382 research outputs found
Steps in the Negative-Differential-Conductivity Regime of a Superconductor
Current-voltage characteristics were measured in the mixed state of
Y1Ba2Cu3O(7-delta) superconducting films in the regime where flux flow becomes
unstable and the differential conductivity dj/dE becomes negative. Under
conditions where its negative slope is steep, the j(E) curve develops a
pronounced staircase like pattern. We attribute the steps in j(E) to the
formation of a dynamical phase consisting of the succesive nucleation of
quantized distortions in the local vortex velocity and flux distribution within
the moving flux matter.Comment: 5 pages, 3 figure
Nucleation of Stable Superconductivity in YBCO-Films
By means of the linear dynamic conductivity, inductively measured on
epitaxial films between 30mHz and 30 MHz, the transition line to
generic superconductivity is studied in fields between B=0 and 19T. It follows
closely the melting line described recently in terms of a blowout of
thermal vortex loops in clean materials. The critical exponents of the
correlation length and time near , however, seem to be dominated by
some intrinsic disorder. Columnar defects produced by heavy-ion irradiation up
to field-equivalent-doses of lead to a disappointing reduction
of while for the generic line of the pristine film
is recovered. These novel results are also discussed in terms of a loop-driven
destruction of generic superconductivity.Comment: 11 pages including 7 EPS figures, accepted for publication in the
Proceedings of the Spring Meeting of the German Physical Society, Muenster
1999,Festkoerperprobleme/Advances in Solid State Physics 199
Possibility of the new type phase transition
The scalar field theory and the scalar electrodynamics quantized in the flat
gap are considered. The dynamical effects arising due to the boundary presence
with two types of boundary conditions (BC) satisfied by scalar fields are
studied. It is shown that while the Neumann BC lead to the usual scalar field
mass generation, the Dirichlet BC give rise to the dynamical mechanism of
spontaneous symmetry breaking. Due to the later, there arises the possibility
of the new type phase transition from the normal to spontaneously broken phase.
The decreasing in the characteristic size of the quantization region (the gap
size here) and increasing in the temperature compete with each other, tending
to transport the system in the spontaneously broken and in the normal phase,
respectively. The system evolves with a combined parameter, simultaneously
reflecting the change in temperature and in the size. As a result, at the
critical value of this parameter there occurs the phase transition from the
normal phase to the spontaneously broken one. In particular, the usual massless
scalar electrodynamics transforms to the Higgs model
Investigation of the kinetics of sulphuric acid leaching of zinc from calamine
This article aims at the research of kinetics of the sulphuric acid leaching of zinc from calamine (hemimorphite) of Shaimerden deposits. The ratio of zinc extraction from calamine to water-soluble zinc sulphate was determined at various leaching durations and its temperatures. The concentration of the sulfuric acid solution, the flow rate of this solution and the size of the calamine particles, selected in the course of this work for leaching zinc from this mineral with the specified solution, made it possible to establish the value of the “apparent” activation energy of the reaction of calamine with sulfuric acid, amounting to 3,075 kJ / mol
Observation of the parallel-magnetic-field-induced superconductor-insulator transition in thin amorphous InO films
We study the response of a thin superconducting amorphous InO film with
variable oxygen content to a parallel magnetic field. A field-induced
superconductor-insulator transition (SIT) is observed that is very similar to
the one in normal magnetic fields. As the boson-vortex duality, which is the
key-stone of the theory of the field-induced SIT, is obviously absent in the
parallel configuration, we have to draw conclusion about the theory
insufficiency.Comment: 3 pages, 4 figure
Transport and Magnetic Properties of R1-xAxCoO3 (R=La, Pr and Nd; A=Ba, Sr and Ca)
Transport and magnetic measurements have been carried out on perovskite
Co-oxides R1-xAxCoO3 (R=La, Pr, and Nd; A=Ba, Sr and Ca; 0<x<0.5: All sets of
the R and A species except Nd1-xBaxCoO3 have been studied.). With increasing
the Sr- or Ba-concentration x, the system becomes metallic ferromagnet with
rather large magnetic moments. For R=Pr and Nd and A=Ca, the system approaches
the metal- insulator phase boundary but does not become metallic. The magnetic
moments of the Ca-doped systems measured with the magnetic field H=0.1 T are
much smaller than those of the Ba- and Sr-doped systems. The thermoelectric
powers of the Ba- and Sr-doped systems decrease from large positive values of
lightly doped samples to negative ones with increasing doping level, while
those of Ca-doped systems remain positive. These results can be understood by
considering the relationship between the average ionic radius of R1-xAx and the
energy difference between the low spin and intermediate spin states. We have
found the resistivity-anomaly in the measurements of Pr1-xCaxCoO3 under
pressure in the wide region of x, which indicates the existence of a phase
transition different from the one reported in the very restricted region of
x~0.5 at ambient pressure [Tsubouchi et al. Phys. Rev. B 66 (2002) 052418.]. No
indication of this kind of transition has been observed in other species of R.Comment: 9 pages, 8 figures. J. Phys. Soc. Jpn. 72 (2003) No.
Defect-unbinding and the Bose-glass transition in layered superconductors
The low-field Bose-glass transition temperature in heavy-ion irradiated
Bi_2Sr_2CaCu_2O_8+d increases progressively with increasing density of
irradiation-induced columnar defects, but saturates for densities in excess of
1.5 x10^9 cm^-2. The maximum Bose-glass temperature corresponds to that above
which diffusion of two-dimensional pancake vortices between different vortex
lines becomes possible, and above which the ``line-like'' character of vortices
is lost. We develop a description of the Bose-glass line that is in excellent
quantitative agreement with the experimental line obtained for widely different
values of track density and material parameters.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
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