arXiv subjects

Niels R. Walet

Publications and source records attributed to Niels R. Walet.

At least 37 records · Page 2Linked to original sources

arXiv2017

Edge modes and non local conductance in graphene superlattices

We study the existence of edge modes in gapped Moiré superlattices in graphene monolayer ribbons. We find that the superlattice bands acquire finite Chern numbers, which lead to a Valley Hall Effect. The presence of dispersive edge modes is confirmed by calculations of the band structure of realistic nanoribbons using tight binding methods. These edge states are only weakly sensitive to disorder, as short-range scattering processes lead to mean free paths of the order of microns. The results explain the existence of edge currents when the chemical potential lies within the bulk superlattice gap, and offer an explanation for existing non-local resistivity measurements in graphene ribbons on boron nitride.

cond-mat.mes-hall↗

arXiv2017

Effective Interactions in a Graphene Layer Induced by the Proximity to a Ferromagnet

The proximity-induced couplings in graphene due to the vicinity of a ferromagnetic insulator are analyzed. We combine general symmetry principles and simple tight-binding descriptions to consider different orientations of the magnetization. We find that, in addition to a simple exchange field, a number of other terms arise. Some of these terms act as magnetic orbital couplings, and others are proximity-induced spin-orbit interactions. The couplings are of similar order of magnitude, and depend on the orientation of the magnetization. A variety of phases, and anomalous Hall effect regimes, are possible.

cond-mat.mes-hall↗

arXiv2017

Majorana Zero-Modes in a Two-Dimensional $p$-Wave Superconductor

We analyze the formation of Majorana zero-modes at the edge of a two-dimensional topological superconductor. In particular, we study a time-reversal-invariant triplet phase that is likely to exist in doped Bi$_2$Se$_3$. Upon the introduction of an in-plane magnetic field to the superconductor, a gap is opened in the surface modes, which induces localized Majorana modes. The position of these modes can be simply manipulated by changing the orientation of the applied field, yielding novel methods for braiding these states with possible applications to topological quantum computation.

cond-mat.supr-con↗

arXiv2017

Water confined between graphene layers: the case for a square ice

Water confined between two graphene layers with a small separation forms a two-dimensional ice structure,with an apparent square symmetry [Algara-Siller et al., Nature (London) 519, 443 (2015)], which is poorly understood. A density functional approach is applied to the water, but not to the water graphene interactions, since the two crystals are incommensurate. We thus need use a potential model for the interaction between water and graphene. We analyze the models for confinement of water by graphene, and find that even though the general features are well established, the detail is not so well understood. Using a representative range of potential models, we perform density-functional calculations and show that many ice-like configurations exist. In some cases these are unstable with respect to decay into a bi-layer structure, but we expect tunneling between such structures to be slow. It is shown that there is one good candidate for a square crystal, which is a peculiar anti-ferroelectric arrangement of water molecules.

cond-mat.mes-hall↗

arXiv2017

Electronic correlations in the Hubbard model on a bi-partite lattice

In this work we study the Hubbard model on a bi-partite lattice using the coupled-cluster method (CCM). We first investigate what, within this approach, allows us to reproduce the zero order parameter in the 1D model, as predicted by the exact solution. We show that we need a critical correlation in some of the coupled-cluster model coefficients to reproduce this result, as can be obtained by applying some very accurate results using the CCM for the Heisenberg model. Using the same approach we then tackle the 2D Hubbard model on a square and a honeycomb lattice, both of which can be thought to represent 2D materials. We also analyse the charge and spin excitations, with reasonable results.

cond-mat.str-el↗

arXiv2016

Description of light nuclei in pionless effective field theory using the stochastic variational method

We construct a coordinate-space potential based on pionless effective field theory with a Gaussian regulator. Charge-symmetry breaking is included through the Coulomb potential and through two- and three-body contact interactions. Starting with the effective field theory potential, we apply the stochastic variational method to determine the ground states of nuclei with mass number $A\leq 4$. At next-to-next-to-leading order, two out of three independent three-body parameters can be fitted to the three-body binding energies. To fix the remaining one, we look for a simultaneous description of the binding energy of $^4$He and the charge radii of $^3$He and $^4$He. We show that at the order considered we can find an acceptable solution, within the uncertainty of the expansion. We find that the EFT expansion shows good agreement with empirical data within the estimated uncertainty, even for a system as dense as $^4$He.

nucl-th↗

arXiv2016

Interaction between point charges, dipoles and graphene layers

We analyse the interaction between charges and graphene layers. The electric polarisability of graphene induces a force, that can be described by an image charge. The analysis shows that graphene can be described as an imperfect conductor with a finite dielectric constant, $ε_r$, for weak coupling, and it behaves as a metal for strong fields. As a consequence, the interaction between polar molecules and graphene layer(s) tends to align the molecular dipole along the direction normal to the graphene, and quantitative estimates of the energy gain are given The strength of this interaction can be sufficient to overcome thermal effects when the molecule is close to the layer, even at room temperature. Hence, boundary effects play a significant role in determining the structure of systems such as water confined in atomically narrow van der Waals heterostructures.

cond-mat.mes-hall↗

arXiv2016

The role of attitudinal factors in mathematical on-line assessments: a study of undergraduate STEM students

This study explores student attitudes to the use of substantive on-line assessments that require mathematical answers. Our goal is to learn what are the important aspects in a design of more effective e-assessments that support learning of mathematical subjects in a higher education setting. To that end we analyse the effects of a variety of attitudinal factors towards such assessments amongst a cross-section of 1st year students in an English University. These students were all previously exposed to on-line assessments containing substantial mathematical work, including testing of and feedback on the algebraic structure of their answers. They were provided with detailed online feedback, and we therefore specifically examine the effect of formative feedback on the usage of educational technology. Our results suggest that students find on-line feedback more enjoyable and useful than traditional feedback. 'Attitude' and 'Enjoyment' are the two most important factors influencing their usage intention. Our results also show that, even for this digital generation, confidence in using computers and the availability of support for using information technology are important factors in making effective use of on-line assessments.

physics.ed-ph↗

arXiv2013

On the nature of the phase transitions in two-dimensional type II superconductors

We have simulated the thermodynamics of vortices in a thin film of a type-II superconductor. We make the lowest Landau level approximation, and use quasi-periodic boundary conditions. Our work is consistent with the results of previous simulations where evidence was found for an apparent first order transition between the vortex liquid state and the vortex crystal state. We show, however, that these results are just an artifact of studying systems which are too small. There are substantial difficulties in simulating larger systems using traditional approaches. By means of the optimal energy diffusion algorithm we have been able to study systems containing up to about one thousand vortices, and for these larger systems the evidence for a first order transition disappears. By studying both crystalline and hexatic order, we show that the KTHNY scenario seems to apply, where melting from the crystal is first to the hexatic liquid state and next to the normal vortex liquid, in both cases via a continuous transition.

cond-mat.supr-con↗

arXiv2013

Functional renormalisation group for few-nucleon systems: SU(4) symmetry and its breaking

We apply the functional renormalisation group to few-nucleon systems. Our starting point is a local effective action that includes three- and four-nucleon interactions, expressed in terms of nucleon and two-nucleon boson fields. The evolution of the coupling constants in this action is described by a renormalisation group flow. We derive these flow equations both in the limit of exact Wigner SU(4) symmetry and in the realistic case of broken symmetry. In the symmetric limit we find that the renormalisation flow equations decouple, and can be combined into two sets, one of which matches the known results for bosons, and the other result matches the one for fermions with spin degrees only. The equations show universal features in the unitary limit, which is obtained when the two-body scattering length tends to infinity. We calculate the spin-quartet neutron-deuteron scattering length and the deuteron-deuteron scattering lengths in the spin-singlet and quintet channels.

nucl-th↗

arXiv2012

Gender differences in conceptual understanding of Newtonian mechanics: a UK cross-institution comparison

We present results of a combined study from three UK universities where we investigate the existence and persistence of a performance gender gap in conceptual understanding of Newtonian mechanics. Using the Force Concept Inventory, we find that students at all three universities exhibit a statistically significant gender gap, with males outperforming females. This gap is narrowed but not eliminated after instruction, using a variety of instructional approaches. Furthermore, we find that before instruction the quartile with the lowest performance on the diagnostic instrument comprises a disproportionately high fraction (~50%) of the total female cohort. The majority of these students remain in the lowest-performing quartile post-instruction. Analysis of responses to individual items shows that male students outperform female students on practically all items on the instrument. Comparing the performance of the same group of students on end-of-course examinations, we find no statistically significant gender gaps.

physics.ed-ph↗

arXiv2010

Convergence of a renormalization group approach to dimer-dimer scattering

We study the convergence of a functional renormalisation group technique by looking at the ratio between the fermion-fermion scattering length and the dimer-dimer scattering length for a system of nonrelativistic fermions. We find that in a systematic expansion in powers of the fields there is a rapid convergence of the result that agrees with know exact results.

cond-mat.quant-gas↗

arXiv2010

The extended coupled cluster method and the pairing problem

We study the application of various forms of the coupled cluster method to systems with paired fermions. The novel element of the analysis is the study of the breaking and eventual restoration of particle number in the CCM variants. We specifically include Arponen's extended coupled cluster method, which describes the normal Hartree-Fock-Bogoliubov mean field at lowest level of truncation. We show that all methods converge to the exact results as we increase the order of truncation, but that the breaking of particle number at an intermediate level means that this convergence occurs in a surprising way. We argue that the most straightforward form of the method seems to be the most stable approach to implement for realistic (large number of particles) pairing problems

quant-ph↗

arXiv2010

Renormalization group, dimer-dimer scattering, and three-body forces

We study the ratio between the fermion-fermion scattering length and the dimer-dimer scattering length for systems of nonrelativistic fermions, using the same functional renormalisation technique as previously applied to fermionic matter. We find a strong dependence on the cutoff function used in the renormalisation flow for a two-body truncation of the action. Adding a simple three-body term substantially reduces this dependence.

cond-mat.quant-gas↗

arXiv2005

Shape mixing in the A 70 region

We investigate the mixing of different shapes in the A~70 region using the adiabatic self-consistent collective method in a rotating nuclei. The calculation is done for 68^Se and 72-78^Kr which are known to show oblate-prolate shape coexistence at low angular momentum. A pairing-plus-quadrupole Hamiltonian, which is known to reproduce reproduce the structure nuclei in this mass region, is used. We calculate the collective path between the oblate and prolate minima and discuss how the collective behaviour of the system changes with increasing angular momentum.

nucl-th↗

arXiv2005

A simple model of the charge transfer in DNA-like substances

We present a very simple model for the study of charge transport in a molecule patterned on B-DNA. In this model we use a discrete non-linear Schrödinger equation to describe electrons propagating along the sugar-phosphate backbone of the DNA molecule. We find that in this model, for a given nonlinearity, the transport is controlled by $J$, a parameter which relates to the electronic coupling between different molecules on the backbone. For smaller values of $J$ we have localised states while at higher values of $J$ the soliton field is spread out and through its interaction with the lattice it has stronger effects on the distortion of the lattice.

cond-mat.soft↗

arXiv2005

Splitting The Gluon?

In the strongly correlated environment of high-temperature cuprate superconductors, the spin and charge degrees of freedom of an electron seem to separate from each other. A similar phenomenon may be present in the strong coupling phase of Yang-Mills theories, where a separation between the color charge and the spin of a gluon could play a role in a mass gap formation. Here we study the phase structure of a decomposed SU(2) Yang-Mills theory in a mean field approximation, by inspecting quantum fluctuations in the condensate which is formed by the color charge component of the gluon field. Our results suggest that the decomposed theory has an involved phase structure. In particular, there appears to be a phase which is quite reminiscent of the superconducting phase in cuprates. We also find evidence that this phase is separated from the asymptotically free theory by an intermediate pseudogap phase.

hep-ph↗

arXiv2005

Removal of Spurious Admixture in a Self-consistent Theory of Adiabatic Large Amplitude Collective Motion

In this article we analyse, for a simple model, the properties of a practical implementation of a fully self-consistent theory of adiabatic large-amplitude collective motion using the local harmonic approach. We show how we can deal with contaminations arising from spurious modes, caused by standard simplifying approximations. This is done both at zero and finite angular momentum. We analyse in detail the nature of the collective coordinate in regions where they cross spurious modes and mixing is largest.

nucl-th↗