Study of Thermal and Magnetic Properties in Strongly Correlated Materials

Abstract

The search for quantum materials is always an exciting field in condensed matter physics. The strongly correlated materials are one of the most intensively studied systems for decades. Due to the complex interplay between the electronic, magnetic, and structural degrees of freedom, the strongly correlated materials display a broad range of interesting phenomena. With the introduction of the concept of topology, topological quantum materials have attracted tremendous attention in recent years. Among them, superconductors with non-trivial topologies are one of the most studied materials due to the existence of gapless boundary states, such as the zero-energy bound states, in them, which have the potential of realizing fault-tolerant quantum computations. A new type of measurement technique with high sensitivity is urgently demanded to reveal the complicated electronic and magnetic properties in quantum materials. We develop a highly sensitive torque differential magnetometry using the qPlus mode of a quartz tuning fork. We observe a sharp resonance of the quartz tuning fork at low temperatures down to 20 mK. We calibrate our torque differential magnetometry by measuring the angular dependence of the hysteresis loop in single-crystal Fe0.25TaS2. Furthermore, we demonstrate thehigh sensitivity of the torque differential magnetometry by measuring the quantum oscilla- tions of a bismuth single crystal. To use the tuning fork magnetometry in a wet cryogenic system, we also make vacuum cells for the tuning forks which could hold a high vacuum at liquid Helium temperature. We also demonstrate the application of tuning fork magne- tometry in a pulsed magnetic field up to 65 T by measuring the hysteresis loop and melting field of underdoped high-temperature superconductors YBa2Cu3Oy. We conduct thermal transport study in two strongly correlated materials, vanadium dioxides (VO2) and SmBaMn2O6. We investigate the thermal conductivity across the phases transitions in VO2 and SmBaMn2O6 single crystals and get one-order-of magnitude enhancement in the thermal conductivity within the metal-insulator transition. These experiments shed light on the role played by phonon across the first-order struc- tural transition. These experiments also solve the thermal management issues in solid-state materials and could bring potential applications in electronic devices. To reveal the superconducting gap structure of a topological superconductor candidate, we conduct the heat capacity measurement in the Nb-doped Bi2Se3 single crystals. The heat capacity shows an exponential decay when T approaches zero, which indicates a nodeless superconducting gap structure. Both the nematic order observed in the torque magnetometry measurement and the nodeless gap structure obtained by the heat capacity measurement indicate an odd parity topological superconductor. We also present a study of the Nernst effect in an iron-based superconductor with a non-trivial band topology Fe1+yTe1−xSex. A non-zero Nernst signal is observed in a narrow temperature region around the superconducting transition temperature Tc at a zero field. This anomalous Nernst signal shows symmetric dependence on the external magnetic field and indicates an unconventional vortex contribution in an s-wave superconductor with a strong spin-orbit coupling, which is originated from the local magnetic moments of the interstitial Fe atoms. Our experiments also provide the first evidence of a locally brokentime-reversal symmetry in bulk Fe1+yTe1−xSex single crystals. In summary, my Ph.D. thesis focuses on the development of new measurement tech- niques such as the torque differential magnetometry and the thermal measurement setup compatible with PPMS, which are capable of resolving novel properties of many solid-state materials.