Wang Xu

Suppression of superconductivity in epitaxial NbN ultrathin films

This paper studies the suppression of superconducting transition temperature (T(c)) of ultrathin NbN film. We fabricated epitaxial NbN superconducting thin films of thicknesses ranging from 2.5 to 100 nm on single crystal MgO (100) substrates by dc magnetron sputtering. We performed structure analyses and measured their electric and far infrared properties. The experimental results were compared with several mechanisms of the suppression of superconductivity proposed in the literature, including the weak localization effect, the proximity effect, and quantum size effect (electron wave leakage model). We found that the electron wave leakage model matches best to the experimental data

Harmonic mixing at mm waveband in a high temperature superconducting grain boundary Josephson junction

Using a high Tc superconducting grain boundary Josephson junction, harmonic mixing experiments in the mm waveband were carried out, aiming at as large a harmonic number and as high a signal frequency as possible. The dependencies of intermediate frequency output on dc bias, harmonic number, frequency of local oscillator (LO), and other parameters were carefully studied. Until now, our best result was the mixing between the signal at 95 GHz and the 105th harmonic of LO at about 900 MHz. Preliminary experiments using a high Tc harmonic mixer and phase‐locking loop were tried to stabilize the frequency of a mm wave source.

Bias dependent tunneling in ferromagnetic junctions and inversion of the tunneling magnetoresistance from a quantum mechanical point of view

In the framework of the free-electron approximation, we have developed a quantum mechanical treatment for describing the bias dependent tunneling in FM/I/FM ferromagnetic junctions. In our theory, the Slonczewski model is extended to include the bias effect. In the barrier region, the Wentzel–Kramers–Brillouin wave function is used following Harrison. The main point of our treatment is to match the wave functions at both sides of the electrode/barrier interfaces quantum mechanically. We find that apart from the usual density of states effect, there exists a quantum coherent factor D(Ex,V)=κR2(Ex,V)−kR↑(Ex,V)kR↓(Ex,V), which decreases monotonously with the increasing applied bias and could change its sign at a sufficiently high bias. The characteristic of this coherent factor can explain the observed rapid decrease of tunneling magnetoresistance (TMR) with increasing bias and the sign change of TMR in some ferromagnetic junctions. Furthermore, numerical results for asymmetry barrier junctions provide a goo...

Magneto-transport properties of heavy-fermion systems

Starting from the two-conduction-band Anderson lattice model, the magneto-transport properties of heavy-fermion systems are studied in the slave-boson mean-field theory. The residual magnetoresistivities induced by different kinds of impurity are calculated, and the experimentally detected positive maximum structure in the residual magnetoresistance of heavy-fermion systems is reproduced. The transition of field-dependent resistivity from non-monotonic to monotonic behaviour with increasing temperature can be explained naturally by including the charge fluctuation effect. The influences of applied pressure are also investigated.

The electrical resistivity of the two-conduction-band heavy-fermion alloy system

The electrical resistivity of the heavy-fermion alloys is calculated through a two-conduction-band slave boson model using the self-consistent coherent potential approximation method. The results indicate that for very low temperatures the resistivity of the alloy system follows the rho 0+AT2 law and the coefficient A changes from negative values to positive values as the alloy concentration increases. The occurrence of the resistivity maximum at a finite temperature is also obtained on increasing the concentration of heavy-fermion alloys.

Normal-state properties of heavy-fermion systems with conduction-band impurities

The effects of conduction-band (CB) impurities in heavy-fermion systems are studied on the basis of the periodic Anderson model in the framework of a slave-boson mean-field theory within the single-site coherent-potential approximation (CPA). We provide a simplified CPA formalism for the treatment of mixing disorder induced by CB alloying. The density of states for conduction and localized f electrons can be calculated self-consistently in the whole impurity-concentration range. The concentration dependence of the specific-heat coefficient, static magnetic susceptibility and resistivity at low temperatures as well as the Kondo temperature in the alloys is obtained. The results indicate that the experimental observations in upon doping with Au, Ag and Al can be qualitatively explained as the effects of mixing disorder.

On the thermodynamic properties of heavy-fermion systems in a magnetic field

Using a two-conduction-band periodic Anderson model, the influence of a magnetic field on the thermodynamic properties of heavy-fermion systems is studied by means of the slave-boson mean-field theory. A depression of the heavy-fermion state by the magnetic field is found. A theoretical explanation for the anomalous volume magnetostriction of heavy-fermion systems is also given.