203 research outputs found
Nanoengineering Carbon Allotropes from Graphene
Monolithic structures can be built into graphene by the addition and
subsequent re-arrangement of carbon atoms. To this end, ad-dimers of carbon are
a particularly attractive building block because a number of emerging
technologies offer the promise of precisely placing them on carbon surfaces. In
concert with the more common Stone-Wales defect, repeating patterns can be
introduced to create as yet unrealized materials. The idea of building such
allotropes out of defects is new, and we demonstrate the technique by
constructing two-dimensional carbon allotropes known as haeckelite. We then
extend the idea to create a new class of membranic carbon allotropes that we
call \emph{dimerite}, composed exclusively of ad-dimer defects.Comment: 5 pages, 5 figure
Quantum Pumping in the Magnetic Field: Role of Discrete Symmetries
We consider an effect of the discrete spatial symmetries and magnetic field
on the adiabatic charge pumping in mesoscopic systems. In general case, there
is no symmetry of the pumped charge with respect to the inversion of magnetic
field Q(B) \neq Q(-B). We find that the reflection symmetries give rise to
relations Q(B)=Q(-B) or Q(B)=-Q(-B) depending on the orientation of the
reflection axis. In presence of the center of inversion, Q(B) = 0. Additional
symmetries may arise in the case of bilinear pumping.Comment: 4 page
Superfluid Flow Past an Array of Scatterers
We consider a model of nonlinear superfluid flow past a periodic array of
point-like scatterers in one dimension. An application of this model is the
determination of the critical current of a Josephson array in a regime
appropriate to a Ginzburg-Landau formulation. Here, the array consists of short
normal-metal regions, in the presence of a Hartree electron-electron
interaction, and embedded within a one-dimensional superconducting wire near
its critical temperature, . We predict the critical current to depend
linearly as , while the coefficient depends sensitively on the
sizes of the superconducting and normal-metal regions and the strength and sign
of the Hartree interaction. In the case of an attractive interaction, we find a
further feature: the critical current vanishes linearly at some temperature
less than , as well as at itself. We rule out a simple
explanation for the zero value of the critical current, at this temperature
, in terms of order parameter fluctuations at low frequencies.Comment: 23 pages, REVTEX, six eps-figures included; submitted to PR
The Basics of Water Waves Theory for Analogue Gravity
This chapter gives an introduction to the connection between the physics of
water waves and analogue gravity. Only a basic knowledge of fluid mechanics is
assumed as a prerequisite.Comment: 36 pages. Lecture Notes for the IX SIGRAV School on "Analogue
Gravity", Como (Italy), May 201
Mesoscopic scattering in the half-plane: squeezing conductance through a small hole
We model the 2-probe conductance of a quantum point contact (QPC), in linear
response. If the QPC is highly non-adiabatic or near to scatterers in the open
reservoir regions, then the usual distinction between leads and reservoirs
breaks down and a technique based on scattering theory in the full
two-dimensional half-plane is more appropriate. Therefore we relate conductance
to the transmission cross section for incident plane waves. This is equivalent
to the usual Landauer formula using a radial partial-wave basis. We derive the
result that an arbitrarily small (tunneling) QPC can reach a p-wave channel
conductance of 2e^2/h when coupled to a suitable reflector. If two or more
resonances coincide the total conductance can even exceed this. This relates to
recent mesoscopic experiments in open geometries. We also discuss reciprocity
of conductance, and the possibility of its breakdown in a proposed QPC for atom
waves.Comment: 8 pages, 3 figures, REVTeX. Revised version (shortened), accepted for
publication in PR
Edge magnetoplasmons in periodically modulated structures
We present a microscopic treatment of edge magnetoplasmons (EMP's) within the
random-phase approximation for strong magnetic fields, low temperatures, and
filling factor , when a weak short-period superlattice potential is
imposed along the Hall bar. The modulation potential modifies both the spatial
structure and the dispersion relation of the fundamental EMP and leads to the
appearance of a novel gapless mode of the fundamental EMP. For sufficiently
weak modulation strengths the phase velocity of this novel mode is almost the
same as the group velocity of the edge states but it should be quite smaller
for stronger modulation. We discuss in detail the spatial structure of the
charge density of the renormalized and the novel fundamental EMP's.Comment: 8 pages, 4 figure
Interstellar MHD Turbulence and Star Formation
This chapter reviews the nature of turbulence in the Galactic interstellar
medium (ISM) and its connections to the star formation (SF) process. The ISM is
turbulent, magnetized, self-gravitating, and is subject to heating and cooling
processes that control its thermodynamic behavior. The turbulence in the warm
and hot ionized components of the ISM appears to be trans- or subsonic, and
thus to behave nearly incompressibly. However, the neutral warm and cold
components are highly compressible, as a consequence of both thermal
instability in the atomic gas and of moderately-to-strongly supersonic motions
in the roughly isothermal cold atomic and molecular components. Within this
context, we discuss: i) the production and statistical distribution of
turbulent density fluctuations in both isothermal and polytropic media; ii) the
nature of the clumps produced by thermal instability, noting that, contrary to
classical ideas, they in general accrete mass from their environment; iii) the
density-magnetic field correlation (or lack thereof) in turbulent density
fluctuations, as a consequence of the superposition of the different wave modes
in the turbulent flow; iv) the evolution of the mass-to-magnetic flux ratio
(MFR) in density fluctuations as they are built up by dynamic compressions; v)
the formation of cold, dense clouds aided by thermal instability; vi) the
expectation that star-forming molecular clouds are likely to be undergoing
global gravitational contraction, rather than being near equilibrium, and vii)
the regulation of the star formation rate (SFR) in such gravitationally
contracting clouds by stellar feedback which, rather than keeping the clouds
from collapsing, evaporates and diperses them while they collapse.Comment: 43 pages. Invited chapter for the book "Magnetic Fields in Diffuse
Media", edited by Elisabete de Gouveia dal Pino and Alex Lazarian. Revised as
per referee's recommendation
Skymrion lattice melting in the quantum Hall system
The melting and magnetic disordering of the skyrmion lattice in the quantum
Hall system at filling factor are studied. A
Berezinskii-Kosterlitz-Thouless renormalization group theory is employed to
describe the coupled magnetic and translational degrees of freedom. The
non-trivial magnetic properties of the skyrmion system stem from the in-plane
components of the non-collinear magnetization in the vicinity of skyrmions,
which are described by an antiferromagnetic XY model. In a Coulomb gas
formulation the `particles' are the topological defects of the XY model
(vortices) and of the lattice (dislocations and disclinations). The latter
frustrate the antiferromagnetic order and acquire fractional vorticity in order
to minimize their energy. We find a number of melting/disordering scenarios for
various lattice types. While these results do not depend on a particular model,
we also consider a simple classical model for the skyrmion system. It results
in a rich T=0 phase diagram. We propose that the triangular and square skyrmion
lattices are generically separated by a centered rectangular phase in the
quantum Hall system.Comment: 15 pages with 5 figures. Minor revisions. Important reference to M.
Rao, S. Sengupta, and R. Shankar, Phys. Rev. Lett. 79, 3998 (1997) adde
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