1,431 research outputs found
Absence of a Slater Transition in The Two-Dimensional Hubbard Model
We present well-controlled results on the metal to insulator transition (MIT)
within the paramagnetic solution of the dynamical cluster approximation (DCA)
in the two-dimensional Hubbard model at half-filling. In the strong coupling
regime, a local picture describes the properties of the model; there is a large
charge gap . In the weak-coupling regime, we find a symbiosis
of short-range antiferromagnetic correlations and moment formation cause a gap
to open at finite temperature as in one dimension. Hence, this excludes the
mechanism of the MIT proposed by Slater long ago.Comment: 4 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
Spectral changes in layered -electron systems induced by Kondo hole substitution in the boundary-layer
We investigate the effect of disorder on the dynamical spectrum of layered
-electron systems. With random dilution of -sites in a single Kondo
insulating layer, we explore the range and extent to which Kondo hole
incoherence can penetrate into adjacent layers. We consider three cases of
neighboring layers: band insulator, Kondo insulator and simple metal. The
disorder-induced spectral weight transfer, used here for quantification of the
proximity effect, decays algebraically with distance from the boundary layer.
Further, we show that the spectral weight transfer is highly dependent on the
frequency range considered as well as the presence of interactions in the clean
adjacent layers. The changes in the low frequency spectrum are very similar
when the adjacent layers are either metallic or Kondo insulating, and hence are
independent of interactions. In stark contrast, a distinct picture emerges for
the spectral weight transfers across large energy scales. The spectral weight
transfer over all energy scales is much higher when the adjacent layers are
non-interacting as compared to when they are strongly interacting Kondo
insulators. Thus, over all scales, interactions screen the disorder effects
significantly. We discuss the possibility of a crossover from non-Fermi liquid
to Fermi liquid behavior upon increasing the ratio of clean to disordered
layers in particle-hole asymmetric systems.Comment: 14 pages, 9 figure
The Structure of the Pairing Interaction in the 2D Hubbard Model
Dynamic cluster Monte Carlo calculations for the doped two-dimensional
Hubbard model are used to study the irreducible particle-particle vertex
responsible for pairing in this model. This vertex increases with
increasing momentum transfer and decreases when the energy transfer exceeds a
scale associated with the spin susceptibility. Using an exact
decomposition of this vertex into a fully irreducible two-fermion vertex and
charge and magnetic exchange channels, the dominant part of the effective
pairing interaction is found to come from the magnetic, spin S=1 exchange
channel.Comment: Published version. 4 pages, 4 figure
Gap States in Dilute Magnetic Alloy Superconductors
We study states in the superconducting gap induced by magnetic impurities
using self-consistent quantum Monte Carlo with maximum entropy and formally
exact analytic continuation methods. The magnetic impurity susceptibility has
different characteristics for T_{0} \alt T_{c0} and T_{0} \agt T_{c0}
(: Kondo temperature, : superconducting transition temperature)
due to the crossover between a doublet and a singlet ground state. We
systematically study the location and the weight of the gap states and the gap
parameter as a function of and the concentration of the
impurities.Comment: 4 pages in ReVTeX including 4 encapsulated Postscript figure
Dual-fermion approach to the Anderson-Hubbard model
We apply the recently developed dual fermion algorithm for disordered
interacting systems to the Anderson-Hubbard model. This algorithm is compared
with dynamical cluster approximation calculations for a one-dimensional system
to establish the quality of the approximation in comparison with an established
cluster method. We continue with a three-dimensional (3d) system and look at
the antiferromagnetic, Mott and Anderson localization transitions. The dual
fermion approach leads to quantitative as well as qualitative improvement of
the dynamical mean-field results and it allows one to calculate the hysteresis
in the double occupancy in 3d taking into account nonlocal correlations
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