Peter A. Schulz

Inner and outer edge states in graphene rings: A numerical investigation

We numerically investigate quantum rings in graphene and find that their electronic properties may be strongly influenced by the geometry, the edge symmetries, and the structure of the corners. Energy spectra are calculated for different geometries (triangular, hexagonal, and rhombus-shaped graphene rings) and edge terminations (zigzag, armchair, as well as the disordered edge of a round geometry). The states localized at the inner edges of the graphene rings describe different evolution as a function of magnetic field when compared to those localized at the outer edges. We show that these different evolutions are the reason for the formation of subbands of edge-states energy levels, separated by gaps (anticrossings). It is evident from mapping the charge densities that the anticrossings occur due to the coupling between inner and outer edge states.

Third edge for a graphene nanoribbon: A tight-binding model calculation

The electronic and transport properties of an extended linear defect embedded in a zigzag nanoribbon of realistic width are studied, within a tight binding model approach. Our results suggest that such defect profoundly modify the properties of the nanoribbon, introducing new conductance quantization values and modifying the conductance quantization thresholds. The linear defect along the nanoribbon behaves as an effective third edge of the system, which shows a metallic behavior, giving rise to new conduction pathways that could be used in nanoscale circuitry as a quantum wire.

Tunable resonances due to vacancies in graphene nanoribbons

The coherent electron transport along zigzag and metallic armchair graphene nanoribbons in the presence of one or two vacancies is investigated. Having in mind atomic scale tunability of the conductance fingerprints, the primary focus is on the effect of the distance to the edges and inter vacancies spacing. An involved interplay of vacancies sublattice location and nanoribbon edge termination, together with the spacing parameters lead to a wide conductance resonance line shape modification. Turning on a magnetic field introduces a new length scale that unveils counter-intuitive aspects of the interplay between purely geometric aspects of the system and the underlying atomic scale nature of graphene.

Revisiting country research profiles: learning about the scientific cultures

In the present work we analyze the Country Profiles, open access data from ISI Thomson Reuter’s Science Watch. The country profiles are rankings of the output (indexed in Web of Science) in different knowledge fields during a determined time span for a given country. The analysis of these data permits defining a Country Profile Index, a tool for diagnosing the activity of the scientific community of a country and their possible strengths and weakness. Furthermore, such analysis also enables the search for identities among research patterns of different countries, time evolution of such patterns and the importance of the adherence to the database journals portfolio in evaluating the productivity in a given knowledge field.

Valley polarization effects on localization in graphene Landau levels

Effects of disorder and valley polarization in graphene are investigated in the quantum Hall regime. We find anomalous localization properties for the lowest Landau level (LL), where disorder can induce wavefunction delocalization (instead of localization), both for white-noise and gaussian-correlated disorder. We quantitatively identify the contribution of each sublattice to wavefunction amplitudes. Following the valley (sublattice) polarization of states within LLs for increasing disorder we show: (i) valley mixing in the lowest LL is the main effect behind the observed anomalous localization properties, (ii) the polarization suppression with increasing disorder depends on the localization for the white-noise model, while, (iii) the disorder induces a partial polarization in the higher Landau levels for both disorder models.

Robust signatures in the current–voltage characteristics of DNA molecules oriented between two graphene nanoribbon electrodes

In this work, we numerically calculate the electric current through three kinds of DNA sequences (telomeric, λ-DNA and p53-DNA) described by different heuristic models. A bias voltage is applied between two zigzag edged graphene contacts attached to the DNA segments, while a gate terminal modulates the conductance of the molecule. Calculation of the current is performed by integrating the transmission function (calculated using the lattice Greens function) over the range of energies allowed by the chemical potentials. We show that a telomeric DNA sequence, when treated as a quantum wire in the fully coherent low-temperature regime, works as an excellent semiconductor. Clear steps are apparent in the current–voltage curves of telomeric sequences and are present independent of length and sequence initialization at the contacts. We also find that the molecule–electrode coupling can drastically influence the magnitude of the current. The difference between telomeric DNA and other DNAs, such as λ-DNA and DNA for the tumour suppressor p53, is particularly visible in the length dependence of the current.

Quasienergy spectra of graphene dots in intense ac fields: Field anisotropy and photon-dressed quantum rings

A graphene quantum dot under intense ac field and static low magnetic field is investigated. From a tight-binding perspective, applying a Fourier-Floquet transformation and renormalization process, we observe that graphene -intrinsically anisotropic- reveals field polarization signatures in the quasi-density of states. For the ac field polarized along the armchair direction, the dressed electronic structure shows an emergent property: an ac field induced quantum ring. This is inferred by the orientation-dependent formation of a miniband of energy states periodically modulated with increasing magnetic field, exactly analogous to the behavior of a quantum ring spectrum.

Wave-function mapping conditions in open quantum dot structures

We discuss the minimal conditions for wave-function spectroscopy, using resonant tunneling as the measurement tool, in open quantum dots. The present results establish a parameter region where the wave-function spectroscopy by resonant tunneling can be achieved. A breakdown of the mapping condition is related to a change into a double quantum dot structure induced by the local probing potential. The precise control over shape and extension of the potential probes is irrelevant for wave-function mapping. Moreover, the present system is a realization of a tunable Fano system beyond the wave-function mapping regime, as well as a system where the states can be selectively manipulated.

Visualization of ranking data: Geographical signatures in international collaboration, leadership and research impact

In this work we address the comprehensive Scimago Institutions Ranking 2012, proposing a data visualization of the listed bibliometric indicators for the 509 Higher Education Institutions among the 600 largest research institutions ranked according to their outputs. We focus on research impact, internationalization and leadership indicators, which became important benchmarks in a worldwide discussion about research quality and impact policies for universities. Our data visualization reveals a qualitative difference between the behavior of Northern American and Western European Higher Education Institutions concerning International collaboration levels. Chinese universities show still a systematic low international collaboration levels which are positively linked to the low research impact. The data suggests that research impact can be related directly to internationalization only to rather low values for both indicators. Above world average, other determinants may become relevant in fostering further impact. The leadership indicator provides further insights to the collaborative environment of universities in different geographical regions, as well as the optimized collaboration portfolio for enhancing research impact.

Quantifying the levitation picture of extended states in lattice models

The behavior of extended states is quantitatively analyzed for two-dimensional lattice models. A levitation picture is established for both white-noise and correlated disorder potentials. In a continuum limit window of the lattice models we find simple quantitative expressions for the extended states levitation, suggesting an underlying universal behavior. On the other hand, these results point out that the quantum Hall phase diagrams may be disorder dependent.