Ultrafast coherent electron dynamics in solids

Due to recent developments in high-field laser systems, intense sub-cycle pulses can be generated on a routine basis in laser laboratories around the world. The electric fields originating from such few-femtosecond laser pulses can be on the same order of magnitude as the internal electric fields in bulk, crystalline solids. Due to the short duration of the pulses the laser fluence can remain below the damage threshold of the material. This paves the way for exploring strong-field effects in solids in a non-destructive regime experimentally, and hence motivates theoretical investigations in this field. This thesis is about numerical studies of strong-field effects in insulators and semiconductors. In particular, calculations are performed at a quantum mechanical level in order to examine the importance of quantum coherence in light-matter interactions in the strong-field regime. The dynamics of electrons in one-dimensional spatially periodic potentials excited by laser pulses was simulated. Upon introducing phenomenological decoherence into the dynamical equations, it was found that the optical responses calculated from geometric phases of mixed quantum system were in excellent agreement with conventional approaches for evaluating the optically induced current and polarization response. The excellent agreement even extended to highly non-linear, strong-field regimes, and motivated the development of a numerical method to simulate open quantum mechanical systems governed by spatially periodic Hamiltonians subject to perturbations with broken translation symmetry. Density functional theory was also employed to obtain wave functions from first principles for a number of materials, for which time-resolved optical responses were calculated. Field-induced intraband motion was found to modify the interband transitions significantly at high field strengths for transitions that would otherwise be resonant at low field strengths. For semiconducting materials like GaAs, where the transition elements are strongly peaked at the centre of the Brillouin zone, a step-like excitation mechanism was revealed at field strengths on the order of 0.5 V/Å. Similar ab initio methods were used to model the optical Faraday effect in the insulating, wide band gap material Al2O3 for few-cycle pulses. The magnitude of the effect was predicted using non-perturbative methods. Time-dependent calculations confirmed that a near-instantaneous response is to be expected.Es wurde die Dynamik von Elektronen in Festkörpern, die durch intensive, Subzykluslaserpulse erregt werden numerisch untersucht. Die Berechnungen wurden auf der quantenmechanischen Ebene und in verschiedenen, unabhängigen elektromagnetischen Eichungen ausgeführt. Zuerst wurde die Dynamik der Elektronen in eindimensionalen periodischen Potentialen berechnet um die Gültigket von neuen numerischen Verfahren zu bestätigen. Eines dieser Verfahren ermöglicht Simulationen von räumlich periodischen, gemischten Quantensystemen mit Hamilton-Operatoren mit gebrochener Translationssymmetrie. Durch Anwendung der Dichtefunktionaltheorie wurden Wellenfunktionen für Halbleiter und Insulatoren hergeleitet. Danach konnt der zeitliche Verlauf des optisch induzierten Strom nach ersten Prinzipien bestimmt werden. Die Bedeutung von intraband Bewegungen für Elektronen im halbleitenden Material GaAs wurde ebenfalls untersucht. Bei Erregung mit resonanten Pulsen konnte ein stufenförmiger Anregungsmechanismus beobachtet werden. Ähnliche Methoden wurden verwendet, um die Größe des optischen Faraday-Effektes in einem Insulator mit einer Bandlücke, grösser der Fotonenergie beider Pulse, zu bestimmen. Diese Berechnungen deuten darauf hin, dass ultraschnelle Kontrolle der optisch induzierten Chiralität möglich ist

Design Synthesis and Optimization of Permanent Magnet Synchronous Machines Based on Computationally-Efficient Finite Element Analysis

In this dissertation, a model-based multi-objective optimal design of permanent magnet ac machines, supplied by sine-wave current regulated drives, is developed and implemented. The design procedure uses an efficient electromagnetic finite element-based solver to accurately model nonlinear material properties and complex geometric shapes associated with magnetic circuit design. Application of an electromagnetic finite element-based solver allows for accurate computation in intricate performance parameters and characteristics. The first contribution of this dissertation is the development of a rapid computational method that allows accurate and efficient exploration of large multi-dimensional design spaces in search of optimum design(s). The computationally efficient finite element-based approach developed in this work provides a framework of tools that allow rapid analysis of synchronous electric machines operating under steady-state conditions. In the developed modeling approach, major steady-state performance parameters such as, winding flux linkages and voltages, average, cogging and ripple torques, stator core flux densities, core losses, efficiencies and saturated machine winding inductances, are calculated with minimum computational effort. In addition, the method includes means for rapid estimation of distributed stator forces and three-dimensional effects of stator and/or rotor skew on the performance of the machine. The second contribution of this dissertation is the development of the design synthesis and optimization method based on a differential evolution algorithm. The approach relies on the developed finite element-based modeling method for electromagnetic analysis and is able to tackle large-scale multi-objective design problems using modest computational resources. Overall, computational time savings of up to two orders of magnitude are achievable, when compared to current and prevalent state-of-the-art methods. These computational savings allow one to expand the optimization problem to achieve more complex and comprehensive design objectives. The method is used in the design process of several interior permanent magnet industrial motors. The presented case studies demonstrate that the developed finite element-based approach practically eliminates the need for using less accurate analytical and lumped parameter equivalent circuit models for electric machine design optimization. The design process and experimental validation of the case-study machines are detailed in the dissertation

Models of wave-function collapse, underlying theories, and experimental tests

We describe the state of the art in preparing, manipulating and detecting coherent molecular matter. We focus on experimental methods for handling the quantum motion of compound systems from diatomic molecules to clusters or biomolecules.Molecular quantum optics offers many challenges and innovative prospects: already the combination of two atoms into one molecule takes several well-established methods from atomic physics, such as for instance laser cooling, to their limits. The enormous internal complexity that arises when hundreds or thousands of atoms are bound in a single organic molecule, cluster or nanocrystal provides a richness that can only be tackled by combining methods from atomic physics, chemistry, cluster physics, nanotechnology and the life sciences.We review various molecular beam sources and their suitability for matter-wave experiments. We discuss numerous molecular detection schemes and give an overview over diffraction and interference experiments that have already been performed with molecules or clusters.Applications of de Broglie studies with composite systems range from fundamental tests of physics up to quantum-enhanced metrology in physical chemistry, biophysics and the surface sciences.Nanoparticle quantum optics is a growing field, which will intrigue researchers still for many years to come. This review can, therefore, only be a snapshot of a very dynamical process

Beam-based Correction and Optimization for Accelerators

This book provides systematic coverage of the beam-based techniques that accelerator physicists use to improve the performance of large particle accelerators, including synchrotrons and linacs. It begins by discussing the basic principles of accelerators, before exploring the various error sources in accelerators and their impact on the machine's performances. The book then demonstrates the latest developments of beam-based correction techniques that can be used to address such errors and covers the new and expanding area of beam-based optimization. This book is an ideal, accessible reference book for physicists working on accelerator design and operation, and for postgraduate studying accelerator physics. Features: Entirely self-contained, exploring the theoretic background, including algorithm descriptions, and providing application guidance Accompanied by source codes of the main algorithms and sample codes online Uses real-life accelerator problems to illustrate principles, enabling readers to apply techniques to their own problems Xiaobiao Huang is an accelerator physicist at the SLAC National Accelerator Laboratory at Stanford University, USA. He graduated from Tsinghua University with a Bachelor of Science in Physics and a Bachelor of Engineering in Computer Science in 1999. He earned a PhD in Accelerator Physics from Indiana University, Bloomington, Indiana, USA, in 2005. He spent three years on thesis research work at Fermi National Accelerator Laboratory from 2003-2005. He has worked at SLAC as a staff scientist since 2006. He became Accelerator Physics Group Leader of the SPEAR3 Division, Accelerator Directorate in 2015. His research work in accelerator physics ranges from beam dynamics, accelerator design, and accelerator modelling and simulation to beam based measurements, accelerator control, and accelerator optimization. He has taught several courses at US Particle Accelerator School (USPAS), including Beam Based Diagnostics, Accelerator Physics, Advanced Accelerator Physics, and Special Topics in Accelerator Physics

Ultrafast coherent electron dynamics in solids

Due to recent developments in high-field laser systems, intense sub-cycle pulses can be generated on a routine basis in laser laboratories around the world. The electric fields originating from such few-femtosecond laser pulses can be on the same order of magnitude as the internal electric fields in bulk, crystalline solids. Due to the short duration of the pulses the laser fluence can remain below the damage threshold of the material. This paves the way for exploring strong-field effects in solids in a non-destructive regime experimentally, and hence motivates theoretical investigations in this field. This thesis is about numerical studies of strong-field effects in insulators and semiconductors. In particular, calculations are performed at a quantum mechanical level in order to examine the importance of quantum coherence in light-matter interactions in the strong-field regime. The dynamics of electrons in one-dimensional spatially periodic potentials excited by laser pulses was simulated. Upon introducing phenomenological decoherence into the dynamical equations, it was found that the optical responses calculated from geometric phases of mixed quantum system were in excellent agreement with conventional approaches for evaluating the optically induced current and polarization response. The excellent agreement even extended to highly non-linear, strong-field regimes, and motivated the development of a numerical method to simulate open quantum mechanical systems governed by spatially periodic Hamiltonians subject to perturbations with broken translation symmetry. Density functional theory was also employed to obtain wave functions from first principles for a number of materials, for which time-resolved optical responses were calculated. Field-induced intraband motion was found to modify the interband transitions significantly at high field strengths for transitions that would otherwise be resonant at low field strengths. For semiconducting materials like GaAs, where the transition elements are strongly peaked at the centre of the Brillouin zone, a step-like excitation mechanism was revealed at field strengths on the order of 0.5 V/Å. Similar ab initio methods were used to model the optical Faraday effect in the insulating, wide band gap material Al2O3 for few-cycle pulses. The magnitude of the effect was predicted using non-perturbative methods. Time-dependent calculations confirmed that a near-instantaneous response is to be expected.Es wurde die Dynamik von Elektronen in Festkörpern, die durch intensive, Subzykluslaserpulse erregt werden numerisch untersucht. Die Berechnungen wurden auf der quantenmechanischen Ebene und in verschiedenen, unabhängigen elektromagnetischen Eichungen ausgeführt. Zuerst wurde die Dynamik der Elektronen in eindimensionalen periodischen Potentialen berechnet um die Gültigket von neuen numerischen Verfahren zu bestätigen. Eines dieser Verfahren ermöglicht Simulationen von räumlich periodischen, gemischten Quantensystemen mit Hamilton-Operatoren mit gebrochener Translationssymmetrie. Durch Anwendung der Dichtefunktionaltheorie wurden Wellenfunktionen für Halbleiter und Insulatoren hergeleitet. Danach konnt der zeitliche Verlauf des optisch induzierten Strom nach ersten Prinzipien bestimmt werden. Die Bedeutung von intraband Bewegungen für Elektronen im halbleitenden Material GaAs wurde ebenfalls untersucht. Bei Erregung mit resonanten Pulsen konnte ein stufenförmiger Anregungsmechanismus beobachtet werden. Ähnliche Methoden wurden verwendet, um die Größe des optischen Faraday-Effektes in einem Insulator mit einer Bandlücke, grösser der Fotonenergie beider Pulse, zu bestimmen. Diese Berechnungen deuten darauf hin, dass ultraschnelle Kontrolle der optisch induzierten Chiralität möglich ist

Beam-based Correction and Optimization for Accelerators

This book provides systematic coverage of the beam-based techniques that accelerator physicists use to improve the performance of large particle accelerators, including synchrotrons and linacs. It begins by discussing the basic principles of accelerators, before exploring the various error sources in accelerators and their impact on the machine's performances. The book then demonstrates the latest developments of beam-based correction techniques that can be used to address such errors and covers the new and expanding area of beam-based optimization. This book is an ideal, accessible reference book for physicists working on accelerator design and operation, and for postgraduate studying accelerator physics. Features: Entirely self-contained, exploring the theoretic background, including algorithm descriptions, and providing application guidance Accompanied by source codes of the main algorithms and sample codes online Uses real-life accelerator problems to illustrate principles, enabling readers to apply techniques to their own problems Xiaobiao Huang is an accelerator physicist at the SLAC National Accelerator Laboratory at Stanford University, USA. He graduated from Tsinghua University with a Bachelor of Science in Physics and a Bachelor of Engineering in Computer Science in 1999. He earned a PhD in Accelerator Physics from Indiana University, Bloomington, Indiana, USA, in 2005. He spent three years on thesis research work at Fermi National Accelerator Laboratory from 2003-2005. He has worked at SLAC as a staff scientist since 2006. He became Accelerator Physics Group Leader of the SPEAR3 Division, Accelerator Directorate in 2015. His research work in accelerator physics ranges from beam dynamics, accelerator design, and accelerator modelling and simulation to beam based measurements, accelerator control, and accelerator optimization. He has taught several courses at US Particle Accelerator School (USPAS), including Beam Based Diagnostics, Accelerator Physics, Advanced Accelerator Physics, and Special Topics in Accelerator Physics

Cooperative surmounting of bottlenecks

The physics of activated escape of objects out of a metastable state plays a key role in diverse scientific areas involving chemical kinetics, diffusion and dislocation motion in solids, nucleation, electrical transport, motion of flux lines superconductors, charge density waves, and transport processes of macromolecules, to name but a few. The underlying activated processes present the multidimensional extension of the Kramers problem of a single Brownian particle. In comparison to the latter case, however, the dynamics ensuing from the interactions of many coupled units can lead to intriguing novel phenomena that are not present when only a single degree of freedom is involved. In this review we report on a variety of such phenomena that are exhibited by systems consisting of chains of interacting units in the presence of potential barriers. In the first part we consider recent developments in the case of a deterministic dynamics driving cooperative escape processes of coupled nonlinear units out of metastable states. The ability of chains of coupled units to undergo spontaneous conformational transitions can lead to a self-organised escape. The mechanism at work is that the energies of the units become re-arranged, while keeping the total energy conserved, in forming localised energy modes that in turn trigger the cooperative escape. We present scenarios of significantly enhanced noise-free escape rates if compared to the noise-assisted case. The second part deals with the collective directed transport of systems of interacting particles overcoming energetic barriers in periodic potential landscapes. Escape processes in both time-homogeneous and time-dependent driven systems are considered for the emergence of directed motion. It is shown that ballistic channels immersed in the associated high-dimensional phase space are the source for the directed long-range transport

Dynamics in Cold Atomic Gases: Resonant Behaviour of the Quantum Delta-Kicked Accelerator and Bose-Einstein Condensates in Ring Traps

In this thesis, the dynamics of cold, trapped atomic gases are investigated, and the prospects for exploiting their nonlinear dynamics for inertial sensing are discussed. In the first part, the resonant and antiresonant dynamics of the atom-optical quantum delta-kicked accelerator with an initial symmetric momentum distribution are considered. The system is modelled as an ideal, non-interacting atomic gas, with a temperature-dependence governed by the width of the initial momentum distribution. The existence of resonant and antiresonant behaviour is established, and analytic expressions describing the dynamics of momentum moments of the time-evolved momentum distribution are derived. In particular, the momentum moment dynamics in both the resonant and antiresonant regimes depend strongly on the width of the initial momentum distribution. The resonant dynamics of all even-ordered momentum moments are shown to exhibit a power-law growth with an exponent given by the order of the moment in the zero-temperature regime, whereas for a broad, thermal initial momentum distribution the exponent is reduced by one. The cross-over in the intermediate regime is also examined, and a characteristic time is determined up to which the system exhibits dynamics associated with the zero-temperature regime. A similar analysis is made for the temperature-dependence of the antiresonant dynamics. This general behaviour is demonstrated explicitly by considering a Maxwell-Boltzmann and uniform momentum distribution, allowing exact expressions describing the dynamics of the second- and fourth-order momentum moments, and momentum cumulants, to be obtained. The relevance of these results to the potential of using this system in accurate determinations of the local gravitational acceleration is discussed. In the second part, the dynamics of one- and two-component Bose-Einstein Condensates prepared in a counter-rotating superposition of flows in a quasi-1D toroidal trap are studied. Particular attention is paid to the dynamical stability of the initial state in the presence of atom-atom interactions, included via a mean-field description within the Gross-Pitaevskii equation. A broad regime of dynamical stability using a two-component BEC is identified, in which a typical implementation using Rb-87 is predicted to lie. A proof-of-principle Sagnac atom-interferometer using a two-component Rb-87 BEC is then presented, and the accumulation of the Sagnac phase is shown to be possible via relative population measurement or, alternatively, through the continuous monitoring the precession of atomic density fringes. In contrast to conventional Sagnac interferometers, the accumulation of the Sagnac phase is independent of the enclosed area of the interferometer. The prospects of using this system for high-precision determinations of rotation is discussed