Y.M. Ni

Suppression of superconducting critical current density by small flux jumps in MgB2 thin films

By doing magnetization measurements during magnetic field sweeps on thin films of the new superconductor MgB2, it is found that in a low-temperature and low-field region small flux jumps are taking place. This effect strongly Suppresses the central magnetization peak leading to reduced nominal superconducting critical current density at low temperatures, A borderline for this effect to occur is determined on the field-temperature (H-T) phase diagram. It is suggested that the small size of the flux jumps in films is due to the higher density of small defects and the relatively easy thermal diffusion in thin films in comparison with bulk samples.

Magnetic relaxation and critical current density of the new superconductor MgB2

Magnetic relaxation rate, critical current density and transport properties have been investigated on MgB2 bulks from 1.6 K to Tc at magnetic fields up to 8 T. A vortex phase diagram is depicted based on these measurements. A large separation between the bulk irreversibility field Hirr(T) and the upper critical field Hc2(T) has been found. It is thus proposed that there is a quantum vortex liquid due to strong quantum fluctuation of vortices at 0 K. It is also found that the magnetic relaxation rate is weakly dependent on temperature but strongly dependent on field indicating a trivial influence of thermal fluctuation on the vortex depinning process. Therefore, the phase line Hirr(T) is attributed to quantum vortex melting in the rather clean system at a finite temperature.

Crystal structures and superconductivity of superconducting phases in the Tl-Ba-Ca-Cu-O system

Abstract We found a series of superconducting phases in the Tl-Ba-Ca-Cu-O system and determined the crystal structures of these superconducting phases by means of the X-ray powder diffraction method. All of these superconducting phases belong to the tetragonal symmetry. They have the same lattice constant a and different lattice constant c . These superconducting phases can be divided into two types. The space group of type I is P4/mmm. The space group of type II is 14/mmm. The chemical formulae of type I and type II can be generalized as TlBa 2 Ca n −1 Cu n O 2 n +2.5 ( n =2, 3, 4) and Tl 2 Ba 2 Ca n −1 Cu n O 2 n +4 ( n =1, 2, 3), respectively. T c increases as n increases. On the condition that n is the same, type II has a higher T c than type I. The relations among the crystal structures of the superconducting phases are discussed. We proposed a method which can deduce the crystal structure of the superconducting phase from the d 1 -value of the first diffraction line (00 l ) or lattice parameter c and which is confirmed by experimental data.

The structural change and superconductivity in MgCxNi3 (x=0.5–1.55) and MgxCyNi3 (x=0.75–1.55, y=0.85, 1.0 and 1.45) and the phase diagram of Ni-rich region in Mg–C–Ni ternary system

The structural change and superconductivity in the nominal composition MgCxNi3 (x = 0.5-1.55) and MgxCyNi3 (x = 0.75-1.55, y = 0.85, 1.0 and 1.45) were investigated and the phase diagram of Ni-rich region in Mg-C-Ni ternary system was determined. It was found that in MgCxNi3 system there are two phases: alpha-MgCxNi3 and beta-MgCNi3. alpha-MgCxNi3 phase exists in the MgCxNi3 samples with x 1.0 prepared at the temperature of 900985 degreesC and in the samples with x < 1.0 prepared at the temperature higher than 965 degreesC. It has a large and constant lattice parameter of 0.3810 run. The beta-MgCNi3 phase is superconducting. The investigation results indicate that a homogeneous region with changeable content of Mg does not exist in MgxCyNi3 (x = 0.75-1.55, y = 0.85, 1.0 and 1.45) systems

Crystal growth and superconductivity of heavily La-doped Bi-2201 single crystals

Abstract A number of La-doped Bi-2201 single crystals were prepared by self-flux method. The qualities of the crystals have been checked by X-ray diffraction patterns (XRD) and electron diffraction patterns from transmission electron microscopy (TEM). The correlation between the actual composition of La determined by Energy-Dispersive X-ray (EDX) analysis and the c -axis parameter determined from XRD shows a linear behavior, being consistent with previous reports. A new superconducting phase with T c as high as 36 K has been discovered for the first time in heavily La-doped Bi 2 Sr 2− x La x CuO 6+ y single crystals ( x ≥0.8) in which one normally could not observe the superconductivity. Since our samples have been post-annealed in flowing oxygen and heavily La-doped samples are normally in the region with an antiferromagnetic background, it is natural to attribute the appearance of the high T c superconductivity to the electronic phase separation induced by the cooperative interactions between the antiferromagnetic background and the mobile holes (given by excess oxygen).

Flux dynamics and vortex phase diagram of the new superconductor MgB2

Magnetic relaxation, critical current density and transport properties have been investigated on MgB2 bulks from 1.6 K to T-c at magnetic fields up to 8 T. A vortex phase diagram is depicted based on these measurements. Two phase boundaries H-irr(bulk) (T) and H-irr(g)(T) characterizing different irreversible flux motions are found. The H-irr(bulk)(T) is characterized by the appearance of the linear resistivity. A large separation between the bulk irreversibility field at 0 K and the upper critical field H-c2(0) has been found, it is interpreted as either due to a quantum vortex liquid induced by strong quantum fluctuation of vortices at 0 K, or flux flow through weak-link channels. It is further found that the magnetic relaxation rate is weakly dependent on temperature but strongly dependent on field indicating a trivial influence of thermal fluctuation on the vortex depinning process. Therefore the phase line H-irr(bulk) (T) may be attributed to quantum vortex melting in the rather clean system at a finite temperature. The second boundary H-irr(g)(T) reflects the irreversible flux motion in some local regions due to either very strong pinning or the surface barrier on the tiny grains

Superconducting properties and c-axis superstructure of Mg1xAlxB2

The superconducting and structural properties of a series of Mg1-xAlxB2 samples have been investigated. X-ray diffraction results confirmed the existence of a structural transition associated with a significant change in interboron layer distance as reported previously by Slusky Moreover, transmission-electron-microscopy observations revealed the existence of a superstructure with doubled lattice constant along the c-axis direction. We propose that this superstructure is probably related to the structural transition. The modifications of the superconducting transition temperature T-c, the normal-state resistivity, and the upper critical field B-c2(0) by Al doping are discussed in terms of Al-substitution-induced changes in the electronic structure at the Fermi energy.

Magnetic relaxation and critical current density of MgB2 thin films

The tunneling splitting in biaxial ferrimagnetic particles at excited states with an explicit calculation of the prefactor of exponent is obtained in terms of periodic instantons that are responsible for tunneling at excited states and is shown as a function of magnetic field applied along an arbitrary direction in the plane of hard and medium axes. Using complex time path integral we demonstrate the oscillation of tunnel splitting with respect to the magnitude and the direction of the magnetic field due to the quantum phase interference of two tunneling paths of opposite windings. The oscillation is gradually smeared and in the end the tunnel splitting monotonously increases with the magnitude of the magnetic field when the direction of the magnetic field tends to the medium axis. The oscillation behavior is similar to the recent experimental observation with Fe-8 molecular clusters. A candidate of possible experiments to observe the effect of quantum phase interference in the ferrimagnetic particles is proposed.

Single crystal growth of superconducting compound Bi-Ca-Sr-Cu-O

We have prepared two kinds of single crystals in Bi-Sr-Ca-Cu oxides by flux method;plate-like single crystals which exhibit metal to superconductor at 93 K, being the high-Tclayered compound BiSr (Ca0.35Sr0.25Bi0.40)2Cu2Oy with orthorhombic structure with a=5.42 A, b=5.44 A and c=30.78 A respectively, and another kind of bar-shapedbismuth-poor single crystals which is superconducting at the temperature below 82K whilebeing semiconducting above this temperature.

Linear temperature dependence of lower critical field in MgB2

The lower critical field