Some superconducting and normal state properties of niobium nitride thin films

We have measured the temperature dependence of the resistance, the superconducting transition temperature, and the upper critical field of niobium nitride thin films. Our samples were made by sputtering in a nitrogen-argon ion beam. The pressure during deposition is typically one to two orders of magnitude lower than that of standard sputtering techniques.     

The CESR and spin wave studies of A-15 films in normal and superconducting states

The electron spin resonance studies have been reported for A-15 superconductors, namely Nb₃Ge, Nb₃Si and V₃Si possessing different T_c values and CESR, Platzmann-Wolff type spin waves, and spin waves of antiferromagnetic type are observed in all the samples. It is found that T_c of Nb₃Ge depends upon the presence and separation of spin wave absorptions from the CESR, and T_c is found to increase when the separation is reduced. It is concluded that the exchange interactions in the conduction band, as manifested by the behaviour of spin waves, are of antiferromagnetic type and they are responsible for superconductivity in A-15 materials studied.     

Nucleation and collapse of the superconducting phase in type-I superconducting films

The phase transition between the intermediate and normal states in type-I superconducting films is investigated using magneto-optical imaging. Magnetic hysteresis with different transition fields for collapse and nucleation of superconducting domains is found. This is accompanied by topological hysteresis characterized by the collapse of circular domains and the appearance of lamellar domains. Magnetic hysteresis is shown to arise from supercooled and superheated states. Domain-shape instability resulting from long-range magnetic interaction accounts well for topological hysteresis.     

Paraconductivity of superconducting tin films

Experimental results on paraconductivity of superconducting thin tin films are compared with theoretical predictions of the Maki-Thompson theory which includes a small correction term proposed by Imamura et al. It is found that the corrected Maki-Thompson expression for paraconductivity of thin superconducting films improves the fit between theory and experimental data at relatively higher temperatures. It also gives a smaller value of δ, a fitting variable in the M-T theory which is known as the pair- breaking interaction parameter, than the value obtained in the unmodified M-T expression for the paraconductivity.     

Gauge fluctuations in superconducting films

In this paper we consider a type-I superconducting film modeled by the Ginzburg-Landau model, confined between two parallel planes a distance L apart from one another. Our approach is based on the Gaussian effective potential in the transverse unitarity gauge, which allows to treat gauge contributions in a compact form. Using techniques from dimensional and -function regularizations, modified by the confinement conditions, we investigate the critical temperature as a function of the film thickness L. The contributions from the scalar self-interaction and from the gauge fluctuations are clearly identified. The model suggests the existence of a minimal critical thickness below which superconductivity is suppressed. A comparison with present experimental observations is done.Received: 19 December 2003, Published online: 9 April 2004PACS: 74.20.-z Theories and models of superconducting state - 11.15.Ex Spontaneous breaking of gauge symmetries - 05.10.Cc Renormalization group methods     

MMW detection by superconducting films-applications of high temperature superconducting films

Conclusions Superconductors exhibit two basic modes of radiation detection —a thermal or bolometer mode and a rectification mode which depends on a nonlinear voltage-current characteristic. The SIS detector has been shown to have a quantum limited current responsivity of e/hf. By analogy, weak link detectors may have a quantum limited voltage responsivity of about 2eR/hf. Granular films behave like arrays of weak links.Both thermal and rectification modes have been observed in HTOS films. They can be characterized by the dependence of film resistance on temperature and current. Analysis of the temperature dependence of film resistance indicates that bolometer responsivities are limited by the broader resistive transition observed in HTOS films. HTOS films show a strong nonlinear dependence of resistance on current at lower temperatures. In this region, we expect voltage responsivities greater than 1 KV/W. This is a very promising mode for detection and mixing of far infrared and millimeter waves.     

Dimensionality of extremely thin superconducting films

Superconducting fluctuations above the transition temperature are strongly dependent on the dimensionality of the samples. Our experiments on extremely thin quench condensed aluminum films show transitions from zero- to one-, two- and three-dimensional behaviour at an average film thickness of 20, 50 and 100 Å respectively.     

Kosterlitz-Thouless-like transition in granular superconducting films

A Kosterlitz-Thouless-like transition in granular superconducting films is considered. A magnetic field fluctuations influence on thermodynamic properties is analyzed. A renormalization of an interaction constant because of “spin-wave” fluctuations is calculated. A role of a weak disorder is considered. A relationship between the transition temperature and the film sheet resistance is found to be in good agreement with experimental data. If the transition temperature is not too low the results are applicable to uniform superconducting films.     

Resistive superconducting transition of vanadium films

The resistive superconducting transition of vanadium films is studied experimentally. The behavior of the resistance is well explained by fluctuation theory. In parallel magnetic fields the pair-breaking parameter increases with the increase of the strength of the magnetic field.     

Pair-breaking current of inhomogeneous superconducting films

The influence of the inhomogeneities in the cross section and in the mean free path on the pair-breaking current I_c is considered using the Ginzburg-Landau theory. In the limiting case of “point” inhomogeneities, the simple analytic dependences of I_c on the parameters of the inhomogeneities and on their concentration are found. The temperature dependence of I_c is obtained.     

Precursor diamagnetism in superconducting Al films

The precursor diamagnetism is studied in a stack of Al films whose thickness is much smaller than the coherence length. The magnetization shows the (T ? T_c)^?1 behavior in a finite range of temperature above T_c. At T_c, in low magnetic fields constant magnetization independent of the field strength is observed. The results are in agreement with the theories of the two-dimensional superconductor.     

Vortices in superconducting bulk, films and SQUIDs

The properties of the ideal periodic vortex lattice in bulk superconductors and in films of any thickness can be calculated from Ginzburg-Landau theory by an iteration method using Fourier series. The London theory yields general analytic expressions for the magnetic field and energy of arbitrary arrangements of straight or curved vortex lines. The elasticity of the vortex lattice is highly nonlocal. The magnetic response of superconductors of realistic shapes like thin and thick strips and disks or thin rectangular plates or films, containing pinned vortices, can be computed within continuum theory by solving an integral equation. A useful example is a thin square with a central hole and a radial slit, used as superconducting quantum interference device (SQUID).