arXiv subjects

J. K. Freericks

Publications and source records attributed to J. K. Freericks.

At least 181 records · Page 10Linked to original sources

arXiv2002

Optimizing thermal transport in the Falicov-Kimball model: binary-alloy picture

We analyze the thermal transport properties of the Falicov-Kimball model concentrating on locating regions of parameter space where the thermoelectric figure-of-merit ZT is large. We focus on high temperature for power generation applications and low temperature for cooling applications. We constrain the static particles (ions) to have a fixed concentration, and vary the conduction electron concentration as in the binary-alloy picture of the Falicov-Kimball model. We find a large region of parameter space with ZT>1 at high temperature and we find a small region of parameter space with ZT>1 at low temperature for correlated systems, but we believe inclusion of the lattice thermal conductivity will greatly reduce the low-temperature figure-of-merit.

cond-mat.str-el↗

arXiv2002

Inelastic X-ray scattering in correlated (Mott) insulators

We calculate the inelastic light scattering from X-rays, which allows the photon to transfer both energy and momentum to the strongly correlated charge excitations. We find that the charge transfer peak and the low energy peak both broaden and disperse through the Brillouin zone similar to what is seen in experiments in materials like Ca_2 Cu O_2 Cl_2.

cond-mat.str-el↗

arXiv2002

Observation of an unconventional metal-insulator transition in overdoped CuO_2 compounds

The electron dynamics in the normal state of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ is studied by inelastic light scattering over a wide range of doping. A strong anisotropy of the electron relaxation is found which cannot be explained by single-particle properties alone. The results strongly indicate the presence of an unconventional quantum-critical metal-insulator transition where "hot" (antinodal) quasiparticles become insulating while "cold" (nodal) quasiparticles remain metallic. A phenomenology is developed which allows a quantitative understanding of the Raman results and provides a scenario which links single- and many-particle properties.

cond-mat.supr-con↗

arXiv2002

Stripe phases in the two-dimensional Falicov-Kimball model

The observation of charge stripe order in the doped nickelate and cuprate materials has motivated much theoretical effort to understand the underlying mechanism of the stripe phase. Numerical studies of the Hubbard model show two possibilities: (i) stripe order arises from a tendency toward phase separation and its competition with the long-range Coulomb interaction or (ii) stripe order inherently arises as a compromise between itinerancy and magnetic interactions. Here we determine the restricted phase diagrams of the two-dimensional Falicov-Kimball model and see that it displays rich behavior illustrating both possibilities in different regions of the phase diagram.

cond-mat.str-el↗

arXiv2002

Phase separation in the combined Falicov-Kimball and static Holstein model

The Falicov-Kimball model has been known to phase separate when the correlation strength is large and the temperature is low. We examine the stability of phase separation under the influence of an additional electron-phonon interaction that tries to force the electron density to be homogeneous. We find that the electron-phonon interaction eventually destroys the phase-separated state once it becomes larger than some critical value. The results of this work may have an influence on theories of collossal magentoresistance materials.

cond-mat.str-el↗

arXiv2002

Optimizing the speed of a Josephson junction

We review the application of dynamical mean-field theory to Josephson junctions and study how to maximize the characteristic voltage IcRn which determines the width of a rapid single flux quantum pulse, and thereby the operating speed in digital electronics. We study a wide class of junctions ranging from SNS, SCmS (where Cm stands for correlated metal), SINIS (where the insulating layer is formed from a screened dipole layer), and SNSNS structures. Our review is focused on a survey of the physical results; the formalism has been developed elsewhere.

cond-mat.supr-con↗

arXiv2002

Nonconstant electronic density of states tunneling inversion for A15 superconductors: Nb3Sn

We re-examine the tunneling data on A15 superconductors by performing a generalized McMillan-Rowell tunneling inversion that incorporates a nonconstant electronic density of states obtained from band-structure calculations. For Nb3Sn, we find that the fit to the experimental data can be slightly improved by taking into account the sharp structure in the density of states, but it is likely that such an analysis alone is not enough to completely explain the superconducting tunneling characteristics of this material. Nevertheless, the extracted Eliashberg function displays a number of features expected to be present for the highest quality Nb3Sn samples.

cond-mat.supr-con↗

arXiv2001

Suppression of the "quasiclassical" proximity gap in correlated-metal--superconductor structures

We study the energy and spatial dependence of the local density of states in a superconductor--correlated-metal--superconductor Josephson junction, where the correlated metal is a non-Fermi liquid (described by the Falicov-Kimball model). Many-body correlations are treated with dynamical mean-field theory, extended to inhomogeneous systems. While quasiclassical theories predict a minigap in the spectrum of a disordered Fermi liquid which is proximity-coupled within a mesoscopic junction, we find that increasing electron correlations destroy any minigap that might be opened in the absence of many-body correlations.

cond-mat.supr-con↗

arXiv2001

Phase separation due to quantum mechanical correlations

Can phase separation be induced by strong electron correlations? We present a theorem that affirmatively answers this question in the Falicov-Kimball model away from half-filling, for any dimension. In the ground state the itinerant electrons are spatially separated from the classical particles.

cond-mat.str-el↗

arXiv2001

Theory of valence transitions in Ytterbium and Europium intermetallics

The exact solution of the multi-component Falicov-Kimball model in infinite-dimensions is presented and used to discuss a new fixed point of valence fluctuating intermetallics with Yb and Eu ions. In these compounds, temperature, external magnetic field, pressure, or chemical pressure induce a transition between a metallic state with the f-ions in a mixed-valent (non-magnetic) configuration and a semi-metallic state with the f-ions in an integral-valence (paramagnetic) configuration. The zero-field transition occurs at the temperature Tv, while the zero-temperature transition sets in at the critical field Hc. We present the thermodynamic and dynamic properties of the model for an arbitrary concentration of d- and f-electrons. For large U, we find a MI transition, triggered by the temperature or field-induced change in the f-occupancy.

cond-mat.str-el↗

arXiv2001

Thermal transport in the Falicov-Kimball model

We prove the Jonson-Mahan theorem for the thermopower of the Falicov-Kimball model by solving explicitly for the correlation functions in the large dimensional limit. We prove a similar result for the thermal conductivity. We separate the results for thermal transport into the pieces of the heat current that arise from the kinetic energy and those that arise from the potential energy. Our method of proof is specific to the Falicov-Kimball model, but illustrates the near cancellations between the kinetic-energy and potential-energy pieces of the heat current implied by the Jonson-Mahan theorem.

cond-mat.str-el↗

arXiv2001

Reduction of Josephson critical current in short ballistic SNS weak links

We present fully self-consistent calculations of the thermodynamic properties of three-dimensional clean SNS Josephson junctions, where S is an s-wave short-coherence-length superconductor and N is a clean normal metal. The junction is modeled on an infinite cubic lattice such that the transverse width of the S is the same as that of the N, and its thickness is tuned from the short to long limit. Both the reduced order parameter near the SN boundary and the short coherence length depress the critical Josephson current $I_c$, even in short junctions. This is contrasted with recent measurements on SNS junctions finding much smaller $I_cR_N$ products than expected from the standard (non-self consistent and quasiclassical) predictions. We also find unusual current-phase relations, a ``phase anti-dipole'' spatial distribution of the self-consistently determined contribution to the macroscopic phase, and an ``unexpected'' minigap in the local density of states within the N region.

cond-mat.supr-con↗

arXiv2001

Exact solution of the multi-component Falicov-Kimball model in infinite dimensions

The exact solution for the thermodynamic and dynamic properties of the infinite-dimensional multi-component Falicov-Kimball model for arbitrary concentration of d- and f-electrons is presented. The emphasis is on a descriptive derivation of important physical quantities by the equation of motion technique. We provide a thorough discussion of the f-electron Green function and of the susceptibility to spontaneous hybridization. The solutions are used to illustrate different physical systems ranging from the high-temperature phase of the YbInCu4 family of materials to an examination of classical intermediate valence systems that can develop a spontaneous hybridization at T=0.

cond-mat.str-el↗

arXiv2001

Equilibrium Properties of Double-Screened-Dipole-Barrier SINIS Josephson Junctions

We report on a self-consistent microscopic study of the DC Josephson effect in $SINIS$ junctions where screened dipole layers at the $SN$ interfaces generate a double-barrier multilayered $SIN$ structure. Our approach starts from a microscopic Hamiltonian defined on a simple cubic lattice, with an attractive Hubbard term accounting for the short coherence length superconducting order in the semi-infinite leads, and a spatially extended charge distribution (screened dipole layer) induced by the difference in Fermi energies of the superconductor $S$ and the clean normal metal interlayer $N$. By employing the temperature Green function technique, in a continued fraction representation, the influence of such spatially inhomogeneous barriers on the proximity effect, current-phase relation, critical supercurrent and normal state junction resistance, is investigated for different normal interlayer thicknesses and barrier heights. These results are of relevance for high-$T_c$ grain boundary junctions, and also reveal one of the mechanisms that can lead to low critical currents of apparently ballistic $SNS$ junctions while increasing its normal state resistance in a much weaker fashion. When the $N$ region is a doped semiconductor, we find a substantial change in the dipole layer (generated by a small Fermi level mismatch) upon crossing the superconducting critical temperature, which is a new signature of proximity effect and might be related to recent Raman studies in Nb/InAs bilayers.

cond-mat.supr-con↗

arXiv2001

Raman scattering through a metal-insulator transition

The exact solution for nonresonant A1g and B1g Raman scattering is presented for the simplest model that has a correlated metal-insulator transition--the Falicov-Kimball model, by employing dynamical mean field theory. In the general case, the A1g response includes nonresonant, resonant, and mixed contributions, the B1g response includes nonresonant and resonant contributions (we prove the Shastry-Shraiman relation for the nonresonant B1g response) while the B2g response is purely resonant. Three main features are seen in the nonresonant B1g channel: (i) the rapid appearance of low-energy spectral weight at the expense of higher-energy weight; (b) the frequency range for this low-energy spectral weight is much larger than the onset temperature, where the response first appears; and (iii) the occurrence of an isosbestic point, which is a characteristic frequency where the Raman response is independent of temperature for low temperatures. Vertex corrections renormalize away all of these anomalous features in the nonresonant A1g channel. The calculated results compare favorably to the Raman response of a number of correlated systems on the insulating side of the quantum-critical point (ranging from Kondo insulators, to mixed-valence materials, to underdoped high-temperature superconductors). We also show why the nonresonant B1g Raman response is ``universal'' on the insulating side of the metal-insulator transition.

cond-mat.str-el↗

arXiv2001

Nonresonant B1g Raman scattering in the Hubbard model

The numerically exact solution for nonresonant B1g Raman scattering is presented for the half-filled Hubbard model in infinite dimensions. This solution illustrates the modifications of the Raman response (in a system tuned through the quantum-critical point of a metal-insulator transition) due to Fermi-liquid properties in the metallic phase. In the insulating phase, we recover the predicted universal behavior, while we find the Raman response is quite anomalous on the metallic side of the transition. Our calculated results are similar to those measured in FeSi, SmB6, and underdoped cuprates.

cond-mat.str-el↗

arXiv2001

Dynamical mean-field theory of a double-exchange model with diagonal disorder

We present a simplified model for the colossal magnetoresistance in doped manganites by exactly solving a double-exchange model (with Ising-like local spins) and quenched binary disorder within dynamical mean field theory. We examine the magnetic properties and the electrical and thermal transport. Our solution illustrates three different physical regimes: (i) a weak-disorder regime, where the system acts like a renormalized double-exchange system (which is insufficient to describe the behavior in the manganites); (ii) a strong-disorder regime, where the system is described by strong-coupling physics about an insulating phase (which is the most favorable for large magnetoresistance); and (iii) a transition region of moderate disorder, where both double-exchange and strong-coupling effects are important. We use the thermopower as a stringent test for the applicability of this model to the manganites and find that the model is unable to properly account for the sign change of the thermopower seen in experiment.

cond-mat.str-el↗

arXiv2001

Tuning a Josephson junction through a quantum critical point

We tune the barrier of a Josephson junction through a zero-temperature metal-insulator transition and study the thermodynamic behavior of the junction in the proximity of the quantum-critical point. We examine a short-coherence-length superconductor and a barrier (that is described by a Falicov-Kimball model) using the local approximation and dynamical mean-field theory. The inhomogeneous system is self-consistently solved by performing a Fourier transformation in the planar momentum and exactly inverting the remaining one-dimensional matrix with the renormalized perturbation expansion. Our results show a delicate interplay between oscillations on the scale of the Fermi wavelength and pair-field correlations on the scale of the coherence length, variations in the current-phase relationship, and dramatic changes in the characteristic voltage as a function of the barrier thickness or correlation strength (which can lead to an ``intrinsic'' pinhole effect).

cond-mat.str-el↗