Ir L3 edge Resonant Inelastic X-ray Scattering (RIXS) and La2 Resonant X-ray Emission Spectroscopy (RXES) have been used to investigate A2IrO3 (A= Na or Li) and CuIr2S4. First principle calculations show that the low-energy dd-excitations in the RIXS spectra of A2IrO3 originate from a split jeff=3/2 to jeff=1/2 excitation. A magnetic excitation is observed in Na2IrO3, whose dispersion is consistent with the theoretical prediction including a bond-dependent Kitaev interaction between jeff=1/2 magnetic moments. In CuIr2S4, RXES measurements reveal that the unoccupied Ir t2g level shift upward in energy as a result of the metal-insulator transition, confirming density functional theory calculations. In the x-ray irradiation induced phase of CuIr2S4, a mid-infrared peak emerges in the RIXS spectrum around 0.4 eV, suggesting that both the crystal structure and the electronic structure are modified due to x-ray irradiation. The size of the magnetic moments in Fe based superconductors has been investigated using Fe Kb X-ray Emission Spectroscopy (XES). Local magnetic moments are found in Fe pnictides and chalcogenides, independent of temperature or carrier concentration. This result supports the notion of local moments coexisting with metallic bands. Our investigation of Ca{1-x}RExFe2As2 (RE=La, Pr, and Nd) shows results consistent with a spin-state transition. We argue that the gradual change of the spin-state over a wide temperature range reveals the importance of multiorbital physics, and in particular the competition between the crystal field split Fe 3d orbitals and the Hund's rule coupling.

Ir L3 edge Resonant Inelastic X-ray Scattering (RIXS) and La2 Resonant X-ray Emission Spectroscopy (RXES) have been used to investigate A2IrO3 (A= Na or Li) and CuIr2S4. First principle calculations show that the low-energy dd-excitations in the RIXS spectra of A2IrO3 originate from a split jeff=3/2 to jeff=1/2 excitation. A magnetic excitation is observed in Na2IrO3, whose dispersion is consistent with the theoretical prediction including a bond-dependent Kitaev interaction between jeff=1/2 magnetic moments. In CuIr2S4, RXES measurements reveal that the unoccupied Ir t2g level shift upward in energy as a result of the metal-insulator transition, confirming density functional theory calculations. In the x-ray irradiation induced phase of CuIr2S4, a mid-infrared peak emerges in the RIXS spectrum around 0.4 eV, suggesting that both the crystal structure and the electronic structure are modified due to x-ray irradiation. The size of the magnetic moments in Fe based superconductors has been investigated using Fe Kb X-ray Emission Spectroscopy (XES). Local magnetic moments are found in Fe pnictides and chalcogenides, independent of temperature or carrier concentration. This result supports the notion of local moments coexisting with metallic bands. Our investigation of Ca{1-x}RExFe2As2 (RE=La, Pr, and Nd) shows results consistent with a spin-state transition. We argue that the gradual change of the spin-state over a wide temperature range reveals the importance of multiorbital physics, and in particular the competition between the crystal field split Fe 3d orbitals and the Hund's rule coupling.

Experiments and analysis in this thesis advance the understanding of critical issues in the carrier transport properties of InAs and InAs/GaAs core/shell heterostructure nanowires (diameter 30-60 nm) grown by molecular beam epitaxy. Effects of robust sub-band quantization structure on the gate-voltage dependence of conductance are observed up to 77 K in a single InAs nanowire with diameter 34\pm2 nm. Electronic field effect mobility at 300 K and 30 K are typically 2000-4000 cm^2V^-1s^-1 and 10000-20000 cm^2V^-1s^-1. Strain induced by lattice mismatch in epitaxial core/shell InAs/GaAs heterostructure nanowires is found to relax by formation of dislocations, correlated with nearly one order of magnitude suppression of room temperature field effect mobility compared with bare InAs nanowires. The carrier transport properties of Mn-doped ZnO nanowires were also investigated, where despite the large bandgap, conductivity is not thermally activated, and carrier mobility is consistent with strong degeneracy of the electron gas at 10 K. A novel method was developed providing the first experimental characterization of the quasi-equilibrium gate-voltage dependent surface potential in nanowire field-effect transistors, based on statistics of charging/discharging of a single Coulomb impurity evident in a random telegraph signal, which succeeds in nanostructures with tiny (attofarad) gate capacitance, where similar capacitance-voltage methods are challenging or impossible. We find that the evolution of channel potential with gate voltage is suppressed in the transistor's accumulation regime due to the screening effects of surface states with D_ss=1-2\times10^{12} cm^-2eV^-1. The gate voltage dependence of the random telegraph signals were used as a novel probe to spectroscopically study strong carrier reflection by single Coulomb impurities in nanowires. Reflection probabilities R=0.98-0.999 approach unity for an electron gas with density n=30-10 /micron in 30 nm diameter, 1 micron long InAs nanowires at 30 K. Results were compared with microscopic theory of electron scattering by Coulomb impurities in nanowires with dielectric confinement, i.e low dielectric constant surroundings. The latter, which is known to enhance the bare Coulomb interaction and excitonic binding energy, is an essential ingredient for the strong scattering in this regime, and in small diameter nanowires causes a breakdown in linear screening. Extending this, we show that InAs nanowires can operate is extremely sensitive charge sensors with sensitivity 60 micro eHz^-1/2 at high temperatures (200 K), a combination of characteristics that is not achieved by existing technology. Strong electrostatic coupling of a single charge to the conducting electron gas in the nanowire is enabled by miniaturization of nanowire diameter, operation in a regime of carrier density where the electronic screening length exceeds the nanowire diameter, and dielectric confinement. Finally, single ZnSe nanowire photodetectors are fabricated and studied. Peak responsivity at 2.0 V bias is 20 A/W at room temperature, similar to that of the best epitaxial ZnSe photodetectors. The high responsivity is due to a photoconductive gain g=500, the ratio of carrier lifetime to carrier transit time. The former is enhanced at room temperature due to rapid selective trapping of one species of excited carriers by surface states.

Quantum phase transitions occur as a result of competing ground states. The focus of the present work is to understand quantum criticality and its consequences when the competition is between insulating and metallic ground states. Metal-insulator transitions are studied by means of electronic transport measurements and quantum critical points are approached by applying hydrostatic pressure in two different compounds namely Eu$_2$Ir$_2$O$_7$ and FeCrAs. The former is a ternary metal oxide and the latter is a ternary metal pnictide. A major component of this work was the development of the ultra-high pressure measurements by means of Anvil cells. A novel design is introduced which minimizes the alignment accessory components hence, making the cell more robust and easier to use. Eu$_2$Ir$_2$O$_7$ is a ternary metal oxide and a member of the pyrochlore iridate family. Resistivity measurements under pressure in moissanite anvil cells show the evolution of the ground state of the system from insulating to metallic. The quantum phase transition at $P_c\sim6$ GPa appears to be continuous. A remarkable correspondence is revealed between the effect of the hydrostatic pressure on Eu$_2$Ir$_2$O$_7$ and the effect of chemical pressure by changing the R size in the R$_2$Ir$_2$O$_7$ series. This suggests that in both cases the tuning parameter controls the $t_{2\textrm{g}}$ bandwidth of the iridium $5d$ electrons. Moreover, hydrostatic pressure unveils a curious cross-over from incoherent to conventional metallic behaviour at a $T^* >$ 150 K in the neighbourhood of $P_c$, suggesting a connection between the high and low temperature phases. The possibility of a topological semi-metallic ground state, predicted in recent theoretical studies, is explained. FeCrAs is a ternary metal pnictide with Fermi liquid specific heat and susceptibility behaviour but non-metallic non-Fermi liquid resistivity behaviour. Characteristic properties of the compound are explained and compared to those of superconducting pnictides. Antiferromagnetic (AFM) order sets in at $\sim125$ K with the magnetic moments residing on the Cr site. Pressure measurements are carried out in moissanite and diamond anvil cells in order to suppress the AFM order and resolve the underlying electronic transport properties. While AFM order is destroyed by pressure, the non-metallic non-Fermi liquid behaviour is shown to be robust against pressure.

Quantum phase transitions occur as a result of competing ground states. The focus of the present work is to understand quantum criticality and its consequences when the competition is between insulating and metallic ground states. Metal-insulator transitions are studied by means of electronic transport measurements and quantum critical points are approached by applying hydrostatic pressure in two different compounds namely Eu$_2$Ir$_2$O$_7$ and FeCrAs. The former is a ternary metal oxide and the latter is a ternary metal pnictide. A major component of this work was the development of the ultra-high pressure measurements by means of Anvil cells. A novel design is introduced which minimizes the alignment accessory components hence, making the cell more robust and easier to use. Eu$_2$Ir$_2$O$_7$ is a ternary metal oxide and a member of the pyrochlore iridate family. Resistivity measurements under pressure in moissanite anvil cells show the evolution of the ground state of the system from insulating to metallic. The quantum phase transition at $P_c\sim6$ GPa appears to be continuous. A remarkable correspondence is revealed between the effect of the hydrostatic pressure on Eu$_2$Ir$_2$O$_7$ and the effect of chemical pressure by changing the R size in the R$_2$Ir$_2$O$_7$ series. This suggests that in both cases the tuning parameter controls the $t_{2\textrm{g}}$ bandwidth of the iridium $5d$ electrons. Moreover, hydrostatic pressure unveils a curious cross-over from incoherent to conventional metallic behaviour at a $T^* >$ 150 K in the neighbourhood of $P_c$, suggesting a connection between the high and low temperature phases. The possibility of a topological semi-metallic ground state, predicted in recent theoretical studies, is explained. FeCrAs is a ternary metal pnictide with Fermi liquid specific heat and susceptibility behaviour but non-metallic non-Fermi liquid resistivity behaviour. Characteristic properties of the compound are explained and compared to those of superconducting pnictides. Antiferromagnetic (AFM) order sets in at $\sim125$ K with the magnetic moments residing on the Cr site. Pressure measurements are carried out in moissanite and diamond anvil cells in order to suppress the AFM order and resolve the underlying electronic transport properties. While AFM order is destroyed by pressure, the non-metallic non-Fermi liquid behaviour is shown to be robust against pressure.

Experiments and analysis in this thesis advance the understanding of critical issues in the carrier transport properties of InAs and InAs/GaAs core/shell heterostructure nanowires (diameter 30-60 nm) grown by molecular beam epitaxy. Effects of robust sub-band quantization structure on the gate-voltage dependence of conductance are observed up to 77 K in a single InAs nanowire with diameter 34\pm2 nm. Electronic field effect mobility at 300 K and 30 K are typically 2000-4000 cm^2V^-1s^-1 and 10000-20000 cm^2V^-1s^-1. Strain induced by lattice mismatch in epitaxial core/shell InAs/GaAs heterostructure nanowires is found to relax by formation of dislocations, correlated with nearly one order of magnitude suppression of room temperature field effect mobility compared with bare InAs nanowires. The carrier transport properties of Mn-doped ZnO nanowires were also investigated, where despite the large bandgap, conductivity is not thermally activated, and carrier mobility is consistent with strong degeneracy of the electron gas at 10 K. A novel method was developed providing the first experimental characterization of the quasi-equilibrium gate-voltage dependent surface potential in nanowire field-effect transistors, based on statistics of charging/discharging of a single Coulomb impurity evident in a random telegraph signal, which succeeds in nanostructures with tiny (attofarad) gate capacitance, where similar capacitance-voltage methods are challenging or impossible. We find that the evolution of channel potential with gate voltage is suppressed in the transistor's accumulation regime due to the screening effects of surface states with D_ss=1-2\times10^{12} cm^-2eV^-1. The gate voltage dependence of the random telegraph signals were used as a novel probe to spectroscopically study strong carrier reflection by single Coulomb impurities in nanowires. Reflection probabilities R=0.98-0.999 approach unity for an electron gas with density n=30-10 /micron in 30 nm diameter, 1 micron long InAs nanowires at 30 K. Results were compared with microscopic theory of electron scattering by Coulomb impurities in nanowires with dielectric confinement, i.e low dielectric constant surroundings. The latter, which is known to enhance the bare Coulomb interaction and excitonic binding energy, is an essential ingredient for the strong scattering in this regime, and in small diameter nanowires causes a breakdown in linear screening. Extending this, we show that InAs nanowires can operate is extremely sensitive charge sensors with sensitivity 60 micro eHz^-1/2 at high temperatures (200 K), a combination of characteristics that is not achieved by existing technology. Strong electrostatic coupling of a single charge to the conducting electron gas in the nanowire is enabled by miniaturization of nanowire diameter, operation in a regime of carrier density where the electronic screening length exceeds the nanowire diameter, and dielectric confinement. Finally, single ZnSe nanowire photodetectors are fabricated and studied. Peak responsivity at 2.0 V bias is 20 A/W at room temperature, similar to that of the best epitaxial ZnSe photodetectors. The high responsivity is due to a photoconductive gain g=500, the ratio of carrier lifetime to carrier transit time. The former is enhanced at room temperature due to rapid selective trapping of one species of excited carriers by surface states.

Master of Science / Department of Physics / Bruce M. Law / Interest in the properties of room-temperature ionic liquids is rapidly expanding. Although there have been numerous studies concerning their preparation, their use as a reaction medium and their physical properties, Ionic Liquids (ILs) are so new that many of their bulk physicochemical properties, optical properties, surface properties, toxicities etc. are unknown or only just beginning to be characterized. The highly polar nature of the ILs causes the surfaces of the liquids to become highly ordered in comparison with the surfaces of many other types of organic liquids. Surface structuring at the liquid-vapor interface of the imidazolium based ILs can be examined by using Brewster Angle Ellipsometry and Contact Angle Measurement. The preliminary observation of Ellipsometric measurement shows that there is an interfacial order-disorder transition at temperature Tc=385 K. This result is not analyzed yet but the initial thought behind this is an indication of a ferroelectric transition at the liquid-vapor interface of dipole moment of ILs. From the contact angle measurement it is shown that there is a remarkable change in the contact angle of the imidazolium based ILs over short interval of time ([similar to] ten minutes). Also study of the spreading of the ILs on hard surface shows that there is some definite structural dependence

This thesis describes experiments carried out on two novel strongly correlated electron systems. The first, FeCrAs, is a new material that has not been studied before, while the second, Sr3Ru2O7, has been previously shown to have a very novel so-called ‘nematic’ phase around the metamagnetic quantum critical end point (QCEP). For these studies, a new variation on an established method for measuring the field dependence of susceptibility in a BeCu clamp cell has been developed, and is described, as is a relaxation heat capacity cell that works from 4 K down to 300 mK. A method of growing stoichiometric crystals of the hexagonal iron-pnictide FeCrAs has been developed, and transport and thermodynamic measurements carried out. The in-plane resistivity shows an unusual “non-metallic” dependence on temperature T, rising continuously with decreasing T from ∼800 K to below 100 mK. The c-axis resistivity is similar, except for a sharp drop upon entry into an antiferromagnetic state at T_N ∼ 125 K. Below 10 K the resistivity follows a non-Fermi-liquid power law, ρ(T) = ρ_0 − AT^x with x < 1, while the specific heat shows Fermi liquid behaviour with a large Sommerfeld coefficient, γ ∼ 30 mJ/molK^2. The high temperature properties are reminiscent of those of the parent compounds of the new layered iron-pnictide superconductors, however the T → 0 K properties suggest a new class of non-Fermi liquid. The metamagnetic critical end point temperature T^∗ in Sr3Ru2O7 as a function of hydrostatic pressure with H//ab has been studied using the ac susceptibility. It is found that T^∗ falls monotonically with increasing pressure, going to zero at Pc = 14±0.3 kbar. One sign of the nematic phase observed in the field-angle tuning, i.e. T^∗ rises as the novel phase emerges, has not been seen in our study. However, we see a slope change in T^∗ vs P at ∼12.8 kbar, and a shoulder at the upper field side of the peak in χ′ from ∼12.8 kbar to ∼16.7 kbar. These new features indicate that some new physics sets in near the pressure-tuned QCEP.

Doctor of Philosophy / Department of Physics / Amit Chakrabarti / We carry out Brownian Dynamics Simulations to study the self-assembly of ligated gold nanoparticles for various ligand chain lengths. First, we develop a phenomenological model for an effective nanoparticle-nanoparticle pair potential by treating the ligands as flexible polymer chains. Besides van der Waals interactions, we incorporate both the free energy of mixing and elastic contributions from compression of the ligands in our effective pair potentials. The separation of the nanoparticles at the potential minimum compares well with experimental results of gold nanoparticle superlattice constants for various ligand lengths. Next, we use the calculated pair potentials as input to Brownian dynamics simulations for studying the formation of nanoparticle assembly in three dimensions. For dodecanethiol ligated nanoparticles in toluene, our model gives a relatively shallower well depth and the clusters formed after a temperature quench are compact in morphology. Simulation results for the kinetics of cluster growth in this case are compared with phase separations in binary mixtures. For decanethiol ligated nanoparticles, the model well depth is found to be deeper, and simulations show hybrid, fractal-like morphology for the clusters. Cluster morphology in this case shows a compact structure at short length scales and a fractal structure at large length scales. Growth kinetics for this deeper potential depth is compared with the diffusion-limited cluster-cluster aggregation (DLCA) model. We also did simulation studies of nanoparticle supercluster (NPSC) nucleation from a temperature quenched system. Induction periods are observed with times that yield a reasonable supercluster interfacial tension via classical nucleation theory (CNT). However, only the largest pre-nucleating clusters are dense and the cluster size can occasionally range greater than the critical size in the pre-nucleation regime until a cluster with low enough energy occurs, then nucleation ensues. Late in the nucleation process the clusters display a crystalline structure that is a random mix of fcc and hcp lattices and indistinguishable from a randomized icosahedra structure. Next, we present results from detailed three-dimensional Brownian dynamics simulations of the self-assembly process in quenched short-range attractive colloids. Clusters obtained in the simulations range from dense faceted crystals to fractal aggregates which show ramified morphology on large length scales but close-packed crystalline morphology on short length scales. For low volume fractions of the colloids, the morphology and crystal structure of a nucleating cluster are studied at various times after the quench. As the volume fraction of the colloids is increased, growth of clusters is controlled by cluster diffusion and cluster-cluster interactions. For shallower quenches and low volume fractions, clusters are compact and the growth-law exponent agrees well with Binder–Stauffer predictions and with recent experimental results. As the volume fraction is increased, clusters do not completely coalesce when they meet each other and the kinetics crosses over to diffusion-limited cluster-cluster aggregation (DLCA) limit. For deeper quenches, clusters are fractals even at low volume fractions and the growth kinetics asymptotically reaches the irreversible DLCA case.

Solving quantum dynamics is an exponentially difficult problem. Thus, an exact numerical solution is inaccessible for any condensed matter system. A promising approach is to divide the system into a quantum subsystem containing degrees of freedom which are of greater interest or those which have more profound quantum character (e.g., have smaller mass) and a classical bath containing the rest of the system. Imposing such a partition and treating the bath classically results in quantum-classical dynamics. The quantum-classical Liouville equation is a general equation in the Hilbert space of quantum degrees of freedom while it resides in the phase space of the classical degrees of freedom. Any numerical solution to this equation requires representation of the quantum subsystem in some basis. Solutions to this equation have been already proposed in the subsystem, adiabatic and force bases, each with its own cons and pros. In this work, the quantum-classical equations of motion are cast in the subsystem basis and subsequently mapped to a number of fictitious harmonic oscillators. The result is quantum-classical dynamics in the mapping basis which treats both quantum and classical degrees of freedom on the same footing, i.e., in phase space. Neglecting a portion of the back reaction of the quantum-subsystem to classical bath results in an expression for the time evolution of an operator (density matrix) equal to its Poisson bracket with the Hamiltonian. This equation can be solved in terms of characteristics to provide a computationally tractable method for calculating quantum-classical dynamical properties. The expressions for expectation values and correlation functions in this formalism are derived. Calculations on spin-boson system, barrier crossing models---the so called Tully models---and the Fenna-Mathews-Olson pigments show very good agreement between the results of this method and numerical solutions to the Schrödinger equation.