High Frequency Probes of Superconductivity and Magnetism in Anisotropic Materials in Very High Magnetic Field

In this dissertation, I present a study of a wide range of organic and inorganic materials using radio frequency (rf) measurement methods. The organic samples under study were λ-(BETS )2 GaCl4 and λ-(BETS )2 FeCl4 . In the λ-(BETS )2 GaCl4 , the H-T superconductivity phase diagram was studied using the tunnel diode oscillator (TDO) method and compared with simultaneous four terminals resistivity measurements. These simultaneous measurements show signs of para-conductivity in this material. The same method was used to study the λ-( BETS )2 FeCl4 sample which is a field induced superconductor (FISC). The inorganic materials that I have studied include Ba 0.55K0.45Fe2 As2 and USb2 . In Ba0.55K0.45Fe 2As2 (which belongs to the recently discovered Pnictide superconductors family), I have studied the H-T phase diagram for magnetic fields applied parallel and perpendicular to the crystallographic c-axis up to 65 tesla and in temperature as low as 4 K . Ba0.55K0.45 Fe2As2 was studied by a new rf technique that I have developed recently (PDO≡Proximity Detector Oscillator). The rf measurements of Ba0.55 K0.45Fe2 As2 from my work support the prediction of an unconventional multigap superconductivity in this material. In the USb 2 sample, a Fermi surfaces measurement was performed by the TDO rf probe and by a torque magnetometer for comparison purposes in high magnetic fields up to 65 tesla and in temperatures above 0.5 K . I found that both the rf and the torque measurements reveal a cylindrical Fermi surface with approximately the same effective mass. However, the rf and the torque measurements reveal some differences in the frequencies obtained from the FFT obtained for each method. In this dissertation, most of the measurements were performed using rf probes like the TDO or the PDO. The PDO method has successfully replaced the TDO method to perform rf measurements in all different kinds of magnets (dc and pulsed).