179 research outputs found
Metallic phase in a two-dimensional disordered Fermi system with singular interactions
We consider a disordered system of gapless fermions interacting with a
singular transverse (2+1)-dimensional gauge-field. We study quantum corrections
to fermion conductivity and show that they are very different from those in a
Fermi liquid with non-singular interactions. In particular, the
weak-localization effect is suppressed by magnetic field fluctuations. We argue
that these fluctuations can be considered static at time scales of fermionic
diffusion. By inducing fluxes through diffusive loops that contribute to weak
localization, they dephase via the Aharonov-Bohm effect. It is shown that while
the flux-flux correlator due to thermal fluctuations of magnetic field is
proportional to the area enclosed by the loop, the correlator due to quantum
fluctuations is proportional to the perimeter of the loop. The possibility of
dephasing due to these quasistatic configurations and the corresponding rates
are discussed. We also study interaction induced effects and show that
perturbation theory contains infrared divergent terms originating from
unscreened magnetic interactions. These singular (Hartree) terms are related to
scattering of a fermion off of the static potential created by the other
fermions. We show that due to singular small-angle scattering, the
corresponding contributions to the density of states and conductivity are very
large and positive indicating that the fermion-gauge system remains metallic at
low temperatures.Comment: 12 pages, 4 figures; changes in the abstract and the text, references
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Gutzwiller-projected wave functions for the pseudogap state of underdoped high-temperature superconductors
Recent experiments strongly suggest that a Fermi surface reconstruction and
multiple Fermi pockets are important common features of the underdoped
high-temperature cuprate superconductors. A related theoretical work [Phys.
Rev. B 79, 134512 (2009)] has demonstrated that a number of hallmark phenomena
observed in the underdoped cuprates appear naturally in the scenario of a
paired electron pocket co-existing with unpaired hole pockets. We propose
Gutzwiller-projected wave-functions to describe this two-fluid state as well as
two competing states in its vicinity. It is argued that a pseudogap state
constructed from these wave-functions may be selected by energetics at finite
temperatures due to spin fluctuations.Comment: 4 pages, 3 figure
Quasiclassical Eilenberger theory of the topological proximity effect in a superconducting nanowire
We use the quasiclassical Eilenberger theory to study the topological
superconducting proximity effects between a segment of a nanowire with a p-wave
order parameter and a metallic segment. This model faithfully represents key
qualitative features of an experimental setup, where only a part of a nanowire
is in immediate contact with a bulk superconductor, inducing topological
superconductivity. It is shown that the Eilenberger equations represent a
viable alternative to the Bogoliubov-de Gennes theory of the topological
superconducting heterostructures and provide a much simpler quantitative
description of some observables. For our setup, we obtain exact analytical
solutions for the quasiclassical Green's functions and the density of states as
a function of position and energy. The correlations induced by the boundary
involve terms associated with both p-wave and odd-frequency pairing, which are
intertwined and contribute to observables on an equal footing. We recover the
signatures of the standard Majorana mode near the end of the superconducting
segment, but find no such localized mode induced in the metallic segment.
Instead, the zero-bias feature is spread out across the entire metallic part in
accordance with the previous works. In shorter wires, the Majorana mode and
delocalized peak split up away from zero energy. For long metallic segments,
non-topological Andreev bound states appear and eventually merge together,
giving rise to a gapless superconductor.Comment: 11 pages, 8 figure
Enriched axial anomaly in Weyl materials
While quantum anomalies are often associated with the breaking of a classical
symmetry in the quantum theory, their anomalous contributions to observables
remain distinct and well-defined even when the symmetry is broken from the
outset. This paper explores such anomalous contributions to the current,
originating from the axial anomaly in a Weyl semimetal, and in the presence of
a generic Weyl node-mixing term. We find that apart from the familiar anomalous
divergence of the axial current proportional to a product of electric and
magnetic fields, there is another anomalous term proportional to a product of
the electric field and the orientation of a spin-dependent node-mixing vector.
We obtain this result both by a quantum field-theoretic analysis of an
effective Weyl action and solving an explicit lattice model. The extended
spin-mixing mass terms, and the enriched axial anomaly they entail, could arise
as mean-field or proximity-induced order parameters in spin-density-wave phases
in Weyl semimetals or be generated dynamically within a Floquet theory.Comment: 5 pages, 3 figure
Critical Viscosity of a Fluctuating Superconductor
We consider a fluctuating superconductor in the vicinity of the transition
temperature, . The fluctuation shear viscosity is calculated. In two
dimensions, the leading correction to viscosity is negative and scales as
. Critical hydrodynamics of the fluctuating
superconductor involves two fluids -- a fluid of fluctuating pairs and a
quasiparticle fluid of single-electron excitations. The pair viscosity
(Aslamazov-Larkin) term is shown to be zero. The (density of states) correction
to viscosity of single-electron excitations is negative, which is due to
fluctuating pairing that results in a reduction of electron density. Scattering
of electrons off of the fluctuations gives rise to an enhanced quasiparticle
scattering and another (Maki-Thomson) negative correction to viscosity. Our
results suggest that fluctuating superconductors provide a promising platform
to investigate low-viscosity electronic media and may potentially host
fermionic/electronic turbulence. Some experimental probes of two-fluid critical
hydrodynamics are proposed such as time-of-flight measurement of turbulent
energy cascades in critical cold atom superfluids and magnetic dynamos in
three-dimensional fluctuating superconductors.Comment: Published version. 6+7 pages, 2+1 figure
Drag viscosity of metals and its connection to Coulomb drag
Recent years have seen a surge of interest in studies of hydrodynamic
transport in electronic systems. We investigate the electron viscosity of
metals and find a new component that is closely related to Coulomb drag. Using
the linear response theory, viscosity, a transport coefficient for momentum,
can be extracted from the retarded correlation function of the momentum flux,
i.e., the stress tensor. There exists a previously overlooked contribution to
the shear viscosity from the interacting part of the stress tensor which
accounts for the momentum flow induced by interactions. This contribution,
which we dub drag viscosity, is caused by the frictional drag force due to
long-range interactions. It is therefore linked to Coulomb drag which also
originates from the interaction induced drag force. Starting from the Kubo
formula and using the Keldysh technique, we compute the drag viscosity of 2D
and 3D metals along with the drag resistivity of double-layer 2D electronic
systems. Both the drag resistivity and drag viscosity exhibit a crossover from
quadratic-in-T behavior at low temperatures to a linear one at higher
temperatures. Although the drag viscosity appears relatively small compared
with the normal Drude component for the clean metals, it may dominate
hydrodynamic transport in some systems, which are discussed in the conclusion.Comment: Published version. 16 pages, 4 figure
A Strongly-Interacting Dirac Liquid on the Surface of a Topological Kondo Insulator
A topological Kondo insulator (TKI) is a strongly-correlated material, where
hybridization between the conduction electrons and localized f-electrons gives
rise to a crossover from a metallic behavior at high temperatures to a
topologically non-trivial insulating state at low temperatures. The existing
description of the TKIs is based on a slave-boson mean-field theory, which
neglects dynamic fluctuation phenomena. Here, we go beyond the mean-field
theory and investigate the role of Kondo fluctuations on the topological
surface states. We derive an effective theory of the Dirac surface states
coupled to fluctuations and show that the latter mediate strong repulsive
interactions between surface excitations. We show that these effects
renormalize the plasmon spectrum on the surface. We also argue that
Kondo-mediated interactions may drive a magnetic instability of the surface
spectrum.Comment: 6 pages, 1 figure; Minor changes. Published versio
Divergence of the effective mass near a density wave instability in a MOSFET system
We study the renormalization of the Fermi-liquid parameters in the vicinity
of a density wave quantum phase transition, which should occur in MOSFET
systems at low densities. First, using a perturbative RPA treatment of
fluctuations, we calculate the electronic self-energy and show that the
effective mass diverges at the density wave transition point. Second, we go
beyond perturbation theory, making use of the exact Pitaevskii identities.
Within this exact analysis, we also find a divergence of the effective mass,
which occurs at higher densities in the fluctuation region, as compared to the
perturbation theory. This result signals the break-down of conventional
Fermi-liquid description in the vicinity of the transition point. The
divergence of the effective mass gives rise to a singular behavior of the
electronic compressibility. We suggest that the experimentally observed
enhancement of the effective mass is a precursor to a second order
thermodynamic phase transition into a glassy density wave state.Comment: 8 pages, no figure
Moving solitons in a one-dimensional fermionic superfluid
A fully analytical theory of a traveling soliton in a one-dimensional
fermionic superfluid is developed within the framework of time-dependent
self-consistent Bogoliubov-de Gennes equations, which are solved exactly in the
Andreev approximation. The soliton manifests itself in a kink-like profile of
the superconducting order parameter and hosts a pair of Andreev bound states in
its core. They adjust to soliton's motion and play an important role in its
stabilization. A phase jump across the soliton and its energy decrease with
soliton's velocity and vanish at the critical velocity, corresponding to the
Landau criterion, where the soliton starts emitting quasiparticles and becomes
unstable. The "inertial" and "gravitational" masses of the soliton are
calculated and the former is shown to be orders of magnitude larger than the
latter. This results in a slow motion of the soliton in a harmonic trap,
reminiscent to the observed behavior of a soliton-like texture in related
experiments in cold fermion gases [T. Yefsah et al., Nature 499, 426, (2013)].
Furthermore, we calculate the full non-linear dispersion relation of the
soliton and solve the classical equations of motion in a trap. The strong
non-linearity at high velocities gives rise to anharmonic oscillatory motion of
the soliton. A careful analysis of this anharmonicity may provide a means to
experimentally measure the non-linear soliton spectrum in superfluids.Comment: 12 pages and 5 figures. Minor changes. Updated references. Published
versio
Anomalous Coulomb Drag in Electron-Hole Bilayers due to the Formation of Excitons
Several recent experiments have reported an anomalous temperature dependence
of the Coulomb drag effect in electron-hole bilayers. Motivated by these
puzzling data, we study theoretically a low-density electron-hole bilayer,
where electrons and holes avoid quantum degeneracy by forming excitonic
molecules. We describe the ionization-recombination crossover between the
electron-hole plasma and exciton gas and calculate both the intralayer and drag
resistivity as a function of temperature. The latter exhibits a minimum
followed by a sharp upturn at low temperatures in a qualitative agreement with
the experimental observations [see, e.g., J. A. Seamons et al., Phys. Rev.
Lett. 102, 026804 (2009)]. Importantly, the drag resistivity in the proposed
scenario is found to be rather insensitive to a mismatch in electron and hole
concentrations in sharp contrast to the scenario of electron-hole Cooper
pairing.Comment: 7 pages, 4 figures. Minor changes. Published versio
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