253 research outputs found
Structure and consequences of vortex-core states in p-wave superfluids
It is now well established that in two-dimensional chiral p-wave paired
superfluids, the vortices carry zero-energy modes which obey non-abelian
exchange statistics and can potentially be used for topological quantum
computation. In such superfluids there may also exist other excitations below
the bulk gap inside the cores of vortices. We study the properties of these
subgap states, and argue that their presence affects the topological protection
of the zero modes. In conventional superconductors where the chemical potential
is of the order of the Fermi energy of a non-interacting Fermi gas, there is a
large number of subgap states and the mini-gap towards the lowest of these
states is a small fraction of the Fermi energy. It is therefore difficult to
cool the system to below the mini-gap and at experimentally available
temperatures, transitions between the subgap states, including the zero modes,
will occur and can alter the quantum states of the zero-modes. We show that
compound qubits involving the zero-modes and the parity of the occupation
number of the subgap states on each vortex are still well defined. However,
practical schemes taking into account all subgap states would nonetheless be
difficult to achieve. We propose to avoid this difficulty by working in the
regime of small chemical potential mu, near the transition to a strongly paired
phase, where the number of subgap states is reduced. We develop the theory to
describe this regime of strong pairing interactions and we show how the subgap
states are ultimately absorbed into the bulk gap. Since the bulk gap vanishes
as mu -> 0 there is an optimum value mu_c which maximises the combined gap. We
propose cold atomic gases as candidate systems where the regime of strong
interactions can be explored, and explicitly evaluate mu_c in a Feshbach
resonant K-40 gas.Comment: 19 pages, 10 figures; v2: main text as published version, additional
detail included as appendice
Simple Quantum Systems in Spacetimes with Closed Timelike Curves
Three simple examples illustrate properties of path integral amplitudes in
fixed background spacetimes with closed timelike curves: non-relativistic
potential scattering in the Born approximation is non-unitary, but both an
example with hard spheres and the exact solution of a totally discrete model
are unitary.Comment: 15 pages, CALT-68-180
Increased dissolved terrestrial input to the coastal ocean during the last deglaciation
Here we present the first downcore results for a new paleoproxy, the Mn/Ca ratio of foraminiferal calcite, applied to sediment accumulated in the extreme Eastern Tropical North Pacific (ETNP) over the last 30,000 years. The Mn/Ca results are compared to oxygen isotopes and sea surface temperature calculated from Mg/Ca. We determined metal ratios using flow-through time-resolved analysis to minimize the effects of secondary mineralization. The foraminiferal species used for this study calcify at different depths. Core top ratios of these variant species change in proportion to the concentration of dissolved manganese in the water column at the depth of calcification. Since terrestrial input and oxidation reduction reactions control the levels of dissolved Mn in the oceans today, it therefore should be possible to use the Mn/Ca ratios of foraminifera as a proxy for these processes in the past. Mn/Ca of a mixed-layer species (G. ruber) suggest that dissolved terrestrial input to the surface waters of the ETNP during the last glacial maximum was lower than today but began to increase with initial sea level rise and reached a maximum at 15 ka B.P. before coming down to present-day levels at the end of sea level rise in the mid-Holocene (7–5 ka). Ratios of a deeper calcifying species (N. dutertrei) mimic those of G. ruber over this same time period, consistent with shoaling of the 18°C thermocline. Mn/Ca of a benthic species (U. peregrina) does not show a maximum at 15 ka, suggesting that Mn was efficiently remineralized in the water column during deglaciation. Assuming that the period from the last glacial until the mid-Holocene was a time of increased productivity, as elevated Mn might imply, the oxygen minimum zone (OMZ) was at least as well developed during deglaciation as it is today. Expansion of the OMZ may have contributed to the Mn/Ca trends we observe through time
No Time Machine Construction in Open 2+1 Gravity with Timelike Total Energy Momentum
It is shown that in 2+1 dimensional gravity an open spacetime with timelike
sources and total energy momentum cannot have a stable compactly generated
Cauchy horizon. This constitutes a proof of a version of Kabat's conjecture and
shows, in particular, that not only a Gott pair cannot be formed from processes
such as the decay of a single cosmic string as has been shown by Carroll et
al., but that, in a precise sense, a time machine cannot be constructed at all.Comment: 7 pages. Several changes and 3 figures added. To appear in Phys. Rev.
Geochemical proxies of North American freshwater routing during the Younger Dryas cold event
Author Posting. © The Authors, 2006. This is the author's version of the work. It is posted here by permission of National Academy of Sciences for personal use, not for redistribution. The definitive version was published in Proceedings of the National Academy of Sciences 104 (2007): 6556-6561, doi:10.1073/pnas.0611313104.The Younger Dryas cold interval represents a time when much of the Northern
Hemisphere cooled from ~12.9 to 11.5 kiloyears before present. The cause of this event,
which has long been viewed as the canonical example of abrupt climate change, was
initially attributed to the routing of freshwater to the St. Lawrence River with an attendant
reduction in Atlantic meridional overturning circulation. However, this mechanism has
recently been questioned because current proxies and dating techniques have been unable
to confirm that eastward routing with an increase in freshwater flux occurred during the
Younger Dryas. Here we use new geochemical proxies (ΔMg/Ca, U/Ca & 87Sr/86Sr)
measured in planktonic foraminifera at the mouth of the St. Lawrence Estuary as tracers
of freshwater sources to further evaluate this question. Our proxies, combined with
planktonic δ18Oseawater and δ13C, confirm that routing of runoff from western Canada to the
St. Lawrence River occurred at the start of the Younger Dryas, with an attendant increase
in freshwater flux of 0.06 ± 0.02 Sverdrup (1 Sverdrup (Sv) = 106 m3 s-1). This base
discharge increase is sufficient to have reduced Atlantic meridional overturning circulation
and caused the Younger Dryas cold interval. In addition, our data indicate subsequent
fluctuations in the freshwater flux to the St. Lawrence River of ~0.06 to 0.12 Sv, thus
explaining the variability in the overturning circulation and climate during the Younger
Dryas.This research was funded by the NSF Paleoclimate Program (P.U.C.) and the NSF (G.P.K.)
Scattering in the Presence of Electroweak Phase Transition Bubble Walls
We investigate the motion of fermions in the presence of an electro\-weak
phase transition bubble wall. We derive and solve the Dirac equation for such
fer\-mions, and compute the transmission and reflection coefficients for
fermions traveling from the symmetric to the asymmetric phases separated by the
domain wall.Comment: TPI--MINN--54, NUC--MINN--93/30--T, UMN--TH--1226/93, LaTex, 29 page
From wormhole to time machine: Comments on Hawking's Chronology Protection Conjecture
The recent interest in ``time machines'' has been largely fueled by the
apparent ease with which such systems may be formed in general relativity,
given relatively benign initial conditions such as the existence of traversable
wormholes or of infinite cosmic strings. This rather disturbing state of
affairs has led Hawking to formulate his Chronology Protection Conjecture,
whereby the formation of ``time machines'' is forbidden. This paper will use
several simple examples to argue that the universe appears to exhibit a
``defense in depth'' strategy in this regard. For appropriate parameter regimes
Casimir effects, wormhole disruption effects, and gravitational back reaction
effects all contribute to the fight against time travel. Particular attention
is paid to the role of the quantum gravity cutoff. For the class of model
problems considered it is shown that the gravitational back reaction becomes
large before the Planck scale quantum gravity cutoff is reached, thus
supporting Hawking's conjecture.Comment: 43 pages,ReV_TeX,major revision
A Self-Consistent Vacuum for Misner Space and the Chronology Protection Conjecture
In this paper we find a self-consistent vacuum for Misner space. For this
"adapted" Rindler vacuum the renormalized stress-energy tensor is zero
throughout the Misner space. A point-like particle detector traveling on a
timelike geodesic in a Misner space with this vacuum detects nothing. Misner
space with this vacuum thus creates no problems for time travel in and of
itself but a time traveler may pose a danger to himself and to the spacetime.Comment: 5 pages, received September 5, 1997, with a note added November 24,
199
Averaged Energy Conditions in 4D Evaporating Black Hole Backgrounds
Using Visser's semi-analytical model for the stress-energy tensor
corresponding to the conformally coupled massless scalar field in the Unruh
vacuum, we examine, by explicitly evaluating the relevant integrals over
half-complete geodesics, the averaged weak (AWEC) and averaged null (ANEC)
energy conditions along with Ford-Roman quantum inequality-type restrictions on
negative energy in the context of four dimensional evaporating black hole
backgrounds. We find that in all cases where the averaged energy conditions
fail, there exist quantum inequality bounds on the magnitude and duration of
negative energy densities.Comment: Revtex, 13 pages, to appear in Phy. Rev.
Restrictions on Negative Energy Density in Flat Spacetime
In a previous paper, a bound on the negative energy density seen by an
arbitrary inertial observer was derived for the free massless, quantized scalar
field in four-dimensional Minkowski spacetime. This constraint has the form of
an uncertainty principle-type limitation on the magnitude and duration of the
negative energy density. That result was obtained after a somewhat complicated
analysis. The goal of the current paper is to present a much simpler method for
obtaining such constraints. Similar ``quantum inequality'' bounds on negative
energy density are derived for the electromagnetic field, and for the massive
scalar field in both two and four-dimensional Minkowski spacetime.Comment: 17 pages, including two figures, uses epsf, minor revisions in the
Introduction, conclusions unchange
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