Quantum decay of metastable states in small magnetic particles

We present an imaginary-time path-integral study of the problem of quantum decay of a metastable state of a uniaxial magnetic particle placed in the magnetic field at an arbitrary angle. Our findings agree with earlier results of Zaslavskii obtained by mapping the spin Hamiltonian onto a particle Hamiltonian. In the limit of low barrier, weak dependence of the decay rate on the angle is found, except for the field which is almost normal to the anisotropy axis, where the rate is sharply peaked, and for the field approaching the parallel orientation, where the rate rapidly goes to zero. This distinct angular dependence, together with the dependence of the rate on the field strength, provides an independent test for macroscopic spin tunneling.

Quantum critical effects in mean-field glassy systems

We consider the effects of quantum fluctuations in mean-field quantum spin-glass models with pairwise interactions. We examine the nature of the quantum glass transition at zero temperature in a transverse field. In models (such as the random orthogonal model) where the classical phase transition is discontinuous an analysis using the static approximation reveals that the transition becomes continuous at zero temperature.

Development of semiconductor metal oxide gas sensors for the detection of NO2 and H2S gases

One of the main challenges in the development of metal-oxide gas sensors is enhancement of selectivity to a particular gas. Currently, two general approaches exist for enhancing the selective properties of sensors. The first one is aimed at preparing a material that is specifically sensitive to one compound and has low or zero cross-sensitivity to other compounds that may be present in the working atmosphere. To do this, the optimal temperature, doping elements, and their concentrations are investigated. Nonetheless, it is usually very difficult to achieve an absolutely selective metal oxide gas sensor in practice. Another approach is based on the preparation of materials for discrimination between several analyte in a mixture. It is impossible to do this by using one sensor signal. Therefore, it is usually done either by modulation of sensor temperature or by using sensor arrays. The present work focus on the characterization of n-type semiconducting metal oxides like Tungsten oxide (WO3), Zinc Oxide (ZnO) and Indium oxide (In2O3) for the gas sensing purpose. For the purpose of gas sensing thick as well as thin films were fabricated. Two different gases, NO2 and H2S gases were selected in order to study the gas sensing behaviour of these metal oxides. To study the problem associated with selectivity the metal oxides were doped with metals and the gas sensing characteristics were investigated. The present thesis is entitled “Development of semiconductor metal oxide gas sensors for the detection of NO2 and H2S gases” and consists of six chapters.

Quantum dynamics of crystals of molecular nanomagnets inside a resonant cavity

It is found that crystals of molecular nanomagnets exhibit enhanced magnetic relaxation when placed inside a resonant cavity. A strong dependence of the magnetization curve on the geometry of the cavity has been observed, providing indirect evidence of the coherent microwave radiation by the crystals. A similar dependence has been found for a crystal placed between the Fabry-Perot superconducting mirrors.

Quantum suppression of shot noise in field emitters

We have analyzed the shot noise of electron emission under strong applied electric fields within the Landauer-Bttiker scheme. In contrast to the previous studies of vacuum-tube emitters, we show that in new generation electron emitters, scaled down to the nanometer dimensions, shot noise much smaller than the Schottky noise is observable. Carbon nanotube field emitters are among possible candidates to observe the effect of shot-noise suppression caused by quantum partitioning.

Fabrication of a quantum well heterostructure based on plasma polymerized aniline and its characterization using STM/STS

Two-dimensional electronic systems play a crucial role in modern electronics and offer a multitude of opportunities to study the fundamental phenomena at low dimensional physics. A quantum well heterostructure based on polyaniline (P) and iodine doped polyaniline (I) thin films were fabricated using radio frequency plasma polymerization on indium tin oxide coated glass plate. Scanning probe microscopy and scanning electron microscopy studies were employed to study the morphology and roughness of the polymer thin films. Local electronic density of states (LDOS) of the P–I–P heterostructures is probed using scanning tunnelling spectroscopy (STS). A step like LDOS is observed in the P–I–P heterostructure and is attributed to the quantum well confinement of electrons in the polymer heterostructure.

Thermo- Optic and Nonlinear-Optical studies on CdSe Quantum Dots for Photonic Applications

In general, linear- optic, thermo- optic and nonlinear- optical studies on CdSe QDs based nano uids and their special applications in solar cells and random lasers have been studied in this thesis. Photo acous- tic and thermal lens studies are the two characterization methods used for thermo- optic studies whereas Z- scan method is used for nonlinear- optical charecterization. In all these cases we have selected CdSe QDs based nano uid as potential photonic material and studied the e ect of metal NPs on its properties. Linear optical studies on these materials have been done using vari- ous characterization methods and photo induced studies is one of them. Thermal lens studies on these materials give information about heat transport properties of these materials and their suitability for applica- tions such as coolant and insulators. Photo acoustic studies shows the e ect of light on the absorption energy levels of the materials. We have also observed that these materials can be used as optical limiters in the eld of nonlinear optics. Special applications of these materials have been studied in the eld of solar cell such as QDSSCs, where CdSe QDs act as the sensitizing materials for light harvesting. Random lasers have many applications in the eld of laser technology, in which CdSe QDs act as scattering media for the gain.

Model calculations on the influence of quantum electrodynamical effects on the chemistry of superheavy elements.

Using a phenomenological model, the influence of quantum electrodynamical effects on the prediction of the chemical behavior of superheavy elements within a relativistic Dirac-Slater calculation was investigated. This influence will be small and nondetectable for elements up to Z = 114. For elements near Z = 164 some changes in the ground state configurations occur but the chemical behavior will not change. Using this heuristic model, it is also possible to calculate elements beyond Z = 175. As an example we have chosen element E184 and are now able to make more valid speculations about the chemical behavior of the element than Penneman and co-workers could.

Entanglement analysis of atomic processes and quantum registers

During recent years, quantum information processing and the study of N−qubit quantum systems have attracted a lot of interest, both in theory and experiment. Apart from the promise of performing efficient quantum information protocols, such as quantum key distribution, teleportation or quantum computation, however, these investigations also revealed a great deal of difficulties which still need to be resolved in practise. Quantum information protocols rely on the application of unitary and non–unitary quantum operations that act on a given set of quantum mechanical two-state systems (qubits) to form (entangled) states, in which the information is encoded. The overall system of qubits is often referred to as a quantum register. Today the entanglement in a quantum register is known as the key resource for many protocols of quantum computation and quantum information theory. However, despite the successful demonstration of several protocols, such as teleportation or quantum key distribution, there are still many open questions of how entanglement affects the efficiency of quantum algorithms or how it can be protected against noisy environments. To facilitate the simulation of such N−qubit quantum systems and the analysis of their entanglement properties, we have developed the Feynman program. The program package provides all necessary tools in order to define and to deal with quantum registers, quantum gates and quantum operations. Using an interactive and easily extendible design within the framework of the computer algebra system Maple, the Feynman program is a powerful toolbox not only for teaching the basic and more advanced concepts of quantum information but also for studying their physical realization in the future. To this end, the Feynman program implements a selection of algebraic separability criteria for bipartite and multipartite mixed states as well as the most frequently used entanglement measures from the literature. Additionally, the program supports the work with quantum operations and their associated (Jamiolkowski) dual states. Based on the implementation of several popular decoherence models, we provide tools especially for the quantitative analysis of quantum operations. As an application of the developed tools we further present two case studies in which the entanglement of two atomic processes is investigated. In particular, we have studied the change of the electron-ion spin entanglement in atomic photoionization and the photon-photon polarization entanglement in the two-photon decay of hydrogen. The results show that both processes are, in principle, suitable for the creation and control of entanglement. Apart from process-specific parameters like initial atom polarization, it is mainly the process geometry which offers a simple and effective instrument to adjust the final state entanglement. Finally, for the case of the two-photon decay of hydrogenlike systems, we study the difference between nonlocal quantum correlations, as given by the violation of the Bell inequality and the concurrence as a true entanglement measure.

Effect of Auger recombination and leakage on the droop in InGaN/GaN quantum well LEDs

We investigate the effect of the epitaxial structure and the acceptor doping profile on the efficiency droop in InGaN/GaN LEDs by the physics based simulation of experimental internal quantum efficiency (IQE) characteristics. The device geometry is an integral part of our simulation approach. We demonstrate that even for single quantum well LEDs the droop depends critically on the acceptor doping profile. The Auger recombination was found to increase stronger than with the third power of the carrier density and has been found to dominate the droop in the roll over zone of the IQE. The fitted Auger coefficients are in the range of the values predicted by atomistic simulations.

Controlling the transport of an ion: classical and quantum mechanical solutions

The accurate transport of an ion over macroscopic distances represents a challenging control problem due to the different length and time scales that enter and the experimental limitations on the controls that need to be accounted for. Here, we investigate the performance of different control techniques for ion transport in state-of-the-art segmented miniaturized ion traps. We employ numerical optimization of classical trajectories and quantum wavepacket propagation as well as analytical solutions derived from invariant based inverse engineering and geometric optimal control. The applicability of each of the control methods depends on the length and time scales of the transport. Our comprehensive set of tools allows us make a number of observations. We find that accurate shuttling can be performed with operation times below the trap oscillation period. The maximum speed is limited by the maximum acceleration that can be exerted on the ion. When using controls obtained from classical dynamics for wavepacket propagation, wavepacket squeezing is the only quantum effect that comes into play for a large range of trapping parameters. We show that this can be corrected by a compensating force derived from invariant based inverse engineering, without a significant increase in the operation time.