2,108 research outputs found
Dynamical CPA approach to an itinerant fermionic spin glass model
We study a fermionic version of the Sherrington-Kirkpatrick model including
nearest-neighbor hopping on a -dimensional simple cubic lattices. The
problem is reduced to one of free fermions moving in a dynamical effective
random medium. By means of a CPA method we derive a set of self-consistency
equations for the spin glass order parameter and for the Fourier components of
the local spin susceptibility. In order to solve these equations numerically we
employ an approximation scheme which restricts the dynamics to a feasible
number of the leading Fourier components. From a sequence of systematically
improved dynamical approximations we estimate the location of the quantum
critical point.Comment: 9 pages, 6 figures, revised versio
Semi-fermionic representation of SU(N) Hamiltonians
We represent the generators of the SU(N) algebra as bilinear combinations of
Fermi operators with imaginary chemical potential. The distribution function,
consisting of a minimal set of discrete imaginary chemical potentials, is found
for arbitrary N. This representation leads to the conventional temperature
diagram technique with standard Feynman codex, except that the Matsubara
frequencies are determined by neither integer nor half-integer numbers. The
real-time Schwinger-Keldysh formalism is formulated in the framework of complex
distribution functions. We discuss the continuous large N and SU(2) large spin
limits. We illustrate the application of this technique for magnetic and
spin-liquid states of the Heisenberg model.Comment: 11 pages, 7 EPS figures included, extended versio
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
New technique for replica symmetry breaking with application to the SK-model at and near T=0
We describe a novel method which allows the treatment of high orders of
replica-symmetry-breaking (RSB) at low temperatures as well as at T=0 directly,
without a need for approximations or scaling assumptions. It yields the low
temperature order function q(a,T) in the full range and is
complete in the sense that all observables can be calculated from it. The
behavior of some observables and the finite RSB theory itself is analyzed as
one approaches continuous RSB. The validity and applicability of the
traditional continuous formulation is then scrutinized and a new continuous RSB
formulation is proposed
Dynamical solutions of a quantum Heisenberg spin glass model
We consider quantum-dynamical phenomena in the ,
infinite-range quantum Heisenberg spin glass. For a fermionic generalization of
the model we formulate generic dynamical self-consistency equations. Using the
Popov-Fedotov trick to eliminate contributions of the non-magnetic fermionic
states we study in particular the isotropic model variant on the spin space.
Two complementary approximation schemes are applied: one restricts the quantum
spin dynamics to a manageable number of Matsubara frequencies while the other
employs an expansion in terms of the dynamical local spin susceptibility. We
accurately determine the critical temperature of the spin glass to
paramagnet transition. We find that the dynamical correlations cause an
increase of by 2% compared to the result obtained in the spin-static
approximation. The specific heat exhibits a pronounced cusp at .
Contradictory to other reports we do not observe a maximum in the -curve
above .Comment: 8 pages, 7 figure
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
Designing Supportive Chatbots for Blood Donors
Healthcare systems worldwide rely on voluntary blood donations, as blood cannot be produced artificially, but is needed for many surgeries and treatments. In countries like South Africa or Ghana, currently less than 1 % of the population donates blood. Donor education, mobilisation and management are therefore crucial. Chatbots offer easy access to information for all types of donors as well as for blood services to educate (potential) donors. By applying the design science research approach and grounding our research on behaviour change models, we have developed a chatbot for all donor types in South Africa and Ghana. In this work, we present an instantiation of the chatbot and its positive evaluation with non-, first-time, lapsed and regular donors of both countries
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
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
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.
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