— We report the first systematic study of the effect of oxidizing annealing on the superconducting transition temperature T c of Bi 2 Sr 2 – x La x CuO 6 + δ crystals. Using a standard heat treatment procedure, we have obtained detailed T c ( x ) data for Bi 2 Sr 2 – x La x CuO 6 + δ crystals in the range x = 0.35–0.75. Comparison of the shape of the T c ( x ) curve for Bi 2 Sr 2 – x La x CuO 6 + δ with that for La 2 – x Sr x CuO 4 demonstrates a factor of 4 reduction in the dependence of hole concentration in the CuO 2 planes from a La 3+ substitution on the Sr 2+ sites becouse oxygen content (or index) varies. This finding can be accounted for in terms of changes in the amount of oxygen vacancies in the SrO planes. In particular, the oxygen index in Bi 2 Sr 2 – x La x CuO 6 + δ varies by 0.3 per formula unit.

We continued the elaboration of the wet etching method for the fabrication of standalone Bi2Sr2CaCu2O8+x (BSCCO) mesastructures, allowing us to get mesas with large thicknesses of the order of 1–15 μm. Here we performed the next step: fabricating such mesas on a thick copper substrate using the electroplating technique. This allowed us to get rigid structures that resist mechanical damage and also have a reliable heat sink, thus reducing the overheating of samples at large bias currents. Samples with a 30 μm thick copper layer were fabricated. Current–voltage curves of superconducting samples at various cryogenic temperatures were measured and studied, and a significant dependence on the thermal interface was observed. For one mesa, radiation of the order 0.1 μW was detected outside the cryostat by a Golay cell.

At present, the question of creating generators of subTHz and THz ranges based on Bi2Sr2CaCu2O8+x (BSCCO) mesastructures is of great interest [1]. BSCCO is a high-temperature superconductor with strong anisotropy, which leads to the appearance of an internal Josephson effect [2]. In other words, CuO layers represent a series of Josephson chains formed on an atomic scale. The first success in observing of electromagnetic radiation from BSCCO structures was achieved in 2007 [3]. The observed radiation had a power of the order of 0.5 μW and a frequency up to 0.85 THz. Since that time the radiation power has been increased to a value of hundreds of microwatts [4]. These achievements make it possible to use BSCCO mesostructures as sources for noise spectroscopy [5, 6]. In our group the technology of electrolytic buildup of a BSCCO/Cu heterostructures is worked out, representing copper covering over a thin layer of gold, that allows to get rigid single BSCCO mesastructures with a good heatsink. To obtain the described samples, we used a BSCCO single crystal, fabricated at ISSP RAS, of several microns thick with lateral dimensions of several millimeters. On one of its surfaces, a layer of 50 nm thick gold was deposited by thermal deposition. After this a layer of copper with thickness of about 30 microns was electrolytically built up on top of the gold layer. Then a layer of 50 nm thick gold was deposited on BSSCO second surface by thermal deposition. After that, using the original method of fast chemical etching [7], developed in our team, the cylindrical single mesastructures were fabricated. The obtained sample was glued to the copper holder, Fig. 1, after which it is taped to the contact pads and installed in a cryostat.

The proposed technology of wet etching provides obtaining of stand-alone Bi2Sr2CaCu2O8 mesa structures thicker than ones usually being studied, but still allows to get mesas below 1 μm. The time required for the fabrication is much shorter than the standard method of ion milling. The development of technology is aimed to increasing of the copper plating up to 30 microns thick over a thin gold layer to improve heat removal. The measurements of fabricated samples over a wide temperature range from 6 to 77 K were performed. Also experimental study of spectrometer with the oscillator based on the BSCCO mesa structure obtained on the basis of standard ion milling technology was performed. In particular the BSCCO generator can be used to develop compact THz spectrometer with high resolution.

A systematic study of NaxCoO2 (x=0.50 - 0.90) and NaxCoO2 yH(D)2O (x=0.26 - 0.42, y=1.3) has been performed to determine phase stability and the effect of hydration on the structural and superconducting properties of this system. We show that a careful control of the Na deintercalation process and hydration dynamics is possible in single crystals of this system. Furthermore, we give experimental evidence that the dependence of the superconducting transition temperature on Na content is much weaker than reported earlier. Implications of this effect for the understanding of the superconducting phase diagram are discussed.

The technology of wet etching allowing fabrication of stand-alone BSCCO mesa structures was proposed. The produced mesas can be made much thicker than ones usually being studied. The time required for the fabrication is much smaller in comparison with the standard method of ion milling. The process used is controllable which provides acceptable precision of mesa fabrication. The IV characteristics of the sample showing Josephson nature were obtained. The qualitative comparison with characteristics of similar structures fabricated by other groups was carried out.

The vortex structure of Bi2Sr1.65La0.35CuO6+δ single crystals in tilted magnetic fields has been studied by the decoration method. From the observed pattern of vortex chains in the basal plane, the parameter of anisotropy in the superconducting state has been estimated as γS = 460 ± 40. The electric resistance of Bi2Sr1.65La0.35CuO6+δ single crystals has been studied in a broad range of temperatures (T c < T < 300 K) and magnetic fields (up to 16 T). The ratio of resistivities in the direction perpendicular to the basal plane and in plane near the critical temperature T c amounted to ρ⊥/ρ‖ = 3.2 × 105. A possible relationship between the anisotropy in the normal and superconducting states is discussed.

We perform a detailed comparison of magnetotunneling in conventional low-T-c Nb/Al-AlOx/Nb junctions with that in slightly overdoped Bi2-yPbySr2CaCu2O8+delta [Bi(Pb)-2212] intrinsic Josephson junctions and with microscopic calculations. It is found that both types of junctions behave in a qualitatively similar way. Both magnetic field and temperature suppress superconductivity in the state-conserving manner. This leads to the characteristic sign change of tunneling magnetoresistance from the negative at the subgap to the positive at the sum-gap bias. We derived theoretically and verified experimentally scaling laws of magnetotunneling characteristics and employ them for accurate extraction of the upper critical field H-c2. For Nb an extended region of surface superconductivity at H-c2 < H < H-c3 is observed. The parameters of Bi(Pb)-2212 were obtained from self-consistent analysis of magnetotunneling data at different levels of bias, dissipation powers, and for different mesa sizes, which precludes the influence of self-heating. It is found that H-c2(0) for Bi(Pb)-2212 is similar or equal to 70 T and decreases significantly at T -> T-c. The amplitude of subgap magnetoresistance is suppressed exponentially at T > T-c/2, but remains negative, although very small, above T-c. This may indicate the existence of an extended fluctuation region, which, however, does not destroy the general second-order type of the phase transition at T-c.

We study Fiske steps in small Bi2Sr2CaCu2O8+x mesa structures, containing only a few stacked intrinsic Josephson junctions. Careful alignment of magnetic field prevents penetration of Abrikosov vortices and facilitates observation of a large variety of high-quality geometrical resonances, including superluminal with velocities larger than the slowest velocity of electromagnetic waves. A small number of junctions limits the number of resonant modes and allows accurate identification of modes and velocities. It is shown that superluminal geometrical resonances can be excited by subluminal fluxon motion and that flux flow itself becomes superluminal at high magnetic fields. We argue that observation of high-quality superluminal geometrical resonances is crucial for realization of the coherent flux-flow oscillator in the terahertz frequency range.

It is shown that the relaxation of a magnetic flux in superconductors is significantly affected by the flux configuration and the sample shape. In particular, variation in the magnetic flux configuration in YBCO plates makes it possible not only to change the relaxation rate but also significantly delay the flux relaxation.

We report the estimated intergrowth concentration of Bi2Sr2CaCu2O8+delta (Bi-2212) layers embedded in Bi2Sr2Ca2Cu3O10+delta (Bi-2223) single crystals grown using the traveling-solvent-floating-zone technique. Possible phase compatibilities in the liquidus region were discussed using the chemical analysis data of the quenched molten zone of real-growth experiments. The growth of Bi-2223 single crystals can be processed in atmosphere with oxygen content of 5, 10 and 20% O-2. The content of 2.5% was found to be the lower oxygen limit. X-ray diffractometry was used to reveal the intergrowth behavior. The values of both the 00L peaks shift and width oscillate as a function of the Bragg index. These oscillations are induced by random intergrowth of Bi-2212 layers in the Bi-2223 matrix. The performed calculations, based on the experimentally determined oscillation amplitude in the 1/d vs. L axes, have enabled us to estimate the concentration of the Bi-2212 layers in the Bi-2223 matrix to be about 2%. (c) 2006 Elsevier B.V. All rights reserved.

The phase equilibria in the Cu-rich part of the Na-Cu-O phase diagram have been investigated by DTA-TG and powder X-ray diffraction (XRD) methods at different oxygen pressures. Part of the preliminary Na-Cu-O phase diagram has been built up, and the low-stability-limit of the NaCu2O2 phase was established. Based on these data single crystals of NaCu2O2 compound were obtained for the first time by the self-flux technique. Powder and single crystal XRD measurements verify the high quality of prepared crystals. All crystals have the orthorhombic structure: a = 6.2087(1) angstrom, b = 2.9343(1) angstrom and c = 13.0648(3) angstrom.The magnetic susceptibility and heat capacity measurements carried out on the NaCu2O2 single crystals in the temperature range 2-325K showed clear evidence of antiferromagnetism at T = 12.25 K. (C) 2004 Elsevier B.V. All rights reserved.

Phase equilibria and growth mechanisms of compositions near the Bi2Sr2Ca2Cu3Ox stoichiometry were investigated by high-temperature optical microscopy in air atmosphere and 6.5% O2/93.5% Ar atmosphere. The resulting phases were determined by energy-dispersive X-ray spectroscopy. No primary crystallization phase field of Bi2Sr2Ca2Cu3Ox was found. Bi2Sr2Ca2Cu3Ox crystallizes through a peritectic reaction of Bi-rich melt with (Sr,Ca)14Cu24Ox crystals in air or with (Sr,Ca)CuOx crystals in 6.5% O2/93.5% Ar atmosphere at 870 °C. So Bi2Sr2Ca2Cu3Ox can be grown only for initial compositions within the compatibility tetrahedrons Bi2Sr2Ca1Cu2Ox−Bi11(Sr,Ca)9Cu5Ox−(Sr,Ca)14Cu24Ox−CuO (air atmosphere) or Bi2Sr2Ca1Cu2Ox−Bi11(Sr,Ca)9Cu5Ox−(Sr,Ca)CuOx−CuO (6.5% O2/93.5% Ar atmosphere) even though the Bi2Sr2Ca2Cu3Ox composition does not fall within these compatibility tetrahedra.

Spectral ellipsometry is used to determine the dielectric function of an untwinned crystal of LaMnO3 in the range 0.5-5.6 eV at temperatures 50<or=T<or=300 K. A pronounced redistribution of spectral weight is found at the Ne el temperature T(N)=140 K. The anisotropy of the spectral weight transfer matches the magnetic ordering pattern. A superexchange model quantitatively describes spectral weight transfer induced by spin correlations. This analysis implies that the lowest-energy transitions around 2 eV are intersite d-d transitions, and that LaMnO3 is a Mott-Hubbard insulator.

The fundamental building block of the copper oxide superconductors is a Cu 4 O 4 square plaquette. The plaquettes in most of these materials are slightly distorted to form a rectangular lattice, for which an influential theory predicts that high-transition-temperature (high- T c ) superconductivity is nucleated in ‘stripes’ aligned along one of the axes 1 , 2 , 3 . This theory received strong support from experiments that indicated a one-dimensional character for the magnetic excitations in the high- T c material YBa 2 Cu 3 O 6.6 (ref. 4 ). Here we report neutron scattering data on ‘untwinned’ YBa 2 Cu 3 O 6+ x crystals, in which the orientation of the rectangular lattice is maintained throughout the entire volume. Contrary to the earlier claim 4 , we demonstrate that the geometry of the magnetic fluctuations is two-dimensional. Rigid stripe arrays therefore appear to be ruled out over a wide range of doping levels in YBa 2 Cu 3 O 6+ x , but the data may be consistent with liquid-crystalline stripe order 5 . The debate about stripes has therefore been reopened.

In situ observation of ferroelastic domain switching of the phase transformation in YBCO was carried out using high temperature optical microscopy. Techniques of uniaxial compression of 2.5×107 N/m2 and quenching the tetragonal phase of YBCO at above the phase transition temperature were applied to obtain single domain samples. Both orthorhombic and macroscopically tetragonal or tweed structures of YBCO are obtained with twin-free and superconducting.

An experimental study of magnetic flux penetration under crossed magnetic fields in Bi2212:Pb single crystals is performed by the magneto-optic technique. The anisotropy of the flux creep rate induced by the in-plane magnetic field is observed at T<54±2 K. This observation confirms the existence of the three-dimensional flux line structure in Bi2212:Pb at low temperatures. An asymmetry of the flux relaxation with respect to the direction of the in-plane field is found. This effect can be attributed to the influence of the laminar structure on the pinning in Bi2212:Pb single crystals.

A systematic study of ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}\phantom{\rule{0.3em}{0ex}}(x=0.50--0.90)$ and ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}y\mathrm{H}{(\mathrm{D})}_{2}\mathrm{O}$ ($x=0.26--0.42$, $y=1.3$) has been performed to determine phase stability and the effect of hydration on the structural and superconducting properties of this system. We show that a careful control of the Na deintercalation process and hydration dynamics is possible in single crystals of this system. Furthermore, we give experimental evidence that the dependence of the superconducting transition temperature on Na content is much weaker than reported earlier. Implications of this effect for the understanding of the superconducting phase diagram are discussed.