Modelling evaporation and phase behaviour of particle suspensions

We present two statistical mechanics based methods for simulating the evaporation of droplets of nanoparticle suspensions from upon a heterogeneous surface. These are based on a generalised lattice-gas model. Properties such as wettability and the dynamic contact angle, are determined by the attraction strength parameters between particles and from the dynamic mobility coefficients. Both models incorporate the effects of surface roughness and slip at the surface. The two approaches used are Monte Carlo (MC) computer simulations and Dynamical Density Functional Theory (DDFT). We calculate the bulk fluid phase behaviour including the influence of the suspended nanoparticles, comparing results from the two approaches. We also calculate thermodynamic quantities such as the surface tensions. Our results show that the presence of steps in the surface can be crucial in controlling dewetting from heterogeneous surfaces. We also observe that coffee ring stains can be formed via the coupling of evaporation to phase separation and that the advective hydrodynamics within the droplets in not required for ring stains to form

Hierarchical dynamics of individual RNA helix base pair formation and disruption

This thesis explores the RNA folding problem using single-molecule field effect transistors (smFETs) to measure the lifetimes of individual RNA base-pairing rearrangements. In the course of this research, considerable computational, chemical, and engineering contributions were developed so that the single-molecule measurements could be conducted and quantified. These advancements have allowed, on the basis of the smFET data collected herein, the quantification of a kinetic model for RNA stem-loop structures which has been generalized to quantitatively explore the phenomenological observation that an RNA found in the bacillus subtilis strain acts as a metabolite-sensing switch, allowing RNA polymerase to transcribe the messenger RNA when the metabolite is present and preventing transcription when the metabolite is absent. Together, the data presented quantify a simple model for the base pairing rearrangements that underlie RNA folding

On exciton-vibration and exciton-photon interactions in organic semiconductors

Organic semiconductors are materials that are promising for novel optoelectronic applications, such as more efficient solar cells and LEDs. The optoelectronic response of these materials is dominated by bound electron-hole pairs called excitons, which are often strongly affected by hundreds of possible molecular vibrations. Although quantum theory contains all the ingredients to describe these complex phenomena, in practice it is only possible to solve the corresponding equations in small systems with few vibrations. As a result, it has been common to assume weak exciton-vibration interactions and to employ perturbative approaches. Similarly, exciton-photon interactions have almost universally been treated in the so-called weak coupling regime. However, in recent years it has become increasingly clear that these approximations can break down in organic semiconductors, placing an important roadblock towards the novel energy-harvesting technologies that could be based on these materials. In this thesis we address this issue by developing methods to treat exciton-photon and exciton-vibration interactions, without relying on any approximation regarding their magnitude. We propose a first principles description of hybrid exciton-light (polariton) states that result from strong exciton-photon interactions. We discuss a method to treat strong exciton-vibration interactions, showing that the spatial extent of exciton states controls their magnitude. Subsequently, we present a beyond Born-Oppenheimer method based on tensor networks to study real-time exciton dynamics. By using these methods, we show how selective excitation of vibrational modes can enhance charge transfer. Moreover, through rigorous comparison to experiments, we highlight that tensor network methods are highly accurate, and we generate a `movie' of the photophysical process of singlet fission, which occurs during early light-harvesting by organic molecules and has the potential to increase solar cell efficiencies. Finally, we construct a singlet fission model including the effects of excess energy, vibrations and the solvent of molecules concurrently, demonstrating that the fission mechanism can be qualitatively changed in a controlled manner, allowing for its acceleration by an order of magnitude.Winton Programme for the Physics of Sustainabilit

Non-Hermitian Topological Magnonics

Dissipation in mechanics, optics, acoustics, and electronic circuits is nowadays recognized to be not always detrimental but can be exploited to achieve non-Hermitian topological phases or properties with functionalities for potential device applications. As elementary excitations of ordered magnetic moments that exist in various magnetic materials, magnons are the information carriers in magnonic devices with low-energy consumption for reprogrammable logic, non-reciprocal communication, and non-volatile memory functionalities. Non-Hermitian topological magnonics deals with the engineering of dissipation and/or gain for non-Hermitian topological phases or properties in magnets that are not achievable in the conventional Hermitian scenario, with associated functionalities cross-fertilized with their electronic, acoustic, optic, and mechanic counterparts, such as giant enhancement of magnonic frequency combs, magnon amplification, (quantum) sensing of the magnetic field with unprecedented sensitivity, magnon accumulation, and perfect absorption of microwaves. In this review article, we address the unified approach in constructing magnonic non-Hermitian Hamiltonian, introduce the basic non-Hermitian topological physics, and provide a comprehensive overview of the recent theoretical and experimental progress towards achieving distinct non-Hermitian topological phases or properties in magnonic devices, including exceptional points, exceptional nodal phases, non-Hermitian magnonic SSH model, and non-Hermitian skin effect. We emphasize the non-Hermitian Hamiltonian approach based on the Lindbladian or self-energy of the magnonic subsystem but address the physics beyond it as well, such as the crucial quantum jump effect in the quantum regime and non-Markovian dynamics. We provide a perspective for future opportunities and challenges before concluding this article.Comment: 101 pages, 35 figure

Dynamics of ion Coulomb crystals

The field of quantum simulations has achieved a remarkable success through the development of highly controllable and accessible quantum platforms, which pro- vide insights into the microscopic properties of complex large-scale systems that are otherwise difficult to analyze. Many of the platforms utilized in this pursuit are derived from the field of atomic, molecular, and optical physics. One particularly popular candidate is provided by trapped ions, whose vibrational and electronic degrees of freedom can be effectively combined through laser pulses to engineer desired model Hamiltonians or quantum circuits. Trapped ions constitute as well the basis for modern atomic clocks, the most precise frequency standards currently available. They find further applications in metrology, geodesy, and fundamental physics experiments. In this Thesis, we investigate the dynamics of vibrational modes in trapped ion crystals, utilizing them as a versatile platform to explore various many-body phenomena. We first focus on the expansion dynamics of local excitations and on heat transport within ion crystals hosting structural defects that undergo a sliding- to-pinned transition. We observe a significant reduction in conductivity when the crystal symmetry is spontaneously broken during the transition, and show that resonances between crystal eigenmodes lead to distinct softening signatures associated with energy localization. We then delve into the effects of thermal and quantum fluctuations on the vibrational modes of ion crystals near two distinct structural transitions. We observe the emergence of a prolonged symmetric phase stabilized by thermal and quantum fluctuations, and develop effective theories that reduce the degrees of freedom to the modes that drive the transitions. Finally, we discuss how to engineer spin-orbit coupling and on-site interaction energies for vibrational quantum excitations using two different external driving schemes. While the simulation of spin models with ions typically involves the use of two electronic states, we propose interpreting the two local oscillation modes in an ion crystal as a pseudospin. We show how using Floquet engineering ideas allows for spin flips in Coulomb-induced vibron hopping, resulting in a non-trivial coupling between spatial motion and spin evolution, that results in a markedly non-Abelian dynamics. Subsequently, we explore the simulation of Hubbard models in trapped ions by coupling the vibrational Fock states to an internal level system. Our findings include the observation of bound states in the strong interaction limit of the resulting Jaynes-Cummings-Hubbard model. By investigating these topics, we aim to contribute to the understanding of vibrational dynamics in trapped ion crystals, and shed light on their potential for simulating condensed matter systems, offering insights into phenomena that are otherwise challenging to explore.DFG/Sonderforschungsbereich 1227 DQ-mat/274200144/E