184,415 research outputs found
Genesis of the Floquet Hofstadter butterfly
We investigate theoretically the spectrum of a graphene-like sample
(honeycomb lattice) subjected to a perpendicular magnetic field and irradiated
by circularly polarized light. This system is studied using the Floquet
formalism, and the resulting Hofstadter spectrum is analyzed for different
regimes of the driving frequency. For lower frequencies, resonances of various
copies of the spectrum lead to intricate formations of topological gaps. In the
Landau-level regime, new wing-like gaps emerge upon reducing the driving
frequency, thus revealing the possibility of dynamically tuning the formation
of the Hofstadter butterfly. In this regime, an effective model may be
analytically derived, which allows us to retrace the energy levels that exhibit
avoided crossings and ultimately lead to gap structures with a wing-like shape.
At high frequencies, we find that gaps open for various fluxes at , and
upon increasing the amplitude of the driving, gaps also close and reopen at
other energies. The topological invariants of these gaps are calculated and the
resulting spectrum is elucidated. We suggest opportunities for experimental
realization and discuss similarities with Landau-level structures in non-driven
systems.Comment: 8 pages, 4 figure
Tuning edge state localization in graphene nanoribbons by in-plane bending
The electronic properties of graphene are influenced by both geometric
confinement and strain. We study the electronic structure of in-plane bent
graphene nanoribbons, systems where confinement and strain are combined. To
understand its electronic properties, we develop a tight-binding model that has
a small computational cost and is based on exponentially decaying hopping and
overlap parameters. Using this model, we show that the edge states in zigzag
graphene nanoribbons are sensitive to bending and develop an effective
dispersion that can be described by a one-dimensional atomic chain model.
Because the velocity of the electrons at the edge is proportional to the slope
of the dispersion, the edge states become gradually delocalized upon increasing
the strength of bending.Comment: 11 pages, 8 figure
Can non-private channels transmit quantum information?
We study the power of quantum channels with little or no capacity for private
communication. Because privacy is a necessary condition for quantum
communication, one might expect that such channels would be of little use for
transmitting quantum states. Nevertheless, we find strong evidence that there
are pairs of such channels that, when used together, can transmit far more
quantum information than the sum of their individual private capacities.
Because quantum transmissions are necessarily private, this would imply a large
violation of additivity for the private capacity. Specifically, we present
channels which display either (1) A large joint quantum capacity but very small
individual private capacities or (2) a severe violation of additivity for the
Holevo information.Comment: We both think so. 4 pages and 3 figures explain wh
Effects of J-gate potential and interfaces on donor exchange coupling in the Kane quantum computer architecture
We calculate the electron exchange coupling for a phosphorus donor pair in
silicon perturbed by a J-gate potential and the boundary effects of the silicon
host geometry. In addition to the electron-electron exchange interaction we
also calculate the contact hyperfine interaction between the donor nucleus and
electron as a function of the varying experimental conditions. Donor
separation, depth of the P nuclei below the silicon oxide layer and J-gate
voltage become decisive factors in determining the strength of both the
exchange coupling and the hyperfine interaction - both crucial components for
qubit operations in the Kane quantum computer. These calculations were
performed using an anisotropic effective-mass Hamiltonian approach. The
behaviour of the donor exchange coupling as a function of the device parameters
varied provides relevant information for the experimental design of these
devices.Comment: 15 pages, 15 figures. Accepted for Journal of Physics: Condensed
Matte
X-ray emission from the field of the hyperluminous IRAS galaxy IRASF15307+3252
We report on a 20-ks observation of the z = 0.93 hyperluminous galaxy IRAS F15307+3252 with the ROSAT HRI. No X-ray source is detected at the position of F15307+3252 at an upper limit of ∼4 × 10⁴³ erg s⁻¹. This is less than 2 × 10⁻⁴ of the bolometric luminosity of the object, and indicates either that the nucleus emits an unusually small fraction of its total power in X-rays, or that little of the nuclear X-ray flux is scattered into our line of sight by electrons. The lack of an X-ray detection around F15307+3252 also rules out it being at the centre of a cluster, such as is observed for IRAS P09104+4109. A weak, possibly extended, X-ray source is detected 13 arcsec south of the galaxy, spatially coincident with a clump of faint objects visible in a Keck K_s-band image of the field. This may be the core of a cluster near the line of sight to F15307+3252
An accurate formula for the period of a simple pendulum oscillating beyond the small-angle regime
A simple approximation formula is derived here for the dependence of the
period of a simple pendulum on amplitude that only requires a pocket calculator
and furnishes an error of less than 0.25% with respect to the exact period. It
is shown that this formula describes the increase of the pendulum period with
amplitude better than other simple formulas found in literature. A good
agreement with experimental data for a low air-resistance pendulum is also
verified and it suggests, together with the current availability/precision of
timers and detectors, that the proposed formula is useful for extending the
pendulum experiment beyond the usual small-angle oscillations.Comment: 15 pages and 4 figures. to appear in American Journal of Physic
The Formation and Fragmentation of Disks around Primordial Protostars
The very first stars to form in the Universe heralded an end to the cosmic
dark ages and introduced new physical processes that shaped early cosmic
evolution. Until now, it was thought that these stars lived short, solitary
lives, with only one extremely massive star, or possibly a very wide binary
system, forming in each dark matter minihalo. Here we describe numerical
simulations that show that these stars were, to the contrary, often members of
tight multiple systems. Our results show that the disks that formed around the
first young stars were unstable to gravitational fragmentation, possibly
producing small binary and higher-order systems that had separations as small
as the distance between the Earth and the Sun.Comment: This manuscript has been accepted for publication in Science. This
version has not undergone final editing. Please refer to the complete version
of record at http://www.sciencemag.org
RR Lyrae in XSTPS: The halo density profile in the North Galactic Cap
We present a catalog of RR Lyrae stars (RRLs) observed by the Xuyi Schmidt
Telescope Photometric Survey (XDSS). The area we consider is located in the
North Galactic Cap, covering 376.75 sq deg at RA 150 deg and Dec
27 deg down to a magnitude limit of i 19. Using the
variability information afforded by the multi-epoch nature of our XDSS data,
combined with colors from the Sloan Digital Sky Survey, we are able to identify
candidate RRLs. We find 318 candidates, derive distances to them and estimate
the detection efficiency. The majority of our candidates have more than 12
observations and for these we are able to calculate periods. These also allows
us to estimate our contamination level, which we predict is between 30% to 40%.
Finally we use the sample to probe the halo density profile in the 9-49 kpc
range and find that it can be well fitted by a double power law. We find good
agreement between this model and the models derived for the South Galactic Cap
using the Watkins et al. (2009) and Sesar et al. (2010) RRL data-sets, after
accounting for possible contamination in our data-set from Sagittarius stream
members. We consider non-spherical double power law models of the halo density
profile and again find agreement with literature data-sets, although we have
limited power to constrain the flattening due to our small survey area. Much
tighter constraints will be placed by current and future wide-area surveys,
most notably ESA's astrometric Gaia mission. Our analysis demonstrates that
surveys with a limited number of epochs can effectively be mined for RRLs. Our
complete sample is provided as accompanying online material.Comment: 14 pages, ApJ (in press
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