18 research outputs found
Electromagnetic properties of viscous charged fluids
We provide a general theoretical framework to describe the electromagnetic
properties of viscous charged fluids, consisting for example of electrons in
certain solids or plasmas. We confirm that finite viscosity leads to multiple
modes of evanescent electromagnetic waves at a given frequency, one of which is
characterized by a negative index of refraction, as previously discussed in a
simplified model by one of the authors. In particular we explain how optical
spectroscopy can be used to probe the viscosity. We concentrate on the impact
of this on the coefficients of refraction and reflection at the sample-vacuum
interface. Analytical expressions are obtained relating the viscosity parameter
to the reflection and transmission coefficients of light. We demonstrate that
finite viscosity has the effect to decrease the reflectivity of a metallic
surface, while the electromagnetic field penetrates more deeply. While on a
phenomenological level there are similarities to the anomalous skin effect, the
model presented here requires no particular assumptions regarding the
corpuscular nature of the charge liquid. A striking consequence of the
branching phenomenon into two degenerate modes is the occurrence in a
half-infinite sample of oscillations of the electromagnetic field intensity as
a function of distance from the interface.Comment: 12 pages, 5 figure
A theory of the strain-dependent critical field in Nb3Sn, based on anharmonic phonon generation
We propose a theory to explain the strain dependence of the critical
properties in A15 superconductors. Starting from the strong-coupling formula
for the critical temperature, and assuming that the strain sensitivity stems
mostly from the electron-phonon alpha^2F function, we link the strain
dependence of the critical properties to a widening of alpha^2F. This widening
is attributed to the nonlinear generation of phonons, which takes place in the
anharmonic deformation potential induced by the strain. Based on the theory of
sum- and difference-frequency wave generation in nonlinear media, we obtain an
explicit connection between the widening of alpha^2F and the anharmonic energy.
The resulting model is fit to experimental datasets for Nb3Sn, and the
anharmonic energy extracted from the fits is compared with first-principles
calculations.Comment: 10 pages, 3 figure
Correlation between phase stiffness and condensation energy across the non-Fermi to Fermi-liquid crossover in the Yukawa-Sachdev-Ye-Kitaev model on a lattice
We construct and analyze a lattice generalization of the
Yukawa-Sachdev-Ye-Kitaev model, where spinful fermions experience on-site,
random, all-to-all interactions with an Einstein bosonic mode, and random
intersite coherent hopping. We obtain the exact self-consistent numerical
solution of the model at mean-field level, and analytical approximations, for
all values of fermion-boson coupling and hopping, under the spin-singlet ansatz
and at particle-hole symmetry, both in the normal and superconducting states,
thus tracing the entire phase diagram. In the normal state, the competition
between hopping and coupling leads to crossovers between Fermi-liquid and
non-Fermi liquid states, as reflected by the fermionic and bosonic spectral
functions and the normal-state entropy. We calculate the finite phase stiffness
of the superconducting state through the equilibrium electromagnetic response.
Furthermore, we study the critical temperature , as well as the spectral
functions, the quasiparticle weight, the gap, and the condensation energy in
the superconducting state. At weak coupling, we retrieve a disordered
generalization of Bardeen-Cooper-Schrieffer theory. At strong coupling,
asymptotically saturates but the stiffness decreases, which suggests
strong superconducting fluctuations. is maximum in the single-dot limit,
while the stiffness peaks exactly at the crossover between non-Fermi liquid and
Fermi-liquid phases. We discover that the quasiparticle weight, the stiffness,
and the condensation energy, are all correlated as a function of coupling,
reminiscent of the correlations observed in high-temperature cuprate
superconductors.Comment: 53 pages, 22 figures; companion paper: arXiv:2302.1313
BCS to incoherent superconductivity crossovers in the Yukawa-SYK model on a lattice
We provide a quantitative and controlled analysis of the phase diagram of the
the Yukawa-SYK model on a lattice, in the normal and superconducting states. We
analyze the entire crossover from BCS/weak-coupling to Eliashberg/strong
coupling superconductivity, as a function of fermion-boson interaction strength
and hopping parameter. Cooper pairs of sharp Fermi-liquid quasiparticles at
weak coupling evolve into pairing of fully incoherent fermions in the non-Fermi
liquid regime. The crossovers leave observable traces in the critical
temperature, the zero-temperature and zero-energy gap, the entropy, and the
phase stiffness.Comment: 7 pages, 4 figures; companion paper: arXiv:2302.1313
Quantum discontinuity fixed point and renormalization group flow of the Sachdev-Ye-Kitaev model
We determine the global renormalization group (RG) flow of the Sachdev-Ye-Kitaev (SYK) model. From a controlled truncation of the infinite hierarchy of the exact functional RG flow equations, we identify several fixed points. Apart from a stable fixed point, associated with the celebrated non-Fermi liquid state of the model, we
find another stable fixed point related to an integer-valence state. These stable fixed points are separated by a discontinuity fixed point with one relevant direction, describing a quantum first-order transition. Most notably,
the fermionic spectrum continues to be quantum critical even at the discontinuity fixed point. This rules out a description of the transition in terms of a local effective Ising variable as is established for classical transitions.
We propose an entangled quantum state at phase coexistence as a possible physical origin of this critical behavior
Kinetic theory of the nonlocal electrodynamic response in anisotropic metals: Skin effect in 2D systems
The electrodynamic response of ultrapure materials at low temperatures becomes spatially nonlocal. This nonlocality gives rise to phenomena such as hydrodynamic flow in transport and the anomalous skin effect in optics. In systems characterized by an anisotropic electronic dispersion, the nonlocal dynamics becomes dependent on the relative orientation of the sample with respect to the applied field, in ways that go beyond the usual, homogeneous response. Such orientational dependence should manifest itself not only in transport experiments, as recently observed, but also in optical spectroscopy. In this paper, we develop a kinetic theory for the distribution function and the transverse conductivity of two- and three-dimensional Fermi systems with anisotropic electronic dispersion. By expanding the collision integral into the eigenbasis of a collision operator, we include momentum-relaxing scattering as well as momentum-conserving collisions. We examine the isotropic 2D case as a reference, as well as anisotropic hexagonal and square Fermi-surface shapes. We apply our theory to the quantitative calculation of the skin depth and the surface impedance, in all regimes of skin effect. We find qualitative differences between the frequency dependence of the impedance in isotropic and anisotropic systems. Such differences are shown to persist even for more complex 2D Fermi surfaces, including the “supercircle” geometry and an experimental parametrization for PdCoO, which deviate from an ideal polygonal shape. We study the orientational dependence of skin effect due to Fermi-surface anisotropy, thus providing guidance for the experimental study of nonlocal optical effects
Cyclotron resonance and quantum oscillations of critical Fermi surfaces
Kohn's theorem places strong constraints on the cyclotron response of Fermi
liquids. Recent observations of a doping dependence in the cyclotron mass of
LaSrCuO (Legros et al., Phys. Rev. B 106, 195110 (2022)) are
therefore surprising because the cyclotron mass can only be renormalized by
large momentum umklapp interactions which are not expected to vary
significantly with doping. We show that a version of Kohn's theorem continues
to apply to disorder-free non-Fermi liquids with a critical boson near zero
momentum. However, marginal Fermi liquids arising from a spatially random
Yukawa coupling between the electrons and bosons do give rise to significant
corrections to the cyclotron mass which we compute. This is the same theory
which yields linear-in-temperature resistivity and other properties of strange
metals at zero fields (Patel et al., Science 381, 790 (2023)).Comment: 55 pages, 29 figures. (v2) Added new authors and new results. (v3)
added panel (b) for Fig.1