Here we report the growth of sub-millimeter MgB 2 single crystals of various shapes under high pressure in Mg-B-N system. Structure refinement using a singlecrystal X-ray diffraction analysis gives lattice parameters a=3.0851 (5) . It was found that as sintering temperature increased, the sample quality was improved, but for the heat treatment above 950 o C the gold started to melt and to adhere strongly to MgB 2. Preparation of high-density bulk MgB 2 materials at a higher temperature (950-1250 o C) and 3.5 GPa was reported in BN crucibles 7) . However, the samples prepared at 1250 o C apparently showed inhomogeneous structure in macroscopic scale, indicating the occurrence of contamination from the crucible wall. PACSThese results are not quite favorable for crystal growth of MgB 2 under high-pressure.In this letter, we report the successful growth of high-quality MgB 2 single crystals under high pressure in the Mg-B-N system. Using the crystals, we carried out structure refinement and measured anisotropic superconducting properties of magnesium diboride. were used for refinement after absorption correction by a ψ-scan. The structure was refined by the full-matrix least-square procedure.The temperature dependence of the magnetization M(T) was measured by a SQUID (MPMS XL, Quantum Design) for 37 crystals of different shape (total weight 0.25mg) aligned with the c-axis perpendicular to the sample holder. In-plane electrical resistivity was measured for several plate-like crystals of typical size of 500x100x20 µm 3 in a four-probe configuration using a low frequency (17.8Hz) ac technique with a voltage resolution of less than 0.3nV. To study the anisotropic superconducting properties, the resistivity measurements were performed in the magnetic fields up to 7Tesla applied parallel and perpendicular to the Mg and B planes.The Mg-B-N system has been extensively studied during last 40 years [8][9][10] . In this system Mg and Mg-containing compounds act as the catalysts for the 4 transformation of boron nitride (BN) from hexagonal to cubic form under high pressure and the formation of MgB 2 and MgB 6 crystals has been observed 11). These compounds were formed as the intermediate phases in Mg-BN system, during the synthesis of cubic boron nitride, but MgB 2 was decomposed after prolong heat treatment.We found that the lower limit for MgB 2 crystal growth in "pressure- , using polycrystalline and powdered sample, refined structural data were previously unavailable. It has been proved to be very difficult to avoid the formation of impurity phases (MgO and MgB 4 ) in polycrystalline samples and thus the precise structure refinement is almost impossible. In this study well-formed single crystals were chosen for structure analysis to minimize of absorption effects. The refined crystallographic data, atomic coordinates, and thermal parameters are listed in Table 1 .In the final part of this letter we briefly characterize the superconducting properties of our MgB 2 single crystals. .In Fig.5(a) we present the resistive supe...

We report the first observation of the intrinsic multiple Andreev reflections effect (IMARE) in S-n-S-…-S-arrays (S = superconductor, n = normal metal) formed by "break-junction" technique in GdO(F)FeAs superconductor (T C = 48 ÷ 53 K). We show that superconducting gap peculiarities at dI/dV-spectra sharpen dramatically in the arrays as compared with that in the single-contact spectra; this enables to improve significantly accuracy of the bulk superconducting parameters determination. Using IMARE, we determined the large and the small gap values Δ L = 11.0 ± 1.1 meV and Δ S = 2.6 ± 0.4 meV. The BCS-ratio 2Δ L /k B T C local = 5.0 ÷ 5.9 > 3.52 (T C local is the contact area critical temperature) evidences for a strong electron-boson coupling. The results obtained agree well with our previous data by Andreev spectroscopy for single SnS-contacts.

We report on study of the electronic anisotropy of the newly discovered MgB

We discuss the important aspects of synthesis and crystal growth of MgB2 under high pressure (P) and temperature (T) in Mg-B-N system, including the optimisation of P -T conditions for reproducible crystal growth, the role of liquid phases in this process, the temperature dependence of crystal size and the effect of growing instabilities on single crystals morphology. Extensive experiments have been carried out on s ingle crystals with slightly different lattice constants and defects concentration, which revealed and possible effects of Mg-deficiency and lattice strain on the superconducting properties of MgB 2 (T c , J c , residual resistivity ratio, anisotropy etc.).

The out-of-plane magnetoconductivity Ds c (BkIkcrystal c axis) of two YBa 2 Cu 3 O 72d single crystals has been measured in magnetic fields up to 12 T at temperatures ranging from T c 1 3 K to T c 1 40 K. A change of sign, from negative Ds c near T c to positive at higher temperatures, is observed in a metallic sample ͑dr c ͞dT . 0͒. Recent fluctuation theories can describe the main features of these observations with parameters that are in good agreement with other studies. These results suggest that fluctuations in the density of states have been observed in the magnetoconductivity. [S0031-9007(96)00941-6] PACS numbers: 73.50.Jt, 74.40.+k, 74.72.Bk It has recently become increasingly clear that the c-axis properties of anisotropic high-temperature superconductors are essential for understanding the anomalous superconducting as well as normal-state properties. A problematic observation, for instance, is a positive dr͞dT for the ab-plane resistivity combined with a strongly negative c-axis dr͞dT in underdoped YBa 2 Cu 3 O 72d and in Bi-based materials. Attempted descriptions include various tunneling models, fluctuations, interplanar disorder, and hopping due to resonant tunneling [1][2][3][4][5].Studies of superconducting fluctuations in the magnetoconductivity, Ds͑B, T͒ 1͞r͑B, T ͒ 2 1͞r͑0, T ͒, represent a powerful experimental technique to address related problems. The considerable effects of the shortlived electron pairs over a wide range of temperatures above T c , together with the possibility to use strong magnetic fields, provide for stringent comparisons with theories. In addition to reliable estimates of the coherence lengths and the phase-breaking time [6], measurements of Ds may allow for conclusions about the pairing state [7,8], or clarify the nature of the impurity state in certain doped alloy systems [9,10]. There is also a rapid development of the fluctuation theories, with continued discussions about what terms and what limits of existing theories to use, and with an increasing number of suggested additions and new theories [11 -13].In the present study we have measured the c-axis magnetoconductivity Ds c ͑BkIkc͒ of two YBa 2 Cu 3 O 72d single crystals in the temperature range T c 1 3 K to T c 1 40 K. A change of sign was observed, from negative Ds c near T c to positive above T c 1 10 K. In the theory by Dorin and co-workers [13], such a sign change could occur as a result of fluctuations in the normal density of states. We have adapted this theory for comparisons with experiments. Excellent descriptions of the observations with reasonable parameter values were obtained in the region where Ds c # 0, including the sign change at approximately the correct temperature. In the region where Ds c . 0, the uncertainties are somewhat larger.Two single crystals of YBa 2 Cu 3 O 72d were grown by the self-flux method [14]. Sample A was oxygenated at 460, 400, 350, and finally 300 ± C, for a few days at each temperature. Sample B was oxygenated at 400 ± C. Sample A was heavily twinned, whereas sample B ...