Emergent Transport Phenomena in Low-Dimensional Conductors, Superconductors and Correlated Electron Materials
Yu, Liuqi (author)
Xiong, Peng (professor directing dissertation)
Strouse, Geoffrey F. (university representative)
Bonesteel, Nicholas E. (committee member)
Chiorescu, Irinel (committee member)
Tabor, Samuel L. (committee member)
Department of Physics (degree granting department)
Florida State University (degree granting institution)
2014
text
The exploration and understanding of new states of matter resulting from reduced dimensionality and electron correlations has been at the forefront of condensed matter physics research for decades. The goal of this dissertation is to study the magnetotransport properties of several electronic materials including strongly correlated electronic systems (EuB6 and manganites), 2D and 1D disordered superconductors, and magnetically-doped topological insulators. The central aim of the research is the study of emergent phases, phase competition, and phase transitions due to disorder, magnetic and electronic interactions, and dimensional confinement in these materials. The activities are of fundamental interest in the basic physics of magnetism, superconductivity, magnetoelectronic and spin-dependent transport, and they may produce new insights on several pertinent and longstanding open questions in condensed matter physics. The results may also have practical implications for potential applications in spintronics and superconducting micro- or nano-electronics devices. In the past few years, the surface states of topological insulators have attracted intense interest. The unique properties of the surface states provide a fertile ground to explore emergent phenomenon, such as Majorana fermion-like excitations and quantum anomalous Hall effect. They are also considered attractive candidates for spintronic devices with pure spin current that can be controlled by electric field. Hall effect and magnetoresistance measurements were performed on single crystals of the semimetallic ferromagnet EuB6 and thin films of anisotropically strained ferromagnetic perovskite La0.67Ca0.33MnO3 (LCMO). Unusual nonlinear features in the Hall resistivity are observed in both materials. Specifically, in the paramagnetic phase at any given temperature, the Hall resistivity in both materials is found to exhibit two linear regions with a distinct change in its slope at a well-defined magnetic field. The nonlinear Hall effect does not stem from the conventional anomalous Hall effect. Rather, the switching field increases with increasing temperature but they always correspond to the same magnetization. It is inferred that both EuB6 and LCMO undergo a magnetic field driven percolative phase transition. Thereby, the switching of Hall resistivity at a constant critical magnetization is interpreted as a signature of the percolative phase transition and electronic phase separation in the paramagnetic phase. Moreover, in the strained LCMO thin films, a peak in the (negative) magnitude of Hall resistivity is observed near the magnetic phase transition. The appearance of the peak is correlated with the emergence of the antiferromagnetic insulating ground state induced by the anisotropic strain. An analogy is made to the giant Hall effect observed in composition driven percolative granular metal films. The peak in the Hall resistivity of LCMO is thus identified as a result of the enhancement of the Hall coefficient near the percolation point, which is a manifestation of magnetic field driven percolative phase transition in an insulating electronic background. Magnetotransport measurements on low dimensional superconductors and topological insulators were carried out in a customized dilution refrigerator which hosts an assortment of in situ capabilities, including film growth, sample rotation, electrical measurements, and magnetic impurity deposition. Thosein situ capabilities enabled a systematic study of the superconductor-insulator transitions in low dimensional superconducting systems, and led to the discovery of giant enhancement of superconductivity by a parallel magnetic field in ultrathin Pb films. The enhancement effect has a clear thickness and magnetic impurity dependence. The mean field HC of the amorphous Pb films can be enhanced by more than 13% over the zero field value. A similar but weaker effect is also observed in the LaAlO3/SrTiO3 heteroepitaxial interfacial superconductor. We found the enhancement effect depends on the strength of the spin-orbit coupling of the material: the stronger the spin-orbit coupling, the larger the enhancement effect. There is currently no theoretical model that would account for all aspects of the observed enhancement effect. In this dissertation, several theoretical scenarios will be discussed and compared with the experimental results, and the constraints on a viable theoretical model imposed by our experiments will be described. The incremental in situ magnetic impurity deposition was employed for a systematic examination of the evolution of the spin-helical surface states of a 3D topological insulator in the presence of increasing spin-flip scattering. Hall effect and magnetoresistance measurements were conducted on Bi2Se3 and (Bi0.5Sb0.5)2Te3 in the presence of electrostatic gating and magnetic impurity. We will focus on (Bi0.5Sb0.5)2Te3, since its Fermi level lies within the bulk band gap without the application of a gate voltage. Ambipolar field effect is observed on (Bi0.5Sb0.5)2Te3 implying effective tuning of the Fermi level and identification of the surface transport in the topological transport regime. Cr atoms were quench-condensed onto the sample surface. Our results show the Cr effectively yields electron doping and reduces the gating efficiency. The weak antilocalization (WAL) effect is found to be surprisingly robust against the magnetic impurity; the cusp-like negative magnetoconductivity remains even at the highest Cr concentration and no apparent weak localization was observed as expected from the theoretically predicted gap opening at the Dirac point. Our analyses reveal an unusual loss of the electron density due to the Cr deposition, which possibly results from the local gap opening by the random magnetic moments. Possible mechanism of gap opening at the Dirac point and the transport manifestations will be discussed.
July 11, 2014.
A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Includes bibliographical references.
Peng Xiong, Professor Directing Dissertation; Geoffrey F. Strouse, University Representative; Nicholas E. Bonesteel, Committee Member; Irinel Chiorescu, Committee Member; Samuel L. Tabor, Committee Member.
Florida State University
FSU_migr_etd-9120
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