Experimental microwave properties of innovative superconductors

Abstract

This thesis summarizes a three-year experimental study. The work is focused on the investigation of the high frequency electrical transport properties of complex superconducting systems. The experimental part included the development, setup and testing of new experimental systems, the improvement of the performance of the existing setups. First, a new dielectric resonator (DR), which operates on the TE011 mode at 8.2 GHz was designed and put into operation. Second, the existing system, based on a sapphire DR excited on the TE011 at the frequency 48 Hz was upgraded for transmission measurements. Third, the design of a rectangular DR for the study of anisotropic properties was investigated and a prototype was tested at low temperature. Two classes of complex superconducting structures were then studied: ferromagnetic/superconductor/ferromagnetic (S/F/S) Nb/Pd0.81Ni0.19/Nb multilayers, driven by the interest in the phenomena occurring due to the interaction of ferromagnetism and superconductivity, and YBa2Cu3O7-x (YBCO) samples with nanosize BaZrO3 (BZO) columnar pinning centers because of the interest in the potential applications. The investigation of Nb/PdNi/Nb and bulk Nb film samples with different PdNi layer thickness in the temperature range T=2.4-15 K and magnetic field range H=0- 3 T was performed by combining wideband (2-20 GHz) and resonant (8 GHz) measurements. The temperature dependencies of the surface resistivity and of the London penetration depth were measured, and the role of structural disorder was assessed by EXAFS spectroscopy. The evolution of the field dependence of the finite resistivity due to the free flow of magnetic flux lines was studied and compared with present theories. A previously unreported field dependence was detected and characterized for various ferromagnetic layer thickness. It was, finally, determined that for the samples with larger ferromagnetic layer, a reduction of the superfluid density induces a reduction in magnetic fields lines pinning and points to a reduction of the superfluid. YBCO/BZO samples in the temperature range T=60-120 K in magnetic field up to 0.8T were studied using resonator technique at 48 GHz. Samples prepared using different growing techniques (Pulsed laser deposition method, PLD, and Metalorganic decomposition method, MOD, were studied). The effect of the BZO concentration on flux pinning in YBCO samples was studied. The study of the field orientation revealed the flux pinning, important for the reduction of the losses was due to a dynamic (“flux caging”) effect. In summary, it was shown that microwave technique is the effective tool for the study of the microscopic properties of the various superconducting systems, which are important as for the application as for understanding of the nature of the superconductivity.