809 research outputs found
An inverse Prandtl–Ishlinskii model based decoupling control methodology for a 3-DOF flexure-based mechanism
A modified Prandtl–Ishlinskii (P–I) hysteresis model is developed to form the feedforward controller for a 3-DOF flexure-based mechanism. To improve the control accuracy of the P–I hysteresis model, a hybrid structure that includes backlash operators, dead-zone operators and a cubic polynomial function is proposed. Both the rate-dependent hysteresis modeling and adaptive dead-zone thresholds selection method are investigated. System identification was used to obtain the parameters of the newly-developed hysteresis model. Closed-loop control was added to reduce the influence from external disturbances such as vibration and noise, leading to a combined feedforward/feedback control strategy. The cross-axis coupling motion of the 3-DOF flexure-based mechanism has been explored using the established controller. Accordingly, a decoupling feedforward/feedback controller is proposed and implemented to compensate the coupled motion of the moving platform. Experimental tests are reported to examine the tracking capability of the whole system and features of the controller. It is demonstrated that the proposed decoupling control methodology can distinctly reduce the coupling motion of the moving platform and thus improve the positioning accuracy and trajectory tracking capability
Active Brownian Particles. From Individual to Collective Stochastic Dynamics
We review theoretical models of individual motility as well as collective
dynamics and pattern formation of active particles. We focus on simple models
of active dynamics with a particular emphasis on nonlinear and stochastic
dynamics of such self-propelled entities in the framework of statistical
mechanics. Examples of such active units in complex physico-chemical and
biological systems are chemically powered nano-rods, localized patterns in
reaction-diffusion system, motile cells or macroscopic animals. Based on the
description of individual motion of point-like active particles by stochastic
differential equations, we discuss different velocity-dependent friction
functions, the impact of various types of fluctuations and calculate
characteristic observables such as stationary velocity distributions or
diffusion coefficients. Finally, we consider not only the free and confined
individual active dynamics but also different types of interaction between
active particles. The resulting collective dynamical behavior of large
assemblies and aggregates of active units is discussed and an overview over
some recent results on spatiotemporal pattern formation in such systems is
given.Comment: 161 pages, Review, Eur Phys J Special-Topics, accepte
Thermal stability of metastable magnetic skyrmions
Magnetic skyrmions are two-dimensional, localized, particle-like, topologically non-trivial magnetic spin textures. In recent years, they have attracted a lot of interest as potential candidates for novel spintronics applications. Isolated skyrmions are metastable excitations of the ferromagnetic ground state. They are separated from it by an activation energy, which may be overcome at finite temperature under the effect of thermal fluctuations. In this thesis, we study the thermal stability of metastable magnetic skyrmions on the two-dimensional square lattice, for which we use an atomistic spin model.
This task is firstly carried out via a numerical implementation of Langer's statistical theory for the decay of metastable states. The paths of minimum energy that lead to the skyrmion annihilation are computed via the geodesic nudged elastic band method. The transition state at the barrier top, which is a saddle point (SP), is precisely identified by a climbing image algorithm. We focus on chiral magnetic skyrmions and we look at two types of annihilation mechanisms: collapse, in which the skryrmion progressively shrinks in size until it annihilates, and escape through a boundary. We find that the thermally significant modes are the modes localized to the skyrmion, in contrast to the rest of the collective spin-wave modes, which extend to the entire lattice and contribute weakly. Important variations of the attempt frequency over several orders of magnitude are found, depending on the mechanism and on the value of the external magnetic field. They originate from strong entropic effects which come from the difference in configurational entropy between the metastable skyrmion state and the saddle point. In the cases we studied, the configurational entropy decreases at the SP (Delta S < 0), which results in lowered attempt frequencies, and enhanced skyrmion stability. We refer to this effect as entropic narrowing in the SP region. The strong entropic contribution mainly originates from the skyrmion's internal modes, and is generally more pronounced for collapse mechanisms.
Next, we use forward flux sampling (FFS) to compute skyrmion collapse rates as a function of the applied field, and compare them with the previous results from Langer's theory. This is an important step, because the use of Langer's theory is based on many assumptions. We obtain a good agreement between both methods, thus confirming the strong dependence of the attempt frequency on the external field. While in magnetism, it is common practice to only focus on activation barriers and assume a characteristic value of the prefactor in the gigahertz regime, we conclude that due to a strong entropic contribution, internal energy barriers are not enough in order to correctly predict the lifetime of magnetic skyrmions, and it is essential to also evaluate a rate prefactor.
Lastly, we look at paths to annihilation of first- and second-order skyrmions and antiskyrmions on the frustrated square lattice. Frustrated exchange has been found to arise from interface effects in certain systems where nanoscale interface skyrmions have been observed. We find that, in certain regions of parameter space, the annihilation of skyrmionic solutions no longer occurs through an isotropic type of collapse, and instead involves the injection of the opposite topological charge into the system, by means of the nucleation of merons and antimerons. Alternatively, the second-order (anti)skyrmion may split into a bound (anti)skyrmion pair, which involves no change in the total topological charge
Cooperative quantum phenomena
Quantum cooperativity is evident in light-matter platforms where quantum
emitter ensembles are interfaced with confined optical modes and are coupled
via the ubiquitous electromagnetic quantum vacuum. Cooperative aspects such as
dipole-dipole interactions and subradiance find applications in the design of
nanoscale coherent light sources and highly-reflective quantum metasurfaces
made up of hundreds of optically trapped atoms, in the implementation of
topological quantum optics on subwavelength arrays of emitters, in quantum
metrology and quantum information. The quick bursts of radiation from a
collection of quasi-indistiguishable emitters provides an alternative approach
to standard lasers by introducing superradiant lasers operating at extremely
low intracavity power. This tutorial provides a set of theoretical tools to
tackle the behavior responsible for the onset of cooperativity in light-matter
systems by extending open quantum system dynamics methods, such as the master
equation and quantum Langevin equations, to electron-photon interactions in
strongly coupled and correlated quantum emitters ensembles. These analytical
approaches are then also extended to frequency disordered or vibronically
coupled quantum emitter ensembles with wide relevance ranging from atoms in
optical lattices, quantum dots in solid state environments or molecular quantum
systems
Hydration, polymorphism and disorder in organic solids, including materials of pharmaceutical relevance
The studies described in this thesis are concerned with understanding and rationalising the inter-relationships between structural and other physical properties of organic solids encompassing hydrates, polymorphs and homologous series. This is achieved through assessment of the crystal packing arrangements and the nature of the intermolecular interactions, which may have a profound impact on the stability and physical properties of organic materials. Such an understanding is critically important when selecting or designing materials that demonstrate specific defined properties. The first part of the thesis describes the relationships between hydrate and anhydrate phases, exemplified by tetraphenyl phosphonium bromide as a model system. The relative inter-relationships between the two main phases of this material are rationalised by characterisation of both structural and dynamic properties. The second aspect is focused on the influence of short-range interactions on the long-range periodicity and subsequent properties of materials using 4-hydroxy benzoic acid esters as a homologous series to explore odd-even alternations, seen for other series containing long chain functionalities. In this study, the nature of short-range interactions is found to correlate with the observed odd-even alternation in material properties, with deviations from these observations shown to correspond to the presence of significant disorder in the structures. The third aspect concerns the properties of a polymorphic pharmaceutical material, AZD7140. The crystal structures for both polymorphs were solved and assessment of hydrogen bonding motifs and thermodynamic evaluation using phase diagrams allowed the selection of a robust polymorph as a suitable development material. The final aspect of the thesis concerns strategies that may be employed to solve structures of disordered systems directly from XRPD data using direct space methodology.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
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
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