Bath-induced decay of Stark many-body localization

We investigate the relaxation dynamics of an interacting Stark-localized system coupled to a dephasing bath, and compare its behavior to the conventional disorder-induced many body localized system. Specifically, we study the dynamics of population imbalance between even and odd sites, and the growth of the von Neumann entropy. For a large potential gradient, the imbalance is found to decay on a time scale that grows quadratically with the Wannier-Stark tilt. For the non-interacting system, it shows an exponential decay, which becomes a stretched exponential decay in the presence of finite interactions. This is different from a system with disorder-induced localization, where the imbalance exhibits a stretched exponential decay also for vanishing interactions. As another clear qualitative difference, we do not find a logarithmically slow growth of the von-Neumann entropy as it is found for the disordered system. Our findings can immediately be tested experimentally with ultracold atoms in optical lattices

Doctor of Philosophy

dissertationPerylene tetracarboxylic diimide (PTCDI) derivatives, typical n-type organic semiconductors with high thermal- and photostability, have been extensively investigated for one-dimensional (1D) self-assembly and their applications in electronic and opto-electronic devices. Unfortunately, the intrinsically low electrical conductivity of PTCDI-based materials hinders further development of functions and applications. To solve this problem, covalently linked electron donor-acceptor (D-A) PTCDI molecules were designed, synthesized, and assembled into nanofibers in this work. Their electrical properties and the thermal- and photo-effects have been systematically studied. In addition to providing an improved understanding of the basic properties of the materials, these studies also open new and potential applications of the D-A PTCDI nanofibers. We designed a PTCDI molecule with 1-methylpiperidine (MP) substituted as electron donor to construct self-doped semiconductors, through one-dimensional (1D) self-assembly of the molecules into a nanofiber structure. The resultant nanofibers exhibit much higher conductivity than the other reported PTCDI molecules. The mechanism studies demonstrate that the MP moieties reduce the adjacent PTCDI core into an anionic radical, which acts as the n-type dopant in the PTCDI lattice. Such highly conductive nanofiber materials can be used as chemiresistive sensor for vapor detection of hydrogen peroxide. Our further study on the MP-PTCDI nanofibers reveals a persistent photoconductivity (PPC) effect, which is sustained conductivity after light illumination is terminated. Systematic study demonstrates the PPC effect is predominantly caused by the D-A structure of PTCDI. This study helps understand the PPC mechanism, and guide the design of new material structures for sustained charge separation to further enhance the photovoltaic and photocatalytic efficiency of organic semiconductor materials. Thermoactivated conductivity was studied in the nanofiber materials assembled from other two D-A PTCDI molecules both in the dark and under visible light illumination. A symmetric n-dodecyl side chain substituted PTCDI nanofiber was used as a control for the comparative study. The charge transport properties are strongly dependent on the PTCDI molecular structure and packing states within the nanofibers. The comprehensive understanding of the thermoactivated conductivity in PTCDI nanofibers can assist in designing new D-A molecules that can be fabricated into nanofibers to be used as temperature sensor with increased sensitivity

A Case Study of the Influence of Cross-Cultural Learning and Teaching Experiences on Pre-Service Teachers’ Perception of Teachers’ Professional Standards

Under the aegis of a Canadian SSHRC project, The Reciprocal Learning in Teacher Education and School Education between China and Canada (RLTESECC), a group of Chinese pre-service teachers joined a three-month exchange immersion program in Canada, and had opportunities to attend university teacher education courses, work with local secondary teachers, and participant in education-related activities and events. This cross-cultural learning and teaching experience not only enriched these Chinese pre-service teachers’ cultural understanding, strengthen their English expression abilities, enrich their pedagogical knowledge and skills, but also changed their opinions on teachers between eastern and western more or less, leading to their new perspectives regarding teaching profession. This paper aims to explore the impact of this cross-cultural program on pre-service teachers’ perception of teachers’ qualities and teaching professional standards. Through surveys by questionnaire, interview, and participants’ reflections, the study found that some changes happened in pre-service teachers’ perception and understanding of elementary and secondary teachers’ qualities before and after they went abroad. And the causes of these changes are also discussed in this study. Findings of this study have practical implications for construction and implementation of teacher professional standards and pre-service teacher education for both China and Canada