185 research outputs found
Pre-formed Cooper pairs and Bose-Einstein condensation in cuprate superconductors
A two-dimensional (2D) assembly of noninteracting, temperature-dependent,
pre-formed Cooper pairs in chemical/thermal equilibrium with unpaired fermions
is examined in a binary boson-fermion statistical model as the Bose-Einstein
condensation (BEC) singularity temperature is approached from above.
Compared with BCS theory (which is {\it not} a BEC theory) substantially higher
's are obtained without any adjustable parameters, that fall roughly
within the range of empirical 's for quasi-2D cuprate superconductors.Comment: 4 page
Nonlinear interference in a mean-field quantum model
Using similar nonlinear stationary mean-field models for Bose-Einstein
Condensation of cold atoms and interacting electrons in a Quantum Dot, we
propose to describe the original many-particle ground state as a one-particle
statistical mixed state of the nonlinear eigenstates whose weights are provided
by the eigenstate non-orthogonality. We search for physical grounds in the
interpretation of our two main results, namely, quantum-classical nonlinear
transition and interference between nonlinear eigenstates.Comment: RevTeX (pdfLaTeX), 7 pages with 5 png-figures include
The long-term fate of the hip arthrodesis: does it remain a valid procedure for selected cases in the 21st century?
Diquark Bose-Einstein condensation
Bose-Einstein condensation (BEC) of composite diquarks in quark matter (the
color superconductor phase) is discussed using the quasi-chemical equilibrium
theory at a relatively low density region near the deconfinement phase
transition, where dynamical quark-pair fluctuations are assumed to be described
as bosonic degrees of freedom (diquarks). A general formulation is given for
the diquark formation and particle-antiparticle pair-creation processes in the
relativistic flamework, and some interesting properties are shown, which are
characteristic for the relativistic many-body system. Behaviors of transition
temperature and phase diagram of the quark-diquark matter are generally
presented in model parameter space, and their asymptotic behaviors are also
discussed. As an application to the color superconductivity, the transition
temperatures and the quark and diquark density profiles are calculated in case
with constituent/current quarks, where the diquark is in bound/resonant state.
We obtained MeV for constituent quarks and MeV
for current quarks at a moderate density (). The method
is also developed to include interdiquark interactions into the quasi-chemical
equilibrium theory within a mean-field approximation, and it is found that a
possible repulsive diquark-diquark interaction lowers the transition
temperature by nearly 50%.Comment: 21 pages, 23 figure
Two-dimensional Bose-Einstein Condensation in Cuprate Superconductors
Transition temperatures calculated using the BCS model
electron-phonon interaction without any adjustable parameters agree with
empirical values for quasi-2D cuprate superconductors. They follow from a
two-dimensional gas of temperature-dependent Cooper pairs in chemical and
thermal equilibrium with unpaired fermions in a boson-fermion (BF) statistical
model as the Bose-Einstein condensation (BEC) singularity temperature is
approached from above. The {\it linear} (as opposed to quadratic) boson
dispersion relation due to the Fermi sea yields substantially higher 's
with the BF model than with BCS or pure-boson BEC theories.Comment: 7 pages including 2 figure
The BCS-Bose Crossover Theory
We contrast {\it four} distinct versions of the BCS-Bose statistical
crossover theory according to the form assumed for the electron-number equation
that accompanies the BCS gap equation. The four versions correspond to
explicitly accounting for two-hole-(2h) as well as two-electron-(2e) Cooper
pairs (CPs), or both in equal proportions, or only either kind. This follows
from a recent generalization of the Bose-Einstein condensation (GBEC)
statistical theory that includes not boson-boson interactions but rather 2e-
and also (without loss of generality) 2h-CPs interacting with unpaired
electrons and holes in a single-band model that is easily converted into a
two-band model. The GBEC theory is essentially an extension of the
Friedberg-T.D. Lee 1989 BEC theory of superconductors that excludes 2h-CPs. It
can thus recover, when the numbers of 2h- and 2e-CPs in both BE-condensed and
noncondensed states are separately equal, the BCS gap equation for all
temperatures and couplings as well as the zero-temperature BCS
(rigorous-upper-bound) condensation energy for all couplings. But ignoring
either 2h- {\it or} 2e-CPs it can do neither. In particular, only {\it half}
the BCS condensation energy is obtained in the two crossover versions ignoring
either kind of CPs. We show how critical temperatures from the original
BCS-Bose crossover theory in 2D require unphysically large couplings for the
Cooper/BCS model interaction to differ significantly from the s of
ordinary BCS theory (where the number equation is substituted by the assumption
that the chemical potential equals the Fermi energy).Comment: thirteen pages including two figures. Physica C (in press, 2007
Phenomenological theory of cuprate superconductivity
Reasonably good agreement with the superconducting transitiontemperatures of the cuprate high‐T c superconductors can be obtained on the basis of an approximate phenomenological theory. In this theory, two criteria are used to calculate the superconducting transitiontemperature. One is that the quantum wavelength is of the order of the electron‐pair spacing. The other is that a fraction of the normal carriers exist as Cooper pairs at T c . The resulting simple equation for T c contains only two parameters: the normal carrier density and effective mass. We calculate specific transition temperatures for 12 cuprate superconductors
Magnetic Properties of a Bose-Einstein Condensate
Three hyperfine states of Bose-condensed sodium atoms, recently optically
trapped, can be described as a spin-1 Bose gas. We study the behaviour of this
system in a magnetic field, and construct the phase diagram, where the
temperature of the Bose condensation increases with magnetic field.
In particular the system is ferromagnetic below and the magnetization
is proportional to the condensate fraction in a vanishing magnetic field.
Second derivatives of the magnetisation with regard to temperature or magnetic
field are discontinuous along the phase boundary.Comment: 5 pages, 5 figures included, to appear in Phys. Rev.
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