2,513 research outputs found
Phase diagram of H2 adsorbed on graphene
The phase diagram of the first layer of H adsorbed on top of a single
graphene sheet has been calculated by means of a series of diffusion Monte
Carlo (DMC) simulations. We have found that, as in the case of He, the
ground state of molecular hydrogen is a commensurate
structure, followed, upon a pressure increase, by an incommensurate triangular
solid. A striped phase of intermediate density was also considered, and found
lying on top of the equilibrium curve separating both commensurate and
incommensurate solids.Comment: 5 pages, 3 figure
Quantized vortices around wavefront nodes, 2
Quantized vortices can occur around nodal points in wavefunctions. The derivation depends only on the wavefunction being single valued, continuous, and having continuous first derivatives. Since the derivation does not depend upon the dynamical equations, the quantized vortices are expected to occur for many types of waves such as electromagnetic and acoustic. Such vortices have appeared in the calculations of the H + H2 molecular collisions and play a role in the chemical kinetics. In a companion paper, it is shown that quantized vortices occur when optical waves are internally reflected from the face of a prism or particle beams are reflected from potential energy barriers
Supersolidity in quantum films adsorbed on graphene and graphite
Using quantum Monte Carlo we have studied the superfluid density of the first
layer of He and H adsorbed on graphene and graphite. Our main focus has
been on the equilibrium ground state of the system, which corresponds to a
registered phase. The perfect solid phase of H shows
no superfluid signal whereas He has a finite but small superfluid fraction
(0.67%). The introduction of vacancies in the crystal makes the superfluidity
increase, showing values as large as 14% in He without destroying the
spatial solid order.Comment: 5 pages, accepted for publication in PR
Phase transitions of H2 adsorbed on the surface of single carbon nanotubes
By means of Diffusion Monte Carlo calculations, we obtained the complete
phase diagrams of H adsorbed on the outer surface of isolated armchair
carbon nanotubes of radii ranging from 3.42 to 10.85 \AA. We only considered
density ranges corresponding to the filling of the first adsorption layer in
these curved structures. In all cases, the zero-temperature ground state was
found to be an incommensurate solid, except in the widest tube, in which the
structure with lowest energy is an analogous of the
phase found in planar substrates. Those incommensurate solids result form the
arrangement of the hydrogen molecules in circumferences whose plane is
perpendicular to the main axis of the carbon nanotube. For each tube, there is
only one of such phases stable in the density range considered, except in the
case of the (5,5) and (6,6) tubes, in which two of these incommensurate solids
are separated by novel first order phase transitions.Comment: 5 pages. to appear in Phys. Rev.
Flickering in FU Orionis
We analyze new and published optical photometric data of FU Orionis, an
eruptive pre-main sequence star. The outburst consists of a 5.5 mag rise at B
with an e-folding timescale of roughly 50 days. The rates of decline at B and V
are identical, 0.015 +- 0.001 mag per yr. Random fluctuations superimposed on
this decline have an amplitude of 0.035 +- 0.005 mag at V and occur on
timescales of 1 day or less. Correlations between V and the color indices U-B,
B-V, and V-R indicate that the variable source has the optical colors of a G0
supergiant. We associate this behavior with small amplitude flickering of the
inner accretion disk.Comment: 19 pages of text, 3 tables, and 6 figures to be published in the
Astrophysical Journal, 10 March 200
Do we need a basis bolus concept for sedoanalgesia of mechanically ventilated patients in ICU?
Hydrodynamics of the stream-disk impact in interacting binaries
We use hydrodynamic simulations to provide quantitative estimates of the
effects of the impact of the accretion stream on disks in interacting binaries.
For low accretion rates, efficient radiative cooling of the hotspot region can
occur, and the primary consequence of the stream impact is stream overflow
toward smaller disk radii. The stream is well described by a ballistic
trajectory, but larger masses of gas are swept up and overflow at smaller, but
still highly supersonic, velocities. If cooling is inefficient, overflow still
occurs, but there is no coherent stream inward of the disk rim. Qualitatively,
the resulting structure appears as a bulge extending downstream along the disk
rim. We calculate the mass fraction and velocity of the overflowing component
as a function of the important system parameters, and discuss the implications
of the results for X-ray observations and doppler tomography of cataclysmic
variables, low-mass X-ray binaries and supersoft X-ray sources.Comment: 16 pages, including 8 figures. 1 color figure as a jpeg. ApJ, in
pres
On Quartet Superfluidity of Fermionic Atomic Gas
Possibility of a quartet superfluidity in fermionic systems is studied as a
new aspect of atomic gas at ultra low temperatures. The four-fold degeneracy of
hyperfine state and moderate coupling is indispensable for the quartet
superfluidity to occur. Possible superconductivity with quartet condensation in
electron systems is discussed.Comment: 7 pages, 1 figure. J. Phys. Soc. Jpn. vol.74 (2005) No.7, in press;
Note added for related previous works; some typographic errors revise
Possible role of 3He impurities in solid 4He
We use a quantum lattice gas model to describe essential aspects of the
motion of 4He atoms and of 3He impurities in solid 4He. This study suggests
that 3He impurities bind to defects and promote 4He atoms to interstitial sites
which can turn the bosonic quantum disordered crystal into a metastable
supersolid. It is suggested that defects and interstitial atoms are produced
during the solid 4He nucleation process where the role of 3He impurities (in
addition to the cooling rate) is known to be important even at very small (1
ppm) impurity concentration. It is also proposed that such defects can form a
glass phase during the 4He solid growth by rapid cooling.Comment: 4 two-column Revtex pages, 4 figures. Europhysics Letters (in Press
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