5,313 research outputs found
Comment on "Clock Shift in High Field Magnetic Resonance of Atomic Hydrogen"
In this Comment, we reanalyze the experiments on the collision frequency
shift of the b-c and a-d hyperfine transitions in three-dimensional atomic
hydrogen in the presence of, respectively, a and b-state atoms. Accurate
consideration of the symmetry of the spatial and spin part of the diatomic
wavefunction yields the difference a_T-a_S=0.30(5) \AA between the triplet and
singlet s-wave scattering lengths of hydrogen atoms. This corrects the
factor-of two error of the commented work [Phys. Rev. Lett. 101, 263003
(2008)].Comment: 1 pag
Hyperfine frequency shift in two-dimensional atomic hydrogen
We propose the explanation of a surprisingly small hyperfine frequency shift
in the two-dimensional (2D) atomic hydrogen bound to the surface of superfluid
helium below 0.1 K. Owing to the symmetry considerations, the microwave-induced
triplet-singlet transitions of atomic pairs in the fully spin-polarized sample
are forbidden. The apparent nonzero shift is associated with the
density-dependent wall shift of the hyperfine constant and the pressure shift
due to the presence of H atoms in the hyperfine state not involved in the
observed transition. The interaction of adsorbed atoms with one
another effectively decreases the binding energy and, consequently, the wall
shift by the amount proportional to their density. The pressure shift of the
resonance comes from the fact that the impurity -state atoms
interact differently with the initial -state and final -state atoms and
is also linear in density. The net effect of the two contributions, both
specific for 2D hydrogen, is comparable with the experimental observation. To
our knowledge, this is the first mentioning of the density-dependent wall
shift. We also show that the difference between the triplet and singlet
scattering lengths of H atoms, pm, is exactly twice smaller
than the value reported by Ahokas {\it et al.}, Phys. Rev. Lett. {\bf101},
263003 (2008).Comment: 4 pages, no figure
Thermal compression of two-dimensional atomic hydrogen to quantum degeneracy
We describe experiments where 2D atomic hydrogen gas is compressed thermally
at a small "cold spot" on the surface of superfluid helium and detected
directly with electron-spin resonance. We reach surface densities up to 5e12
1/cm^2 at temperatures of approximately 100 mK corresponding to the maximum 2D
phase-space density of about 1.5. By independent measurements of the surface
density and its decay rate we make the first direct determination of the
three-body recombination rate constant and get the value of 2e-25 cm^4/s for
its upper bound, which is an order of magnitude smaller than previously
reported experimental results.Comment: 4 pages, 4 postscript figures, bibliography (.bbl) file, submitted to
PR
Adsorption and two-body recombination of atomic hydrogen on He-He mixture films
We present the first systematic measurement of the binding energy of
hydrogen atoms to the surface of saturated He-He mixture films.
is found to decrease almost linearly from 1.14(1) K down to 0.39(1) K, when the
population of the ground surface state of He grows from zero to
cm, yielding the value K cm
for the mean-field parameter of H-He interaction in 2D. The experiments
were carried out with overall He concentrations ranging from 0.1 ppm to 5 %
as well as with commercial and isotopically purified He at temperatures
70...400 mK. Measuring by ESR the rate constants and for
second-order recombination of hydrogen atoms in hyperfine states and we
find the ratio to be independent of the He content and to
grow with temperature.Comment: 4 pages, 4 figures, all zipped in a sigle file. Submitted to Phys.
Rev. Let
Applicability of the piecewise-linear approximation of the potential profile of undoped MQW heterostructures
Speckle Interferometry with CMOS Detector
In 2022 we carried out an upgrade of the speckle polarimeter (SPP) -- the
facility instrument of the 2.5-m telescope of the Caucasian Observatory of the
SAI MSU. During the overhaul, CMOS Hamamatsu ORCA-Quest qCMOS C15550-20UP was
installed as the main detector, some drawback of the previous version of the
instrument were eliminated. In this paper, we present a description of the
instrument, as well as study some features of the CMOS detector and ways to
take them into account in speckle interferometric processing. Quantitative
comparison of CMOS and EMCCD in the context of speckle interferometry is
performed using numerical simulation of the detection process. Speckle
interferometric observations of 25 young variable stars are given as an example
of astronomical result. It was found that BM And is a binary system with a
separation of 273 mas. The variability of the system is dominated by the
brightness variations of the main component. A binary system was also found in
NSV 16694 (TYC 120-876-1). The separation of this system is 202 mas.Comment: 24 pages, 27 figures. Minor style differences with respect to version
accepted to Astrophys. Bull. V. 78, no.
Education management in Ukraine in the context of global economic transformations
The article identifies the features of the management of higher education in Ukraine and in Europe in the context of global economic transformations. In the process of analysis, it was determined that the financing of higher education in European countries is carried out almost equally: at the expense of the State budget and at the expense of individuals.
At the same time, the outflow of students from the CIS countries to European countries can be explained by the proposed concepts of providing educational services, the main difference among which consists, firstly, in orientation towards the needs of the state, and secondly, in orientation towards the needs of business structures and various market subjects.
It was also determined that the impact of economic factors on the level of education is rather low, and at the same time, this indicator largely depends on the indicator of the social capital.
In this regard, the following social trends have been identified that need to be implemented for the successful education management: expanding the population education program, as well as providing lifelong education, ensuring equal access to quality education, strengthening the role of the state in ensuring equity in education, efficient and effective use of education costs, humanization and democratization of education, updating the content, forms, methods and means of training, enhancing the professional competence of teachers, transparency of education systems, the formation of state-public forms of education management.peer-reviewe
Low-Temperature Mobility of Surface Electrons and Ripplon-Phonon Interaction in Liquid Helium
The low-temperature dc mobility of the two-dimensional electron system
localized above the surface of superfluid helium is determined by the slowest
stage of the longitudinal momentum transfer to the bulk liquid, namely, by the
interaction of surface and volume excitations of liquid helium, which rapidly
decreases with temperature. Thus, the temperature dependence of the
low-frequency mobility is \mu_{dc} = 8.4x10^{-11}n_e T^{-20/3} cm^4 K^{20/3}/(V
s), where n_e is the surface electron density. The relation
T^{20/3}E_\perp^{-3} << 2x10^{-7} between the pressing electric field (in
kV/cm) and temperature (in K) and the value \omega < 10^8 T^5 K^{-5}s^{-1} of
the driving-field frequency have been obtained, at which the above effect can
be observed. In particular, E_\perp = 1 kV/cm corresponds to T < 70 mK and
\omega/2\pi < 30 Hz.Comment: 4 pages, 1 figur
Fluctuation-dissipation considerations and damping models for ferromagnetic materials
The role of fluctuation-dissipation relations (theorems) for the
magnetization dynamics with Landau-Lifshitz-Gilbert and Bloch-Bloembergen
damping terms are discussed. We demonstrate that the use of the Callen-Welton
fluctuation-dissipation theorem that was proven for Hamiltonian systems can
give an inconsistent result for magnetic systems with dissipation
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