681 research outputs found
Magnetic shape-memory effects in La2-xSrxCuO4 crystals
The magnetic field affects the motion of electrons and the orientation of
spins in solids, but it is believed to have little impact on the crystal
structure. This common perception has been challenged recently by ferromagnetic
shape-memory alloys, where the spin-lattice coupling is so strong that
crystallographic axes even in a fixed sample are forced to rotate, following
the direction of moments. One would, however, least expect any structural
change to be induced in antiferromagnets where spins are antiparallel and give
no net moment. Here we report on such unexpected magnetic shape-memory effects
that take place ironically in one of the best-studied 2D antiferromagnets,
La2-xSrxCuO4 (LSCO). We find that lightly-doped LSCO crystals tend to align
their b axis along the magnetic field, and if the crystal orientation is fixed,
this alignment occurs through the generation and motion of crystallographic
twin boundaries. Both resistivity and magnetic susceptibility exhibit curious
switching and memory effects induced by the crystal-axes rotation; moreover,
clear kinks moving over the crystal surfaces allow one to watch the crystal
rearrangement directly with a microscope or even bare eyes.Comment: 3 pages, 4 figures; shortend version of this paper has been published
in Nature as a Brief Communicatio
Two mechanisms of pseudogap formation in Bi-2201: Evidence from the c-axis magnetoresistance
Measurements of the c-axis resistivity and magnetoresistance have been used
to investigate the pseudogap (PG) behavior in Bi_{2+z}Sr_{2-x-z}La_xCuO_y
(Bi-2201) crystals at various hole densities. While the PG opening temperature
T* increases with decreasing hole doping, the magnetic-field sensitivity of the
PG is found to have a very different trend: it appears at lower temperatures in
more underdoped samples and vanishes in non-superconducting samples. These data
suggest that besides the field-insensitive pseudogap emerging at T*, a distinct
one is formed above T_c as a precursor to superconductivity.Comment: 7 pages, 6 figures, accepted for publication in Europhysics Letters
(initially submitted to PRL on 14 June 2000
Normal-state resistivity anisotropy in underdoped RBa_2Cu_3O_{6+x} crystals
We have revealed new features in the out-of-plane resistivity rho_c of
heavily underdoped RBa_2Cu_3O_{6+x} (R=Tm,Lu) single crystals, which give
evidence for two distinct mechanisms contributing the c-axis transport. We have
observed a crossover towards "metal-like" (d rho_c/d T > 0) behavior at the
temperature T_m which quickly increases with decreasing doping. The
"metal-like" conductivity contribution dominates at T < T_m and provides a
saturation of the resistivity anisotropy, rho_c / rho_{ab}. The
antiferromagnetic ordering is found to block this "metal-like" part of the
c-axis conductivity and complete decoupling of CuO_2 planes, which may be the
reason of superconductivity disappearance.Comment: RevTex, 4 pages including 4 eps figures. To be published in
Phys.Rev.Let
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
Ising-like Spin Anisotropy and Competing Antiferromagnetic - Ferromagnetic Orders in GdBaCo_{2}O_{5.5} Single Crystals
In RBaCo_{2}O_{5+x} compounds (R is rare earth), a
ferromagnetic-antiferromagnetic competition is accompanied by a giant
magnetoresistance. We study the magnetization of detwinned GdBaCo_{2}O_{5.5}
single crystals, and find a remarkable uniaxial anisotropy of Co^{3+} spins
which is tightly linked with the chain oxygen ordering in GdO_{0.5} planes.
Reflecting the underlying oxygen order, CoO_2 planes also develop a spin-state
order consisting of Co^{3+} ions in alternating rows of S=1 and S=0 states. The
magnetic structure appears to be composed of weakly coupled ferromagnetic
ladders with Ising-like moments, which gives a simple picture for
magnetotransport phenomena.Comment: 5 pages, 4 figures, accepted to Phys.Rev.Let
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
Study of Thermal Performance of Modern Design of the Drum-type Batch Furnace
The report focuses on the layout and features of the thermal performance of the drum-type batch furnace for heating of metal products for hardening. Technical characteristics of the furnace, some results of the thermomechanical calculation are given. The computer simulation of the processes of gas flow and heat exchange in the furnace is presented. The study has been carried out using the CAE-system (CAE, Computer Aided Engineering)—the software module ANSYS Fluent. In this module, the flow boundary conditions were specified. To check the repeatability of calculations, the control of current values and calculated temperature discrepancies has been used. The results of the simulation are presented graphically and contain a visualization of the field of temperature and velocity distribution, as well as a vector distribution of gas flow rates. The obtained results of the computer simulation allowed evaluating the efficiency of the thermal and gas-dynamic performance of the developed design of the drum-type batch furnace with a constant temperature of the operating space. The developed design of the furnace for heating of metal billets with moving of billets in the furnace along the drum allows solving of some problems of resource and energy saving; it could also be used for heat treatment of bars, pipes, strip, and bar sections of various shapes.
Keywords: batch furnace with a constant temperature of operating space, recuperative burner, computer simulation, temperature fields, velocity fields, resource saving, burning, heat exchang
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