arXiv subjects

I. I. Mazin

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

At least 145 records · Page 8Linked to original sources

arXiv2006

Unresolved problems in superconductivity of CaC6

We discuss the current status of the theory of the "high-temperature" superconductivity in intercalated graphites YbC6 and CaC6. We emphasize that while the general picture of conventional, phonon-driven superconductivity has already emerged and is generally accepted, there are still interesting problems with this picture, such as weak-coupling regime inferred from specific heat suggesting coupling exclusively with high-energy carbon phonons coming in direct contradiction with the isotope effect measurements suggesting coupling exclusively with the low-energy intercalant modes. At the same time, the first principle calculations, while explaining Tc, contradict both of the experiments above by predicting equal coupling with both groups of phonons.

cond-mat.supr-con↗

arXiv2006

Unconventional superconducting pairing symmetry induced by phonons

The possibility of non-s-wave superconductivity induced by phonons is investigated using a simple model that is inspired by Sr$_2$RuO$_4$. The model assumes a two-dimensional electronic structure, a two-dimensional spin-fluctuation spectrum, and three-dimensional electron-phonon coupling. Taken separately, each interaction favors formation of spin-singlet pairs (of s symmetry for the phonon interaction and d$_{x^2-y^2}$ symmetry for the spin interaction), but in combination, a variety of more unusual singlet and triplet states are found, depending on the interaction parameters. This may have important implications for Sr$_2$RuO$_4$, providing a plausible explanation of how the observed spin fluctuations, which clearly favor d$_{x^2-y^2}$ pairing, may still be instrumental in creating a superconducting state with a different (e.g., p-wave) symmetry. It also suggests an interpretation of the large isotope effect observed in Sr$_2$RuO$_4$. These results indicate that phonons could play a key role in establishing the order-parameter symmetry in Sr$_2$RuO$_4$, and possibly in other unconventional superconductors.

cond-mat.supr-con↗

arXiv2006

Fermi Surface Nesting and the Origin of the Charge Density Wave in NbSe$_2$

We use highly accurate density functional calculations to study the band structure and Fermi surfaces of NbSe2. We calculate the real part of the non-interacting susceptibility, Re chi_0(q), which is the relevant quantity for a charge density wave (CDW) instability and the imaginary part, Im chi_0(q), which directly shows Fermi surface (FS) nesting. We show that there are very weak peaks in Re chi_0(q) near the CDW wave vector, but that no such peaks are visible in Im chi_0(q), definitively eliminating FS nesting as a factor in CDW formation. Because the peak in Re chi_0(q) is broad and shallow, it is unlikely to be the direct cause of the CDW instability. We briefly address the possibility that electron-electron interactions (local field effects) produce additional structure in the total (renormalized) susceptibility, and we discuss the role of electron-ion matrix elements.

cond-mat.mtrl-sci↗

arXiv2005

CeMnNi4: an impostor half-metal

Recent experiments show CeMnNi$_{4}$ 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 thermolectric figure of merit, $ZT,$ are predicted in the ferromagnetic state of the well ordered stoichiometric CeMnNi$_{4}$

cond-mat.mtrl-sci↗

arXiv2005

A critical assessment of the pairing symmetry in NaxCoO2.yH2O

We examine each of the symmetry-allowed pairing states of NaxCoO2.yH2O and compare their properties to what is experimentally and theoretically established about the compound. In this way, we can eliminate the vast majority of states that are technically allowed and narrow the field to two, both of f-wave type states. We discuss the expected features of these states and suggest experiments that can distinguish between them. We also discuss odd-frequency gap pairing and how it relates to available experimental evidence.

cond-mat.supr-con↗

arXiv2005

Critical temperature and giant isotope effect in presence of paramagnons

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 had been rather well understood, the existing approximate formulas fail to describe the correct behavior of $% T_{c}$ for general phonon and paramagnon spectra. Using a controllable approximation, we derive an analytical formula for $T_{c}$ 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 MgCNi$_{3}$

cond-mat.supr-con↗

arXiv2005

Intercalant-Driven Superconductivity in YbC$_{6}$ and CaC$_{6}$

Recently deiscovered superconductivity in YbC$_6$ and CaC$_6$ at temperatures substantially higher than previously known for intercalated graphites, raised several new questions: (1) Is the mechanism considerably different from the previously known intercalated graphites? (2) If superconductivity is conventional, what are the relevant phonons? (3) Given extreme similarity between YbC$_6$ and CaCa$_6$, why their critical temperatures are 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 YbC$_6$ and CaC$_6$, 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.

cond-mat.supr-con↗

arXiv2005

Electrons and Phonons in YbC6

The electronic structure and selected zone-center phonons in Yb graphite intercalation compound (YbC6) are investigated {theoretically} using density functional calculations and LDA+U approximation for Coulomb correlations in the f shell, and experimentally using angle-resolved photoemission. We find that both in LDA and LDA+U approach the Yb f states are fully occupied providing no evidence for mixed-valent behavior. The obtained theoretical results are in good agreement with photoemission experiments. The 4f states are considerably hybridized both with the Yb 5d and C 2p states resulting in a substantial admixture of Yb f at the Fermi level. Soft Yb phonons, given a noticeable presence of the Yb states at the Fermi level, are probably responsible for the superconductivity recently reported in YbC6.

cond-mat.supr-con↗

arXiv2004

Why the magnetic interactions in Na$_{x}$CoO$_{2}$ are 3D

The puzzle of 3D magnetic interactions in the structurally 2D layered oxide Na$_{x}$CoO$_2$ is addressed using first principles calculations and analysis of the exchange mechanisms. The calculations agree with recent neutron results, favoring AFM stacking of FM planes. Superexchange via direct O-O hopping and through intermediate Na $sp^2$ hybrids couples each Co to its nearest and six \textit{next}-nearest interplanar neighbors equally. The individual exchange constants are rather 2D, like the lattice itself, but due to multiple c-axis exchange paths, the magnetism becomes effectively 3D.

cond-mat.str-el↗

arXiv2004

Optical Properties and Correlation Effects in NaxCoO2

We have calculated the optical spectra of Na$_{x}$CoO$_2$ for $x$=0.3, 0.5, and 0.7 within the LDA. We compare our results to available experimental data and show that the important features and trends are reproduced well, but there is a nearly uniform shift of peak positions and poor agreement in intensities. We show, through application of a simple model, that these differences can be attributed to overhybridization between Co and O orbitals and spin fluctuations which renormalize the bandwidth. Applying the LDA+U procedure shifts the optical peaks further from their experimental locations, indicating that this method of incorporating correlation effects is ill-suited for the case NaxCoO2.

cond-mat.supr-con↗

arXiv2004

Electronic Structure and Electron-Phonon Coupling in the 18K Superconductor Y2C3

The electronic structure and electron-phonon coupling in Y2C3 is investigated using density functional calculations. We find that the Fermi level falls in a manifold of mixed character derived from Y d states and antibonding states associated with the C dimers in the structure. Calculations of the electron-phonon coupling for Y and C modes show that the former provide most of the coupling. Modes associated with C-C bond stretching have large matrix elements, but make small contributions to the coupling because of their high phonon frequencies. Substantial electron doping of the C-C antibonding states would yield a large increase in the coupling and critical temperature, perhaps to values comparable to MgB2. However, it seems unlikely that a modification of Y2C3 with much higher filling of the C-C antibonding states can be stabilized.

cond-mat.supr-con↗

arXiv2004

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

We have calculated the one-electron susceptibility of hydrated NaxCoO2 and find strong nesting nearly commensurate with a 2X2 superstructure. The nesting involves about 70% of all electrons at the Fermi level and is robust with respect to doping. This nesting creates a tendency to a charge density wave compatible with the charge order often seen at x approx 0.5, which is usually ascribed to electrostatic repulsion of Na ions. In the spin channel, it gives rise to strong spin-fluctuations, which should be important for superconductivity. The superconducting state most compatible with this nesting structure is an odd-gap triplet s-wave state.

cond-mat.supr-con↗

arXiv2004

Density Functional Calculations near Ferromagnetic Quantum Critical Points

We discuss the application of the density functional theory in the local density approximation (LDA) near a ferromagnetic quantum critical point. The LDA fails to describe the critical fluctuations in this regime. This provides a fingerprint of a materials near ferromagnetic quantum critical points: overestimation of the tendency to magnetism in the local density approximation. This is in contrast to the typical, but not universal, tendency of the LDA to underestimate the tendency to magnetism in strongly Hubbard correlated materials. We propose a method for correcting the local density calculations by including critical spin fluctuations. This is based on (1) Landau expansion for the free energy, evaluated within the LDA, (2) lowest order expansion of the RPA susceptibility in LDA and (3) extraction of the amplitude of the relevant (critical) fluctuations by applying the fluctuation-dissipation theorem to the difference between a quantum-critical system and a reference system removed from the quantum critical point. We illustrate some of the aspects of this by the cases of Ni3Al and Ni3Ga, which are very similar metals on opposite sides of a ferromagnetic quantum critical point. LDA calculations predict that Ni3Ga is the more magnetic system, but we find that due to differences in the band structure, fluctuation effects are larger in Ni3Ga, explaining the fact that experimentally it is the less magnetic of the two materials.

cond-mat.str-el↗

arXiv2003

Magnetism, Critical Fluctuations and Susceptibility Renormalization in Pd

Some of the most popular ways to treat quantum critical materials, that is, materials close to a magnetic instability, are based on the Landau functional. The central quantity of such approaches is the average magnitude of spin fluctuations, which is very difficult to measure experimentally or compute directly from the first principles. We calculate the parameters of the Landau functional for Pd and use these to connect the critical fluctuations beyond the local-density approximation and the band structure.

cond-mat.mtrl-sci↗

arXiv2003

Why Ni$_3$Al is an itinerant ferromagnet but Ni$_3$Ga is not

Ni$_3$Al and Ni$_3$Ga 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 Ni$_3$Ga. So, according to the Stoner theory, it should be more magnetic, and, in LDA calculations, it is. However, experimentally, it is a paramagnet, while Ni$_3$Al is an itinerant ferromagnet. We show that the critical spin fluctuations are stronger than in Ni$_3$Ga, due to a weaker q-dependence of the susceptibility, and this effect is strong enough to reverse the trend. The approach combines LDA calculations with the Landau theory and the fluctuation-dissipation theorem using the same momentum cut-off for both materials. The calculations provide evidence for strong, beyond LDA, spin fluctuations associated with the critical point in both materials, but stronger in Ni$_3$Ga than in Ni$_3$Al.

cond-mat.mtrl-sci↗

arXiv2003

Spin Fluctuations and the Magnetic Phase Diagram of ZrZn2

The magnetic properties of the weak itinerant ferromagnet ZrZn_2 are analyzed using Landau theory based on a comparison of density functional calculations and experimental data as a function of field and pressure. We find that the magnetic properties are strongly affected by the nearby quantum critical point, even at zero pressure; LDA calculations neglecting quantum critical spin fluctuations overestimate the magnetization by a factor of approximately three. Using renormalized Landau theory, we extract pressure dependence of the fluctuation amplitude. It appears that a simple scaling based on the fluctuation-dissipation theorem provides a good description of this pressure dependence.

cond-mat.mtrl-sci↗

arXiv2003

Toward one-band superconductivity in MgB2

The two-gap model for superconductivity in MgB2 predicts that interband impurity scattering should be pair breaking, reducing the critical temperature. This is perhaps the only prediction of the model that has not been confirmed experimentally. It was previously shown theoretically that common substitutional impurities lead to negligible interband scattering - if the lattice is assumed not to distort. Here we report theoretical results showing that certain impurities can indeed produce lattice distortions sufficiently large to create measurable interband scattering. On this basis, we predict that isoelectronic codoping with Al and Na will provide a decisive test of the two-gap model.

cond-mat.supr-con↗

arXiv2003

Point Contact Spin Spectroscopy of Ferromagnetic MnAs Epitaxial Films

We use point contact Andreev reflection spin spectroscopy to measure the transport spin polarization of MnAs epitaxial films grown on (001) GaAs. By analyzing both the temperature dependence of the contact resistance and the phonon spectra of lead acquired simultaneously with the spin polarization measurements, we demonstrate that all the point contacts are in the ballistic limit. A ballistic transport spin polarization of approximately 49% and 44% is obtained for the type A and type B orientations of MnAs, respectively. These measurements are consistent with our density functional calculations, and with recent observations of a large tunnel magnetoresistance in MnAs/AlAs/(Ga,Mn)As tunnel junctions.

cond-mat.str-el↗