Theory of Polar Corrections to Donor Binding

We calculate the optical phonon correction to the binding energy of electrons to donors in cubic materials. Previous theories calculated the Rydberg energy reduced by the effective mass and the static dielectric function. They omitted an important energy term from the long-range polarization of the ionized donor, which vanishes for the neutral donor. They also omitted the donor-phonon interaction. Including these terms yields a new formula for the donor binding energy

Bulk metals with helical surface states

In the flurry of experiments looking for topological insulator materials, it has been recently discovered that some bulk metals very close to topological insulator electronic states, support the same topological surface states that are the defining characteristic of the topological insulator. First observed in spin-polarized ARPES in Sb (D. Hsieh et al. Science 323, 919 (2009)), the helical surface states in the metallic systems appear to be robust to at least mild disorder. We present here a theoretical investigation of the nature of these "helical metals" - bulk metals with helical surface states. We explore how the surface and bulk states can mix, in both clean and disordered systems. Using the Fano model, we discover that in a clean system, the helical surface states are \emph{not} simply absorbed by hybridization with a non-topological parasitic metallic band. Instead, they are pushed away from overlapping in momentum and energy with the bulk states, leaving behind a finite-lifetime surface resonance in the bulk energy band. Furthermore, the hybridization may lead in some cases to multiplied surface state bands, in all cases retaining the helical characteristic. Weak disorder leads to very similar effects - surface states are pushed away from the energy bandwidth of the bulk, leaving behind a finite-lifetime surface resonance in place of the original surface states

Luttinger Liquid at the Edge of a Graphene Vacuum

We demonstrate that an undoped two-dimensional carbon plane (graphene) whose bulk is in the integer quantum Hall regime supports a non-chiral Luttinger liquid at an armchair edge. This behavior arises due to the unusual dispersion of the non-interacting edges states, causing a crossing of bands with different valley and spin indices at the edge. We demonstrate that this stabilizes a domain wall structure with a spontaneously ordered phase degree of freedom. This coherent domain wall supports gapless charged excitations, and has a power law tunneling $I-V$ with a non-integral exponent. In proximity to a bulk lead, the edge may undergo a quantum phase transition between the Luttinger liquid phase and a metallic state when the edge confinement is sufficiently strong relative to the interaction energy scale.Comment: 4 pages, 3 figure

Literary Africa: Spanish Reflections of Morocco, Western Sahara, and Equatorial Guinea in the Contemporary Novel, 1990-2010

This dissertation analyzes the strategies that Spanish and Hispano-African authors employ when writing about Africa in the contemporary novel (1990-2010). Focusing on the former Spanish colonial territories of Morocco, Western Sahara, and Equatorial Guinea, I analyze the post-colonial literary discourse about these regions. This study examines the new ways of conceptualizing Africa that depart from an Orientalist framework as advanced by the novelists Lorenzo Silva, Concha López Sarasúa, Ramón Mayrata, María Dueñas, Fernando Gamboa, Montserrat Abumalham, Javier Reverte, Alberto Vázquez-Figueroa, and Donato Ndongo. Their works are representative of a recent trend in Spanish letters that signals a literary focus on Africa and the African Other. I examine these contemporary novels within their historical context, specifically engaging with the theoretical ideas of Edward Said’s Orientalism (1978), to determine to what extent his analysis of Orientalist discourse still holds value for a study of the Spanish novel of thirty years later. In addition, the work of theorists such as Gil Anidjar, Emmanuel Levinas, James C. Scott, Ryszard Kapuściński, Georges Van den Abbeele and Chandra Mohanty contribute to the analyses of specific works. These theorists provide a theoretical framework for my thesis that contemporary Spanish authors are writing Africa in ways that undermine and circumvent the legacy of Orientalist discourse. I seek to highlight the innovative approaches that these authors are taking towards their literary engagement with Africa. The imaginary that pertains to Africa has served an integral role in the history and creation of modern Spain, and it is illuminating to trace the influences that it continues to exert on Spanish writers. In the last thirty years, Spain’s relationship with Africa has dramatically changed through peace treaties, the independence of nations, migratory patterns, tourism, and in other substantial ways. Within this dissertation, I address these changes by focusing on literary representations of political engagement, gender issues, and travel to highlight how Africa is represented in light of these recent developments. As Spanish authors continue to engage with and to write about Africa, this study hopes to show that Orientalism is no longer a prevalent discourse in the contemporary Spanish novel

Student Centered Approaches in High School Algebra

This quasi-experimental study evaluated the impact of four, student centered approaches on learning outcomes for high school math students, to determine if algebraic understanding improved in an active learning environment. Specifically, this study investigated high school algebra students’ use of learning stations, hands on manipulatives, small group collaboration and mathematical games, to deepen their conceptual understanding of linear equations. Surveys revealed students prefered to work independently while being led by the teacher, yet this approach yielded the least amount of improvement. Pre- and post-tests showed students from the experimental groups out-performed peers in the control group on achievement of standards during each cycle of the experiment, with small group collaboration being the student centered approach yielding the highest mean. In conclusion, research findings reveal students need to become active participants in their learning in order to enrich their learning experience and enhance conceptual understanding in mathematics

Unconventional Hall effect in pnictides from interband interactions

We calculate the Hall transport in a multiband systems with a dominant interband interaction between carriers having electron and hole character. We show that this situation gives rise to an unconventional scenario, beyond the Boltzmann theory, where the quasiparticle currents dressed by vertex corrections acquire the character of the majority carriers. This leads to a larger (positive or negative) Hall coefficient than what expected on the basis of the carrier balance, with a marked temperature dependence. Our results explain the puzzling measurements in pnictides and they provide a more general framework for transport properties in multiband materials.Comment: 5 pages, 2 figure

Quantum pump effect in one-dimensional systems of Dirac fermions

We investigate the behavior of the directed current in one-dimensional systems of Dirac fermions driven by local periodic potentials in the forward as well in backscattering channels. We treat the problem with Keldysh non-equilibrium Green's function formalism. We present the exact solution for the case of an infinite wire and show that in this case the dc current vanishes identically. We also investigate a confined system consistent in an annular arrangement coupled to a particle reservoir. We present a perturbative treatment that allows for the analytical expressions of the dc current in the lowest order of the amplitudes of the potential. We also present results obtained from the exact numerical solution of the problem.Comment: 8 pages, 5 figure

Transport through a molecular quantum dot in the polaron crossover regime

We consider resonant transport through a molecular quantum dot coupled to a local vibration mode. Applying the non-equilibrium Green function technique in the polaron representation, we develop a non-perturbative scheme to calculate the electron spectral function of the molecule in the regime of intermediate electron-phonon coupling. With increasing tunneling coupling to the leads, correlations between polaron clouds become more important at relatively high temperature leading to a strong sharpening of the peak structure in the spectral function. The detection of such features in the current-voltage characteristics is briefly discussed

Non-Markov dynamics and phonon decoherence of a double quantum dot charge qubit

In this paper we investigate decoherence times of a double quantum dot (DQD) charge qubit due to it coupling with acoustic phonon baths. We individually consider the acoustic piezoelectric as well as deformation coupling phonon baths in the qubit environment. The decoherence times are calculated with two kinds of methods. One of them is based on the qusiadiabatic propagator path integral (QUAPI) and the other is based on Bloch equations, and two kinds of results are compared. It is shown that the theoretical decoherence times of the DQD charge qubit are shorter than the experimental reported results. It implies that the phonon couplings to the qubit play a subordinate role, resulting in the decoherence of the qubit.Comment: 5 pages, 4 figure

Phonon-assisted decoherence in the production of polarization-entangled photons in a single semiconductor quantum dot

We theoretically investigate the production of polarization-entangled photons through the biexciton cascade decay in a single semiconductor quantum dot. In the intermediate state the entanglement is encoded in the polarizations of the first emitted photon and the exciton, where the exciton state can be effectively ``measured'' by the solid state environment through the formation of a lattice distortion. We show that the resulting loss of entanglement becomes drastically enhanced if the phonons contributing to the lattice distortion are subject to elastic scatterings at the device boundaries, which might constitute a serious limitation for quantum-dot based entangled-photon devices.Comment: 4 pages, 3 figure, to appear in Physical Review Letter