Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Abstract: Topological solitons are crucial to many branches of physics, such as models of fundamental particles in quantum field theory, information carriers in nonlinear optics, and elementary entities in quantum and classical computations. Chiral magnetic materials are a fertile ground for studying solitons. In the past a few years, a huge number of all kinds of topologically protected localized magnetic solitons have been found. The number is so large, and a proper organization and classification is necessary for their future developments. Here we show that many topological magnetic solitons can be understood from the duality of particle and elastic continuum-medium nature of skyrmions. In contrast to the common belief that a skyrmion is an elementary particle that is indivisible, skyrmions behave like both particle and continuum media that can be tore apart to bury other objects, reminiscing particle-wave duality in quantum mechanics. Skyrmions, like indivisible particles, can be building blocks for cascade skyrmion bags and target skyrmions. They can also act as bags and glues to hold one or more skyrmions together. The principles and rules for stable composite skyrmions are explained and presented, revealing their rich and interesting physics.

Abstract: Based on the algebraic equation of motion (AEOM) method, we investigate the transport properties of a quantum dot. We obtain an analytical expression for the dot electron single-particle Green's function, and based on this expression, we plot the dot electron density of states under different biases. We find that the Kondo resonance splits and is suppressed as the bias is increased. In addition, we calculate the differential conductance of the dot and obtain the zero-bias Kondo resonance at different temperatures, which is found to be suppressed as the temperature is increased.

Abstract: Hybrid van der Waals heterostructures of organic semiconductors and transition metal dichalcogenides (TMDs) are promising candidates for various optoelectronic devices, such as solar cells and biosensors. Energy-transfer processes in these materials are crucial for the efficiency of such devices, yet they are poorly understood. In this work, we develop a fully microscopic theory describing the effect of the F\"{o}rster interaction on exciton dynamics and optics in a WSe$_2$/tetracene heterostack. We demonstrate that the differential absorption and time-resolved photoluminescence can be used to track the real-time evolution of excitons. We predict a strongly unidirectional energy transfer from the organic to the TMD layer. Furthermore, we explore the role temperature has in activating the F\"{o}rster transfer and find a good agreement to previous experiments. Our results provide a blueprint to tune the light-harvesting efficiency through temperature, molecular orientation and interlayer separation in TMD/organic heterostructures.

Abstract: We theoretically investigate the F\"orster transfer of an exciton in an ensemble of self-assembled quantum dots randomly distributed on a circular mesa. We use the stochastic simulation method to solve the equation of~motion for the density matrix with a given decay rate. We express the diffusion in terms of the mean square displacement from the initially excited quantum dot. The mean square displacement follows three time stages: ballistic, normal diffusion, and saturation. In addition, the exciton exhibits power-law localization. Using an approximate analytical approach, we provide the formulas that follow the results of numerical studies.

Abstract: This study investigated the formation of multi-voids in polydimethylsiloxane (PDMS) using a multi-pulse irradiation method and explored the impact of laser energy, number of pulses per micron (writing speed), and laser spot size (NA) on the process. The experimental results revealed that multi-void formation occurred due to multi-pulse irradiation in the bulk of PDMS. Additionally, increasing laser energy led to an increase in the number of voids, while the number of voids did not change with an increase in the number of pulses per micron for a fixed laser parameter. However, the size of the voids increased with the number of pulses per micron, and tighter focusing conditions (higher NA) resulted in smaller voids with a shorter distance between them. Furthermore, Finite-Difference-Time-Domain (FDTD) simulations reproduced the generation of void arrays in PDMS using a similar multi-laser pulse approach. By modeling the voids as concentric spheres with densified shells and simulating the laser interaction with the voids, we showed that void array generation in PDMS is a linear mechanism. This study provides valuable insight into the mechanism behind the formation of void arrays in PDMS. The simulation results agrees well with the experimental results to further validate the model and gain a better understanding of the physical processes involved in the generation of void arrays in PDMS.

Abstract: In two-dimensional superlattice materials, the nonlinear current response to a large applied electric field can feature a strong angular dependence. This nonperturbative regime encodes information about the band dispersion and Berry curvature of isolated electronic Bloch minibands. Within the relaxation-time approximation, we obtain analytic expressions for the current in a band-projected theory with time-reversal and trigonal symmetry, up to infinite order in the driving field. For a fixed field strength, the dependence of the current on the direction of the applied field is given by rose curves whose petal structure is symmetry constrained and is obtained from an expansion in real-space translation vectors. We illustrate our theory with calculations on periodically-buckled graphene and twisted double bilayer graphene, wherein the discussed physics can be accessed at experimentally-relevant field strengths.