156 research outputs found
On orientational relief of inter-molecular potential and the structure of domain walls in fullerite C60
A simple planar model for an orientational ordering of threefold molecules on
a triangular lattice modelling a close-packed (111) plane of fullerite is
considered. The system has 3-sublattice ordered ground state which includes 3
different molecular orientations. There exist 6 kinds of orientational domains,
which are related with a permutation or a mirror symmetry. Interdomain walls
are found to be rather narrow.
The model molecules have two-well orientational potential profiles, which are
slightly effected by a presence of a straight domain wall. The reason is a
stronger correlation between neighbour molecules in triangular lattice versus
previously considered square lattice
A considerable reduction (up to one order) of orientational interwell
potential barrier is found in the core regions of essentially two-dimentional
potential defects, such as a three-domain boundary or a kink in the domain
wall. For ultimately uncorrelated nearest neighbours the height of the
interwell barrier can be reduced even by a factor of 100.Comment: 11 pages, 13 figures, LaTeX, to appear in Low Temperature Physic
Temperature-doping phase diagram of layered superconductors
The superconducting properties of a layered system are analyzed for the cases
of zero- and non-zero angular momentum of the pairs. The effective
thermodynamic potential for the quasi-2D XY-model for the gradients of the
phase of the order parameter is derived from the microscopic superconducting
Hamiltonian. The dependence of the superconducting critical temperature T_c on
doping, or carrier density, is studied at different values of coupling and
inter-layer hopping. It is shown that the critical temperature T_c of the
layered system can be lower than the critical temperature of the
two-dimensional Berezinskii-Kosterlitz-Thouless transition T_BKT at some values
of the model parameters, contrary to the case when the parameters of the
XY-model do not depend on the microscopic Hamiltonian parameters.Comment: To be published in Phys. Rev.
Pseudogap phase formation in the crossover from Bose-Einstein condensation to BCS superconductivity
A phase diagram for a 2D metal with variable carrier density has been
derived. It consists of a normal phase, where the order parameter is absent; a
so-called ``abnormal normal'' phase where this parameter is also absent but the
mean number of composite bosons (bound pairs) exceeds the mean number of free
fermions; a pseudogap phase where the absolute value of the order parameter
gradually increases but its phase is a random value, and finally a
superconducting (here Berezinskii-Kosterlitz-Thouless) phase. The
characteristic transition temperatures between these phases are found. The
chemical potential and paramagnetic susceptibility behavior as functions of the
fermion density and the temperature are also studied. An attempt is made to
qualitatively compare the resulting phase diagram with the features of
underdoped high- superconducting compounds above their critical
temperature.Comment: 26 pages, revtex, 5 EMTeX figures; more discussion and references
added; to be published in JET
Superconductivity in La(1.56)Sr(0.44)CuO(4)/La(2)CuO(4) superlattices
Superlattices of the repeated structure La(1.56)Sr(0.44)CuO(4)/La(2)CuO(4)
(LSCO-LCO), where none of the constituents is superconducting, show a
superconducting transition of T_c \simeq 25 K. In order to elucidate the nature
of the superconducting state we have performed a low-energy muSR study. By
applying a magnetic field parallel (Meissner state) and perpendicular (vortex
state) to the film planes, we could show that superconductivity is sheet like,
resulting in a very anisotropic superconducting state. This result is
consistent with a simple charge-transfer model, which takes into account the
layered structure and the difference in the chemical potential between LCO and
LSCO, as well as Sr interdiffusion. Using a pancake-vortex model we could
estimate a strict upper limit of the London penetration depth to 380 nm in
these superlattices. The temperature dependence of the muon depolarization rate
in field cooling experiments is very similar to what is observed in
intercalated BSCCO and suggests that vortex-vortex interaction is dominated by
electromagnetic coupling but negligible Josephson interaction.Comment: 4 pages, 3 figure
BCS-Bose Crossover in Color Superconductivity
It is shown that the onset of the color superconducting phase occurs in the
BCS-BE crossover region.Comment: 5 pages, LaTeX, references adde
Evidences for Tsallis non-extensivity on CMR manganites
We found, from the analysis of vs. curves of some manganese oxides
(manganites), that these systems do not follow the traditional
Maxwell-Boltzmann statistics, but the Tsallis statistics, within the
\QTR{em}{normalized} formalism. Curves were calculated within the mean field
approximation, for various ferromagnetic samples and the results were compared
to measurements of our own and to various other authors published data, chosen
at random from the literature. The agreement between the experimental data and
calculated vs. curve, where is an effective
temperature, is excellent for all the compounds. The entropic parameter, ,
correlates in a simple way with the experimental value of , irrespect
the chemical composition of the compounds, heat treatment or other details on
sample preparation. Examples include (superextensivity),
(extensivity) and (subextensivity) cases.Comment: 12 pages, 3 figure
Pseudogap from ARPES experiment: three gaps in cuprates and topological superconductivity
A term first coined by Mott back in 1968 a `pseudogap' is the depletion of
the electronic density of states at the Fermi level, and pseudogaps have been
observed in many systems. However, since the discovery of the high temperature
superconductors (HTSC) in 1986, the central role attributed to the pseudogap in
these systems has meant that by many researchers now associate the term
pseudogap exclusively with the HTSC phenomenon. Recently, the problem has got a
lot of new attention with the rediscovery of two distinct energy scales
(`two-gap scenario') and charge density waves patterns in the cuprates. Despite
many excellent reviews on the pseudogap phenomenon in HTSC, published from its
very discovery up to now, the mechanism of the pseudogap and its relation to
superconductivity are still open questions. The present review represents a
contribution dealing with the pseudogap, focusing on results from angle
resolved photoemission spectroscopy (ARPES) and ends up with the conclusion
that the pseudogap in cuprates is a complex phenomenon which includes at least
three different `intertwined' orders: spin and charge density waves and
preformed pairs, which appears in different parts of the phase diagram. The
density waves in cuprates are competing to superconductivity for the electronic
states but, on the other hand, should drive the electronic structure to
vicinity of Lifshitz transition, that could be a key similarity between the
superconducting cuprates and iron based superconductors. One may also note that
since the pseudogap in cuprates has multiple origins there is no need to recoin
the term suggested by Mott.Comment: invited review, more info at http://www.imp.kiev.ua/~kor
On the Theory of the Pseudogap Formation in 2D Attracting Fermion Systems
Two-dimensional system of the fermions with the indirect Einstein
phonon-exchange attraction and added local four-fermion interaction is
considered. It is shown that in such a system at resulting attraction between
particles a new nonsuperconducting phase arises along with the normal and
superconducting phases. In this, called "abnormal normal", or pseudogap, phase
the absolute value of the order parameter is finite but its phase is a random
quantity. It is important that the new phase really exists at low carrier
density only, i.e. it shrinks with doping increasing in the case of phonon
attraction. The relevance of the results for high-temperature superconductors
is speculated.
Key words: 2D metal, arbitrary carrier density, normal phase, abnormal normal
phase, pseudogap, suderconducting phase, Berezinskii-Kosterlitz-Thouless phase,
electron-electron and electron-hole pairingComment: 19 pages, 2 figures (emtex
Математическое моделирование участков переменной жёсткости перед искусственными сооружениями
For the English abstract and full text of the article please see the attached PDF-File (English version follows Russian version).ABSTRACT The article deals with the features of the transition zone from the ballast under-sleeper base to the bridge structure with various types of span structures, as well as the sections of the ballastless track, conjugated with the transient zone. An analytical model is proposed for describing the dynamic behavior of a railway track in the form of a transversely isotropic plate with variable rigidity parameters. Examples of the use of the proposed model for calculating the dynamic depression of a roadbed under the influence of a rolling stock with different freight and speed characteristics are given. Keywords: railway, bridge, roadbed, residual deformation, variable rigidity section, track depression, slope, elastic wave, track profile, transversal-isotropic plate.Текст аннотации на англ. языке и полный текст статьи на англ. языке находится в прилагаемом файле ПДФ (англ. версия следует после русской версии).В статье рассматриваются особенности переходной зоны с балластного подшпального основания на мостовое сооружение с различными типами пролётных строений, а также сопряжённые с зоной участки безбалластного пути. Предложена аналитическая модель для описания динамического поведения железнодорожного пути в виде трансверсально-изотропной пластины с переменными параметрами жёсткости. Приведены примеры использования предложенной модели для вычисления динамической осадки земляного полотна под воздействием подвижного состава с разными грузовыми и скоростными характеристиками
Pseudogap and Superconducting Fluctuation in High-Tc Cuprates: Theory beyond 1-loop Approximation
The pseudogap phenomena induced by the SC fluctuation are investigated in
details. We perform a calculation beyond the 1-loop approximation. The SC
fluctuation is microscopically derived on the basis of the repulsive Hubbard
model. The vertex corrections are collected in the infinite order with use of
the quasi-static approximation. The single-particle excitations, NMR 1/T_{1}T,
spin susceptibility and superconducting transition temperature are discussed.
The important role of the vertex correction is pointed out for the single
particle spectral function. On the other hand, the validity of the 1-loop order
theory is confirmed for other quantities. We shed light on the essential nature
of SC fluctuation leading to the pseudogap from the comparison with spin and
charge fluctuations
- …