Transport and Tunneling Investigation of Strongly Correlated Superconducting and Magnetic Thin Films

Complex physical phenomena, such as superconductivity, colossal magnetoresistance (CMR) effect, multi-ferroics, metal-insulator transition, quantum phase transition, etc. in strongly correlated materials have been enduring topics in condensed matter physics. The overarching theme of this dissertation is the study of transport and electronic states in emergent phases with distinct magnetic and electronic properties in strongly correlated materials in thin film forms. At first, we have investigated anisotropic electronic phase separation (EPS) of optimally doped La2/3Ca1/3MnO3 (LCMO) thin films under various degrees of anisotropic strain by static magnetotransport and dynamic relaxation measurements. Distinct from the prototype perovskite manganite LPCMO with well-known micrometer scale EPS, the bulk optimally doped LCMO does not exhibit the large-scale EPS near a transition from paramagnetic insulating phase (PMI) to ferromagnetic metallic (FMM) phase at a high temperature. Through epitaxial growth of LCMO thin films on NGO (001) substrates and post-growth annealing, an antiferromagnetic insulating phase is induced in the FMM ground state and results in a large-scale EPS of coexisting AFI and FMM phases below the bulk metal insulator transition (MIT). Substantial resistivity anisotropies along the two orthogonal in-plane directions in the EPS region were experimentally probed by static temperature and magnetic field dependent resistivity measurements. More strikingly, with increasing annealing time, resistivity along the tensile-strained [010] direction becomes progressively larger than that along the compressive-strained [100] direction in the EPS region. The enhanced resistivity anisotropy suggests that the EPS is characterized by phase-separated FMM entities with a preferred orientation along [100] direction, possibly due to the deformation and rotation of the MnO6 octahedra under the enhanced anisotropic strain via the post-growth annealing. Furthermore, the EPS was found to exhibit glass-like behavior. The resistivity measured at fixed temperatures relaxes logarithmically over a long period of time. The relaxation behavior also shows a coherent enhancement with increasing annealing time. By fitting the relaxation data to a phenomenological model, the fitted parameters, resistive viscosity and characteristic relaxation time were found to evolve with temperature, showing a close correlation with the static measurements in the EPS states. In another project, we have investigated the superconductor-insulator quantum phase transitions tuned by disorder (d), magnetic impurity (MI) and magnetic field (B) in ultrathin Pb films by electrical transport measurement and single electron tunneling spectroscopy. In the past decade, the investigation of SITs in homogeneous thin films by transport measurement from our group has provided valuable insights to the mechanisms of SITs. There are two main theoretical models to explain SITs. The first one emphasizes that a transition from a superconducting state to a fermionic insulator without the existence of the superconducting order parameter, the formation of Cooper pairs is completely suppressed at the transition. The other one calls for a bosonic insulator with localized Cooper pairs. d-tuned and MI-tuned SITs well fit the fermionic framework, and both share common transport features, such as a sharp resistive transition to the superconducting state, a well-defined phase boundary, and a weakly insulating state near the phase boundary. While B-tuned SITs are a canonical example of the bosonic model. The resistive transition to the superconducting state is broadened by an application of magnetic field. Rather than a clear phase boundary near the transition, emerged resistive reentrance and double reentrance indicate phase fluctuation of the superconducting parameter is the main driving force for the transition, suggesting the survival of Cooper pairs in the insulating phase. Electron tunneling spectroscopy has been proposed to directly probe the existence and evolution of the superconductivity in these SITs. For B-tuned SITs, the existence of Cooper pairs is supposed to be detected even in the global insulating phase of thin films. More importantly, the approach also allows us to compare the evolutions of the normal state density of state among these SITs, particularly for d-tuned and MI-tuned SITs. Transport results show that MI has little influence on the normal state sheet resistance near the transition, while increasing disorder gradually raises the normal state sheet resistance. These observations suggest that the normal state density of states behaves differently in the two transitions. The experimental setup is a dilution refrigerator incorporated with in situ quench condensation, electrical measurement, and sample rotation, enabling us to achieve and tune SITs in the same sample by different parameters, and systematically check and compare the evolution of the density of states in the SITs. Up to now, we have performed transport and tunneling measurements for d-tuned SITs in homogeneous Pb films in a 4He quench probe and the modified dilution refrigerator. The transport results are consistent with previous experiments from our group. Increasing disorder leads to a SIT characterized with a sharp resistive transition to a zero resistance state, a well-defined phase boundary, and a gradual reduction of the superconducting critical temperature. The preliminary tunneling testing in the quench probe successfully reveals the suppression of the superconducting energy gap and the normal state density of state by the increasing disorder. In the modified dilution refrigerator, we still observed a concomitant suppression of the normal state density of states. Unfortunately, we were not able to reproduce valid tunneling spectra to study the evolution of the superconducting energy gap near the Fermi level. Possible reasons for the unsatisfying tunneling results are discussed at the end.