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

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

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

Technical Upgrading and Thermal Performance of Heating Furnace of the Pipe Rolling Workshop

The report is focused on the design and thermal performance of the continuous furnace for heating of pipe billets before piercing operating at ‘ChPRP’ PJSC. The problems arising during operation of the thermal generating unit have been analyzed. To evaluate the efficiency of existing heating system, the heat balance of the continuous furnace has been drawn up. During analysis of the results of calculated studies, disadvantages of existing furnace systems and assemblies have been revealed. In order to improve the quality of metal heating, it is proposed to install the through-type furnace heated by means of regenerative burners, as well as provided with the metal transportation system, ensuring more uniform heating, both along the length and thickness of billets, in place of the existing furnace. When implementing the proposed activities, a significant economic benefit is expected, which is confirmed by the heat balance of the through-type furnace given in the article. Besides, to visualize the distribution of temperature and gas-dynamic flows within the operating space of the proposed through-type furnace, the computer simulation for evaluation of the uniformity of metal heating has been performed. Keywords: continuous furnace, regenerative burner, energy saving, through-type furnace, heat balanc

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&nbsp; 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

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

Competition and coexistence of antiferromagnetism and superconductivity in RBa_2Cu_3O_{6+x} (R = Lu, Y) single crystals

We use c-axis resistivity and magnetoresistance measurements to study the interplay between antiferromagnetic (AF) and superconducting (SC) ordering in underdoped RBa_2Cu_3O_{6+x} (R = Lu, Y) single crystals. Both orders are found to emerge from an anisotropic 3D metallic state, upon which antiferromagnetism opposes superconductivity by driving the doped holes towards localization. Despite the competition, the superconductivity sets in before the AF order is completely destroyed and coexists with latter in a certain range of hole doping. We find also that strong magnetic fields affect the AF-SC interplay by both suppressing the superconductivity and stabilizing the Neel order.Comment: 7 pages, 6 figure