217 research outputs found
Finite size effects for the gap in the excitation spectrum of the one-dimensional Hubbard model
We study finite size effects for the gap of the quasiparticle excitation
spectrum in the weakly interacting regime one-dimensional Hubbard model with
on-site attraction. Two type of corrections to the result of the thermodynamic
limit are obtained. Aside from a power law (conformal) correction due to
gapless excitations which behaves as , where is the number of
lattice sites, we obtain corrections related to the existence of gapped
excitations. First of all, there is an exponential correction which in the
weakly interacting regime () behaves as in the extreme limit of ,
where is the hopping amplitude, is the on-site energy, and
is the gap in the thermodynamic limit. Second, in a finite
size system a spin-flip producing unpaired fermions leads to the appearance of
solitons with non-zero momenta, which provides an extra (non-exponential)
contribution . For moderate but still large values of
, these corrections significantly increase and may
become comparable with the conformal correction. Moreover, in the case
of weak interactions where , the exponential correction
exceeds higher order power law corrections in a wide range of parameters,
namely for , and so does
even in a wider range of . For sufficiently small number of particles,
which can be of the order of thousands in the weakly interacting regime, the
gap is fully dominated by finite size effects.Comment: 17 pages, 5 figure
Vortex structures in rotating Bose-Einstein condensates
We present an analytical solution for the vortex lattice in a rapidly
rotating trapped Bose-Einstein condensate (BEC) in the lowest Landau level and
discuss deviations from the Thomas-Fermi density profile. This solution is
exact in the limit of a large number of vortices and is obtained for the cases
of circularly symmetric and narrow channel geometries. The latter is realized
when the trapping frequencies in the plane perpendicular to the rotation axis
are different from each other and the rotation frequency is equal to the
smallest of them. This leads to the cancelation of the trapping potential in
the direction of the weaker confinement and makes the system infinitely
elongated in this direction. For this case we calculate the phase diagram as a
function of the interaction strength and rotation frequency and identify the
order of quantum phase transitions between the states with a different number
of vortex rows.Comment: 17 pages, 12 figures, with addition
Subgap tunneling through channels of polarons and bipolarons in chain conductors
We suggest a theory of internal coherent tunneling in the pseudogap region
where the applied voltage is below the free electron gap. We consider quasi 1D
systems where the gap is originated by a lattice dimerization like in
polyacethylene, as well as low symmetry 1D semiconductors. Results may be
applied to several types of conjugated polymers, to semiconducting nanotubes
and to quantum wires of semiconductors. The approach may be generalized to
tunneling in strongly correlated systems showing the pseudogap effect, like the
family of High Tc materials in the undoped limit. We demonstrate the evolution
of tunneling current-voltage characteristics from smearing the free electron
gap down to threshold for tunneling of polarons and further down to the region
of bi-electronic tunneling via bipolarons or kink pairs.Comment: 14 pages, 8 postscript figure
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Electrochemical surface treatment of glass by a lead-copper melt
Electrochemical surface treatment of a glass by lead-copper melts was investigated and the contents of the anode elements in the glass were determined. It was shown that the glass colour changes as a function of the treatment time. The variation in the content of lead and copper in the glass was established also as function of the treatment duration. Based on the data obtained a technology for the production of new materials from glass can be developed. The surface of these materials is coloured by the ions of lead and copper
Initial Stage of Star Formation
Abstract In frame of the hydrodynamics wh irlwind mechanis m of b ipolar outflow formation fro m disk system the data of observations of active starburst region a dense molecular cloud of Orion KL in H 2 O maser rad iation is analyzed. It is shown that the theory as a first appro ximation exp lains the basic observant laws of active region, structure accompanying protostar formation: a disk-a b ipolar outflo w, includ ing its mechanis m ejection and self-collimation, excitation maser radiat ion
Analytical stripe phase solution for the Hubbard model
The self-consistent solution for the spin-charge solitonic superstructure in
quasi-one-dimensional electron system is obtained in the framework of the
Hubbard model as a function of a hole doping. Effects of interchain
interactions on the ground state are discussed. Results are used for the
interpretation of the observed stripe phases in doped antiferromagnets.Comment: 9 pages, LaTex file, no figure
Many-body localization transition in a frustrated XY chain
We study many-body localization (MBL) transition in a one-dimensional
isotropic XY chain with a weak next-nearest-neighbor frustration in a random
magnetic field. We perform finite-size exact diagonalization calculations of
level-spacing statistics and fractal dimensions to demonstrate the MBL
transition with increasing the random field amplitude. An equivalent
representation of the model in terms of spinless fermions explains the presence
of the delocalized phase by the appearance of an effective non-local
interaction between the fermions. This interaction appears due to frustration
provided by the next-nearest-neighbor hopping
Project-based method in teaching foreign language for specific purposes
This paper considers the innovative methods of teaching Foreign Language for Specific Purposes. The methods are intensively implemented in teaching 3–4-year students at the Institute of Natural Resources, Tomsk Polytechnic University (TPU), Russia. In particular, the paper presents the international project “Approaching the Arctic” developed in cooperation with NTNU (Norwegian University of Science and Technology), Norway. This paper analyzes the efficiency of the implementation of the project-based method in teaching English for Specific Purposes
Exact solution of new integrable nineteen-vertex models and quantum spin-1 chains
New exactly solvable nineteen vertex models and related quantum spin-1 chains
are solved. Partition functions, excitation energies, correlation lengths, and
critical exponents are calculated. It is argued that one of the non-critical
Hamiltonians is a realization of an integrable Haldane system. The finite-size
spectra of the critical Hamiltonians deviate in their structure from standard
predictions by conformal invariance.Comment: 16 pages, to appear in Z. Phys. B, preprint Cologne-94-474
Radiation Generation with an Existing Demonstrator of an Energy Recovery Continuous Wave Superconducting RF Accelerator
Over the past decades, many accelerator laboratories have put much effort into the development of compact energy recovery linac ERL demonstrators to verify various physical and technical aspects of the generation, acceleration, transport and energy recovery of high brightness and high average current electron beams in a superconducting radio frequency SRF linear accelerator. Beyond these goals, the ERL demonstrator also offers unique opportunities to study novel schemes for THz and X ray radiation generation. In this paper, we discuss feasible options for schemes generating THz and X ray radiation at low energy continuous wave CW SRF ERL demonstrators such as the bERLinPro accelerato
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