Yu. Eltsev

Anisotropic superconducting properties of MgB2 single crystals probed by in-plane electrical transport measurements

We report on the study of the electronic anisotropy of the MgB 2 superconductor using the in-plane resistivity measurements in the magnetic field applied perpendicular and parallel to Mg and B planes of MgB 2 single crystals. The results show the temperature dependent anisotropy of the upper critical field with the anisotropy ratio γ=H c 2 | | /H c 2 increasing from 2.2 close to T c up to about 3 below 30 K. Our estimation of the in-plane and out-of-plane coherence length of about ξ a b (0) = 68 A and ξ c (0) = 23 A and the electronic mean-free path l a b =240 A and l c =60 A, respectively, indicates MgB 2 single crystal approaches the clean limit type-II superconductor.

Crystal growth and characterization of MgB2: the relation between structural and superconducting properties

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 single 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 MgB2 (Tc, Jc, residual resistivity ratio, anisotropy etc.).We discuss the important aspects of synthesis and crystal growth of MgB2 under high pressure (P) and temperature (T) in the Mg–B–N system, including the optimization 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-crystal morphology. Extensive experiments have been carried out on single crystals with slightly different lattice constants and defect concentrations, which have revealed the possible effects of Mg deficiency and lattice strain on the superconducting properties of MgB2 (Tc, Jc, residual resistivity ratio, anisotropy, etc).

Resistive behavior and magnetic phase diagram of MgB2 single crystals

Abstract I report on the resistive behavior below T c and magnetic phase diagram of MgB 2 single crystals. Strong current induced resistive transition broadening has been found at magnetic fields above 1.5–2 T in H ∥ c -axis field orientation. I discuss this behavior in relation with possible effects of the vortex–liquid state, surface superconductivity, excess conductivity as well as double-energy superconducting-gap structure in MgB 2 .

Bulk magnetization and strong intrinsic pinning in Ni-doped BaFe2As2 single crystals

We report on measurements of isothermal irreversible magnetization loops (M(H)) of Ni-doped BaFe2−xNixAs2 single crystals with x = 0.1 and 0.14 in magnetic fields up to 14 T. For both samples in fields H || c-axis and H || ab-plane, the critical current density calculated from the M(H) curves exceeds 106 A cm−2 at low temperatures, suggesting strong intrinsic pinning in our crystals. In a broad temperature range of 2–17 K in both field orientations, a nearly optimally doped single crystal displays a second magnetization peak (fish-tail effect). For H || c-axis, curves of the normalized pinning force, fp, as a function of reduced field, h, measured at different temperatures obey the scaling relation fp ∝ hp(1−h)q with peak position hmax ≈ 0.33 for the BaFe1.86Ni0.14As2 crystal and hmax ≈ 0.4 for the BaFe1.9Ni0.1As2 sample, indicating a single normal point pinning mechanism. In striking contrast, in the H || ab-plane orientation, the scaling of fp(h) curves is absent.

Observation of multi-gap superconductivity in GdO(F)FeAs by Andreev spectroscopy

We have studied current-voltage characteristics of Andreev contacts in polycrystalline GdO0.88F0.12FeAs samples with bulk critical temperature Tc = (52.5 ± 1) K using break-junction technique. The data obtained can- not be described within the single-gap approach and suggests the existence of a multi-gap superconductivity in this compound. The large and small superconducting gap values estimated at T = 4.2 K are ΔL = 10.5 ± 2 meV and Δs = 2.3 ± 0.4 meV, respectively.

YBa2Cu3O7−y single-crystal growth by the method of horizontally directed temperature gradient

YBCO single crystals 1-2-3 were grown from a melted mixture of YBa2Cu3O7-y-BaO-CuO using the method of horizontally directed cooling in the zone melting equipment. This method makes it possible to grow high quality single crystals with dimensions up to 3*2*0.15 mm3, which may then be extracted simply from the crucible. Differences in superconducting properties, parameters of crystal structure and the microstructure were found in single crystals grown in Al2O3 and ZrO2 crucibles. It is possible that these differences are connected with the different stability of the crucibles used and consequently with the presence of impurities in the melted charge during the single crystal growth process.

Single crystals of MgB2 superconductor grown under high-pressure in Mg-B-N system

Abstract Here, we report the growth of magnesium diboride (MgB2) single crystals under high-pressure in the Mg–B–N system. The phase stability limit of MgB2 in P–T space was determined and reaction diagram for the crystal growth of MgB2 under high-pressure was proposed. It was found that utilization of longitudinal temperature gradient and optimisation of reactant pellets locations in the BN high-pressure cell is very helpful for reproducible growth of MgB2 single crystals of various shapes and size of 200–700 μm. The high-quality of the samples was confirmed by single crystal X-ray diffraction and sharp superconducting transitions at T c =38.1–38.3 K obtained in both magnetization (ΔTc=0.6 K) and resistivity (ΔTc=0.3 K) measurements.

Critical currents and microstructures of LPE grown YBCO bicrystal films with large single facet grain boundaries

Abstract The microstructures and electric transport properties across grain boundaries (GBs) are studied for YBaCuO bicrystal films grown by liquid phase epitaxy (LPE) method. Large single facet GBs with typical size of 10–100 μm are obtained in the films on MgO bicrystal substrates. Microstructures and atomic arrangements of GBs with different misorientation angles ϑ are investigated by TEM. The critical current densities across [001] tilt GBs as a function of ϑ show that J c values are almost constant up to ∼12° and then decrease exponentially, as usually observed for bicrystal films grown by physical vapor deposition methods. The presence of a plateau region up to 12° demonstrates a feature of the clean and meandering free GB structure in the LPE bicrystal films. The temperature and field dependencies of I c for 24° GB are presented and discussed based on the microstructures.

Transverse dynamic vortex correlation in a Bi2Sr2CaCu2O8+δ Corbino disk

Abstract In-plane voltage response has been measured in Bi2Sr2CaCu2O8+δ single crystal using a Corbino disk contact configuration. Strong transverse vortex velocity correlation as well as pronounced non-Ohmic behavior and surface barriers dominated vortex motion was found at B

Applicability of the Montgomery type analysis to the anisotropic resistivity of Bi2Sr2CaCu2Ox

Abstract Multi-terminal measurements were performed for a Bi2Sr2CaCu2Ox (Bi-2212) single crystal in magnetic fields along the c-axis, and the applicability of the Montgomery type analysis was systematically tested. In addition to an exact numerical calculation, a new technique to test the applicability was proposed. The Montgomery type analysis is applicable in the normal state and is not just below Tc. Surprisingly, it seems to be applicable again at lower temperatures in low magnetic fields. The origin of the breakdown of the Montgomery type analysis is discussed.