704 research outputs found
Cumulant approach to weakly doped antiferromagnets
We present a new approach to static and dynamical properties of holes and
spins in weakly doped antiferromagnets in two dimensions. The calculations are
based on a recently introduced cumulant approach to ground--state properties of
correlated electronic systems. The present method allows to evaluate hole and
spin--wave dispersion relations by considering hole or spin excitations of the
ground state. Usually, these dispersions are found from time--dependent
correlation functions. To demonstrate the ability of the approach we first
derive the dispersion relation for the lowest single hole excitation at
half--filling. However, the main purpose of this paper is to focus on the
mutual influence of mobile holes and spin waves in the weakly doped system. It
is shown that low-energy spin excitations strongly admix to the ground--state.
The coupling of spin waves and holes leads to a strong suppression of the
staggered magnetization which can not be explained by a simple rigid--band
picture for the hole quasiparticles. Also the experimentally observed doping
dependence of the spin--wave excitation energies can be understood within our
formalism.Comment: REVTEX, 25 pages, 7 figures (EPS), to be published in Phys. Rev.
Stochastic framework for evaluating the effect of displaced antenna elements on DOA estimation
We establish a statistical framework for investigating the influence of correlated random displacements of antenna elements in a uniform circular antenna array (UCA) on the distribution of direction-of-arrival (DOA) estimates. More specifically, we apply a stochastic collocation method formodeling the sparse UCA root-MUSIC-DOA estimates as polynomial expansions of the random displacements. Compared to Monte-Carlo simulations, this approach yields a speedup of about 40 for the case of a displacement of two antenna elements
Transfer Entropy and Cumulant Based Cost as Measures of Nonlinear Causal Relationships in Space Plasmas: Applications to D St.
It is well known that the magnetospheric response to the solar wind is nonlinear. Information theoretical tools such as mutual information, transfer entropy, and cumulant based analysis are able to characterize the nonlinearities in the system. Using cumulant based cost, we show that nonlinear significance of D st peaks at 3–12 hours lags that can be attributed to VBs which also exhibit similar behavior. However, the nonlinear significance that peaks at lags 25, 50, and 90 hours can be attributed to internal dynamics, which may be related to the relaxation of the ring current. These peaks are absent in the linear and nonlinear self-significance of VBs . Our analysis with mutual information and transfer entropy show that both methods can establish that there are a strong correlation and transfer of information from V sw to D st at a time scale that is consistent with that obtained from the cumulant based analysis. However, mutual information also shows that there is a strong correlation in the backward direction, from D st to V sw , which is counterintuitive. In contrast, transfer entropy shows that there is no or little transfer of information from D st to V sw , as expected because it is the solar wind that drives the magnetosphere, not the other way around. Our case study demonstrates that these information theoretical tools are quite useful for space physics studies because these tools can uncover nonlinear dynamics that cannot be seen with the traditional analyses and models that assume linear relationships
Replica Symmetry Breaking in Short-Range Spin Glasses: Theoretical Foundations and Numerical Evidences
We discuss replica symmetry breaking (RSB) in spin glasses. We update work in
this area, from both the analytical and numerical points of view. We give
particular attention to the difficulties stressed by Newman and Stein
concerning the problem of constructing pure states in spin glass systems. We
mainly discuss what happens in finite-dimensional, realistic spin glasses.
Together with a detailed review of some of the most important features, facts,
data, and phenomena, we present some new theoretical ideas and numerical
results. We discuss among others the basic idea of the RSB theory, correlation
functions, interfaces, overlaps, pure states, random field, and the dynamical
approach. We present new numerical results for the behaviors of coupled
replicas and about the numerical verification of sum rules, and we review some
of the available numerical results that we consider of larger importance (for
example, the determination of the phase transition point, the correlation
functions, the window overlaps, and the dynamical behavior of the system).Comment: 48 pages, 21 figures. v2: the published versio
Van der Waals interaction between flux lines in High-T_c Superconductors
In anisotropic or layered superconductors thermal fluctuations as well as
impurities induce a van der Waals (vdW) attraction between flux lines, as has
recently been shown by Blatter and Geshkenbein in the thermal case [Phys. Rev.
Lett. 77, 4958 (1996)] and by Mukherji and Nattermann in the disorder dominated
case [Phys. Rev. Lett. 79, 139 (1997)]. This attraction together with the
entropic or disorder induced repulsion has interesting consequences for the low
field phase diagram. We present two derivations of the vdW attraction, one of
which is based on an intuitive picture, the other one following from a
systematic expansion of the free energy of two interacting flux lines. Both the
thermal and the disorder dominated case are considered. In the thermal case in
the absence of disorder, we use scaling arguments as well as a functional
renormalization of the vortex-vortex interaction energy to calculate the
effective Gibbs free energy on the scale of the mean flux line distance. We
discuss the resulting low field phase diagram and make quantitative predictions
for pure BiSCCO (Bi_2-Sr_2-CaCu_2-O_8). In the case with impurities, the Gibbs
free energy is calculated on the basis of scaling arguments, allowing for a
semi-quantitative discussion of the low-field, low-temperature phase diagram in
the presence of impurities.Comment: 19 pages EPJ style, 9 PostScript figures. Minor additions to the
first submission. Accepted for publication in EPJ
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