Structure and correlation effects in semiconducting SrTiO₃

We have investigated the effects of structure change and electron correlation on SrTiO₃ single crystals using angle-resolved photoemission spectroscopy. We show that the cubic to tetragonal phase transition at 105 K is manifested by a charge transfer from in-plane (dyz and dzx) bands to out-of-plane (dxy) band, which is opposite to the theoretical predictions. Along this second-order phase transition, we find a smooth evolution of the quasiparticle strength and effective masses. The in-plane band exhibits a peak-dip-hump lineshape, indicating a high degree of correlation on a relatively large (170 meV) energy scale, which is attributed to the polaron formation

Morphology of graphene thin film growth on SiC(0001)

Epitaxial films of graphene on SiC(0001) are interesting from a basic physics as well as applications-oriented point of view. Here we study the emerging morphology of in-vacuo prepared graphene films using low energy electron microscopy (LEEM) and angle-resolved photoemission (ARPES). We obtain an identification of single and bilayer of graphene film by comparing the characteristic features in electron reflectivity spectra in LEEM to the PI-band structure as revealed by ARPES. We demonstrate that LEEM serves as a tool to accurately determine the local extent of graphene layers as well as the layer thickness

On the zeros of a minimal realization

AbstractIn an earlier work, the authors have introduced a coordinate-free, module-theoretic definition of zeros for the transfer function G(s) of a linear multivariable system (A,B,C). The first contribution of this paper is the construction of an explicit k[z]-module isomorphism from that zero module, Z(G), to V∗/R∗, where V∗ is the supremal (A,B)-invariant subspace contained in kerC and R∗ is the supremal (A,B)-controllable subspace contained in kerC, and where (A,B,C) constitutes a minimal realization of G(s). The isomorphism is developed from an exact commutative diagram of k-vector spaces. The second contribution is the introduction of a zero-signal generator and the establishment of a relation between this generator and the classic notion of blocked signal transmissions

Antipsychotic‐Induced Hyperprolactinemia

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90238/1/phco.29.1.64.pd

Three Papers in Applied Microeconomics and Econometrics

This dissertation is comprised of three distinct papers covering topics in appliedmicroeconomics and applied econometrics. The first paper addresses a common problem faced by empirical researchers wishing to estimate Markov regime-switching models. For these models, testing for the possible presence of more than one regime requires the use of a non-standard test statistic. The analytic steps needed to implement the test of Markov regime-switching proposed by Cho & White (2007) are derived in detail in Carter & Steigerwald (2013). We summarize those implementation steps and address the computational issues that arise. A new Stata command to compute the regime-switching critical values, rscv, is introduced and presented in the context of empirical economic research. This paper is joint work with Douglas Steigerwald, and has previously appeared in the Stata Journal (Bostwick and Steigerwald, 2014).In the second paper, I address a question in the field of economics of education: that is,whether college students use their choice of major as a signal of unobserved productivityin the labor market. I propose a model of postsecondary education in which major fieldof study can be used by individuals to signal productivity to employers. Under thissignaling model, I show that geographic areas with high access to elite universities resultin fewer science, technology, engineering, and mathematics (STEM) majors among lower ability students at non-elite colleges. Using data from the National Center for Education Statistics' Baccalaureate and Beyond survey, I find evidence that is consistent with the signaling model prediction, specifically a 2.3-3.7 percentage point (or 16-25%) decrease in the probability of choosing a STEM major among lower ability students in areas with greater access to elite colleges. This paper has previously appeared in Economic Inquiry (Bostwick, 2016).In the third paper, I analyze an unexpected consequence of a highly debated educationpolicy. Many school districts are now considering delaying high school start times toaccommodate the sleep schedules of teens. This paper explores whether such policychanges can have an impact on teen car accident rates. This impact could functionboth through a direct effect on teen sleep deprivation and indirectly through changes tothe driving environment, i.e. shifting teen commute times into the high volume, "rushhour" of the morning. I find that, during the morning commute hours, any potentialeffect stemming from avoided sleep deprivation is offset by the effect of shifting teendriving into rush hour, so that a 15 minute delay in high school start times leads to a21% increase in morning teen accidents. However, by focusing on late-night accidents, Ialso find evidence of a persistent sleep effect. By decreasing teen sleep deprivation, a 15 minute delay in school start times leads to a 26% decrease in late-night teen accidents

Anderson Transition in Disordered Graphene

We use the regularized kernel polynomial method (RKPM) to numerically study the effect disorder on a single layer of graphene. This accurate numerical method enables us to study very large lattices with millions of sites, and hence is almost free of finite size errors. Within this approach, both weak and strong disorder regimes are handled on the same footing. We study the tight-binding model with on-site disorder, on the honeycomb lattice. We find that in the weak disorder regime, the Dirac fermions remain extended and their velocities decrease as the disorder strength is increased. However, if the disorder is strong enough, there will be a {\em mobility edge} separating {\em localized states around the Fermi point}, from the remaining extended states. This is in contrast to the scaling theory of localization which predicts that all states are localized in two-dimensions (2D).Comment: 4 page

Controlling the Electronic Structure of Bilayer Graphene

We describe the synthesis of bilayer graphene thin films deposited on insulating silicon carbide and report the characterization of their electronic band structure using angle-resolved photoemission. By selectively adjusting the carrier concentration in each layer, changes in the Coulomb potential led to control of the gap between valence and conduction bands. This control over the band structure suggests the potential application of bilayer graphene to switching functions in atomic-scale electronic devices

Quasiparticle dynamics in graphene

The effectively massless, relativistic behaviour of graphene's charge carriers—known as Dirac fermions—is a result of its unique electronic structure, characterized by conical valence and conduction bands that meet at a single point in momentum space (at the Dirac crossing energy). The study of many-body interactions amongst the charge carriers in graphene and related systems such as carbon nanotubes, fullerenes and graphite is of interest owing to their contribution to superconductivity and other exotic ground states in these systems. Here we show, using angle-resolved photoemission spectroscopy, that electron–plasmon coupling plays an unusually strong role in renormalizing the bands around the Dirac crossing energy—analogous to mass renormalization by electron–boson coupling in ordinary metals. Our results show that electron–electron, electron–plasmon and electron–phonon coupling must be considered on an equal footing in attempts to understand the dynamics of quasiparticles in graphene and related systems

Experimental Determination of the Spectral Function of Graphene

A number of interesting properties of graphene and graphite are postulated to derive from the peculiar bandstructure of graphene. This bandstructure consists of conical electron and hole pockets that meet at a single point in momentum (k) space--the Dirac crossing, at energy $E_{D} = \hbar \omega_{D}$. Direct investigations of the accuracy of this bandstructure, the validity of the quasiparticle picture, and the influence of many-body interactions on the electronic structure have not been addressed for pure graphene by experiment to date. Using angle resolved photoelectron spectroscopy (ARPES), we find that the expected conical bands are distorted by strong electron-electron, electron-phonon, and electron-plasmon coupling effects. The band velocity at $E_{F}$ and the Dirac crossing energy $E_{D}$ are both renormalized by these many-body interactions, in analogy with mass renormalization by electron-boson coupling in ordinary metals. These results are of importance not only for graphene but also graphite and carbon nanotubes which have similar bandstructures.Comment: pdf file, 10 pages, 4 figure

The formation of an energy gap in graphene on ruthenium by controlling the interface

In this work, we have investigated the spectral function of graphene on a monolayer of intercalated gold on Ru(0001) using angle-resolved photoemission spectroscopy (ARPES). The intercalation leads to a decoupling of the graphene film, as documented by emergence of the characteristic linear π-bands near the Fermi level. However, a band gap at the band crossing is observed. We relate this gap opening to the broken symmetry of the two carbon sublattices, induced by the special lattice mismatch of the graphene and the intercalated gold monolayer