1,388 research outputs found
Frequency scaling of photo-induced tunneling
The DC current-voltage characteristics, induced by a driving electric field
with frequency Omega, of a one dimensional electron channel with a tunnel
barrier is calculated. Electron-electron interaction of finite-range is taken
into account. For intermediate interaction strengths, the non-linear
differential conductance shows cusp-like minima at bias voltages integer
multiples of hbar Omega / e that are a consequence of the finite non-zero range
of the interaction but are independent of the shape of the driving electric
field. However, the frequency-scaling of the photo-induced current shows a
cross-over between Omega^{-1} and Omega^{-2}, and depends on the spatial shape
of the driving field and the range of the interaction.Comment: 7 pages, EURO-TeX, 3 figures, to appear in Europhysics Letter
Accelerating Cooperative Planning for Automated Vehicles with Learned Heuristics and Monte Carlo Tree Search
Efficient driving in urban traffic scenarios requires foresight. The
observation of other traffic participants and the inference of their possible
next actions depending on the own action is considered cooperative prediction
and planning. Humans are well equipped with the capability to predict the
actions of multiple interacting traffic participants and plan accordingly,
without the need to directly communicate with others. Prior work has shown that
it is possible to achieve effective cooperative planning without the need for
explicit communication. However, the search space for cooperative plans is so
large that most of the computational budget is spent on exploring the search
space in unpromising regions that are far away from the solution. To accelerate
the planning process, we combined learned heuristics with a cooperative
planning method to guide the search towards regions with promising actions,
yielding better solutions at lower computational costs
A population of high-velocity absorption-line systems residing in the Local Group
Aims. We aim to investigate the ionisation conditions and distances of
Galactic high-velocity clouds (HVCs) in the Galactic halo and beyond in the
direction of the Local Group (LG) barycentre and anti-barycentre, by studying
spectral data of 29 extragalactic background sources obtained with Cosmic
Origins Spectropgraph (COS) installed on the Hubble Space Telescope (HST).
Methods. We model column-densities of low, intermediate, and high ions, such as
Si II, C II, Si III, Si IV, and C IV and use this to construct a set of Cloudy
ionisation models. Results. In total, we found 69 high-velocity absorption
components along the 29 lines of sight. The ones in the direction of the LG
barycentre span the entire range of studied velocities, 100 \lesssim
|v_{LSR}|\lesssim 400 km s^-1, while the anti-barycentre sample has velocities
up to about 300 km s^-1. For 49 components, we infer the gas densities. In the
direction of the LG barycentre, the gas densities exhibit a large range between
log n_H=-3.96 to -2.55, while in the anti-barycentre direction the densities
are systematically higher, log n_H>-3.25. The barycentre absorbers can be split
into two groups based on their density: a high density group with log
n_H>-3.54, which can be affected by the Milky Way radiation field, and a low
density group (log n_H \leq -3.54). The latter has very low thermal pressures
of P/k<7.3 K cm^-3. Conclusions. Our study shows that part of the absorbers in
the LG barycentre direction trace gas at very low gas densities and thermal
pressures. Such properties indicate that these absorbers are located beyond the
virial radius of the Milky Way. Our study also confirms results from earlier,
single-sightline studies, suggesting the presence of a metal-enriched
intragroup medium filling the LG near its barycentre.Comment: Accepted for publication in A&A. 12 pages, 11 figure
Immunity to MHC class I antigen after direct DNA transfer into skeletal muscle.
Plasmid cDNA encoding the alpha-chain of either membrane-bound (pcRT.45) or secreted (pcRQ.B3) RT1Aa MHC class I Ag were transferred to Lewis (RT1(1)) rat skeletal muscle by direct injection. Rats were challenged 7 days later with an ACI (RT1a) heterotropic heart transplant, and cardiac allograft survival, RT1Aa-specific antibody levels, and frequency of ACI-specific CTL were monitored. Graft rejection was accelerated by > or = 2 days in an Ag-specific and dose-dependent manner in pcRT.45-injected rats. The pcRQ.B3-injected rats also rejected grafts more rapidly; however, graft rejection was accelerated by only 1 day, and graft infiltrates were less pronounced than in pcRT.45-injected rats. Injection of pcRT.45 resulted in an increase in ACI-specific CTL precursor frequency 3 days post-transplant, whereas there was no significant change in rats pretreated with pcRQ.B3 injection. Compared with rats injected with a control plasmid encoding firefly luciferase, transfer of pcRT.45 resulted in an increase in RT1Aa-specific IgG and IgM antibody 3 days after heart transplantation. Transfer of pcRQ.B3 resulted in a similar mean increase in RT1Aa-specific IgG and IgM antibody after transplantation, but the variability from rat to rat was greater, with some animals exhibiting strong priming, and others showing little or no priming by gene injection. Our results suggest that skeletal muscle can express either membrane-bound or secreted MHC class I Ag after gene transfer, but that the membrane-bound form is more immunogenic than the secreted form in the high responder Lewis rat. Direct DNA transfer to skeletal muscle provides a rapid and specific approach to studying immunity to allogeneic MHC Ag
Use of donor serum to prevent passive transfer of hyperacute rejection
Organ transplantation in presensitized recipients continues to be contraindicated for heart and kidney recipients due to the risk of hyperacute rejection, which has no known treatment at this time. We tested whether donor serum, which contains soluble MHC class I antigen, is able to neutralize the effect of anti-donor antibody in the recipient and prevent hyperacute or accelerated rejection. A rat model of passive immunization was used to test the role of anti-donor antibody in hyperacute rejection. Seven of 10 recipients of hyperimmune serum (HyS), derived from Lewis rats (RT1l) following 3 ACI (RT1a) skin grafts, developed hyperacute or accelerated rejection. Intravenous injection of ACI serum prior to the HyS administration prevented hyperacute rejection in all recipients tested. When third-party (Wistar-Furth, RT1u) serum was given to Lewis rats injected with HyS, hyperacute rejection was not abrogated. When examining the mechanism of this effect, a simple antibody blocking phenomenon was found to be unlikely since flow cytometry analysis showed that ACI serum needed to be present at > or = 256-fold excess compared to HyS to block anti-ACI antibody binding to RT1.Aa+cells by 50%. We tested whether the RT1.Aa class I antigen in ACI serum had other biologic properties that resulted in the prolonged graft survival. However, removal of RT1.Aa antigen from ACI serum prior to use in the passive transfer model did not abrogate the graft prolongation observed previously. These data suggest that components of donor serum other than MHC class I antigen may be useful for preventing the antibody-mediated component of hyperacute rejection
Magnetophononics: ultrafast spin control through the lattice
Using a combination of first-principles and magnetization-dynamics
calculations, we study the effect of the intense optical excitation of phonons
on the magnetic behavior in insulating magnetic materials. Taking the
prototypical magnetoelectric \CrO\ as our model system, we show that excitation
of a polar mode at 17 THz causes a pronounced modification of the magnetic
exchange interactions through a change in the average Cr-Cr distance. In
particular, the quasi-static deformation induced by nonlinear phononic coupling
yields a structure with a modified magnetic state, which persists for the
duration of the phonon excitation. In addition, our time-dependent
magnetization dynamics computations show that systematic modulation of the
magnetic exchange interaction by the phonon excitation modifies the
magnetization dynamics. This temporal modulation of the magnetic exchange
interaction strengths using phonons provides a new route to creating
non-equilibrium magnetic states and suggests new avenues for fast manipulation
of spin arrangements and dynamics.Comment: 11 pages with 7 figure
An HST/COS legacy survey of intervening SiIII absorption in the extended gaseous halos of low-redshift galaxies
Doubly ionized silicon (SiIII) is a powerful tracer of diffuse ionized gas
inside and outside of galaxies. It can be observed in the local Universe in
ultraviolet (UV) absorption against bright extragalactic background sources. We
here present an extensive study of intervening SiIII-selected absorbers and
their relation to the circumgalactic medium (CGM) of galaxies at low redshift
(z<=0.1), based on the analysis of UV absorption spectra along 303
extragalactic lines of sight obtained with the Cosmic Origins Spectrograph
(COS) on board the Hubble Space Telescope (HST). Along a total redshift path of
Dz=24 we identify 69 intervening SiIII systems that all show associated
absorption from other low and high ions. We derive a bias-corrected number
density of dN/dz(SiIII)=2.5 for absorbers with column densities log
N(SiIII)>12.2. We develop a geometrical model for the absorption-cross section
of the CGM around the local galaxy population and find excellent agreement
between the model predictions and the observations. We further compare
redshifts and positions of the absorbers with that of ~64,000 galaxies using
archival galaxy-survey data. For the majority of the absorbers we identify
possible host galaxies within 300 km/s of the absorbers and derive impact
parameters rho<200 kpc, demonstrating that the spatial distributions of SiIII
absorbers and galaxies are highly correlated. Our study indicates that the
majority of SiIII-selected absorbers in our sample trace the CGM of nearby
galaxies within their virial radii at a typical covering fraction of ~70 per
cent. From a detailed ionization model we estimate that diffuse gas in the CGM
around galaxies, as traced by SiIII, contains substantially more baryonic mass
than their neutral interstellar medium.Comment: 32 pages, 17 figures; final version accepted for publication in A&
Pump frequency resonances for light-induced incipient superconductivity in YBaCuO
Optical excitation in the cuprates has been shown to induce transient
superconducting correlations above the thermodynamic transition temperature,
, as evidenced by the terahertz frequency optical properties in the
non-equilibrium state. In YBaCuO this phenomenon has so far
been associated with the nonlinear excitation of certain lattice modes and the
creation of new crystal structures. In other compounds, like
LaBaCuO, similar effects were reported also for excitation at
near infrared frequencies, and were interpreted as a signature of the melting
of competing orders. However, to date it has not been possible to
systematically tune the pump frequency widely in any one compound, to
comprehensively compare the frequency dependent photo-susceptibility for this
phenomenon. Here, we make use of a newly developed optical parametric
amplifier, which generates widely tunable high intensity femtosecond pulses, to
excite YBaCuO throughout the entire optical spectrum (3 - 750
THz). In the far-infrared region (3 - 25 THz), signatures of non-equilibrium
superconductivity are induced only for excitation of the 16.4 THz and 19.2 THz
vibrational modes that drive -axis apical oxygen atomic positions. For
higher driving frequencies (25 - 750 THz), a second resonance is observed
around the charge transfer band edge at ~350 THz. These observations highlight
the importance of coupling to the electronic structure of the CuO planes,
either mediated by a phonon or by charge transfer.Comment: 47 pages, 21 figures, 2 table
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