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<p>The quark gluon plasma (QGP) forms when matter governed by quantum chromodynamics (QCD) undergoes a transition at high temperature or high density from hadronic bound states to deconfined quarks and gluons. The QGP at high temperature is believed to be experimentally accessible in relativistic heavy-ion collisions, such as those done at the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Lab and in the near future at the Large Hadron Collider (LHC) at CERN. The results obtained so far reveal the production of energetic (hard) partons in the early stages of a heavy-ion collision which propagate through the plasma. Results also show that the QGP produced at RHIC is a nearly ideal fluid and that hard partons may generate conical, Mach-like, disturbances in the QGP. </p><p>This thesis uses theoretical methods to address how the QGP responds to a hard parton that propagates through the plasma and contains the first rigorous derivation of how a hard parton deposits energy and momentum in a QGP which lead to the formation of a Mach cone. A comparison of experimental results with the theory introduced in this thesis could shed light on important properties of the QGP such as its equation of state and transport coefficients like viscosity. I investigate this problem by evaluating the source of energy and momentum generated by the hard parton in the QGP. Formalisms are developed and applied for evaluating the source of energy and momentum in perturbation theory with three different methods: classical kinetic theory, finite temperature field theory, and by including the energy lost by the hard parton to radiation. Having obtained the source of energy and momentum generated by the hard parton, I evaluate the medium response using linearized hydrodynamics. My results show Mach cone formation in the medium. I compare the medium response for different viscosities and speeds of sound, from which I find the Mach cone weakens and broadens as viscosity is increased. By studying the time evolution of the medium response once the source of energy and momentum is turned off, which occurs in a heavy-ion collision during the hadronic phase, I find that the conical disturbance is enhanced relative to diffusive contributions over a time period of several fm/c.</p> / Dissertation

<p> In the past three decades, spintronic devices have played an important technological role. Half-metallic alloys have drawn much attention due to their special properties and promised spintronic applications. This dissertation describes some theoretical techniques used in first-principal calculations of alloys that may be useful for spintronic device applications with an emphasis on half-metallic ferromagnets. I consider three types of simple spintronic materials using a wide range of theoretical techniques. They are (a) transition metal based half-Heusler alloys, like CrMnSb, where the ordering of the two transition metal elements within the unit cell can cause the material to be ferromagnetic semiconductors or semiconductors with zero net magnetic moment, (b) half-Heusler alloys involving Li, like LiMnSi, where the Li stabilizes the structure and increases the magnetic moment of zinc blende half-metals by one Bohr magneton per formula unit, and (c) zinc blende alloys, like CrAs, where many-body techniques improve the fundamental gap by considering the physical effects of the local field. Also, I provide a survey of the theoretical models and numerical methods used to treat the above systems.</p>

<p> Historically our understanding of the microscopic world has been impeded by limitations in systems that behave classically. Even today, understanding simple problems in quantum mechanics remains a difficult task both computationally and experimentally. As a means of overcoming these classical limitations, the idea of using a controllable quantum system to simulate a less controllable quantum system has been proposed. This concept is known as quantum simulation and is the origin of the ideas behind quantum computing. </p><p> In this thesis, experiments have been conducted that address the feasibility of using devices with a circuit quantum electrodynamics (cQED) architecture as a quantum simulator. In a cQED device, a superconducting qubit is capacitively coupled to a superconducting resonator resulting in coherent quantum behavior of the qubit when it interacts with photons inside the resonator. It has been shown theoretically that by forming a lattice of cQED elements, different quantum phases of photons will exist for dierent system parameters. In order to realize such a quantum simulator, the necessary experimental foundation must rst be developed. Here experimental eorts were focused on addressing two primary issues: 1) designing and fabricating low disorder lattices that are readily available to incorporate superconducting qubits, and 2) developing new measurement tools and techniques that can be used to characterize large lattices, and probe the predicted quantum phases within the lattice. </p><p> Three experiments addressing these issues were performed. In the rst experiment a Kagome lattice of transmission line resonators was designed and fabricated, and a comprehensive study on the effects of random disorder in the lattice demonstrated that disorder was dependent on the resonator geometry. Subsequently a cryogenic 3-axis scanning stage was developed and the operation of the scanning stage was demonstrated in the final two experiments. The rst scanning experiment was conducted on a 49 site Kagome lattice, where a sapphire defect was used to locally perturb each lattice site. This perturbative scanning probe microscopy provided a means to measure the distribution of photon modes throughout the entire lattice. The second scanning experiment was performed on a single transmission line resonator where a transmon qubit was fabricated on a separate substrate, mounted to the tip of the scanning stage and coupled to the resonator. Here the coupling strength of the qubit to the resonator was mapped out demonstrating strong coupling over a wide scanning range, thus indicating the potential for a scanning qubit to be used as a local quantum probe.</p>

The subject of this thesis is the production of electromagnetic radiations during relativistic heavy ions collisions. Since they constitute one of the major ways to probe the presence of a quark-gluon plasma (QGP), their evaluation through theoretical models is very important. The photon production at low-to intermediate transverse momentum (pT) is first studied. The photon production rate in a mesonic gas is evaluated within a massive Yang-Mills (MYM) approach. Earlier calculations are reexamined with additional constraints, including new production channels and with the inclusion of form-factors. Adding primordial N-N contribution and existing baryonic and QGP production rates, we can reproduce the photon spectra observed at the Super Proton Synchrotron (SPS). The intermediate to high-p T region is dominated by the physics of jets. A treatment, complete to leading-order in the strong coupling, is used to calculate energy loss in the strongly interacting medium. This approach is convolved with a physical description of the initial spatial distribution of jets and with an expansion of the emission zone. The role played by jet-plasma interactions is highlighted, showing that they dominate in the range 2 &lt; p T &lt; 4 GeV, at the Relativistic Heavy Ion Collider (RHIC). This mechanism has an important impact on both the total photon yield and the photon azimuthal asymmetry, turning the coefficient v 2 negative. Finally, the dilepton production at high p T is calculated with hard-thermal loops (HTL) effects, showing, that in perfect analogy with real photons, jet-plasma interactions also dominate the dilepton yield around pT = 4 GeV.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-05, Section: B, page: 3221. Adviser: Phillip Geil. Includes bibliographical references. Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph. D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-11, Section: B, page: 6874. Adviser: Kenneth S. Schweizer. Includes bibliographical references. Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007. / Source: Dissertation Abstracts International, Volume: 69-02, Section: B, page: 1071. Adviser: Sheldon Katz. Includes bibliographical references (leaves 117-123) Available on microfilm from Pro Quest Information and Learning.

This dissertation provides an overview of what gauge-gravity duality, often called holography, has taught us about quantum condensed matter physics, with particular emphasis on the problem of thermoelectric transport. A comprehensive theory of transport in weakly disordered metals is subsequently developed using a variety of techniques, all of which precisely agree. The theory in its present form may be applied directly to realistic models of transport in strongly interacting strange metallic phases, and also serves as a point of direct contact between gauge-gravity duality and more traditional theories of condensed matter physics. Next, novel techniques are developed to demonstrate the absence of disorder-driven metal-insulator transitions in holographic metals in two spatial dimensions. Finally, experimental evidence is presented for hydrodynamic transport in charge neutral graphene, and new theories are developed to understanding the resulting data. / Physics

When a brittle material is loaded to the limit of its strength, it fails by nucleation and propagation of a crack. The conditions for crack propagation are created by the concentration of a long-range stress field at an atomically sharp crack tip, creating a complex and strongly coupled multiscale system. This thesis reports the results of multiscale simulations of the brittle fracture of silicon on the (111) cleavage plane. The simulations are made possible by combining a quantum mechanical description of the processes taking place near the crack tip with a classical atomistic model that captures the long-range elastic relaxation. The 'Learn on The Fly' technique is used to couple the quantum and classical models, allowing accurate quantum forces to be combined with classical forces using a simple adjustable potential to give stable dynamics. The simulations predict that fracture is unstable on the (111) plane at low speeds; conventionally this has been thought of as the most stable crack plane. The instability is caused by a crack tip reconstruction which triggers a positive feedback 'sinking' mechanism leading to macroscopic, experimentally observable corrugations. Recent experiments have observed crack surface features consistent with these predictions. The instability is the first example in a crystalline material of a fracture instability which onsets below a critical velocity, and shows how subtle atomistic details at the crack tip can control the qualitative macroscopic fracture behaviour.