11,798 research outputs found
Emergence of massless Dirac fermions in graphene's Hofstadter butterfly at switches of the quantum Hall phase connectivity
The fractal spectrum of magnetic minibands (Hofstadter butterfly), induced by
the moir\'e super- lattice of graphene on an hexagonal crystal substrate, is
known to exhibit gapped Dirac cones. We show that the gap can be closed by
slightly misaligning the substrate, producing a hierarchy of conical
singularities (Dirac points) in the band structure at rational values Phi =
(p/q)(h/e) of the magnetic flux per supercell. Each Dirac point signals a
switch of the topological quantum number in the connected component of the
quantum Hall phase diagram. Model calculations reveal the scale invariant
conductivity sigma = 2qe^2 / pi h and Klein tunneling associated with massless
Dirac fermions at these connectivity switches.Comment: 4 pages, 6 figures + appendix (3 pages, 1 figure
Inertial and dimensional effects on the instability of a thin film
We consider here the effects of inertia on the instability of a flat liquid
film under the effects of capillary and intermolecular forces (van der Waals
interaction). Firstly, we perform the linear stability analysis within the long
wave approximation, which shows that the inclusion of inertia does not produce
new regions of instability other than the one previously known from the usual
lubrication case. The wavelength, , corresponding to he maximum
growth, , and the critical (marginal) wavelength do not change at
all. The most affected feature of the instability under an increase of the
Laplace number is the noticeable decrease of the growth rates of the unstable
modes. In order to put in evidence the effects of the bidimensional aspects of
the flow (neglected in the long wave approximation), we also calculate the
dispersion relation of the instability from the linearized version of the
complete Navier-Stokes (N-S) equation. Unlike the long wave approximation, the
bidimensional model shows that can vary significantly with inertia
when the aspect ratio of the film is not sufficiently small. We also perform
numerical simulations of the nonlinear N-S equations and analyze to which
extent the linear predictions can be applied depending on both the amount of
inertia involved and the aspect ratio of the film
Andreev reflection from a topological superconductor with chiral symmetry
It was pointed out by Tewari and Sau that chiral symmetry (H -> -H if e
h) of the Hamiltonian of electron-hole (e-h) excitations in an N-mode
superconducting wire is associated with a topological quantum number
Q\in\mathbb{Z} (symmetry class BDI). Here we show that Q=Tr(r_{he}) equals the
trace of the matrix of Andreev reflection amplitudes, providing a link with the
electrical conductance G. We derive G=(2e^2/h)|Q| for |Q|=N,N-1, and more
generally provide a Q-dependent upper and lower bound on G. We calculate the
probability distribution P(G) for chaotic scattering, in the circular ensemble
of random-matrix theory, to obtain the Q-dependence of weak localization and
mesoscopic conductance fluctuations. We investigate the effects of chiral
symmetry breaking by spin-orbit coupling of the transverse momentum (causing a
class BDI-to-D crossover), in a model of a disordered semiconductor nanowire
with induced superconductivity. For wire widths less than the spin-orbit
coupling length, the conductance as a function of chemical potential can show a
sequence of 2e^2/h steps - insensitive to disorder.Comment: 10 pages, 5 figures. Corrected typo (missing square root) in
equations A13 and A1
Extended topological group structure due to average reflection symmetry
We extend the single-particle topological classification of insulators and
superconductors to include systems in which disorder preserves average
reflection symmetry. We show that the topological group structure of bulk
Hamiltonians and topological defects is exponentially extended when this
additional condition is met, and examine some of its physical consequences.
Those include localization-delocalization transitions between topological
phases with the same boundary conductance, as well as gapless topological
defects stabilized by average reflection symmetry.Comment: 8 pages, 5 figures, 1 table; improved section 4 "Extended topological
classification" incl. example of stacked QSH layer
From car to bike. Marketing and dialogue as a driver of change
The Paris Climate Agreement has sent a key message to the international community regarding the need to increase efforts to move towards a low-carbon economy and help slow climate change, while underpinning global long-term economic growth and sustainable development. COP 21 recognizes the social, economic and environmental value of voluntary mitigation actions and their co-benefits for adaptation, health and sustainable development. In this framework, the PTP Cycle project, running from 2013 to 2016 and funded by the European Commission through the Intelligent Energy Europe program, introduces a non-market approach through voluntary participation in the adoption of sustainable transport modes such as cycling, based on marketing to potential customers through Personalized Travel Plans. The medium-sized city of Burgos (Spain) and the cities of Ljubljana, Riga, Antwerp and London
(boroughs of Haringey and Greenwich) developed a new policy instrument (Personalized Travel Plans) in order to increase bike patronage. Beyond potential savings of CO2, the results show that PTP as a form of Active Mobility Consultancy is a suitable instrument to influence modal shift to public transport, walking and cycling, and to address the challenges of climate change, while fostering sustainable transportation by changing mobility behaviour. These results, matching with the state-of-the-art of studies and pilot applications in other countries, allows deriving differentiated results for medium-size and large urban areas
Bimodal conductance distribution of Kitaev edge modes in topological superconductors
A two-dimensional superconductor with spin-triplet p-wave pairing supports
chiral or helical Majorana edge modes with a quantized (length -independent)
thermal conductance. Sufficiently strong anisotropy removes both chirality and
helicity, doubling the conductance in the clean system and imposing a
super-Ohmic decay in the presence of disorder. We explain the
absence of localization in the framework of the Kitaev Hamiltonian, contrasting
the edge modes of the two-dimensional system with the one-dimensional Kitaev
chain. While the disordered Kitaev chain has a log-normal conductance
distribution peaked at an exponentially small value, the Kitaev edge has a
bimodal distribution with a second peak near the conductance quantum. Shot
noise provides an alternative, purely electrical method of detection of these
charge-neutral edge modes.Comment: 11 pages, 13 figure
Pilot study of vegetation in the Alchichica-Perote region by remote sensing
A study of the application of satellite images to the identification of vegetation in a small area corresponding to the arid zone of Veracruz and part of Puebla is presented. This study is accomplished by means of images from the LANDSAT satellite obtained on January 19 and May 23, 1973. The interpretation of the different maps is made on the basis of information from the data bank of the Flora de Veracruz program, and various surveys made by land and air
- …