2,517 research outputs found
Paramagnetic Breakdown of Superconductivity in Ultrasmall Metallic Grains
We study the magnetic-field-induced breakdown of superconductivity in
nm-scale metal grains having a mean electron level spacing (bulk gap). Using a generalized variational BCS approach that
yields good qualitative agreement with measured spectra, we argue that Pauli
paramagnetism dominates orbital diamagnetism, as in the case of thin films in a
parallel magnetic field. However, the first-order transition observed for the
latter can be made continuous by finite size effects. The mean-field procedure
of describing the system by a single pairing parameter breaks down for
.Comment: 4 pages of revtex, 3 postscript figures, uses psfrag.sty, epsfig.sty.
Slightly revised and improved version, matching published versio
Ultraviolet Imagery of NGC 6752: A Test of Extreme Horizontal Branch Models
We present a 1620 A image of the nearby globular cluster NGC 6752 obtained
with the Ultraviolet Imaging Telescope (UIT) during the Astro-2 mission of the
Space Shuttle Endeavour in 1995 March. An ultraviolet-visible color-magnitude
diagram (CMD) is derived for 216 stars matched with the visible photometry of
Buonanno et al. (1986). This CMD provides a nearly complete census of the hot
horizontal branch (HB) population with good temperature and luminosity
discrimination for comparison with theoretical tracks. The observed data show
good agreement with the theoretical zero-age horizontal branch (ZAHB) of
Sweigart (1996) for an assumed reddening of E(B-V) = 0.05 and a distance
modulus of 13.05. The observed HB luminosity width is in excellent agreement
with the theoretical models and supports the single star scenario for the
origin of extreme horizontal branch (EHB) stars. However, only four stars can
be identified as post-EHB stars, whereas almost three times this many are
expected from the HB number counts. If this effect is not a statistical
anomaly, then some non-canonical effect may be decreasing the post-EHB
lifetime. The recent non-canonical models of Sweigart (1996), which have
helium-enriched envelopes due to mixing along the red giant branch, cannot
explain the deficit of post-EHB stars, but might be better able to explain
their luminosity distribution.Comment: 14 pages, AASTeX, includes 4 EPS figures ApJ Letters accepte
Parity Effect in Ground State Energies of Ultrasmall Superconducting Grains
We study the superconductivity in small grains in the regime when the quantum
level spacing is comparable to the gap . As
is increased, the system crosses over from superconducting
to normal state. This crossover is studied by calculating the dependence of the
ground state energy of a grain on the parity of the number of electrons. The
states with odd numbers of particles carry an additional energy ,
which shows non-monotonic dependence on . Our predictions
can be tested experimentally by studying the parity-induced alternation of
Coulomb blockade peak spacings in grains of different sizes.Comment: 4 pages, revtex, multicol.st
Thermoelectric refrigerator
A refrigerator is provided which combines the benefits of superinsulation materials with thermoelectric devices and phase change materials to provide an environmentally benign system that is energy efficient and can maintain relatively uniform temperatures for extended periods of time with relatively low electrical power requirements. The refrigerator includes a thermoelectric assembly having a thermoelectric device with a hot sink and a cold sink. The superinsulation materials include a plurality of vacuum panels. The refrigerator is formed from an enclosed structure having a door. The vacuum panels may be contained within the walls of the enclosed structure and the door. By mounting the thermoelectric assembly on the door, the manufacturer of the enclosed structure is simplified and the overall R rating of the refrigerator increased. Also an electrical motor and propellers may be mounted on the door to assist in the circulation of air to improve the efficiency of the cold sink and the hot sink. A propeller and/or impeller is preferably mounted within the refrigerator to assist in establishing the desired air circulation flow path
Superconductivity in Ultrasmall Metallic Grains
We develop a theory of superconductivity in ultrasmall (nm-scale) metallic
grains having a discrete electronic eigenspectrum with a mean level spacing of
order of the bulk gap. The theory is based on calculating the eigenspectrum
using a generalized BCS variational approach, whose applicability has been
extensively demonstrated in studies of pairing correlations in nuclear physics.
We discuss how conventional mean field theory breaks down with decreasing
sample size, how the so-called blocking effect weakens pairing correlations in
states with non-zero total spin, and how this affects the discrete
eigenspectrum's behavior in a magnetic field, which favors non-zero total spin.
In ultrasmall grains, spin magnetism dominates orbital magnetism, just as in
thin films in a parallel field; but whereas in the latter the magnetic-field
induced transition to a normal state is known to be first-order, we show that
in ultrasmall grains it is softened by finite size effects. Our calculations
qualitatively reproduce the magnetic-field dependent tunneling spectra for
individual aluminum grains measured recently by Ralph, Black and Tinkham. We
argue that previously-discussed parity effects for the odd-even ground state
energy difference are presently not observable for experimental reasons, and
propose an analogous parity effect for the pair-breaking energy that should be
observable provided that the grain size can be controlled sufficiently well.
Finally, experimental evidence is pointed out that the dominant role played by
time-reversed pairs of states, well-established in bulk and in dirty
superconductors, persists also in ultrasmall grains.Comment: 21 pages RevTeX, 12 EPS figures included, uses epsf.st
What makes you not a Sikh? : a preliminary mapping of values
This study sets out to establish which Sikh values contrasted with or were shared by non-Sikh adolescents. A survey of attitude toward a variety of Sikh values was fielded in a sample of 364 non-Sikh schoolchildren aged between 13 and 15 in London. Values where attitudes were least positive concerned Sikh duties/code of conduct, festivals, rituals, prayer Gurdwara attendance, listening to scripture recitation, the amrit initiation. Sikh values empathized with by non-Sikhs concerned family pride, charity, easy access to ordination and Gurdwaras, maintaining the five Ks, seeing God in all things, abstaining from meat and alcohol and belief in the stories of Guru Nanak. Further significant differences of attitude toward Sikhism were found in comparisons by sex, age and religious affiliation. Findings are applied to teaching Sikhism to pupils of no faith adherence. The study recommends the extension of values mapping to specifically Sikh populations
Re-entrant spin susceptibility of a superconducting grain
We study the spin susceptibility chi of a small, isolated superconducting
grain. Due to the interplay between parity effects and pairing correlations,
the dependence of chi on temperature T is qualitatively different from the
standard BCS result valid in the bulk limit. If the number of electrons on the
grain is odd, chi shows a re-entrant behavior as a function of temperature.
This behavior persists even in the case of ultrasmall grains where the mean
level spacing is much larger than the BCS gap. If the number of electrons is
even, chi(T) is exponentially small at low temperatures.Comment: 9 pages, 3 figures. To be published in PR
Evaluation of Beam Losses and Energy Depositions for a Possible Phase II Design for LHC Collimation
The LHC beams are designed to have high stability and to be stored for many hours. The nominal beam intensity lifetime is expected to be of the order of 20h. The Phase II collimation system has to be able to handle particle losses in stable physics conditions at 7 TeV in order to avoid beam aborts and to allow correction of parameters and restoration to nominal conditions. Monte Carlo simulations are needed in order to evaluate the behavior of metallic high-Z collimators during operation scenarios using a realistic distribution of losses, which is a mix of the three limiting halo cases. Moreover, the consequences in the IR7 insertion of the worst (case) abnormal beam loss are evaluated. The case refers to a spontaneous trigger of the horizontal extraction kicker at top energy, when Phase II collimators are used. These studies are an important input for engineering design of the collimation Phase II system and for the evaluation of their effect on adjacent components. The goal is to build collimators that can survive the expected conditions during LHC stable physics runs, in order to avoid quenches of the SC magnets and to protect other LHC equipments
Weak localization in disordered systems at the ballistic limit
The weak localization (WL) contribution to the two-level correlation function
is calculated for two-dimensional disordered conductors. Our analysis extends
to the nondiffusive (ballistic) regime, where the elastic mean path is of order
of the size of the system. In this regime the structure factor (the Fourier
transform of the two-point correlator) exhibits a singular behavior consisting
of dips superimposed on a smooth positive background. The strongest dips appear
at periods of the periodic orbits of the underlying clean system. Somewhat
weaker singularities appear at times which are sums of periods of two such
orbits. The results elucidate various aspects of the weak localization physics
of ballistic chaotic systems.Comment: 13 pages, 13 figure
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