52 research outputs found
Energy versus information based estimations of dissipation using a pair of magnetic colloidal particles
Using the framework of stochastic thermodynamics, we present an experimental
study of a doublet of magnetic colloidal particles which is manipulated by a
time-dependent magnetic field. Due to hydrodynamic interactions, each bead
experiences a state-dependent friction, which we characterize using a
hydrodynamic model. In this work, we compare two estimates of the dissipation
in this system: the first one is energy based since it relies on the measured
interaction potential, while the second one is information based since it uses
only the information content of the trajectories. While the latter only offers
a lower bound of the former, we find it to be simple to implement and of
general applicability to more complex systems.Comment: Main text: 5 pages, 4 figures. Supplementary material: 5 pages, 5
figure
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Task-Irrelevant Novel Sounds have Antithetical Effects on Visual Target Processing in Young and Old Adults
In young adults, primary visual task processing can be either enhanced or disrupted by novel auditory stimuli preceding target events, depending on task demands. Little is known about this phenomenon in older individuals, who, in general, are more susceptible to distraction. In the current study, age-related differences in the electrophysiological effects of task-irrelevant auditory stimuli on visual target processing were examined. Under both low and high primary task loads, the categorization/updating process in response to visual targets preceded by auditory novels, as indexed by the target P3 component, was enhanced in young, but diminished in old adults. In both age groups, the alerting/orienting response to novel auditory stimuli, as measured by the P3a, was smaller under high task load, whereas redirecting attention to the visual task after a novel auditory event, as indexed by the reorienting negativity (RON), tended to be augmented under high task load. Old subjects generated a smaller P3a and RON. We conclude that task irrelevant novel auditory stimuli have the opposite effect on the processing of visual targets in young and old adults. This finding may help explain age-related increases in the disruption of primary task activity by irrelevant, but salient auditory events
Electronic and magnetic properties of metallic phases under coexisting short-range interaction and diagonal disorder
We study a three-dimensional Anderson-Hubbard model under the coexistence of
short-range interaction and diagonal disorder within the Hartree-Fock
approximation. We show that the density of states at the Fermi energy is
suppressed in the metallic phases near the metal-insulator transition as a
proximity effect of the soft Hubbard gap in the insulating phases. The
transition to the insulator is characterized by a vanishing DOS in contrast to
formation of a quasiparticle peak at the Fermi energy obtained by the dynamical
mean field theory in pure systems. Furthermore, we show that there exist frozen
spin moments in the paramagnetic metal.Comment: 4 pages, 2 figures, published versio
The absence of finite-temperature phase transitions in low-dimensional many-body models: a survey and new results
After a brief discussion of the Bogoliubov inequality and possible
generalizations thereof, we present a complete review of results concerning the
Mermin-Wagner theorem for various many-body systems, geometries and order
parameters. We extend the method to cover magnetic phase transitions in the
periodic Anderson Model as well as certain superconducting pairing mechanisms
for Hubbard films. The relevance of the Mermin-Wagner theorem to approximations
in many-body physics is discussed on a conceptual level.Comment: 33 pages; accepted for publication as a Topical Review in Journal of
Physics: Condensed Matte
Magnetic Correlations in the Two Dimensional Anderson-Hubbard Model
The two dimensional Hubbard model in the presence of diagonal and
off-diagonal disorder is studied at half filling with a finite temperature
quantum Monte Carlo method. Magnetic correlations as well as the electronic
compressibility are calculated to determine the behavior of local magnetic
moments, the stability of antiferromagnetic long range order (AFLRO), and
properties of the disordered phase. The existence of random potentials
(diagonal or ``site'' disorder) leads to a suppression of local magnetic
moments which eventually destroys AFLRO. Randomness in the hopping elements
(off-diagonal disorder), on the other hand, does not significantly reduce the
density of local magnetic moments. For this type of disorder, at half-filling,
there is no ``sign-problem'' in the simulations as long as the hopping is
restricted between neighbor sites on a bipartite lattice. This allows the study
of sufficiently large lattices and low temperatures to perform a finite-size
scaling analysis. For off-diagonal disorder AFLRO is eventually destroyed when
the fluctuations of antiferromagnetic exchange couplings exceed a critical
value. The disordered phase close to the transition appears to be
incompressible and shows an increase of the uniform susceptibility at low
temperatures.Comment: 10 pages, REVTeX, 14 figures included using psfig.st
Insulating phases of the infinite-dimensional Hubbard model
A theory is developed for the T=0 Mott-Hubbard insulating phases of the
infinite-dimensional Hubbard model at half-filling, including both the
antiferromagnetic (AF) and paramagnetic (P) insulators. Local moments are
introduced explicitly from the outset, enabling ready identification of the
dominant low energy scales for insulating spin- flip excitations. Dynamical
coupling of single-particle processes to the spin-flip excitations leads to a
renormalized self-consistent description of the single-particle propagators
that is shown to be asymptotically exact in strong coupling, for both the AF
and P phases. For the AF case, the resultant theory is applicable over the
entire U-range, and is discussed in some detail. For the P phase, we consider
in particular the destruction of the Mott insulator, the resultant critical
behaviour of which is found to stem inherently from proper inclusion of the
spin-flip excitations.Comment: 13 pages Revtex, 12 postscript figure
Disorder-enhanced delocalization and local-moment quenching in a disordered antiferromagnet
The interplay of disorder and spin-fluctuation effects in a disordered
antiferromagnet is studied. In the weak-disorder regime (W \le U), while the
energy gap decreases rapidly with disorder, the sublattice magnetization,
including quantum corrections, is found to remain essentially unchanged in the
strong correlation limit. Magnon energies and Neel temperature are enhanced by
disorder in this limit. A single paradigm of disorder-enhanced delocalization
qualitatively accounts for all these weak disorder effects. Vertex corrections
and magnon damping, which appear only at order (W/U)^4, are also studied. With
increasing disorder a crossover is found at W \sim U, characterized by a rapid
decrease in sublattice magnetization due to quenching of local moments, and
formation of spin vacancies. The latter suggests a spin-dilution behavior,
which is indeed observed in softened magnon modes, lowering of Neel
temperature, and enhanced transverse spin fluctuations.Comment: 12 pages, includes 8 postscript figures. To appear in Physical Review
B. References adde
Constrained-path quantum Monte Carlo simulations of the zero-temperature disordered two-dimensional Hubbard model
We study the effects of disorder on long-range antiferromagnetic correlations
in the half-filled, two dimensional, repulsive Hubbard model at T=0. A mean
field approach is first employed to gain a qualitative picture of the physics
and to guide our choice for a trial wave function in a constrained path quantum
Monte Carlo (CPQMC) method that allows for a more accurate treatment of
correlations. Within the mean field calculation, we observe both Anderson and
Mott insulating antiferromagnetic phases. There are transitions to a paramagnet
only for relatively weak coupling, U < 2t in the case of bond disorder, and U <
4t in the case of on-site disorder. Using ground-state CPQMC we demonstrate
that this mean field approach significantly overestimates magnetic order. For
U=4t, we find a critical bond disorder of Vc = (1.6 +- 0.4)t even though within
mean field theory no paramagnetic phase is found for this value of the
interaction. In the site disordered case, we find a critical disorder of Vc =
(5.0 +- 0.5)t at U=4t.Comment: Revtex, 13 pages, 15 figures. Minor changes to title and abstract,
discussion and references added, figures 5, 6, 8, 9 replaced with easier to
read version
Disorder and Impurities in Hubbard-Antiferromagnets
We study the influence of disorder and randomly distributed impurities on the
properties of correlated antiferromagnets. To this end the Hubbard model with
(i) random potentials, (ii) random hopping elements, and (iii) randomly
distributed values of interaction is treated using quantum Monte Carlo and
dynamical mean-field theory. In cases (i) and (iii) weak disorder can lead to
an enhancement of antiferromagnetic (AF) order: in case (i) by a
disorder-induced delocalization, in case (iii) by binding of free carriers at
the impurities. For strong disorder or large impurity concentration
antiferromagnetism is eventually destroyed. Random hopping leaves the local
moment stable but AF order is suppressed by local singlet formation. Random
potentials induce impurity states within the charge gap until it eventually
closes. Impurities with weak interaction values shift the Hubbard gap to a
density off half-filling. In both cases an antiferromagnetic phase without
charge gap is observed.Comment: 16 pages, 9 figures, latex using vieweg.sty (enclosed); typos
corrected, references updated; to appear in "Advances in Solid State
Physics", Vol. 3
Two-Particle-Self-Consistent Approach for the Hubbard Model
Even at weak to intermediate coupling, the Hubbard model poses a formidable
challenge. In two dimensions in particular, standard methods such as the Random
Phase Approximation are no longer valid since they predict a finite temperature
antiferromagnetic phase transition prohibited by the Mermin-Wagner theorem. The
Two-Particle-Self-Consistent (TPSC) approach satisfies that theorem as well as
particle conservation, the Pauli principle, the local moment and local charge
sum rules. The self-energy formula does not assume a Migdal theorem. There is
consistency between one- and two-particle quantities. Internal accuracy checks
allow one to test the limits of validity of TPSC. Here I present a pedagogical
review of TPSC along with a short summary of existing results and two case
studies: a) the opening of a pseudogap in two dimensions when the correlation
length is larger than the thermal de Broglie wavelength, and b) the conditions
for the appearance of d-wave superconductivity in the two-dimensional Hubbard
model.Comment: Chapter in "Theoretical methods for Strongly Correlated Systems",
Edited by A. Avella and F. Mancini, Springer Verlag, (2011) 55 pages.
Misprint in Eq.(23) corrected (thanks D. Bergeron
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