86 research outputs found
Precursor Pairing Correlations and Pseudogaps
I begin by briefly reviewing various experimental results on the pseudogap
phenomena in underdoped cuprates. I argue that, taken together, all of these
lead to a picture of singlet pairing above . I then explore the idea that
the pseudogap is a normal state precursor of the superconducting gap due to
local, dynamic pairing correlations in a state without long range phase
coherence. Early work on simple model systems which exhibit pseudogap anomalies
in the normal state of 2D superconductors in a low density, small pair size
regime is reviewed and critically re-examined in view of more recent
developments. I also describe recent studies of how the underlying d-wave
superconducting ground state affects the anisotropy of the pseudogap and the
destruction of the Fermi surface.Comment: 23 pages, 9 postscript figures, RevTex; Varenna Lectures, 199
Universal Short-Distance Structure of the Single-Particle Spectral Function of Dilute Fermi Gases
We show that the universal tail in the momentum distribution of
dilute Fermi gases implies that the spectral function A(\kk,\omega) must have
weight below the chemical potential for large momentum , with
observable consequences in RF spectroscopy experiments. We find that this
incoherent spectral weight is centered about \omega \simeq - \epsilon(\kk) in
a range of energies of order . This "bending back" in the dispersion,
while natural for superfluids, is quite surprising for normal gases. This
universal structure is present in the hard-sphere gas as well as the Fermi
liquid ground state of the highly imbalanced, attractive gas near unitarity. We
argue that, even in the BCS superfluid, this bending back at large is
dominated by interaction effects which do not reflect the pairing gap.Comment: 4 pages, 4 figure
BCS-BEC Crossover and the Unitary Fermi Gas
The crossover from weak coupling Bardeen-Cooper-Schrieffer (BCS) pairing to a
Bose-Einstein condensate (BEC) of tightly bound pairs, as a function of the
attractive interaction in Fermi systems, has long been of interest to
theoretical physicists. The past decade has seen a series of remarkable
experimental developments in ultracold Fermi gases that has realized the
BCS-BEC crossover in the laboratory, bringing with it fresh new insights into
the very strongly interacting unitary regime in the middle of this crossover.
In this review, we start with a pedagogical introduction to the crossover and
then focus on recent progress in the strongly interacting regime. While our
focus is on new theoretical developments, we also describe three key
experiments that probe the thermodynamics, transport and spectroscopy of the
unitary Fermi gas. We discuss connections between the unitary regime and other
areas of physics -- quark-gluon plasmas, gauge-gravity duality and high
temperature superconductivity -- and conclude with open questions about
strongly interacting Fermi gases.Comment: 33 pages, 6 figures. Invited article for Annual Reviews of Condensed
Matter Physics. v3: Published versio
High Tc Superconductors: New Insights from Angle-Resolved Photoemission
Recent angle-resolved photoemission (ARPES) studies of the high Tc
superconductors are reviewed. Amongst the topics discussed are: the spectral
function interpretation of ARPES data and sum rules; studies of the momentum
distribution and the Fermi surface (FS); dispersion of electronic states, flat
bands and superlattice effects; unusual lineshapes and their temperature
dependence; the question of bilayer splitting; detailed studies of the
superconducting gap and its anisotropy; and, finally, studies of the pseudogap
and evolution of the FS with doping in the underdoped materials.Comment: 18 pages, RevTex, 14 postscript figures; Varenna Lectures, 199
BCS-BEC crossover with unequal mass fermions
We investigate the crossover from BCS pairing to molecular BEC in an atomic
gas with two fermion species with masses m_\up \ne m_\dn tuned through a
Feshbach resonance. We present results for the T=0 equation of state as a
function of the scattering length including the effects of Gaussian
fluctuations about the mean field ground state. We compute the ground state
energy as a function of m_\up/m_\dn at unitarity and find excellent agreement
with the quantum Monte Carlo result for m_\up/m_\dn = 6.67 for a
K-Li mixture. We show that the dimer scattering length in the BEC
limit as a function of m_\up/m_\dn compares well with exact four-body results
of Petrov {\it et al}. We also derive the condition for trapping frequencies to
obtain an unpolarized gas in a harmonic trap.Comment: 4 pages, 3 figure
Theory of Kerr and Faraday rotations and linear dichroism in Topological Weyl Semimetals
We consider the electromagnetic response of a topological Weyl semimetal
(TWS) with a pair of Weyl nodes in the bulk and corresponding Fermi arcs in the
surface Brillouin zone. We compute the frequency-dependent complex
conductivities and also take into account the
modification of Maxwell equations by the topological -term to obtain
the Kerr and Faraday rotations in a variety of geometries. For TWS films
thinner than the wavelength, the Kerr and Faraday rotations, determined by the
separation between Weyl nodes, are significantly larger than in topological
insulators. In thicker films, the Kerr and Faraday angles can be enhanced by
choice of film thickness and substrate refractive index. We show that, for
radiation incident on a surface with Fermi arcs, there is no Kerr or Faraday
rotation but the electric field develops a longitudinal component inside the
TWS, and there is magnetic linear dichroism. Our results have implications for
probing the TWS phase in various experimental systems
Strong correlations lead to protected low energy excitations in disordered d-wave superconductors
We show that strong correlations play a vital role in protecting low energy
excitations in disordered high temperature superconductors. The
impurity-induced low-energy density of states (DOS) is greatly reduced in the
strongly correlated superconductor compared to d-wave Bogoliubov-deGennes
theory. The gapless nodal quasiparticles, and the resulting `V' in the
low-energy DOS, are much more robust against disorder compared to the large-gap
antinodal excitations. We discuss the relevance of our results to
angle-resolved photoemission and scanning tunneling spectroscopy experiments.Comment: 4 pages, 4 figure
Quantum oscillations in a d-wave vortex liquid
The observation of quantum oscillations in underdoped cuprates has generated
intense debate about the nature of the field-induced resistive state and its
implications for the `normal state' of high T_c superconductors. Quantum
oscillations suggest an underlying Fermi liquid state at high magnetic fields H
and low temperatures, in contrast with the high-temperature, zero-field
pseudogap state seen in spectroscopy. Recent heat capacity measurements show
quantum oscillations together with a large and singular field-dependent
suppression of the electronic density of states (DOS), which suggests a
resistive state that is affected by the d-wave superconducting gap. We present
a theoretical analysis of the electronic excitations in a vortex-liquid state,
with short range pairing correlations in space and time, that is able to
reconcile these seemingly contradictory observations. We show that phase
fluctuations lead to large suppression of the DOS that goes like at
low fields, in addition to quantum oscillations with a period determined by a
Fermi surface reconstructed by a competing order parameter.Comment: 9 pages, 3 figure
Ferromagnetic exchange, spin-orbit coupling and spiral magnetism at the LaAlO_3/SrTiO_3 interface
The electronic properties of the polar interface between insulating oxides is
a subject of great current interest. An exciting new development is the
observation of robust magnetism at the interface of two non-magnetic materials
LaAlO_3 (LAO) and SrTiO_3 (STO). Here we present a microscopic theory for the
formation and interaction of local moments, which depends on essential features
of the LAO/STO interface. We show that correlation-induced moments arise due to
interfacial splitting of orbital degeneracy. We find that gate-tunable Rashba
spin-orbit coupling at the interface influences the exchange interaction
mediated by conduction electrons. We predict that the zero-field ground state
is a long-wavelength spiral and show that its evolution in an external field
accounts semi-quantitatively for torque magnetometry data. Our theory describes
qualitative aspects of the scanning SQUID measurements and makes several
testable predictions for future experiments.Comment: 9 pages, 4 figures, a typo corrected from the previous versio
Charged fermions coupled to gauge fields: Superfluidity, confinement and emergent Dirac fermions
We consider a 2+1 dimensional model of charged fermions coupled to a
gauge field, and study the confinement transition in this
regime. To elucidate the phase diagram of this model, we introduce a method to
handle the Gauss law constraint within sign problem free determinantal quantum
Monte Carlo, at any charge density. For generic charge densities,
gauge fluctuations mediate pairing and the ground state is a
gapped superfluid. Superfluidity also appears in the confined phase. This is
reminiscent of the BCS-BEC crossover, except that a true zero temperature
transition occurs here, with the maximum achieved near the transition. At
half-filling also one obtains a large Fermi surface which is gapped at zero
temperature. However, on increasing fermion hopping a -flux phase is
spontaneously generated, with emergent Dirac fermions that are stable against
pairing. In contrast to a Fermi liquid of electrons, the change in Fermi
surface volumes of the fermions occurs without the breaking of
translation symmetry. Unexpectedly, the numerics indicate a single continuous
transition between the deconfined Dirac phase and the confined superfluid, in
contrast to the naive expectation of a split transition, where a gap to
fermions precedes confinement.Comment: 16 pages, 14 figure
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