17,610 research outputs found
Semiclassical (Quantum Field Theory) and Quantum (String) de Sitter Regimes: New Results
We compute the quantum string entropy S_s(m, H) from the microscopic string
density of states rho_s (m,H) of mass m in de Sitter space-time. We find for
high m, a {\bf new} phase transition at the critical string temperature T_s=
(1/2 pi k_B)L c^2/alpha', higher than the flat space (Hagedorn) temperature
t_s. (L = c/H, the Hubble constant H acts at the transition as producing a
smaller string constant alpha' and thus, a higher tension). T_s is the precise
quantum dual of the semiclassical (QFT Hawking-Gibbons) de Sitter temperature
T_sem = hbar c /(2\pi k_B L). We find a new formula for the full de Sitter
entropy S_sem (H), as a function of the usual Bekenstein-Hawking entropy
S_sem^(0)(H). For L << l_{Planck}, ie. for low H << c/l_Planck,
S_{sem}^{(0)}(H) is the leading term, but for high H near c/l_Planck, a new
phase transition operates and the whole entropy S_sem (H) is drastically
different from the Bekenstein-Hawking entropy S_sem^(0)(H). We compute the
string quantum emission cross section by a black hole in de Sitter (or
asymptotically de Sitter) space-time (bhdS). For T_sem ~ bhdS << T_s, (early
evaporation stage), it shows the QFT Hawking emission with temperature T_sem ~
bhdS, (semiclassical regime). For T_sem ~ bhdS near T_{s}, it exhibits a phase
transition into a string de Sitter state of size L_s = l_s^2/L}, l_s=
\sqrt{\hbar alpha'/c), and string de Sitter temperature T_s. Instead of
featuring a single pole singularity in the temperature (Carlitz transition), it
features a square root branch point (de Vega-Sanchez transition). New bounds on
the black hole radius r_g emerge in the bhdS string regime: it can become r_g =
L_s/2, or it can reach a more quantum value, r_g = 0.365 l_s.Comment: New original materia
Recursive parametrization of Quark flavour mixing matrices
We examine quark flavour mixing matrices for three and four generations using
the recursive parametrization of and matrices developed by some
of us in Refs.[2] and [3]. After a brief summary of the recursive
parametrization, we obtain expressions for the independent rephasing invariants
and also the constraints on them that arise from the requirement of mod
symmetry of the flavour mixing matrix
Semiclassical and Quantum Black Holes and their Evaporation, de Sitter and Anti-de Sitter Regimes, Gravitational and String Phase Transitions
An effective string theory in physically relevant cosmological and black hole
space times is reviewed. Explicit computations of the quantum string entropy,
partition function and quantum string emission by black holes (Schwarzschild,
rotating, charged, asymptotically flat, de Sitter dS and AdS space times) in
the framework of effective string theory in curved backgrounds provide an
amount of new quantum gravity results as: (i) gravitational phase transitions
appear with a distinctive universal feature: a square root branch point
singularity in any space time dimensions. This is of the type of the de Vega -
Sanchez transition for the thermal self-gravitating gas of point particles.
(ii) There are no phase transitions in AdS alone. (iii) For background,
upper bounds of the Hubble constant H are found, dictated by the quantum string
phase transition.(iv) The Hawking temperature and the Hagedorn temperature are
the same concept but in different (semiclassical and quantum) gravity regimes
respectively. (v) The last stage of black hole evaporation is a microscopic
string state with a finite string critical temperature which decays as usual
quantum strings do in non-thermal pure quantum radiation (no information
loss).(vi) New lower string bounds are given for the Kerr-Newman black hole
angular momentum and charge, which are entirely different from the upper
classical bounds. (vii) Semiclassical gravity states undergo a phase transition
into quantum string states of the same system, these states are duals of each
other in the precise sense of the usual classical-quantum (wave-particle)
duality, which is universal irrespective of any symmetry or isommetry of the
space-time and of the number or the kind of space-time dimensions.Comment: review paper, no figures. to appear in Int Jour Mod Phys
Semiclassical (QFT) and Quantum (String) anti - de Sitter Regimes: New Results
We compute the quantum string entropy S_s(m, H) from the microscopic string
density of states of mass m in Anti de Sitter space-time. For high m, (high Hm
-->c/\alpha'), no phase transition occurs at the Anti de Sitter string
temperature T_{s} which is higher than the flat space (Hagedorn) temperature
t_{s}. (the Hubble constant H acts as producing a smaller string constant and
thus, a higher tension). T_s is the precise quantum dual of the semiclassical
(QFT) Anti de Sitter temperature scale . We compute the quantum string emission
by a black hole in Anti de Sitter space-time (bhAdS). In the early evaporation
stage, it shows the QFT Hawking emission with temperature T_{sem~bhAdS},
(semiclassical regime). For T_{sem~bhAdS}--> T_{s}, it exhibits a phase
transition into a Anti de Sitter string state. New string bounds on the black
hole emerge in the bhAdS string regime. We find a new formula for the full
(quantum regime included) Anti de Sitter entropy S_{sem}, as a function of the
usual Bekenstein-Hawking entropy S_{sem}^(0). For low H (semiclassical regime),
S_{sem}^(0) is the leading term but for high H (quantum regime), no phase
transition operates, in contrast to de Sitter space, and the entropy S_{sem} is
very different from the Bekenstein-Hawking term S_{sem}^(0).Comment: Comments 26 pages; no figure
Semiclassical (QFT) and Quantum (String) Rotating Black Holes and their Evaporation: New Results
Combination of both quantum field theory (QFT) and string theory in curved
backgrounds in a consistent framework, the string analogue model, allows us to
provide a full picture of the Kerr-Newman black hole and its evaporation going
beyond the current picture. We compute the quantum emission cross section of
strings by a Kerr-Newmann black hole (KNbh). It shows the black hole emission
at the Hawking temperature T_{sem} in the early evaporation and the new string
emission featuring a Hagedorn transition into a string state of temperature T_
s at the last stages. New bounds on the angular momentum J and charge Q emerge
in the quantum string regime. The last state of evaporation of a semiclassical
KNbh is a string state of temperature T_s, mass M_s, J = 0 = Q, decaying as a
quantum string into all kinds of particles.(There is naturally, no loss of
information, (no paradox at all)). We compute the microscopic string entropy
S_s(m, j) of mass m and spin mode j. (Besides the usual transition at T_s), we
find for high j, (extremal string states) a new phase transition at a
temperature T_{sj} higher than T_s. We find a new formula for the Kerr black
hole entropy S_{sem}, as a function of the usual Bekenstein-Hawking entropy .
For high angular momentum, (extremal J = GM^2/c), a gravitational phase
transition operates and the whole entropy S_{sem} is drastically different from
the Bekenstein-Hawking entropy. This new extremal black hole transition occurs
at a temperature T_{sem J} higher than the Hawking temperature T_{sem}.Comment: New articl
First-principles study of the atomic and electronic structure of the Si(111)-(5x2-Au surface reconstruction
We present a systematic study of the atomic and electronic structure of the
Si(111)-(5x2)-Au reconstruction using first-principles electronic structure
calculations based on the density functional theory. We analyze the structural
models proposed by Marks and Plass [Phys. Rev. Lett.75, 2172 (1995)], those
proposed recently by Erwin [Phys. Rev. Lett.91, 206101 (2003)], and a
completely new structure that was found during our structural optimizations. We
study in detail the energetics and the structural and electronic properties of
the different models. For the two most stable models, we also calculate the
change in the surface energy as a function of the content of silicon adatoms
for a realistic range of concentrations. Our new model is the energetically
most favorable in the range of low adatom concentrations, while Erwin's "5x2"
model becomes favorable for larger adatom concentrations. The crossing between
the surface energies of both structures is found close to 1/2 adatoms per 5x2
unit cell, i.e. near the maximum adatom coverage observed in the experiments.
Both models, the new structure and Erwin's "5x2" model, seem to provide a good
description of many of the available experimental data, particularly of the
angle-resolved photoemission measurements
Magneto-mechanical interplay in spin-polarized point contacts
We investigate the interplay between magnetic and structural dynamics in
ferromagnetic atomic point contacts. In particular, we look at the effect of
the atomic relaxation on the energy barrier for magnetic domain wall migration
and, reversely, at the effect of the magnetic state on the mechanical forces
and structural relaxation. We observe changes of the barrier height due to the
atomic relaxation up to 200%, suggesting a very strong coupling between the
structural and the magnetic degrees of freedom. The reverse interplay is weak,
i.e. the magnetic state has little effect on the structural relaxation at
equilibrium or under non-equilibrium, current-carrying conditions.Comment: 5 pages, 4 figure
Chemical abundances of stars with brown-dwarf companions
It is well-known that stars with giant planets are on average more metal-rich
than stars without giant planets, whereas stars with detected low-mass planets
do not need to be metal-rich. With the aim of studying the weak boundary that
separates giant planets and brown dwarfs (BDs) and their formation mechanism,
we analyze the spectra of a sample of stars with already confirmed BD
companions both by radial velocity and astrometry. We employ standard and
automatic tools to perform an EW-based analysis and to derive chemical
abundances from CORALIE spectra of stars with BD companions. We compare these
abundances with those of stars without detected planets and with low-mass and
giant-mass planets. We find that stars with BDs do not have metallicities and
chemical abundances similar to those of giant-planet hosts but they resemble
the composition of stars with low-mass planets. The distribution of mean
abundances of -elements and iron peak elements of stars with BDs
exhibit a peak at about solar abundance whereas for stars with low-mass and
high-mass planets the [X/H] and [X/H] peak abundances
remain at ~dex and ~dex, respectively. We display these
element abundances for stars with low-mass and high-mass planets, and BDs
versus the minimum mass, , of the most-massive substellar companion
in each system, and we find a maximum in -element as well as Fe-peak
abundances at jupiter masses. We discuss the
implication of these results in the context of the formation scenario of BDs in
comparison with that of giant planets.Comment: Accepted for publication in Astronomy & Astrophysic
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