1,301 research outputs found
Selforganized 3-band structure of the doped fermionic Ising spin glass
The fermionic Ising spin glass is analyzed for arbitrary filling and for all
temperatures. A selforganized 3-band structure of the model is obtained in the
magnetically ordered phase. Deviation from half filling generates a central
nonmagnetic band, which becomes sharply separated at T=0 by (pseudo)gaps from
upper and lower magnetic bands. Replica symmetry breaking effects are derived
for several observables and correlations. They determine the shape of the
3-band DoS, and, for given chemical potential, influence the fermion filling
strongly in the low temperature regime.Comment: 13 page
Real-time propagator eigenstates
Obtaining a numerical solution of the time-dependent Schrödinger equation requires an initial state for the time evolution. If the system Hamiltonian can be split into a time-independent part and a time-dependent perturbation, the initial state is typically chosen as an eigenstate of the former. For propagation using approximate methods such as operator splitting, we show that both imaginary-time evolution and diagonalization of the time-independent Hamiltonian produce states that are not exactly stationary in absence of the perturbation. In order to avoid artifacts from these non-stationary initial states, we propose an iterative method for calculating eigenstates of the real-time propagator. We compare the performance of different initial states by simulating ionization of a model atom in a short laser pulse and we demonstrate that much lower noise levels can be achieved with the real-time propagator eigenstates
One-step replica symmetry breaking solution for a highly asymmetric two-sublattice fermionic Ising spin glass model in a transverse field
The one-step replica symmetry breaking (RSB) is used to study a
two-sublattice fermionic infinite-range Ising spin glass (SG) model in a
transverse field . The problem is formulated in a Grassmann path
integral formalism within the static approximation. In this model, a parallel
magnetic field breaks the symmetry of the sublattices. It destroys the
antiferromagnetic (AF) order, but it can favor the nonergodic mixed phase
(SG+AF) characterizing an asymmetric RSB region. In this region,
intra-sublattice disordered interactions increase the difference between
the RSB solutions of each sublattice. The freezing temperature shows a higher
increase with when enhances. A discontinue phase transition from the
replica symmetry (RS) solution to the RSB solution can appear with the presence
of an intra-sublattice ferromagnetic average coupling. The field
introduces a quantum spin flip mechanism that suppresses the magnetic orders
leading them to quantum critical points. Results suggest that the quantum
effects are not able to restore the RS solution. However, in the asymmetric RSB
region, can produce a stable RS solution at any finite temperature for
a particular sublattice while the other sublattice still presents RSB solution
for the special case in which only the intra-sublattice spins couple with
disordered interactions.Comment: 11 pages, 8 figures, accepted for publication in Phys. Rev.
Mass-ratio dependent strong-field dissociation of artificial helium hydride isotopologues
We study the effect of the nuclear-mass ratio in a diatomic molecular ion on the dissociation dynamics in strong infrared laser pulses. A molecular ion is a charged system, in which the dipole moment depends on the reference point and therefore on the position of the nuclear center of mass, so that the laser-induced dynamics is expected to depend on the mass asymmetry. Whereas usually both the reduced mass and the mass ratio are varied when different isotopologues are compared, we fix the reduced mass and artificially vary the mass ratio in a model system. This allows us to separate effects related to changes in the resonance frequency, which is determined by the reduced mass, from those that arise due to the mass asymmetry. Numerical solutions of the time-dependent Schrödinger equation are compared with classical trajectory simulations. We find that at a certain mass ratio, vibrational excitation is strongly suppressed, which decreases the dissociation probability by many orders of magnitude
Spin - glass transition in Kondo lattice with quenched disorder
We use the Popov-Fedotov representation of spin operators to construct an
effective action for a Kondo lattice model with quenched disorder at finite
temperatures. We study the competition between the Kondo effect and frozen spin
order in Ising-like spin glass. We present the derivation of new mean-field
equations for the spin-glass order parameter and analyze the effects of
screening of localized spins by conduction electrons on the spin-glass phase
transition.Comment: 6 pages, jetpl style included, to appear in JETP Letter
Antiferromagnetic Ising spin glass competing with BCS pairing interaction in a transverse field
The competition among spin glass (SG), antiferromagnetism (AF) and local
pairing superconductivity (PAIR) is studied in a two-sublattice fermionic Ising
spin glass model with a local BCS pairing interaction in the presence of an
applied magnetic transverse field . In the present approach, spins in
different sublattices interact with a Gaussian random coupling with an
antiferromagnetic mean and standard deviation . The problem is
formulated in the path integral formalism in which spin operators are
represented by bilinear combinations of Grassmann variables. The saddle-point
Grand Canonical potential is obtained within the static approximation and the
replica symmetric ansatz. The results are analysed in phase diagrams in which
the AF and the SG phases can occur for small ( is the strength of the
local superconductor coupling written in units of ), while the PAIR phase
appears as unique solution for large . However, there is a complex line
transition separating the PAIR phase from the others. It is second order at
high temperature that ends in a tricritical point. The quantum fluctuations
affect deeply the transition lines and the tricritical point due to the
presence of .Comment: 16 pages, 6 figures, accepted Eur. Phys. J.
Tricritical behaviour of Ising spin glasses with charge fluctuations
We show that tricritical points displaying unusal behaviour exist in phase
diagrams of fermionic Ising spin glasses as the chemical potential or the
filling assumes characteristic values. Exact results for infinite range
interaction and a one loop renormalization group analysis of thermal
tricritical fluctuations for finite range models are presented. Surprising
similarities with zero temperature transitions and a new tricritical
point of metallic quantum spin glasses are derived.Comment: 4 pages, 1 Postscript figure, minor change
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
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
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
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