arXiv subjects

I. I. Mazin

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

At least 163 records · Page 9Linked to original sources

arXiv2003

Comment on "First-principles calculation of the superconducting transition in MgB2 within the anisotropic Eliashberg formalism"

Choi et al. [Phys. Rev. B 66, 020513 (2002)] recently presented first principles calculations of the electron-phonon coupling and superconductivity in MgB2, emphasizing the importance of anisotropy and anharmonicity. We point out that (1) variation of the superconducting gap inside the sigma- or the pi-bands can hardly be observed in real samples, and (2) taking the anisotropy of the Coulomb repulsion into account influences the size of the small gap, Delta_pi.

cond-mat.supr-con↗

arXiv2003

Calculation of magnetic anisotropy energy in SmCo5

SmCo5 is an important hard magnetic material, due to its large magnetic anisotropy energy (MAE). We have studied the magnetic properties of SmCo5 using density functional theory (DFT) calculations where the Sm f-bands, which are difficult to include in DFT calculations, have been treated within the LDA+U formalism. The large MAE comes mostly from the Sm f-shell anisotropy, stemming from an interplay between the crystal field and the spin-orbit coupling. We found that both are of similar strengths, unlike some other Sm compounds, leading to a partial quenching of the orbital moment (f-states cannot be described as either pure lattice harmonics or pure complex harmonics), an optimal situation for enhanced MAE. A smaller portion of the MAE can be associated with the Co-d band anisotropy, related to the peak in the density of states at the Fermi energy. Our result for the MAE of SmCo5, 21.6 meV/f.u., agrees reasonably with the experimental value of 13-16 meV/f.u., and the calculated magnetic moment (including the orbital component) of 9.4 mu_B agrees with the experimental value of 8.9 mu_B.

cond-mat.mtrl-sci↗

arXiv2002

Correlated metals and the LDA+U method

While LDA+U method is well established for strongly correlated materials with well localized orbitals, its application to weakly correlated metals is questionable. By extending the LDA Stoner approach onto LDA+U, we show that LDA+U enhances the Stoner factor, while reducing the density of states. Arguably the most important correlation effects in metals, fluctuation-induced mass renormalization and suppression of the Stoner factor, are missing from LDA+U. On the other hand, for {\it moderately} correlated metals LDA+U may be useful. With this in mind, we derive a new version of LDA+U that is consistent with the Hohenberg-Kohn theorem and can be formulated as a constrained density functional theory. We illustrate all of the above on concrete examples, including the controversial case of magnetism in FeAl.

cond-mat.str-el↗

arXiv2002

Electronic Structure, Electron-Phonon Coupling, and Multiband Effects in MgB2

We review the current situation in the theory of superconducting and transport properties of MgB2. First principle calculations of of the electronic structure and electron-phonon coupling are discussed and compared with the experiment. We also present a brief description of the multiband effects in superconductivity and transport, and how these manifest themselves in MgB2. We also mention some yet open questions.

cond-mat.supr-con↗

arXiv2002

Interpretation of the de Haas - van Alphen experiments in MgB2

Recent reports on quantum oscillations in MgB2 provide valuable information on three important aspects of this material: (1) electronic structure near the Fermi level, (2) disparity of the electron-phonon interaction between the two systems of bands (3) renormalization of spin susceptibility. However, extraction of most of this information requires highly accurate band structure calculations of the relevant quantities. In this paper we provide such calculations and use them to analyze the experimental data.

cond-mat.supr-con↗

arXiv2002

Two-gap superconductivity in MgB$_{2}$: clean or dirty?

A large number of experimental facts and theoretical arguments favor a two-gap model for superconductivity in MgB$_{2}$. 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.

cond-mat.supr-con↗

arXiv2001

Manifestation of multiband optical properties of MgB$_2$

The optical conductivity of MgB$_2$ has been determined on a dense polycrystalline sample in the spectral range 6 meV - 4.6 eV using a combination of ellipsometric and normal incidence reflectivity measurements. $σ_{1}(ω)$ features a narrow Drude peak with anomalously small plasma frequency (1.4 eV) and a very broad "dome" structure, which comprises the bulk of the low-energy spectral weight. This fact can be reconciled with the results of band structure calculations by assuming that charge carriers from the 2D $σ$-bands and the 3D $π$-bands have principally different impurity scattering rates and negligible interband scattering. This also explains a surprisingly small correlation between the defect concentration and $T_c$, expected for a two-gap superconductor. The large 3D carrier scattering rate suggests their proximity to the localization limit.

cond-mat.supr-con↗

arXiv2001

Superconductivity in compressed iron: Role of spin fluctuations

The recent discovery of superconductivity in hexagonal iron under pressure poses a question about whether it is of conventional (phonon) or unconventional (magnetic?) origin. We present first-principles calculations of the electron-phonon coupling in iron at $P\agt 15$ GPa, and argue that a conventional mechanism can explain the appearance of superconductivity, but not its rapid disappearance at $P\agt 30$ GPa. We suggest that spin fluctuations, ferro- and/or antiferromagnetic, play a crucial role in superconductivity in this case.

cond-mat.supr-con↗

arXiv2001

Competition of Spin-Fluctuations and Phonons in Superconductivity of ZrZn2

It has been long suspected that spin fluctuations in the weak itinerant ferromagnet ZrZn2 may lead to a triplet superconductivity in this material. Here we point out another possibility, a spatially inhomogeneous singlet superconducting state (a Fulde-Ferrell-Larkin-Ovchinnikov state). We report detailed electronic structure calculations, as well as calculations of the zone center phonons and their coupling with electrons. We find that the exchange splitting is nonuniform and may allow for gap formation at some parts of the Fermi surface. We also find that there is substantial coupling of Zr rattling modes with electrons, which can, in principle, provide the necessary pairing in the s-channel.

cond-mat.supr-con↗

arXiv2001

Pressure Dependence of the Raman Spectrum, Lattice Parameters and Superconducting Critical Temperature of MgB2

We have observed a strongly broadened Raman band of MgB2 that shows anomalously large pressure dependence of its frequency. This band and its pressure dependence can be interpreted as the E2g zone center phonon, which is strongly anharmonic because of coupling to electronic excitations. The pressure dependence of Tc was measured to 14 GPa in hydrostatic conditions and can be explained only when a substantial pressure dependence of the Hopfield parameter h=N(0) ~(V0/V)^2.3(6)is taken into account.

cond-mat.supr-con↗

arXiv2001

Superconductivity and Electronic Structure of Perovskite MgCNi3

The electronic structure, stability, electron phonon coupling and superconductivity of the non-oxide perovskite MgCNi$_3$ are studied using density functional calculations. The band structure is dominated by a Ni $d$ derived density of states peak just below the Fermi energy, which leads to a moderate Stoner enhancement, placing MgCNi$_3$ in the range where spin fluctuations may noticeably affect transport, specific heat and superconductivity, providing a mechanism for reconciling various measures of the coupling $λ$. Strong electron phonon interactions are found for the octahedral rotation mode and may exist for other bond angle bending modes. The Fermi surface contains nearly cancelling hole and electron sheets that give unusual behavior of transport quantities particularly the thermopower. The results are discussed in relation to the superconductivity of MgCNi$_3$.

cond-mat.supr-con↗

arXiv2001

Surface electronic structure of Sr2RuO4

We have addressed the possibility of surface ferromagnetism in Sr2RuO4 by investigating its surface electronic states by angle-resolved photoemission spectroscopy (ARPES). By cleaving samples under different conditions and using various photon energies, we have isolated the surface from the bulk states. A comparison with band structure calculations indicates that the ARPES data are most readily explained by a nonmagnetic surface reconstruction.

cond-mat.supr-con↗

arXiv2001

NMR relaxation rates and Knight shifts in MgB2

We calculate ab initio the NMR relaxation rates and the Knight shifts in MgB_2. We show that the dominant relaxation mechanism at the B nucleus is the interaction with the electronic orbital moment, and we give a simple explanation of that using a simple $sp$ tight binding model. When Stoner enhancement (also calculated ab-initio) is accounted for, we obtain good agreement with reported experimental values. For the 25Mg nucleus, we predict that the dominant relaxation mechanism is the Fermi-contact interaction, which also dominates the Mg Knight shift.

cond-mat.supr-con↗

arXiv2001

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 $λ$ in the clean limit is calculated to be significantly larger. The $E_{2g}$ phonons are strongly anharmonic, and the non-linear contribution to the coupling between the $E_{2g}$ modes and the p$_{x,y}$ bands is significant.

cond-mat.supr-con↗

arXiv2001

Structural, Electronic, and Magnetic Properties of MnO

We calculate the structural, electronic, and magnetic properties of MnO from first principles, using the full-potential linearized augmented planewave method, with both local-density and generalized-gradient approximations to exchange and correlation. We find the ground state to be of rhombohedrally distorted B1 structure with compression along the [111] direction, antiferromagnetic with type-II ordering, and insulating, consistent with experiment. We show that the distortion can be understood in terms of a Heisenberg model with distance dependent nearest-neighbor and next-nearest-neighbor couplings determined from first principles. Finally, we show that magnetic ordering can induce significant charge anisotropy, and give predictions for electric field gradients in the ground-state rhombohedrally distorted structure.

cond-mat↗

arXiv2001

Superconductivity of metallic boron in MgB_2

Boron in MgB_2 forms layers of honeycomb lattices 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 sizeable electron-phonon coupling. Using the rigid atomic sphere approximation and an analogy to Al, we estimate the coupling constant lambda to be of order 1. 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 reporting Tc=39 K (J. Akimitsu et al., to be published). Thus MgB_2 can be viewed as an analog of the long sought, but still hypothetical, superconducting metallic hydrogen.

cond-mat.supr-con↗

arXiv2000

Probing Spin Polarization with Andreev Reflection: A Theoretical Basis

Andreev reflection at the interface between a ferromagnet and a superconductor has become a foundation of a versatile new technique of measuring the spin polarization of magnetic materials. In this paper we will briefly outline a general theory of Andreev reflection for spin-polarized systems and arbitrary Fermi surface in two limiting cases of ballistic and diffusive transport

cond-mat.mtrl-sci↗

arXiv2000

Tunneling of Bloch electrons through vacuum barrier

Tunneling of Bloch electrons through a vacuum barrier introduces new physical effects in comparison with the textbook case of free (plane wave) electrons. For the latter, the exponential decay rate in the vacuum is minimal for electrons with the parallel component of momentum ${\bf k}_\parallel=0$, and the prefactor is defined by the electron momentum component in the normal to the surface direction. However, the decay rate of Bloch electrons may be minimal at an arbitrary ${\bf k}_\parallel$ (``hot spots''), and the prefactor is determined by the electron's group velocity, rather than by its quasimomentum.

cond-mat.mtrl-sci↗