Colloidal particles provide convenient and useful building blocks for creating ordered and disordered structures with length scales on the order of a micrometer. These structures are useful materials in their own right, and also serve as excellent scale models for exploring properties of atomic materials that would otherwise be inaccessible to direct experiment. In this dissertation, we explore structure formation in hard-sphere colloidal systems using templated sedimentation techniques, and then use colloidal crystals and glasses formed in this way to study the development of extended defects in single crystals and shear defects in glasses. We find that it is possible to form large, defect-free colloidal single crystals extremely rapidly by centrifugation onto a deterministic template. On non-deterministic templates, we find a critical deposition flux above which the material always crosses over to forming a glass. With this understanding of the effects of template and deposition flux, we designed and tested amorphous templates that allow us to make colloidal glasses by sedimentation under gravity, as well as more complex structures. In face-centered cubic colloidal single crystals grown on (100) templates, extended defects (dislocations and stacking faults) can nucleate and grow if the crystal exceeds a critical thickness that depends on the lattice misfit with the template spacing. We account for the experimental observations of the density of misfit dislocations using the Frank-van der Merwe theory, adapted for the depth-dependent variation of lattice spacing and elastic constants that results from the gravitational pressure. In the second part of the thesis, we report the first results of a detailed study of reversible and irreversible deformation of colloidal glasses. We show that shear defects exist and are active in both sheared and quiescent colloidal glasses and that these defects behave as Eshelby inclusions. We observe a decrease in the shear modulus of the glass, which corresponds to a small dilatation, which, in turn, lowers the activation barrier for shear. / Physics

This thesis describes electronic transport experiments in graphene from the hopping to the ballistic regime. The first experiment studies dual-gated bilayer graphene devices. By applying an electric field with these dual gates, we can open a band gap in bilayer graphene and observe an increase in resistance of over six orders of magnitude as well as a strongly non-linear behavior in the transport characteristics. A temperature-dependence study of resistance at large electric field at the charge neutrality point shows the change in the transport mechanism from a hopping dominated regime at low temperature to a diffusive regime at high temperature. / Physics

In this thesis, I discuss engineering colloidal particles to have specific, isotropic interactions and studying their cluster geometries in equilibrium. I discuss light scattering experiments showing that a highly specific protein, Dscam, is unstable against thermal aggregation. This result lead me to use DNA instead to control interparticle specificity. I coated 1-micron diameter polystyrene particles uniformly with DNA. I used fluorescence microscopy with oxygen-scavenging enzymes to observe these particles self-assembling in clusters. These experiments show that a packing of 6 spheres that is rarely seen in a single-component system is observed very often in an optimized 3-species system. Then I show experiments using the same 3 species but 9 total particles, finding that the equilibrium yields of the most likely cluster relative to other stable clusters are lower than at 6 particles. I conclude from these experiments that optimizing the assembly of an otherwise unlikely configuration may require nearly as many species as particles. Finally, I investigate the scalability of self-assembly of particles with isotropic and specific interactions theoretically. I use both exact and approximate partition functions to show that spheres with specific interactions can have energy landscapes with thermodynamically large numbers of strictly local minima relative to the number of their ground states. Compared to single-component systems, these systems of many different species may spend much more time in kinetic traps and never reach their ground states. Finally, I discuss briefly some directions for further study, including questions of how the results in this thesis may be related to protein folding and complex formation. / Engineering and Applied Sciences

Over the past twenty-five years, condensed matter physics has been developing materials with novel electronic characteristics for a wide range of future applications. Two research directions have shown particular promise: topological insulators, and high temperature copper based superconductors (cuprates). Topological insulators are a newly discovered class of materials that can be manipulated for spintronic or quantum computing devices. However there is a poor spectroscopic understanding of the current topological insulators and emerging topological insulator candidates. In cuprate superconductors, the challenge lies in raising the superconducting transition temperature to temperatures accessible in non-laboratory settings. This effort has been hampered by a poor understanding of the superconducting mechanism and its relationship with a mysterious pseudogap phase. In this thesis, I will describe experiments conducted on topological insulators and cuprate superconductors using scanning tunneling microscopy and spectroscopy, which provide nanoscale spectroscopic information in these materials. / Physics

Low-dimensional systems in condensed matter physics exhibit a rich array of correlated electronic phases. One-dimensional systems stand out in this regard. Electrons cannot avoid each other in 1D, enhancing the effects of interactions. The resulting correlations leave distinct spatial imprints on the electronic density that can be imaged with scanning probes. Disorder, however, can destroy these delicate interacting states by breaking up the electron liquid into localized pieces. Thus, to generate fragile interacting quantum states, one requires an extremely clean system in which disorder does not overcome interactions, as well as a high degree of tunability to design potential landscapes. Furthermore, to directly measure the resulting spatial correlations, one requires an exceptionally sensitive scanning probe, but the most sensitive probes presently available are also invasive, perturbing the system and screening electron-electron interactions. / Engineering and Applied Sciences

Plasmonic waves in solid-state are caused by collective oscillation of mobile charges inside or at the surface of conductors. In particular, surface plasmonic waves propagating at the skin of metals have recently attracted interest, as they reduce the wavelength of electromagnetic waves coupled to them by up to ~10 times, allowing one to create miniaturized wave devices at optical frequencies. In contrast, plasmonic waves on two-dimensional (2D) conductors appear at much lower infrared and THz-GHz frequencies, near or in the electronics regime, and can achieve far stronger wavelength reduction factor reaching well above 100. In this thesis, we study the unique machinery of 2D plasmonic waves behind this ultra-subwavelength confinement and explore how it can be used to create various interesting devices. To this end, we first develop a physically intuitive theoretical formulation of 2D plasmonic waves, whose two main components---the Coulomb restoration force and inertia of the collectively oscillating charges---are combined into a transmission-line-like model. We then use this formulation to create various ultra-subwavelength 2D plasmonic devices. For the 2D conductor, we first choose GaAs/AlGaAs heterostructure---a 2D electron gas consisting of massive (m*>0) electrons---demonstrating plasmonic bandgap crystals, interferometers, and negatively refracting metamaterials. We then examine a 2D plasmonic device based on graphene, a 2D electron gas consisting of effectively massless (m*=0) electrons. We theoretically show and experimentally demonstrate that the massless electrons in graphene can surprisingly exhibit a collective mass when subjected to a collective excitation, providing the inertia that is essential for the propagation of 2D plasmonic waves. Lastly, we theoretically investigate the thermal current fluctuation behaviors in massive and massless electron gases. While seemingly unrelated on first sight, we show that the thermal current fluctuation is actually intimately linked to the collective mass of the massive or massless electron gas. Thus, we show that the thermal current fluctuation behaviors can also be described by the same theoretical framework introduced earlier, suggesting a possibility to design new concept devices and experiments based on this linkage. / Engineering and Applied Sciences

High-quality platelet single crystals of Tl₂Ba₂CuO₆±δ (Tl-2201) have been grown using a novel time-varying encapsulation scheme, minimizing the thallium oxide loss that has plagued other attempts and reducing cation substitution. This encapsulation scheme allows the melt to be decanted from the crystals, a step previously impossible, and the remaining cation substitution is homogenized via a high-temperature anneal. Oxygen annealing schemes were developed to produce sharp superconducting transitions from 5 to 85 K without damaging the crystals. The crystals' high homogeneity and high degree of crystalline perfection are further evidenced by narrow rocking curves; the crystals are comparable to YSZ-grown YBa₂Cu₃O₆₊δ by both metrics. Electron probe microanalysis (EPMA) ascertained the crystals' composition to be Tl₁.₉₂₀₍₂₎Ba₁.₉₆₍₂₎Cu₁.₀₈₀₍₂₎O₆₊δ; X-ray diffraction found the composition of a Tc = 75 K crystal to be Tl₁.₉₁₄₍₁₄₎Ba₂Cu₁.₀₈₆₍₁₄₎O₆.₀₇₍₅₎, in excellent agreement. X-ray refinement of the crystal structure found the crystals orthorhombic at most dopings, and their structure to be in general agreement with previous powder data. Cation-substituted Tl-2201 can be orthorhombic, orthorhombic crystals can be prepared, and these superconduct, all new results. X-ray diffraction also found evidence of an as yet unidentified commensurate superlattice modulation. The Tl-2201 crystals' electronic structure were studied by X-ray absorption and emission spectroscopies (XAS/XES). The Zhang-Rice singlet band gains less intensity on overdoping than expected, suggesting a breakdown of the Zhang-Rice singlet approximation, and one thallium oxide band does not disperse as expected. The spectra correspond very closely with LDA band structure calculations, and do not exhibit the upper Hubbard bands arising from strong correlations seen in other cuprates. The spectra are noteworthy for their unprecedented (in the high-Tc cuprates) simplicity. The startling degree to which the electronic structure can be explained bodes well for future research in the cuprates. The overdoped cuprates, and Tl-2201 in particular, may offer a unique opportunity for understanding in an otherwise highly confusing family of materials.

This thesis will present research that studies the role of disorder, geometric frustration and the long range dipolar interaction on the collective behaviour of several insulating, rare earth magnets. Experiments were performed at low temperatures to measure the specific heat and magnetic susceptibility of several materials. Susceptibility was measured with a SQUID magnetometer that has been designed and constructed primarily for the study of slow dynamics in glassy systems. Specifically, this thesis will discuss three distinct topics. The first is the series of materials LiHo(x)Y(1-x)F(4), which are manifestations of the dilute, dipolar coupled Ising model. The low-x portion of the phase diagram has become a rather contentious issue in recent years with both theoretical and experimental groups disagreeing on the existence of a spin glass freezing transition and one experimental group arguing for the existence of an exotic "antiglass'' or spin liquid state resulting from quantum entanglement at x=0.045. We present specific heat and dynamical susceptibility measurements on four stoichiometries in this series: x = 0.018, 0.045, 0.080 and 0.012. No evidence of an unusual antiglass state is observed. Instead, our results show evidence, at all dilution levels studied, of a spin glass freezing transition. Interpretation of experimental data is found to be complicated by the anomalously slow dynamics in these materials. The relaxation time scales are found to increase as the concentration of Ho(3+) ions is reduced, an effect which can be attributed to single-ion physics and the importance of the nuclear hyperfine coupling in this system. A second set of materials studied here is a series of several Gd garnet materials, the most famous of which is Gd(3)Ga(5)O(12) (GGG), a material previously argued to be a disorder-free spin glass. Our specific heat experiments reproduce previous experiments on GGG and show that the homologous Gd garnets Gd(3)Te(2)Li(3)O(12) and Ga(3)Al(5)O(12) do not share the same glassy physics but exhibit sharp ordering features. By experimenting with the introduction of random site dilution, it is concluded that a 1-2% off-stoichiometry inherent in GGG is likely a special kind of disorder that is particularly effective in inducing random frustration and the formation of a spin glass. Finally, specific heat measurements on the pyrochlore antiferromagnet Gd(2)Sn(2)O(7) (GSO) are presented. While GSO has generally been found to be a well behaved and well understood model magnet, with long range order developing at around 1 K, like many other geometrically frustrated magnets, it has been discovered to possess persistent spin dynamics down to very low temperatures as measured by μSR and Mössbauer spectroscopy. Measurement of the low temperature limit of the specific heat when compared with linear spin-wave theory, however, presents a consistent picture of gapped magnon excitations that freeze out at low temperatures and make the existence of the proposed dynamic ground state unlikely.

<p>The high-throughput framework AFLOW that has been developed and used successfully over the last decade is improved to include fully-integrated software for crystallographic symmetry characterization. The standards used in the symmetry algorithms conform with the conventions and prescriptions given in the International Tables of Crystallography (ITC). A standard cell choice with standard origin is selected, and the space group, point group, Bravais lattice, crystal system, lattice system, and representative symmetry operations are determined. Following the conventions of the ITC, the Wyckoff sites are also determined and their labels and site symmetry are provided. The symmetry code makes no assumptions on the input cell orientation, origin, or reduction and has been integrated in the AFLOW high-throughput framework for materials discovery by adding to the existing code base and making use of existing classes and functions. The software is written in object-oriented C++ for flexibility and reuse. A performance analysis and examination of the algorithms scaling with cell size and symmetry is also reported.</p> / Dissertation

<p>Granular impact, where a foreign object strikes a granular material like sand, is common in nature and industry. Due to experimental difficulties in obtaining sufficiently fast data at the scale of a single grain, a description of this process which connects to physics at the grain-scale is lacking. In this thesis, I will present data from a series of two-dimensional granular impact experiments. By cutting each grain out of a photoelastic material and using a very fast camera, we obtain data on the intruder trajectory, as well as the particle flow and force response of the granular material. Past experiments have shown that the decelerating force on an intruder moving through a granular medium is often well captured by a force law which is dominated by a velocity-squared drag force. Using the intruder trajectories, as well as the flow and force response of the granular material, I will demonstrate that, while these force laws describe the intruder trajectories on slow time scales, the instantaneous force on the intruder is highly fluctuating in space and time. I will particularly focus on the velocity-squared drag force, showing that it arises from random, locally normal collisions with chain-like clusters of particles which send energy and momentum away into the granular material. In this regime, the particles and intruder reach a kind of adiabatic steady state, where the particle motion scales linearly with the intruder speed. However, for impact velocities which are fast compared to the rate of momentum transfer within the granular material, the system response qualitatively changes, behaving like an elastic solid with a shock-like response at impact.</p> / Dissertation