207 research outputs found
Radiation studies for GaAs in the ATLAS Inner Detector
We estimate the hardness factors and the equivalent 1 MeV neutron fluences
for hadrons fluences expected at the GaAs positions wheels in the ATLAS Inner
Detector. On this basis the degradation of the GaAs particle detectors made
from different substrates as a function of years LHC operation is predicted.Comment: 11 pages, 6 Postscript figures, uses elsart.cls, submitted to Nucl.
Inst. and Met
Conductance of the single-electron transistor: A comparison of experimental data with Monte Carlo calculations
We report on experimental results for the conductance of metallic
single-electron transistors as a function of temperature, gate voltage and
dimensionless conductance. In contrast to previous experiments our transistor
layout allows for a direct measurement of the parallel conductance and no ad
hoc assumptions on the symmetry of the transistors are necessary. Thus we can
make a comparison between our data and theoretical predictions without any
adjustable parameter. Even for rather weakly conducting transistors significant
deviations from the perturbative results are noted. On the other hand, path
integral Monte Carlo calculations show remarkable agreement with experiments
for the whole range of temperatures and conductances.Comment: 8 pages, 7 figures, revtex4, corrected typos, submitted to PR
Self-Energy Correction to the Two-Photon Decay Width in Hydrogenlike Atoms
We investigate the gauge invariance of the leading logarithmic radiative
correction to the two-photon decay width in hydrogenlike atoms. It is shown
that an effective treatment of the correction using a Lamb-shift "potential"
leads to equivalent results in both the length as well as the velocity gauges
provided all relevant correction terms are taken into account. Specifically,
the relevant radiative corrections are related to the energies that enter into
the propagator denominators, to the Hamiltonian, to the wave functions, and to
the energy conservation condition that holds between the two photons; the form
of all of these effects is different in the two gauges, but the final result is
shown to be gauge invariant, as it should be. Although the actual calculation
only involves integrations over nonrelativistic hydrogenic Green functions, the
derivation of the leading logarithmic correction can be regarded as slightly
more complex than that of other typical logarithmic terms. The dominant
radiative correction to the 2S two-photon decay width is found to be -2.020536
(alpha/pi) (Zalpha)^2 ln[(Zalpha)^-2] in units of the leading nonrelativistic
expression. This result is in agreement with a length-gauge calculation [S. G.
Karshenboim and V. G. Ivanov, e-print physics/9702027], where the coefficient
was given as -2.025(1).Comment: 9 pages, RevTe
AC-coupled GaAs microstrip detectors with a new type of integrated bias resistors
Full size single-sided GaAs microstrip detectors with integrated coupling
capacitors and bias resistors have been fabricated on 3'' substrate wafers.
PECVD deposited SiO_2 and SiO_2/Si_3N_4 layers were used to provide coupling
capacitaces of 32.5 pF/cm and 61.6 pF/cm, respectively. The resistors are made
of sputtered CERMET using simple lift of technique. The sheet resistivity of 78
kOhm/sq. and the thermal coefficient of resistance of less than 4x10^-3 /
degree C satisfy the demands of small area biasing resistors, working on a wide
temperature range.Comment: 20 pages, 9 figures, to be published in NIM
Coulomb blockade in one-dimensional arrays of high conductance tunnel junctions
Properties of one-dimensional (1D) arrays of low Ohmic tunnel junctions (i.e.
junctions with resistances comparable to, or less than, the quantum resistance
k) have been studied experimentally
and theoretically. Our experimental data demonstrate that -- in agreement with
previous results on single- and double-junction systems -- Coulomb blockade
effects survive even in the strong tunneling regime and are still clearly
visible for junction resistances as low as 1 k. We have developed a
quasiclassical theory of electron transport in junction arrays in the strong
tunneling regime. Good agreement between the predictions of this theory and the
experimental data has been observed. We also show that, due to both heating
effects and a relatively large correction to the linear relation between the
half-width of the conductance dip around zero bias voltage, , and the
measured electronic temperature, such arrays are inferior to those
conventionally used in the Coulomb Blockade Thermometry (CBT). Still, the
desired correction to the half-width, , can be determined
rather easily and it is proportional to the magnitude of the conductance dip
around zero bias voltage, . The constant of proportionality is a
function of the ratio of the junction and quantum resistances, ,
and it is a pure strong tunneling effect.Comment: LaTeX file + five postscript figure
On a biphononic origin of the 1125 cm^(-1) absorption band in cuprous oxide
We report on the IR spectroscopic studies in both reflection (50-900 cm^{-1})
and transmission (900-3000 cm^{-1}) mode of the vibration spectrum of the
cuprous oxide. A detailed analysis based on a comparison of the temperature
dependences of the absorption band at 1125 cm^{-1} and of IR and Raman active
fundamental vibrations results in assignment of the former to a biphonon.Comment: 5 pages, 5 figures (to appear in Phys.Lett. A
Nonresonant effects in one- and two-photon transitions
We investigate nonresonant contributions to resonant Rayleigh scattering
cross sections of atoms. The problematic nonresonant contributions set a limit
to the accuracy to which atomic spectra determine energy levels. We discuss the
off-resonance effects in one-photon transitions. We also show that
off-resonance contributions for the 1S-2S two-photon transition in atomic
hydrogen are negligible at current and projected levels of experimental
accuracy. The possibility of a differential measurement for the detection of
off-resonance effects in one-photon transitions in atomic hydrogen is
discussed.Comment: 13 pages, LaTeX, 3 figures; submitted to Can. J. Phys. (Oct 2001);
discussion of one-photon transitions enhance
Magnetic field effects in energy relaxation mediated by Kondo impurities
We study the energy distribution function of quasiparticles in voltage biased
mesoscopic wires in presence of magnetic impurities and applied magnetic field.
The system is described by a Boltzmann equation where the collision integral is
determined by coupling to spin 1/2 impurities. We derive an effective coupling
to a dissipative spin system which is valid well above Kondo temperature in
equilibrium or for sufficiently smeared distribution functions in
non-equilibrium. For low magnetic field an enhancement of energy relaxation is
found whereas for larger magnetic fields the energy relaxation decreases again
meeting qualitatively the experimental findings by Anthore et al.
(cond-mat/0109297). This gives a strong indication that magnetic impurities are
in fact responsible for the enhanced energy relaxation in copper wires. The
quantitative comparison, however, shows strong deviations for energy relaxation
with small energy transfer whereas the large energy transfer regime is in
agreement with our findings.Comment: 14 pages, 8 figure
Ly alpha escape during cosmological hydrogen recombination: the 3d-1s and 3s-1s two-photon processes
We give a formulation of the radiative transfer equation for Lyman alpha
photons which allows us to include the two-photon corrections for the 3s-1s and
3d-1s decay channels during cosmological hydrogen recombination. We use this
equation to compute the corrections to the Sobolev escape probability for Lyman
alpha photons during hydrogen recombination, which then allow us to calculate
the changes in the free electron fraction and CMB temperature and polarization
power spectra. We show that the effective escape probability changes by DP/P ~+
11% at z~1400 in comparison with the one obtained using the Sobolev
approximation. This speeds up of hydrogen recombination by DN_e/N_e ~- 1.6% at
z~1190, implying |DC_l/C_l| ~1%-3% at l >~ 1500 with shifts in the positions of
the maxima and minima in the CMB power spectra. These corrections will be
important for the analysis of future CMB data.
The total correction is the result of the superposition of three independent
processes, related to (i) time-dependent aspects of the problem, (ii)
corrections due to quantum mechanical deviations in the shape of the emission
and absorption profiles in the vicinity of the Lyman alpha line from the normal
Lorentzian, and (iii) a thermodynamic correction factor, which occurs to be
very important. All these corrections are neglected in the
Sobolev-approximation, but they are important in the context of future CMB
observations. All three can be naturally obtained in the two-photon formulation
of the Lyman alpha absorption process. However, the corrections (i) and (iii)
can also be deduced in the normal '1+1' photon language, without necessarily
going to the two-photon picture. Therefore only (ii) is really related to the
quantum mechanical aspects of the two-photon process (abridged)Comment: 30 pages, 21 figures, submitted to A&
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