4,774 research outputs found
Correlations in a band insulator
We study a model of a covalent band insulator with on-site Coulomb repulsion
at half-filling using dynamical mean-field theory. Upon increasing the
interaction strength the system undergoes a discontinuous transition from a
correlated band insulator to a Mott insulator with hysteretic behavior at low
temperatures. Increasing the temperature in the band insulator close to the
insulator-insulator transition we find a crossover to a Mott insulator at
elevated temperatures. Remarkably, correlations decrease the energy gap in the
correlated band insulator. The gap renormalization can be traced to the
low-frequency behavior of the self-energy, analogously to the quasiparticle
renormalization in a Fermi liquid. While the uncorrelated band insulator is
characterized by a single gap for both charge and spin excitations, the spin
gap is smaller than the charge gap in the correlated system.Comment: 7 pages, 7 figure
Breakdown of a topological phase: Quantum phase transition in a loop gas model with tension
We study the stability of topological order against local perturbations by
considering the effect of a magnetic field on a spin model -- the toric code --
which is in a topological phase. The model can be mapped onto a quantum loop
gas where the perturbation introduces a bare loop tension. When the loop
tension is small, the topological order survives. When it is large, it drives a
continuous quantum phase transition into a magnetic state. The transition can
be understood as the condensation of `magnetic' vortices, leading to
confinement of the elementary `charge' excitations. We also show how the
topological order breaks down when the system is coupled to an Ohmic heat bath
and discuss our results in the context of quantum computation applications.Comment: 5 pages, 7 figure
Environment-assisted quantum transport in a 10-qubit network
The way in which energy is transported through an interacting system governs
fundamental properties in many areas of physics, chemistry, and biology.
Remarkably, environmental noise can enhance the transport, an effect known as
environment-assisted quantum transport (ENAQT). In this paper, we study ENAQT
in a network of coupled spins subject to engineered static disorder and
temporally varying dephasing noise. The interacting spin network is realized in
a chain of trapped atomic ions and energy transport is represented by the
transfer of electronic excitation between ions. With increasing noise strength,
we observe a crossover from coherent dynamics and Anderson localization to
ENAQT and finally a suppression of transport due to the quantum Zeno effect. We
found that in the regime where ENAQT is most effective the transport is mainly
diffusive, displaying coherences only at very short times. Further, we show
that dephasing characterized by non-Markovian noise can maintain coherences
longer than white noise dephasing, with a strong influence of the spectral
structure on the transport effciency. Our approach represents a controlled and
scalable way to investigate quantum transport in many-body networks under
static disorder and dynamic noise.Comment: Mai
Analysis of aggregated tick returns: evidence for anomalous diffusion
In order to investigate the origin of large price fluctuations, we analyze
stock price changes of ten frequently traded NASDAQ stocks in the year 2002.
Though the influence of the trading frequency on the aggregate return in a
certain time interval is important, it cannot alone explain the heavy tailed
distribution of stock price changes. For this reason, we analyze intervals with
a fixed number of trades in order to eliminate the influence of the trading
frequency and investigate the relevance of other factors for the aggregate
return. We show that in tick time the price follows a discrete diffusion
process with a variable step width while the difference between the number of
steps in positive and negative direction in an interval is Gaussian
distributed. The step width is given by the return due to a single trade and is
long-term correlated in tick time. Hence, its mean value can well characterize
an interval of many trades and turns out to be an important determinant for
large aggregate returns. We also present a statistical model reproducing the
cumulative distribution of aggregate returns. For an accurate agreement with
the empirical distribution, we also take into account asymmetries of the step
widths in different directions together with crosscorrelations between these
asymmetries and the mean step width as well as the signs of the steps.Comment: 9 pages, 10 figures, typos correcte
Multiple scaling regimes in simple aging models
We investigate aging in glassy systems based on a simple model, where a point
in configuration space performs thermally activated jumps between the minima of
a random energy landscape. The model allows us to show explicitly a subaging
behavior and multiple scaling regimes for the correlation function. Both the
exponents characterizing the scaling of the different relaxation times with the
waiting time and those characterizing the asymptotic decay of the scaling
functions are obtained analytically by invoking a `partial equilibrium'
concept.Comment: 4 pages, 3 figure
Universal scaling behavior of the single electron box in the strong tunneling limit
We perform a numerical analysis of recently proposed scaling functions for
the single electron box. Specifically, we study the ``magnetic'' susceptibility
as a function of tunneling conductance and gate charge, and the effective
charging energy at zero gate charge as a function of tunneling conductance in
the strong tunneling limit. Our Monte Carlo results confirm the accuracy of the
theoretical predictions.Comment: Published versio
Enhanced Electron-Phonon Coupling and its Irrelevance to High T Superconductivity
It is argued that the origin of the buckling of the CuO planes in
certain cuprates as well as the strong electron-phonon coupling of the
phonon is due to the electric field across the planes induced by atoms with
different valence above and below. The magnitude of the electric field is
deduced from new Raman results on YBaCuO and
BiSr(CaY)CuO with different O and Y
doping, respectively. In the latter case it is shown that the symmetry breaking
by replacing Ca partially by Y enhances the coupling by an order of magnitude,
while the superconducting drops to about two third of its original value.Comment: 4 pages, 2 figures. This and other papers can be downloaded from
http://gwis2.circ.gwu.edu/~tp
Database management and analysis of fisheries in Illinois: Final report, 1 March 1999-28 February 2002
Issued May 2002; F-69-RReport issued on: May 200
Magnetic moments of W 5d in Ca2CrWO6 and Sr2CrWO6 double perovskites
We have investigated the magnetic moment of the W ion in the ferrimagnetic
double perovskites Sr2CrWO6 and Ca2CrWO6 by X-ray magnetic circular dichroism
(XMCD) at the W L(2,3) edges. In both compounds a finite negative spin and
positive orbital magnetic moment was detected. The experimental results are in
good agreement with band-structure calculations for (Sr/Ca)2CrWO6 using the
full-potential linear muffin-tin orbital method. It is remarkable, that the
magnetic ordering temperature, TC, is correlated with the magnetic moment at
the 'non-magnetic' W atom.Comment: accepted for publicatio
Nonthermal Melting of Néel Order in the Hubbard Model
Symmetry-broken states such as magnetic order and superconductivity are the hallmark of complex properties in solids. An intriguing new direction in condensed-matter physics is to manipulate these properties on the fastest possible time scales using ultrashort laser pulses. Theoretically, however, the collective many-particle dynamics that is responsible for the formation and melting of long-range order is associated with many open questions.Here, we combine two state-of-the-art numerical techniques—time-dependent density matrix renormalization group and nonequilibrium dynamical mean-field theory—to create a model system that represents interacting electrons on a bipartite lattice in which electrons can tunnel between sites. We prepare this model such that particles on neighboring sites initially align their magnetic moments in an antiparallel manner (i.e., representing antiferromagnetic order). The particles can then move between lattice sites, which leads to the melting of the magnetic order. We theoretically show that the precise movement mechanism depends strongly on the interaction between the particles: For strong interactions, the system behaves like a collection of localized magnetic moments. For weak interactions, on the other hand, the dynamics reflects the existence of coherent quasiparticles, which are typically restricted to excitations close to the ground state. In our case, these quasiparticles prevail on short times even though the system is strongly excited.Our setup, which is well suited for experiments using cold atoms, has the ability to reveal the crossover between localized and itinerant behavior. In the future, similar studies of systems with several active orbitals may make it possible to better understand how complex solids can relax into entirely new—and possibly thermodynamically hidden—phases
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