Transport in bilayer graphene near charge neutrality: Which scattering mechanisms are important?

Using the semiclassical quantum Boltzmann equation (QBE), we numerically calculate the DC transport properties of bilayer graphene near charge neutrality. We find, in contrast to prior discussions, that phonon scattering is crucial even at temperatures below 40K. Nonetheless, electron-electron scattering still dominates over phonon collisions allowing a hydrodynamic approach. We introduce a simple two-fluid hydrodynamic model of electrons and holes interacting via Coulomb drag and compare our results to the full QBE calculation. We show that the two-fluid model produces quantitatively accurate results for conductivity, thermopower, and thermal conductivity.Comment: 10 pages, 3 figure

Quantum Boltzmann equation for bilayer graphene

A-B stacked bilayer graphene has massive electron and hole-like excitations with zero gap in the nearest-neighbor hopping approximation. In equilibrium, the quasiparticle occupation approximately follows the usual Fermi-Dirac distribution. In this paper we consider perturbing this equilibrium distribution so as to determine DC transport coefficients near charge neutrality. We consider the regime $\beta |\mu| \lesssim 1$ (with $\beta$ the inverse temperature and $\mu$ the chemical potential) where there is not a well formed Fermi surface. Starting from the Kadanoff-Baym equations, we obtain the quantum Boltzmann equation of the electron and hole distribution functions when the system is weakly perturbed out of equilibrium. The effect of phonons, disorder, and boundary scattering for finite sized systems are incorporated through a generalized collision integral. The transport coefficients, including the electrical and thermal conductivity, thermopower, and shear viscosity, are calculated in the linear response regime. We also extend the formalism to include an external magnetic field. We present results from numerical solutions of the quantum Boltzmann equation. Finally, we derive a simplified two-fluid hydrodynamic model appropriate for this system, which reproduces the salient results of the full numerical calculations.Comment: 27 pages, 7 figures, fixed typos, add a section on a two-fluid mode

Financial reporting by European foundations

A Work Project, presented as part of the requirements for the Award of a Masters Degree in Finance from the NOVA – School of Business and EconomicsDespite a significant increase in importance of the third sector in Europe, only little data is available of non-profit institutions like foundations and specifically their financial reporting. Therefore, this research analyzes the financial reporting by European foundations and draws conclusions concerning their approach in respective legal frameworks. Accounting standards and practices currently in use are compared and possible characteristics, relationships and patterns are investigated. The conclusion of this analysis confirms the lack of comparability, harmonization and standardization amongst European foundations. Existing reporting standards as well as legal frameworks differ within each country, making a comparison among European foundations very difficult

AlScN: A III-V semiconductor based ferroelectric

Ferroelectric switching is unambigiously demonstrated for the first time in a III-V semiconductor based material: AlScN -- A discovery which could help to satisfy the urgent demand for thin film ferroelectrics with high performance and good technological compatibility with generic semiconductor technology which arises from a multitude of memory, micro/nano-actuator and emerging applications based on controlling electrical polarization. The appearance of ferroelectricity in AlScN can be related to the continuous distortion of the original wurtzite-type crystal structure towards a layered-hexagonal structure with increasing Sc content and tensile strain, which is expected to be extendable to other III-nitride based solid solutions. Coercive fields which are systematically adjustable by more than 3 MV/cm, high remnant polarizations in excess of 100 \mu C/cm$^2$ which constitute the first experimental estimate of the previously inaccessible spontaneous polarization in a III-nitride based material, an almost ideally square-like hysteresis resulting in excellent piezoelectric linearity over a wide strain interval from -0.3% to +0.4% as well as a paraelectric transition temperature in excess of 600{\deg}C are confirmed. This intriguing combination of properties is to our knowledge as of now unprecedented in the field of polycrystalline ferroelectric thin films and promises to significantly advance the commencing integration of ferroelectric functionality to micro- and nanotechnology, while at the same time providing substantial insight to one of the central open questions of the III-nitride semiconductors - that of their actual spontaneous polarization

Interaction effects and charge quantization in single-particle quantum dot emitters

We discuss a theoretical model of an on-demand single-particle emitter that employs a quantum dot, attached to an integer or fractional quantum Hall edge state. Via an exact mapping of the model onto the spin-boson problem we show that Coulomb interactions between the dot and the chiral quantum Hall edge state, unavoidable in this setting, lead to a destruction of precise charge quantization in the emitted wave-packet. Our findings cast doubts on the viability of this set-up as a single-particle source of quantized charge pulses. We further show how to use a spin-boson master equation approach to explicitly calculate the current pulse shape in this set-up.Comment: 5+5 pages, 3 figures, fixed typos, update Supplement Material and update figure

Behavior of Random Dynamical Systems of a Complex Variable

In this thesis we examine some methods of adding noise to the discrete dynamical system z → z^2 + c, in the complex plane. We compare the ; Traditional Random Iteration ; : choosing a sequence of c-values and applying that sequence of maps to the entire plane, versus what we introduce as ; Noisy Random Iteration ; : for each z and for each iterate calculated, we choose a different c-value. We examine two methods of choices for c: (1) Uniform distribution on a neighborhood of c, versus (2) a Bernoulli choice from two values {a,b}, with varying probability p in [0,1] that c=a. We show the results of computer investigations, provide definitions and prove some initial results about Noisy Random Iteration. Finally, we leave the audience with some open questions and directions for future research

Parties’ views on decentralisation are determined by their economic and cultural positions

Decentralisation has been one of the principal forces driving constitutional change in European democracies over the past few decades. Using data covering 284 parties in 31 separate countries, Simon Toubeau and Markus Wagner assess the link between policies on decentralisation, and parties’ ideology on economic and cultural matters. They illustrate that parties form their stance on decentralisation with close reference to their economic and cultural value positions. This effect is influenced, however, by factors such as the degree of self-rule afforded to regions, the existence of regional economic disparities, and the ideological distance of regionalist parties at the national level

Party support in multi-level elections: the influence of economic perceptions and vertical congruence

© 2018 Elsevier Ltd How does the economy influence party support in multi-level states? Using regional-level survey data from Canada, Germany and Spain, we show that there is a ‘cross-level’ effect of economic evaluations. Citizens thus take into consideration both the regional and the national economy when determining their support for incumbents at either level. However, the way in which they do so depends on whether the same party is in office at the two levels or not. If so, standard reward-punishment patterns apply. If different parties are in office at the two levels, incumbents at one level are rewarded for bad and punished for good economic outcomes at the other level. Overall, the influence of cross-level economic perceptions is about half as large as that of same-level economic perceptions. Our findings have important normative implications for the signalling function of elections

Application of Ewald's Method for Efficient Summation of Dyon Long-Range Potentials

We study a model of dyons for SU(2) Yang-Mills theory at finite temperature T < T_c, in particular its ability to generate a confining force between a static quark antiquark pair. The interaction between dyons corresponds to a long-range 1/r potential, which in naive treatments with a finite number of dyons typically gives rise to severe finite volume effects. To avoid such effects we apply the so-called Ewald method, which has its origin in solid state physics. The basic idea of Ewald's method is to consider a finite number of dyons inside a finite cubic volume and enforce periodicity of this volume. We explain the technicalities of Ewald's method and outline how the method can be applied to a wider class of 1/r^p long-range potentials.Comment: 8 pages, 4 figures, contribution to conference "Confinement X