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I. I. Mazin

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

At least 127 records · Page 7Linked to original sources
arXiv2008

NMR relaxation rate in superconducting pnictides: extended $s_{\pm}$ scenario

Recently, several measurements of the nuclear spin lattice relaxation rate $T_{1}^{-1}$ in the newly discovered superconducting Fe-pnictides have been reported. These measurements generally show no coherence peak below $T_{c}$ and indicate a low temperature power law behavior, the characteristics commonly taken as evidence of unconventional superconductivity with lines of nodes crossing the Fermi surface. In this work we show that (i) the lack of a coherence peak is fully consistent with the previously proposed nodeless extended $s_{\pm}$-wave symmetry of the order parameter (whether in the clean or dirty limit) and (ii) the low temperature power law behavior can be also explained in the framework of the same model, but requires going beyond the Born model.

cond-mat.supr-con↗
arXiv2008

The electronic structure of the Na$_x$CoO$_2$ surface

The idea that surface effects may play an important role in suppressing $e_g'$ Fermi surface pockets on Na$_x$CoO$_2$ $(0.333 \le x \le 0.75)$ has been frequently proposed to explain the discrepancy between LDA calculations (performed on the bulk compound) which find $e_g$' hole pockets present and ARPES experiments, which do not observe the hole pockets. Since ARPES is a surface sensitive technique it is important to investigate the effects that surface formation will have on the electronic structure of Na$_{1/3}$CoO$_2$ in order to more accurately compare theory and experiment. We have calculated the band structure and Fermi surface of cleaved Na$_{1/3}$CoO$_2$ and determined that the surface non-trivially affects the fermiology in comparison to the bulk. Additionally, we examine the likelihood of possible hydroxyl cotamination and surface termination. Our results show that a combination of surface formation and contamination effects could resolve the ongoing controversy between ARPES experiments and theory.

cond-mat.mtrl-sci↗
arXiv2008

Unconventional sign-reversing superconductivity in LaFeAsO_(1-x)F_x

We argue that the newly discovered superconductivity in a nearly magnetic, Fe-based layered compound is unconventional and mediated by antiferromagnetic spin fluctuations, though different from the usual superexchange and specific to this compound. This resulting state is an example of extended s-wave pairing with a sign reversal of the order parameter between different Fermi surface sheets. The main role of doping in this scenario is to lower the density of states and suppress the pair-breaking ferromagnetic fluctuations.

cond-mat.supr-con↗
arXiv2008

A key role for unusual spin dynamics in ferropnictides

The 2008 discovery of superconducting ferropnictides with Tc~26K-56K introduced a new family of materials into the category of high Tc superconductors. The ongoing project of understanding the superconducting mechanism and pairing symmetry has already revealed a complicated and often contradictory underlying picture of the structural and magnetic properties. There is an almost unprecedented sensitivity of the calculated magnetism and Fermi surface to structural details that prohibits correspondence with experiment. Furthermore, experimental probes of the order parameter symmetry are in surprisingly strong disagreement, even considering the relative immaturity of the field. Here we outline all of the various and seemingly contradictory evidences, both theoretical and experimental, and show that they can be rectified if the system is assumed to be highly magnetic with a spin density wave that is well-defined but with magnetic twin and anti-phase boundaries that are dynamic on the time-scale of experiments. Under this assumption, we find that our calculations can accurately reproduce even very fine details of the structure, and a natural explanation for the temperature separation of structural and magnetic transitions is provided. Thus, our theory restores agreement between experiment and theory in crucial areas, making further cooperative progress possible on both fronts. We believe that fluctuating magnetic domains will be an essential component of unravelling the interplay between magnetic interactions and superconductivity in these newest high Tc superconductors.

cond-mat.supr-con↗
arXiv2008

The challenge of unravelling magnetic properties in LaFeAsO

First principles calculations of magnetic and, to a lesser extent, electronic properties of the novel LaFeAsO-based superconductors show substantial apparent controversy, as opposed to most weakly or strongly correlated materials. Not only do different reports disagree about quantitative values, there is also a schism in terms of interpreting the basic physics of the magnetic interactions in this system. In this paper, we present a systematic analysis using four different first principles methods and show that while there is an unusual sensitivity to computational details, well-converged full-potential all-electron results are fully consistent among themselves. What makes results so sensitive and the system so different from simple local magnetic moments interacting via basic superexchange mechanisms is the itinerant character of the calculated magnetic ground state, where very soft magnetic moments and long-range interactions are characterized by a particular structure in the reciprocal (as opposed to real) space. Therefore, unravelling the magnetic interactions in their full richness remains a challenging, but utterly important task.

cond-mat.mtrl-sci↗
arXiv2008

The origin of a$_{1g}$ and e$_g$' orderings in Na$_x$CoO$_2$

It has often been suggested that correlation effects suppress the small e_g' Fermi surface pockets of NaxCoO_2 that are predicted by LDA, but absent in ARPES measurements. It appears that within the dynamical mean field theory (DMFT) the ARPES can be reproduced only if the on-site energy of the eg' complex is lower than that of the a1g complex at the one-electron level, prior to the addition of local correlation effects. Current estimates regarding the order of the two orbital complexes range from -200 meV to 315 meV in therms of the energy difference. In this work, we perform density functional theory calculations of this one-electron splitting Δ= ε_a1g-ε_e_g' for the full two-layer compound, Na2xCo2O4, accounting for the effects of Na ordering, interplanar interactions and octahedral distortion. We find that εa_1g-εe_g' is negative for all Na fillings and that this is primarily due to the strongly positive Coulomb field created by Na+ ions in the intercalant plane. This field disproportionately affects the a_1g orbital which protrudes farther upward from the Co plane than the e_g' orbitals. We discuss also the secondary effects of octahedral compression and multi-orbital filling on the value of Δas a function of Na content. Our results indicate that if the e_g' pockets are indeed suppressed that can only be due to nonlocal correlation effects beyond the standard DMFT.

cond-mat.supr-con↗
arXiv2008

Superconductivity in Ca-intercalated bilayer graphene

Recent observation of proximity effect \cite{Morpurgo:2007} has ignited interest in superconductivity in graphene and its derivatives. We consider Ca-intercalated graphene bilayer and argue that it is a superconductor, and likely with a sizeable $T_{c}$. We find substantial and suggestive similarities between Ca-intercalated bilayer (C$_{6}$CaC$_{6}$), and CaC$_{6} $, an established superconductor with $T_{c}$ = 11.5 K. In particular, the nearly free electron band, proven to be instrumental for superconductivity in intercalated graphites, does cross the chemical potential in (C$_{6}$CaC$% _{6}$), despite the twice smaller doping level, satisfying the so-called \textquotedblleft Cambridge criterion\textquotedblright . Calculated properties of zone-center phonons are very similar to those of CaC$%_{6}.$ This suggests that the critical temperature would probably be on the same scale as in CaC$_{6}$.

cond-mat.supr-con↗
arXiv2008

Fermi surface nesting and the origin of Charge Density Waves in metals

The concept of a CDW induced by Fermi-surface nesting originated from the Peierls idea of electronic instabilities in purely 1D metals and is now often applied to charge ordering in real low-dimensional materials. The idea is that if Fermi surface contours coincide when shifted along the observed CDW wave vector, then the CDW is considered to be nesting-derived. We show that in most cases this procedure has no predictive power, since Fermi surfaces either do not nest at the right wave vector, or nest more strongly at the wrong vector. We argue that only a tiny fraction, if any, of the observed charge ordering phase transitions are true analogues of the Peierls instability because electronic instabilities are easily destroyed by even small deviations from perfect nesting conditions. Using prototypical CDW materials NbSe$_2$, TaSe$_2$, and CeTe$_3$, we show that such conditions are hardly ever fulfilled, and that the CDW phases are actually structural phase transitions, driven by the concerted action of electronic and ionic subsystems, \textit{i.e.,} \textbf{q}-dependent electron-phonon coupling plays an indispensable part. We also show mathematically that the original Peierls construction is so fragile as to be unlikely to apply to real materials. We argue that no meaningful distinction between a CDW and an incommensurate lattice transition exists.

cond-mat.mtrl-sci↗
arXiv2007

Electron-phonon interaction and charge carrier mass enhancement in SrTiO3

We report a comprehensive THz, infrared and optical study of Nb doped SrTiO3 as well as DC conductivity and Hall effect measurements. Our THz spectra at 7K show the presence of a very narrow (<2 meV) Drude peak, the spectral weight of which shows approximately a factor of three enhancement of the band mass for all carrier concentrations. The missing spectral weight is regained in a broad 'mid-infrared' band which originates from electron-phonon coupling. We find no evidence of a particularly large electron-phonon coupling that would result in small polaron formation. Analysis of the results yields an electron-phonon coupling parameter of an intermediate strength, $α\approx 4$.

cond-mat.str-el↗
arXiv2007

Self-consistent theory of phonon renormalization and electron-phonon coupling near a 2D Kohn singularity

We show that the usual expression for evaluating electron-phonon coupling and the phonon linewidth in 2D metals with a cylindrical Fermi surface cannot be applied near the wave vector corresponding to the Kohn singularity. Instead, the Dyson equation for phonons has to be solved self-consistently. If a self-consistent procedure is properly followed, there is no divergency in either the coupling constant or the phonon linewidth near the offending wave vectors, in contrast to the standard expression.

cond-mat.supr-con↗
arXiv2007

Orbital degeneracy removed by charge order in triangular antiferromagnet AgNiO2

We report a high-resolution neutron diffraction study on the orbitally-degenerate spin-1/2 hexagonal antiferromagnet AgNiO2. A structural transition to a tripled unit cell with expanded and contracted NiO6 octahedra indicates root(3) x root(3) charge order on the Ni triangular lattice. This suggests charge order as a possible mechanism of lifting the orbital degeneracy in the presence of charge fluctuations, as an alternative to Jahn-Teller distortions. A novel magnetic ground state is observed at base temperatures with the electron-rich S = 1 Ni sites arranged in alternating ferromagnetic rows on a triangular lattice, surrounded by a honeycomb network of non-magnetic and metallic Ni ions. We also report first-principles band-structure calculations that explain microscopically the origin of these phenomena.

cond-mat.str-el↗
arXiv2007

Absence of magnetic ordering in NiGa_2S_4

Triangular-layered NiGa2S4, contrary to intuitive expectation, does not form a noncollinear antiferromagnetic structure, as do isoelectronic NaCrO2 and LiCrO2. Instead, the local magnetic moments remain disordered down to the lowest measured temperature. To get more insight into this phenomenon, we have performed first principles calculations of the first, second end third neighbors exchange interactions, and found that the second neighbor exchange is negligible, while the first and the third neighbor exchanges are comparable and antiferromagnetic. Both are rapidly suppressed by the on-site Hubbard repulsion.

cond-mat.str-el↗
arXiv2007

First-principles calculations of spin-orbit effects and NMR in Sr2RuO4

We present a first principles study of NMR and spin orbit effects in the unconventional superconductor Sr2RuO4. We have calculated the uniform magnetic susceptibility, which agrees rather well with the experiment in amplitude, but, as in an earlier model result we found the calculated hard axis to be z, opposite to the experiment. We have also calculated the Knight shifts and the NMR relaxation rates for all atoms, and again found an overall good agreement, but important deviations from the experiment in same particular characteristic, such as the Knight shift anisotropy. Our results suggest that correlations in Sr2RuO4 lead to underestimations of the orbital effects in density-functional based calculations. We also argue that the accepted ``experimental'' value for the relative contribution of orbital polarization in susceptibility, 10-15%, is also an underestimation. We discuss the puzzling invariance of the the O and Ru Knight shift across the superconducting transition for all directions of the applied field. We show that this fact cannot be explained by accidental cancellations or spin-flip scattering, as it happens in some elemental superconductors. We also point out that large contribution of the dipole and orbital hyperfine field into the Knight shifts in Sr2RuO4, combined with the possibility of an orbital-dependent superconductivity, calls for a revision of the standard theory of the Knight shift in the superconducting state.

cond-mat.supr-con↗
arXiv2007

Electronic structure and magnetism in the frustrated antiferromagnet LiCrO2

LiCrO2 is a 2D triangular antiferromagnet, isostructural with the common battery material LiCoO2 and a well-known Jahn-Teller antiferromagnet NaNiO2. As opposed to the latter, LiCrO2 exibits antiferromagnetic exchange in Cr planes, which has been ascribed to direct Cr-Cr d-d overlap. Using LDA and LDA+U first principles calculations I confirm this conjecture and show that (a) direct d-d overlap is indeed enhanced compared to isostructural Ni and Cr compounds, (b) p-d charge transfer gap is also enhanced, thus suppressing the ferromagnetic superexchange, (c) the calculated magnetic Hamiltonian maps well onto the nearest neighbors Heisenberg exchange model and (d) interplanar inteaction is antiferromagnetic.

cond-mat.mtrl-sci↗
arXiv2007

Combining the advantages of superconducting MgB2 and CaC6 in one material: suggestions from first-principles calculations

We show that a recently predicted layered phase of lithium monoboride, Li2B2, combines the key mechanism for strong electron-phonon coupling in MgB2 (i.e., interaction of covalent B sigma bands with B bond-stretching modes) with the dominant coupling mechanism in CaC6 (i.e., interaction of free-electron-like interlayer states with soft intercalant modes). Yet, surprisingly, the electron-phonon coupling in Li2B2 is calculated to be weaker than in either MgB2 or CaC6. We demonstrate that this is due to the accidental absence of B pi states at the Fermi level in Li2B2. In MgB2, the pi electrons play an indirect but important role in strengthening the coupling of sigma electrons. Doping Li2B2 to restore pi electrons at the Fermi level is expected to lead to a new superconductor that could surpass MgB2 in Tc.

cond-mat.supr-con↗
arXiv2006

Comment on " Low-Lying Quasiparticle States and Hidden Collective Charge Instabilities in Parent Cobaltate Superconductors"

Qian et al [1] recently reported angular-resolved photoemission spectroscopy (ARPES) measurements for Na0.8CoO2 that show two concentric Fermi surfaces (FS) split by a delta k_F that varies by a factor of three around the Brillouin zone (BZ). The surfaces occupy 70+/-5% of the full 2D BZ and were interpreted as the bonding and antibonding splitting (BAS) of the a1g bands, with an unspecified effect of magnetic ordering. Below we show that this interpretation is not possible, and, in fact, no valid intepretation of the observed spectra in terms of the bulk electronic structure of Na0.8CoO2 can be found.

cond-mat.mtrl-sci↗
arXiv2006

Formation of an unconventional Ag valence state in Ag2NiO2

The Ag ion in the recently synthesized novel material Ag2NiO2 adopts an extremely unusual valency of 1/2, leaving the Ni ion as 3+, rather than the expected 2+. Using first principles calculations, we show that this mysterious subvalent state emerges due to a strong bonding-antibonding interaction between the two Ag layers which drives the lower band beneath the O p complex, eliminating the possibility of a conventional Ag 1+ valence state. The strong renormalization of the specific heat coefficient, gamma, is likely due to strong spin fluctuations that stem from nearly complete compensation of the ferro- (metallic double exchange and the 90 degree superexchange) and antiferromagnetic (conventional superexchange via Ni-O-Ag-O-Ni path) interactions.

cond-mat.mtrl-sci↗