2,624 research outputs found
Excitations and phase segregation in a two component Bose-Einstein condensate
Bogoliubov-de Gennes (BdG) equations and the excitation spectrum of a
two-component Bose-Einstein condensate (BEC) are derived with an arbitrary
interaction between bosons, including long-range and short range forces. The
nonconverting BEC mixture segregates into two phases for some two-body
interactions. Gross-Pitaevskii (GP) equations are solved for the phase
segregated BEC. A possibility of boundary-surface and other localised
excitations is studied.Comment: 9 pages, 2 figure
Enhanced stability of bound pairs at nonzero lattice momenta
A two-body problem on the square lattice is analyzed. The interaction
potential consists of strong on-site repulsion and nearest-neighbor attraction.
Exact pairing conditions are derived for s-, p-, and d-symmetric bound states.
The pairing conditions are strong functions of the total pair momentum K. It is
found that the stability of pairs increases with K. At weak attraction, the
pairs do not form at the -point but stabilize at lattice momenta close
to the Brillouin zone boundary. The phase boundaries in the momentum space,
which separate stable and unstable pairs are calculated. It is found that the
pairs are formed easier along the direction than along the
direction. This might lead to the appearance of ``hot pairing
spots" on the Kx and Ky axes.Comment: 7 RevTEX pages, 5 figure
Isotope effects in high-Tc cuprate superconductors: Ultimate proof for bipolaron theory of superconductivity
Developing a theory of high-temperature superconductivity in copper oxides is
one of the outstanding problems in physics. Twenty-five years after its
discovery, no consensus on the microscopic theory has been reached despite
tremendous theoretical and experimental efforts. Attempts to understand this
problem are hindered by the subtle interplay among a few mechanisms and the
presence of several nearly degenerate and competing phases in these systems.
Here we provide unified parameter-free explanation of the observed
oxygen-isotope effects on the critical temperature, the magnetic-field
penetration depth, and on the normal-state pseudogap for underdoped cuprate
superconductors within the framework of the bipolaron theory compatible with
the strong Coulomb and Froehlich interactions, and with many other independent
observations in these highly polarizable doped insulators. Remarkably, we also
quantitatively explain measured critical temperatures and magnitudes of the
magnetic-field penetration depth. The present work thus represents an ultimate
proof of the bipolaron theory of high-temperature superconductivity, which
takes into account essential Coulomb and electron-phonon interactions.Comment: 8 pages, 2 figure
Nonnegative/binary matrix factorization with a D-Wave quantum annealer
D-Wave quantum annealers represent a novel computational architecture and
have attracted significant interest, but have been used for few real-world
computations. Machine learning has been identified as an area where quantum
annealing may be useful. Here, we show that the D-Wave 2X can be effectively
used as part of an unsupervised machine learning method. This method can be
used to analyze large datasets. The D-Wave only limits the number of features
that can be extracted from the dataset. We apply this method to learn the
features from a set of facial images
The "normal" state of superconducting cuprates might really be normal after all
High magnetic field studies of cuprate superconductors revealed a non-BCS
temperature dependence of the upper critical field determined
resistively by several groups.
These determinations caused some doubts on the grounds of both the
contrasting effect of the magnetic field on the in-plane and out-of-plane
resistances reported for large Bi2212 sample and the large Nernst signal
\emph{well above} .
Here we present both and of tiny Bi2212 crystals
in magnetic fields up to 50 Tesla.
None of our measurements revealed a situation when on the field increase
reaches its maximum while remains very small if not zero.
The resistive %upper critical fields estimated from the in-plane and
out-of-plane estimated from and are
approximately the same. Our results support any theory of cuprates that
describes the state above the resistive phase transition as perfectly normal
with a zero off-diagonal order parameter. In particular, the anomalous Nernst
effect above the resistive phase transition in high- cuprates can be
described quantitatively as a normal state phenomenon in a model with itinerant
and localised fermions and/or charged bosons
Vortex matter in the charged Bose liquid at absolute zero
The Gross-Pitaevskii-type equation is solved for the charge Bose liquid in
the external magnetic field at zero temperature. There is a vortex lattice with
locally broken charge neutrality. The boson density is modulated in real space
and each vortex is charged. Remarkably, there is no upper critical field at
zero temperature, so the density of single flux-quantum vortices monotonously
increases with the magnetic field up to B=infinity and no indication of a phase
transition. The size of each vortex core decreases as about 1/sqrt(B) keeping
the system globally charge neutral. If bosons are composed of two fermions, a
phase transition to a spin-polarized Fermi liquid at some magnetic field larger
than the pair-breaking field is predicted.Comment: 4 pages, 4 figures, references update
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