259 research outputs found
Bond excitations in the pseudogap phase of the Hubbard Model
Using the dynamical cluster approximation, we calculate the correlation
functions associated with the nearest neighbor bond operator which measure the
z component of the spin exchange in the two-dimensional Hubbard model with
equal to the bandwidth. We find that in the pseudogap region, the local bond
susceptibility diverges at T=0. This shows the existence of degenerate bond
spin excitation and implies quantum criticality and bond order formation when
long range correlations are considered. The strong correlation between
excitations on parallel neighboring bonds suggests bond singlet dimerization.
The suppression of divergence for implies that tor these
model parameters this is quantum critical point which separates the
unconventional pseudogap region characterized by bond order from a conventional
Fermi liquid.Comment: 5 pages, 5 figure
Transition Temperature of a Magnetic Semiconductor with Angular Momentum j
We employ dynamical mean-field theory to identify the materials properties
that optimize Tc for a generalized double-exchange (DE) model. We reach the
surprising conclusion that Tc achieves a maximum when the band angular momentum
j equals 3/2 and when the masses in the 1/2 and 3/2 sub-bands are equal.
However, we also find that Tc is significantly reduced as the ratio of the
masses decreases from one. Consequently, the search for dilute magnetic
semiconductors (DMS) materials with high Tc should proceed on two fronts. In
semiconductors with p bands, such as the currently studied Mn-doped Ge and GaAs
semiconductors, Tc may be optimized by tuning the band masses through strain
engineering or artificial nanostructures. On the other hand, semiconductors
with s or d bands with nearly equal effective masses might prove to have higher
Tc's than p-band materials with disparate effective masses.Comment: 5 pages, 4 figure
A Maximum Entropy Method of Obtaining Thermodynamic Properties from Quantum Monte Carlo Simulations
We describe a novel method to obtain thermodynamic properties of quantum
systems using Baysian Inference -- Maximum Entropy techniques. The method is
applicable to energy values sampled at a discrete set of temperatures from
Quantum Monte Carlo Simulations. The internal energy and the specific heat of
the system are easily obtained as are errorbars on these quantities. The
entropy and the free energy are also obtainable. No assumptions as to the
specific functional form of the energy are made. The use of a priori
information, such as a sum rule on the entropy, is built into the method. As a
non-trivial example of the method, we obtain the specific heat of the
three-dimensional Periodic Anderson Model.Comment: 8 pages, 3 figure
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