Universal Borromean Binding in Spin-Orbit Coupled Ultracold Fermi Gases

Borromean rings and Borromean binding, a class of intriguing phenomena as three objects are linked (bound) together while any two of them are unlinked (unbound), widely exist in nature and have been found in systems of biology, chemistry and physics. Previous studies have suggested that the occurrence of such a binding in physical systems typically relies on the microscopic details of pairwise interaction potentials at short-range, and is therefore non-universal. Here, we report a new type of Borromean binding in ultracold Fermi gases with Rashba spin-orbit coupling, which is {\it universal} against short-range interaction details, with its binding energy only dependent on the s-wave scattering length and the spin-orbit coupling strength. We show that the occurrence of this universal Borromean binding is facilitated by the symmetry of the single-particle dispersion under spin-orbit coupling, and is therefore {\it symmetry-selective} rather than interaction-selective. The state is robust over a wide range of mass ratio between composing fermions, which are accessible by Li-Li, K-K and K-Li mixtures in cold atoms experiments. Our results reveal the importance of symmetry factor in few-body physics, and shed light on the emergence of new quantum phases in a many-body system with exotic few-body correlations.Comment: 6+1.5 pages, 5 figures, published versio

APPROACHING DNA METHYLATION AT THE NANOSCALE

Epigenetics involves a variety of biochemical modifications occurring on chromatin that are able to regulate and fine-tune genetic activities without altering the underlying DNA sequence. So far, four types of epigenetic modulation have been extensively studied: DNA methylation, histone post-translational modification, non-coding RNA, and nuclear organization. A fast-growing body of evidence suggests that epigenetic mechanism plays a fundamental role in physiology and pathology. Of the elucidated epigenetic processes, DNA methylation is the one under intense research due to its direct impact on gene expression, which dynamically bridges the microscale genotype and macroscale phenotype. However, to date our understanding of DNA methylation has primarily come from ensemble and end-point measurements using a population of cells or bulky samples. Unlike genetic aberrations (e.g., amplification, deletion, translocation, and mutation) that are rare to occur and resistant to reverse, events relating to epigenetic modifications take place in a time- and context-dependent manner, thus necessitating nanoscale tools to dissect epigenetic dynamics at a finer spatiotemporal resolution. In my Ph.D. work, a group of advanced single-molecule fluorescence tools, in addition to methodologies in bioengineering, nanotechnology, molecular biology, and bioinformatics, is implemented to approach the DNA methylation-related activities. Furthermore, part of my research aims to translate the new discoveries from single-cell experiments to biomedical applications. Another focus of my dissertation is on effective manipulation of DNA methylation by novel techniques including nanomaterials and optogenetics. Taken together, these efforts lay the groundwork for better understanding of DNA methylation and provide ample avenues for improving epigenetic therapies

Solidification orientation relationships in Al and Mg alloys

This thesis explores solidification orientation relationships (ORs) in intermetallic compounds (IMCs) and Al and Mg alloys. In the Al3Ti-TiB2 system, it is found that the nucleation of Al3Ti on TiB2 and the pushing and engulfment of TiB2 particles by growing Al3Ti crystals both form reproducible ORs during solidification. The nucleation OR is identified by solidifying multiple small Al3Ti crystals on one large (0001) facet of TiB2. Pushing and engulfment ORs are investigated by statistical analysis of EBSD measurements, DFT calculations of interface energies, and imaging of TiB2 particles being pushed and engulfed by Al3Ti facets. It is shown that the lowest energy OR is formed by nucleation as well as by pushing/engulfment. The higher energy ORs, formed by pushing and engulfment, correspond to local interfacial energy minima and can be explained by rotation of TiB2 particles on Al3Ti facets during pushing. ORs formed by cyclic twinning of low symmetry IMCs is studied in Al3Ti, Ag3Sn, Al45Cr7 and Al13Fe4. It is argued that deeper undercooling induced by higher cooling rate favours the nucleation of metastable phases and/or the formation of short-range order with high symmetry in the melt, which then nucleated/transformed into stable phases with all orientation variants to the higher-symmetry parent phases. This thesis then applies the new understanding developed in the previous chapters to explore the formation mechanism for the above-random proportion of special grain boundaries in FCC-Al and HCP-Mg after equiaxed solidification. Two main mechanisms are examined and, by combining statistical EBSD analysis and DFT calculations, it is found that the measured preferred grain boundaries with twin ORs correspond to local interfacial energy minima and, for the alloy systems studied here, it is likely due to the rotation and movement between neighbouring grains during solidification instead of nucleation from icosahedral quasicrystals and/or icosahedral short-range order.Open Acces