15,443 research outputs found
Surface plasma resonance in small rare gas clusters by mixing IR and VUV laser pulses
The ionization dynamics of a Xenon cluster with 40 atoms is analyzed under a
pum p probe scenario of laser pulses where an infrared laser pulse of 50 fs
length follows with a well defined time delay a VUV pulse of the same length
and peak intensity. The mechanism of resonant energy absorption due to the
coinc idence of the IR laser frequency with the frequency of collective motion
of quasi free electrons in the cluster is mapped out by varying the time delay
between the pulses
Electron spin relaxation in graphene: the role of the substrate
Theory of the electron spin relaxation in graphene on the SiO substrate
is developed. Charged impurities and polar optical surface phonons in the
substrate induce an effective random Bychkov-Rashba-like spin-orbit coupling
field which leads to spin relaxation by the D'yakonov-Perel' mechanism.
Analytical estimates and Monte Carlo simulations show that the corresponding
spin relaxation times are between micro- to milliseconds, being only weakly
temperature dependent. It is also argued that the presence of adatoms on
graphene can lead to spin lifetimes shorter than nanoseconds.Comment: 5 pages, 4 figure
The running coupling from the four-gluon vertex in Landau gauge Yang-Mills theory
We consider the running coupling from the four-gluon vertex in Landau gauge,
SU() Yang-Mills theory as given by a combination of dressing functions of
the vertex and the gluon propagator. We determine these functions numerically
from a coupled set of Dyson-Schwinger equations. We reproduce asymptotic
freedom in the ultraviolet momentum region and find a coupling of order one at
mid-momenta. In the infrared we find a nontrivial (i.e. nonzero) fixed point
which is three orders of magnitude smaller than the corresponding fixed point
in the coupling of the ghost-gluon vertex. This result explains why the
Dyson-Schwinger and the functional renormalization group equations for the two
point functions can agree in the infrared, although their structure is quite
different. Our findings also support Zwanziger's notion of an infrared
effective theory driven by the Faddeev-Popov determinant.Comment: 25 pages, 4 figures; v2: minor clarifications added and typos
corrected, version accepted by PR
Thermal Quantum Fields without Cut-offs in 1+1 Space-time Dimensions
We construct interacting quantum fields in 1+1 dimensional Minkowski space,
representing neutral scalar bosons at positive temperature. Our work is based
on prior work by Klein and Landau and Hoegh-KrohnComment: 48 page
Matching factors for Delta S=1 four-quark operators in RI/SMOM schemes
The non-perturbative renormalization of four-quark operators plays a
significant role in lattice studies of flavor physics. For this purpose, we
define regularization-independent symmetric momentum-subtraction (RI/SMOM)
schemes for Delta S=1 flavor-changing four-quark operators and provide one-loop
matching factors to the MS-bar scheme in naive dimensional regularization. The
mixing of two-quark operators is discussed in terms of two different classes of
schemes. We provide a compact expression for the finite one-loop amplitudes
which allows for a straightforward definition of further RI/SMOM schemes.Comment: 22 pages, 5 figure
Half-Metallic Ferromagnetism in the Heusler Compound CoFeSi revealed by Resistivity, Magnetoresistance, and Anomalous Hall Effect measurements
We present electrical transport data for single-crystalline CoFeSi which
provide clear-cut evidence that this Heusler compound is truly a half-metallic
ferromagnet, i.e. it possesses perfect spin-polarization. More specifically,
the temperature dependence of is governed by electron scattering off
magnons which are thermally excited over a sizeable gap
() separating the electronic majority states at the Fermi level
from the unoccupied minority states. As a consequence, electron-magnon
scattering is only relevant at but freezes out at lower
temperatures, i.e., the spin-polarization of the electrons at the Fermi level
remains practically perfect for . The gapped magnon population
has a decisive influence on the magnetoresistance and the anomalous Hall effect
(AHE): i) The magnetoresistance changes its sign at , ii) the
anomalous Hall coefficient is strongly temperature dependent at and compatible with Berry phase related and/or side-jump electronic
deflection, whereas it is practically temperature-independent at lower
temperatures
Supercurrent-phase relationship of a Nb/InAs(2DES)/Nb Josephson junction in overlapping geometry
Superconductor/normal conductor/superconductor (SNS) Josephson junctions with
highly transparent interfaces are predicted to show significant deviations from
sinusoidal supercurrent-phase relationships (CPR) at low temperatures. We
investigate experimentally the CPR of a ballistic Nb/InAs(2DES)/Nb junction in
the temperature range from 1.3 K to 9 K using a modified Rifkin-Deaver method.
The CPR is obtained from the inductance of the phase-biased junction. Transport
measurements complement the investigation. At low temperatures, substantial
deviations of the CPR from conventional tunnel-junction behavior have been
observed. A theoretical model yielding good agreement to the data is presented.Comment: RevTex4, 4 pages including 3 figure
Chirality sensitive effect on surface states in chiral p-wave superconductors
We study the local density of states at the surface of a chiral p-wave
superconductor in the presence of a weak magnetic field. As a result, the
formation of low-energy Andreev bound states is either suppressed or enhanced
by an applied magnetic field, depending on its orientation with respect to the
chirality of the p-wave superconductor. Similarly, an Abrikosov vortex, which
is situated not too far from the surface, leads to a zero-energy peak of the
density of states, if its chirality is the same as that of the superconductor,
and to a gap structure for the opposite case. We explain the underlying
principle of this effect and propose a chirality sensitive test on
unconventional superconductors.Comment: 4 pages, 2 figure
Calculating the mass fraction of primordial black holes
We reinspect the calculation for the mass fraction of primordial black holes (PBHs) which are formed from primordial perturbations, finding that performing the calculation using the comoving curvature perturbation c in the standard way vastly overestimates the number of PBHs, by many orders of magnitude. This is because PBHs form shortly after horizon entry, meaning modes significantly larger than the PBH are unobservable and should not affect whether a PBH forms or not - this important effect is not taken into account by smoothing the distribution in the standard fashion. We discuss alternative methods and argue that the density contrast, Δ, should be used instead as super-horizon modes are damped by a factor k2. We make a comparison between using a Press-Schechter approach and peaks theory, finding that the two are in close agreement in the region of interest. We also investigate the effect of varying the spectral index, and the running of the spectral index, on the abundance of primordial black holes
Episodic excursions of low-mass protostars on the Hertzsprung-Russell diagram
Following our recent work devoted to the effect of accretion on the
pre-main-sequence evolution of low-mass stars, we perform a detailed analysis
of episodic excursions of low-mass protostars in the Hertzsprung-Russell (H-R)
diagram triggered by strong mass accretion bursts typical of FU Orionis-type
objects (FUors). These excursions reveal themselves as sharp increases in the
stellar total luminosity and/or effective temperature of the protostar and can
last from hundreds to a few thousands of years, depending on the burst strength
and characteristics of the protostar. During the excursions, low-mass
protostars occupy the same part of the H-R diagram as young intermediate-mass
protostars in the quiescent phase of accretion. Moreover, the time spent by
low-mass protostars in these regions is on average a factor of several longer
than that spent by the intermediate-mass stars in quiescence. During the
excursions, low-mass protostars pass close to the position of most known FUors
in the H-R diagram, but owing to intrinsic ambiguity the model stellar
evolutionary tracks are unreliable in determining the FUor properties. We find
that the photospheric luminosity in the outburst state may dominate the
accretion luminosity already after a few years after the onset of the outburst,
meaning that the mass accretion rates of known FUors inferred from the
bolometric luminosity may be systematically overestimated, especially in the
fading phase.Comment: 15 pages, 12 figure
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