2,560 research outputs found
Spin bipolaron in the framework of emery model for high-T(sub c) copper oxide superconductors
The high-T(sub c) oxide compounds discovered recently exhibit a number of interesting physical properties. Two-dimensional antiferromagnetic spin order has been observed in these materials at the oxygen deficiency. This fact can be explained by strong correlation of the spins, situated on Cu sites in the conducting planes of the oxide superconductors. The doping or the oxygen deficiency lead to the occurrence of holes, occupying the oxygen p-orbitals according to the Emery model. At the small hole concentration they can move along the antiferromagnetic lattice of spins, localized on Cu sites. Researchers consider the two holes situation and describe in what way their behavior depends on the antiferromagnetic exchange interation J. It is known that in the framework of Hubbard model with strong on-site Coulomb repulsion, a single hole can form a spin polaron of the large radius. It is reasonable to admit that two holes with parallel spins (triplet) form the spin bipolaron complex owing to the hole excitations' capability to polarize Cu spin surroundings. Such an excitation was considered in the phenomenological way. Here the problem is discussed on the basis of the microscopic approach in the framework of the variational principle. A special kind of wave function is used for such a purpose. The wave function is constructed by generalizing the trial functions proposed in over two holes excitation situation (triplet) and then the region of spin bipolaron existance in the framework of Emery model is studied. In this model the Hamiltonian can be easily rewritten by forming the oxygen states transforming as the irreducible representations of the group D(sub 4)
Ratchet effects in two-dimensional systems with a lateral periodic potential
Radiation-induced ratchet electric currents have been studied theoretically
in graphene with a periodic noncentrosymmetric lateral potential. The ratchet
current generated under normal incidence is shown to consist of two
contributions, one of them being polarization-independent and proportional to
the energy relaxation time, and another controlled solely by elastic scattering
processes and sensitive to both the linear and circular polarization of
radiation. Two realistic mechanisms of electron scattering in graphene are
considered. For short-range defects, the ratchet current is helicity-dependent
but independent of the direction of linear polarization. For the Coulomb
impurity scattering, the ratchet current is forbidden for the radiation
linearly polarized in the plane perpendicular to the lateral-potential
modulation direction. For comparison, the ratchet currents in a quantum well
with a lateral superlattice are calculated at low temperatures with allowance
for the dependence of the momentum relaxation time on the electron energy.Comment: 8 pages, 4 figure
New cantacaderid lace bugs from Dominician amber (Heteroptera: Tingidae, Catacaderiane)
New fossil tingids, representatives of the small subfamily Cantacaderinae (Tingidae) from the Oligocene Dominican amber, are described and discussed. The fossil species Eocader babyrussus n. sp. belongs to the recent neotropical genus Eocader of the tribe Phatnomini
Shot noise in superconducting junctions with weak link formed by Anderson impurity
A theory is developed to study shot noise in superconducting (SAS) and hybrid
(SAN) junctions with singly occupied Anderson impurity (A) as a weak link. The
zero-frequency DC component of the shot noise spectral density is calculated at
zero temperature as a function of the bias at different Coulomb repulsion
strengths U, and show a remarkable structure resulting from combination of
electron-electron interaction and Andreev reflections.Comment: 4 two column pages including 4 .eps figure
Electric field control of spin-orbit splittings in GaAs/AlGaAs coupled quantum wells
Electron spin dynamics is investigated in n-i-n GaAs/AlGaAs coupled quantum
wells. The electron spin dephasing time is measured as a function of an
external electrical bias under resonant excitation of the 1sHH intrawell
exciton using a time-resolved Kerr rotation technique. It is found a strong
electron spin dephasing time anisotropy caused by an interference of the
structure inversion asymmetry and the bulk inversion asymmetry. This anisotropy
is shown to be controlled by an electrical bias. A theoretical analysis of
electron spin dephasing time anisotropy is developed. The ratio of Rashba and
Dresselhaus spin splittings is studied as a function of applied bias.Comment: 4 pages, 3 figure
Deformed Wigner crystal in a one-dimensional quantum dot
The spatial Fourier spectrum of the electron density distribution in a finite
1D system and the distribution function of electrons over single-particle
states are studied in detail to show that there are two universal features in
their behavior, which characterize the electron ordering and the deformation of
Wigner crystal by boundaries. The distribution function has a -like
singularity at the Fermi momentum . The Fourier spectrum of the density
has a step-like form at the wavevector , with the harmonics being absent
or vanishing above this threshold. These features are found by calculations
using exact diagonalization method. They are shown to be caused by Wigner
ordering of electrons, affected by the boundaries. However the common Luttinger
liquid model with open boundaries fails to capture these features, because it
overestimates the deformation of the Wigner crystal. An improvement of the
Luttinger liquid model is proposed which allows one to describe the above
features correctly. It is based on the corrected form of the density operator
conserving the particle number.Comment: 10 pages, 11 figures. Misprints fixe
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