arXiv subjects

I. I. Mazin

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

196 records · Page 11Linked to original sources

arXiv1997

Nonlocal density functionals and the linear response of the homogeneous electron gas

The known and usable truly nonlocal functionals for exchange-correlation energy of the inhomogeneous electron gas are the ADA (average density approximation) and the WDA (weighted density approximation). ADA, by design, yields the correct linear response function of the uniform electron gas. WDA is constructed so that it is exact in the limit of one-electron systems. We derive an expression for the linear response of the uniform gas in the WDA, and calculate it for several flavors of WDA. We then compare the results with the Monte-Carlo data on the exchange-correlation local field correction, and identify the weak points of conventional WDA in the homogeneous limit. We suggest how the WDA can be modified to improve the response function. The resulting approximation is a good one in both opposite limits, and should be useful for practical nonlocal density functional calculations.

cond-mat.mtrl-sci↗

arXiv1997

Electronic structure and magnetism in Ru based perovskites

The magnetic properties of (Ca,Sr)RuO_3 and Sr_2YRuO_6 are studied within the context of band structure based Stoner theory. LSDA calculations without recourse to strong correlation effects yield the correct magnetic behavior and order in all cases. Insulating character of Sr_2YRuO_6 is reproduced. The different magnetic states of SrRuO_3 and CaRuO_3 are shown to be due to the different structural distortions. CaRuO_3 is found to be on the verge of a ferromagnetic instability. O p states hybridize strongly with Ru d states in all three compounds, and O, through this hybridization plays an unusually large role in the magnetic properties. Transport properties of CaRuO_3 and SrRuO_3 are analyzed using the calculated Fermiology. Unusually large magnon and paramagnon couplings are found, which are consistent with reported measurements of the low temperature specific heat and the resistivity coefficient. We also demonstrate how the calculated band structures can be understood in terms of a nearest neighbor tight binding model.

cond-mat.mtrl-sci↗

arXiv1997

Ferromagnetic spin fluctuation induced superconductivity in Sr_2RuO_4

We propose a quantitative model for triplet superconductivity in Sr_2RuO_4 based on first principles calculations for the electronic structure and magnetic susceptibility. The superconductivity is due to ferromagnetic spin fluctuations, that are strong at small wave vectors. The calculated effective mass renormalization, renormalized susceptibility, and superconducting critical temperature are all in good agreement with experiment. The most stable superconducting solution has order parameters of comparable magnitude on all three sheets of the Fermi surface. We also propose an explanation of the discrepancy between the Fermi surface topology observed in the photoemission and in de Haas-van Alphen experiments.

cond-mat.supr-con↗

arXiv1997

Where are holes in Y_{1-x}Pr_xBa2Cu3O7 ?

Recent experiments by Mertz et al (Phys. Rev. B55, 9160, 1997) demonstrated that the loss of superconductivity upon Pr doping is associated with the change of the character of the oxygen holes from $p_σ$ to $p_π.$ This experiment sheds new light onto the long-standing problem of $T_{c}$ suppression by Pr, and helps to rule out a number of theoretical models, leaving only those which predict such a transfer of the O holes. To distinguish between the two models that do predict such an effect, one has to access the ratio of the planar and axial character of the holes in both models. We do so in this paper.

cond-mat.supr-con↗

arXiv1997

Calculated thermoelectric properties of La-filled skutterudites

The thermoelectric properties of La-filled skutterudites are discussed from the point of view of their electronic structures. These are calculated from first principles within the local density approximation. The electronic structure is in turn used to determine transport related quantities. Virtual crystal calculations for La(Fe,Co)_4Sb_{12} show that the system obeys near rigid band behavior with varying Co concentration, and has a substantial band gap at a position corresponding to the composition LaFe_3% CoSb_{12}. The valence band maximum occurs at the $Γ$ point and is due to a singly degenerate dispersive band, which by itself would not be favorable for high thermopower. However, very flat transition metal derived bands occur in close proximity and become active as the doping level is increased, giving a non-trivial dependence of the properties on carrier concentration and explaining the favorable thermoelectric properties.

cond-mat.mtrl-sci↗

arXiv1997

Effect of magnetic and non-magnetic impurities on highly anisotropic superconductivity

We generalize Abrikosov-Gor'kov solution of the problem of weakly coupled superconductor with impurities on the case of a multiband superconductor with arbitrary interband order parameter anisotropy, including interband sign reversal of the order parameter. The solution is given in terms of the effective (renormalized) coupling matrix and describes not only $T_c$ suppression but also renormalization of the superconducting gap basically at all temperatures. In many limiting cases we find analytical solutions for the critical temperature suppression. We illustrate our results by numerical calculations for two-band model systems.

cond-mat.supr-con↗

arXiv1997

Quantum and Classical Orientational Ordering in Solid Hydrogen

We present a unified view of orientational ordering in phases I, II, and III of solid hydrogen. Phases II and III are orientationally ordered, while the ordering objects in phase II are angular momenta of rotating molecules, and in phase III the molecules themselves. This concept provides quantitative explanation of the vibron softening, libron and roton spectra, and increase of the IR vibron oscillator strength in phase III. The temperature dependence of the effective charge parallels the frequency shifts of the IR and Raman vibrons. All three quantities are linear in the order parameter.

cond-mat↗

arXiv1996

Insulating gap in FeO: correlations and covalency

We report calculations of the electronic structure of FeO in the LDA and LDA+U approximation with and without rhombohedral distortion. In both cases LDA renders an antiferromagnetic metal, and LDA+U opens a Hubbard gap. However, the character of the gap is qualitatively different in the two structure, and the difference can be traced down to underlying LDA band structure. An analysis of the calculations gives a new insight on the origin of the insulating gap in 3d monoxides and on the role of the k-dependency of U, missing in the contemporary LDA+U method.

cond-mat.str-el↗

arXiv1996

Neutron scattering and superconducting order parameter in YBa2Cu3O7

We discuss the origin of the neutron scattering peak at 41 meV observed in YBa$_2$Cu$_3$O$_7$ below $T_c$. The peak may occur due to spin-flip electron excitations across the superconducting gap which are enhanced by the antiferromagnetic interaction between Cu spins. In this picture, the experiment is most naturally explained if the superconducting order parameter has $s$-wave symmetry and opposite signs in the bonding and antibonding electron bands formed within a Cu$_2$O$_4$ bilayer.

cond-mat↗

arXiv1996

Notes on the static dielectric response function in the density functional theory

We discuss several aspects of the dielectric response theory application to the density functional theory. This field has been an unceasing source of confusion during several decades. The most frequent reasons for this confusion are (a) uncritical tranfer of the results, especially regarding so-called local field corrections, obtained in many-body perturbation theory onto density functional theory, and (b) mixing up the statements true for the exact density functional theory with those applicable to the local density approximation only. In these notes we try to draw an appropriate lines between those theories. We also discuss a newly introduced (X. Gonze, Ph. Ghosez, and R.W. Godby, Phys. Rev. Lett., 74, 4035, 1995) "polarization+density functional" and show that within a given (e.g., local density) approximation to the exchange-correlation energy the Gonze et al approach is exactly equivalent to the conventional one.

cond-mat↗

arXiv1996

Impurity scattering in highly anisotropic superconductors and interband sign reversal of the order parameter

We discuss various mechanisms that can lead to interband sign reversal of the order parameter in a multiband superconductor. In particular, we generalize Abrikosov-Gor'kov solution of the problem of weakly coupled superconductor with magnetic and nonmagnetic impurities on the case of arbitary order parameter anisotropy, including extreme cases as $d-$pairing or interband sign reversal of the order parameter, and show that interband scattering by magnetic impurities can stabilize an interband sign-reversal state. We discuss a possibility of such state in YBa$_2$Cu$_3$O$_7$ in the context of various experiments: Josephson tunneling, neutron scattering, isotope effect measurements.

cond-mat↗

arXiv1995

Insulator-metal transition in solid hydrogen: Implication of electronic structure calculations for recent experiments

Electronic structure calculations for compressed molecular hydrogen are performed to provide more insight into the diversity of phenomena recently observed experimentally. We perform full-potential LAPW calculations and analyze them in terms of a molecular tight-binding model. We show that $σ_g$ and $σ_u$ bands overlap occurs at rather low pressure, while an insulating state persists at specific orientations up to 200 GPa, in accord with previous work, and is due to opening of hybridization gaps at the Fermi level. We also present calculations of electronic properties such as plasma frequencies and electron-vibron coupling.

mtrl-th↗

arXiv1995

Hohenberg-Kohn theorem is valid

In a recent PRL, Gonze et al claim that the density functional theory in Hohenberg-Kohn formulation is not valid for crystalline solids in a homogenious electric field. They introduce another theory, density-and-polarization functional theory, which has to substitute the DFT in the calculations of the long-wave dielectric response. We show that the theorem proven by Gonzo et al holds in addition to, and not instead of, the Hohenberg-Kohn theorem.

cond-mat↗

arXiv1995

"Chain scenario" for Josephson tunneling with pi-shift in YBa2Cu3O7

We point out that all current Josephson-junction experiments probing directly the symmetry of the superconducting state in YBa2Cu3O7, can be interpreted in terms of the bilayer antiferromagnetic spin fluctuation model, which renders the superconducting state with the order parameters of extended $s$ symmetry, but with the opposite signs in the bonding and antibonding Cu-O plane bands. The essential part of our interpretation includes the Cu-O chain band which would have the order parameter of the same sign as antibonding plane band. We show that in this case net Josephson currents along and perpendicular to the chains have the phase shift equal to pi.

cond-mat↗

arXiv1995

Sign reversal of the order parameter in s-wave superconductors

We show that in a superconductor where two or more bands cross the Fermi level it is possible, in the framework of the conventional (s-wave) BCS theory, that the sign of the superconducting gap is different on the different sheets of the Fermi surface. At least one of two conditions has to be satisfied: (1) The interband pairing interaction is weaker than the Coulomb pseudopotential, while the intraband one is stronger, or (2) there is strong interband scattering by magnetic impurities. In the case of YBa$_2$% Cu$_3$O$_7$ we shall argue that the first condition is possibly satisfied, and the second one very likely satisfied. In many aspects such a sign-reversal $s-$wave superconductor is similar to a d-wave superconductor, and thus demands revising recent experiments aimed to distinguish between the $s-$ and $d-$wave superconductivity in this compound.

cond-mat↗

arXiv1995

s-wave superconductivity from antiferromagnetic spin-fluctuation model for bilayer materials

It is usually believed that the spin-fluctuation mechanism for high-temperature superconductivity results in d-wave pairing, and that it is destructive for the conventional phonon-mediated pairing. We show that in bilayer materials, due to nearly perfect antiferromagnetic spin correlations between the planes, the stronger instability is with respect to a superconducting state whose order parameters in the even and odd plane-bands have opposite signs, while having both two-dimensional $s$-symmetry. The interaction of electrons with Raman- (infrared-) active phonons enhances (suppresses) the instability.

cond-mat↗