4,293 research outputs found
Adiabatic Phase Diagram of an Ultracold Atomic Fermi Gas with a Feshbach Resonance
We determine the adiabatic phase diagram of a resonantly-coupled system of
Fermi atoms and Bose molecules confined in the harmonic trap by using the local
density approximation. The adiabatic phase diagram shows the fermionic
condensate fraction composed of condensed molecules and Cooper pair atoms. The
key idea of our work is conservation of entropy through the adiabatic process,
extending the study of Williams et al. [Williams et al., New J. Phys. 6, 123
(2004)] for an ideal gas mixture to include the resonant interaction in a
mean-field theory. We also calculate the molecular conversion efficiency as a
function of initial temperature. Our work helps to understand recent
experiments on the BCS-BEC crossover, in terms of the initial temperature
measured before a sweep of the magnetic field.Comment: 13 pages, 8 figures. In press, "Journal of the Physical Society of
Japan", Vol.76, No.
The International X-ray Observatory
The International X-ray Observatory (IXO) is a joint ESA-JAXA-NASA effort to
address fundamental and timely questions in astrophysics: What happens close to
a black hole? How did supermassive black holes grow? How does large scale
structure form? What is the connection between these processes? To address
these questions IXO will employ optics with 3 sq m collecting area and 5 arc
sec angular resolution - 20 times more collecting area at 1 keV than any
previous X-ray observatory. Focal plane instruments will deliver a 100-fold
increase in effective area for high-resolution spectroscopy, deep spectral
imaging over a wide field of view, unprecedented polarimetric sensitivity,
microsecond spectroscopic timing, and high count rate capability. The mission
is being planned for launch in 2021 to an L2 orbit, with a five-year lifetime
and consumables for 10 years.Comment: 6 pages, 3 figures, for conference "X-ray Astronomy 2009 Present
status, multi-wavelength approach and future perspectives
Electric Field Modulation of Galvanomagnetic Properties of Mesoscopic Graphite
Electric field effect devices based on mesoscopic graphite are fabricated for
galvanomagnetic measurements. Strong modulation of magneto-resistance and Hall
resistance as a function of gate voltage is observed as sample thickness
approaches the screening length. Electric field dependent Landau level
formation is detected from Shubnikov de Haas oscillations in
magneto-resistance. The effective mass of electron and hole carriers has been
measured from the temperature dependant behavior of these oscillations.Comment: 4 pages, 4 figures included, submitted to Phys. Rev. Let
First-Principles Study of Electronic and Vibrational Properties of BaHfN
The transition metal nitride BaHfN, which consists of weakly bonded
neutral slabs of closed shell ions, has structural and chemical similarities to
other layered nitrides which have impressive superconducting T when
electron doped: AHfNCl, AZrNCl, ATiNCl, with ,
and K, respectively for appropriate donor (A) concentrations . These
similarities suggest the possibility of BaHfN being another relatively high
T nitride upon doping, with effects of structure and the role of specific
transition metal ions yet to be understood. We report first-principles
electronic structure calculations for stoichiometric BaHfN using density
functional theory with plane-wave basis sets and separable dual-space Gaussian
pseudopotentials. An indirect band gap of 0.8 eV was obtained and the lowest
conduction band is primarily of Hf 5 character, similar to
-ZrNCl and -TiNCl. The two N sites, one in the Hf layer and
another one in the Ba layer, were found to have very anisotropic Born effective
charges (BEC):deviations from the formal charge (-3) are opposite for the two
sites, and opposite for the two orientations (in-plane, out of plane). LO-TO
splittings and comparison of BECs and dielectric constant tensors to those of
related compounds are discussed, and the effect of electron doping on the
zone-center phonons is reported.Comment: 11 pages, 5 figure
Superfluid transition temperature in a trapped gas of Fermi atoms with a Feshbach resonance
We investigate strong coupling effects on the superfluid phase transition in
a gas of Fermi atoms with a Feshbach resonance. The Feshbach resonance
describes a composite quasi-Boson, which can give rise to an additional pairing
interaction between the Fermi atoms. This attractive interaction becomes
stronger as the threshold energy of the Feshbach resonance two-particle bound
state is lowered. In a recent paper, we showed that in the uniform Fermi gas,
this tunable pairing interaction naturally leads to a BCS-BEC crossover of the
Nozi`eres and Schmitt-Rink kind, in which the BCS-type superfluid phase
transition continuously changes into the BEC-type as the threshold energy is
decreased. In this paper, we extend our previous work by including the effect
of a harmonic trap potential, treated within the local density approximation
(LDA). We also give results for both weak and strong coupling to the Feshbach
resonance. We show that the BCS-BEC crossover phenomenon strongly modifies the
shape of the atomic density profile at the superfluid phase transition
temperature Tc, reflecting the change of the dominant particles going from
Fermi atoms to composite Bosons. In the BEC regime, these composite Bosons are
shown to first appear well above Tc. We also discuss the "phase diagram" above
Tc as a function of the tunable threshold energy. We introduce a characteristic
temperature T* describing the effective crossover in the normal phase from a
Fermi gas of atoms to a gas of stable molecules.Comment: 43 pages, 13 figures (submitted to PRA
Comments on differential cross section of phi-meson photoproduction at threshold
We show that the differential cross section d_sigma/d_t of gamma p --> \phi p
reaction at the threshold is finite and its value is crucial to the mechanism
of the phi meson photoproduction and for the models of phi-N interaction.Comment: 8 pages, 2 figure
Recoil effects of photoelectrons in a solid
High energy resolution C 1 photoelectron spectra of graphite were measured
at the excitation energy of 340, 870, 5950 and 7940eV using synchrotron
radiation. On increasing the excitation energy, i.e., increasing kinetic energy
of the photoelectron, the bulk origin C 1 peak position shifts to higher
binding energies. This systematic shift is due to the kinetic energy loss of
the high-energy photoelectron by kicking the atom, and is clear evidence of the
recoil effect in photoelectron emission. It is also observed that the
asymmetric broadening increases for the higher energy photoelectrons. All these
recoil effects can be quantified in the same manner as the M\"ossbauer effect
for -ray emission from nuclei embedded in crystals.Comment: 4 pages, 2 figure
On the Nature of AX J2049.6+2939 and AX J2050.0+2914
AX J2049.6+2939 is a compact X-ray source in the vicinity of the southern
blow-up region of the Cygnus Loop supernova remnant (Miyata et al. 1998a). This
source was the brightest X-ray source inside the
Cygnus Loop observed during the ASCA survey project. The X-ray spectrum was
well fitted by a power-law function with a photon index of . Short-term timing analysis was performed and no coherent pulsation
was found. Follow-up observations with ASCA have revealed a large variation in
X-ray intensity by a factor of 50, whereas the spectral shape did not
change within the statistical uncertainties. In the second ASCA observation, we
found another X-ray source, AX J2050.0+2941, at the north east of AX
J2049.6+2939. During the three ASCA observations, the X-ray intensity of AX
J2050.0+2941 varied by a factor of 4. No coherent pulsations could be
found for AX J2050.0+2941.
We have performed optical photometric and spectroscopic observations in the
vicinity of AX J2049.6+2939 at the Kitt Peak National Observatory
(KPNO). As a result, all objects brighter than -band magnitude of 22 in
the error box can be identified with normal stars. Combined with the
X-ray results and the fact that there are no radio counterparts, AX
J2049.6+2939 is not likely to be either an ordinary rotation-powered pulsar
or an AGN. The nature of AX J2049.6+2939 is still unclear and further
observations over a wide energy band are strongly required.
As to AX J2050.0+2941, the long-term X-ray variability and the radio
counterpart suggests that it is an AGN.Comment: 23 pages, 4 figures, Accepted for publication by Astrophysical
Journa
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