1,682 research outputs found
The Doped Two Chain Hubbard Model
The properties of the two-chain Hubbard Model doped away from half-filling
are investigated. The charge gap is found to vanish, but a finite spin gap
exists over a range of interchain hopping strength . In this range,
there are modified --like pairing correlations whose strength is
correlated with the size of the spin gap. It is found that the pair field
correlations are enhanced by the onsite Coulomb interaction U.Comment: 10 pages and 5 postscript figures, RevTeX 3.0, UCI-CMTHE-94-0
Study of the charge correlation function in one-dimensional Hubbard heterostructures
We study inhomogeneous one-dimensional Hubbard systems using the density
matrix renormalization group method. Different heterostructures are
investigated whose configuration is modeled varying parameters like the on-site
Coulomb potential and introducing local confining potentials. We investigate
their Luttinger liquid properties through the parameter K_rho, which
characterizes the decay of the density-density correlation function at large
distances. Our main goal is the investigation of possible realization of
engineered materials and the ability to manipulate physical properties by
choosing an appropriate spatial and/or chemical modulation.Comment: 6 pages, 7 figure
Float zone experiments in space
The molten zone/freezing crystal interface system and all the mechanisms were examined. If Marangoni convection produces oscillatory flows in the float zone of semiconductor materials, such as silicon, then it is unlikely that superior quality crystals can be grown in space using this process. The major goals were: (1) to determine the conditions for the onset of Marangoni flows in molten tin, a model system for low Prandtl number molten semiconductor materials; (2) to determine whether the flows can be suppressed by a thin oxide layer; and (3) based on experimental and mathematical analysis, to predict whether oscillatory flows will occur in the float zone silicon geometry in space, and if so, could it be suppressed by thin oxide or nitride films. Techniques were developed to analyze molten tin surfaces in a UHV system in a disk float zone geometry to minimize buoyancy flows. The critical Marangoni number for onset of oscillatory flows was determined to be greater than 4300 on atomically clean molten tin surfaces
The Density Matrix Renormalization Group applied to single-particle Quantum Mechanics
A simplified version of White's Density Matrix Renormalization Group (DMRG)
algorithm has been used to find the ground state of the free particle on a
tight-binding lattice. We generalize this algorithm to treat the tight-binding
particle in an arbitrary potential and to find excited states. We thereby solve
a discretized version of the single-particle Schr\"odinger equation, which we
can then take to the continuum limit. This allows us to obtain very accurate
results for the lowest energy levels of the quantum harmonic oscillator,
anharmonic oscillator and double-well potential. We compare the DMRG results
thus obtained with those achieved by other methods.Comment: REVTEX file, 21 pages, 3 Tables, 4 eps Figure
Condensation of magnons and spinons in a frustrated ladder
Motivated by the ever-increasing experimental effort devoted to the
properties of frustrated quantum magnets in a magnetic field, we present a
careful and detailed theoretical analysis of a one-dimensional version of this
problem, a frustrated ladder with a magnetization plateau at m=1/2. We show
that even for purely isotropic Heisenberg interactions, the magnetization curve
exhibits a rather complex behavior that can be fully accounted for in terms of
simple elementary excitations. The introduction of anisotropic interactions
(e.g., Dzyaloshinskii-Moriya interactions) modifies significantly the picture
and reveals an essential difference between integer and fractional plateaux. In
particular, anisotropic interactions generically open a gap in the region
between the plateaux, but we show that this gap closes upon entering fractional
plateaux. All of these conclusions, based on analytical arguments, are
supported by extensive Density Matrix Renormalization Group calculations.Comment: 15 pages, 15 figures. minor changes in tex
State Estimation with Sets of Densities considering Stochastic and Systematic Errors
In practical applications, state estimation requires the consideration of stochastic and systematic errors. If both error types are present, an exact probabilistic description of the state estimate is not possible, so that common Bayesian estimators have to be questioned. This paper introduces a theoretical concept, which allows for incorporating unknown but bounded errors into a Bayesian inference scheme by utilizing sets of densities. In order to derive a tractable estimator, the Kalman filter is applied to ellipsoidal sets of means, which are used to bound additive systematic errors. Also, an extension to nonlinear system and observation models with ellipsoidal error bounds is presented. The derived estimator is motivated by means of two example applications
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