1,075 research outputs found
Renormalization group and Ward identities in quantum liquid phases and in unconventional critical phenomena
By reviewing the application of the renormalization group to different
theoretical problems, we emphasize the role played by the general symmetry
properties in identifying the relevant running variables describing the
behavior of a given physical system. In particular, we show how the constraints
due to the Ward identities, which implement the conservation laws associated
with the various symmetries, help to minimize the number of independent running
variables. This use of the Ward identities is examined both in the case of a
stable phase and of a critical phenomenon. In the first case we consider the
problems of interacting fermions and bosons. In one dimension general and
specific Ward identities are sufficient to show the non-Fermi-liquid character
of the interacting fermion system, and also allow to describe the crossover to
a Fermi liquid above one dimension. This crossover is examined both in the
absence and presence of singular interaction. On the other hand, in the case of
interacting bosons in the superfluid phase, the implementation of the Ward
identities provides the asymptotically exact description of the acoustic
low-energy excitation spectrum, and clarifies the subtle mechanism of how this
is realized below and above three dimensions. As a critical phenomenon, we
discuss the disorder-driven metal-insulator transition in a disordered
interacting Fermi system. In this case, through the use of Ward identities, one
is able to associate all the disorder effects to renormalizations of the Landau
parameters. As a consequence, the occurrence of a metal-insulator transition is
described as a critical breakdown of a Fermi liquid.Comment: 47 pages, 11 figure
Theory of the spin galvanic effect at oxide interfaces
The spin galvanic effect (SGE) describes the conversion of a non-equilibrium
spin polarization into a transverse charge current. Recent experiments have
demonstrated a large conversion efficiency for the two-dimensional electron gas
formed at the interface between two insulating oxides, LaAlO and SrTiO.
Here we analyze the SGE for oxide interfaces within a three-band model for the
Ti t orbitals which displays an interesting variety of effective
spin-orbit couplings in the individual bands that contribute differently to the
spin-charge conversion. Our analytical approach is supplemented by a numerical
treatment where we also investigate the influence of disorder and temperature,
which turns out to be crucial to provide an appropriate description of the
experimental data.Comment: 5 pages, 3 figure
Density inhomogeneities and Rashba spin-orbit coupling interplay in oxide interfaces
There is steadily increasing evidence that the two-dimensional electron gas
(2DEG) formed at the interface of some insulating oxides like LaAlO3/SrTiO3 and
LaTiO3/SrTiO3 is strongly inhomogeneous. The inhomogeneous distribution of
electron density is accompanied by an inhomogeneous distribution of the
(self-consistent) electric field confining the electrons at the interface. In
turn this inhomogeneous transverse electric field induces an inhomogeneous
Rashba spin-orbit coupling (RSOC). After an introductory summary on two
mechanisms possibly giving rise to an electronic phase separation accounting
for the above inhomogeneity,we introduce a phenomenological model to describe
the density-dependent RSOC and its consequences. Besides being itself a
possible source of inhomogeneity or charge-density waves, the density-dependent
RSOC gives rise to interesting physical effects like the occurrence of
inhomogeneous spin-current distributions and inhomogeneous quantum-Hall states
with chiral "edge" states taking place in the bulk of the 2DEG. The
inhomogeneous RSOC can also be exploited for spintronic devices since it can be
used to produce a disorder-robust spin Hall effect.Comment: 13 pages, 15 figure
Quantum Ising model in a transverse random field: A density-matrix renormalization group analysis
The spin-1/2 quantum Ising chain in a transverse random magnetic field is
studied by means of the density-matrix renormalization group. The system
evolves from an ordered to a paramagnetic state as the amplitude of the random
field is increased. The dependence of the magnetization on a uniform magnetic
field in the z direction and the spontaneous magnetization as a function of the
amplitude of the transverse random magnetic field are determined. The behavior
of the spin-spin correlation function both above and at criticality is studied.
The scaling laws for magnetization and correlation functions are tested against
previous numerical and renormalization-group results.Comment: 5 pages with 7 figures inside them, proper format of authors' names
use
Signatures of nematic quantum critical fluctuations in the Raman spectra of lightly doped cuprates
We consider the lightly doped cuprates YCaBaCuO
and LaSrCuO (with ,0.04), where the presence of a
fluctuating nematic state has often been proposed as a precursor of the stripe
(or, more generically, charge-density wave) phase, which sets in at higher
doping. We phenomenologically assume a quantum critical character for the
longitudinal and transverse nematic, and for the charge-ordering fluctuations,
and investigate the effects of these fluctuations in Raman spectra. We find
that the longitudinal nematic fluctuations peaked at zero transferred momentum
account well for the anomalous Raman absorption observed in these systems in
the channel, while the absence of such effect in the channel
may be due to the overall suppression of Raman response at low frequencies,
associated with the pseudogap. While in YCaBaCuO the
low-frequency lineshape is fully accounted by longitudinal nematic collective
modes alone, in LaSrCuO also charge-ordering modes with finite
characteristic wavevector are needed to reproduce the shoulders observed in the
Raman response. This different involvement of the nearly critical modes in the
two materials suggests a different evolution of the nematic state at very low
doping into the nearly charge-ordered state at higher doping.Comment: 12 pages with 10 figures, to appear in Phys. Rev. B 201
Effective medium theory for superconducting layers: A systematic analysis including space correlation effects
We investigate the effects of mesoscopic inhomogeneities on the
metal-superconductor transition occurring in several two-dimensional electron
systems. Specifically, as a model of systems with mesoscopic inhomogeneities,
we consider a random-resistor network, which we solve both with an exact
numerical approach and by the effective medium theory. We find that the width
of the transition in these two-dimensional superconductors is mainly ruled by
disorder rather than by fluctuations. We also find that "tail" features in
resistivity curves of interfaces between LaAlO3 or LaTiO3 and SrTiO3 can arise
from a bimodal distribution of mesoscopic local Tc's and/or substantial space
correlations between the mesoscopic domains.Comment: 12 pages, 10 figure
Multi-band superconductivity and nanoscale inhomogeneity at oxide interfaces
The two-dimensional electron gas at the LaTiO3/SrTiO3 or LaAlO3/SrTiO3 oxide
interfaces becomes superconducting when the carrier density is tuned by gating.
The measured resistance and superfluid density reveal an inhomogeneous
superconductivity resulting from percolation of filamentary structures of
superconducting "puddles" with randomly distributed critical temperatures,
embedded in a non-superconducting matrix. Following the evidence that
superconductivity is related to the appearance of high-mobility carriers, we
model intra-puddle superconductivity by a multi-band system within a weak
coupling BCS scheme. The microscopic parameters, extracted by fitting the
transport data with a percolative model, yield a consistent description of the
dependence of the average intra-puddle critical temperature and superfluid
density on the carrier density.Comment: 7 pages with 3 figures + supplemental material (4 pages and 5
figures
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