We introduce a new method for calculating the broadening of atomic levels as a function of the atom's position outside the surface. The surface is studied using a cluster model, and the adsorbate-cluster eigenproblem is solved using quantum chemistry codes. The resulting density of states is projected on the adsorbate orbitals, revealing the broadening of adsorbate energy levels into resonances. We extract the width of these resonances from the projected density of states to calculate the broadening. Arbitrary lateral adsorbate positions and surface geometries can be explored by specifying different atom-cluster configurations.

As the size of a physical system decreases toward the nanoscale, quantum mechanical effects such as the discretization of energy levels and the interactions of the electronic spins become readily observable. To understand what happens when an isolated quantum mechanical object, such as an individual molecule, is coupled to a classical object, such as a macroscopic piece of metal, is one of the goals of modern condensed matter physics. The central question of our research is: How do the degrees of freedom of a single molecule (both electronic and mechanical) interact with an electrostatic environment under a constrained geometry? We have chosen to answer this question by looking at the electronic transport through single molecule transistors (SMTs), nanometer-scale transistors in which charge transport occurs through individual molecular states. We use an electromigration technique to fabricate SMTs based on C60 and transition metal coordination complexes (TMCCs). In these devices, the molecule of interest is constrained between two metallic electrodes which act as reservoirs of electrons and energy. At low temperatures, each transistor acts as a single-electron device in the Coulomb blockade regime. Our experimental results suggest that the vibrational modes of the molecules contribute to the transport characteristics of the SMTs. From measurements of the differential conductance of these devices, we observe direct tunneling features that are consistent with vibrational excitations of the molecules. In the TMCC-based SMTs, we also observe inelastic cotunneling features that correspond energetically to vibrational excitations of the molecule, as determined by Raman and infrared spectroscopy. This is a form of gate-modulated inelastic tunneling spectroscopy. In some of the SMTs we observe conductance features characteristic of the Kondo effect, a coherent many-body state comprising an unpaired spin on the molecule coupled by electronic correlation effects to the conduction electrons of the leads. The inferred Kondo temperature in these devices typically exceeds 50 K. In TMCC-based SMTs that exhibit the Kondo effect we observe unusual transport characteristics that deviate from the simplest model of Kondo physics in single electron devices. We suggest possible mechanisms, including strong intramolecular exchange and electron-phonon interaction, that may explain the observed deviation.

We develop a transport formalism for interacting electrons in the presence of quenched disorder. Quantum effects on transport, due both to quantum interference and interaction effects, are incorporated through non-analytic terms in the irreducible interactions and appropriate contributions to the electron self-energy. Perturbatively, our approach recovers the standard results on quantum corrections to the Drude conductivity. We argue the strong coupling fixed point is a magnetic metal beyond perturbation theory. Extensions of the theory are outlined.

Using the RPA, we calculate the plasmon frequencies of an electron gas on a two-dimensional spherical surface in the presence of a weak magnetic field. We show that the magnetic field results in a coupling between electronic states with different angular momentum numbers. This coupling results in a blueshift of the dipolar plasmon resonance with increasing magnetic field. We also investigate how the plasmon energies vary as a function of the number of electrons and radius of the sphere.

The effect of spin injection into d-wave, p-wave, and s-wave superconductors is studied here. A theory is developed which treats the interplay between the bulk and boundary spin transport, as well as the interplay between pair and single-particle transport at the boundary. This theory is used to study the relationship between Andreev reflection and the linear-response spin-injection characteristics. Among the quantities of interest are, the amount of injected magnetization (m), the induced spin-dependent current (I s), and the induced boundary voltage (Vs). In general, Andreev reflection makes each of these three quantities depend on a different combination of the boundary and bulk contributions. The conditions are identified under which some of these quantities that depend solely on the bulk properties can be isolated. A correlation between the Andreev bound states and the spin-injection characteristics is established, which implies a strong directional dependence of spin injection into high-temperature superconductors and in p-wave superconductors.

This thesis will be divided into two parts. In the first part, theory and results of a novel system in which a superconducting single-electron transistor (S-SET) coupled to a two-dimensional electron gas (2DEG) serving as a tunable electromagnetic environment for the S-SET will be discussed, including effects of dissipation, resonant tunneling with photon emission, and photon-assisted tunneling. In the second part, we discuss the techniques for which the SET is incorporated in an RF resonant circuit, resulting in an ultra high charge sensitivity and bandwidth. After the 2DEG is confined into a quantum dot, random telegraph signals (RTS) caused by individual electrons tunneling on and off the dot have been observed. In the equilibrium configuration, the occupational probabilities of the charge states of the dot can be directly measured from the RTS and were found to follow a Fermi distribution. In the non-equilibrium configuration, the RTS correctly detected the onset of the current through the dot.

In the fast collision approximation, the scattering amplitude operator of the quasi-elastic scattering is expressed as the summation of multipole moment operators $M\sp{(k)}(l\sb{i},s\sb{i})$ of the valence shell involved in the resonance$\sp1$ with distinct polarization factors. Each multipole moment operator is expressed as the sum of an orbital moment operator and two spin-orbital moment operators with unique coefficients. The explicit form of these coefficients is obtained and the numerical values are calculated. For the transitions to continuous bands, the explicit forms of $M\sp{(k)}(l\sb{i},s\sb{i})$ are extended from electric dipole transitions to any electric multipole transitions. Within the manifolds of good total L and good total S, the $k\sp{\rm th}$ rank multipole moment operator $M\sp{(k)}(l\sb{i},s\sb{i})$ can be expressed in terms of the $k\sp{\rm th}$ rank spin-orbital moments $M\sp{(k)}({\bf L,S})$ of the total L- and total ${\bf S}$-operators of the valence shell involved in the resonance. Furthermore, within the manifolds of good total J, $M\sp{(k)}(l\sb{i},s\sb{i})$ can be further simplified in terms of the spherical tensor operators of the total J of the resonance valence shell. For Hund's rule ground states, the corresponding proportionality coefficients for both cases were obtained. For rare earths, we obtained the thermal expectation value of $M\sp{(k)}(l\sb{i},s\sb{i})$ at T = 0 for coherent elastic scattering. These results are inconsistent with Hamrick's single electron method$\sp2$ for the second half of the rare earth series. For the first half of the rare earth series, we showed that the single electron method is an approximation of our theory. In spiral antiferromagnets, such as holmium, the magnetic sensitivity results in a series of magnetic satellites distributed at each side of Bragg peak. This behavior can be understood on the basis of the XRES electric multipole transition theory we developed. As temperature increases, the higher order harmonics decrease more rapidly than the lower order harmonics, which can be qualitatively explained by mean-field theory. Just above the Neel temperature, there is weak magnetic scattering which can be understood as the short range moment-moment correlations of different spin-orbital multipole moment operators. ftn $\sp1$J. Luo, J. P. Hannon, G. T. Trammell, Phys. Rev. Lett., 71 287 (1993). $\sp2$M. Hamrick, M.A. Thesis, Physics Department, Rice University, 1991.

Quantum critical properties of strongly correlated metals in heavy fermion systems are investigated. Based on an extended dynamic mean field theory of the Kondo lattice model, two types of quantum phase transitions are found to exist in these materials: the conventional spin density wave transition and a novel locally critical quantum phase transition where the local dynamics is also critical. The associated quantum impurity model, the Bose-Fermi Kondo model, is extensively studied with an epsilon-expansion renormalization group analysis and a large N method. A local quantum critical point is identified in all these approaches, when the bosonic bath has a sub-ohmic spectrum; the results guarantee that a self-consistent solution of the locally critical type is a robust solution to the Kondo lattice model. Quantum critical properties such as thermodynamics are also theoretically investigated for both pictures. A universally diverging Gruneisen ratio is discovered at any quantum critical point, which can be used to characterize different classes of quantum phase transitions.

Intersubband resonances in InAs/AlSb are an ideal tool for optically pumped terahertz (THz) generation because of their enormous tunability and their strength at room temperature. We have carried out a systematic temperature-dependent study of intersubband absorption in InAs/AlSb quantum wells from S to 10 nm well width. The resonance energy redshifts with increasing temperature from 10 to 300 K, and the amount of redshift increases with decreasing well width. We have also observed intersubband absorption in wells as narrow as 3 nm, investigated the carrier distribution in the wells and its influence on intersubband absorption, and performed temperature-dependent cyclotron resonance using a THz quantum cascade laser. We have observed multiple intersubband resonances in coupled quantum well structures designed for THz difference frequency generation. We have modeled the resonances using eight-band k·p theory combined with semiconductor Bloch equations, including the main many-body effects. Temperature is incorporated via band filling and nonparabolicity.

This paper discusses the investigation and manipulation by scanning tunneling microscopy of new fullerene derivative molecules synthesized specifically to achieve nanoscale motion. Two, three, and four-fullerene molecules with connecting oligo (phenylene-ethynylene) structures have been studied statically, and manipulated dynamically to ascertain the type of motion they undergo under direct tip-manipulation and thermal excitation. The dimer molecules were found to have a low surface-diffusion barrier on a Au(111) surface, and were seen to pivot around a single fullerene between scans. Trimer molecules were heated to temperatures where pivoting motion was observed over time spans of minutes, but did not illustrate significant translational motion. Quadrimers, or nanocars, were both directly tip-manipulated and thermally annealed to examine their surface-mechanics, and were found to prefer motion along an axis perpendicular to the oligo (phenylene ethynylene) axle structure, suggesting a coordinated rolling of the fullerenes.