Strong Quantum Fluctuation of Vortices in Bulk Samples of the New Superconductor MgB_(2)

By using transport and magnetic measurement,the upper critical field H_(c2)(T)and the irreversibility line Hirr(T)has been determined.A big separation between H_(c2)(0)and Hirr(0)has been found showing the existence of a quantum vortex liquid state induced by quantum fluctuation of vortices in the new superconductor MgB_(2).Further investigation on the magnetic relaxation shows that both the quantum tunneling and the thermally activated flux creep depends weakly on temperature.However,when the melting field Hirr is approached,a drastic rise of the relaxation rate is observed.This may indicate that the melting of the vortex matter at a finite temperature is also induced by the quantum fluctuation of vortices.

Preparation and Characterization of the New Superconductor MgB_(2)

Polycrystalline samples of the new superconductor MgB_(2) are prepared by solid state reaction using Mg and B powders.The resistance measurement shows that the onset transition temperature and zero resistance transition temperature are Tonsetc=43.8 K and Tc0=37 K,respectively.The Tc0 depends on measuring current strongly.The diamagnetic transition temperature of~38 K is obtained by the ac susceptibility measurement.X-ray powder diffraction spectra of MgB_(2) can be indexed using hexagonal structure with space group P6/mmm and lattice parameters a=0.30864 and c=0.35212 nm.A second phase,probably MgO,is also found.

Superconducting Properties of MgB_(2) Prepared by High Pressure and Ambient Pressure

The new superconductor MgB_(2) has been prepared by two ways: high pressure and ambient pressure synthesis. Their superconducting properties were measured and compared. It is found that the sample prepared by high pressure is much denser than that prepared under ambient pressure. Accordingly the high pressure sample has a very narrow transition width and a much higher bulk critical current density.

Possible Superconductivity at 37K in Graphite-Sulphur Composite

Sulphur intercalated graphite composites with diamagnetic transitions at 6.7 and 37K are prepared. The magnetization hysteresis loops (MHL), x-ray diffraction (XRD) patterns, and resistance have been measured. From the MHL, a slight superconducting-like penetration process is observed at 15K in the low field region. The XRD shows no large difference from the mixture of graphite and sulphur indicating that the volume of the superconducting phase (if any) is very small. The temperature dependence of resistance shows a typical semi-conducting behaviour with a saturation in the low-temperature region. This saturation is either induced by the delocalization of conducting electrons or by possible superconductivity in this system.

Upper Critical Field and Irreversibility Line Determined by Transport Measurement of the New Superconductor MgB_(2)

Resistive transitions of the new superconductor MgB_(2) are carried out under magnetic fields from 0 to 8 T.It is found that the irreversibility field Hirr(T)and the upper critical field Hc2(T)obtained are very close to those determined in the magnetic measurement.By using the Arrhenius activation lawρ=ρ0exp(-U/kBT),the activation barrier U has been determined by the low dissipation part of R(T)curves.It is found that U■kBT manifests a very weak thermal activation and thermal fluctuation effect.This may further indicate that the flux dynamics near Hirr(T)is induced by strong quantum fluctuation and tunneling of vortices.

Influence of Weak Self-Field Effect on Calculated Activation Energy for Superconducting Thin Film

When the applied field Ba=μ0 H_(a)(H_(a)>H_(p))is perpendicular to the surface of a disk-shaped superconducting thin film with the full penetrated field H_(p),the radius R,and the thickness d,a quantitative relation between the average fieldand the magnetization M is described by=μ_(0)[H_(a)+(0.81 d/R)M]for R»d,where the termμ0(0.81 d/R)M also is the average self-field of the film.Combining this relation with the magnetic flux conservation equation and the Arrhenius relation,the effective activation energy is extracted from the experimental magnetization relaxation and the dynamic magnetization relaxation,respectively.

Field and Current Distribution in Superconducting Film Under Partial Field Penetration

Using finite-element analysis in a self-consistent way with the self-field of the current considered for superconducting thin film,the field and current distributions under partial field penetration are calculated based on the field dependent critical current density function.The field penetrated depth is non-linear with the applied field.Due to the limited resolution of the vibrating sample magnetometer(VSM),the full penetrated field measured by VSM may be much smaller than its actual value.