5,446 research outputs found
Ideal Bose gas in fractal dimensions and superfluid He in porous media
Physical properties of ideal Bose gas with the fractal dimensionality between
D=2 and D=3 are theoretically investigated. Calculation shows that the
characteristic features of the specific heat and the superfluid density of
ideal Bose gas in fractal dimensions are strikingly similar to those of
superfluid Helium-4 in porous media. This result indicates that the geometrical
factor is dominant over mutual interactions in determining physical properties
of Helium-4 in porous media.Comment: 13 pages, 6 figure
Broadband high-resolution x-ray frequency combs
Optical frequency combs have had a remarkable impact on precision
spectroscopy. Enabling this technology in the x-ray domain is expected to
result in wide-ranging applications, such as stringent tests of astrophysical
models and quantum electrodynamics, a more sensitive search for the variability
of fundamental constants, and precision studies of nuclear structure.
Ultraprecise x-ray atomic clocks may also be envisaged. In this work, an x-ray
pulse-shaping method is put forward to generate a comb in the absorption
spectrum of an ultrashort high-frequency pulse. The method employs an
optical-frequency-comb laser, manipulating the system's dipole response to
imprint a comb on an excited transition with a high photon energy. The
described scheme provides higher comb frequencies and requires lower
optical-comb peak intensities than currently explored methods, preserves the
overall width of the optical comb, and may be implemented by presently
available x-ray technology
Lorentz meets Fano spectral line shapes: A universal phase and its laser control
Symmetric Lorentzian and asymmetric Fano line shapes are fundamental
spectroscopic signatures that quantify the structural and dynamical properties
of nuclei, atoms, molecules, and solids. This study introduces a universal
temporal-phase formalism, mapping the Fano asymmetry parameter q to a phase
{\phi} of the time-dependent dipole-response function. The formalism is
confirmed experimentally by laser-transforming Fano absorption lines of
autoionizing helium into Lorentzian lines after attosecond-pulsed excitation.
We also prove the inverse, the transformation of a naturally Lorentzian line
into a Fano profile. A further application of this formalism amplifies
resonantly interacting extreme-ultraviolet light by quantum-phase control. The
quantum phase of excited states and its response to interactions can thus be
extracted from line-shape analysis, with scientific applications in many
branches of spectroscopy.Comment: 11 pages, 4 figure
Association Between {HIV}-1 Coreceptor Usage and Resistance to Broadly Neutralizing Antibodies
Background: Recently discovered broadly neutralizing antibodies have revitalized hopes of developing a universal vaccine against HIV-1. Mainly responsible for new infections are variants only using CCR5 for cell entry, whereas CXCR4-using variants can become dominant in later infection stages. Methods: We performed a statistical analysis on two different previously published data sets. The first data set was a panel of 199 diverse HIV-1 isolates for which IC50 neutralization titers were determined for the broadly neutralizing antibodies VRC01, VRC-PG04, PG9, and PG16. The second data set contained env sequences of viral variants extracted from HIV-1–infected humanized mice treated with the antibody PGT128 and from untreated control mice. Results: For the panel of 199 diverse HIV-1 isolates, we found a statistically significant association between viral resistance to PG9 and PG16 and CXCR4 coreceptor usage (P = 0.0011 and P = 0.0010, respectively). Our analysis of viral variants from HIV-1–infected humanized mice under treatment with the broadly neutralizing antibody PGT128 indicated that certain antibodies might drive a viral population toward developing CXCR4 coreceptor usage capability (P = 0.0011 for the comparison between PGT128 and control measurement). Conclusions: These analyses highlight the importance of accounting for a possible coreceptor usage bias pertaining to the effectiveness of an HIV vaccine and to passive antibody transfer as therapeutic approach
Phase reconstruction of strong-field excited systems by transient-absorption spectroscopy
We study the evolution of a V-type three-level system, whose two resonances
are coherently excited and coupled by two ultrashort laser pump and probe
pulses, separated by a varying time delay. We relate the quantum dynamics of
the excited multi-level system to the absorption spectrum of the transmitted
probe pulse. In particular, by analyzing the quantum evolution of the system,
we interpret how atomic phases are differently encoded in the
time-delay-dependent spectral absorption profiles when the pump pulse either
precedes or follows the probe pulse. We experimentally apply this scheme to
atomic Rb, whose fine-structure-split 5s\,^2S_{1/2}\rightarrow 5p\,^2P_{1/2}
and 5s\,^2S_{1/2}\rightarrow 5p\,^2P_{3/2} transitions are driven by the
combined action of a pump pulse of variable intensity and a delayed probe
pulse. The provided understanding of the relationship between quantum phases
and absorption spectra represents an important step towards full time-dependent
phase reconstruction (quantum holography) of bound-state wave-packets in
strong-field light-matter interactions with atoms, molecules and solids.Comment: 5 pages, 4 figure
Precise reducer of increased durability
The results of works on creation of precise reducer being a part of spacecrafts have been presented. Modular composition of reducer construction on the basis of wave gear with intermediate rolling bodies was described. Reducer construction with modular composition of kinematic circuit was offered. In this circuit the gears with adaptive generators supporting elastic tightness in interlock were applie
Coherent control for the spherical symmetric box potential in short and intensive XUV laser fields
Coherent control calculations are presented for a spherically symmetric box
potential for non-resonant two photon transition probabilities. With the help
of a genetic algorithm (GA) the population of the excited states are maximized
and minimized. The external driving field is a superposition of three intensive
extreme ultraviolet (XUV) linearly polarized laser pulses with different
frequencies in the femtosecond duration range. We solved the quantum mechanical
problem within the dipole approximation. Our investigation clearly shows that
the dynamics of the electron current has a strong correlation with the
optimized and neutralizing pulse shape.Comment: 11 Pages 3 Figure
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