170 research outputs found
Vortices and quasiparticles near the "superconductor-insulator" transition in thin films
We study the low temperature behavior of an amorphous superconducting film
driven normal by a perpendicular magnetic field (B). For this purpose we
introduce a new two-fluid formulation consisting of fermionized field induced
vortices and electrically neutralized Bogoliubov quasiparticles (spinons)
interacting via a long-ranged statistical interaction. This approach allows us
to access a novel non-Fermi liquid phase which naturally interpolates between
the low B superconductor and the high B normal metal. We discuss the transport,
thermodynamic, and tunneling properties of the resulting "vortex metal" phase.Comment: 4 pages, 1 figure, references adde
Electron-induced massive dynamics of magnetic domain walls
We study the dynamics of domain walls (DWs) in a metallic, ferromagnetic nanowire. We develop a Keldysh collective coordinate technique to describe the effect of conduction electrons on rigid magnetic structures. The effective Lagrangian and Langevin equations of motion for a DW are derived. The DW dynamics is described by two collective degrees of freedom: position and tilt-angle. The coupled Langevin equations therefore involve two correlated noise sources, leading to a generalized fluctuation-dissipation theorem (FDT). The DW response kernel due to electrons contains two parts: one related to dissipation via FDT, and another `inertial\u27 part. We prove that the latter term leads to a mass for both degrees of freedom, even though the intrinsic bare mass is zero. The electron-induced mass is present even in a clean system without pinning or specifically engineered potentials. The resulting equations of motion contain rich dynamical solutions and point toward a new way to control domain wall motion in metals via the electronic system properties. We discuss two observable co nsequences of the mass, hysteresis in the DW dynamics and resonant response to ac current
Spin-Mediated Mott Excitons
Motivated by recent experiments on Mott insulators, in both iridates and
ultracold atoms, we theoretically study the effects of magnetic order on the
Mott-Hubbard excitons. In particular, we focus on spin-mediated doublon-holon
pairing in Hubbard materials. We use several complementary theoretical
techniques: mean-field theory to describe the spin degrees of freedom, the
self-consistent Born approximation to characterize individual charge
excitations across the Hubbard gap, and the Bethe-Salpeter equation to identify
bound states of doublons and holons. The binding energy of the Hubbard exciton
is found to increase with increasing the N{\'e}el order parameter, while the
exciton mass decreases. We observe that these trends rely significantly on the
retardation of the effective interaction, and require consideration of multiple
effects from changing the magnetic order. Our results are consistent with the
key qualitative trends observed in recent experiments on iridates. Moreover,
the findings could have direct implications on ultracold atom Mott insulators,
where the Hubbard model is the exact description of the system and the
microscopic degrees of freedom can be directly accessed.Comment: 11 pages, 11 figure
Temperature dependent spin susceptibility in a two-dimensional metal
We consider a two-dimensional electron system with Coulomb interaction
between particles at a finite temperature T. We show that the dynamic Kohn
anomaly in the response function at 2K_F leads to a linear-in-T correction to
the spin susceptibility, same as in systems with short-range interaction.
We show that the singularity of the Coulomb interaction at q=0 does not
invalidate the expansion in powers of r_s, but makes the expansion
non-analytic. We argue that the linear temperature dependence is consistent
with the general structure of Landau theory and can be viewed as originating
from the non-analytic component of the Landau function near the Fermi surface.Comment: 4 pages, no figure
Low-Frequency Quantum Oscillations due to Strong Electron Correlations
The normal-state energy spectrum of the two-dimensional - model in a
homogeneous perpendicular magnetic field is investigated. The density of states
at the Fermi level as a function of the inverse magnetic field
reveals oscillations in the range of hole concentrations . The
oscillations have both high- and low-frequency components. The former
components are connected with large Fermi surfaces, while the latter with van
Hove singularities in the Landau subbands, which traverse the Fermi level with
changing . The singularities are related to bending the Landau subbands due
to strong electron correlations. Frequencies of these components are of the
same order of magnitude as quantum oscillation frequencies observed in
underdoped cuprates.Comment: 10 pages, 3 figures, Proc. NSS-2013, Yalta. arXiv admin note: text
overlap with arXiv:1308.056
Two-component Coulomb Glass in Disordered Superconducting Films
Motivated by evidence of local electron-electron attraction in experiments on
disordered insulating films, we propose a new two-component Coulomb glass model
that combines strong disorder and long-range Coulomb repulsion with the
additional possibility of local pockets of a short-range inter-electron
attraction. This model hosts a variety of interesting phenomena, in particular
a crucial modification of the Coulomb gap previously believed to be universal.
Tuning the short-range interaction to be repulsive, we find non-monotonic humps
in the density of states within the Coulomb gap. We further study
variable-range hopping transport in such systems by extending the standard
resistor network approach to include the motion of both single electrons and
local pairs. In certain parameter regimes the competition between these two
types of carriers results in a distinct peak in resistance as a function of the
local attraction strength, which can be tuned by a magnetic field.Comment: 12 pages, 6 figure
Critical disorder effects in Josephson-coupled quasi-one-dimensional superconductors
Effects of non-magnetic randomness on the critical temperature T_c and
diamagnetism are studied in a class of quasi-one dimensional superconductors.
The energy of Josephson-coupling between wires is considered to be random,
which is typical for dirty organic superconductors. We show that this
randomness destroys phase coherence between the wires and T_c vanishes
discontinuously when the randomness reaches a critical value. The parallel and
transverse components of the penetration depth are found to diverge at
different critical temperatures T_c^{(1)} and T_c, which correspond to
pair-breaking and phase-coherence breaking. The interplay between disorder and
quantum phase fluctuations results in quantum critical behavior at T=0,
manifesting itself as a superconducting-normal metal phase transition of
first-order at a critical disorder strength.Comment: 4 pages, 2 figure
Majorana path integral for nonequilibrium dynamics of two-level systems
We present a new field-theoretic approach to anaylize non-equilirbium
dynamics of two-level systems (TLS), which is based on a correspondence between
a driven TLS and a Majorana fermion field theory coupled to bosonic fields.
This approach allows us to calculate analytically properties of non-linear TLS
dynamics with an arbitrary accuracy. We apply our method to analyze specific
TLS dynamics under a monochromatic periodic drive that is relevant to the
problem of decoherence in Josephson junction qubits. It is demonstrated that
the method gives the precise positions of the resonance peaks in the non-linear
dielectric response function that are in agreement with numerical simulations.Comment: 8 pages, 5 figures; References added; Accepted for publication in
Phys. Rev.
Self-consistent treatment of the self-energy in nuclear matter
The influence of hole-hole propagation in addition to the conventional
particle-particle propagation, on the energy per nucleon and the momentum
distribution is investigated. The results are compared to the
Brueckner-Hartree-Fock (BHF) calculations with a continuous choice and
conventional choice for the single-particle spectrum. The Bethe-Goldstone
equation has been solved using realistic interactions. Also, the structure
of nucleon self-energy in nuclear matter is evaluated. All the self-energies
are calculated self-consistently. Starting from the BHF approximation without
the usual angle-average approximation, the effects of hole-hole contributions
and a self-consistent treatment within the framework of the Green function
approach are investigated. Using the self-consistent self-energy, the hole and
particle self-consistent spectral functions including the particle-particle and
hole-hole ladder contributions in nuclear matter are calculated using realistic
interactions. We found that, the difference in binding energy between both
results, i.e. BHF and self-consistent Green function, is not large. This
explains why is the BHF ignored the 2h1p contribution.Comment: Preprint 20 pages including 15 figures and one tabl
Quantum oscillations from Fermi arcs
When a metal is subjected to strong magnetic field B nearly all measurable
quantities exhibit oscillations periodic in 1/B. Such quantum oscillations
represent a canonical probe of the defining aspect of a metal, its Fermi
surface (FS). In this study we establish a new mechanism for quantum
oscillations which requires only finite segments of a FS to exist. Oscillations
periodic in 1/B occur if the FS segments are terminated by a pairing gap. Our
results reconcile the recent breakthrough experiments showing quantum
oscillations in a cuprate superconductor YBCO, with a well-established result
of many angle resolved photoemission (ARPES) studies which consistently
indicate "Fermi arcs" -- truncated segments of a Fermi surface -- in the normal
state of the cuprates.Comment: 8 pages, 5 figure
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