595 research outputs found
UV continuum emission and diagnostics of hydrogen-containing non-equilibrium plasmas
For the first time the emission of the radiative dissociation continuum of
the hydrogen molecule ( electronic
transition) is proposed to be used as a source of information for the
spectroscopic diagnostics of non-equilibrium plasmas. The detailed analysis of
excitation-deactivation kinetics, rate constants of various collisional and
radiative transitions and fitting procedures made it possible to develop two
new methods of diagnostics of: (1) the ground state
vibrational temperature from the relative intensity
distribution, and (2) the rate of electron impact dissociation
(d[\mbox{H_{2}}]/dt)_{\text{diss}} from the absolute intensity of the
continuum. A known method of determination of from relative
intensities of Fulcher- bands was seriously corrected and simplified
due to the revision of transition probabilities and cross sections of
electron impact excitation. General considerations are illustrated
with examples of experiments in pure hydrogen capillary-arc and H+Ar
microwave discharges.Comment: REVTeX, 25 pages + 12 figures + 9 tables. Phys. Rev. E, eprint
replaced because of resubmission to journal after referee's 2nd repor
Origin of the large thermoelectric power in oxygen-variable RBaCo_{2}O_{5+x} (R=Gd, Nd)
Thermoelectric properties of GdBaCo_{2}O_{5+x} and NdBaCo_{2}O_{5+x} single
crystals have been studied upon continuous doping of CoO_2 planes with either
electrons or holes. The thermoelectric response and the resistivity behavior
reveal a hopping character of the transport in both compounds, providing the
basis for understanding the recently found remarkable divergence of the Seebeck
coefficient at x=0.5. The doping dependence of the thermoelectric power evinces
that the configurational entropy of charge carriers, enhanced by their spin and
orbital degeneracy, plays a key role in the origin of the large thermoelectric
response in these correlated oxides.Comment: 5 pages, 4 figures, accepted for publication in PR
Magnetoresistance Anomalies in Antiferromagnetic YBa_2Cu_3O_{6+x}: Fingerprints of Charged Stripes
We report novel features in the in-plane magnetoresistance (MR) of heavily
underdoped YBa_2Cu_3O_{6+x}, which unveil a developed ``charged stripe''
structure in this system. One of the striking features is an anisotropy of the
MR with a "d-wave" symmetry upon rotating the magnetic field H within the ab
plane, which is caused by the rotation of the stripes with the external field.
With decreasing temperature, a hysteresis shows up below ~20 K in the MR curve
as a function of H and finally below 10 K the magnetic-field application
produces a persistent change in the resistivity. This "memory effect" is caused
by the freezing of the directionally-ordered stripes.Comment: 4 pages, 6 figures, final version, to appear in 4 October 1999 issue
of PR
Magnetic order in lightly doped La_{2-x}Sr_{x}CuO_{4}
We study long wavelength magnetic excitations in lightly doped
La_{2-x}Sr_{x}CuO_{4} (x < 0.03) detwinned crystals. The lowest energy magnetic
anisotropy induced gap can be understood in terms of the antisymmetric spin
interaction inside the antiferromagnetic (AF) phase. The second magnetic
resonace, analyzed in terms of in-plane spin anisotropy, shows unconventional
behavior within the AF state and led to the discovery of collective spin
excitations pertaining to a field induced magnetically ordered state. This
state persists in a 9 T field to more than 100 K above the N\'{e}el temperature
in x = 0.01.Comment: 5 pages, 5 figure
Magnetoresistance in Heavily Underdoped YBa_2Cu_3O_{6+x}: Antiferromagnetic Correlations and Normal-State Transport
We report on a contrasting behavior of the in-plane and out-of-plane
magnetoresistance (MR) in heavily underdoped antiferromagnetic (AF)
YBa_2Cu_3O_{6+x} (x<0.37). The out-of-plane MR (I//c) is positive over most of
the temperature range and shows a sharp increase, by about two orders of
magnitude, upon cooling through the Neel temperature T_N. A contribution
associated with the AF correlations is found to dominate the out-of-plane MR
behavior for H//c from far above T_N, pointing to the key role of spin
fluctuations in the out-of-plane transport. In contrast, the transverse
in-plane MR (I//a(b);H//c) appears to be small and smooth through T_N, implying
that the development of the AF order has little effect on the in-plane
resistivity.Comment: 4 pages, 5 figures, accepted for publication in Phys.Rev.Let
More on the Subtraction Algorithm
We go on in the program of investigating the removal of divergences of a
generical quantum gauge field theory, in the context of the Batalin-Vilkovisky
formalism. We extend to open gauge-algebrae a recently formulated algorithm,
based on redefinitions of the parameters of the
classical Lagrangian and canonical transformations, by generalizing a well-
known conjecture on the form of the divergent terms. We also show that it is
possible to reach a complete control on the effects of the subtraction
algorithm on the space of the gauge-fixing parameters. A
principal fiber bundle with a connection
is defined, such that the canonical transformations are gauge
transformations for . This provides an intuitive geometrical
description of the fact the on shell physical amplitudes cannot depend on
. A geometrical description of the effect of the subtraction
algorithm on the space of the physical parameters is
also proposed. At the end, the full subtraction algorithm can be described as a
series of diffeomorphisms on , orthogonal to
(under which the action transforms as a scalar), and gauge transformations on
. In this geometrical context, a suitable concept of predictivity is
formulated. We give some examples of (unphysical) toy models that satisfy this
requirement, though being neither power counting renormalizable, nor finite.Comment: LaTeX file, 37 pages, preprint SISSA/ISAS 90/94/E
Resistive Transition and Upper Critical Field in Underdoped YBa_2Cu_3O_{6+x} Single Crystals
A superconducting transition in the temperature dependence of the ab-plane
resistivity of underdoped YBa_2Cu_3O_{6+x} crystals in the range T_c<30 K has
been investigated. Unlike the case of samples with the optimal level of doping,
the transition width increased insignificantly with magnetic field, and in the
range T_c<13 K it decreased with increasing magnetic field. The transition
point T_c(B) was determined by analyzing the fluctuation conductivity. The
curves of B_{c2}(T) measured in the region T/T_c>0.1 did not show a tendency to
saturation and had a positive second derivative everywhere, including the
immediate neighborhood of T_c. The only difference among the curves of
B_{c2}(T) for different crystal states is the scales of T and B, so they can be
described in terms of a universal function, which fairly closely follows
Alexandrov's model of boson superconductivity.Comment: 10 Revtex pages, 6 figures, uses psfig.st
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