been prepared by means of an ion exchange reaction from NaxCoO2. Transmission electron microscopy (TEM) measurements revealed a rich variety of structural phenomena resulting from cation ordering, structural distortion, and twinning. Systematic structural analysis, in combination with the experimental data of NaxCoO2 (0.15x0.8) and SrxCoO2 (1.5x0.4) systems, suggests that there are two common well-defined cation ordered states corresponding, respectively, to the orthorhombic superstructure at around x=1/2 and the 3^1/2a×3^1/2a superstructure at around x=1/3 in this kind of system. Multiple ordered states, phase separation, and incommensurate structural modulations commonly appear in the materials with 0.33<x<0.5. The TEM observations also reveal an additional periodic structural distortion with q2=a^*/2 in materials for x0.35. This structural modulation also appears in the remarkable superconducting phase Na0.33CoO2·1.3H2O.

Acoustic emission across the magnetostructural transition of the giant magnetocaloric Gd₅Si₂Ge₂

Francisco-José Pérez-Reche, Fèlix Casanova, Eduard Vives, Lluís Mañosa, Antoni Planes, Jordi Marcos, Xavier Batlle, and Amílcar Labarta

Published 20 January 2006 (5 pages)
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We report on the existence of acoustic emission during the paramagnetic-monoclinicferromagnetic-orthorhombic magnetostructural phase transition in the giant magnetocaloric Gd5Si2Ge2 compound. The transition kinetics have been analyzed from the detected acoustic signals. It is shown that this transition proceeds by avalanches between metastable states.

Structure of SiO₂/4H-SiC interface probed by positron annihilation spectroscopy

M. Maekawa, A. Kawasuso, M. Yoshikawa, A. Miyashita, R. Suzuki, and T. Ohdaira

Published 23 January 2006 (9 pages)
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The structure of the SiO2/4H-SiC interface produced by dry oxidation has been studied using positron annihilation spectroscopy using energy-variable slow positron beams. Based on the Doppler broadening shape and wing parameter (S-W) correlation, the interface layer was clearly distinguished from the SiO2 and SiC layers. A single positron lifetime of 451  ps, which is sufficiently longer than that in the SiC substrate (~140  ps) and close to the second lifetime in the SiO2 layer, was obtained when the incident positron energy was adjusted at the interface layer. The electron-positron momentum distribution associated with the interface layer was well explained by a theoretical calculation that considered the annihilation of the positrons by the oxygen valence electrons in the SiO2 layer. The annealing process after the oxidation resulted in the modification of the electron-positron momentum distribution in a manner similar to that of the interface traps, thereby suggesting that the interface traps correlate with the positron annihilation site.

Modulated structure of Rb₂ZnCl₄ in the soliton regime close to the lock-in phase transition

I. Aramburu, K. Friese, J. M. Pérez-Mato, W. Morgenroth, M. Aroyo, T. Breczewski, and G. Madariaga

Published 24 January 2006 (15 pages)
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The structure of the incommensurate phase of Rb2ZnCl4 has been determined at 194  K (2  K above the lock-in transition) within the soliton regime using satellites up to fifth order. The rather anharmonic modulation functions agree with the expected steplike functions supported by theoretical arguments. In addition, the constancy of the ratio between the amplitudes of the fifth-order and first-order harmonics, a relation predicted by theory, indicate the correctness of the model and imply a value of 0.4 for the soliton density ns. A symmetry mode analysis shows that the incommensurate structure is consistent with the one of the lock-in phase in the sense that the displacement pattern of every symmetry mode remains unaltered in the transition except for a global change in the amplitudes.

Three intrinsic relationships of lattice parameters between intermediate monoclinic M_C and tetragonal phases in ferroelectric Pb[(Mg_1/3Nb_2/3)_1–xTi_x]O₃ and Pb[(Zn_1/3Nb_2/3)_1–xTi_x]O₃ near morphotropic phase boundaries

Yu U. Wang

Published 25 January 2006 (13 pages)
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Systematic analysis of extensive experimental data confirms the theoretical prediction of three intrinsic relationships of lattice parameters between the recently discovered intermediate monoclinic MC phase and the conventional tetragonal phase in ferroelectric Pb[(Mg1/3Nb2/3)1–xTix]O3 and Pb[(Zn1/3Nb2/3)1–xTix]O3 near the morphotropic phase boundaries. These intrinsic relationships of lattice parameters are fulfilled by experimental data reported in the literature for different temperatures, compositions, and electric fields. They present quantitative evidence that the intermediate monoclinic MC phase is a mixed state of nanometer-sized twin-related domains of the conventional ferroelectric tetragonal phase. The analysis supports the concept recently proposed by Khachaturyan and co-workers [Phys. Rev. Lett. 91, 197601 (2003)] that the intermediate monoclinic MC phase is adaptive ferroelectric and ferroelastic phase, which is homogeneous only on the macroscale while inhomogeneous on the nanoscale. Due to the small domain size and small ferroelastic strain, the conventional diffraction measurement does not resolve the lattice of individual nanodomains rather instead only perceives the average diffraction effect of nanotwins, yielding the experimentally observed monoclinic symmetry. The result indicates that the electric-field-induced domain-wall movement plays an essential role in the ultrahigh electromechanical responses of Pb[(Mg1/3Nb2/3)1–xTix]O3 and Pb[(Zn1/3Nb2/3)1–xTix]O3, and the high-density domain walls associated with the nanotwins have a significant contribution to the peculiar material properties near the morphotropic phase boundaries.

Gamma-ray spectra resulting from the annihilation of positrons with selected core levels of Cu, Ag, and Au

S. Kim, A. Eshed, S. Goktepeli, P. A. Sterne, A. R. Koymen, W. C. Chen, and A. H. Weiss

Published 26 January 2006 (7 pages)
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The energy spectra of rays resulting from positron annihilation with selected core levels of Cu, Ag, and Au were obtained separately from the total annihilation spectra. The separation was accomplished by measuring the energy of rays detected in time coincidence with Auger electrons emitted consequent to the filling of holes resulting from the annihilation of core electrons. The results of these measurements are compared to the total annihilation spectra and with local-density approximation based theoretical calculations of the core contributions of the selected levels. Good agreement was found between calculated and measured values of the core momentum densities with no adjustable parameters outside of the overall normalization.

Electric-field-, temperature-, and stress-induced phase transitions in relaxor ferroelectric single crystals

Matthew Davis, Dragan Damjanovic, and Nava Setter

Published 27 January 2006 (16 pages)
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Electric-field-induced phase transitions have been evidenced by macroscopic strain measurements at temperatures between 25  °C and 100  °C in [001]C-poled (1–x)Pb(Mg1/3Nb2/3)O3–xPbTiO3  [(PMN-xPT);x=0.25,0.305,0.31] and (1–x)Pb(Zn1/3Nb2/3)O3–xPbTiO3  [(PZN-xPT);x=0.05,0.065,0.085] single crystals. Such measurements provide a convenient way of ascertaining thermal and electrical phase stabilities over a range of compositions and give direct evidence for first-order phase transitions. A pseudorhombohedral (MA)–pseudo-orthorhombic (MC)–tetragonal (T) polarization rotation path is evidenced by two first-order-like, hysteretic discontinuities in strain within the same unipolar electric field cycle for PZN-5PT, PMN-30.5PT, and PMN-31PT whereas, in PMN-25PT, a single first-order-like MA-T transition is observed. This agrees well with in situ structural studies reported elsewhere. Electric-field-temperature (E-T) phase diagrams are constructed showing general trends for MA, MC, and T phase stabilities for varying temperatures and electric fields in poled samples over the given range of compositions. The complex question of whether the MA and MC states constitute true phases, or rather piezoelectrically distorted versions of their rhombohedral (R) and orthorhombic (O) parents, is discussed. Finally, stress-induced phase transitions are evidenced in [001]C-poled PZN-4.5PT by application of a moderate compressive stress (<100  MPa) both along and perpendicularly to the poling direction (longitudinal and transverse modes, respectively). The rotation path is likely R-MB-O, via a first-order, hysteretic rotation within the MB monoclinic plane. The results are presented alongside a thorough review of previously reported electric-field-induced and stress-induced phase transitions in PMN-xPT and PZN-xPT.

Hysteretic elasticity in damaged concrete: Quantitative analysis of slow and fast dynamics

M. Bentahar, H. El Aqra, R. El Guerjouma, M. Griffa, and M. Scalerandi

Published 27 January 2006 (10 pages)
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Fast and slow dynamics in the elastic response of damaged materials to external excitations show evidence of an anomalous (hysteretic) elastic behavior. Experimental observations may be used to detect and eventually characterize the damaged state of structural components. At the same time, reliable theoretical models might support data interpretation and development of new experiments. Here, we analyze the resonance frequency shift in undamaged and damaged concrete bars, comparing experimental results and theoretical predictions obtained from a Preisach-Mayergoyz based elastic model.

Phonons in the -tin, Imma, and sh phases of silicon from ab initio calculations

Katalin Gaál-Nagy and Dieter Strauch

Published 30 January 2006 (4 pages)
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We study the lattice statics and dynamics of Si near the -tinImmash phase transitions. The pressure at and the order of the phase transitions can be determined precisely from the dynamical rather than the static properties. We also present an interpretation of measured Raman frequencies of a high-pressure structure of silicon correcting a previous assignment to the -tin phase, since the Imma phase was unknown. With the new assignment, the ab initio phonon frequencies display an excellent agreement with the experimental data. The shImma transition is accompanied by soft modes which should be accessible in future experiments. The phonon-dispersion curves and density of states point at irregularities of the behavior of the superconducting temperature as a function of the pressure for these phases.

Inhomogeneous, disordered, and partially ordered systems

Intermediate-range order and collective dynamics in an aqueous solution of trivalent cations

Mauro C. C. Ribeiro

Published 10 January 2006 (7 pages)
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Molecular dynamics (MD) simulations of a concentrated aqueous solution of high-valence cations M³⁺ demonstrate the occurrence of intermediate-range order (IRO) by a pronounced prepeak at k~1.0  Å^–1 in the static structure factor. The IRO effect on the high-frequency collective dynamics of acoustic modes is investigated in a model system which mimics a 2.8M EuCl3 aqueous solution. A single branch in the dispersion relation of longitudinal acoustic (LA) modes in solution is obtained up to k~0.5  Å^–1, and then it splits into two branches for higher wave vectors. The low-frequency branch of LA modes in solution is not the well-known transverselike branch seen in MD simulation of pure water. It is assigned instead to a subsystem of M³⁺ cations plus water molecules belonging to the cations' first hydration shell. The excitation energy of the low-frequency branch reaches a maximum in a k range half of the prepeak value, so that IRO plays the role of a pseudo–Brillouin zone for the cation subsystem. The high-frequency branch of the LA modes in solution follows the corresponding one in pure water, and in fact a subsystem of "bulk" water molecules is identified in solution showing high-frequency short-range structural relaxation similar to that of pure water.

One-particle conductance of an open quasi-two-dimensional Fermi system: Evidence of the parallel-magnetic-field-induced mode reduction effect

Yu. V. Tarasov

Published 10 January 2006 (7 pages)
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The conductance of an open quench-disordered two-dimensional (2D) electron system subject to an in-plane magnetic field is calculated within the framework of conventional Fermi liquid theory actually applied to a three-dimensional system of spinless electrons confined to a highly anisotropic (planar) near-surface potential well. Using the calculation method suggested earlier [Phys. Rev. B 71, 125112 (2005)], the magnetic field piercing a finite range of an infinitely long laterally confined system of carriers is treated (technically) as introducing the additional highly nonlocal scattering region which separates the circuit thereby modeled into three parts—the system as such and two perfect leads. The transverse quantization spectrum of the inner part of the electron waveguide thus constructed can be effectively tuned by means of the magnetic field, even though the least transverse dimension of the waveguide is small compared to the magnetic length. The initially finite (metallic) value of the conductance, which is attributed to the existence of extended modes of the transverse quantization, decreases rapidly as the magnetic field grows. This decrease is due to the mode number reduction effect produced by the magnetic field. The closing of the last current-carrying mode, which is slightly sensitive to the disorder level, is suggested as the conceivable origin of the magnetic-field-driven metal-to-insulator transition widely observed in 2D systems.

Mechanical hole-burning spectroscopy: Demonstration of hole burning in the terminal relaxation regime

Xiangfu Shi and Gregory B. McKenna

Published 19 January 2006 (11 pages)
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We have developed a mechanical spectral hole-burning (MSHB) scheme that is analogous to dielectric and magnetic spectral hole-burning techniques. Previous dielectric nonresonant spectral hole-burning experiments have been performed close to the glass temperatures of glass-forming materials and interpreted in terms of dynamic heterogeneity. The present study focuses on polymeric systems far above the glass temperature and in the terminal (reptation) regime. Theoretically, we examine Kaye-Bernstein-Kearsley-Zapas and Bernstein-Shokooh nonlinear viscoelastic constitutive models, which do not invoke an explicit heterogeneous dynamics for the relaxation response, to study MSHB, and find that both models fail to capture the subtle mechanical holes observed in the experiments. Experimentally, we successfully burned mechanical holes and show that the hole intensities vary as a function of "waiting time" and pump amplitude. The results suggest that MSHB is a potentially powerful tool with which to examine the dynamics of complex fluids.

Interaction of a discrete soliton with a surface mode

M. I. Molina

Published 24 January 2006 (6 pages)
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We examine the formation of bound state(s) near the surface of a semi-infinite tight-binding chain, when both, a linear surface mode and a nonlinear impurity located at distance d from the surface, are present. By using the formalism of lattice Green functions, we obtain an exact equation for the allowed bound states, and find that in general, up to five bound states are possible, although not all of them are stable. The presence of a surface mode can alter considerably the critical nonlinearity needed to form a bound state on the nonlinear impurity site, when the impurity is close to the surface. When the surface state is "on," interference effects between its tail and the discrete soliton tail can lead to a destabilization of the latter.

Low-temperature breakdown of coherent tunneling in amorphous solids induced by the nuclear quadrupole interaction

A. L. Burin, I. Ya. Polishchuk, P. Fulde, and Y. Sereda

Published 26 January 2006 (17 pages)
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We consider the effect of the internal nuclear quadrupole interaction on quantum tunneling in complex multiatomic two-level systems. Two distinct regimes of strong and weak interactions are found. The regimes depend on the relationship between a characteristic energy of the nuclear quadrupole interaction * and a bare tunneling coupling strength 0. When 0>*, the internal interaction is negligible and tunneling remains coherent determined by 0. When 0<*, coherent tunneling breaks down and an effective tunneling amplitude decreases by an exponentially small overlap factor ^*1 between internal ground states of left and right wells of a tunneling system. This affects thermal and kinetic properties of tunneling systems at low temperatures T<*. The theory is applied for interpreting the anomalous behavior of the resonant dielectric susceptibility in amorphous solids at low temperatures T5  mK, where the nuclear quadrupole interaction breaks down coherent tunneling. We suggest the experiments with external magnetic fields to test our predictions and to clarify the internal structure of tunneling systems in amorphous solids.

Dynamics, dynamical systems, lattice effects, quantum solids

Two interacting electrons in one-dimensional deterministic aperiodic systems: Fibonacci and Thue-Morse lattices

P. E. de Brito, E. S. Rodrigues, and H. N. Nazareno

Published 12 January 2006 (11 pages)
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We investigate the influence of the interaction on propagating properties of two electrons with opposite spins in one-dimensional deterministic aperiodic systems, namely the Fibonacci and Thue-Morse lattices, where the interaction was assumed along the Hubbard model. We solve the time dependent Schrödinger equation for different values of the interaction strength and degrees of disorder and we vary the initial conditions. We follow the time evolution of the particles and give the mean-square displacement as well as determine the trajectories of the center of the wave packets associated with each of the particles. The interplay between the degree of disorder and the strength of the interaction is analyzed. Both structures present superdiffusive propagation enhanced by the strength of the interaction. We obtained the time exponent for the mean-square displacement. Finally, the action of a dc electric field was included, giving a place to the phenomenon of dynamical localization.

Quasielastic neutron scattering study of hydrogen motion in A15-type Nb₃AlH_x

A. V. Skripov, L. S. Voyevodina, and R. Hempelmann

Published 12 January 2006 (8 pages)
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In order to study the mechanism and parameters of hydrogen diffusion in A15-type intermetallic Nb3Al, we have performed high-resolution quasielastic neutron scattering measurements in Nb3AlHx (x=0.13 and 1.77) over the temperature range 10–407  K. The experimental results are consistent with a coexistence of two hydrogen jump processes: the fast H motion along the chains formed by interstitial d (Nb4) sites and the slower H jumps from one chain to another. It is found that the modified Chudley–Elliott model taking into account the structure of the d-site sublattice gives a qualitative description of the experimental data for Nb3AlH0.13. The faster jump process in Nb3AlH0.13 corresponding to the long-range H diffusion along the d-site chains is characterized by the activation energy of 194  meV. In Nb3AlH1.77 the d-site chains are more than half filled, and the faster jump process is well described in terms of a model of local H jumps within pairs of nearest-neighbor d sites on the chains.

Determination of pseudorotation rates in Ee Jahn-Teller systems

Ian D. Hands, Janette L. Dunn, and Colin A. Bates

Published 13 January 2006 (9 pages)
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A simple method employing the time-evolution operator is used to investigate the dynamical nature of the Jahn-Teller effect in the Ee system. This method allows analytical expressions for the rate of pseudorotation between potential wells to be determined as a function of the vibronic coupling parameters. A comparison is made with experimental data in the literature.

Adiabatic-nonadiabatic transition in the diffusive Hamiltonian dynamics of a classical Holstein polaron

Alex A. Silvius, Paul E. Parris, and Stephan De Bièvre

Published 23 January 2006 (12 pages)
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We study the Hamiltonian dynamics of a free particle injected onto a chain containing a periodic array of harmonic oscillators in thermal equilibrium. The particle interacts locally with each oscillator, with an interaction that is linear in the oscillator coordinate and independent of the particle's position when it is within a finite interaction range. At long times the particle exhibits diffusive motion, with an ensemble averaged mean-squared displacement that is linear in time. The diffusion constant at high temperatures follows a power law D~T^5/2 for all parameter values studied. At low temperatures particle transport changes to a hopping process in which the particle is bound for considerable periods of time to a single oscillator before it is able to escape and explore the rest of the chain. A different power law, D~T^3/4, emerges in this limit. A thermal distribution of particles exhibits thermally activated diffusion at low temperatures as a result of classically self-trapped polaronic states.

Effects of entanglement on elastic and inelastic scattering functions for neutron scattering from a pair of nuclei in solids

Hidehiko Sugimoto, Aiko Okumura, and Hiroyuki Yuuki

Published 25 January 2006 (9 pages)
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We calculate the scattering function of neutrons, S(,), from a pair of nuclei in entangled states caused by the quantum-mechanical indistinguishability of particles. From the calculation, the conditions for detecting the entanglement of protons in solids by neutron scattering experiments are clarified. Our conclusion is that the entangled state of a proton pair, if it exists, can be observed by performing neutron scattering experiments at very low temperatures for elastic scattering and with high energy resolution for inelastic scattering.

Lattice vibrations in La(Ce)Fe₄Sb₁₂ and CoSb₃: Inelastic neutron scattering and theory

J. L. Feldman, Pengcheng Dai, T. Enck, B. C. Sales, D. Mandrus, and D. J. Singh

Published 27 January 2006 (11 pages)
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We present results of time-of-flight inelastic neutron scattering phonon density of states measurements on (La,Ce)0.9Fe4Sb12 and CoSb3 as well as of a detailed comparison with lattice dynamical models in the literature. The MARI experimental setup is replicated by a theory for scattering from a polycrystalline material. The model considered for the filled materials is a local density approximation (LDA) based short-range force constant model and those considered for CoSb3 are the LDA-based model with the La-related parameters removed and a semiempirical model. We show that the presence of La significantly affects the shape of the spectrum. We also conclude that upon filling the Sb intrasquare force constants are weakened and that the transition-metal Sb bonds are almost unchanged.

Magnetism

Ferromagnetic polarons in the one-dimensional ferromagnetic Kondo model with quantum mechanical S=3/2 core spins

Danilo R. Neuber, Maria Daghofer, Hans Gerd Evertz, Wolfgang von der Linden, and Reinhard M. Noack

Published 3 January 2006 (7 pages)
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We present an extensive numerical study of the ferromagnetic Kondo lattice model with quantum mechanical S=3/2 core spins. We treat one orbital per site in one dimension using the density-matrix renormalization group and include on-site Coulomb repulsion between the electrons. We examine parameters relevant to manganites, treating the range of low to intermediate doping, 0x<0.5. In particular, we investigate whether quantum fluctuations favor phase separation over the ferromagnetic polarons observed in a model with classical core spins. We obtain very good agreement of the quantum model with previous results for the classical model, finding separated polarons, which are repulsive at short distance for finite t2g superexchange J. Taking on-site Coulomb repulsion into account, we observe phase separation for small but finite superexchange J, whereas for larger J, polarons are favored in accordance with simple energy considerations previously applied to classical spins. We discuss the interpretation of compressibilities and present a phase diagram with respect to doping and the t2g superexchange parameter J with and without Coulomb repulsion.

Multiple magnetic singlet-singlet excitations in intermetallic PrNiSn

K. A. McEwen, J. Jensen, E. D. Beirne, J. P. Allen, K. Habicht, D. T. Adroja, R. I. Bewley, and D. Fort

Published 3 January 2006 (7 pages)
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Inelastic neutron-scattering experiments have been carried out on a polycrystalline sample of PrNiSn, and seven of the eight excited crystal-field singlets of the Pr ions were detected. The system stays paramagnetic, at least down to 0.9  K, and the three principal susceptibility components have been measured on a PrNiSn single crystal between room temperature and 1.7  K. The crystal-field excitations of the single crystal at low temperatures have been studied by triple-axis neutron spectroscopy. Dispersive effects are observed for three different levels of singlet-singlet excitations. The results are analyzed in terms of a mean-field/random phase approximation model, and it is concluded that the exchange interaction is highly anisotropic and of long range. The critical ratio between the maximum of the exchange interaction and that required for inducing a magnetic ordering of the singlet ground-state system is derived to be 0.48, leading to a predicted ordering temperature of 6  mK for the combined electron-nuclear magnetic system.

Thermodynamic modeling of phase separation in manganites

J. Sacanell, F. Parisi, J. C. P. Campoy, and L. Ghivelder

Published 5 January 2006 (7 pages)
014403 Full Text: PDF (446 kB) | Buy Article

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We present a phenomenological model based on the thermodynamics of the phase separated state of manganites, accounting for its static and dynamic properties. Through calorimetric measurements on La0.225Pr0.4Ca0.375MnO3 the low temperature free energies of the coexisting ferromagnetic and charge ordered phases are evaluated. The phase separated state is modeled by free energy densities uniformly spread over the sample volume. The calculations contemplate the out of equilibrium features of the coexisting phase regime, to allow a comparison between magnetic measurements and the predictions of the model. A phase diagram including the static and dynamic properties of the system is constructed, showing the existence of blocked and unblocked regimes which are characteristics of the phase separated state in manganites.

Deviations from unidirectional anisotropy in layered exchange-bias systems due to breakdown of rigid spin rotations

M. Gruyters

Published 9 January 2006 (5 pages)
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The magnetic behavior of CoO/Co bilayers has been investigated by magnetic hysteresis loop and torque measurements. The latter show a purely unidirectional anisotropy for small-angle rotations of the Co layer magnetization from the bias direction. For larger-angle rotations, high-order anisotropies have to be included. The deviation from a purely unidirectional anisotropy is attributed to a breakdown of rigid spin rotations in the CoO, which reveals spin-glass-like properties owing to a random magnetic anisotropy.

Spin, charge, and orbital correlations in the one-dimensional t_2g-orbital Hubbard model

J. C. Xavier, H. Onishi, T. Hotta, and E. Dagotto

Published 10 January 2006 (9 pages)
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We present the zero-temperature phase diagram of the one-dimensional t2g-orbital Hubbard model, obtained using the density-matrix renormalization group and Lanczos techniques. Emphasis is given to the case of the electron density n=5 corresponding to five electrons per site, while several other cases for electron densities between n=3 and 6 are also studied. At n=5, our results indicate a first-order transition between a paramagnetic (PM) insulator phase, with power-law slowly decaying correlations, and a fully polarized ferromagnetic (FM) state by tuning the Hund's coupling. The results also suggest a transition from the n=5 PM insulator phase to a metallic regime by changing the electron density, either via hole or electron doping. The behavior of the spin, charge, and orbital correlation functions in the FM and PM states are also described in the text and discussed. The robustness of these two states against varying parameters suggests that they may be of relevance in quasi-one-dimensional Co-oxide materials, or even in higher dimensional cobaltite systems as well.

Magnetic inhomogeneity and magnetotransport in electron-doped Ca_1–xLa_xMnO₃ (0x0.10)

C. Chiorescu, J. J. Neumeier, and J. L. Cohn

Published 12 January 2006 (6 pages)
014406 Full Text: PDF (366 kB) | Buy Article

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The dc magnetization (M) and electrical resistivity () as functions of magnetic field and temperature are reported for a series of lightly electron doped Ca1–xLaxMnO3 (0x0.10) specimens for which magnetization [Phys. Rev. B 61, 14319 (2000)] and scattering studies [Phys. Rev. B 68, 134440 (2003)] indicate an inhomogeneous magnetic ground state composed of ferromagnetic (FM) droplets embedded in a G-type antiferromagnetic matrix. A change in the magnetic behavior near x=0.02 has been suggested to be the signature of a crossover to a long-ranged spin-canted phase. The data reported here provide further detail about this crossover in the magnetization, and additional insight into the origin of this phenomenon through its manifestation in the magnetotransport. In the paramagnetic phase (T125  K) we find a magnetoresistance /=–C(M/MS)² (MS is the low-T saturation magnetization), as observed in many manganites in the ferromagnetic (FM), colossal magnetoresistance (CMR) region of the phase diagram, but with a value of C that is two orders of magnitude smaller than observed for CMR materials. The doping behavior C(x) follows that of MS(x), indicating that electronic inhomogeneity associated with FM fluctuations occurs well above the magnetic ordering transition.

Analysis of measured transport properties of domain walls in magnetic nanowires and films

L. Berger

Published 12 January 2006 (5 pages)
014407 Full Text: PDF (80 kB) | Buy Article

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Existing data for soft magnetic materials of critical current for domain-wall motion, wall speed driven by a magnetic field, and wall electrical resistance show that all three observable properties are related through a single parameter: the wall mobility µ. The reciprocal of µ represents the strength of viscous friction between domain wall and conduction-electron gas. And µ is a function of the wall width, which depends in turn on the aspect ratio t/w, where t and w are the thickness and width of the sample. Over four orders of magnitude of µ, the data for nanowires show µ(t/w)^–2.2. This dependence is in approximate agreement with the prediction of the 1984 Berger theory based on s-d exchange. On the other hand, it is inconsistent with the prediction of the 2004 Tatara and Kohno theory, and of the 2004 Zhang and Li theory.

Current-induced macrospin versus spin-wave excitations in spin valves

Arne Brataas, Yaroslav Tserkovnyak, and Gerrit E. W. Bauer

Published 13 January 2006 (8 pages)
014408 Full Text: PDF (167 kB) | Buy Article

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The mode dependence of current-induced magnetic excitations in spin valves is studied theoretically. The torque exerted on the magnetization by transverse spin currents as well as the Gilbert damping constant are found to depend strongly on the wavelength of the excitation (spin wave). Analytic expressions are presented for the critical currents that excite a selected spin wave. The onset of macrospin (zero wavelength) vs finite wavelength instabilities depends on the device parameters and the current direction, in agreement with recent experimental findings.

Frustrated three-leg spin tubes: From spin 1/2 with chirality to spin 3/2

J.-B. Fouet, A. Läuchli, S. Pilgram, R. M. Noack, and F. Mila

Published 13 January 2006 (7 pages)
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Motivated by the recent discovery of the spin tube [(CuCl2tachH)3Cl]Cl2, we investigate the properties of a frustrated three-leg spin tube with antiferromagnetic intra- and inter-ring couplings. We pay special attention to the evolution of the properties from weak to strong inter-ring coupling and show on the basis of extensive density matrix renormalization group and exact diagonalization calculations that the system undergoes a first-order phase transition between a dimerized gapped phase at weak coupling that can be described by the usual spin-chirality model and a gapless critical phase at strong coupling that can be described by an effective spin-3/2 model. We also show that there is a magnetization plateau at 1/3 in the whole gapped phase and slightly beyond. The implications for [(CuCl2tachH)3Cl]Cl2 are discussed, with the conclusion that this system behaves essentially as a spin-3/2 chain.

Renormalization algorithm for the calculation of spectra of interacting quantum systems

D. Porras, F. Verstraete, and J. I. Cirac

Published 13 January 2006 (7 pages)
014410 Full Text: PDF (214 kB) | Buy Article

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We present an algorithm for the calculation of eigenstates with definite linear momentum in quantum lattices. Our method is related to the density matrix renormalization group, and makes use of the distribution of multipartite entanglement to build variational wave functions with translational symmetry. The algorithm is applied to the study of bilinear-biquadratic S=1 chains, in particular to the region of phase space between the dimerized and ferromagnetic phases.

Thermodynamic properties of ferrimagnetic spin chains in the presence of a magnetic field

J. Abouie, S. A. Ghasemi, and A. Langari

Published 17 January 2006 (7 pages)
014411 Full Text: PDF (106 kB) | Buy Article

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We have implemented three approaches to describe the thermodynamic properties of ferrimagnetic (S=5/2, s=2) spin chains. The application of cumulant expansion has been generalized to the ferrimagnetic chain in the presence of an external magnetic field. Using cumulants, we have obtained the field-dependent effective Hamiltonian in terms of the classical variables up to the second order of quantum corrections. Thermodynamic functions, the internal energy, the specific heat, and the magnetic susceptibility are obtained from the effective Hamiltonian. We have also examined the modified spin-wave theory to derive the same physical properties. Finally, we have studied our model using quantum Monte Carlo simulation to obtain accurate results. The comparison of the above results and also the high temperature series expansion shows that cumulant expansion gives good results for moderate and high temperature regions while the modified spin wave theory is good for low temperatures. Moreover, the convergence regions of the cumulant expansion and the modified spin-wave theory overlap each other which propose these two as a set of complement methods to get the thermodynamic properties of spin models.

Spherical 2+p spin-glass model: An analytically solvable model with a glass-to-glass transition

A. Crisanti and L. Leuzzi

Published 17 January 2006 (20 pages)
014412 Full Text: PDF (254 kB) | Buy Article

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We present the detailed analysis of the spherical s+