190 research outputs found
Theory of a Narrow roton Absorption Line in the Spectrum of a Disk-Shaped SHF Resonator
We calculate the probability of the birth of a circular phonon (c-phonon) in
He II by a c-photon of the resonator. It is shown that this probability has
sharp maxima at frequencies, where the effective group velocity of the c-phonon
is equal to zero; the density of states of c-phonons strongly grows at such
frequencies. For He II, these frequencies correspond to a roton and a maxon.
From the probability of the c-roton birth, we calculate the roto line width
which is found to approximately agree with the experimental one. We conclude
that the roton line observed in the super-high-frequency (SHF) absorption
spectrum of helium is related to the birth of c-rotons. A possible
interpretation of the Stark effect observed for the roton line is also
proposed.Comment: 13 pages, 1 figure, v2: journal variant, several minor correction
On the mutual polarization of two He-4 atoms
We propose a simple method based on the standard quantum-mechanical
perturbation theory to calculate the mutual polarization of two atoms He^4.Comment: 9 pages, 1 table; the article is revised and the calculation is
essentially refined; v4: final version, the Introduction is delete
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.
On the theory of Bose-condensate fluctuations in finite size systems
An asymptotic expansions for the grand partition function of ideal Bose gas
in the canonical ensemble with arbitrary number of particles is obtained. It is
shown that the expressions found are valid in the whole temperature region, the
critical temperature included. A comparison between the asymptotic formulas for
Bose-condensate fluctuations and the exact ones is carried out and their
quantitative agreement is established
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
Spin transfer and current-induced switching in antiferromagnets
We present theoretical description of the precessional switching processes
induced by simultaneous application of spin-polarized current and external
magnetic field to antiferromagnetic component of the "pinned" layer. We found
stability ranges of different static and dynamic regimes. We showed the
possibility of steady current-induced precession of antiferromagnetic vector
with frequency that linearly depends on the bias current. Furthermore, we found
an optimal duration of current pulse required for switching between different
orientations of antiferromagnetic vector and current and field dependence of
switching time. Our results reveal the difference between dynamics of ferro-
and antiferromagnets subjected to spin transfer torques.Comment: 7 pages, 4 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
Повышение информативности оценки содержания железнодорожного пути
[For the English abstract and full text of the article please see the attached PDF-File (English version follows Russian version)].ABSTRACT Using the methods of statistical analysis based on the theory of random process emissions, the theory of random point processes, the method of least squares, the method of least modules, filtering algorithms, the choice of a type of a theoretical function and its parameters that allow reducing the error of the measured characteristics of defects and deviations of the track from the design position, a mathematical model is formulated that provides an increase in the informative value of the assessment of the actual state of the railway track. High information value is needed primarily for formation of adequate control actions on the track in order to ensure traffic safety. Keywords: railway track, measured parameters, information-measuring system, defect detection, statistical analysis, theory of random processes, mathematical model.Текст аннотации на англ. языке и полный текст статьи на англ. языке находится в прилагаемом файле ПДФ (англ. версия следует после русской версии).С помощью методов статистического анализа, основанных на теории выбросов случайных процессов, теории случайных точечных процессов, методе наименьших квадратов, методе наименьших модулей, алгоритмах фильтрации, выборе вида теоретической функции и её параметров, позволяющих уменьшить ошибку измеряемых характеристик дефектов и отступлений пути от проектного положения, формулируется математическая модель, обеспечивающая повышение информативности оценки фактического состояния железнодорожного пути. Высокая информативность нужна прежде всего для формирования адекватных управляющих воздействий на путь с целью обеспечения безопасности движения
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
LARGE-SCALE RESEARCH FACILITIES «MID-LATITUDE GEOPHYSICAL OBSERVATION COMPLEX «MIKHNEVO»
Presented here are the descriptions of the Large-Scale Research Facilities (LSRF) «Mid-latitude complex of geophysical observations "Mikhnevo"» at the Institute of Geospheres Dynamics of the Russian Academy of Sciences. The composition of the measuring complex, equipment and applied techniques are provided. The paper also presents some of the scientific results obtained at the LSRF «Mikhnevo» in recent years
- …