Localisation and finite-size effects in graphene flakes

We show that electron states in disordered graphene, with an onsite potential that induces inter-valley scattering, are localised for all energies at disorder as small as of the band width of clean graphene. We clarify that, in order for this Anderson-type localisation to be manifested, graphene flakes of size or larger are needed. For smaller samples, due to the surprisingly large extent of the electronic wave functions, a regime of apparently extended (or even critical) states is identified. Our results complement earlier studies of macroscopically large samples and can explain the divergence of results for finite-size graphene flakes

Contextual welfare chauvinism: Left-wing governments and immigrant welfarerights in Western Europe.

In Western Europe, as immigration flows increase – or at least become more salient – and austerity measures place welfare states under pressure, policy reforms that extend or restrict access to the welfare state for immigrants are highly contested. Much academic attention has been paid to restrictive or ‘welfare chauvinist’ policy reforms and the role played by far-right parties and sympathisers in the policy-making process. Yet, left-wing parties, often considered the most susceptible to the ‘progressive’s dilemma’ between open borders and strong welfare states, remain under-researched. Using new data on immigrant welfare rights for 14 European countries from 1980 to 2018, and differentiating between social democrats, the greens and far-left parties, we show that social democrats engage in both reforms that restrict as well as expand, but on average, they tend to be negatively associated with immigrant welfare rights. However, our evidence shows that context matters: We find that that social democrats are less likely to retrench immigrant welfare rights when they share power with the far left, and become more likely to retrench as unemployment rises

Transport Properties of a One-Dimensional Two-Component Quantum Liquid with Hyperbolic Interactions

We present an investigation of the sinh-cosh (SC) interaction model with twisted boundary conditions. We argue that, when unlike particles repel, the SC model may be usefully viewed as a Heisenberg-Ising fluid with moving Heisenberg-Ising spins. We derive the Luttinger liquid relation for the stiffness and the susceptibility, both from conformal arguments, and directly from the integral equations. Finally, we investigate the opening and closing of the ground state gaps for both SC and Heisenberg-Ising models, as the interaction strength is varied.Comment: 10 REVTeX pages + 4 uuencoded figures, UoU-002029

Combined position & force control of a robotic manipulator

The ARM is a 6 DOF robotic manipulator used by disabled people with a severe handicap at the upper extremities The present ARM is position and velocity controlled. The desired position of the robot is given by the user. However, in constraint scenario's, manipulation becomes too difficult and an assistant-controller is wanted. This assistant is based on external forces on the gripper of the robot, measured using a force-torque sensor. A new control strategy is designed for measured forces and user input. The basic principle of this strategy is derived from the way that humans steer their hand. Sensed forces are followed until they are not present anymore, except when the user wants to do a manipulation in that direction. Therefore a combined position/force controller was designed. All 6 DOF of the robot can be steered by both the user and the force controller at the same time. Beside the design of the control strategy, it is also implemented on the ARM and tested in four test-cases

Correlation-Strength Driven Anderson Metal-Insulator Transition

The possibility of driving an Anderson metal-insulator transition in the presence of scale-free disorder by changing the correlation exponent is numerically investigated. We calculate the localization length for quasi-one-dimensional systems at fixed energy and fixed disorder strength using a standard transfer matrix method. From a finite-size scaling analysis we extract the critical correlation exponent and the critical exponent characterizing the phase transition.Comment: 3 pages; 2 figure

General Localization Lengths for Two Interacting Particles in a Disordered Chain

The propagation of an interacting particle pair in a disordered chain is characterized by a set of localization lengths which we define. The localization lengths are computed by a new decimation algorithm and provide a more comprehensive picture of the two-particle propagation. We find that the interaction delocalizes predominantly the center-of-mass motion of the pair and use our approach to propose a consistent interpretation of the discrepancies between previous numerical results.Comment: 4 pages, 2 epsi figure

Interacting particles at a metal-insulator transition

We study the influence of many-particle interaction in a system which, in the single particle case, exhibits a metal-insulator transition induced by a finite amount of onsite pontential fluctuations. Thereby, we consider the problem of interacting particles in the one-dimensional quasiperiodic Aubry-Andre chain. We employ the density-matrix renormalization scheme to investigate the finite particle density situation. In the case of incommensurate densities, the expected transition from the single-particle analysis is reproduced. Generally speaking, interaction does not alter the incommensurate transition. For commensurate densities, we map out the entire phase diagram and find that the transition into a metallic state occurs for attractive interactions and infinite small fluctuations -- in contrast to the case of incommensurate densities. Our results for commensurate densities also show agreement with a recent analytic renormalization group approach.Comment: 8 pages, 8 figures The original paper was splitted and rewritten. This is the published version of the DMRG part of the original pape

Rigidity analysis of HIV-1 protease

We present a rigidity analysis on a large number of X-ray crystal structures of the enzyme HIV-1 protease using the 'pebble game' algorithm of the software FIRST. We find that although the rigidity profile remains similar across a comprehensive set of high resolution structures, the profile changes significantly in the presence of an inhibitor. Our study shows that the action of the inhibitors is to restrict the flexibility of the beta-hairpin flaps which allow access to the active site. The results are discussed in the context of full molecular dynamics simulations as well as data from NMR experiments.Comment: 4 pages, 3 figures. Conference proceedings for CMMP conference 2010 which was held at the University of Warwic

The random phase property and the Lyapunov Spectrum for disordered multi-channel systems

A random phase property establishing in the weak coupling limit a link between quasi-one-dimensional random Schrödinger operators and full random matrix theory is advocated. Briefly summarized it states that the random transfer matrices placed into a normal system of coordinates act on the isotropic frames and lead to a Markov process with a unique invariant measure which is of geometric nature. On the elliptic part of the transfer matrices, this measure is invariant under the unitaries in the hermitian symplectic group of the universality class under study. While the random phase property can up to now only be proved in special models or in a restricted sense, we provide strong numerical evidence that it holds in the Anderson model of localization. A main outcome of the random phase property is a perturbative calculation of the Lyapunov exponents which shows that the Lyapunov spectrum is equidistant and that the localization lengths for large systems in the unitary, orthogonal and symplectic ensemble differ by a factor 2 each. In an Anderson-Ando model on a tubular geometry with magnetic field and spin-orbit coupling, the normal system of coordinates is calculated and this is used to derive explicit energy dependent formulas for the Lyapunov spectrum

Two interacting particles in a random potential

We study the scaling of the localization length of two interacting particles in a one-dimensional random lattice with the single particle localization length. We obtain several regimes, among them one interesting weak Fock space disorder regime. In this regime we derive a weak logarithmic scaling law. Numerical data support the absence of any strong enhancement of the two particle localization length