4,317 research outputs found
Dual-mode operation of a neutron source, a concept
Pulsed neutron source operates in conjunction with a photomultiplier tube coupled to a gamma ray scintillation crystal. This allows measurements of gamma radiation from both inelastic scattering and thermal neutron capture in a single experiment
Neutron die-away experiment for lunar and planetary surface analysis Final report, 26 Jul. 1966 - 26 Mar. 1967
Neutron and gamma ray die-away experiment for lunar and planetary surface analysi
Positronium S state spectrum: analytic results at O(m alpha^6)
We present an analytic calculation of the O(m alpha^6) recoil and radiative
recoil corrections to energy levels of positronium nS states and their
hyperfine splitting. A complete analytic formula valid to O(m alpha^6) is given
for the spectrum of S states. Technical aspects of the calculation are
discussed in detail. Theoretical predictions are given for various energy
intervals and compared with experimental results.Comment: 29 pages, revte
The "recoil" correction of order to hyperfine splitting of positronium ground state
The "recoil" correction of order to the hyperfine splitting of
positronium ground state was found. The formalism employed is based on the
noncovariant perturbation theory in QED. Equation for two-particle component of
full (many-body) wave function is used, in which effective Hamiltonian depends
on the energy of a system. The effective Hamiltonian is not restricted to the
nonrelativistic region, so there is no need in any regularization. To evaluate
integrals over loop momenta, they are divided into "hard" and "soft" parts,
coming from large and small momenta respectively. Soft contributions were found
analytically, and hard ones are evaluated by numerical integration. Some soft
terms due to the retardation cancel each other. To calculate the "hard"
contributions, a great number of noncovariant graphs is replaced by only a few
covariant ones. The hard contribution was found in two ways. The first way is
to evaluate contributions of separate graphs, using the Coulomb gauge. The
second one is to calculate full hard contribution as a whole using the Feynman
gauge. The final result for the "recoil" correction is 0.381(6) m\al^6 and
agrees with those of previous papers. Diagram-to-diagram comparison with the
revised results of Adkins&Sapirstein was done. All the results agree, so the
"recoil" correction is now firmly established. This means a considerable
disagreement with the experimental data.Comment: 28 pages, latex including latex figure
Resistivity peak values at transition between fractional quantum Hall states
Experimental data available in the literature for peak values of the diagonal
resistivity in the transitions between fractional quantum Hall states are
compared with the theoretical predictions. It is found that the majority of the
peak values are close to the theoretical values for two-dimensional systems
with moderate mobilities.Comment: 3 pages, 1 figur
The metallic resistance of a dilute two-dimensional hole gas in a GaAs quantum well: two-phase separation at finite temperature?
We have studied the magnetotransport properties of a high mobility
two-dimensional hole gas (2DHG) system in a 10nm GaAs quantum well (QW) with
densities in range of 0.7-1.6*10^10 cm^-2 on the metallic side of the
zero-field 'metal-insulator transition' (MIT). In a parallel field well above
B_c that suppresses the metallic conductivity, the 2DHG exhibits a conductivity
g(T)~0.3(e^2/h)lnT reminiscent of weak localization. The experiments are
consistent with the coexistence of two phases in our system: a metallic phase
and a weakly insulating Fermi liquid phase having a percolation threshold close
to B_c
Complete Result for Positronium Energy Levels at Order alpha^6 m
We have completed theoretical predictions for positronium energy levels
through order alpha^6 m by the calculation of the spin independent, radiative
recoil correction. This contribution is significant and amounts to 10.64 MHz
for the 1S state. We further perform detailed comparison of theoretical
predictions to experimental results for 1S-2S and 2S-2P transitions.Comment: 9 pages, 2 tables. Email: [email protected]
Spin-orbit interaction and the 'metal-insulator' transition observed in two-dimensional hole systems
We present calculations of the spin and phase relaxation rates in GaAs/AlGaAs
-type quantum wells. These rates are used to derive the temperature
dependence of the weak-localization correction to the conductivity. In -type
quantum wells both weak localization and weak anti-localization are present due
to the strong spin-orbit interaction. When determining the total conductivity
correction one also have to include the term due to hole-hole interaction. The
magnitude of the latter depends on the ratio between the thermal energy and the
Fermi energy, and whether the system can be considered
as ballistic or diffusive (). We argue that due to the relatively low Fermi energy
and the moderate mobilities, in the -type systems in question, the
conductivity correction arising from hole-hole interactions is negligible at
the highest temperatures accessible in the experiments. Hence the
'metal-insulator' transition observed at these relatively high temperatures
could be caused by interference effects. We compare our calculations of the
weak anti-localization correction with the experimental results from different
independent groups with special emphasis on the experiments by Simmons et al.
We find good agreement between predicted and observed transistion density
.Comment: 6 pages, 4 figures. Accepted to PRB (15 June, 2002
Scanning-probe spectroscopy of semiconductor donor molecules
Semiconductor devices continue to press into the nanoscale regime, and new
applications have emerged for which the quantum properties of dopant atoms act
as the functional part of the device, underscoring the necessity to probe the
quantum structure of small numbers of dopant atoms in semiconductors[1-3].
Although dopant properties are well-understood with respect to bulk
semiconductors, new questions arise in nanosystems. For example, the quantum
energy levels of dopants will be affected by the proximity of nanometer-scale
electrodes. Moreover, because shallow donors and acceptors are analogous to
hydrogen atoms, experiments on small numbers of dopants have the potential to
be a testing ground for fundamental questions of atomic and molecular physics,
such as the maximum negative ionization of a molecule with a given number of
positive ions[4,5]. Electron tunneling spectroscopy through isolated dopants
has been observed in transport studies[6,7]. In addition, Geim and coworkers
identified resonances due to two closely spaced donors, effectively forming
donor molecules[8]. Here we present capacitance spectroscopy measurements of
silicon donors in a gallium-arsenide heterostructure using a scanning probe
technique[9,10]. In contrast to the work of Geim et al., our data show
discernible peaks attributed to successive electrons entering the molecules.
Hence this work represents the first addition spectrum measurement of dopant
molecules. More generally, to the best of our knowledge, this study is the
first example of single-electron capacitance spectroscopy performed directly
with a scanning probe tip[9].Comment: In press, Nature Physics. Original manuscript posted here; 16 pages,
3 figures, 5 supplementary figure
Benchmark problems for robust control design
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76996/1/AIAA-20949-475.pd
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