7,324 research outputs found
Mechanically-Induced Transport Switching Effect in Graphene-based Nanojunctions
We report a theoretical study suggesting a novel type of electronic switching
effect, driven by the geometrical reconstruction of nanoscale graphene-based
junctions. We considered junction struc- tures which have alternative
metastable configurations transformed by rotations of local carbon dimers. The
use of external mechanical strain allows a control of the energy barrier
heights of the potential profiles and also changes the reaction character from
endothermic to exothermic or vice-versa. The reshaping of the atomic details of
the junction encode binary electronic ON or OFF states, with ON/OFF
transmission ratio that can reach up to 10^4-10^5. Our results suggest the
possibility to design modern logical switching devices or mechanophore sensors,
monitored by mechanical strain and structural rearrangements.Comment: 10 pages, 4 figure
Z-graded weak modules and regularity
It is proved that if any Z-graded weak module for vertex operator algebra V
is completely reducible, then V is rational and C_2-cofinite. That is, V is
regular. This gives a natural characterization of regular vertex operator
algebras.Comment: 9 page
-cofiniteness of 2-cyclic permutation orbifold models
In this article, we consider permutation orbifold models of -cofinite
vertex operator algebras of CFT type. We show the -cofiniteness of the
2-cyclic permutation orbifold model for an arbitrary
-cofinite simple vertex operator algebra of CFT type. We also give a
proof of the -cofiniteness of a -orbifold model of the
lattice vertex operator algebra associated with a rank one positive
definite even lattice by using our result and the -cofiniteness of
.Comment: 25 pages, no figure, some typo are correcte
Cheon's anholonomies in Floquet operators
Anholonomies in the parametric dependences of the eigenvalues and the
eigenvectors of Floquet operators that describe unit time evolutions of
periodically driven systems, e.g., kicked rotors, are studied. First, an
example of the anholonomies induced by a periodically pulsed rank-1
perturbation is given. As a function of the strength of the perturbation, the
perturbed Floquet operator of the quantum map and its spectrum are shown to
have a period. However, we show examples where each eigenvalue does not obey
the periodicity of the perturbed Floquet operator and exhibits an anholonomy.
Furthermore, this induces another anholonomy in the eigenspaces, i.e., the
directions of the eigenvectors, of the Floquet operator. These two anholonomies
are previously observed in a family of Hamiltonians [T. Cheon, Phys. Lett. A
248, 285 (1998)] and are different from the phase anholonomy known as geometric
phases. Second, the stability of Cheon's anholonomies in periodically driven
systems is established by a geometrical analysis of the family of Floquet
operators. Accordingly, Cheon's anholonomies are expected to be abundant in
systems whose time evolutions are described by Floquet operators. As an
application, a design principle for quantum state manipulations along adiabatic
passages is explained
The Mass of the Milky Way: Limits from a Newly Assembled Set of Halo Objects
We set new limits on the mass of the Milky Way, making use of the latest
kinematic information for Galactic satellites and halo objects. In order to
bind these sample objects to the Galaxy, their rest-frame velocities must be
lower than their escape velocities at their estimated distances. This
constraint enables us to show that the mass estimate of the Galaxy is largely
affected by several high-velocity objects (Leo I, Pal 3, Draco, and a few FHB
stars), not by a single object alone (such as Leo I), as has often been the
case in past analyses. We also find that a gravitational potential that gives
rise to a declining rotation curve is insufficient to bind many of our sample
objects to the Galaxy; a possible lower limit on the mass of the Galaxy is
about 2.2 x 10^12 Msolar. To be more quantitative, we adopt a Bayesian
likelihood approach to reproduce the observed distribution of the current
positions and motions of the sample, in a prescribed Galactic potential that
yields a flat rotation curve. This method enables a search for the most likely
total mass of the Galaxy, without undue influence in the final result arising
from the presence or absence of Leo I, provided that both radial velocities and
proper motions are used. The most likely total mass derived from this method is
2.5^+0.5_-1.0 x 10^12 Msolar (including Leo I), and 1.8^+0.4_-0.7 x 10^12
Msolar (excluding Leo I).Comment: 14 pages, including 9 figures and 3 tables, accepted for publication
in Astronomy and Astrophysic
Interplay Between Chaotic and Regular Motion in a Time-Dependent Barred Galaxy Model
We study the distinction and quantification of chaotic and regular motion in
a time-dependent Hamiltonian barred galaxy model. Recently, a strong
correlation was found between the strength of the bar and the presence of
chaotic motion in this system, as models with relatively strong bars were shown
to exhibit stronger chaotic behavior compared to those having a weaker bar
component. Here, we attempt to further explore this connection by studying the
interplay between chaotic and regular behavior of star orbits when the
parameters of the model evolve in time. This happens for example when one
introduces linear time dependence in the mass parameters of the model to mimic,
in some general sense, the effect of self-consistent interactions of the actual
N-body problem. We thus observe, in this simple time-dependent model also, that
the increase of the bar's mass leads to an increase of the system's chaoticity.
We propose a new way of using the Generalized Alignment Index (GALI) method as
a reliable criterion to estimate the relative fraction of chaotic vs. regular
orbits in such time-dependent potentials, which proves to be much more
efficient than the computation of Lyapunov exponents. In particular, GALI is
able to capture subtle changes in the nature of an orbit (or ensemble of
orbits) even for relatively small time intervals, which makes it ideal for
detecting dynamical transitions in time-dependent systems.Comment: 21 pages, 9 figures (minor typos fixed) to appear in J. Phys. A:
Math. Theo
Lifshitz transition and van Hove singularity in a Topological Dirac Semimetal
A topological Dirac semimetal is a novel state of quantum matter which has
recently attracted much attention as an apparent 3D version of graphene. In
this paper, we report critically important results on the electronic structure
of the 3D Dirac semimetal Na3Bi at a surface that reveals its nontrivial
groundstate. Our studies, for the first time, reveal that the two 3D Dirac
cones go through a topological change in the constant energy contour as a
function of the binding energy, featuring a Lifshitz point, which is missing in
a strict 3D analog of graphene (in other words Na3Bi is not a true 3D analog of
graphene). Our results identify the first example of a band saddle point
singularity in 3D Dirac materials. This is in contrast to its 2D analogs such
as graphene and the helical Dirac surface states of a topological insulator.
The observation of multiple Dirac nodes in Na3Bi connecting via a Lifshitz
point along its crystalline rotational axis away from the Kramers point serves
as a decisive signature for the symmetry-protected nature of the Dirac
semimetal's topological groundstate.Comment: 5 pages, 4 Figures, Related papers on topological Fermi arcs and Weyl
Semimetals (WSMs) are at
http://physics.princeton.edu/zahidhasangroup/index.htm
Global monopole solutions in Horava gravity
In Horava's theory of gravity coupled to a global monopole source, we seek
for static, spherically symmetric spacetime solutions for general values of
. We obtain the explicit solutions with deficit solid angles, in the
IR modified Horava gravity model, at the IR fixed point and at the
conformal point . For the other values of we also
find special solutions to the inhomogenous equation of the gravity model with
detailed balance, and we discuss an possibility of astrophysical applications
of the solution that has a deficit angle for a finite range.Comment: 7 pages, added reference
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