521 research outputs found
Formation of correlations and energy-conservation at short time scales
The formation of correlations due to collisions in an interacting nucleonic
system is investigated. Results from one-time kinetic equations are compared
with the Kadanoff and Baym two-time equation with collisions included in Born
approximation. A reasonable agreement is found for a proposed approximation of
the memory effects by a finite duration of collisions. This form of collision
integral is in agreement with intuitive estimates from Fermi's golden rule. The
formation of correlations and the build up time is calculated analytically for
the high temperature and the low temperature limit. Different approximate
expressions are compared with the numerical results. We present analytically
the time dependent interaction energy and the formation time for Gau\ss{}- and
Yukawa type of potentials.Comment: Europ. Lournal Physics A accepte
Phase diagram for interacting Bose gases
We propose a new form of the inversion method in terms of a selfenergy
expansion to access the phase diagram of the Bose-Einstein transition. The
dependence of the critical temperature on the interaction parameter is
calculated. This is discussed with the help of a new condition for
Bose-Einstein condensation in interacting systems which follows from the pole
of the T-matrix in the same way as from the divergence of the medium-dependent
scattering length. A many-body approximation consisting of screened ladder
diagrams is proposed which describes the Monte Carlo data more appropriately.
The specific results are that a non-selfconsistent T-matrix leads to a linear
coefficient in leading order of 4.7, the screened ladder approximation to 2.3,
and the selfconsistent T-matrix due to the effective mass to a coefficient of
1.3 close to the Monte Carlo data
Nonlinear relaxation field in charged systems under high electric fields
The influence of an external electric field on the current in charged systems
is investigated. The results from the classical hierarchy of density matrices
are compared with the results from the quantum kinetic theory. The kinetic
theory yields a systematic treatment of the nonlinear current beyond linear
response. To this end the dynamically screened and field-dependent
Lenard-Balescu equation is integrated analytically and the nonlinear relaxation
field is calculated. The classical linear response result known as Debye -
Onsager relaxation effect is only obtained if asymmetric screening is assumed.
Considering the kinetic equation of one specie the other species have to be
screened dynamically while the screening with the same specie itself has to be
performed statically. Different other approximations are discussed and
compared.Comment: language correction
The Nonlinear Debye-Onsager Relaxation Effect in Weakly Ionized Plasmas
A weakly ionized plasma under the influence of a strong electric field is
considered. Supposing a local Maxwellian distribution for the electron momenta
the plasma is described by hydrodynamic equations for the pair distribution
functions. These equations are solved and the relaxation field is calculated
for an arbitrary field strength. It is found that the relaxation effect becomes
lower with increasing strength of the electrical field.Comment: 4 pages, 1 figur
The concept of correlated density and its application
The correlated density appears in many physical systems ranging from dense
interacting gases up to Fermi liquids which develop a coherent state at low
temperatures, the superconductivity. One consequence of the correlated density
is the Bernoulli potential in superconductors which compensates forces from
dielectric currents. This Bernoulli potential allows to access material
parameters. Though within the surface potential these contributions are largely
canceled, the bulk measurements with NMR can access this potential. Recent
experiments are explained and new ones suggested. The underlying quantum
statistical theory in nonequilibrium is the nonlocal kinetic theory developed
earlier.Comment: 14 pages, CMT30 proceeding
Surface energy and magneto-capacitance of superconductors under electric field bias
A superconducting layer exposed to a perpendicular electric field and a
parallel magnetic field is considered within the Ginzburg-Landau (GL) approach.
The GL equation is solved near the surface and the surface energy is
calculated. The nucleation critical field of superconducting state at the
surface depends on the magnetic and electric fields. Special consideration is
paid to the induced magnetic-field effect cause d by diamagnetic surface
currents. The latter effect is strongly dependent on the thickness of the
sample. The effective inverse capacitance determines the effective penetration
depth. It is found that the capacitance exhibits a jump at the surface critical
field. An experiment is suggested for determining the change in the effective
capacitance of the layer
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