81 research outputs found
P-T phase diagram of iron arsenide superconductor NdFeAsO0.88F0.12
NdFeAsO0.88F0.12 belongs to the recently discovered family of high-TC
iron-based superconductors. The influence of high pressure on transport
properties of this material has been studied. Contrary to La-based compounds,
we did not observe a maximum in TC under pressure. Under compression, TC drops
rapidly as a linear function of pressure with the slope k = -2.8 \pm 0.1 K /
GPa. The extrapolated value of TC at zero pressure is about TC (0) = 51.7 \pm
0.4 K. At pressures higher than ~18.4 GPa, the superconducting state disappears
at all measured temperatures. The resistance changes slope and shows a turn-up
behavior, which may be related to the Kondo effect or a weak localization of
two-dimensional carriers below ~45 K that is above TC and thus competing with
the superconducting phase. The behavior of the sample is completely reversible
at the decompression. On the bases of our experimental data, we propose a
tentative P-T phase diagram of NdFeAsO0.88F0.12
Anomalous diffusion in the dynamics of complex processes
Anomalous diffusion, process in which the mean-squared displacement of system
states is a non-linear function of time, is usually identified in real
stochastic processes by comparing experimental and theoretical displacements at
relatively small time intervals. This paper proposes an interpolation
expression for the identification of anomalous diffusion in complex signals for
the cases when the dynamics of the system under study reaches a steady state
(large time intervals). This interpolation expression uses the chaotic
difference moment (transient structural function) of the second order as an
average characteristic of displacements. A general procedure for identifying
anomalous diffusion and calculating its parameters in real stochastic signals,
which includes the removal of the regular (low-frequency) components from the
source signal and the fitting of the chaotic part of the experimental
difference moment of the second order to the interpolation expression, is
presented. The procedure was applied to the analysis of the dynamics of
magnetoencephalograms, blinking fluorescence of quantum dots, and X-ray
emission from accreting objects. For all three applications, the interpolation
was able to adequately describe the chaotic part of the experimental difference
moment, which implies that anomalous diffusion manifests itself in these
natural signals. The results of this study make it possible to broaden the
range of complex natural processes in which anomalous diffusion can be
identified. The relation between the interpolation expression and a diffusion
model, which is derived in the paper, allows one to simulate the chaotic
processes in the open complex systems with anomalous diffusion.Comment: 47 pages, 15 figures; Submitted to Physical Review
Superconducting properties of [BaCuO_x]_2/[CaCuO_2]_n artificial structures with ultrathick CaCuO_2 blocks
The electrical transport properties of [BaCuO_x]_2/[CaCuO_2]_n
(CBCCO-2xn)underdoped high temperature superconducting superlattices grown by
Pulsed Laser Deposition have been investigated. Starting from the optimally
doped CBCCO-2x2 superlattice, having three CuO_2 planes and T_c around 80 K, we
have systematically increased the number n up to 15 moving toward the
underdoped region and hence decreasing T_c. For n>11 the artificial structures
are no longer superconducting, as expected, for a uniformly distributed charge
carriers density inside the conducting block layer. The sheet resistance of
such artificial structures (n nearly equal to 11) turns out to be quite
temperature independent and close to the 2D quantum resistance 26 kOhm. A
further increase of the number of CuO_2 planes results in an insulator-type
dependence of R(T) in the wide range of temperatures from room temperature to 1
K. The value of the sheet resistance separating the Superconducting and the
Insulating regimes supports the fermionic scenario of the
Superconductor-Insulator transition in these systems.Comment: 12 pages, 5 figures. Corresponding author: [email protected]
Mesoscopic Superconducting Disc with Short-Range Columnar Defects
Short-range columnar defects essentially influence the magnetic properties of
a mesoscopic superconducting disc.They help the penetration of vortices into
the sample, thereby decrease the sample magnetization and reduce the upper
critical field. Even the presence of weak defects split a giant vortex state
(usually appearing in a clean disc in the vicinity of the transition to a
normal state) into a number of vortices with smaller topological charges. In a
disc with a sufficient number of strong enough defects vortices are always
placed onto defects. The presence of defects lead to the appearance of
additional magnetization jumps related to the redistribution of vortices which
are already present on the defects and not to the penetration of new vortices.Comment: 14 pgs. RevTex, typos and figures corrected. Submitted to Phys. Rev.
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