954 research outputs found
True Dielectric and Ideal Conductor in Theory of the Dielectric Function for Coulomb System
On the basis of the exact relations the general formula for the static
dielectric permittivity e(q,0) for Coulomb system is found in the region of
small wave vectors q. The obtained formuladescribes the dielectric function
e(q,0) of the Coulomb system in both states in the "metallic" state and in the
"dielectric" one. The parameter which determines possible states of the Coulomb
system - from the "true" dielectric till the "ideal" conductor is found. The
exact relation for the pair correlation function for two-component system of
electrons and nuclei g_ei(r) is found for the arbitrary thermodynamic
parameters.Comment: 5 pages, no figure
Angular Schmidt Modes in Spontaneous Parametric Down-Conversion
We report a proof-of-principle experiment demonstrating that appropriately
chosen set of Hermite-Gaussian modes constitutes a Schmidt decomposition for
transverse momentum states of biphotons generated in the process of spontaneous
parametric down conversion. We experimentally realize projective measurements
in Schmidt basis and observe correlations between appropriate pairs of modes.
We perform tomographical state reconstruction in the Schmidt basis, by direct
measurement of single-photon density matrix eigenvalues.Comment: 5 pages, 4 figure
Competition of multiband superconducting and magnetic order in ErNi2B2C observed by Andreev reflection
Point contacts (PC) Andreev reflection dV/dI spectra for the
antiferromagnetic (T_N =6K) superconductor (Tc=11K) ErNi2B2C have been measured
for the two main crystallographic directions. Observed retention of the Andreev
reflection minima in dV/dI up to Tc directly points to unusual superconducting
order parameter (OP) vanishing at Tc. Temperature dependence of OP was obtained
from dV/dI using recent theory of Andreev reflection including pair-breaking
effect. For the first time existence of a two superconducting OPs in ErNi2B2C
is shown. A distinct decrease of both OPs as temperature is lowered below T_N
is observed.Comment: 5 pages, 5 figs, to be published in Europhys. Let
Identity of electrons and ionization equilibrium
It is perhaps appropriate that, in a year marking the 90th anniversary of
Meghnad Saha seminal paper (1920), new developments should call fresh attention
to the problem of ionization equilibrium in gases. Ionization equilibrium is
considered in the simplest "physical" model for an electronic subsystem of
matter in a rarefied state, consisting of one localized electronic state in
each nucleus and delocalized electronic states considered as free ones. It is
shown that, despite the qualitative agreement, there is a significant
quantitative difference from the results of applying the Saha formula to the
degree of ionization. This is caused by the fact that the Saha formula
corresponds to the "chemical" model of matter.Comment: 9 pages, 2 figure
Observation of anisotropic effect of antiferromagnetic ordering on the superconducting gap in ErNi2B2C
The point-contact (PC) spectra of the Andreev reflection dV/dI curves of the
superconducting rare-earth nickel borocarbide ErNi2B2C (Tc=11 K) have been
analyzed in the "one-gap" and "two-gap" approximations using the generalized
Blonder-Tinkham-Klapwijk (GBTK) model and the Beloborod'ko (BB) model allowing
for the pair-breaking effect of magnetic impurities. Experimental and
calculated curves have been compared not only in shape, but in magnitude as
well, which provide more reliable data for determining the temperature
dependence of the energy gap (or superconducting order parameter) \Delta(T).
The anisotropic effect of antiferromagnetic ordering at T_N =6 K on the
superconducting gap/order parameter has been determined: as the temperature is
lowered, \Delta(T) decreases by 25% in the c-direction and only by 4% in the
ab-plane. It is found that the pair-breaking parameter increases in the
vicinity of the magnetic transitions, the increase being more pronounced in the
c-direction. The efficiency of the models was tested for providing \Delta(T)
data for ErNi2B2C from Andreev reflection spectra.Comment: 16 two column pages, 20 figs., will be published in Fiz. Nizk. Temp.
N10, 2010; V2: added - "Acknowledgement" & "Note added in proof
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