122 research outputs found
Two-mode Bose-Einstein condensate in a high-frequency driving field that directly couples the two modes
A two-mode Bose-Einstein condensate coupled by a high-frequency modulation
field is found to display rich features. An effective stationary Hamiltonian
approach reveals the emergence of additional degenerate eigenstates as well as
new topological structures of the spectrum. Possible applications, such as the
suppression of nonlinear Landau-Zener tunneling, are discussed. An interesting
phenomenon, which we call "deterministic symmetry-breaking trapping" associated
with separatrix crossing, is also found in an adiabatic process.Comment: 5 pages, 3 figures, revised version, to appear in Phys. Rev.
Hierarchical Theory of Quantum Adiabatic Evolution
Quantum adiabatic evolution is a dynamical evolution of a quantum system
under slow external driving. According to the quantum adiabatic theorem, no
transitions occur between non-degenerate instantaneous eigen-energy levels in
such a dynamical evolution. However, this is true only when the driving rate is
infinitesimally small. For a small nonzero driving rate, there are generally
small transition probabilities between the energy levels. We develop a
classical mechanics framework to address the small deviations from the quantum
adiabatic theorem order by order. A hierarchy of Hamiltonians are constructed
iteratively with the zeroth-order Hamiltonian being determined by the original
system Hamiltonian. The th-order deviations are governed by a th-order
Hamiltonian, which depends on the time derivatives of the adiabatic parameters
up to the th-order. Two simple examples, the Landau-Zener model and a
spin-1/2 particle in a rotating magnetic field, are used to illustrate our
hierarchical theory. Our analysis also exposes a deep, previously unknown
connection between classical adiabatic theory and quantum adiabatic theory.Comment: 10 pages, 6 figures, 29 reference
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Ultrafast Spectroscopic Studies of Solution-Processing Organic Photovoltaic Materials and Devices
Organic solar cells (OSCs) have potential applications in wearable electronics as well as in-door and building-integrated photovoltaics, owing to features such as low-temperature processing, light weight and flexibility, and synthetic versatility. Due to the development of new materials systems with unique optoelectronic properties, a breakthrough in the OSC performance has been achieved. However, knowledge of the underlying mechanism still lags behind. Such understanding is essential for further development of organic photovoltaics. The needed comprehensive investigation mainly comes from studying the carrier dynamics on promising OSC systems. To this aim, the used tool kits, ultrafast spectroscopic methods, are the subject of this thesis.
First three chapters of the thesis present the general paradigm of photoconversion in OSCs and outlines the key models used to describe their photovoltaic performance. The ability of OSCs to generate electrical current in response to absorbing sunlight greatly differs, depending on materials and the following device fabrication methods. In the device operation, the performance is related to the conversion efficiency from molecular excited states into charge carriers. Chapter 2 presents a historical review of different material combination, device architecture and key processes relevant for solar cell performance. This leads to the key questions that this thesis addresses: which molecular properties are most important for photoconversion in OSCs? How much energy is required to convert from molecular excited states to free charges? What are the optimal preparation procedure and microstructure of OSC devices? Chapter 3 gives an overview of spectroscopic tools used to address these questions.
Chapters 4 to 8 target above questions in application to different material systems, from the ‘classical’ broadly studied polythiophene-fullerene blends to the high-performance OSC devices based on non-fullerene acceptors (NFAs). Firstly, the rate of exciton dissociation is studied in state-of-the-art organic blends with NFAs. The charge generation is found to be slow (ps), in sharp contrast with the ultrafast timescales in traditional blends based on fullerene acceptors. Secondly, a new technique is developed, namely temperature-dependent pump-push photocurrent spectroscopy, which is able to measure the strength of interaction between charges in working OSCs. This technique is applied to research which factors govern the dissociation of molecular excited states in fullerene-based blends. Thirdly, recombination processes are studied using a set of ultrafast techniques, focusing on their effect on device performance, and the dependence on processing conditions and used materials. Finally, a new method ‘via sequential deposition’ is demonstrated for fabrication of efficient NFA-based devices. We discuss the properties of OSCs fabricated by this method and the potential for its commercialisation.China Scholarship Council (No. 201503170255
All-optical Imprinting of Geometric Phases onto Matter Waves
Traditional optical phase imprinting of matter waves is of a dynamical
nature. In this paper we show that both Abelian and non-Abelian geometric
phases can be optically imprinted onto matter waves, yielding a number of
interesting phenomena such as wavepacket re-directing and wavepacket splitting.
In addition to their fundamental interest, our results open up new
opportunities for robust optical control of matter waves.Comment: 5 pages, 2 figures, to appear in Phys. Rev.
Quantum Geometric Tensor in -Symmetric Quantum Mechanics
A series of geometric concepts are formulated for -symmetric
quantum mechanics and they are further unified into one entity, i.e., an
extended quantum geometric tensor (QGT). The imaginary part of the extended QGT
gives a Berry curvature whereas the real part induces a metric tensor on
system's parameter manifold. This results in a unified conceptual framework to
understand and explore physical properties of -symmetric systems
from a geometric perspective. To illustrate the usefulness of the extended QGT,
we show how its real part, i.e., the metric tensor, can be exploited as a tool
to detect quantum phase transitions as well as spontaneous
-symmetry breaking in -symmetric systems.Comment: main text of 5 pages, plus supplementary material of 8 page
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