Cosmological observations have precisely measured the amount of cold dark matter (CDM) in the Universe. The best fit value corresponds to around 23% of the Universe being composed of CDM. Nothing in the Standard Model (SM) is able to account for this cold dark matter. This provides unambiguous evidence for physics beyond the SM. From particle physics, the hierarchy between the electroweak and Planck scales within the SM provides motivation to consider new physics beyond the SM. In this thesis, I investigated the origin of CDM, analyzed various prospects for indirect detection, and studied its collider implications.We focused on two such models: the Left Right Twin Higgs (LRTH) model and the Inert Doublet model (IDM). Both of these models contain a neutral scalar that is stable and a good CDM candidate. We performed a CDM analysis, and identified regions of parameter space that can account for all of the CDM in the Universe.CDM can become trapped around massive objects such as the Sun, Earth, and galactic center. Over time, these CDM particles can annihilate to produce neutrinos and photons. Within the IDM framework, we analyzed the neutrino signal from the Sun and Earth and the photon signal from the galactic center.Due to the nature of new particles within the IDM, there are implications for signals at high energy colliders, such as the Large Hadron Collider (LHC). These particles are produced and can subsequently decay to CDM, jets, and leptons. Within the framework of the IDM, we performed a dilepton signal analysis at the LHC.There exists a synergy between particle physics and cosmology. The study of the interplay between these two fields could provide valuable insights and bring a better understanding of Nature within our grasp. It is an exciting time for physics.

Dopant diffusion in semiconductors is an interestingphenomenon from both technological and scientific points ofview. Firstly, dopant diffusion is taking place during most ofthe steps in electronic device fabrication and, secondly,diffusion is related to fundamental properties of thesemiconductor, often controlled by intrinsic point defects:self-interstitials and vacancies. This thesis investigates thediffusion of P, B and Sb in Si as well as in strained andrelaxed SiGe. Most of the measurements have been performedusing secondary ion mass spectrometry on high purityepitaxially grown samples, having in-situ incorporated dopantprofiles, fabricated by reduced pressure chemical vapordeposition or molecular beam epitaxy. The samples have beenheat treated both under close-to-equilibrium conditions (i. e.,long time annealings in an inert ambient) and conditions whichresulted in non-equilibrium diffusion (i. e., vacuum annealing,oxidation, short annealing duration, and protonirradiation). Equilibrium P and B diffusion coefficients in Si asdetermined in this thesis differ from a substantial part ofpreviously reported values. This deviation may be attributed toslow transients before equilibrium concentrations of pointdefects are established, which have normally not been takeninto account previously. Also an influence of extrinsic dopingconditions may account for the scattering of the diffusivityvalues reported in literature. B and Sb diffusion in Si underproton irradiation at elevated temperatures was found to obeythe so-called intermittent model. Parameters describing themicroscopic diffusion process were derived in terms of theintermittent diffusion mechanism, and it was found also thatthe presence of Sb strongly affected the B diffusion and viceversa. In relaxed Si1-xGex-alloys, which has the same lattice structure as Sibut a larger lattice constant, P diffusion is found to increasewith increasing Ge content (x≤ 0.2). In Si/SiGe/Si heterostructures, wherethe SiGe layer is biaxially strained in order to comply withthe smaller lattice parameter of Si, P diffusion in thestrained layer is retarded as compared with relaxed materialhaving the same Ge content. In addition, P is found tosegregate into the Si layer via the Si/SiGe interface and thesegregation coefficient increases with increasing Ge content inthe SiGe layer. / <p>QCR 20161027</p>

The understanding of the electronic systems of materials has not been only the essential, but the driving force, behind the progress of technology for over 100 years. This year marks the 60th anniversary of the revolutionary Bardeen-Cooper-Schrieffer, or BCS, theory which described the creation of Cooper pairs from a Fermi liquid ‘normal’ state through a coupling of conduction elections to phonons. Despite this, it wasn’t until the cuprate La2−xBaxCuO4, the first high-temperature superconductor, was discovered in the late 1980’s [1] that the dream of a room temperature superconductor seemed attainable and the ‘Age of the Superconductor’ began. However, the unique properties for which these high-temperature, unconventional superconductors are prized have also obstructed thorough investigation of the electronic behavior underlying their superconductivity and demanded extremely intense magnetic fields, very low temperatures, and thermodynamic measurements in extreme environments in order to fully characterize their electronic systems. It is, therefore, no small thing to flesh out the phase diagrams of these materials whose exotic electronic properties may eventually lead to faster, more compact devices and new methods of digital computation. Despite the difficulties in collecting usefully data on high-temperature superconductors, a vast body of work has amassed and grown with the increasingly intense magnetic fields available. As a result, quasiparticle mass enhancement near optimal doping was recently observed in two major classes of high-temperature superconductors, cuprates [2] and pnictides [3–5]. Because an effective quasiparticle mass accounts for the interactions between an electron and surrounding particles, it is an experimental indicator of enhanced electronic interactions. Enhancement of the quasiparticle effective mass, or increased electronic interactions, is believed to accompany quantum criticality, and the observation of mass enhancement in two very different classes of high-temperature superconductors makes quantum criticality the most promising candidate for universality across the high-temperature superconductors. The study outlined here is an investigation of the properties of three high-temperature superconductors, La2−xSrxCuO4, YBa2Cu3Oδ , and BaFe2(As1−xPx)2, through specific heat and resistivity measurements at very low temperatures, 1.5 K ≤ T ≤ 20 K, and magnetic fields up to 35 T. Such measurements required the construction of instrumentation specifically designed to deal with these extreme environments, and the low thermodynamic signals which are a signature of the cuprate superconductors. In order to understand the unprecedented data collected, novel analysis techniques based on Volovik phenomenology were developed. The procedures for specific heat measurements and the analysis of the resulting data developed for this study and outlined in the following thesis stand as the model for measurement of the normal state density of states of correlated superconductors. I report the observation of a saturation of the specific heat as a function of applied magnetic field in all three compounds, La2−xSrxCuO4, YBa2Cu3Oδ , and BaFe2(As1−xPx)2, indicating superconductivity has been suppressed and from which an effective mass, or sum of quasiparticle masses can be determined. I report that the onset of the normal state corresponds to the onset of finite resistance in La2−xSrxCuO4 and BaFe2(As1−xPx)2. I report enhancement in the sum of quasiparticle masses with doping in BaFe2(As1−xPx)2 that diverges near the predicted quantum critical point at optimum doping and that the dramatic enhancement evidences an orbital selective coupling to quantum fluctuations when compared to previous studies. / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Summer Semester 2017. / July 10, 2017. / Includes bibliographical references. / Gregory S. Boebinger, Professor Directing Dissertation; Theo Siegrist, University Representative; Arkady Shekhter, Committee Member; Nicholas Bonesteel, Committee Member; Mark Riley, Committee Member.

Superfluid 4He (He II) has been widely used as a coolant material in many engineering applications. Its unique heat transfer mode is the so-called thermal counterflow. The study of thermal counterflow will contribute to the design of He II based cooling devices and our understanding of quantum turbulence. However, due to the lack of effective visualization and velocimetry techniques, studying the fluid dynamics in superfluid 4He is very challenging. In this dissertation, we discussed the development of a novel flow-visualization technique in He II based on the generation and imaging of thin lines of metastable tracer molecules. These molecular tracers are created via femtosecond-laser field-ionization of helium atoms and can be imaged using a laser-induced fluorescence technique. In steady state thermal counterflow measurement, we demonstrated that such tracer molecules are entrained by the normal fluid component. We revealed for the first time a laminar to turbulent transition in the normal fluid component. We found that the profile of the normal fluid in the laminar flow can exhibit quite different velocity profile compared to the laminar Poiseuille profile of classical fluid in a channel. In the turbulent flow state, the turbulence intensity is found to be much higher than that in classical channel flow. This turbulence intensity appears to depend primarily on temperature. We also found that the form of the second order transverse structure function deviates more strongly from that found in classical turbulence as the steady state heat flux increases, suggesting novel energy spectrum. In decaying counterflow turbulence, we studied the normal fluid flow via flow visualization and measured the quantized vortex line density using 2nd sound attenuation. Comparing the decay behavior of both fluids, we were able to produce a theoretical model to explain the puzzling decay behavior of the vortices. We were also able to determine the effective kinematic viscosity in a wide temperature range. Some preliminary results in the study of decaying grid turbulence were obtained, which allows us to examine the intermittent behavior of superfluid turbulence. / A Dissertation submitted to the Department of Mechanical Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Summer Semester 2017. / April 28, 2017. / Flow visualization, Quantum turbulence, Thermal counterflow / Includes bibliographical references. / Wei Guo, Professor Directing Dissertation; Hui Li, University Representative; Emmanuel G. Collins, Committee Member; Kunihiko Taira, Committee Member; Sastry V. Pamidi, Committee Member.

Crystalline materials that include 4f- and 5f-electron elements frequently exhibit a variety of intriguing phenomena including spin and charge orderings, spin and valence fluctuations, heavy fermion behavior, breakdown of Fermi liquid behavior, and unconventional superconductivity. [5, 6, 7, 8, 9, 10, 11, 12, 13] Amongst such materials, the Kondo lattice system URu2Si2 stands out as being particularly unusual. [14, 15, 16] While at high temperature it exhibits behavior that is typical for an f-electron lattice immersed in a sea of conduction electrons, at T0 = 17:5 K there is a second order phase transition that is followed by unconventional superconductivity near Tc 1:5 K. [15] Despite three decades of work, the order parameter for the transition at T0 remains unknown and hence, it has been named "hidden order". There have been a multitude of experimental attempts to unravel hidden order, mainly through tuning of the electronic state via pressure, applied magnetic field, and chemical substitution. [17, 18] While these strategies reveal interesting phase diagrams, a longstanding challenge is that any such approach explores the phase space along an unknown vector: i.e., many different factors are affected. To address this issue, we developed a new organizational map for the U-based ThCr2Si2-type compounds that are related to URu2Si2 and thus guided, we explored a new chemical tuning axis: Si -> P. Our studies were enabled by the development of a new molten metal crystal growth method for URu2Si2 which produces high quality single crystals and allows us to introduce high vapor pressure elements, such as phosphorous. [19, 20] Si -> P tuning reveals that while the high temperature Kondo lattice behavior is robust, the low temperature phenomena are remarkably sensitive to electronic tuning. [21, 22] In the URu2Si2-xPx phase diagram we find that while hidden order is monotonically suppressed and destroyed for x < 0.035, the superconducting strength evolves non-monotonically with a maximum near x = 0.01 and that superconductivity is destroyed near x = 0.028. For 0.03 < x < 0.26 there is a region with Kondo coherence but no ordered state. Antiferromagnetism abruptly appears for x = 0.26. This phase diagram differs significantly from those produced by most other tuning strategies in URu2Si2, including applied pressure, and isoelectronic chemical substitution (i.e. Ru -> Fe and Os), where hidden order and magnetism share a common phase boundary. [2, 23, 24] We discuss implications for understanding hidden order, its relationship to magnetism, and prospects for uncovering novel sibling electronic states. / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Summer Semester 2017. / May 19, 2017. / correlated electrons, heavy fermion, hidden order, molten metal flux, URu2Si2 / Includes bibliographical references. / Ryan Baumbach, Professor Co-Directing Dissertation; Stephen Hill, Professor Co-Directing Dissertation; Theo Siegrist, University Representative; Greg Boebinger, Committee Member; Jorge Piekarewicz, Committee Member.

This dissertation is focused on electrical spin injection and detection at the nanoscale dimensions that semiconductor nanowires offer. Semiconductor spintronics is the natural extension of metallic spintronics for applications in semiconductor industry. After the tremendous impact of the giant magnetoresistance effect (GMR) in hard disk read heads, semiconductor spintronics has been thought as the key ingredient for the realization of spin field-effect transistors (Spin-FETs). The advantages of spintronic devices would include non-volatility, enhanced data processing speeds, decreased electric power consumption and facilitation of quantum computation. The primary goal of this research is to study spin dynamics and spin-polarized transport in semiconductor nanowire (NW) channels, specifically in phosphorus (P) doped silicon (Si) nanowires (NWs). The interest in one-dimensional (1D) nanoscopic devices is driven by the rich spin-dependent physics quantum confinement engenders, and the eventual miniaturization of the spintronic devices down to nanoscales. One of the most important aspects to achieve efficient spin injection from a ferromagnet (FM) into a semiconductor (SC) is the interface between the two materials. This study is focused primarily on this effect and how it can be tuned. In this work, we peform systematic spin transport measurements on a unique type of P-doped Si NWs which exhibit an inherent doping gradient along the axial direction. On a single NW, we place a series of FM electrodes, which form contacts that evolve from Ohmic-like to Schottky barriers of increasing heights and widths due to the pronounced doping gradient. This facilitates rigorous investigation of the dependence of the spin signal on the nature of the FM/SC interface. The advantage of using a single NW to study the afformentioned effects is that possible complications during the fabrication process are minimized compared to experiments that use multiple different devices to perform such experiments. 2-terminal (2T), nonlocal 4-terminal (4T) and 3-terminal (3T) spin valve (SV) measurements using different configurations of FM electrodes were performed on the Si NWs. In addition, 3T and nonlocal 4T Hanle measurements were performed. The collected data reveal distinct correlations between the spin signals and the injector and detector interfacial properties. These results were possible due to the unique inhomogeneous doping profile of our Si NWs. This study reveals a distinct correlation between the spin signals and the FM/Si NW injector interfacial properties. Specifically, we observe a decreasing injected current spin polarization due to diminishing contribution of the d-electrons, thus the necessary tunneling contact for efficient spin injection and its properties are being investigated and analyzed. The results demonstrate that there is an optimal window of interface resistance parameters for maximum injected current spin polarization. In addition, they suggest a new approach for maximizing the spin signals by making devices with asymmetric interfaces. To the best of our knowledge, this is the first report of electrical spin injection in SC channels with asymmetric interfaces. / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Summer Semester 2018. / May 23, 2018. / nanowires, Schottky barriers, silicon, spin injection, spintronics, spin valve devices / Includes bibliographical references. / Peng Xiong, Professor Directing Dissertation; Steven Lenhert, University Representative; Stephen Hill, Committee Member; Volker Crede, Committee Member; Pedro Schlottmann, Committee Member.

The discovery of graphene marked a turning point in research and interest towards 2 -D materials. Among them, Transition Metal Dichalcogenides (TMDs) and Metal Monochalcogenides (MM) have seen an upturn in interest owing to their versatile properties. Although, they have been studied for many years in bulk form, recent advances in nano-technology enabled new opportunities to study the role of atomically thin materials. In recent years much work has been dedicated to development of their application for the next generation of electronic and optoelectronic devices, and we are witnessing the dawn of the exploration of their properties. In Chapter 1 a brief introduction of highlighted properties of the newly emerged 2 -D materials and their heterostructures is provided. Chapter 2 focuses on field-effect transistor response of few atomic layers of MoSe2, MoTe2 and WSe2. In contrast to previous reports on MoSe2 FETs electrically contacted with Ni, MoSe2 FETs electrically contacted with Ti display ambipolar behavior with current ON to OFF ratios up to 10^6 for both hole and electron channels when applying a small excitation voltage. For both channels the Hall effect indicates Hall mobilities H = 250 cm^2/V.s. Our MoTe2 field-effect transistors are observed to be hole-doped, displaying ON/OFF ratios surpassing 10^6 and typical subthreshold swings of ~140mV per decade. Both field-effect and Hall mobilities indicate maximum values approaching or surpassing 10 cm^2/V.s, which are comparable to figures previously reported for single or bilayer MoS2 and/or for MoSe2 exfoliated onto SiO2 at room temperature and without the use of dielectric engineering. Temperature dependent comparison between field-effect and Hall mobilities in field effect transistors based on few-layered WSe2 exfoliated onto SiO2 is also reported. We observe maximum hole mobilities approaching 350 cm^2/V.s at T = 300 K. The hole Hall mobility reaches a maximum value of 650 cm^2/V.s as T is lowered below ~150 K, indicating that insofar WSe2- based field-effect transistors (FETs) display the largest Hall mobilities among the transition metal dichalcogenides. Chapter 3 evaluates electrostatically gated p-n junctions based on MoSe2 and the photovoltaic response of electrostatically generated p-n junctions composed of approximately 10 atomic layers of MoSe2 stacked onto dielectric h-BN is presented. In addition to ideal diode-like response, we find that these junctions can yield photovoltaic effciencies exceeding 14% under standard solar simulator spectrum with fill factors values of about 70 %. Chapter 4 presents electrical and optical characterization of monolayer and bilayer lateral heterostructures of MoS2-WS2 and MoSe2-WSe2, grown by a one-pot chemical vapor deposition (CVD) synthesis approach, using a single heterogeneous solid source, a newly developed CVD growth method that eliminates the need for the exchange of multiple sources which leads to sample air exposure. The structures show a diode like response which is enhanced under optical illumination. Additionally, bilayer lateral heterostructures exhibit a clear photovoltaic response to optical excitation. / A Dissertation submitted to the Department Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Spring Semester 2018. / April 17, 2018. / 2-D Materials, Field Effect Transistor, Photovoltaics, p-n Junctions, Transition Metal Dichalcogenides / Includes bibliographical references. / Luis Balicas, Professor Co-Directing Thesis; Per Arne Rickvold, Professor Co-Directing Thesis; Mykhailo Shatruk, University Representative; Efstratios Manousakis, Committee Member; Jianming Cao, Committee Member; Sergio Almaraz-Calderon, Committee Member.

Two heavy Fermion superconducting compounds, URu₂Si₂ and PuRhIn₅, were investigated by the techniques of chemical substitution and application of high magnetic fields. These materials are particularity interesting for their unique electronic and ordered state behavior (hidden order and superconducting phases) and proximity to magnetism. It is thought the superconductivity of these materials is unconventional and that they exhibit some features that are associated with quantum criticality, especially for PuRhIn₅. In earlier work URu₂Si₂ was doped with the non-isoelectric element phosphorus to produce the doping series URu₂Si₂₋ₓPₓ. During the current study, single crystals of the series were placed in high pulsed magnetic fields (up to 65T) and the evolution of the field induced phases was observed. The parent compound exhibits five unique phases in field as does the doped series up to approximately x=0.3. At this concentration and at zero field the hidden order phase is destroyed and any higher doping exhibit no ordered ground state. Over this x-range there is only one field induced state. Further increasing x (x > 0.26) pushes the system into an antiferromagnetic ground state, which has some high field ordering but at a higher magnetic fields than the lower doped compounds. This behavior is similar to the effect of Co, Rh and Ir substitution, which are also non-isoelectronic dopants that add electrons. This is in contrast to isoelectric doping (using Fe or Os) in which produce effects in the material similar to applied pressure. From this, it appears that the effects of non-isoelectric dopants might be attributed to band filling. The hidden order state of the parent compound URu₂Si₂ was also investigated with an optical magnetostriction technique in high magnetic fields. A transition from a quadratic to linear field response is seen in the signal while still in the hidden order state. This behavior is unusual and possible explanations include partial polarization of the Fermi surface and quadrupolar interactions. The Pu based superconductor PuRhIn₅ was doped with Cd and placed in high magnetic fields. Pu is both a radiological and toxicity hazard. As a result, a significant part of this project was spent controlling these hazards while enabling measurements. The phase diagram of PuRh(In₁₋ₓCdₓ)₅ in the T-x-H phase diagram was mapped and the optical magnetostriction technique was applied to the parent compound. From this data the electronic Grüneisen ratio was determined. Applying scaling arguments it was shown that the Grüneisen data is consistent with proximity to a quantum critical point, which is though to figure heavily in unconventional superconducting systems. / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Spring Semester 2018. / April 10, 2018. / Includes bibliographical references. / Greg Boebinger, Professor Directing Dissertation; Theo Siegrist, University Representative; Ross McDonald, Committee Member; Ryan Baumbach, Committee Member; Vladimir Dobrosavljevic, Committee Member; Jianming Cao, Committee Member.

Complex physical phenomena, such as superconductivity, colossal magnetoresistance (CMR) effect, multi-ferroics, metal-insulator transition, quantum phase transition, etc. in strongly correlated materials have been enduring topics in condensed matter physics. The overarching theme of this dissertation is the study of transport and electronic states in emergent phases with distinct magnetic and electronic properties in strongly correlated materials in thin film forms. At first, we have investigated anisotropic electronic phase separation (EPS) of optimally doped La2/3Ca1/3MnO3 (LCMO) thin films under various degrees of anisotropic strain by static magnetotransport and dynamic relaxation measurements. Distinct from the prototype perovskite manganite LPCMO with well-known micrometer scale EPS, the bulk optimally doped LCMO does not exhibit the large-scale EPS near a transition from paramagnetic insulating phase (PMI) to ferromagnetic metallic (FMM) phase at a high temperature. Through epitaxial growth of LCMO thin films on NGO (001) substrates and post-growth annealing, an antiferromagnetic insulating phase is induced in the FMM ground state and results in a large-scale EPS of coexisting AFI and FMM phases below the bulk metal insulator transition (MIT). Substantial resistivity anisotropies along the two orthogonal in-plane directions in the EPS region were experimentally probed by static temperature and magnetic field dependent resistivity measurements. More strikingly, with increasing annealing time, resistivity along the tensile-strained [010] direction becomes progressively larger than that along the compressive-strained [100] direction in the EPS region. The enhanced resistivity anisotropy suggests that the EPS is characterized by phase-separated FMM entities with a preferred orientation along [100] direction, possibly due to the deformation and rotation of the MnO6 octahedra under the enhanced anisotropic strain via the post-growth annealing. Furthermore, the EPS was found to exhibit glass-like behavior. The resistivity measured at fixed temperatures relaxes logarithmically over a long period of time. The relaxation behavior also shows a coherent enhancement with increasing annealing time. By fitting the relaxation data to a phenomenological model, the fitted parameters, resistive viscosity and characteristic relaxation time were found to evolve with temperature, showing a close correlation with the static measurements in the EPS states. In another project, we have investigated the superconductor-insulator quantum phase transitions tuned by disorder (d), magnetic impurity (MI) and magnetic field (B) in ultrathin Pb films by electrical transport measurement and single electron tunneling spectroscopy. In the past decade, the investigation of SITs in homogeneous thin films by transport measurement from our group has provided valuable insights to the mechanisms of SITs. There are two main theoretical models to explain SITs. The first one emphasizes that a transition from a superconducting state to a fermionic insulator without the existence of the superconducting order parameter, the formation of Cooper pairs is completely suppressed at the transition. The other one calls for a bosonic insulator with localized Cooper pairs. d-tuned and MI-tuned SITs well fit the fermionic framework, and both share common transport features, such as a sharp resistive transition to the superconducting state, a well-defined phase boundary, and a weakly insulating state near the phase boundary. While B-tuned SITs are a canonical example of the bosonic model. The resistive transition to the superconducting state is broadened by an application of magnetic field. Rather than a clear phase boundary near the transition, emerged resistive reentrance and double reentrance indicate phase fluctuation of the superconducting parameter is the main driving force for the transition, suggesting the survival of Cooper pairs in the insulating phase. Electron tunneling spectroscopy has been proposed to directly probe the existence and evolution of the superconductivity in these SITs. For B-tuned SITs, the existence of Cooper pairs is supposed to be detected even in the global insulating phase of thin films. More importantly, the approach also allows us to compare the evolutions of the normal state density of state among these SITs, particularly for d-tuned and MI-tuned SITs. Transport results show that MI has little influence on the normal state sheet resistance near the transition, while increasing disorder gradually raises the normal state sheet resistance. These observations suggest that the normal state density of states behaves differently in the two transitions. The experimental setup is a dilution refrigerator incorporated with in situ quench condensation, electrical measurement, and sample rotation, enabling us to achieve and tune SITs in the same sample by different parameters, and systematically check and compare the evolution of the density of states in the SITs. Up to now, we have performed transport and tunneling measurements for d-tuned SITs in homogeneous Pb films in a 4He quench probe and the modified dilution refrigerator. The transport results are consistent with previous experiments from our group. Increasing disorder leads to a SIT characterized with a sharp resistive transition to a zero resistance state, a well-defined phase boundary, and a gradual reduction of the superconducting critical temperature. The preliminary tunneling testing in the quench probe successfully reveals the suppression of the superconducting energy gap and the normal state density of state by the increasing disorder. In the modified dilution refrigerator, we still observed a concomitant suppression of the normal state density of states. Unfortunately, we were not able to reproduce valid tunneling spectra to study the evolution of the superconducting energy gap near the Fermi level. Possible reasons for the unsatisfying tunneling results are discussed at the end. / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Spring Semester 2018. / April 17, 2018. / Electronic Phase Separation, Magnetic Material, Quantum Phase Transition, Strongly Correlated System, Superconductor / Includes bibliographical references. / Peng Xiong, Professor Directing Dissertation; Jingjiao Guan, University Representative; Todd Adams, Committee Member; Irinel Chiorescu, Committee Member; Pedro Schlottmann, Committee Member.

This dissertation presents the recent developments and experiments performed using the third generation femtosecond electron diffractometer in Professor Jianming Cao's group as well as experiments performed using the previous second generation diffractometer now located at Shanghai Jiao Tong University. Two techniques of ultrafast electron diffraction (UED), time-resolved reflection high energy electron diffraction (Tr-RHEED) and time-resolved transmission electron diffraction (Tr-TED) were developed and applied to study the ultrafast lattice dynamics in semiconductor nanostructures. Tr-RHEED provides the ability to directly monitor the thermal transport across an interface inside a semiconductor quantum well (QW) by measuring the temperature evolution of the first few atomic layers. Tr-TED allows for a measurement of the laser-induced ultrafast structural dynamics of 5 nm PbSe quantum dots (QDs) in real time by diffracting through the entire sample thickness. In the first project, the setup of the first Tr-RHEED experiments and the first successful collection of Tr-RHEED data in our laboratory's history is discussed. The ultrafast temperature evolution of the GaAs nanofilm was measured and numerically modeled using the well known heat conduction equation and also a three-temperature model. These models were fit to the experimental data, allowing for the extraction of the thermal boundary conductance (TBC) and providing a method of measuring TBC in epitaxially grown semiconductor heterostructures. Surprisingly, the TBC was found to increase with increasing temperature even for temperatures above the Debye temperature, opening up questions about the exact mechanisms governing heat transfer at interfaces between very similar semiconductor nanoscale materials. In the second project, we directly monitored the lattice dynamics in PbSe quantum dots induced by laser excitation using Tr-TED. The energy relaxation between the carriers and the lattice took place within 10 ps, showing no evidence of any significant phonon bottleneck effect. Meanwhile, the lattice dilation exhibited some unusual features that could not be explained by the available mechanisms of photon-induced acoustic vibrations in semiconductors alone. The heat transport between the QDs and the substrate deviates significantly from Fourier's Law, which furthers studies about the heat transfer under nonequilibrium conditions in nanoscale materials. In addition to the UED projects, femtosecond transient spectroscopy (FTS) experiments were set up and tested on 20 nm gold nanofilms for various optical excitation laser fluences. The experimental data obtained agrees well with many previous published results. The well known two-temperature model (TTM) was used to describe the temperature evolution and the energy redistribution from the electronic to lattice systems. Using similar experimental and data analysis techniques to the ones developed in this dissertation will pave the way for future FTS experiments performed in conjunction to UED experiments to gain a more complete picture of the ultrafast dynamics in carriers and phonons in complex materials. / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester 2018. / October 19, 2018. / GaAs/AlGaAs Quantum Wells, Lattice Dynamics, Nanoscale Thermal Transport, Phonon Bottleneck, Thermal Boundary Conductance, Ultrafast Electron Diffraction / Includes bibliographical references. / Jianming Cao, Professor Directing Dissertation; Wei Yang, University Representative; Peng Xiong, Committee Member; Mark Riley, Committee Member; Nicholas Bonesteel, Committee Member.