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I I Mazin

Publications and source records attributed to I I Mazin.

9 recordsLinked to original sources
PubMed2005

Critical temperature and enhanced isotope effect in the presence of paramagnons in phonon-mediated superconductors.

We reconsider the long-standing problem of the effect of spin fluctuations on the critical temperature and isotope effect in a phonon-mediated superconductor. Although the general physics of the interplay between phonons and paramagnons has been rather well understood, the existing approximate formulas fail to describe the correct behavior of Tc for general phonon and paramagnon spectra. Using a controllable approximation, we derive an analytical formula for Tc which agrees well with exact numerical solutions of the Eliashberg equations for a broad range of parameters. Based on both numerical and analytical results, we predict a strong enhancement of the isotope effect when the frequencies of spin fluctuation and phonons are of the same order. This effect may have important consequences for near-magnetic superconductors such as MgCNi3.

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PubMed2005

Intercalant-driven superconductivity in YbC6 and CaC6.

Recently discovered superconductivity in YbC6 and CaC6, at temperatures substantially higher than previously known for intercalated graphites, raises several new questions. (1) Is the mechanism considerably different from that of previously known intercalated graphites? (2) If superconductivity is conventional, what are the relevant phonons? (3) Given the extreme similarity between YbC6 and CaC6, why are their critical temperatures so different? We address these questions on the basis of first-principles calculations and conclude that coupling with intercalant phonons is likely to be the main force for superconductivity in YbC6 and CaC6, but not in alkaline-intercalated compounds, and explain the difference in T(c) by the "isotope effect" due to the difference in Yb and Ca atomic masses.

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PubMed2004

Nesting, spin fluctuations, and odd-gap superconductivity in NaxCoO2.yH2O.

We calculated the one-electron susceptibility of hydrated NaxCoO2 and find strong nesting, involving about 70% of all electrons at the Fermi level and nearly commensurate with a 2 x 2 superstructure. This nesting creates a tendency to a charge density wave compatible with the charge order often seen at x approximately 0.5 and usually ascribed to electrostatic repulsion of Na ions. In the spin channel, it leads to strong spin fluctuations, which should be important for superconductivity. The state most compatible with this nesting structure is an odd-gap triplet s-wave state.

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PubMed2004

Why Ni3Al is an itinerant ferromagnet but Ni3Ga is not.

Ni3Al and Ni3Ga are closely related materials on opposite sides of a ferromagnetic quantum critical point. The Stoner factor of Ni is virtually the same in both compounds and the density of states is larger in Ni3Ga. Thus in Stoner theory it should be more magnetic, and in local-density approximation (LDA) calculations it is. However, experimentally it is a paramagnet, while Ni3Al is an itinerant ferromagnet. We show that critical spin fluctuations are stronger in Ni3Ga, due to weaker q dependence of the susceptibility, and this effect is enough to reverse the trend. The approach combines LDA calculations with Landau theory and the fluctuation-dissipation theorem using the same momentum cutoff for both compounds. The calculations provide evidence for strong, beyond LDA, spin fluctuations associated with the critical point in both materials, but stronger in Ni3Ga than in Ni3Al.

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PubMed2002

Superconductivity in MgB2: clean or dirty?

A large number of experimental facts and theoretical arguments favor a two-gap model for superconductivity in MgB2. However, this model predicts strong suppression of the critical temperature by interband impurity scattering and, presumably, a strong correlation between the critical temperature and the residual resistivity. No such correlation has been observed. We argue that this fact can be understood if the band disparity of the electronic structure is taken into account, not only in the superconducting state, but also in normal transport.

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PubMed2002

Competition of spin fluctuations and phonons in superconductivity of ZrZn(2).

It has long been suspected that spin fluctuations in ZrZn2 may lead to a triplet superconductivity. We point out another possibility, an inhomogeneous singlet (Fulde-Ferrell) state. We calculated the electronic structure, as well as the zone center phonons and their coupling with electrons. We find that the exchange splitting is nonuniform and the Fermi surface exhibits substantial nesting. Both factors favor a Fulde-Ferrell state at parts of the Fermi surface. We find a substantial coupling of Zr rattling modes with electrons, which can provide the necessary pairing in the s-channel.

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PubMed2001

Beyond Eliashberg superconductivity in MgB2: anharmonicity, two-phonon scattering, and multiple gaps.

Density-functional calculations of the phonon spectrum and electron-phonon coupling in MgB (2) are presented. The E(2g) phonons, which involve in-plane B displacements, couple strongly to the p(x,y) electronic bands. The isotropic electron-phonon coupling constant is calculated to be about 0.8. Allowing for different order parameters in different bands, the superconducting lambda in the clean limit is calculated to be significantly larger. The E(2g) phonons are strongly anharmonic, and the nonlinear contribution to the coupling between the E(2g) modes and the p(x,y) bands is significant.

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PubMed2001

Superconductivity of metallic boron in MgB2.

Boron in MgB2 forms stacks of honeycomb layers with magnesium as a space filler. Band structure calculations indicate that Mg is substantially ionized, and the bands at the Fermi level derive mainly from B orbitals. Strong bonding with an ionic component and considerable metallic density of states yield a sizable electron-phonon coupling. Together with high phonon frequencies, which we estimate via zone-center frozen phonon calculations to be between 300 and 700 cm(-1), this produces a high critical temperature, consistent with recent experiments. Thus MgB2 can be viewed as an analog of the long sought, but still hypothetical, superconducting metallic hydrogen.

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