446 research outputs found
Thermal history of the string universe
Thermal history of the string universe based on the Brandenberger and Vafa's
scenario is examined. The analysis thereby provides a theoretical foundation of
the string universe scenario. Especially the picture of the initial oscillating
phase is shown to be natural from the thermodynamical point of view. A new tool
is employed to evaluate the multi state density of the string gas. This
analysis points out that the well-known functional form of the multi state
density is not applicable for the important region , and derives a
correct form of it.Comment: 39 pages, no figures, use revtex.sty, aps.sty, aps10.sty &
preprint.st
A comprehensive survey on quantum computer usage: How many qubits are employed for what purposes?
Quantum computers (QCs), which work based on the law of quantum mechanics,
are expected to be faster than classical computers in several computational
tasks such as prime factoring and simulation of quantum many-body systems. In
the last decade, research and development of QCs have rapidly advanced. Now
hundreds of physical qubits are at our disposal, and one can find several
remarkable experiments actually outperforming the classical computer in a
specific computational task. On the other hand, it is unclear what the typical
usages of the QCs are. Here we conduct an extensive survey on the papers that
are posted in the quant-ph section in arXiv and claim to have used QCs in their
abstracts. To understand the current situation of the research and development
of the QCs, we evaluated the descriptive statistics about the papers, including
the number of qubits employed, QPU vendors, application domains and so on. Our
survey shows that the annual number of publications is increasing, and the
typical number of qubits employed is about six to ten, growing along with the
increase in the quantum volume (QV). Most of the preprints are devoted to
applications such as quantum machine learning, condensed matter physics, and
quantum chemistry, while quantum error correction and quantum noise mitigation
use more qubits than the other topics. These imply that the increase in QV is
fundamentally relevant, and more experiments for quantum error correction, and
noise mitigation using shallow circuits with more qubits will take place.Comment: 14 pages, 5 figures, figures regenerate
Method of determining the optimal settings of automatic excitation regulators of synchronous machines in EPS
The stability of the electric power system can be improved by forming of the correct settings of automatic exciting regulators. Currently, there is no unified methodology of automatic exciting regulators, so analysis of their impact is still an urgent task. The article describes the approach to solving above-mentioned problem, which combines several methods. Research based on Hybrid Real Time Simulator of EPS developed in Tomsk Polytechnic University
Long-Term Density Trend in the Mesosphere and Lower Thermosphere from Occultations of the Crab Nebula with X-Ray Astronomy Satellites
We present long-term density trends of the Earth's upper atmosphere at
altitudes between 71 and 116 km, based on atmospheric occultations of the Crab
Nebula observed with X-ray astronomy satellites, ASCA, RXTE, Suzaku, NuSTAR,
and Hitomi. The combination of the five satellites provides a time period of 28
yr from 1994 to 2022. To suppress seasonal and latitudinal variations, we
concentrate on the data taken in autumn (49< doy <111) and spring (235< doy
<297) in the northern hemisphere with latitudes of 0--40 degrees. With this
constraint, local times are automatically limited either around noon or
midnight. We obtain four sets (two seasons times two local times) of density
trends at each altitude layer. We take into account variations due to a linear
trend and the 11-yr solar cycle using linear regression techniques. Because we
do not see significant differences among the four trends, we combine them to
provide a single vertical profile of trend slopes. We find a negative density
trend of roughly -5 %/decade at every altitude. This is in reasonable agreement
with inferences from settling rate of the upper atmosphere. In the 100--110 km
altitude, we found an exceptionally high density decline of about -12 %/decade.
This peak may be the first observational evidence for strong cooling due to
water vapor and ozone near 110 km, which was first identified in a numerical
simulation by Akmaev et al. (2006). Further observations and numerical
simulations with suitable input parameters are needed to establish this
feature.Comment: 38 pages, 9 figures, accepted for publication in Journal of
Geophysical Research - Space Physic
Quasiparticle excitation in and around the vortex core of underdoped YBa_2Cu_4O_8 studied by site-selective NMR
We report a site-selective ^{17}O spin-lattice relaxation rate T_1^{-1} in
the vortex state of underdoped YBa_2Cu_4O_8. We found that T_1^{-1} at the
planar sites exhibits an unusual nonmonotonic NMR frequency dependence. In the
region well outside the vortex core, T_1^{-1} cannot be simply explained by the
density of states of the Doppler-shifted quasiparticles in the d-wave
superconductor. Based on T_1^{-1} in the vortex core region, we establish
strong evidence that the local density of states within the vortex core is
strongly reduced.Comment: 5 pages, 3 figure
The falling chain of Hopkins, Tait, Steele and Cayley
A uniform, flexible and frictionless chain falling link by link from a heap
by the edge of a table falls with an acceleration if the motion is
nonconservative, but if the motion is conservative, being the
acceleration due to gravity. Unable to construct such a falling chain, we use
instead higher-dimensional versions of it. A home camcorder is used to measure
the fall of a three-dimensional version called an -slider. After
frictional effects are corrected for, its vertical falling acceleration is
found to be . This result agrees with the theoretical
value of for an ideal energy-conserving -slider.Comment: 17 pages, 5 figure
Single-top production at future ep colliders
The production of top quarks in single mode at future ep colliders is
studied, the attention being mainly focused to the case of the proposed LEPXLHC
collider. We are motivated to reanalyse such a process following the discovery
of the top quark at Fermilab. Thanks to the measurement of its mass one is now
able to establish more accurately the relevance of single top production for
itself and for many other processes to which it may act as a background. In
addition, the recent improvement of our knowledge of the quark and gluon
dynamics inside the proton now allows one to pin down the dependence of single
top production on the partonic structure functions. Both the leptonic and
hadronic decay channels of the top quark are studied and compared to the yield
of the corresponding irreducible background in presence of b-taggingComment: 28 pages, latex, epsfig, 10 postscript figures, complete paper
available at ftp://axpa.hep.phy.cam.ac.uk/moretti/cavendish_9704 and at
http://www.hep.phy.cam.ac.uk/theory/papers
Signature of band inversion in the antiferromagnetic phase of axion insulator candidate EuIn2As2
We have performed angle-resolved photoemission spectroscopy on EuIn2As2 which
is predicted to be an axion insulator in the antiferromagnetic state. By
utilizing soft-x-ray and vacuum-ultraviolet photons, we revealed a
three-dimensional hole pocket centered at the Gamma point of bulk Brillouin
zone together with a heavily hole-doped surface state in the paramagnetic
phase. Upon entering the antiferromagnetic phase, the band structure exhibits a
marked reconstruction characterized by the emergence of a "M"-shaped bulk band
near the Fermi level. The qualitative agreement with first-principles
band-structure calculations suggests the occurrence of bulk-band inversion at
the Gamma point in the antiferromagnetic phase. We suggest that EuIn2As2
provides a good opportunity to study the exotic quantum phases associated with
possible axion-insulator phase.Comment: 8 pages, 5 figure
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