723 research outputs found
Pairing Correlations on t-U-J Ladders
Pairing correlations on generalized t-U-J two-leg ladders are reported. We
find that the pairing correlations on the usual t-U Hubbard ladder are
significantly enhanced by the addition of a nearest-neighbor exchange
interaction J. Likewise, these correlations are also enhanced for the t-J model
when the onsite Coulomb interaction is reduced from infinity. Moreover, the
pairing correlations are larger on a t-U-J ladder than on a t-Jeff ladder in
which Jeff has been adjusted so that the two models have the same spin gap at
half-filling. This enhancement of the pairing correlations is associated with
an increase in the pair-binding energy and the pair mobility in the t-U-J model
and point to the importance of the charge transfer nature of the cuprate
systems
First record of the non-native suckermouth armored catfish \u3cem\u3eHypostomus cf. niceforoi\u3c/em\u3e (Fowler 1943) (Siluriformes: Loricariidae) from Central America
We document the first record of Hypostomus cf. niceforoi in Central America. Two specimens of these suckermouth armored catfishes were collected in Lake Nicaragua (Nicaragua) and identified as H. cf. niceforoi. Hypostomus niceforoi is endemic to Andean streams of Colombia, Venezuela, Ecuador, and Peru. We hypothesize that its introduction in Central America is related to the aquarium trade, as is the case of other armored catfish species introductions
Signatures of Spin and Charge Energy Scales in the Local Moment and Specific Heat of the Two-Dimensional Hubbard Model
Local moment formation driven by the on--site repulsion is one of the
most fundamental features in the Hubbard model. At the simplest level, the
temperature dependence of the local moment is expected to have a single
structure at , reflecting the suppression of the double occupancy. In
this paper we show new low temperature Quantum Monte Carlo data which emphasize
that the local moment also has a signature at a lower energy scale which
previously had been thought to characterize only the temperatures below which
moments on {\it different} sites begin to correlate locally. We discuss
implications of these results for the structure of the specific heat, and
connections to quasiparticle resonance and pseudogap formation in the density
of states.Comment: 13 pages, 19 figure
Neutrons from multiplicity-selected La-La and Nb-Nb collisions at 400A MeV and La-La collisions at 250A MeV
Triple-differential cross sections for neutrons from high-multiplicity La-La
collisions at 250 and 400 MeV per nucleon and Nb-Nb collisions at 400 MeV per
nucleon were measured at several polar angles as a function of the azimuthal
angle with respect to the reaction plane of the collision. The reaction plane
was determined by a transverse-velocity method with the capability of
identifying charged-particles with Z=1, Z=2, and Z > 2. The flow of neutrons
was extracted from the slope at mid-rapidity of the curve of the average
in-plane momentum vs the center-of-mass rapidity. The squeeze-out of the
participant neutrons was observed in a direction normal to the reaction plane
in the normalized momentum coordinates in the center-of-mass system.
Experimental results of the neutron squeeze-out were compared with BUU
calculations. The polar-angle dependence of the maximum azimuthal anisotropy
ratio was found to be insensitive to the mass of the colliding
nuclei and the beam energy. Comparison of the observed polar-angle dependence
of the maximum azimuthal anisotropy ratio with BUU calculations for
free neutrons revealed that is insensitive also to the
incompressibility modulus in the nuclear equation of state.Comment: ReVTeX, 16 pages, 17 figures. To be published in Physical Review
Band Calculation for Ce-compounds on the basis of Dynamical Mean Field Theory
The band calculation scheme for electron compounds is developed on the
basis of the dynamical mean field theory (DMFT) and the LMTO method. The
auxiliary impurity problem is solved by a method named as NCAv', which
includes the correct exchange process of the virtual
excitation as the vertex correction to the non-crossing approximation (NCA) for
the fluctuation. This method leads to the correct magnitude
of the Kondo temperature, , and makes it possible to carry out
quantitative DMFT calculation including the crystalline field (CF) and the
spin-orbit (SO) splitting of the self-energy. The magnetic excitation spectra
are also calculated to estimate . It is applied to Ce metal and CeSb
at T=300 K as the first step. In Ce metal, the hybridization intensity (HI)
just below the Fermi energy is reduced in the DMFT band. The photo-emission
spectra (PES) have a conspicuous SO side peak, similar to that of experiments.
is estimated to be about 70 K in -Ce, while to be about
1700 K in -Ce. In CeSb, the double-peak-like structure of PES is
reproduced. In addition, which is not so low is obtained because HI
is enhanced just at the Fermi energy in the DMFT band.Comment: 30pages, 18 figure
High-Pressure Amorphous Nitrogen
The phase diagram and stability limits of diatomic solid nitrogen have been
explored in a wide pressure--temperature range by several optical spectroscopic
techniques. A newly characterized narrow-gap semiconducting phase has
been found to exist in a range of 80--270 GPa and 10--510 K. The vibrational
and optical properties of the phase produced under these conditions
indicate that it is largely amorphous and back transforms to a new molecular
phase. The band gap of the phase is found to decrease with pressure
indicating possible metallization by band overlap above 280 GPa.Comment: 5 pages, 4 figure
Is mindfulness Buddhist? (and why it matters).
Modern exponents of mindfulness meditation promote the therapeutic effects of "bare attention"--a sort of non-judgmental, non-discursive attending to the moment-to-moment flow of consciousness. This approach to Buddhist meditation can be traced to Burmese Buddhist reform movements of the first half of the 20th century, and is arguably at odds with more traditional Theravāda Buddhist doctrine and meditative practices. But the cultivation of present-centered awareness is not without precedent in Buddhist history; similar innovations arose in medieval Chinese Zen (Chan) and Tibetan Dzogchen. These movements have several things in common. In each case the reforms were, in part, attempts to render Buddhist practice and insight accessible to laypersons unfamiliar with Buddhist philosophy and/or unwilling to adopt a renunciatory lifestyle. In addition, these movements all promised astonishingly quick results. And finally, the innovations in practice were met with suspicion and criticism from traditional Buddhist quarters. Those interested in the therapeutic effects of mindfulness and bare attention are often not aware of the existence, much less the content, of the controversies surrounding these practices in Asian Buddhist history
Quantum Monte Carlo Study of Hole Binding and Pairing Correlations in the Three-Band Hubbard Model
We simulated the 3-band Hubbard model using the Constrained Path Monte Carlo
(CPMC) method in search for a possible superconducting ground state. The CPMC
is a ground state method which is free of the exponential scaling of computing
time with system size. We calculated the binding energy of a pair of holes for
systems up to unit cells. We also studied the pairing correlation
functions versus distance for both the d-wave and extended s-wave channels in
systems up to . We found that holes bind for a wide range of
parameters and that the binding increased as the system size is increased.
However, the pairing correlation functions decay quickly with distance.
For the extended s channel, we found that as the Coulomb interaction on
the Cu sites is increased, the long-range part of the correlation functions is
suppressed and fluctuates around zero. For the channel, we
found that the correlations decay rapidly with distance towards a small
positive value. However, this value becomes smaller as the interaction or
the system size is increased.Comment: 21 pages, 13 Postscript figures, Submitted to Phys. Rev.
- …