729 research outputs found
Evanescent Modes and Step-like Acoustic Black Holes
We consider a model of an acoustic black hole formed by a quasi-one
dimensional Bose-Einstein condensate with a step-like horizon. This system is
analyzed by solving the corresponding Bogoliubov-de Gennes equation with an
appropriate matching condition at the jump. When the step is between a subsonic
and supersonic flow, a sonic horizon develops and in addition to the scattering
coefficients we compute the distribution of the accompanying analogue Hawking
radiation. Additionally, in response to the abrupt variation in flow and
non-linear Bogoliubov dispersion relation, evanescent solutions of the
Bogoliubov-de Gennes equation also appear and decay out from the horizon. We
bound this decay length and show that these modes produce a modulation of
observables outside the event horizon by their interference with outgoing
Hawking flux. We go further and find specific superpositions of ingoing
eigenmodes which exhibit coherent cancellation of the Hawking flux outside the
horizon but nevertheless have evanescent support outside the black hole. We
conclude by speculating that when quasiparticle interactions are included,
evanescent modes may yield a leakage of information across the event horizon
via interactions between the real outgoing Hawking flux and the virtual
evanescent modes, and that we may expect this as a generic feature of models
which break Lorentz invariance at the UV (Planck) scale.Comment: 28 pages, 7 figures (including Appendices) Expanded upon and
reformatte
Absence of Weak Localization on Negative Curvature Surfaces
The interplay between disorder and quantum interference leads to a wide
variety of physical phenomena including celebrated Anderson localization -- the
complete absence of diffusive transport due to quantum interference between
different particle trajectories. In two dimensions, any amount of disorder is
thought to induce localization of all states at long enough length scales,
though this may be prevented if bands are topological or have strong spin-orbit
coupling. In this note, we present a simple argument providing another
mechanism for disrupting localization: by tuning the underlying curvature of
the manifold on which diffusion takes place. We show that negative curvature
manifolds contain a natural infrared cut off for the probability of self
returning paths. We provide explicit calculations of the Cooperon -- directly
related to the weak-localization corrections to the conductivity -- in
hyperbolic space. It is shown that constant negative curvature leads to a rapid
growth in the number of available trajectories a particle can coherently
traverse in a given time, reducing the importance of interference effects and
restoring classical diffusive behavior even in the absence of inelastic
collisions. We conclude by arguing that this result may be amenable to
experimental verification through the use of quantum simulators.Comment: 4 pages, 1 figur
Dynamics of photo-induced ferromagnetism in oxides with orbital degeneracy
By using intense coherent electromagnetic radiation, it may be possible to
manipulate the properties of quantum materials very quickly, or even induce new
and potentially useful phases that are absent in equilibrium. For instance,
ultrafast control of magnetic dynamics is crucial for a number of proposed
spintronic devices and can also shed light on the possible dynamics of
correlated phases out of equilibrium. Inspired by recent experiments on
spin-orbital ferromagnet YTiO we consider the nonequilibrium dynamics of
Heisenberg ferromagnetic insulator with low-lying orbital excitations. We model
the dynamics of the magnon excitations in this system following an optical
pulse which resonantly excites infrared-active phonon modes. As the phonons
ring down they can dynamically couple the orbitals with the low-lying magnons,
leading to a dramatically modified effective bath for the magnons. We show this
transient coupling can lead to a dynamical acceleration of the magnetization
dynamics, which is otherwise bottlenecked by small anisotropy. Exploring the
parameter space more we find that the magnon dynamics can also even completely
reverse, leading to a negative relaxation rate when the pump is blue-detuned
with respect to the orbital bath resonance. We therefore show that by using
specially targeted optical pulses, one can exert a much greater degree of
control over the magnetization dynamics, allowing one to optically steer
magnetic order in this system. We conclude by discussing interesting parallels
between the magnetization dynamics we find here and recent experiments on
photo-induced superconductivity, where it is similarly observed that depending
on the initial pump frequency, an apparent metastable superconducting phase
emerges.Comment: 16 pages, 11 figures + 5 pages, no figure
Probing Electromagnetic Nonreciprocity with Quantum Geometry of Photonic States
Reciprocal and nonreciprocal effects in dielectric and magnetic materials
provide crucial information about the microscopic properties of electrons.
However, experimentally distinguishing the two has proven to be challenging,
especially when the associated effects are extremely small. To this end, we
propose a contact-less detection using a cross-cavity device where a material
of interest is placed at its centre. We show that the optical properties of the
material, such as Kerr and Faraday rotation, or, birefringence, manifest in the
coupling between the cavities' electromagnetic modes and in the shift of their
resonant frequencies. By calculating the dynamics of a geometrical photonic
state, we formulate a measurement protocol based on the quantum metric and
quantum process tomography that isolates the individual components of the
material's complex refractive index and minimizes the quantum mechanical
Cram\'er-Rao bound on the variance of the associated parameter estimation. Our
approach is expected to be applicable across a broad spectrum of experimental
platforms including Fock states in optical cavities, or, coherent states in
microwave and THz resonators.Comment: 9 pages, 4 figure
A map of the day-night contrast of the extrasolar planet HD 189733b
"Hot Jupiter" extrasolar planets are expected to be tidally locked because
they are close (<0.05 astronomical units, where 1 AU is the average Sun-Earth
distance) to their parent stars, resulting in permanent daysides and
nightsides. By observing systems where the planet and star periodically eclipse
each other, several groups have been able to estimate the temperatures of the
daysides of these planets. A key question is whether the atmosphere is able to
transport the energy incident upon the dayside to the nightside, which will
determine the temperature at different points on the planet's surface. Here we
report observations of HD 189733, the closest of these eclipsing planetary
systems, over half an orbital period, from which we can construct a 'map' of
the distribution of temperatures. We detected the increase in brightness as the
dayside of the planet rotated into view. We estimate a minimum brightness
temperature of 973 +/- 33 K and a maximum brightness temperature of 1212 +/- 11
K at a wavelength of 8 microns, indicating that energy from the irradiated
dayside is efficiently redistributed throughout the atmosphere, in contrast to
a recent claim for another hot Jupiter. Our data indicate that the peak
hemisphere-integrated brightness occurs 166 degrees before opposition,
corresponding to a hot spot shifted east of the substellar point. The secondary
eclipse (when the planet moves behind the star) occurs 120 +/- 24 s later than
predicted, which may indicate a slightly eccentric orbit.Comment: To appear in the May 10 2007 issue of Nature, 10 pages, 2 black and
white figures, 1 colo
Contrasting Ecosystem-Effects of Morphologically Similar Copepods
Organisms alter the biotic and abiotic conditions of ecosystems. They can modulate the availability of resources to other species (ecosystem engineering) and shape selection pressures on other organisms (niche construction). Very little is known about how the engineering effects of organisms vary among and within species, and, as a result, the ecosystem consequences of species diversification and phenotypic evolution are poorly understood. Here, using a common gardening experiment, we test whether morphologically similar species and populations of Diaptomidae copepods (Leptodiaptomus ashlandi, Hesperodiaptomus franciscanus, Skistodiaptomus oregonensis) have similar or different effects on the structure and function of freshwater ecosystems. We found that copepod species had contrasting effects on algal biomass, ammonium concentrations, and sedimentation rates, and that copepod populations had contrasting effects on prokaryote abundance, sedimentation rates, and gross primary productivity. The average size of ecosystem-effect contrasts between species was similar to those between populations, and was comparable to those between fish species and populations measured in previous common gardening experiments. Our results suggest that subtle morphological variation among and within species can cause multifarious and divergent ecosystem-effects. We conclude that using morphological trait variation to assess the functional similarity of organisms may underestimate the importance of species and population diversity for ecosystem functioning
Stabilizing fluctuating spin-triplet superconductivity in graphene via induced spin-orbit coupling
A recent experiment showed that proximity induced Ising spin-orbit coupling
enhances the spin-triplet superconductivity in Bernal bilayer graphene. Here,
we show that, due to the nearly perfect spin rotation symmetry of graphene, the
fluctuations of the spin orientation of the triplet order parameter suppress
the superconducting transition to nearly zero temperature. Our analysis shows
that both Ising spin-orbit coupling and in-plane magnetic field can eliminate
these low-lying fluctuations and can greatly enhance the transition
temperature, consistent with the recent experiment. Our model also suggests the
possible existence of a phase at small anisotropy and magnetic field which
exhibits quasi-long-range ordered spin-singlet charge 4e superconductivity,
even while the triplet 2e superconducting order only exhibits short-ranged
correlations. Finally, we discuss relevant experimental signatures.Comment: 6 pages, 2 figures + 8 pages, 1 figure supplementa
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