Single-Particle and Pair Tunnelling Studies of Superconducting Compounds and Two-Dimensional Films.
I. Experimental Investigation of Vortex-Anti -Vortex Unbinding in Two-Dimensional Superconducting Thin Films. Kosterlitz and Thouless have proposed a theoretical model which suggests the occurrence of phase transitions in several two-dimensional (2D) systems. In particular, it has been argued that the broadening of the resistive transition in 2D superconducting thin films is due to the onset of vortex-anti-vortex unbinding as the temperature approaches the phase transition temperature T(,KT) from below. According to the Kosterlitz-Thouless theory, the super-electron sheet density n(,s)('2D) vanishes above T(,KT) while at T(,KT), a universal relationship exists between n(,s)('2D) (T(,KT)) and T(,KT). To date, all experimental efforts to verify the static Kosterlitz-Thouless universal relationship have involved dynamic measurements of n(,s)('2D). In Sec. I, an alternative procedure using superconducting quantum interferometry is proposed. It is discussed how this technique may be employed to detect the disappearance of quasi-long -range order at the Kosterlitz-Thouless transition and to determine in a static fashion the temperature dependence of the transverse penetration depth (lamda)(,(PERP))(T), whose value at the transition should reflect the universal jump in the superfluid density. The results of measurements using this technique on granular aluminum thin films are presented. In addition, the application of this technique to study ultra-thin, single crystal aluminum films is discussed. II. Single-Particle Tunnelling Studies of the Superconducting A-15 Compound V(,3)Si. Because of the significant technological applications of high-T(,c) materials such as the A-15 compounds it is important that the superconductivity of these materials be well understood. In this section, a discussion of single-particle tunnelling experiments on bulk samples of V(,3)Si is given. The short sampling depth in tunnelling experiments on materials such as the A-15 compounds (about one coherence length (xi) (TURN) 40-50 (ANGSTROM)), requires that the sample surface be well characterized and that suitable tunnelling barrier techniques be developed. For these experiments, the surfaces were prepared using mechanical polishing and ion etching sequences and the junctions were formed using artificial barrier techniques. Auger Electron Spectroscopy was employed for quantitative analysis during sample preparation. It was found that only proximity-effect junctions could be prepared using these techniques. Phonon structure from V(,3)Si was not observed in second derivative measurements. Theories of proximity-effect tunnelling are briefly reviewed and are used to make a determination of the energy gap parameter (DELTA) for V(,3)Si. This analysis yields 2(DELTA)/k(,B)T(,c) = 3.61 (+OR-) 0.01. III. Critical Magnetic Field Measurements on ErRh(,4)B(,4) Thin Films. The interaction of magnetism and superconductivity has attracted considerable theoretical and experimental interest in recent years. The compound ErRh(,4)B(,4) is one member of a series of rare earth rhodium borides which has been found to exhibit superconducting and magnetic behavior at low temperatures. For these experiments, thin film samples of ErRh(,4)B(,4) were prepared using a dc cryo-getter sputtering technique. Critical magnetic field measurements were carried out for both a parallel and perpendicular orientation. It was found from these measurements that the perpendicular critical field is larger than the parallel critical field. This may be the first observation of such an effect in superconductors. It is shown how this unusual result is consistent with a simple mean field model which considers the effect of the large magnetization induced in the film by the applied magnetic field as a consequence of the significant concentration of magnetic ions. Consistency of these calculations requires a value for the magnetic moment of Er('3+) (mu)(,eff) = 8.1(mu)(,B) which is consistent with recent Mossbauer measurements. Analysis of the data using two more detailed microscopic models were not successful. It is discussed how a procedure of fitting the theory to data from one field orientation and using the model parameterization so obtained to calculate the critical field for the other orientation provides a means of critically testing the internal consistency of the theoretical models.
- Pub Date:
- Physics: Condensed Matter