31 research outputs found
Mechanism of collisionless sound damping in dilute Bose gas with condensate
We develop a microscopic theory of sound damping due to Landau mechanism in
dilute gas with Bose condensate. It is based on the coupled evolution equations
of the parameters describing the system. These equations have been derived in
earlier works within a microscopic approach which employs the
Peletminskii-Yatsenko reduced description method for quantum many-particle
systems and Bogoliubov model for a weakly nonideal Bose gas with a separated
condensate. The dispersion equations for sound oscillations were obtained by
linearization of the mentioned evolution equations in the collisionless
approximation. They were analyzed both analytically and numerically. The
expressions for sound speed and decrement rate were obtained in high and low
temperature limiting cases. We have shown that at low temperature the
dependence of the obtained quantities on temperature significantly differs from
those obtained by other authors in the semi-phenomenological approaches.
Possible effects connected with non-analytic temperature dependence of
dispersion characteristics of the system were also indicated.Comment: 17 pages, 7 figure
Coexistence of photonic and atomic Bose-Einstein condensates in ideal atomic gases
We have studied conditions of photon Bose-Einstein condensate formation that
is in thermodynamic equilibrium with ideal gas of two-level Bose atoms below
the degeneracy temperature. Equations describing thermodynamic equilibrium in
the system were formulated; critical temperatures and densities of photonic and
atomic gas subsystems were obtained analytically. Coexistence conditions of
these photonic and atomic Bose-Einstein condensates were found. There was
predicted the possibility of an abrupt type of photon condensation in the
presence of Bose condensate of ground-state atoms: it was shown that the
slightest decrease of the temperature could cause a significant gathering of
photons in the condensate. This case could be treated as a simple model of the
situation known as "stopped light" in cold atomic gas. We also showed how
population inversion of atomic levels can be created by lowering the
temperature. The latter situation looks promising for light accumulation in
atomic vapor at very low temperatures.Comment: 18 pages, 3 figure
On the spatially periodic ordering in the system of electrons above the surface of liquid helium in an external electric field
A theory of equilibrium states of electrons above a liquid helium surface in
the presence of an external clamping field is built based on the first
principles of quantum statistics for the system of many identical
Fermi-particles. The approach is based on the variation principle modified for
the considered system and on Thomas-Fermi model. In terms of the developed
theory we obtain the self-consistency equations that connect the parameters of
the system description, i.e., the potential of a static electric field, the
distribution function of electrons and the surface profile of a liquid
dielectric. The equations are used to study the phase transition of the system
to a spatially periodic state. To demonstrate the capabilities of the proposed
method, the characteristics of the phase transition of the system to a
spatially periodic state of a trough type are analyzed.Comment: 11 pages, 2 figures, 1 tabl
Zero sound in a quantum gas of spin-3/2 atoms with multipole exchange interaction
In the context of quantum gases, we obtain a many-body Hamiltonian for
spin-3/2 atoms with general multipole (spin, quadrupole, and octupole) exchange
interaction by employing the apparatus of irreducible spherical tensor
operators. This Hamiltonian implies the finite-range interaction, whereas, for
zero-range (contact) potentials parameterized by the -wave scattering
length, the multipole exchange interaction becomes irrelevant. Following the
reduced description method for quantum systems, we derive the quantum kinetic
equation for spin-3/2 atoms in a magnetic field and apply it to examine the
high-frequency oscillations known as zero sound.Comment: 21 pages, 2 figure
Broken-axisymmetry state and magnetic state diagram of spin-1 condensate through the prism of quadrupole degrees of freedom
We theoretically study a weakly interacting gas of spin-1 atoms with
Bose-Einstein condensate in external magnetic field within the Bogoliubov
approach. To this end, in contrast to previous studies, we employ the general
Hamiltonian, which includes both spin and quadrupole exchange interactions as
well as the couplings of the spin and quadrupole moment with the external
magnetic field (the linear and quadratic Zeeman terms). The latter is
responsible for the emergence of the broken-axisymmetry state. We also
re-examine ferromagnetic, quadrupolar, and paramagnetic states employing the
proposed Hamiltonian. For all magnetic states, we find the relevant
thermodynamic characteristics such as magnetization, quadrupole moment,
thermodynamic potential, as well as excitation energies for broken-axisymmetry
state. We show that the broken-axisymmetry state can be prepared at three
different regimes of applied magnetic field. We also present the magnetic state
diagrams for each regime of realizing the broken-axisymmetry state.Comment: 14 pages, 2 figures, 4 table