arXiv subjects

Gabriel Kotliar

Publications and source records attributed to Gabriel Kotliar.

At least 109 records · Page 6Linked to original sources

arXiv2013

Tuning the charge-transfer energy in hole-doped cuprates

Chemical substitution, combined with strain, allows the charge-transfer energy in hole-doped cuprates to be broadly tuned. We theoretically characterize the structural and electronic properties of the family of compounds $R_2$CuO$_2$S$_2$, constructed by sulfur replacement of the apical oxygens and rare earth substitutions in the parent cuprate La$_2$CuO$_4$. Additionally, the enthalpies of formation for possible synthesis pathways are determined.

cond-mat.supr-con↗

arXiv2013

Kondo effect in monolayer and bilayer graphene: physical realizations of the multi-channel Kondo models

We perform a general group-theoretical study of the Kondo problem in monolayer and bilayer graphene around the charge neutrality point. Utilizing the group representation theory, we derive from symmetry considerations a family of the Kondo models for all symmetric placements with either 3- or 6-fold rotational axis of an impurity atom in an arbitrary orbital state. We find six possible classes of the partially anisotropic four-channel Kondo model. As the key result, we argue several possibilities to realize the regime of the dominant channel-symmetric two-channel Kondo effect, protected by the local symmetry and specifics of the graphene band structure. Our findings open prospects for the observation of the rich multi-channel Kondo physics in graphene and the associated non-Fermi-liquid behavior.

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arXiv2013

Hidden Fermi Liquid, Scattering Rate Saturation and Nernst Effect: a DMFT Perspective

We investigate the transport properties of a correlated metal within dynamical mean field theory. Canonical Fermi liquid behavior emerges only below a very low temperature scale $T_{FL}$. Surprisingly the quasiparticle scattering rate follows a quadratic temperature dependence up to much higher temperatures and crosses over to saturated behavior around a temperature scale $T_{sat}$. We identify these quasiparticles as constituents of the hidden Fermi liquid. The non-Fermi liquid transport above $T_{FL}$, in particular the linear-in-$T$ resistivity, is shown to be a result of a strongly temperature dependent band dispersion. We derive simple expressions for resistivity, Hall angle, thermoelectric power and Nernst coefficient in terms of a temperature dependent renormalized band structure and the quasiparticle scattering rate. We discuss possible tests of the DMFT picture of transport using ac measurements.

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arXiv2013

A magnetic origin for high temperature superconductivity in iron pnictides

In conventional Bardeen-Cooper-Schrieffer (BCS) superconductors, superconductivity occurs when electrons form coherent Cooper pairs below the superconducting transition temperature Tc. Although the kinetic energy of paired electrons increases in the superconducting state relative to the normal state, the reduction in the ion lattice energy is sufficient to give the superconducting condensation energy. For iron pnictide superconductors derived from electron or hole doping of their antiferromagnetic (AF) parent compounds, the microscopic origin for supercnductivity is unclear. Here we use neutron scattering to show that high-Tc superconductivity only occurs for iron pnictides with low-energy itinerant electron-spin excitation coupling and high energy spin excitations. Since our absolute spin susceptibility measurements for optimally hole-doped iron pnictide reveal that the change in magnetic exchange energy below and above Tc can account for the superconducting condensation energy, we conclude that the presence of both high-energy spin excitations giving rise to a large magnetic exchange coupling J and low-energy spin excitations coupled to the itinerant electrons is essential for high-Tc superconductivity in iron pnictides.

cond-mat.supr-con↗

arXiv2013

Impurity model for non-equilibrium steady states

We propose an out-of-equilibrium impurity model for the dynamical mean-field description of the Hubbard model driven by a finite electric field. The out-of-equilibrium impurity environment is represented by a collection of equilibrium reservoirs at different chemical potentials. We discuss the validity of the impurity model and propose a non-perturbative method, based on a quantum Monte Carlo solver, which provides the steady-state solutions of the impurity and original lattice problems. We discuss the relevance of this approach to other non-equilibrium steady-state contexts.

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arXiv2013

How bad metals turn good: spectroscopic signatures of resilient quasiparticles

We investigate transport in strongly correlated metals. Within dynamical mean-field theory, we calculate the resistivity, thermopower, optical conductivity and thermodynamic properties of a hole-doped Mott insulator. Two well-separated temperature scales are identified: T_FL below which Landau Fermi liquid behavior applies, and T_MIR above which the resistivity exceeds the Mott-Ioffe-Regel value and `bad-metal' behavior is found. We show that quasiparticle excitations remain well-defined above T_FL and dominate transport throughout the intermediate regime T_FL < T_MIR. The lifetime of these `resilient quasiparticles' is longer for electron-like excitations, and this pronounced particle-hole asymmetry has important consequences for the thermopower. The crossover into the bad-metal regime corresponds to the disappearance of these excitations, and has clear signatures in optical spectroscopy.

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arXiv2013

Importance of many body effects in the kernel of hemoglobin for ligand binding

We propose a mechanism for binding of diatomic ligands to heme based on a dynamical orbital selection process. This scenario may be described as bonding determined by local valence fluctuations. We support this model using linear-scaling first-principles calculations, in combination with dynamical mean-field theory, applied to heme, the kernel of the hemoglobin metalloprotein central to human respiration. We find that variations in Hund's exchange coupling induce a reduction of the iron 3d density, with a concomitant increase of valence fluctuations. We discuss the comparison between our computed optical absorption spectra and experimental data, our picture accounting for the observation of optical transitions in the infrared regime, and how the Hund's coupling reduces, by a factor of five, the strong imbalance in the binding energies of heme with CO and O_2 ligands.

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arXiv2012

Scaling of the transition temperature of hole-doped cuprate superconductors with the charge-transfer energy

We use first-principles calculations to extract two essential microscopic parameters, the charge-transfer energy and the inter-cell oxygen-oxygen hopping, which correlate with the maximum superconducting transition temperature $\Tcmax$ across the cuprates. We explore the superconducting state in the three-band model of the copper-oxygen planes using cluster Dynamical Mean-Field Theory. We find that the variation in the charge-transfer energy largely accounts for the empirical trend in $\Tcmax$, resolving a long-standing contradiction with theoretical calculations.

cond-mat.str-el↗

arXiv2012

Vanadium dioxide : A Peierls-Mott insulator stable against disorder

Vanadium dioxide undergoes a first order metal-insulator transition at 340 K. In this work, we develop and carry out state of the art linear scaling DFT calculations refined with non-local dynamical mean-field theory. We identify a complex mechanism, a Peierls-assisted orbital selection Mott instability, which is responsible for the insulating M$_1$ phase, and furthermore survives a moderate degree of disorder.

cond-mat.str-el↗

arXiv2012

Orbital selective Fermi surface shifts and mechanism of high T$_c$ superconductivity in correlated AFeAs (A=Li,Na)

Based on the dynamical mean field theory (DMFT) and angle resolved photoemission spectroscopy (ARPES), we have investigated the mechanism of high $T_c$ superconductivity in stoichiometric LiFeAs. The calculated spectrum is in excellent agreement with the observed ARPES measurement. The Fermi surface (FS) nesting, which is predicted in the conventional density functional theory method, is suppressed due to the orbital-dependent correlation effect with the DMFT method. We have shown that such marginal breakdown of the FS nesting is an essential condition to the spin-fluctuation mediated superconductivity, while the good FS nesting in NaFeAs induces a spin density wave ground state. Our results indicate that fully charge self-consistent description of the correlation effect is crucial in the description of the FS nesting-driven instabilities.

cond-mat.supr-con↗

arXiv2012

Dimensional Crossover Driven by an Electric Field

We study the steady-state dynamics of the Hubbard model driven out-of-equilibrium by a constant electric field and coupled to a dissipative heat bath. For very strong field, we find a dimensional reduction: the system behaves as an equilibrium Hubbard model in lower dimensions. We derive steady-state equations for the dynamical mean field theory in the presence of dissipation. We discuss how the electric field induced dimensional crossover affects the momentum resolved and integrated spectral functions, the energy distribution function, as well as the steady current in the non-linear regime.

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arXiv2012

Nature of magnetic excitations in superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$

Since the discovery of the metallic antiferromagnetic (AF) ground state near superconductivity in iron-pnictide superconductors, a central question has been whether magnetism in these materials arises from weakly correlated electrons, as in the case of spin-density-wave in pure chromium, requires strong electron correlations, or can even be described in terms of localized electrons such as the AF insulating state of copper oxides. Here we use inelastic neutron scattering to determine the absolute intensity of the magnetic excitations throughout the Brillouin zone in electron-doped superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ ($T_c=20$ K), which allows us to obtain the size of the fluctuating magnetic moment $ $, and its energy distribution. We find that superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ and AF BaFe$_2$As$_2$ both have fluctuating magnetic moments $ \approx3.2\ μ_B^2$ per Fe(Ni), which are similar to those found in the AF insulating copper oxides. The common theme in both classes of high temperature superconductors is that magnetic excitations have partly localized character, thus showing the importance of strong correlations for high temperature superconductivity.

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arXiv2011

Hexadecapolar Kondo effect in URu$_2$Si$_2$?

We derive the coupling of a localized hexadecapolar mode to conduction electrons in tetragonal symmetry. The derivation can be easily adapted to arbitrary multipoles in arbitrary environment. We relate our model to the two-channel Kondo (2CK) model and show that for an $f^2$-configuration, a relevant crystal field splitting in addition to the 2CK interaction is intrinsic to tetragonal symmetry. We discuss possible realizations of a hexadecapolar Kondo effect in URu$_2$Si$_2$. Solving our model we find good agreement with susceptibility and specific heat measurements in Th$_{1-x}$U$_x$Ru$_2$Si$_2$ ($x\ll1$).

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arXiv2011

The magnetic excitation spectra in BaFe$_{2}$As$_{2}$: a two-particle approach within DFT+DMFT

We study the magnetic excitation spectra in the paramagnetic state of BaFe$_{2}$As$_{2}$ from the \textit{ab initio} perspective. The one-particle excitation spectrum is determined within the combination of the density functional theory and the dynamical mean-field theory method. The two-particle response function is extracted from the local two-particle vertex function, also computed by the dynamical mean field theory, and the polarization function. This method reproduces all the experimentally observed features in inelastic neutron scattering (INS), and relates them to both the one particle excitations and the collective modes. The magnetic excitation dispersion is well accounted for by our theoretical calculation in the paramagnetic state without any broken symmetry, hence nematic order is not needed to explain the INS experimental data.

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arXiv2011

High frequency thermoelectric response in correlated electronic systems

We derive a general formalism for evaluating the high-frequency limit of the thermoelectric power of strongly correlated materials, which can be straightforwardly implemented in available first principles LDA+DMFT programs. We explore this formalism using model Hamiltonians and we investigate the validity of approximating the static thermoelectric power $S_0$, by its high-temperature limit, $S^*$. We point out that the behaviors of $S^*$ and $S_0$ are qualitatively different for a correlated Fermi liquid near the Mott transition, when the temperature is in the coherent regime. When the temperature is well above the coherent regime, e.g., when the transport is dominated by incoherent excitations, $S^*$ provides a good estimation of $S_0$.

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arXiv2011

The coherence-incoherence crossover and the mass-renormalization puzzles in Sr2RuO4

We calculate the electronic structure of Sr2RuO4, treating correlations in the framework of dynamical mean-field theory. The approach successfully reproduces several experimental results and explains the key properties of this material: the anisotropic mass renormalization of quasiparticles and the crossover into an incoherent regime at a low temperature. While the orbital differentiation originates from the proximity of the van Hove singularity, strong correlations are caused by the Hund's coupling. The generality of this mechanism for other correlated materials is pointed out.

cond-mat.str-el↗

arXiv2011

Electronic Correlation and Transport Properties of Nuclear Fuel Materials

Actinide elements, such as uranium and plutonium, and their compounds are best known as nuclear materials. When engineering optimal fuel materials for nuclear power, important thermophysical properties to be considered are melting point and thermal conductivity. Understanding the physics underlying transport phenomena due to electrons and lattice vibrations in actinide systems is a crucial step toward the design of better fuels. Using first principle LDA+DMFT method, we conduct a systematic study on the correlated electronic structures and transport properties of select actinide carbides, nitrides, and oxides, many of which are nuclear fuel materials. We find that different mechanisms, electrons--electron and electron--phonon interactions, are responsible for the transport in the uranium nitride and carbide, the best two fuel materials due to their excellent thermophysical properties. Our findings allow us to make predictions on how to improve their thermal conductivities.

cond-mat.str-el↗

arXiv2011

Neutron magnetic form factor in strongly correlated materials

We introduce a formalism to compute the neutron magnetic form factor Fm(q) within a first-principles Density Functional Theory (DFT) + Dynamical Mean Field Theory (DMFT). The approach treats spin and orbital interactions on the same footing and reduces to earlier methods in the fully localized or the fully itinerant limit. We test the method on various actinides of current interest NpCoGa5, PuSb and PuCoGa5; we show that PuCoGa5 is in mixed valent state, which naturally explains the measured magnetic form factor.

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