5,907 research outputs found
Remarks on Existence of Proper Action for Reducible Gauge Theories
In the field-antifield formalism, we review existence and uniqueness proofs
for the proper action in the reducible case. We give two new existence proofs
based on two resolution degrees called "reduced antifield number" and "shifted
antifield number", respectively. In particular, we show that for every choice
of gauge generators and their higher stage counterparts, there exists a proper
action that implements them at the quadratic order in the auxiliary variables.Comment: 37 pages, LaTeX. v2,v3: Minor corrections. v4: Added reference. To
appear in IJMP
Quantized Transport in Two-Dimensional Spin-Ordered Structures
We study in detail the transport properties of a model of conducting
electrons in the presence of double-exchange between localized spins arranged
on a 2D Kagome lattice, as introduced by Ohgushi, Murakami, and Nagaosa (2000).
The relationship between the canting angle of the spin texture and the
Berry phase field flux per triangular plaquette is derived explicitly
and we emphasize the similarities between this model and Haldane's honeycomb
lattice version of the quantum Hall effect (Haldane, 1988). The quantization of
the transverse (Hall) conductivity is derived explicitly from the
Kubo formula and a direct calculation of the longitudinal conductivity
shows the existence of a metal-insulator transition as a function
of the canting angle (or flux density ). This transition might
be linked to that observable in the manganite compounds or in the pyrochlore
ones, as the spin ordering changes from ferromagnetic to canted.Comment: 17 pages, 12 figure
Fast and Accurate Modeling of Molecular Atomization Energies with Machine Learning
We introduce a machine learning model to predict atomization energies of a
diverse set of organic molecules, based on nuclear charges and atomic positions
only. The problem of solving the molecular Schr\"odinger equation is mapped
onto a non-linear statistical regression problem of reduced complexity.
Regression models are trained on and compared to atomization energies computed
with hybrid density-functional theory. Cross-validation over more than seven
thousand small organic molecules yields a mean absolute error of ~10 kcal/mol.
Applicability is demonstrated for the prediction of molecular atomization
potential energy curves
Spin currents in rough graphene nanoribbons: Universal fluctuations and spin injection
We investigate spin conductance in zigzag graphene nanoribbons and propose a
spin injection mechanism based only on graphitic nanostructures. We find that
nanoribbons with atomically straight, symmetric edges show zero spin
conductance, but nonzero spin Hall conductance. Only nanoribbons with
asymmetrically shaped edges give rise to a finite spin conductance and can be
used for spin injection into graphene. Furthermore, nanoribbons with rough
edges exhibit mesoscopic spin conductance fluctuations with a universal value
of .Comment: 4 pages, 5 figures, PdfLaTeX, accepted for publication in Physical
Review Letter
Edge effects in graphene nanostructures: II. Semiclassical theory of spectral fluctuations and quantum transport
We investigate the effect of different edge types on the statistical
properties of both the energy spectrum of closed graphene billiards and the
conductance of open graphene cavities in the semiclassical limit. To this end,
we use the semiclassical Green's function for ballistic graphene flakes that we
have derived in Reference 1. First we study the spectral two point correlation
function, or more precisely its Fourier transform the spectral form factor,
starting from the graphene version of Gutzwiller's trace formula for the
oscillating part of the density of states. We calculate the two leading order
contributions to the spectral form factor, paying particular attention to the
influence of the edge characteristics of the system. Then we consider transport
properties of open graphene cavities. We derive generic analytical expressions
for the classical conductance, the weak localization correction, the size of
the universal conductance fluctuations and the shot noise power of a ballistic
graphene cavity. Again we focus on the effects of the edge structure. For both,
the conductance and the spectral form factor, we find that edge induced
pseudospin interference affects the results significantly. In particular
intervalley coupling mediated through scattering from armchair edges is the key
mechanism that governs the coherent quantum interference effects in ballistic
graphene cavities
- …