62 research outputs found
Depth of interaction and bias voltage depenence of the spectral response in a pixellated CdTe detector operating in time-over-threshold mode subjected to monochromatic X-rays
High stopping power is one of the most important figures of merit for X-ray detectors. CdTe is a promising material but suffers from: material defects, non-ideal charge transport and long range X-ray fluorescence. Those factors reduce the image quality and deteriorate spectral information. In this project we used a monochromatic pencil beam collimated through a 20μm pinhole to measure the detector spectral response in dependance on the depth of interaction. The sensor was a 1mm thick CdTe detector with a pixel pitch of 110μm, bump bonded to a Timepix readout chip operating in Time-Over-Threshold mode. The measurements were carried out at the Extreme Conditions beamline I15 of the Diamond Light Source. The beam was entering the sensor at an angle of \texttildelow20 degrees to the surface and then passed through \texttildelow25 pixels before leaving through the bottom of the sensor. The photon energy was tuned to 77keV giving a variation in the beam intensity of about three orders of magnitude along the beam path. Spectra in Time-over-Threshold (ToT) mode were recorded showing each individual interaction. The bias voltage was varied between -30V and -300V to investigate how the electric field affected the spectral information. For this setup it is worth noticing the large impact of fluorescence. At -300V the photo peak and escape peak are of similar height. For high bias voltages the spectra remains clear throughout the whole depth but for lower voltages as -50V, only the bottom part of the sensor carries spectral information. This is an effect of the low hole mobility and the longer range the electrons have to travel in a low field
Directed percolation with a single defect site
In a recent study [arXiv:1011.3254] the contact process with a modified
creation rate at a single site was shown to exhibit a non-universal scaling
behavior with exponents varying with the creation rate at the special site. In
the present work we argue that the survival probability decays according to a
stretched exponential rather than a power law, explaining previous
observations.Comment: 8 pages, 3 figure
Magnetic impurities in the one-dimensional spin-orbital model
Using one-dimensional spin-orbital model as a typical example of quantum spin
systems with richer symmetries, we study the effect of an isolated impurity on
its low energy dynamics in the gapless phase through bosonization and
renormalization group methods. In the case of internal impurities, depending on
the symmetry, the boundary fixed points can be either an open chain with a
residual spin or (and) orbital triplet left behind, or a periodic chain.
However, these two fixed points are indistinguishable in the sense that in both
cases, the lead-correction-to-scaling boundary operators (LCBO) only show
Fermi-liquid like corrections to thermodynamical quantities. (Except the
possible Curie-like contributions from the residual moments in the latter
cases.) In the case of external (Kondo) impurities, the boundary fixed points,
depending on the sign of orbital couplings, can be either an open chain with an
isolated orbital doublet due to Kondo screening or it will flow to an
intermediate fixed point with the same LCBO as that of the two-channel Kondo
problem. Comparison with the Kondo effect in one-dimensional (1D) Heisenberg
spin chain and multi-band Hubbard models is also made.Comment: 7 pages, No figur
Kondo Resonance in a Mesoscopic Ring Coupled to a Quantum Dot: Exact Results for the Aharonov-Bohm/Casher Effects
We study the persistent currents induced by both the Aharonov-Bohm and
Aharonov-Casher effects in a one-dimensional mesoscopic ring coupled to a
side-branch quantum dot at Kondo resonance. For privileged values of the
Aharonov-Bohm-Casher fluxes, the problem can be mapped onto an integrable
model, exactly solvable by a Bethe ansatz. In the case of a pure magnetic
Aharonov-Bohm flux, we find that the presence of the quantum dot has no effect
on the persistent current. In contrast, the Kondo resonance interferes with the
spin-dependent Aharonov-Casher effect to induce a current which, in the
strong-coupling limit, is independent of the number of electrons in the ring.Comment: Replaced with published version; 5 page
Crossover from Isotropic to Directed Percolation
Directed percolation is one of the generic universality classes for dynamic
processes. We study the crossover from isotropic to directed percolation by
representing the combined problem as a random cluster model, with a parameter
controlling the spontaneous birth of new forest fires. We obtain the exact
crossover exponent at using Coulomb gas methods in 2D.
Isotropic percolation is stable, as is confirmed by numerical finite-size
scaling results. For , the stability seems to change. An intuitive
argument, however, suggests that directed percolation at is unstable and
that the scaling properties of forest fires at intermediate values of are
in the same universality class as isotropic percolation, not only in 2D, but in
all dimensions.Comment: 4 pages, REVTeX, 4 epsf-emedded postscript figure
An Exactly Solvable Kondo Problem for Interacting One-Dimensional Fermions
The single impurity Kondo problem in the one-dimensional -potential
Fermi gas is exactly solved for two sets of special coupling constants via
Bethe ansatz. It is found that ferromagnetic Kondo screening does occur in one
case which confirms the Furusaki-Nagaosa conjecture while in the other case it
does not, which we explain in a simple physical picture. The surface energy,
the low temperature specific heat and the Pauli susceptibility induced by the
impurity and thereby the Kondo temperature are derived explicitly.Comment: 8 pages, LATEX, REVTE
Particle transport in density gradient driven TE mode turbulence
The turbulent transport of main ion and trace impurities in a tokamak device
in the presence of steep electron density gradients has been studied. The
parameters are chosen for trapped electron (TE) mode turbulence, driven
primarily by steep electron density gradients relevant to H-mode physics, but
with a transition to temperature gradient driven turbulence as the density
gradient flattens. Results obtained through non-linear (NL) and quasilinear
(QL) gyrokinetic simulations using the GENE code are compared with results
obtained from a fluid model. Main ion and impurity transport is studied by
examining the balance of convective and diffusive transport, as quantified by
the density gradient corresponding to zero particle flux (peaking factor).
Scalings are obtained for the impurity peaking with the background electron
density gradient and the impurity charge number. It is shown that the impurity
peaking factor is weakly dependent on impurity charge and significantly smaller
than the driving electron density gradient.Comment: 11 pages, 6 figures. Submitted to Nuclear Fusion SP
Sliding blocks with random friction and absorbing random walks
With the purpose of explaining recent experimental findings, we study the
distribution of distances traversed by a block that
slides on an inclined plane and stops due to friction. A simple model in which
the friction coefficient is a random function of position is considered.
The problem of finding is equivalent to a First-Passage-Time
problem for a one-dimensional random walk with nonzero drift, whose exact
solution is well-known. From the exact solution of this problem we conclude
that: a) for inclination angles less than \theta_c=\tan(\av{\mu})
the average traversed distance \av{\lambda} is finite, and diverges when
as \av{\lambda} \sim (\theta_c-\theta)^{-1}; b) at
the critical angle a power-law distribution of slidings is obtained:
. Our analytical results are confirmed by
numerical simulation, and are in partial agreement with the reported
experimental results. We discuss the possible reasons for the remaining
discrepancies.Comment: 8 pages, 8 figures, submitted to Phys. Rev.
Kondo Effect in a Luttinger Liquid: Exact Results from Conformal Field Theory
We report on exact results for the low-temperature thermodynamics of a
spin- magnetic impurity coupled to a one-dimensional interacting
electron system. By using boundary conformal field theory, we show that there
are only two types of critical behaviors consistent with the symmetries of the
problem: {\em either} a local Fermi liquid, {\em or} a theory with an anomalous
response identical to that recently proposed by Furusaki and Nagaosa.
Suppression of back scattering off the impurity leads to the same critical
properties as for the two-channel Kondo effect.Comment: 9 pages, REVTeX, uses amsfonts, accepted for publication in Phys.
Rev. Let
Scaling and criticality of the Kondo effect in a Luttinger liquid
A quantum Monte Carlo simulation method has been developed and applied to
study the critical behavior of a single Kondo impurity in a Luttinger liquid.
This numerically exact method has no finite-size limitations and allows to
simulate the whole temperature range. Focusing on the impurity magnetic
susceptibility, we determine the scaling functions, in particular for
temperatures well below the Kondo temperature. In the absence of elastic
potential scattering, we find Fermi-liquid behavior for strong
electron-electron interactions, g_c < 1/2, and anomalous power laws for 1/2<g_c
< 1, where g_c is the correlation parameter of the Luttinger liquid. These
findings resolve a recent controversy. If elastic potential scattering is
present, we find a logarithmically divergent impurity susceptibility at g_c<1/2
which can be rationalized in terms of the two-channel Kondo model.Comment: 11 pages REVTeX, incl. 9 PS figures, subm. to PR
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