The Development of Tunnel Diode Oscillators And High Magnetic Field Studies of Unconventional Superconductor UTe2: Unveiling the Phase Diagram And Unconventional Hall Effect

Abstract

This dissertation presents an extensive investigation of UTe2 under various magnetic field and pressure conditions by utilizing the Tunnel Diode Oscillator (TDO) and electrical transport. The study is primarily divided into two parts. The first part focuses on the utilization of the TDO to establish a magnetic susceptibility measurement device. The TDO's frequency, highly sensitive to inductance changes due to the negative I-V curve regime of the tunnel diode, is harnessed to create a measurement circuit compatible with low-temperature refrigerators. The design and development process of these devices are thoroughly detailed. The second part unveils the comprehensive (H, T, P) phase diagram of UTe2 under magnetic fields reaching 41 T along the crystallographic b-axis, combined with applied pressures of up to 18.8 kbar. Utilizing magnetoresistance and tunnel diode oscillator measurements, we investigated the pressure-induced evolutions of multiple phases. By monitoring the field-induced transition between superconducting and magnetic field-polarized phases across various pressures (up to 18.8 kbar), we track the suppression of this transition with increasing pressure. This suppression culminates in the disappearance of superconductivity near 16 kbar, concurrently leading to a distinct, pressure-induced magnetic ordered state that is stable at zero field. The evolution of a second superconducting phase and its upper critical field under pressure are also investigated, leading to insights into the confinement of superconductivity by magnetic phases and the boundaries of triplet superconductivity.