Dynamical correlation functions and the related physical effects in three-dimensional Weyl/Dirac semimetals

We present a unified derivation of the dynamical correlation functions including density-density, density-current and current-current, of three-dimensional Weyl/Dirac semimetals by use of the Passarino-Veltman reduction scheme at zero temperature. The generalized Kramers-Kronig relations with arbitrary order of subtraction are established to verify these correlation functions. Our results lead to the exact chiral magnetic conductivity and directly recover the previous ones in several limits. We also investigate the magnetic susceptibilities, the orbital magnetization and briefly discuss the impact of electron interactions on these physical quantities within the random phase approximation. Our work could provide a starting point for the investigation of the nonlocal transport and optical properties due to the higher-order spatial dispersion in three-dimensional Weyl/Dirac semimetals.Comment: 21 pages, 3+1 figures, 1 table. Accepted in PR

RKKY interaction in three-dimensional electron gases with linear spin-orbit coupling

We theoretically study the impacts of linear spin-orbit coupling (SOC) on the Ruderman-Kittel-Kasuya-Yosida interaction between magnetic impurities in two kinds of three-dimensional noncentrosymmetric systems. It has been found that linear SOCs lead to the Dzyaloshinskii-Moriya interaction and the Ising interaction, in addition to the conventional Heisenberg interaction. These interactions possess distinct range functions from three dimensional electron gases and Dirac/Weyl semimetals. In the weak SOC limit, the Heisenberg interaction dominates over the other two interactions in a moderately large region of parameters. Sufficiently strong Rashba SOC makes the Dzyaloshinskii-Moriya interaction or the Ising interaction dominate over the Heisenberg interaction in some regions. The change in topology of the Fermi surface leads to some quantitative changes in periods of oscillations of range functions. The anisotropy of Ruderman-Kittel-Kasuya-Yosida interaction in bismuth tellurohalides family BiTe$X$ ($X$ = Br, Cl, and I) originates from both the specific form of Rashba SOC and the anisotropic effective mass. Our work provides some insights into understanding observed spin textures and the application of these materials in spintronics.Comment: 11 pages, 4 figures, Final Version in PR

Continuous focusing and separation of microparticles with acoustic and magnetic fields

Microfluidics enables a diverse range of manipulations (e.g., focusing, separating, trapping, and enriching) of micrometer-sized objects, and has played an increasingly important role for applications that involve single cell biology and the detection and diagnosis of diseases. In microfluidic devices, methods that are commonly used to manipulate cells or particles include the utilization of hydrodynamic effects and externally applied field gradients that induce forces on cells/particles, such as electrical fields, optical fields, magnetic fields, and acoustic fields. However, these conventional methods often involve complex designs or strongly depend on the properties of the flow medium or the interaction between the fluid and fluidic channels, so this dissertation aims to propose and demonstrate novel and low-cost techniques to fabricate microfluidic devices to separate microparticles with different sizes, materials and shapes by the optimized acoustic and magnetic fields. The first method is to utilize acoustic bubble-enhanced pinched flow for microparticle separation; the microfluidic separation of magnetic particles with soft magnetic microstructures is achieved in the second part; the third technique separates and focuses microparticles by multiphase ferrofluid flows; the fourth method realizes the fabrication and integration of microscale permanent magnets for particle separation in microfluidics; magnetic separation of microparticles by shape is proposed in the fifth technique. The methods demonstrated in this dissertation not only address some of the limitations of conventional microdevices, but also provide simple and efficient method for the separation of microparticles and biological cells with different sizes, materials and shapes, and will benefit practical microfluidic platforms concerning micron sized particles/cells --Abstract, page iv

Evolutionary genomics of dynamic sex chromosomes in the Salicaceae

Identifying the sex-determination region (SDR) and other genomic features of sex chromosomes are of great importance in the studies of the evolution of sex. However, the process of accurately identifying the size and location of the SDR is often difficult, even when a genomic sequence is available. This usually is hindered by large repetitive elements and a lack of recombination in the SDR. In this thesis, I assemble sex chromosomes with whole genomic sequencing data, identify SDRs and explore their genomic features in two sister species from the Salicaceae family. I also develop an interpretation of the lability of the sex configuration in the two species. In Chapter 2, I use quantitative trait locus mapping and a genome-wide association study to characterize the genomic composition of the SDR in a reference genome derived a female Salix purpurea clone. I show that the SDR in S. purpurea has a female heterogametic (ZW) system on chromosome 15. The SDR is inferred to be between 5 to 7 Mb long and overlapping with the centromere. This SDR has several classic features like reduced recombination and high structural polymorphism. Intriguingly, chromosome 19 contains sex-associated markers, which raises the possibility of a translocation of the SDR within the Salicaceae lineage. In Chapter 3, I improve the quality of assembly of sex chromosomes in S. purpurea with long-reads sequencing data and a modified map. Using an improved assembly of the SDR, I show that two consecutive palindromes span over a region of 200 kb, with conspicuous 20 kb stretches of highly conserved homologous sequences among the four arms in the female-specific regions of the SDR. Comparison to the genome of a closely related species S. suchowensis provides evidence for gene conversion occurring among the palindrome arms. The hypothesis of the translocation of the SDR within the Salicaceae could not be rejected. In Chapter 4, I use a similar strategy from Chapter 3 to study the SDR of a male Populus trichocarpa clone. I show that the SDR in P. trichocarpa has a male heterogametic (XY) system on chromosome 19. A cluster of inverted repeats that are homologous with a response regulator gene is present in the male-specific region in the SDR. This research provides important genomic resources for futures studies in these two species as well as the evolution of SDRs in the Salicaceae