1,429 research outputs found
Spectral and Transport Properties of d-Wave Superconductors With Strong Impurities
One of the remarkable features of disordered d-wave superconductors is strong
sensitivity of long range properties to the microscopic realization of the
disorder potential. Particularly rich phenomenology is observed for the --
experimentally relevant -- case of dilute distributions of isolated impurity
centers. Building on earlier diagrammatic analyses, the present paper derives
and analyses a low energy effective field theory of this system. Specifically,
the results of previous diagrammatic T-matrix approaches are extended into the
perturbatively inaccessible low energy regimes, and the long range (thermal)
transport behaviour of the system is discussed. It turns out that in the
extreme case of a half-filled tight binding band and infinitely strong
impurities (impurities at the unitary limit), the system is in a delocalized
phase.Comment: 14 pages, two figures include
Nonanalytic quantum oscillator image of complete replica symmetry breaking
We describe the effect of replica symmetry breaking in the field distribution
function P(h) of the T=0 SK-model as the difference between a split Gaussian
and the first excited state of a weakly anharmonic oscillator with
nonanalytic shift by means of the analogy . New numerical
calculations of the leading 100 orders of replica symmetry breaking (RSB) were
performed in order to obtain P(h), employing the exact mapping between density
of states of the fermionic SK-model and P(h) of the standard model,
as derived by Perez-Castillo and Sherrington. Fast convergence towards a fixed
point function for infinite steps of RSB is observed. A surprisingly
small number of harmonic oscillator wave-functions suffices to represent this
fixed point function. This allows to determine an anharmonic potential V(x)
with nonanalytic shift, whose first excited state represents and
hence P(h). The harmonic potential with unconventional shift yields already a very good approximation, since
anharmonic couplings of decay rapidly with
increasing m. We compare the pseudogap-forming effect of replica symmetry
breaking, hosted by the fermionic SK-model, with the analogous effect in the
Coulomb glass as designed by Davies-Lee-Rice and described by M\"uller-Pankov.Comment: 11 pages, 3 figures, submitted to Phil. Mag., special edition in
honour of David Sherrington's 70th birthda
A remote-control datalogger for large-scale resistivity surveys and robust processing of its signals using a software lock-in approach
We present a new versatile datalogger that can be used for a wide range of
possible applications in geosciences. It is adjustable in signal strength
and sampling frequency, battery saving and can remotely be controlled over
a Global System for Mobile Communication (GSM) connection so that it saves
running costs, particularly in monitoring experiments. The internet connection
allows for checking functionality, controlling schedules and optimizing
pre-amplification. We mainly use it for large-scale electrical resistivity
tomography (ERT), where it independently registers voltage time series on
three channels, while a square-wave current is injected. For the analysis of
this time series we present a new approach that is based on the lock-in (LI)
method, mainly known from electronic circuits. The method searches the
working point (phase) using three different functions based on a mask
signal, and determines the amplitude using a direct current (DC) correlation
function. We use synthetic data with different types of noise to compare the
new method with existing approaches, i.e. selective stacking and a modified
fast Fourier transformation (FFT)-based approach that assumes a 1∕f noise
characteristics. All methods give comparable results, but the LI is better
than the well-established stacking method. The FFT approach can be even
better but only if the noise strictly follows the assumed characteristics.
If overshoots are present in the data, which is typical in the field, FFT
performs worse even with good data, which is why we conclude that the new LI
approach is the most robust solution. This is also proved by a field data
set from a long 2-D ERT profile
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
From second to first order transitions in a disordered quantum magnet
We study the spin-glass transition in a disordered quantum model. There is a
region in the phase diagram where quantum effects are small and the phase
transition is second order, as in the classical case. In another region,
quantum fluctuations drive the transition first order. Across the first order
line the susceptibility is discontinuous and shows hysteresis. Our findings
reproduce qualitatively observations on LiHoYF. We also discuss
a marginally stable spin-glass state and derive some results previously
obtained from the real-time dynamics of the model coupled to a bath.Comment: 4 pages, 3 figures, RevTe
Imprints of magnetic power and helicity spectra on radio polarimetry statistics
Statistical properties of turbulent magnetic fields in radio-synchrotron
sources should imprint on the statistics of polarimetric observables. In search
of these imprints, we calculate correlation and cross-correlation functions
from a set of observables containing the total intensity I, the polarized
intensity P and the Faraday depth phi. The correlation functions are evaluated
for all combinations of observables up to fourth order in the magnetic field B.
We derive these as far as possible analytically and from first principles only
using some basic assumptions such as Gaussian statistics of the underlying
magnetic field in the observed region and statistical homogeneity. We further
assume some simplifications to reduce the complexity of the calculations, as
for a start we were interested in a proof of concept. Using this statistical
approach, we show that it is in principle possible to gain information about
the helical part of the magnetic power spectrum, namely via the correlation
functions and . Using this insight, we
construct an easy-to-use test for helicity, called LITMUS (Local Inference Test
for Magnetic fields which Uncovers heliceS). For now, all calculations are
given in a Faraday-free case, but set up in a way so that Faraday rotational
effects could be included later on.Comment: 24 pages, 4 figures; typos corrected; additional explanations in
section 1 and 2; revised and extended derivation in section 5, results
unchange
Superconducting ``metals'' and ``insulators''
We propose a characterization of zero temperature phases in disordered
superconductors on the basis of the nature of quasiparticle transport. In three
dimensional systems, there are two distinct phases in close analogy to the
distinction between normal metals and insulators: the superconducting "metal"
with delocalized quasiparticle excitations and the superconducting "insulator"
with localized quasiparticles. We describe experimental realizations of either
phase, and study their general properties theoretically. We suggest experiments
where it should be possible to tune from one superconducting phase to the
other, thereby probing a novel "metal-insulator" transition inside a
superconductor. We point out various implications of our results for the phase
transitions where the superconductor is destroyed at zero temperature to form
either a normal metal or a normal insulator.Comment: 18 page
A Theory of Ferroelectric Phase Transition in SrTiO induced by Isotope Replacement
A theory to describe the dielectric anomalies and the ferroelectric phase
transition induced by oxygen isotope replacement in SrTiO is developed. The
proposed model gives consistent explanation between apparently contradictory
experimental results on macroscopic dielectric measurements versus microscopic
lattice dynamical measurements by neutron scattering studies. The essential
feature is described by a 3-state quantum order-disorder system characterizing
the degenerated excited states in addition to the ground state of TiO
cluster. The effect of isotope replacement is taken into account through the
tunneling frequency between the excited states. The dielectric properties are
analyzed by the mean field approximation (MFA), which gives qualitative
agreements with experimental results throughout full range of the isotope
concentration.The phase diagram in the temperature-tunneling
frequencycoordinate is studied by a QMC method to confirm the qualitative
validity of the MFA analysis.Comment: 26 pages, 8 figure
The mixed problem for the Lam\'e system in two dimensions
We consider the mixed problem for the Lam\'e system of elasticity in a
bounded Lipschitz domain . We suppose that the
boundary is written as the union of two disjoint sets, . We take traction data from the space and Dirichlet data from a
Sobolev space and look for a solution of with the
given boundary conditions. We give a scale invariant condition on and find
an exponent so that for , we have a unique solution of this
boundary value problem with the non-tangential maximal function of the gradient
of the solution in . We also establish the existence of a
unique solution when the data is taken from Hardy spaces and Hardy-Sobolev
spaces with in for some
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