280 research outputs found
Solvable glassy system: static versus dynamical transition
A directed polymer is considered on a flat substrate with randomly located
parallel ridges. It prefers to lie inside wide regions between the ridges. When
the transversel width is exponential in the
longitudinal length , there can be a large number of
available wide states. This ``complexity'' causes a phase transition from a
high temperature phase where the polymer lies in the widest lane, to a glassy
low temperature phase where it lies in one of many narrower lanes. Starting
from a uniform initial distribution of independent polymers, equilibration up
to some exponential time scale induces a sharp dynamical transition. When the
temperature is slowly increased with time, this occurs at a tunable
temperature. There is an asymmetry between cooling and heating. The structure
of phase space in the low temperature non-equilibrium glassy phase is of a
one-level tree.Comment: 4 pages revte
Anomalous Density-of-States Fluctuations in Two-Dimensional Clean Metals
It is shown that density-of-states fluctuations, which can be interpreted as
the order-parameter susceptibility \chi_OP in a Fermi liquid, are anomalously
strong as a result of the existence of Goldstone modes and associated strong
fluctuations. In a 2-d system with a long-range Coulomb interaction, a suitably
defined \chi_OP diverges as 1/T^2 as a function of temperature in the limit of
small wavenumber and frequency. In contrast, standard statistics suggest
\chi_OP = O(T), a discrepancy of three powers of T. The reasons behind this
surprising prediction, as well as ways to observe it, are discussed.Comment: 4 pp, revised version contains a substantially expanded derivatio
To maximize or not to maximize the free energy of glassy systems, !=?
The static free energy of glassy systems can be expressed in terms of the
Parisi order parameter function. When this function has a discontinuity, the
location of the step is determined by maximizing the free energy. In dynamics a
transition is found at larger temperature, while the location of the step
satisfies a marginality criterion. It is shown here that in a replica
calculation this criterion minimizes the free energy. This leads to first order
phase transitions at the dynamic transition point. Though the order parameter
function is the same as in the long-time limit of a dynamical analysis,
thermodynamics is different.Comment: 4 pages PostScript, one figur
Pulmonary stretch receptor activity during partial liquid ventilation in cats with healthy lungs
Aim: To study whether pulmonary stretch receptor (PSR) activity in mechanically ventilated young cats with healthy lungs during partial liquid ventilation (PLV) is different from that during gas ventilation (GV). Methods: In 10 young cats (4.4 +/- 0.4 months, 2.3 +/- 0.3 kg; mean B SD), PSR instantaneous impulse frequency (PSR f(imp)) was recorded from single fibres in the vagal nerve during GV and PLV with perfluorocarbon (30 ml/kg) at increasing positive inspiratory pressures (PIP; 1.2, 1.8, 2.2 and 2.7 kPa), and at a positive end-expiratory pressure of 0.5 kPa. Results: All PSRs studied during GV maintained their phasic character with increased impulse frequency during inspiration during PLV. Peak PSR fimp was lower at PIP 1.2 kPa (p < 0.05) and at PIP 2.7 kPa (p = 0.10) during PLV than during GV, giving a lower number of PSR impulses at these two settings during PLV (p < 0.05). Conclusion: The phasic character of PSR activity is similar during GV and PLV. PSR activity is not higher during PLV than during GV in cats with healthy lungs, indicating no extensive stretching of the lung during PLV. Copyright (C) 2004 S. Karger AG, Basel
Finite size effects and localization properties of disordered quantum wires with chiral symmetry
Finite size effects in the localization properties of disordered quantum
wires are analyzed through conductance calculations. Disorder is induced by
introducing vacancies at random positions in the wire and thus preserving the
chiral symmetry. For quasi one-dimensional geometries and low concentration of
vacancies, an exponential decay of the mean conductance with the wire length is
obtained even at the center of the energy band. For wide wires, finite size
effects cause the conductance to decay following a non-pure exponential law. We
propose an analytical formula for the mean conductance that reproduces
accurately the numerical data for both geometries. However, when the
concentration of vacancies increases above a critical value, a transition
towards the suppression of the conductance occurs.
This is a signature of the presence of ultra-localized states trapped in
finite regions of the sample.Comment: 5 figures, revtex
Competition between glassiness and order in a multi-spin glass
A mean-field multi-spin interaction spin glass model is analyzed in the
presence of a ferromagnetic coupling. The static and dynamical phase diagrams
contain four phases (paramagnet, spin glass, ordinary ferromagnet and glassy
ferromagnet) and exhibit reentrant behavior. The glassy ferromagnet phase has
anomalous dynamical properties. The results are consistent with a
nonequilibrium thermodynamics that has been proposed for glasses.Comment: revised version, 4 pages Revtex, 2 eps-figures. Phys. Rev. E, Rapid
Communication, to appea
Metastable states in glassy systems
Truly stable metastable states are an artifact of the mean-field
approximation or the zero temperature limit. If such appealing concepts in
glass theory as configurational entropy are to have a meaning beyond these
approximations, one needs to cast them in a form involving states with finite
lifetimes.
Starting from elementary examples and using results of Gaveau and Schulman,
we propose a simple expression for the configurational entropy and revisit the
question of taking flat averages over metastable states. The construction is
applicable to finite dimensional systems, and we explicitly show that for
simple mean-field glass models it recovers, justifies and generalises the known
results. The calculation emphasises the appearance of new dynamical order
parameters.Comment: 4 fig., 20 pages, revtex; added references and minor change
Resolving the L/T transition binary SDSS J2052-1609 AB
Binaries provide empirical key constraints for star formation theories, like
the overall binary fraction, mass ratio distribution and the separation
distribution. They play a crucial role to calibrate the output of theoretical
models, like absolute magnitudes, colors and effective temperature depending on
mass, metallicity and age. We present first results of our on-going
high-resolution imaging survey of late type brown dwarfs. The survey aims at
resolving tight brown dwarf binary systems to better constrain the T dwarf
binary fraction. We intent to follow-up the individual binaries to determine
orbital parameters. Using NACO at the VLT we performed AO-assisted
near-infrared observations of SDSS J2052-1609. High-spatial resolution images
of the T1 dwarf were obtained in H and Ks filters. We resolved SDSS J2052-1609
into a binary system with a separation of 0.101" \pm 0.001". Archival data from
HST/NICMOS taken one year previous to our observations proves the components to
be co-moving. Using the flux ratio between the components we infer J, H and Ks
magnitudes for the resolved system. From the near-IR colors we estimate
spectral types of T1 +1 -4 and T2.5 \pm 1 for component A and B, respectively.
A first estimate of the total system mass yields Mtot > 78 Mjup, assuming a
circular orbit.Comment: 5 pages, 3 figures, 3 tables, accepted for publication by A&
2MASS J03105986+1648155AB - A new binary at the L/T transition
The transition from the L to the T spectral type of brown dwarfs is marked by
a very rapid transition phase, remarkable brightening in the J-band and a
higher binary frequency. Despite being an active area of inquiry, this
transition regime still remains one of the most poorly understood phases of
brown dwarf evolution. We resolved the L dwarf 2MASS J03105986+1648155 for the
first time into two almost equally bright components straddling the L/T
transition. Since such a co-eval system with common age and composition
provides crucial information of this special transition phase, we monitored the
system over 3 years to derive first orbital parameters and dynamical mass
estimates, as well as a spectral type determination. We obtained resolved high
angular resolution, near-IR images with HST and the adaptive optics instrument
NACO at the VLT including the laser guide star system PARSEC. Based on two
epochs of astrometric data we derive a minimum semi-major axis of 5.2 +- 0.8
AU. The assumption of a face-on circular orbit yields an orbital period of 72
+- 4 years and a total system mass of 30-60 Mjup. This places the masses of the
individual components of the system at the lower end of the mass regime of
brown dwarfs. The achieved photometry allowed a first spectral type
determination of L9 +- 1 for each component. In addition, this seems to be only
the fifth resolved L/T transition binary with a flux reversal. While ultimate
explanations for this effect are still owing, the 2MASS J03105986+1648155
system adds an important benchmark object for improving our understanding of
this remarkable evolutionary phase of brown dwarfs. Additionally, the
observational results of 2MASS J03105986+1648155 AB derived with the new PARSEC
AO system at the VLT show the importance of this technical capability. The
updated AO system allows us to significantly extend the sample of brown dwarfs
observable with high-resolution from the ground and hence to reveal more of
their physical properties.Comment: 6 pages, 2 figures, 3 tables, accepted for publication by A&
Emergence of macroscopic temperatures in systems that are not thermodynamical microscopically: towards a thermodynamical description of slow granular rheology
A scenario for systems with slow dynamics is characterised by stating that
there are several temperatures coexisting in the sample, with a single
temperature shared by all observables at each (widely separate) time-scale.
In preparation for the study of granular rheology, we show within this
framework that glassy systems with driving and friction that are generic and do
not correspond to a thermal bath --- and whose microscopic `fast' motion is
hence not thermal --- have a well-defined macroscopic temperature associated to
the slow degrees of freedom.
This temperature is what a thermometer coupled to the system will measure if
tuned to respond to low frequencies, and since it can be related to the number
of stationary configurations, it is the formalisation of Edwards'
`compactivity' ideas.Comment: Revised version: treatment of `tapping' deferre
- …