Showing papers on "Fermi liquid theory published in 2014"

Charge order and its connection with Fermi-liquid charge transport in a pristine high- T c cuprate

Abstract: Electronic inhomogeneity appears to be an inherent characteristic of the enigmatic cuprate superconductors. Here we report the observation of charge-density-wave correlations in the model cuprate superconductor HgBa2CuO(4+δ) (T(c)=72 K) via bulk Cu L3-edge-resonant X-ray scattering. At the measured hole-doping level, both the short-range charge modulations and Fermi-liquid transport appear below the same temperature of about 200 K. Our result points to a unifying picture in which these two phenomena are preceded at the higher pseudogap temperature by q=0 magnetic order and the build-up of significant dynamic antiferromagnetic correlations. The magnitude of the charge modulation wave vector is consistent with the size of the electron pocket implied by quantum oscillation and Hall effect measurements for HgBa2CuO(4+δ) and with corresponding results for YBa2Cu3O(6+δ), which indicates that charge-density-wave correlations are universally responsible for the low-temperature quantum oscillation phenomenon.

Holographic duality and the resistivity of strange metals

Abstract: We present a strange metal, described by a holographic duality, which reproduces the famous linear resistivity of the normal state of the copper oxides, in addition to the linear specific heat. This holographic metal reveals a simple and general mechanism for producing such a resistivity, which requires only quenched disorder and a strongly interacting, locally quantum critical state. The key is the minimal viscosity of the latter: unlike in a Fermi liquid, the viscosity is very small and therefore is important for the electrical transport. This mechanism produces a resistivity proportional to the electronic entropy.

Ground-state pressure of quasi-2D Fermi and Bose gases.

Abstract: Using an ultracold gas of atoms, we have realized a quasi-two-dimensional Fermi system with widely tunable $s$-wave interactions nearly in a ground state. Pressure and density are measured. The experiment covers physically different regimes: weakly and strongly attractive Fermi gases and a Bose gas of tightly bound pairs of fermions. In the Fermi regime of weak interactions, the pressure is systematically above a Fermi-liquid-theory prediction, maybe due to mesoscopic effects. In the opposite Bose regime, the pressure agrees with a bosonic mean-field scaling in a range beyond simplest expectations. In the strongly interacting regime, measurements disagree with a purely 2D model. Reported data may serve for sensitive testing of theoretical methods applicable across different quantum physics disciplines.

Asymmetry in band widening and quasiparticle lifetimes in SrVO 3 : Competition between screened exchange and local correlations from combined G W and dynamical mean-field theory G W + DMFT

Abstract: The very first dynamical implementation of the combined GW and dynamical mean field scheme "GW+DMFT" for a real material was achieved recently [J.M. Tomczak et al., Europhys. Lett. 100 67001 (2012)], and applied to the ternary transition metal oxide SrVO3. Here, we review and extend that work, giving not only a detailed account of full GW+DMFT calculations, but also discussing and testing simplified approximate schemes. We give insights into the nature of exchange and correlation effects: Dynamical renormalizations in the Fermi liquid regime of SrVO3 are essentially local, and nonlocal correlations mainly act to screen the Fock exchange term. The latter substantially widens the quasi-particle band structure, while the band narrowing induced by the former is accompanied by a spectral weight transfer to higher energies. Most interestingly, the exchange broadening is much more pronounced in the unoccupied part of spectrum. As a result, the GW+DMFT electronic structure of SrVO3 resembles the conventional density functional based dynamical mean field (DFT+DMFT) description for occupied states, but is profoundly modified in the empty part. Our work leads to a reinterpretation of inverse photoemission spectroscopy (IPES) data. Indeed, we assign a prominent peak at about 2.7 eV dominantly to eg states, rather than to an upper Hubbard band of t2g character. Similar surprises can be expected for other transition metal oxides, calling for more detailed investigations of the conduction band states.

Umklapp superradiance with a collisionless quantum degenerate Fermi gas.

Abstract: The quantum dynamics of the electromagnetic light mode of an optical cavity filled with a coherently driven Fermi gas of ultracold atoms strongly depends on the geometry of the Fermi surface. Superradiant light generation and self-organization of the atoms can be achieved at low pumping threshold due to resonant atom-photon umklapp processes, where the fermions are scattered from one side of the Fermi surface to the other by exchanging photon momenta. The cavity spectrum exhibits sidebands that, despite strong atom-light coupling and cavity decay, retain narrow linewidth, due to absorptionless transparency windows outside the atomic particle-hole continuum and the suppression of broadening and thermal fluctuations in the collisionless Fermi gas.

Non-Fermi-liquid behavior of large- N B quantum critical metals

Abstract: The problem of continuous quantum phase transitions in metals involves critical bosons coupled to a Fermi surface. We solve the theory in the limit of a large number, ${N}_{B}$, of bosonic flavors, where the bosons transform in the adjoint representation (a matrix representation), while the fermions are in the fundamental representation (a vector representation) of a global $SU({N}_{B})$ flavor symmetry group. The leading large ${N}_{B}$ solution corresponds to a non-Fermi liquid coupled to Wilson-Fisher bosons. In a certain energy range, the fermion velocity vanishes---resulting in the destruction of the Fermi surface. Subleading $1/{N}_{B}$ corrections correspond to a qualitatively different form of Landau damping of the bosonic critical fluctuations. We discuss the model in $d=3\ensuremath{-}\ensuremath{\epsilon}$ but because of the additional control afforded by large ${N}_{B}$, our results are valid down to $d=2$. In the limit $\ensuremath{\epsilon}\ensuremath{\ll}1$, the large ${N}_{B}$ solution is consistent with the renormalization group analysis of Fitzpatrick et al. [Phys. Rev. B 88, 125116 (2013)].

Attachment of Surface "Fermi Arcs" to the Bulk Fermi Surface: "Fermi-Level Plumbing" in Topological Metals

Abstract: The role of "Fermi arc" surface-quasiparticle states in "topological metals" (where some Fermi surface sheets have non-zero Chern number) is examined They act as "Fermi-level plumbing" conduits that transfer quasiparticles among groups of apparently-disconnected Fermi sheets with non-zero Chern numbers to maintain equality of their chemical potentials, which is required by gauge invariance Fermi arcs have a chiral tangential attachment to the surface projections of sheets of the bulk Fermi Surface: the total Chern number of each projection equals the net chirality of arc-attachments to it Information from the Fermi arcs is needed to unambiguously determine the quantized part of the anomalous Hall effect that is not determined at the bulk Fermi surface

Diagrammatic Monte Carlo Method for Many-Polaron Problems

Abstract: We introduce the first bold diagrammatic Monte Carlo approach to deal with polaron problems at a finite electron density nonperturbatively, i.e., by including vertex corrections to high orders. Using the Holstein model on a square lattice as a prototypical example, we demonstrate that our method is capable of providing accurate results in the thermodynamic limit in all regimes from a renormalized Fermi liquid to a single polaron, across the nonadiabatic region where Fermi and Debye energies are of the same order of magnitude. By accounting for vertex corrections, the accuracy of the theoretical description is increased by orders of magnitude relative to the lowest-order self-consistent Born approximation employed in most studies. We also find that for the electron-phonon coupling typical for real materials, the quasiparticle effective mass increases and the quasiparticle residue decreases with increasing the electron density at constant electron-phonon coupling strength.

Hartree-Fock computation of the high- T c cuprate phase diagram

Abstract: A computation of the cuprate phase diagram is presented Adiabatic deformability back to the density function band structure plus symmetry constraints lead to a Fermi liquid theory with five interaction parameters Two of these are forced to zero by experiment The remaining three are fit to the moment of the antiferromagnetic state at half-filling, the superconducting gap at optimal doping, and the maximum pseudogap The latter is identified as orbital antiferromagnetism Solution of the Hartree-Fock equations gives, in quantitative agreement with experiment, (1) quantum phase transitions at 5% and 16% $p$-type doping, (2) insulation below 5%, (3) a $d$-wave pseudogap quasiparticle spectrum, (4) pseudogap and superconducting gap values as a function of doping, (5) superconducting ${T}_{c}$ versus doping, (6) London penetration depth versus doping, and (7) spin wave velocity The fit points to superexchange mediated by the bonding O atom in the Cu-O plane as the causative agent of all three ordering phenomena

Criterion for stability of Goldstone modes and Fermi liquid behavior in a metal with broken symmetry.

Abstract: There are few general physical principles that protect the low-energy excitations of a quantum phase. Of these, Goldstone’s theorem and Landau–Fermi liquid theory are the most relevant to solids. We investigate the stability of the resulting gapless excitations—Nambu–Goldstone bosons (NGBs) and Landau quasiparticles—when coupled to one another, which is of direct relevance to metals with a broken continuous symmetry. Typically, the coupling between NGBs and Landau quasiparticles vanishes at low energies, leaving the gapless modes unaffected. If, however, the low-energy coupling is nonvanishing, non-Fermi liquid behavior and overdamped bosons are expected. Here we prove a general criterion that specifies when the coupling is nonvanishing. It is satisfied by the case of a nematic Fermi fluid, consistent with earlier microscopic calculations. In addition, the criterion identifies a new kind of symmetry breaking—of magnetic translations—where nonvanishing couplings should arise, opening a previously unidentified route to realizing non-Fermi liquid phases.

Theory of electronic relaxation in a metal excited by an ultrashort optical pump

Abstract: The theory of the electron relaxation in simple metals excited by an ultrashort optical pump is developed on the basis of the solution of the linearized Boltzmann kinetic equation. The kinetic equation includes both the electron-electron and the electron-phonon collision integrals and assumes that Fermi liquid theory is applicable for the description of a simple metal. The widely used two-temperature model follows from the theory as the limiting case when the thermalization due to the electron-electron collisions is fast with respect to the electron-phonon relaxation. It is demonstrated that the energy relaxation has two consecutive processes. The first and most important step describes the emission of phonons by the photoexcited electrons. It leads to the relaxation of 90% of the energy before the electrons become thermalized among themselves. The second step describes electron-phonon thermalization and may be described by the two-temperature model. The second stage is difficult to observe experimentally because it involves the transfer of only a small amount of energy from electrons. Thus the theory explains why the divergence of the relaxation time at low temperatures has never been observed experimentally.

Optical Response of Sr 2 RuO 4 Reveals Universal Fermi-Liquid Scaling and Quasiparticles Beyond Landau Theory

Abstract: We report optical measurements demonstrating that the low-energy relaxation rate (1/τ) of the conduction electrons in Sr(2)RuO(4) obeys scaling relations for its frequency (ω) and temperature (T) dependence in accordance with Fermi-liquid theory. In the thermal relaxation regime, 1/τ ∝ (ħω)(2)+(pπk(B)T)(2) with p = 2, and ω/T scaling applies. Many-body electronic structure calculations using dynamical mean-field theory confirm the low-energy Fermi-liquid scaling and provide quantitative understanding of the deviations from Fermi-liquid behavior at higher energy and temperature. The excess optical spectral weight in this regime provides evidence for strongly dispersing "resilient" quasiparticle excitations above the Fermi energy.

Emergent BCS regime of the two-dimensional fermionic Hubbard model: ground-state phase diagram

Abstract: A significant part of the phase diagram of the two-dimensional fermionic Hubbard model for moderate interactions and filling factors ($U < 4, \, n<0.7$) is governed by effective Fermi liquid physics with weak BCS-type instabilities. We access this regime in a controlled way by a combination of the bold-line diagrammatic Monte Carlo method with an additional ladder-diagram summation trick and semi-analytic treatment of the weak instability in the Cooper channel. We obtain the corresponding ground-state phase diagram in the $(n,U)$ plane describing the competition between the $p-$ and $d-$wave superfluid states. We also claim the values of the dimensionless BCS coupling constants controlling the superfluid $T_c$ at the phase boundaries, which prove to be very small up to $U=4, n = 0.6$.

Phase Diagram of a Three-Orbital Model for High-T c Cuprate Superconductors

Abstract: We study the phase diagram of an effective three-orbital model of the cuprates using variational Monte Carlo calculations on asymptotically large lattices and exact diagonalization on a 24-site cluster. States with ordered orbital current loops (LC), itinerant antiferromagnetism, $d$-wave superconductivity, and the Fermi liquid are investigated using appropriate Slater determinants refined by Jastrow functions for on-site and intersite correlations. We find an LC state stable in the thermodynamic limit for a range of parameters compatible with the Fermi surface of a typical hole doped superconductor provided the transfer integrals between the oxygen atoms have signs determined by the effects of indirect transfer through the $\mathrm{Cu}\text{\ensuremath{-}}4s$ orbitals as suggested by Andersen. The results of the calculations are that the LC phase gives way at lower dopings to an antiferromagnetism phase, and at larger dopings to superconductivity and Fermi liquid phases.

Orbital character and electron correlation effects on two- and three-dimensional Fermi surfaces in KFe2As2 revealed by angle-resolved photoemission spectroscopy

Abstract: We have investigated orbital character and electron correlation effects on Fermi surfaces in the hole-overdoped iron pnictide superconductor KFe2As2, which shows a low Tc of ~4 K, by angle-resolved photoemission spectroscopy From the polarization-dependence of the ARPES spectra, we have determined the orbital character of each Fermi surface Electron mass renormalization of each band is quantitatively consistent with de Haas-van Alphen results The outer beta and middle zeta Fermi surfaces show large renormalization factor of m*/mb ~6-7, while the inner Fermi surface has a smaller factor m*/mb ~2 Middle hole Fermi surface zeta has strong three-dimensionality compared to other Fermi surfaces, indicating the d3z2-r2 orbital character, which may be related to the "octet-line nodes" recently observed by laser ARPES The observed orbital-dependent mass renormalization would give constraints on the pairing mechanism with line nodes of this system

Chiral non-Fermi liquids

Abstract: The authors provide an example of a stable two-dimensional non-Fermi liquid state without time-reversal and parity invariance. It could be realized at an interface of two stacks of quantum Hall layers with opposite chiralities.

Competition between Kondo screening and magnetism at the LaAlO 3 / SrTiO 3 interface

Abstract: We present a theory of magnetic and magnetotransport phenomena at ${\mathrm{LaAlO}}_{3}$/${\mathrm{SrTiO}}_{3}$ interfaces, which as a central ingredient includes coupling between the conduction bands and local magnetic moments originating from charge traps at the interface. Tuning the itinerant electron density in the model drives transitions between a heavy Fermi liquid phase with screened moments and various magnetic states. The dependence of the magnetic phenomena on the electron density or gate voltage stems from competing magnetic interactions between the local moments and the different conduction bands. At low densities only the lowest conduction band, composed of the ${d}_{xy}$ orbitals of Ti, is occupied. Its antiferromagnetic interaction with the local moments leads to screening of the moments at a Kondo scale that increases with density. However, above a critical density, measured in experiments to be ${n}_{c}\ensuremath{\approx}1.7\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, the ${d}_{xz}$ and ${d}_{yz}$ bands begin to populate. Their ferromagnetic interaction with the local moments competes with the antiferromagnetic interaction of the ${d}_{xy}$ band leading to eventual reduction of the Kondo scale with density. We explain the distinct magnetotransport regimes seen in experiments as manifestations of the magnetic phase diagram computed from the model. We present data showing a relation between the anomalous Hall effect and the resistivity in the system. The data strongly suggest that the concentration of local magnetic moments affecting the transport in the system is much lower than the carrier density, in accord with the theoretical model.

Conductance fingerprint of Majorana fermions in the topological Kondo effect

Abstract: We consider an interacting nanowire/superconductor heterostructure attached to metallic leads. The device is described by an unusual low-energy model involving spin-1 conduction electrons coupled to a nonlocal spin-$\frac{1}{2}$ Kondo impurity built from Majorana fermions. The topological origin of the resulting Kondo effect is manifest in distinctive non-Fermi-liquid (NFL) behavior, and the existence of Majorana fermions in the device is demonstrated unambiguously by distinctive conductance line shapes. We study the physics of the model in detail, using the numerical renormalization group, perturbative scaling, and Abelian bosonization. In particular, we calculate the full scaling curves for the differential conductance in ac and dc fields, onto which experimental data should collapse. Scattering $t$ matrices and thermodynamic quantities are also calculated, recovering asymptotes from conformal field theory. We show that the NFL physics is robust to asymmetric Majorana-lead couplings, and here we uncover a duality between strong and weak coupling. The NFL behavior is understood physically in terms of competing Kondo effects. The resulting frustration is relieved by inter-Majorana coupling which generates a second crossover to a regular Fermi liquid.

Critical metal-insulator transition and divergence in a two-particle irreducible vertex in disordered and interacting electron systems

Abstract: We study singularities in the self-energy and a two-particle irreducible vertex connected with the metal-insulator transition of the disordered Falicov-Kimball model. We resort to the dynamical mean-eld approximation. We set general conditions for the existence of a critical metal-insulator transition caused by divergence of the imaginary part of the self-energy. We calculate explicitly the critical behavior of the self-energy for symmetric and asymmetric disorder distributions. We demonstrate that the metal-insulator transition is preceded by a pole in a two-particle irreducible vertex and show that unlike the singularity in the self-energy the divergence in the irreducible vertex has no impact on physical measurable quantities. We reveal universal features of the critical metalinsulator transition that are transferable also to the Mott-Hubbard transition in the local Fermi liquid.

Low-Energy Electronic Properties of Clean CaRuO 3 : Elusive Landau Quasiparticles

Abstract: We have prepared high-quality epitaxial thin films of CaRuO$_3$ with residual resistivity ratios up to 55 Shubnikov-de Haas oscillations in the magnetoresistance and a $T^2$ temperature dependence in the electrical resistivity only below 15 K, whose coefficient is substantially suppressed in large magnetic fields, establish CaRuO$_3$ as a Fermi liquid (FL) with anomalously low coherence scale Non-Fermi liquid (NFL) $T^{3/2}$ dependence is found between 2 and 25 K The high sample quality allows access to the intrinsic electronic properties via THz spectroscopy For frequencies below 06 THz, the conductivity is Drude-like and can be modeled by FL concepts, while for higher frequencies non-Drude behavior, inconsistent with FL predictions, is found This establishes CaRuO$_3$ as a prime example of optical NFL behavior in the THz range

Phase diagram and Fermi liquid properties of the extended Hubbard model on the honeycomb lattice

Abstract: The Hubbard and extended Hubbard models on the honeycomb lattice can be seen as prototype models of a single-layer graphene placed in a high dielectric constant environment that screens the Coulomb interaction. Taking advantage of the absence of a sign problem at half-filling, we study this problem with clusters up to 96 sites with the determinant quantum Monte Carlo method as an impurity solver for the dynamical cluster approximation at finite temperatures. After determining the stability of the semimetallic phase to interaction-induced spin-density wave (SDW), charge-density wave (CDW), and Mott insulating phases, we study the single-particle dynamics of the Dirac fermions. We show that when spontaneous symmetry breaking is avoided, the semimetallic phase is a stable Fermi liquid in the presence of repulsive interactions and that Kondo screening dominates the low temperature regime, even though there is a $\ensuremath{\rho}(\ensuremath{\omega})=|\ensuremath{\omega}|$ type local density of states. We also investigate the impact of the correlation effects on the renormalization of the Fermi velocity ${v}_{F}$. We find that ${v}_{F}$ is not renormalized when only on-site repulsion $U$ is present, but that near-neighbor repulsion $V$ does renormalize ${v}_{F}$. This may explain the variations between different measurements of ${v}_{F}$ in graphene.

Coherent Quasiparticles with a Small Fermi Surface in Lightly Doped Sr 3 Ir 2 O 7

Abstract: We characterize the electron doping evolution of (Sr1−xLax)3Ir2O7 by means of angle-resolved photoemission. Concomitant with the metal insulator transition around x≈0.05 we find the emergence of coherent quasiparticle states forming a closed small Fermi surface of volume 3x/2, where x is the independently measured La concentration. The quasiparticle weight Z remains large along the entire Fermi surface, consistent with the moderate renormalization of the low-energy dispersion, and no pseudogap is observed. This indicates a conventional, weakly correlated Fermi liquid state with a momentum independent residue Z≈0.5 in lightly doped Sr3Ir2O7.

Effects of Spin-Orbit Coupling on the Berezinskii-Kosterlitz-Thouless Transition and the Vortex-Antivortex Structure in Two-Dimensional Fermi Gases

Abstract: We investigate the Berezinskii-Kosterlitz-Thouless (BKT) transition in a 2D Fermi gas with spin-orbit coupling (SOC), as a function of the two-body binding energy and a perpendicular Zeeman field. By including a generic form of the SOC, as a function of Rashba and Dresselhaus terms, we study the evolution between the experimentally relevant equal Rashba-Dresselhaus (ERD) case and the Rashba-only (RO) case. We show that in the ERD case, at a fixed nonzero Zeeman field, the BKT transition temperature ${T}_{\mathrm{BKT}}$ is increased by the presence of SOC for all values of the binding energy. We also find a significant increase in the value of the Clogston limit compared to the case without SOC. Furthermore, we demonstrate that the superfluid density tensor becomes anisotropic (except in the RO case), leading to an anisotropic phase-fluctuation action that describes elliptic vortices and antivortices, which become circular in the RO limit. This deformation constitutes an important experimental signature for superfluidity in a 2D Fermi gas with ERD SOC. Finally, we show that the anisotropic sound velocities exhibit anomalies at low temperatures, in the vicinity of quantum phase transitions between topologically distinct uniform superfluid phases.

Non-Fermi-liquid manifold in a Majorana device.

Abstract: We propose and study a setup realizing a stable manifold of non-Fermi liquid states. The device consists of a mesoscopic superconducting island hosting $N \ge 3$ Majorana bound states tunnel-coupled to normal leads, with a Josephson contact to a bulk superconductor. We find a nontrivial interplay between multi-channel Kondo and resonant Andreev reflection processes, which results in the fixed point manifold. The scaling dimension of the leading irrelevant perturbation changes continuously within the manifold and determines the power-law scaling of the temperature dependent conductance.

Quantum phases of the Shastry-Sutherland Kondo lattice: implications for the global phase diagram of heavy-fermion metals.

Abstract: Considerable recent theoretical and experimental effort has been devoted to the study of quantum criticality and novel phases of antiferromagnetic heavy-fermion metals. In particular, quantum phase transitions have been discovered in heavy-fermion compounds with geometrical frustration. These developments have motivated us to study the competition between the Ruderman-Kittel-Kasuya-Yosida and Kondo interactions on the Shastry-Sutherland lattice. We determine the zero-temperature phase diagram as a function of magnetic frustration and Kondo coupling within a slave-fermion approach. Pertinent phases include the valence bond solid and heavy Fermi liquid. In the presence of antiferromagnetic order, our zero-temperature phase diagram is remarkably similar to the global phase diagram proposed earlier based on general grounds. We discuss the implications of our results for the experiments on Yb2Pt2Pb and related compounds.

Duality between zeroes and poles in holographic systems with massless fermions and a dipole coupling

Abstract: We discuss the zeroes and poles of the determinant of the retarded Green function ($\det G_R$) at zero frequency in a holographic system of charged massless fermions interacting via a dipole coupling. For large negative values of the dipole coupling constant $p$, $\det G_R$ possesses only poles pointing to a Fermi liquid phase. We show that a duality exists relating systems of opposite $p$. This maps poles of $\det G_R$ at large negative $p$ to zeroes of $\det G_R$ at large positive $p$, indicating that the latter corresponds to a Mott insulator phase. This duality suggests that the properties of a Mott insulator can be studied by mapping the system to a Fermi liquid. Finally, for small values of $p$, $\det G_R$ contains both poles and zeroes (pseudo-gap phase).

The enigma of the pseudogap phase of the cuprate superconductors

Abstract: The last few years have seen significant experimental progress in characterizing the copper-based hole-doped high temperature superconductors in the regime of low hole density, p Quantum oscillations, NMR, X-ray, and STM experiments have shed much light on the nature of the ordering at low temperatures We review evidence that the order parameter in the non-Lanthanum-based cuprates is a d-form factor density-wave This novel order acts as an unexpected window into the electronic structure of the pseudogap phase at higher temperatures in zero field: we argue in favor of a `fractionalized Fermi liquid' (FL*) with 4 pockets of spin S=1/2, charge +e fermions enclosing an area specified by p

Nonequilibrium dynamical cluster theory

Abstract: We study the effect of spatially nonlocal correlations on the nonequilibrium dynamics of interacting fermions by constructing the nonequilibrium dynamical cluster theory, a cluster generalization of the nonequilibrium dynamical mean-field theory (DMFT). The formalism is applied to interaction quenches in the Hubbard model in one and two dimensions, and the results are compared with data from single-site DMFT, the time-dependent density matrix renormalization group, and lattice perturbation theory. Both in one and two dimensions the double occupancy quickly thermalizes, while the momentum distribution relaxes only on much longer time scales. For the two-dimensional square lattice we find a strongly momentum-dependent evolution of the momentum distribution around the Fermi energy, with a much faster relaxation near the momenta $(0,\ensuremath{\pi})$ and $(\ensuremath{\pi},0)$ than near $(\ensuremath{\pi}/2,\ensuremath{\pi}/2)$. This result is interpreted as reflecting the momentum-anisotropic quasiparticle lifetime of the marginal Fermi liquid. The method is further applied to the two-dimensional Hubbard model driven by a dc electric field, where the damping of the Bloch oscillation of the current is found to be less effective than predicted by DMFT and lattice perturbation theory.

Fermi liquid breakdown and evidence for superconductivity in YFe2Ge2

Abstract: In the d-electron system YFe2Ge2, an unusually high and temperature dependent Sommerfeld ratio of the specific heat capacity C /T ∼ 100 mJ/(mol K2) and an anomalous power law temperature dependence of the electrical resistivity signal Fermi liquid breakdown, probably connected to a close-by quantum critical point. Full resistive transitions and DC diamagnetic screening fractions of up to 80% suggest that pure samples of YFe2Ge2 superconduct below 1.8 K. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)