61,145 research outputs found
Ground State Properties of the Doped 3-Leg t-J Ladder
Results for a doped 3-leg t-J ladder obtained using the density matrix
renormalization group are reported. At low hole doping, the holes form a dilute
gas with a uniform density. The momentum occupation of the odd band shows a
sharp decrease at a large value of k_F similar to the behavior of a lightly
doped t-J chain, while the even modes appear gapped. The spin-spin correlations
decay as a power law consistent with the absence of a spin gap, but the pair
field correlations are negligible. At larger doping we find evidence for a spin
gap and as x increases further we find 3-hole diagonal domain walls. In this
regime there are pair field correlations and the internal pair orbital has
d_x^2-y^2 - like symmetry. However, the pair field correlations appear to fall
exponentially at large distances.Comment: 14 pages, 11 postscript figure
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The effect of elevated hydrostatic pressure on the spectral absorption of deep-sea fish visual pigments
The effect of hydrostatic pressure (0.1-54 MPa, equivalent to pressures experienced by fish from the ocean's surface to depths of ca. 5400 m) on visual pigment absorption spectra was investigated for rod visual pigments extracted from the retinae of 12 species of deep-sea fish of diverse phylogeny and habitat. The wavelength of peak absorption (λmax) was shifted to longer wavelengths by an average of 1.35 nm at 40 MPa (a pressure approximately equivalent to average ocean depth) relative to measurements made at one atmosphere (ca. 0.1 MPa), but with little evidence of a change in absorbance at the λmax. We conclude that previousλ max measurements of deep-sea fish visual pigments, made at a pressure close to 0.1 MPa, provide a good indication ofλ max values at higher pressures when considering the ecology of vision in the deep-sea. Although not affecting the spectral sensitivity of the animal to any important degree, the observed shift inλ max may be of interest in the context of understanding opsin-chromophore interaction and spectral tuning of visual pigments
The RHIC Zero Degree Calorimeter
High Energy collisions of nuclei usually lead to the emission of evaporation
neutrons from both ``beam'' and ``target'' nuclei. At the RHIC heavy ion
collider with 100GeV/u beam energy, evaporation neutrons diverge by less than
milliradians from the beam axis Neutral beam fragments can be detected
downstream of RHIC ion collisions (and a large aperture Accelerator dipole
magnet) if 4 mr but charged fragments in the same angular range
are usually too close to the beam trajectory.
In this 'zero degree' region produced particles and other secondaries deposit
negligible energy when compared with that of beam fragmentation neutrons.
The purpose of the RHIC zero degree calorimeters (ZDC's) is to detect
neutrons emitted within this cone along both beam directions and measure their
total energy (from which we calculate multiplicity). The ZDC coincidence of the
2 beam directions is a minimal bias selection of heavy ion collisions. This
makes it useful as an event trigger and a luminosity monitor\cite{baltz} and
for this reason we built identical detectors for all 4 RHIC experiments.
The neutron multiplicity is also known to be correlated with event geometry
\cite{appel} and will be used to measure collision centrality in mutual beam
int eractions.Comment: 18 pages, 12 figure
An evaluation of potential locations for AROD ground stations
Airborne Range and Orbit Determination tracking stations potential locations evaluation and accuracy during early minutes of fligh
Frustrated Hubbard ladders and superconductivity in -BEDT-TTF organic compounds
Half-filled two-leg Hubbard ladders have spin-gapped short-range
antiferromagnetic correlations while three-leg ladders have power law
antoferromagnetic correlations, and both systems have d_{x^2-y^2}-power law
pairing correlations when they are doped. Thus these ladders exhibit some of
the phenomenology seen in the layered cuprates. Here we report results for
half-filled frustrated Hubbard ladders, based upon ladder segments taken from a
tight-binding model of kappa-BEDT-TTF. Although these ladders are half-filled,
varying the degree of frustration can drive them across an insulator-metal
transition. We suggest that the spin, charge and pairing correlations of these
frustrated ladders near the insulator-metal transition provide support for the
notion that kappa-BEDT-TTF is a strongly correlated superconductor
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
From an insulating to a superfluid pair-bond liquid
We study an exchange coupled system of itinerant electrons and localized
fermion pairs resulting in a resonant pairing formation. This system inherently
contains resonating fermion pairs on bonds which lead to a superconducting
phase provided that long range phase coherence between their constituents can
be established. The prerequisite is that the resonating fermion pairs can
become itinerant. This is rendered possible through the emergence of two kinds
of bond-fermions: individual and composite fermions made of one individual
electron attached to a bound pair on a bond. If the strength of the exchange
coupling exceeds a certain value, the superconducting ground state undergoes a
quantum phase transition into an insulating pair-bond liquid state. The gap of
the superfluid phase thereby goes over continuously into a charge gap of the
insulator. The change-over from the superconducting to the insulating phase is
accompanied by a corresponding qualitative modification of the dispersion of
the two kinds of fermionic excitations. Using a bond operator formalism, we
derive the phase diagram of such a scenario together with the elementary
excitations characterizing the various phases as a function of the exchange
coupling and the temperature.Comment: 10 pages, 5 figure
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Intense molecular emission from the Lagoon nebula, M8
The discovery is reported of the second strongest source of mm and submm wavelength CO line emission, towards M8, the Lagoon Nebula in Sagittarius. The ~31 M⊙ molecular core has dimensions ~0.2 x 0.3pc and is centred on the O7V star Herschel 36 (H36), near the Hourglass Nebula in the core of M8. Emission from the CO line wings extends to the north and south of the Hourglass, although a lack of near-IR H2 emission indicates that outflow activity is much less prominent than in many active star-formation regions, and suggests that the CO line wings may trace the expanding edge of a cavity around H36. The molecular line data are compared with new near-IR narrow-band, continuum-subtracted images in He I, H2, and H,+ (Brγ) lines and archival HST emission-line images in Hα, [O III], and [S II]. The optical and near-IR data are found to be broadly consistent with previous photo-ionisation models of the Hourglass, which is excited by H 36. However, there are variations in the He I/Brγ line ratio which are difficult to explain
Effects of Nonmagnetic Impurity Doping on Spin Ladder System
Effects of nonmagnetic impurity doping on an AF spin-1/2 Heisenberg ladder
system are studied by the QMC method. A single nonmagnetic impurity induces a
localized spin-1/2 moment accompanied by "static" and enhanced AF correlations
around it. Small and finite concentration of impurities induces a remarkable
change of magnetic and thermodynamic properties with gapless excitations. It
also shows rather sharp but continuous crossover around the concentration of
about 4%. Above the crossover concentration, all the spins are strongly coupled
participating in the enhanced and rather uniform power-law decay of the
antiferromagnetic correlation. Below the crossover, each impurity forms an
antiferromagnetic cluster only weakly coupled each other. For random
distribution of impurities, large Curie-like susceptibility accompanied with
small residual entropy is obtained at low temperatures in agreement with recent
experimental observation in Zn-doped . Temperature dependence of
AF susceptibility shows power-law-like but weaker divergence than the single
chain AFH in the temperature range studied.Comment: 4 pages, LaTeX+epsf.sty, submitted to J.Phys.Soc.Jpn. New results of
AF susceptibility are adde
Topics concerning state variable feedback in automatic control systems. Part 1 - Specification. Part 2 - Sensitivity. Part 3 - Intentional nonlinearities. Part 4 - Unavailable states
Specifications, sensitivity, intentional nonlinearities, and unavailable states concerned with state variable feedback in automatic control system
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