178 research outputs found
Variational Approach to Hydrogen Atom in Uniform Magnetic Field of Arbitrary Strength
Extending the Feynman-Kleinert variational approach, we calculate the
temperature-dependent effective classical potential governing the quantum
statistics of a hydrogen atom in a uniform magnetic at all temperatures. The
zero-temperature limit yields the binding energy of the electron which is quite
accurate for all magnetic field strengths and exhibits, in particular, the
correct logarithmic growth at large fields.Comment: Author Information under this
http://www.physik.fu-berlin.de/~kleinert/institution.html Latest update of
paper also at this http://www.physik.fu-berlin.de/~kleinert/30
Carbon clusters near the crossover to fullerene stability
The thermodynamic stability of structural isomers of ,
, and , including
fullerenes, is studied using density functional and quantum Monte Carlo
methods. The energetic ordering of the different isomers depends sensitively on
the treatment of electron correlation. Fixed-node diffusion quantum Monte Carlo
calculations predict that a isomer is the smallest stable
graphitic fragment and that the smallest stable fullerenes are the
and clusters with and
symmetry, respectively. These results support proposals that a
solid could be synthesized by cluster deposition.Comment: 4 pages, includes 4 figures. For additional graphics, online paper
and related information see http://www.tcm.phy.cam.ac.uk/~prck
Transitions between Inherent Structures in Water
The energy landscape approach has been useful to help understand the dynamic
properties of supercooled liquids and the connection between these properties
and thermodynamics. The analysis in numerical models of the inherent structure
(IS) trajectories -- the set of local minima visited by the liquid -- offers
the possibility of filtering out the vibrational component of the motion of the
system on the potential energy surface and thereby resolving the slow
structural component more efficiently. Here we report an analysis of an IS
trajectory for a widely-studied water model, focusing on the changes in
hydrogen bond connectivity that give rise to many IS separated by relatively
small energy barriers. We find that while the system \emph{travels} through
these IS, the structure of the bond network continuously modifies, exchanging
linear bonds for bifurcated bonds and usually reversing the exchange to return
to nearly the same initial configuration. For the 216 molecule system we
investigate, the time scale of these transitions is as small as the simulation
time scale ( fs). Hence for water, the transitions between each of
these IS is relatively small and eventual relaxation of the system occurs only
by many of these transitions. We find that during IS changes, the molecules
with the greatest displacements move in small ``clusters'' of 1-10 molecules
with displacements of nm, not unlike simpler liquids.
However, for water these clusters appear to be somewhat more branched than the
linear ``string-like'' clusters formed in a supercooled Lennar d-Jones system
found by Glotzer and her collaborators.Comment: accepted in PR
Habitable Zones of Host Stars During the Post-MS Phase
A star will become brighter and brighter with stellar evolution, and the
distance of its habitable zone will become farther and farther. Some planets
outside the habitable zone of a host star during the main sequence phase may
enter the habitable zone of the host star during other evolutionary phases. A
terrestrial planet within the habitable zone of its host star is generally
thought to be suited to life existence. Furthermore, a rocky moon around a
giant planet may be also suited to life survive, provided that the planet-moon
system is within the habitable zone of its host star. Using Eggleton's code and
the boundary flux of habitable zone, we calculate the habitable zone of our
Solar after the main sequence phase. It is found that Mars' orbit and Jupiter's
orbit will enter the habitable zone of Solar during the subgiant branch phase
and the red giant branch phase, respectively. And the orbit of Saturn will
enter the habitable zone of Solar during the He-burning phase for about 137
million years. Life is unlikely at any time on Saturn, as it is a giant gaseous
planet. However, Titan, the rocky moon of Saturn, may be suitable for
biological evolution and become another Earth during that time. For low-mass
stars, there are similar habitable zones during the He-burning phase as our
Solar, because there are similar core masses and luminosities for these stars
during that phase.Comment: 6 pages, 7 figures. Accepted by Ap & S
A first-principles approach to electrical transport in atomic-scale nanostructures
We present a first-principles numerical implementation of Landauer formalism
for electrical transport in nanostructures characterized down to the atomic
level. The novelty and interest of our method lies essentially on two facts.
First of all, it makes use of the versatile Gaussian98 code, which is widely
used within the quantum chemistry community. Secondly, it incorporates the
semi-infinite electrodes in a very generic and efficient way by means of Bethe
lattices. We name this method the Gaussian Embedded Cluster Method (GECM). In
order to make contact with other proposed implementations, we illustrate our
technique by calculating the conductance in some well-studied systems such as
metallic (Al and Au) nanocontacts and C-atom chains connected to metallic (Al
and Au) electrodes. In the case of Al nanocontacts the conductance turns out to
be quite dependent on the detailed atomic arrangement. On the contrary, the
conductance in Au nanocontacts presents quite universal features. In the case
of C chains, where the self-consistency guarantees the local charge transfer
and the correct alignment of the molecular and electrode levels, we find that
the conductance oscillates with the number of atoms in the chain regardless of
the type of electrode. However, for short chains and Al electrodes the even-odd
periodicity is reversed at equilibrium bond distances.Comment: 14 pages, two-column format, submitted to PR
"Author! Author!" : Shakespeare and biography
Original article can be found at: http://www.informaworld.com/smpp/title~content=t714579626~db=all Copyright Informa / Taylor & Francis Group. DOI: 10.1080/17450910902764454Since 1996, not a year has passed without the publication of at least one Shakespeare biography. Yet for many years the place of the author in the practice of understanding literary works has been problematized, and even on occasions eliminated. Criticism reads the âworksâ, and may or may not refer to an author whose âlifeâ contributed to their meaning. Biography seeks the author in the works, the personality that precedes the works and gives them their characteristic shape and meaning. But the form of literary biography addresses the unusual kind of âlifeâ that puts itself into âworksâ, and this is particularly challenging where the âworksâ predominate massively over the salient facts of the âlifeâ. This essay surveys the current terrain of Shakespeare biography, and considers the key questions raised by the medium: can we know anything of Shakespeare's âpersonalityâ from the facts of his life and the survival of his works? What is the status of the kind of speculation that inevitably plays a part in biographical reconstruction? Are biographers in the end telling us as much about themselves as they tell us about Shakespeare?Peer reviewe
Growth chemistry and electrical performance of ultrathin alumina formed by area selective vapor phase infiltration
The growth chemistry and electrical performance of 5 nm alumina films, fabricated via the area-selective vapor phase infiltration (VPI) of trimethylaluminum into poly(2-vinylpyridine), are compared to a conventional plasma enhanced atomic layer deposition (PEALD) process. The chemical properties are assessed via energy dispersive X-ray spectroscopy and hard X-ray photoelectron spectroscopy measurements, while current â voltage dielectric breakdown and capacitance â voltage analysis is undertaken to provide electrical information of these films for the first time. The success and challenges in dielectric formation via polymer VPI, the compatibility of pyridine in such a role, and the ability of the unique and rapid grafting-to polymer brush method in forming coherent metal oxides is evaluated. It was found that VPI made alumina fabricated at temperatures between 200 and 250 °C had a consistent breakdown electrical field, with the best performing devices possessing a Đș value of 5.9. The results indicate that the VPI approach allows for the creation of alumina films that display dielectric properties of a comparable quality to conventional PEALD grown films
Quantifying ionospheric effects on time-domain astrophysics with the Murchison Widefield Array
© 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Refraction and diffraction of incoming radio waves by the ionosphere induce time variability in the angular positions, peak amplitudes and shapes of radio sources, potentially complicating the automated cross-matching and identification of transient and variable radio sources. In this work, we empirically assess the effects of the ionosphere on data taken by the Murchison Widefield Array (MWA) radio telescope. We directly examine 51 h of data observed over 10 nights under quiet geomagnetic conditions (global storm index Kp < 2), analysing the behaviour of short-time-scale angular position and peak flux density variations of around ten thousand unresolved sources. We find that while much of the variation in angular position can be attributed to ionospheric refraction, the characteristic displacements (10-20 arcsec) at 154 MHz are small enough that search radii of 1-2 arcmin should be sufficient for crossmatching under typical conditions. By examining bulk trends in amplitude variability, we place upper limits on the modulation index associated with ionospheric scintillation of 1-3 per cent for the various nights. For sources fainter than ~1 Jy, this variation is below the image noise at typical MWA sensitivities. Our results demonstrate that the ionosphere is not a significant impediment to the goals of time-domain science with the MWA at 154 MHz
Detection of a supernova signature associated with GRB 011121
Using observations from an extensive monitoring campaign with the Hubble Space Telescope, we present the detection of an intermediate-time flux excess that is redder in color relative to the afterglow of GRB 011121, currently distinguished as the gamma-ray burst with the lowest known redshift. The red "bump," which exhibits a spectral rollover at âŒ7200 A, is well described by a redshifted Type le supernova that occurred approximately at the same time as the gamma-ray burst event. The inferred luminosity is about half that of the bright supernova SN 1998bw. These results serve as compelling evidence for a massive star origin of long-duration gamma-ray bursts. Models that posit a supernova explosion weeks to months preceding the gamma-ray burst event are excluded by these observations. Finally, we discuss the relationship between spherical core-collapse supernovae and gamma-ray bursts
GRB 010921: Strong limits on an underlying supernova from the Hubble Space Telescope
GRB 010921 was the first HETE-2 gamma-ray burst (GRB) to be localized via its afterglow emission. The low redshift of the host galaxy, z = 0.451, prompted us to undertake intensive multicolor observations with the Hubble Space Telescope with the goal of searching for an underlying supernova (SN) component. We do not detect any coincident SN to a limit 1.33 mag fainter than SN 1998bw at 99.7% confidence, making this one of the most sensitive searches for an underlying SN. Analysis of the afterglow data allows us to infer that the GRB was situated behind a net extinction (Milky Way and the host galaxy) of Av ⌠1.8 mag in the observer frame. Thus, had it not been for such heavy extinction, our data would have allowed us to probe for an underlying SN with brightness approaching those of more typical Type Ib/c SNe
- âŠ