19,984 research outputs found
Superconductivity in the Kondo lattice model
We study the Kondo lattice model with additional attractive interaction
between the conduction electrons within the dynamical mean-field theory using
the numerical renormalization group to solve the effective quantum impurity
problem. In addition to normal-state and magnetic phases we also allow for the
occurrence of a superconducting phase. In the normal phase we observe a very
sensitive dependence of the low-energy scale on the conduction-electron
interaction. We discuss the dependence of the superconducting transition on the
interplay between attractive interaction and Kondo exchange.Comment: Submitted to ICM 2009 Conference Proceeding
Runtime Verification of Temporal Properties over Out-of-order Data Streams
We present a monitoring approach for verifying systems at runtime. Our
approach targets systems whose components communicate with the monitors over
unreliable channels, where messages can be delayed or lost. In contrast to
prior works, whose property specification languages are limited to
propositional temporal logics, our approach handles an extension of the
real-time logic MTL with freeze quantifiers for reasoning about data values. We
present its underlying theory based on a new three-valued semantics that is
well suited to soundly and completely reason online about event streams in the
presence of message delay or loss. We also evaluate our approach
experimentally. Our prototype implementation processes hundreds of events per
second in settings where messages are received out of order.Comment: long version of the CAV 2017 pape
Another derivation of the geometrical KPZ relations
We give a physicist's derivation of the geometrical (in the spirit of
Duplantier-Sheffield) KPZ relations, via heat kernel methods. It gives a
covariant way to define neighborhoods of fractals in 2d quantum gravity, and
shows that these relations are in the realm of conformal field theory
External Operators and Anomalous Dimensions in Soft-Collinear Effective Theory
It has recently been argued that soft-collinear effective theory for
processes involving both soft and collinear partons contains a new
soft-collinear mode, which can communicate between the soft and collinear
sectors of the theory. The formalism incorporating the corresponding fields
into the effective Lagrangian is extended to include external current and
four-quark operators relevant to weak interactions. An explicit calculation of
the anomalous dimensions of these operators reveals that soft-collinear modes
are needed for correctly describing the ultraviolet behavior of the effective
theory.Comment: 15 pages, 2 figure
Photoemission of BiSe with Circularly Polarized Light: Probe of Spin Polarization or Means for Spin Manipulation?
Topological insulators are characterized by Dirac cone surface states with
electron spins aligned in the surface plane and perpendicular to their momenta.
Recent theoretical and experimental work implied that this specific spin
texture should enable control of photoelectron spins by circularly polarized
light. However, these reports questioned the so far accepted interpretation of
spin-resolved photoelectron spectroscopy. We solve this puzzle and show that
vacuum ultraviolet photons (50-70 eV) with linear or circular polarization
probe indeed the initial state spin texture of BiSe while circularly
polarized 6 eV low energy photons flip the electron spins out of plane and
reverse their spin polarization. Our photoemission calculations, considering
the interplay between the varying probing depth, dipole selection rules and
spin-dependent scattering effects involving initial and final states explain
these findings, and reveal proper conditions for light-induced spin
manipulation. This paves the way for future applications of topological
insulators in opto-spintronic devices.Comment: Submitted for publication (2013
Spitzer Space Telescope Observations of the Nucleus of Comet 103P/Hartley 2
We have used the Spitzer Space Telescope InfraRed Spectrograph (IRS) 22-μm peakup array to observe thermal emission from the nucleus and trail of comet 103P/Hartley 2, the target of NASA’s Deep Impact Extended Investigation (DIXI). The comet was observed on UT 2008 August 12 and 13, while 5.5 AU from the Sun. We obtained two 200 frame sets of photometric imaging over a 2.7 hr period. To within the errors of the measurement, we find no detection of any temporal variation between the two images. The comet showed extended emission beyond a point source in the form of a faint trail directed along the comet’s antivelocity vector. After modeling and removing the trail emission, a NEATM model for the nuclear emission with beaming parameter of 0.95 ± 0.20 indicates a small effective radius for the nucleus of 0.57 ± 0.08 km and low geometric albedo 0.028 ± 0.009 (1σ). With this nucleus size and a water production rate of 3 × 10^(28) molecules s^(-1) at perihelion, we estimate that ~100% of the surface area is actively emitting volatile material at perihelion. Reports of emission activity out to ~5 AU support our finding of a highly active nuclear surface. Compared to Deep Impact’s first target, comet 9P/Tempel 1, Hartley 2’s nucleus is one-fifth as wide (and about one-hundredth the mass) while producing a similar amount of outgassing at perihelion with about 13 times the active surface fraction. Unlike Tempel 1, comet Hartley 2 should be highly susceptible to jet driven spin-up torques, and so could be rotating at a much higher frequency. Since the amplitude of nongravitational forces are surprisingly similar for both comets, close to the ensemble average for ecliptic comets, we conclude that comet Hartley 2 must have a much more isotropic pattern of time-averaged outgassing from its nuclear surface. Barring a catastrophic breakup or major fragmentation event, the comet should be able to survive up to another 100 apparitions (~700 yr) at its current rate of mass loss
Cosmologies with a time dependent vacuum
The idea that the cosmological term, Lambda, should be a time dependent
quantity in cosmology is a most natural one. It is difficult to conceive an
expanding universe with a strictly constant vacuum energy density, namely one
that has remained immutable since the origin of time. A smoothly evolving
vacuum energy density that inherits its time-dependence from cosmological
functions, such as the Hubble rate or the scale factor, is not only a
qualitatively more plausible and intuitive idea, but is also suggested by
fundamental physics, in particular by quantum field theory (QFT) in curved
space-time. To implement this notion, is not strictly necessary to resort to ad
hoc scalar fields, as usually done in the literature (e.g. in quintessence
formulations and the like). A "running" Lambda term can be expected on very
similar grounds as one expects (and observes) the running of couplings and
masses with a physical energy scale in QFT. Furthermore, the experimental
evidence that the equation of state of the dark energy could be evolving with
time/redshift (including the possibility that it might currently behave
phantom-like) suggests that a time-variable Lambda term (possibly accompanied
by a variable Newton's gravitational coupling G=G(t)) could account in a
natural way for all these features. Remarkably enough, a class of these models
(the "new cosmon") could even be the clue for solving the old cosmological
constant problem, including the coincidence problem.Comment: LaTeX, 15 pages, 4 figure
Crystalline Electric Field Effects in CeMIn5: Superconductivity and the Influence of Kondo Spin Fluctuations
We have measured the crystalline electric field (CEF) excitations of the
CeMIn5 (M = Co, Rh, Ir) series of heavy fermion superconductors by means of
inelastic neutron scattering. Fits to a CEF model reproduce the inelastic
neutron scattering spectra and the high temperature magnetic susceptibility.
The CEF parameters, energy level splittings, and wavefunctions are tabulated
for each member of the CeMIn5 series and compared to each other as well as to
the results of previous measurements. Our results indicate that the CEF level
splitting in all three materials is similar, and can be thought of as being
derived from the cubic parent compound CeIn3 in which an excited state quartet
at ~12 meV is split into two doublets by the lower symmetry of the tetragonal
environment of the CeMIn5 materials. In each case, the CEF excitations are
observed as broad lines in the inelastic neutron scattering spectrum. We
attribute this broadening to Kondo hybridization of the localized f moments
with the conduction electrons. The evolution of the superconducting transition
temperatures in the different members of CeMIn5 can then be understood as a
direct consequence of the strength of this hybridization. Due to the importance
of Kondo spin fluctuations in these materials, we also present calculations
within the non-crossing approximation (NCA) to the Anderson impurity model
including the effect of CEF level splitting for the inelastic neutron
scattering spectra and the magnetic susceptibility.Comment: 30 pages, 6 figures, submitted to Phys. Rev.
Stable Magnetostatic Solitons in Yttrium Iron Garnet Film Waveguides for Tilted in-Plane Magnetic Fields
The possibility of nonlinear pulses generation in Yttrium Iron Garnet thin
films for arbitrary direction between waveguide and applied static in-plane
magnetic field is considered. Up to now only the cases of in-plane magnetic
fields either perpendicular or parallel to waveguide direction have been
studied both experimentally and theoretically. In the present paper it is shown
that also for other angles (besides 0 or 90 degrees) between a waveguide and
static in-plane magnetic field the stable bright or dark (depending on
magnitude of magnetic field) solitons could be created.Comment: Phys. Rev. B (accepted, April 1, 2002
Main-belt comets in the Palomar Transient Factory survey – I. The search for extendedness
Cometary activity in main-belt asteroids probes the ice content of these objects and provides clues to the history of volatiles in the inner Solar system. We search the Palomar Transient Factory survey to derive upper limits on the population size of active main-belt comets (MBCs). From data collected from 2009 March through 2012 July, we extracted ∼2 million observations of ∼220 thousand known main-belt objects (40 per cent of the known population, down to ∼1-km diameter) and discovered 626 new objects in multinight linked detections. We formally quantify the ‘extendedness’ of a small-body observation, account for systematic variation in this metric (e.g. due to on-sky motion) and evaluate this method's robustness in identifying cometary activity using observations of 115 comets, including two known candidate MBCs and six newly discovered non-MBCs (two of which were originally designated as asteroids by other surveys). We demonstrate a 66 per cent detection efficiency with respect to the extendedness distribution of the 115 sampled comets, and a 100 per cent detection efficiency with respect to extendedness levels greater than or equal to those we observed in the known candidate MBCs P/2010 R2 (La Sagra) and P/2006 VW_(139). Using a log-constant prior, we infer 95 per cent confidence upper limits of 33 and 22 active MBCs (per million main-belt asteroids down to ∼1-km diameter), for detection efficiencies of 66 and 100 per cent, respectively. In a follow-up to this morphological search, we will perform a photometric (disc-integrated brightening) search for MBCs
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