1,970 research outputs found
Anomalous relaxation kinetics and charge density wave correlations in underdoped BaPb1-xBixO3
Superconductivity often emerges in proximity of other symmetry-breaking
ground states, such as antiferromagnetism or charge-density-wave (CDW) order.
However, the subtle inter-relation of these phases remains poorly understood,
and in some cases even the existence of short-range correlations for
superconducting compositions is uncertain. In such circumstances, ultrafast
experiments can provide new insights, by tracking the relaxation kinetics
following excitation at frequencies related to the broken symmetry state. Here,
we investigate the transient terahertz conductivity of BaPb1-xBixO3 - a
material for which superconductivity is adjacent to a competing CDW phase -
after optical excitation tuned to the CDW absorption band. In insulating BaBiO3
we observed an increase in conductivity and a subsequent relaxation, which are
consistent with quasiparticles injection across a rigid semiconducting gap. In
the doped compound BaPb0.72Bi0.28O3 (superconducting below Tc=7K), a similar
response was also found immediately above Tc. This observation evidences the
presence of a robust gap up to T=40 K, which is presumably associated with
short-range CDW correlations. A qualitatively different behaviour was observed
in the same material fo T>40 K. Here, the photo-conductivity was dominated by
an enhancement in carrier mobility at constant density, suggestive of melting
of the CDW correlations rather than excitation across an optical gap. The
relaxation displayed a temperature dependent, Arrhenius-like kinetics,
suggestive of the crossing of a free-energy barrier between two phases. These
results support the existence of short-range CDW correlations above Tc in
underdoped BaPb1-xBixO3, and provide new information on the dynamical interplay
between superconductivity and charge order.Comment: 19 pages, 4 figure
Integral measurement of the 12C(n, p)12B reaction up to 10 GeV
The integral measurement of the 12C(n, p)12B reaction was performed at the neutron time-offlight facility n TOF at CERN. The total number of 12B nuclei produced per neutron pulse of the n TOF
beam was determined using the activation technique in combination with a time-of-flight technique. The
cross section is integrated over the n TOF neutron energy spectrum from reaction threshold at 13.6 MeV
to 10 GeV. Having been measured up to 1 GeV on basis of the 235U(n, f) reaction, the neutron energy
spectrum above 200 MeV has been re-evaluated due to the recent extension of the cross section reference
for this particular reaction, which is otherwise considered a standard up to 200 MeV. The results from the
dedicated GEANT4 simulations have been used to evaluate the neutron flux from 1 GeV up to 10 GeV. The
experimental results related to the 12C(n, p)12B reaction are compared with the evaluated cross sections
from major libraries and with the predictions of different GEANT4 models, which mostly underestimate
the 12B production. On the contrary, a good reproduction of the integral cross section derived from measurements is obtained with TALYS-1.6 calculations, with optimized parameters.European Atomic Energy Communitys (Euratom) Seventh Framework Programme FP7/2007-2011-CHANDA (No. 605203)Narodowe Centrum Nauki (NCN)-UMO-2012/04/M/ST2/00700Croatian Science Foundation-No. 168
Silicon-based three-dimensional microstructures for radiation dosimetry in hadrontherapy
In this work, we propose a solid-state-detector for use in radiation microdosimetry. This device improves the performance of existing dosimeters using customized 3D-cylindrical microstructures etched inside silicon. The microdosimeter consists of an array of micro-sensors that have 3D-cylindrical electrodes of 15 μm diameter and a depth of 5 μm within a silicon membrane, resulting in a well-defined micrometric radiation sensitive volume. These microdetectors have been characterized using an 241Am source to assess their performance as radiation detectors in a high-LET environment. This letter demonstrates the capability of this microdetector to be used to measure dose and LET in hadrontherapy centers for treatment plan verification as part of their patient-specific quality control program
Strong and auxiliary forms of the semi-Lagrangian method for incompressible flows
We present a review of the semi-Lagrangian method for advection-diusion and incompressible Navier-Stokes equations discretized with high-order methods. In particular, we compare the strong form where the departure points are computed directly via backwards integration with the auxiliary form where an auxiliary advection equation is solved instead; the latter is also referred to as Operator Integration Factor Splitting (OIFS) scheme. For intermediate size of time steps the auxiliary form is preferrable but for large time steps only the strong form is stable
Discontinuous galerkin spectral/hp element modelling of dispersive shallow water systems
Phenomenology of a three-family model with gauge symmetry SU(3)_c X SU(4)_L X U(1)_X
We study an extension of the gauge group SU(3)_c X SU(2)_L X U(1)_Y of the
standard model to the symmetry group SU(3)_c X SU(4)_L X U(1)_X (3-4-1 for
short). This extension provides an interesting attempt to answer the question
of family replication in the sense that models for the electroweak interaction
can be constructed so that anomaly cancellation is achieved by an interplay
between generations, all of them under the condition that the number of
families must be divisible by the number of colours of SU(3)_c. This method of
anomaly cancellation requires a family of quarks transforming differently from
the other two, thus leading to tree-level flavour changing neutral currents
(FCNC) transmitted by the two extra neutral gauge bosons and
predicted by the model. In a version of the 3-4-1 extension, which does not
contain particles with exotic electric charges, we study the fermion mass
spectrum and some aspects of the phenomenology of the neutral gauge boson
sector. In particular, we impose limits on the mixing angle and on the
mass scale of the corresponding physical new neutral gauge boson , and
establish a lower bound on the mass of the additional new neutral gauge boson
. For the analysis we use updated precision electroweak data at
the Z-pole from the CERN LEP and SLAC Linear Collider, and atomic parity
violation data. The mass scale of the additional new neutral gauge boson
is constrained by using updated experimental inputs from neutral meson mixing
in the analysis of the sources of FCNC in the model. The data constrain the
mixing angle to a very small value of O(0.001), and the lower bounds on
and on are found to be of O(1 TeV) and of O(7 TeV),
repectively.Comment: 22 pages, 6 tables, 1 figure. To appear in J. Phys. G: Nuclear and
Particle Physic
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