1,141 research outputs found
Temperature dependence of the nuclear symmetry energy
We have studied the properties of A=54 and A=64 isobars at temperatures T
\leq 2 MeV via Monte Carlo shell model calculations with two different residual
interactions. In accord with empirical indications, we find that the symmetry
energy coefficient, b_{sym}, is independent of temperature to within 0.6 MeV
for T \leq 1 MeV. This is in contrast to a recent suggestion of a 2.5 MeV
increase of b_{sym} for this temperature, which would have significantly
altered the supernova explosion scenario.Comment: 7 pages, including 2 figures, Caltech preprint MAP-17
Shell model Monte Carlo calculations for Dy-170
We present the first auxiliary field Monte Carlo calculations for a rare
earth nucleus, Dy-170. A pairing plus quadrupole Hamiltonian is used to
demonstrate the physical properties that can be studied in this region. We
calculate various static observables for both uncranked and cranked systems and
show how the shape distribution evolves with temperature. We also introduce a
discretization of the path integral that allows a more efficient Monte Carlo
sampling.Comment: 11 pages, figures available upon request, Caltech Preprint No.
MAP-16
Photoconductance Quantization in a Single-Photon Detector
We have made a single-photon detector that relies on photoconductive gain in
a narrow electron channel in an AlGaAs/GaAs 2-dimensional electron gas. Given
that the electron channel is 1-dimensional, the photo-induced conductance has
plateaus at multiples of the quantum conductance 2e/h. Super-imposed on
these broad conductance plateaus are many sharp, small, conductance steps
associated with single-photon absorption events that produce individual
photo-carriers. This type of photoconductive detector could measure a single
photon, while safely storing and protecting the spin degree of freedom of its
photo-carrier. This function is valuable for a quantum repeater that would
allow very long distance teleportation of quantum information.Comment: 4 pages, 4 figure
A high resolution imaging detector for TeV gamma-ray astronomy
Details are presented of an atmospheric Cherenkov telescope for use in very high energy gamma-ray astronomy which consists of a cluster of 109 close-packed photomultiplier tubes at the focus of a 10 meter optical reflector. The images of the Cherenkov flashes generated both by gamma-ray and charged cosmic-ray events are digitized and recorded. Subsequent off-line analysis of the images improves the significance of the signal to noise ratio by a factor of 10 compared with non-imaging techniques
Struggling for food in a time of crisis: Responsibility and paradox
Responsibility is a useful lens through which to examine the current state of food poverty in the UK in the context of the Covidâ19 crisis, noting that this concept contains several paradoxes. Currently, responsibility involves the voluntary sector, the food industry and the state, a situation which the author has been exploring for the last five years in an ethnographic study of food poverty and food aid in the UK. Food aid organizations, especially food banks, have mushroomed during the period of austerity. This reveals the first paradox: namely, that the existence of food banks conveys the message that âsomething is being doneâ, but in actuality this is very far from being sufficient to meet the needs of either the âoldâ or ânewâ food insecure. The second paradox is that at the onset of the crisis, a government which had been responsible for inflicting austerity on the country for 10 years, dramatically reversed some of its policies. However, predictably, this did not change the situation visâĂ âvis food insecurity. The third paradox is that the frequent rhetoric invoking the two world wars has not resulted in lessons being learned â notably, the creation of a ministry to deal with food and rationing, as in the Second World War. The final paradox relates to Brexit and its likely deleterious effects on food security, particularly if no âdealâ is achieved with the European Union, as seems likely. The voluntary food aid sector, try as it may, cannot possibly assume responsibility for the longâstanding and now hugely increased problems of food insecurity. That belongs to the state
Pairing correlations in N~Z pf-shell nuclei
We perform Shell Model Monte Carlo calculations to study pair correlations in
the ground states of nuclei with masses A=48-60. We find that ,
proton-neutron correlations play an important, and even dominant
role, in the ground states of odd-odd nuclei, in agreement with
experiment. By studying pairing in the ground states of Fe, we
observe that the isovector proton-neutron correlations decrease rapidly with
increasing neutron excess. In contrast, both the proton, and trivially the
neutron correlations increase as neutrons are added.
We also study the thermal properties and the temperature dependence of pair
correlations for Mn and Fe as exemplars of odd-odd and even-even
nuclei. While for Fe results are similar to those obtained for
other even-even nuclei in this mass range, the properties of Mn at low
temperatures are strongly influenced by isovector neutron-proton pairing. In
coexistence with these isovector pair correlations, our calculations also
indicate an excess of isoscalar proton-neutron pairing over the mean-field
values. The isovector neutron-proton correlations rapidly decrease with
temperatures and vanish for temperatures above keV, while the isovector
correlations among like nucleons persist to higher temperatures. Related to the
quenching of the isovector proton-neutron correlations, the average isospin
decreases from 1, appropriate for the ground state, to 0 as the temperature
increases
Electrostatic potential profiles of molecular conductors
The electrostatic potential across a short ballistic molecular conductor
depends sensitively on the geometry of its environment, and can affect its
conduction significantly by influencing its energy levels and wave functions.
We illustrate some of the issues involved by evaluating the potential profiles
for a conducting gold wire and an aromatic phenyl dithiol molecule in various
geometries. The potential profile is obtained by solving Poisson's equation
with boundary conditions set by the contact electrochemical potentials and
coupling the result self-consistently with a nonequilibrium Green's function
(NEGF) formulation of transport. The overall shape of the potential profile
(ramp vs. flat) depends on the feasibility of transverse screening of electric
fields. Accordingly, the screening is better for a thick wire, a multiwalled
nanotube or a close-packed self-assembled monolayer (SAM), in comparison to a
thin wire, a single-walled nanotube or an isolated molecular conductor. The
electrostatic potential further governs the alignment or misalignment of
intramolecular levels, which can strongly influence the molecular I-V
characteristic. An external gate voltage can modify the overall potential
profile, changing the current-voltage (I-V) characteristic from a resonant
conducting to a saturating one. The degree of saturation and gate modulation
depends on the metal-induced-gap states (MIGS) and on the electrostatic gate
control parameter set by the ratio of the gate oxide thickness to the channel
length.Comment: to be published in Phys. Rev. B 69, No.3, 0353XX (2004
On the temperature dependence of the symmetry energy
We perform large-scale shell model Monte Carlo (SMMC) calculations for many
nuclei in the mass range A=56-65 in the complete pfg_{9/2}d_{5/2} model space
using an effective quadrupole-quadrupole+pairing residual interaction. Our
calculations are performed at finite temperatures between T=0.33-2 MeV. Our
main focus is the temperature dependence of the symmetry energy which we
determine from the energy differences between various isobaric pairs with the
same pairing structure and at different temperatures. Our SMMC studies are
consistent with an increase of the symmetry energy with temperature. We also
investigate possible consequences for core-collapse supernovae events
Models for Enhanced Absorption in Inhomogeneous Superconductors
We discuss the low-frequency absorption arising from quenched inhomogeneity
in the superfluid density rho_s of a model superconductor. Such inhomogeneities
may arise in a high-T_c superconductor from a wide variety of sources,
including quenched random disorder and static charge density waves such as
stripes. Using standard classical methods for treating randomly inhomogeneous
media, we show that both mechanisms produce additional absorption at finite
frequencies. For a two-fluid model with weak mean-square fluctuations <(d
rho_s)^2 > in rho_s and a frequency-independent quasiparticle conductivity, the
extra absorption has oscillator strength proportional to the quantity <(d
rho_s)^2>/rho_s, as observed in some experiments. Similar behavior is found in
a two-fluid model with anticorrelated fluctuations in the superfluid and normal
fluid densities. The extra absorption typically occurs as a Lorentzian centered
at zero frequency. We present simple model calculations for this extra
absorption under conditions of both weak and strong fluctuations. The relation
between our results and other model calculations is briefly discussed
Pairing correlations and transitions in nuclear systems
We discuss several pairing-related phenomena in nuclear systems, ranging from
superfluidity in neutron stars to the gradual breaking of pairs in finite
nuclei. We describe recent experimental evidence that points to a relation
between pairing and phase transitions (or transformations) in finite nuclear
systems. A simple pairing interaction model is used in order to study and
classify an eventual pairing phase transition in finite fermionic systems such
as nuclei. We show that systems with as few as 10-16 fermions can exhibit clear
features reminiscent of a phase transition.Comment: Proceedings of COMEX1, Sorbonne, Paris, june 10-13 2003. To appear in
Nuclear Physics
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