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I. I. Mazin

Publications and source records attributed to I. I. Mazin.

At least 181 records · Page 10Linked to original sources
arXiv2000

The origin of high transport spin polarization in La$_{0.7}$Sr$_{0.3} $MnO$_{3}$: direct evidence for minority spin states

Using the point contact Andreev reflection technique, we have carried out a systematic study of the spin polarization in the colossal magnetoresistive manganite, La$_{0.7}$Sr$_{0.3}$MnO$_{3}$} (LSMO). Surprisingly, we observed a significant increase in the current spin polarization with the residual resistivity. This counterintuitive trend can be understood as a transition from ballistic to diffusive transport in the contact. Our results strongly suggest that LSMO does have minority spin states at the Fermi level. However, since its current spin polarization is much higher than that of the density of states, this material can mimic the behavior of a true half-metal in transport experiments. Based on our results we call this material a {\it transport} half-metal.

cond-mat.mtrl-sci↗
arXiv2000

Electronic Structure and Fermiology of Sr$_3$Ru$_2$O$_7$

The electronic structure of layered Sr$_{3}$Ru$_{2}$O$_{7}$ in its orthorhombic structure is investigated using density functional calculations. The band structure near the Fermi energy, consists of Ru $% t_{2g}$ states hybridized with O $p$ orbitals. The $yz$ and $xz$ bands, which are largely responsible for the nesting related antiferromagnetic spin fluctuations in Sr$_{2}$RuO$_{4}$ split pairwise into even and odd combinations due to the interlayer coupling reducing the strength of the nesting. The $xy$ bands show much less interplanar coupling as expected, and also less $c$-axis dispersion, so that the barrel like sections are largely intact compared to the single layer material. The zone folding due to orthorhombicity yields small cylindrical lens shaped Fermi surfaces centered at the midpoints of the former tetragonal $Γ-X$ lines. Fixed spin moment calculations indicate that tetragonal Sr$_{3}$Ru$_{2}$O$_{7}$ is borderline ferromagnetic but that orthorhombicity favors magnetism via a substantial magnetoelastic coupling. These results are related to experimental observations particularly in regard to magnetic properties.

cond-mat.supr-con↗
arXiv2000

Lattice dynamics and reduced thermal conductivity of filled skutterudites

The great reduction in thermal conductivity of skutterudites upon filling the ``void'' sites with Rare Earth (RE) ions is key to their favorable thermoelectric properties but remains to be understood. Using lattice dynamic models based on first principles calculations, we address the most popular microscopic mechanism, reduction via rattling ions. The model withstands inelastic neutron scattering and specific heat measurements, and refutes hypotheses of an anharmonic RE potential and of two distinct localized RE vibrations of disparate frequencies. It does indicate a strong hybridization between bare La vibrations and certain Sb-like phonon branches, suggesting anharmonic scattering by harmonic RE motions as an important mechanism for suppression of heat conductivity.

cond-mat.mtrl-sci↗
arXiv2000

Robust half-metallicity and metamagnetism in Fe_{x}Co_{1-x}S_{2}

The Fe_{x}Co_{1-x}S_{2} system is predicted, on the basis of density functional calculations, to be a half metal for a large range of concentrations. Unlike most known half metals, the half metallicity in this system should be very stable with respect to crystallographic disorder and other types of defects. The endmember of the series, CoS_{2}, is not a half metal, but exhibits interesting and unusual magnetic properties which can, however, be reasonably well understood within the density functional theory, particularly with the help of the extended Stoner model. Calculations suggest strong electron-phonon and electron-magnon coupling in the system, and probably a bad metal behavior at high temperatures.

cond-mat.mtrl-sci↗
arXiv1999

Transport spin polarization of Ni_xFe_{1-x}: electronic kinematics and band structure

We present measurements of the transport spin polarization of Ni_xFe_{1-x} (0<x<1) using the recently-developed Point Contact Andreev Reflection technique, and compare them with our first principles calculations of the spin polarization for this system. Surpisingly, the measured spin polarization is almost composition-independent. The results clearly demonstrate that the sign of the transport spin polarization does not coincide with that of the difference of the densities of states at the Fermi level. Calculations indicate that the independence of the spin polarization of the composition is due to compensation of density of states and Fermi velocity in the s- and d- bands.

cond-mat.mtrl-sci↗
arXiv1999

Tight-binding Hamiltonians for Sr filled ruthenates: application to the gap anisotropy and Hall coefficient in Sr2RuO4

Accurate orthogonal tight-binding Hamiltonians are constructed for ferromagnetic SrRuO$_3$ and the layered perovskite superconductor, Sr$_2$RuO$_4$ by fitting to all-electron full-potential local density band structures obtained by the linearized augmented planewave method. These Hamiltonians allow the band structure to be computed on very fine meshes in the Brillouin zone at low cost, and additionally have analytic band velocities, while retaining the accuracy of the full-potential electronic structure calculations. This greatly facilitates calculation of transport and superconducting parameters related to the fermiology. These features are exploited to calculate the Hall coefficient and vortex lattice geometry for Sr$_2$RuO$_4$ with fine integration meshes. We find the lower limit for the interband order parameter anisotropy to be compatible with the observed square geometry. We also find that the sign reversal of the Hall coefficient can be explained in a conventional way if the bands are shifted by a few mRy so as to match the experimental de Haas-van Alphen areas exactly, {\it and} the temperature dependence of the relaxation time is strongly dependent on the band character.

cond-mat.mtrl-sci↗
arXiv1999

Electronic Structure and Heavy Fermion Behavior in LiV_2O_4

First principles density functional calculations of the electronic and magnetic properties of spinel-structure LiV$_{2}$O$_{4}$ have been performed using the full potential linearized augmented planewave method. The calculations show that the electronic structure near the Fermi energy consists of a manifold of 12 bands derived from V $t_{2g}$ states, weakly hybridized with O p states. While the total width of this active manifold is approximately 2 eV, it may be roughly decomposed into two groups: high velocity bands and flatter bands, although these mix in density functional calculations. The flat bands, which are the more atomic-like lead to a high density of states and magnetic instability of local moment character. The value of the on-site exchange energy is sensitive to the exact exchange correlation parameterization used in the calculations, but is much larger than the interaction between neighboring spins, reflecting the weak coupling of the magnetic system with the high velocity bands. A scenario for the observed heavy fermion behavior is discussed in which conduction electrons in the dispersive bands are weakly scattered by local moments associated with strongly correlated electrons in the heavy bands.This is analogous to that in conventional Kondo type heavy fermions, but is unusual in that both the local moments and conduction electrons come from the same d-manifold.

cond-mat.str-el↗
arXiv1999

How to calculate the degree of spin polarization in ferromagnets?

Different ways to define and to calculate the degree of spin polarization in a ferromagnet are discussed, particularly with respect to spin-polarized tunneling and Andreev reflection at the boundary between a superconductor and ferromagnet. As an example, the degrees of spin polarization for different experiments in Fe and Ni are calculated in the framework of the local spin density approximation (LSDA) and used to illustrate the differences between various definitions of spin polarization.

cond-mat.mtrl-sci↗
arXiv1999

Suppression and restoration of superconductivity in PrBa2Cu3O7

I review the principal experimental findings about superconductivity, suppression thereof, and related properties of RE_{1-x}Pr_xBa_2Cu3O7 in a decision-tree manner, eliminating the models substantially incompatible with established experimental facts, eventually focussing on the pfσhybridization models and listing on a verbal level the interpretations of the existing experiments as they emerge from such models.

cond-mat.supr-con↗
arXiv1999

Competitions in layered ruthenates: ferro- vs. antiferromagnetism and triplet vs. singlet pairing

Ru based perovskites demonstrate an amazing richness in their magnetic properties, including 3D and quasi-2D ferromagnetism, antiferromagnetism, and unconventional superconductivity. Tendency to ferromagnetism, stemming from the unusually large involvement of O in magnetism in ruthenates, leads to ferromagnetic spin fluctuations in Sr2RuO4 and eventually to p-wave superconductivity. A related compound Ca2RuO4 was measured to be antiferromagnetic, suggesting a possibility of antiferromagnetic fluctuations in Sr2RuO4 as well. Here we report first principles calculations that demonstrate that in both compounds the ferro- and antiferromagnetic fluctuations coexist, leading to an actual instability in Ca2RuO4 and to a close competition between p-wave and d-wave superconducting symmetries in Sr2RuO4. The antiferromagnetism in this system appears to be mostly related with the nesting, which is the strongest at Q=(2pi/3,2pi/3,0). Surprisingly, for the Fermiology of Sr2RuO4 the p-wave state wins over the d-wave one everywhere except in close vicinity of the antiferromagnetic instability. The most stable state within the d-wave channel has vanishing order parameter at one out of three Fermi surfaces in Sr2RuO4, while in the p channel its amplitude is comparable at all three of them.

cond-mat.supr-con↗
arXiv1998

Theoretical search for Chevrel phase based thermoelectric materials

We investigate the thermoelectric properties of some semiconducting Chevrel phases. Band structure calculations are used to compute thermopowers and to estimate of the effects of alloying and disorder on carrier mobility. Alloying on the Mo site with transition metals like Re, Ru or Tc to reach a semiconducting composition causes large changes in the electronic structure at the Fermi level. Such alloys are expected to have low carrier mobilities. Filling with transition metals was also found to be incompatible with high thermoelectric performance based on the calculated electronic structures. Filling with Zn, Cu, and especially with Li was found to be favorable. The calculated electronic structures of these filled Chevrel phases are consistent with low scattering of carriers by defects associated with the filling. We expect good mobility and high thermopower in materials with the composition close to (Li,Cu)$_4$Mo$_6$Se$_8$, particularly when Li-rich, and recommend this system for experimental investigation.

cond-mat.mtrl-sci↗
arXiv1998

Transport, optical and electronic properties of the half metal CrO2

The electronic structure of CrO_2 is critically discussed in terms of the relation of existing experimental data and well converged LSDA and GGA calculations of the electronic structure and transport properties of this half metal magnet, with a particular emphasis on optical properties. We find only moderate manifestations of many body effects. Renormalization of the density of states is not large and is in the typical for transition metals range. We find substantial deviations from Drude behavior in the far-infrared optical conductivity. These appear because of the unusually low energy of interband optical transitions. The calculated mass renormalization is found to be rather sensitive to the exchange-correlation functional used and varies from 10% (LSDA) to 90% (GGA), using the latest specific heat data. We also find that dressing of the electrons by spin fluctuations, because of their high energy, renormalizes the interband optical transition at as high as 4 eV by about 20%. Although we find no clear indications of strong correlations of the Hubbard type, strong electron-magnon scattering related to the half metallic band structure is present and this leads to a nontrivial temperature dependence of the resistivity and some renormalization of the electron spectra.

cond-mat.mtrl-sci↗
arXiv1998

On the possibility of superconductivity in PrBa2Cu3O7

Recent reports about observations of superconductivity in PrBa2Cu3O7 raise a number of questions: (i) of various theories striving to explain the Tc suppression in PrxY{1-x}Ba2Cu3O7, are there any compatible with possible superconductivity in stoichiometric PrBa2Cu3O7? (ii) if this superconductivity is not an experimental artifact, are the superconducting carriers (holes) of the same character as in the other high-Tc cuprates, or do they represent another electronic subsystem? (iii) is the underlying mechanism the same as in other high-Tc superconductors? I present an answer to the first two questions, while leaving the last one open.

cond-mat.supr-con↗
arXiv1998

Weighted Density Functionals for Ferroelectric Materials

The weighted density approximation, its implementation and its application to ferroelectric materials is discussed. Calculations are presented for several perovskite oxides and related materials. In general the weighted density approximation is found to be superior to either the local density or generalized gradient approximation for the ground state. Electronic structures are little changed. The linear response of the weighted density approximation is calculated for the homogeneous electron gas, and found to be improved relative to the local density result, but not in full agreement with existing Monte Carlo data. It is shown that the agreement can be further improved by a simple modification. Calculations of the ferroelectric soft mode in KNbO$_3$ suggest that the low temperature distortion is approximately 20% smaller than indicated by existing experiments.

cond-mat.mtrl-sci↗
arXiv1998

Electronic Structure, Local Moments and Transport in Fe_2VAl

Local spin density approximation calculations are used to elucidate electronic and magnetic properties of Heusler structure Fe_2VAl. The compound is found to be a low carrier density semimetal. The Fermi surface has small hole pockets derived from a triply degenerate Fe derived state at Gamma compensated by an V derived electron pocket at the X point. The ideal compound is found to be stable against ferromagnetism. Fe impurities on V sites, however, behave as local moments. Because of the separation of the hole and electron pockets the RKKY interaction between such local moments should be rapidly oscillating on the scale of its decay, leading to the likelihood of spin-glass behavior for moderate concentrations of Fe on V sites. These features are discussed in relation to experimental observations of an unusual insulating state in this compound.

cond-mat.mtrl-sci↗
arXiv1997

Phenomenological interpretations of the ac Hall effect in the normal state of YBa2Cu3O7

Ac and dc magnetotransport data in the normal state of YBa2Cu3O7 are analyzed within Fermi-liquid and non-Fermi-liquid models. In the Fermi-liquid analysis we use the Fermi surface deduced from band structure calculations and angular resolved photoemission experiments and assume that the electron relaxation rate varies over the Fermi surface. The non-Fermi-liquid models are the two-dimensional Luttinger liquid model and the charge-conjugation-symmetry model. We find that the existing experimental data can be adequately fitted by any of these models. This work provides a framework for the analysis of experiments that may discriminate among these models.

cond-mat.supr-con↗
arXiv1997

First Principles Study of Zn-Sb Thermoelectrics

We report first principles LDA calculations of the electronic structure and thermoelectric properties of $β$-Zn$_{4}$Sb$_{3}$. The material is found to be a low carrier density metal with a complex Fermi surface topology and non-trivial dependence of Hall concentration on band filling. The band structure is rather covalent, consistent with experimental observations of good carrier mobility. Calculations of the variation with band filling are used to extract the doping level (band filling) from the experimental Hall number. At this band filling, which actually corresponds to 0.1 electrons per 22 atom unit cell, the calculated thermopower and its temperature dependence are in good agreement with experiment. The high Seebeck coefficient in a metallic material is remarkable, and arises in part from the strong energy dependence of the Fermiology near the experimental band filling. Improved thermoelectric performance is predicted for lower doping levels which corresponds to higher Zn concentrations.

cond-mat.mtrl-sci↗