107 research outputs found
Contemporary Methods of Treating Children with Autoimmune Nephric Diseases
The review is dedicated to the analysis of contemporary therapies and efficacy thereof for treating children autoimmune nephric diseases. The authors describe peculiarities of using the conventional therapy in children and discuss alternative treatments using cyclosporine, tacrolimus, budesonide and ursodeoxycholic acid, as well as the need in using the second-line drugs for treating patients with resistant autoimmune nephric diseases. The review touches upon the promising approaches to the treatment of this category of patients
Effective action for Superconductors and BCS-Bose crossover
A standard perturbative expansion around the mean-field solution is used to
derive the low-energy effective action for superconductors at T=0. Taking into
account the density fluctuations at the outset we get the effective action
where the density is the conjugated momentum to the phase of
the order parameter. In the hydrodynamic regime, the dynamics of the
superconductor is described by a time dependent non-linear Schr\"odinger
equation (TDNLS) for the field . The
evolution of the density fluctuations in the crossover from weak-coupling (BCS)
to strong-coupling (Bose condensation of localized pairs) superconductivity is
discussed for the attractive Hubbard model. In the bosonic limit, the TDNLS
equation reduces to the the Gross-Pitaevskii equation for the order parameter,
as in the standard description of superfluidity. The conditions under which a
phase-only action can be derived in the presence of a long-range interaction to
describe the physics of the superconductivity of ``bad metals'' are discussed.Comment: 13 pages, accepted for publication on Phys. Rev.
Современные методы лечения детей с аутоиммунными заболеваниями печени
The review is dedicated to the analysis of contemporary therapies and efficacy thereof for treating children autoimmune nephric diseases. The authors describe peculiarities of using the conventional therapy in children and discuss alternative treatments using cyclosporine, tacrolimus, budesonide and ursodeoxycholic acid, as well as the need in using the second-line drugs for treating patients with resistant autoimmune nephric diseases. The review touches upon the promising approaches to the treatment of this category of patients. В обзоре проанализированы современные схемы терапии и их эффективность у детей с аутоиммунными заболеваниями печени. Описаны особенности применения стандартного режима терапии у детей, рассмотрены альтернативные схемы лечения с использованием циклоспорина, такролимуса, будесонида и урсодезоксихолевой кислоты, а также необходимость применения препаратов второй линии для лечения больных с резистентными случаями аутоиммунных заболеваний печени. Освещены перспективные направления лечения этой категории больных.
Real space inhomogeneities in high temperature superconductors: the perspective of two-component model
The two-component model of high temperature superconductors in its real space
version has been solved using Bogoliubov-de Gennes equations. The disorder in
the electron and boson subsystem has been taken into account. It strongly
modifies the superconducting properties and leads to local variations of the
gap parameter and density of states. The assumption that the impurities mainly
modify boson energies offers natural explanation of the puzzling positive
correlation between the positions of impurities and the values of the order
parameter found in the scanning tunnelling microscopy experiments.Comment: 19 pages, IOPP style include
Berezinskii-Kosterlitz-Thouless transition and BCS-Bose crossover in the two-dimensional attractive Hubbard model
We study the two-dimensional attractive Hubbard model using the mapping onto
the half-filled repulsive Hubbard model in a uniform magnetic field coupled to
the fermion spins. The low-energy effective action for charge and pairing
fluctuations is obtained in the hydrodynamic regime. We recover the action of a
Bose superfluid where half the fermion density is identified as the conjugate
variable of the phase of the superconducting order parameter. By integrating
out charge fluctuations, we obtain a phase-only action. In the zero-temperature
superconducting state, this action describes a collective phase mode smoothly
evolving from the Anderson-Bogoliubov mode at weak coupling to the Bogoliubov
mode of a Bose superfluid at strong coupling. At finite temperature, the
phase-only action can be used to extract an effective XY model and thus obtain
the Berezinskii-Kosterlitz-Thouless (BKT) phase transition temperature. We also
identify a renormalized classical regime of superconducting fluctuations above
the BKT phase transition, and a regime of incoherent pairs at higher
temperature. Special care is devoted to the nearly half-filled case where the
symmetry of the order parameter is enlarged to SO(3) due to strong charge fluctuations. The low-energy effective action is then an
SO(3) non-linear sigma model with a (symmetry breaking) magnetic field
proportional to the doping. In the strong-coupling limit, the attractive
Hubbard model can be mapped onto the Heisenberg model, from which we recover
the Gross-Pitaevskii equation in the low-density limit.Comment: 31 pages, 12 figures, RevTex4; (v2) changes following referees'
comments, references adde
The Phonon Drag Effect in Single-Walled Carbon Nanotubes
A variational solution of the coupled electron-phonon Boltzmann equations is
used to calculate the phonon drag contribution to the thermopower in a 1-D
system. A simple formula is derived for the temperature dependence of the
phonon drag in metallic, single-walled carbon nanotubes. Scattering between
different electronic bands yields nonzero values for the phonon drag as the
Fermi level varies.Comment: 8 pages, 4 figure
Superfluid transition temperature in a trapped gas of Fermi atoms with a Feshbach resonance
We investigate strong coupling effects on the superfluid phase transition in
a gas of Fermi atoms with a Feshbach resonance. The Feshbach resonance
describes a composite quasi-Boson, which can give rise to an additional pairing
interaction between the Fermi atoms. This attractive interaction becomes
stronger as the threshold energy of the Feshbach resonance two-particle bound
state is lowered. In a recent paper, we showed that in the uniform Fermi gas,
this tunable pairing interaction naturally leads to a BCS-BEC crossover of the
Nozi`eres and Schmitt-Rink kind, in which the BCS-type superfluid phase
transition continuously changes into the BEC-type as the threshold energy is
decreased. In this paper, we extend our previous work by including the effect
of a harmonic trap potential, treated within the local density approximation
(LDA). We also give results for both weak and strong coupling to the Feshbach
resonance. We show that the BCS-BEC crossover phenomenon strongly modifies the
shape of the atomic density profile at the superfluid phase transition
temperature Tc, reflecting the change of the dominant particles going from
Fermi atoms to composite Bosons. In the BEC regime, these composite Bosons are
shown to first appear well above Tc. We also discuss the "phase diagram" above
Tc as a function of the tunable threshold energy. We introduce a characteristic
temperature T* describing the effective crossover in the normal phase from a
Fermi gas of atoms to a gas of stable molecules.Comment: 43 pages, 13 figures (submitted to PRA
Non-linear feedback effects in coupled Boson-Fermion systems
We address ourselves to a class of systems composed of two coupled subsystems
without any intra-subsystem interaction: itinerant Fermions and localized
Bosons on a lattice. Switching on an interaction between the two subsystems
leads to feedback effects which result in a rich dynamical structure in both of
them. Such feedback features are studied on the basis of the flow equation
technique - an infinite series of infinitesimal unitary transformations - which
leads to a gradual elimination of the inter-subsystem interaction. As a result
the two subsystems get decoupled but their renormalized kinetic energies become
mutually dependent on each other. Choosing for the inter - subsystem
interaction a charge exchange term (the Boson-Fermion model) the initially
localized Bosons acquire itinerancy through their dependence on the
renormalized Fermion dispersion. This latter evolves from a free particle
dispersion into one showing a pseudogap structure near the chemical potential.
Upon lowering the temperature both subsystems simultaneously enter a
macroscopic coherent quantum state. The Bosons become superfluid, exhibiting a
soundwave like dispersion while the Fermions develop a true gap in their
dispersion. The essential physical features described by this technique are
already contained in the renormalization of the kinetic terms in the respective
Hamiltonians of the two subsystems. The extra interaction terms resulting in
the process of iteration only strengthen this physics. We compare the results
with previous calculations based on selfconsistent perturbative approaches.Comment: 14 pages, 16 figures, accepted for publication in Phys. Rev.
Pi excitation of the t-J model
In this paper, we present analytical and numerical calculations of the pi
resonance in the t-J model. We show in detail how the pi resonance in the
particle-particle channel couples to and appears in the dynamical spin
correlation function in a superconducting state. The contribution of the pi
resonance to the spin excitation spectrum can be estimated from general
model-independent sum rules, and it agrees with our detailed calculations. The
results are in overall agreement with the exact diagonalization studies of the
t-J model. Earlier calculations predicted the correct doping dependence of the
neutron resonance peak in the YBCO superconductor, and in this paper detailed
energy and momentum dependence of the spin correlation function is presented.
The microscopic equations of motion obtained within current formalism agree
with that of the SO(5) nonlinear sigma model, where the pi resonance is
interpreted as a pseudo Goldstone mode of the spontaneous SO(5) symmetry
breaking.Comment: 33 pages, LATEX, 14 eps fig
Aharonov-Bohm spectral features and coherence lengths in carbon nanotubes
The electronic properties of carbon nanotubes are investigated in the
presence of disorder and a magnetic field parallel or perpendicular to the
nanotube axis. In the parallel field geometry, the -periodic
metal-insulator transition (MIT) induced in metallic or semiconducting
nanotubes is shown to be related to a chirality-dependent shifting of the
energy of the van Hove singularities (VHSs). The effect of disorder on this
magnetic field-related mechanism is considered with a discussion of mean free
paths, localization lengths and magnetic dephasing rate in the context of
recent experiments.Comment: 22 pages, 6 Postscript figures. submitted to Phys. Rev.
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