Shao-Xiong Li

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.

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

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.

Linear temperature dependence of lower critical field in MgB2

The lower critical field

Modeling and simulation on the magnetization in field-cooling and zero-field-cooling processes

H_{c1}

Tunneling observation of the coexistence of two distinct gaps in Bi2Sr2CaCu2O8+δ

has been carefully measured on a well shaped cylindrical sample of the new superconductor

Andreev reflection in point-contact tunneling spectroscopy on Bi2Sr1.94La0.06CuO6+δ single crystals

MgB_2

High-Tc superconductor Bi2Sr2CaCu2O8+δ tunnel junction with Zn counterelectrode

fabricated by high pressure synthesis. The penetration depth

Revisit the electronic phase diagram of high temperature superconductors: macroscopic phase separation in heavily overdoped regime

\lambda

Tunneling spectroscopy of Bi2Sr2CaCu2O8+δ/Zn junctions

is calculated from the