819 research outputs found
A new class of exactly solvable interacting fermion models in one dimension
We investigate a model containing two species of one-dimensional fermions
interacting via a gauge field determined by the positions of all particles of
the opposite species. The model can be solved exactly via a simple unitary
transformation. Nevertheless, correlation functions exhibit nontrivial
interaction-dependent exponents. A similar model defined on a lattice is
introduced and solved. Various generalizations, e.g. to the case of internal
symmetries of the fermions, are discussed. The present treatment also clarifies
certain aspects of Luttinger's original solution of the ``Luttinger model''.Comment: 11 pages, revtex 3.0, no figures, some typos correcte
Drude weight and total optical weight in a t-t'-J model
We study the Drude weight D and the total optical weight K for a t-t'-J model
on a square lattice that exhibits a metallic phase-modulated antiferromagnetic
ground state close to half-filling. Within a suitable 1/N expansion that
includes leading quantum-fluctuation effects, D and K are found to increase
linearly with small hole doping away from the Mott metal-insulator transition
point at half-filling. The slow zero-sound velocity near the latter transition
identifies with the velocity of the lower-energy branch of the twofold
excitation spectrum. At higher doping values, D and K eventually saturate and
then start to decrease. These features are in qualitative agreement with
optical conductivity measurements in doped antiferromagnets.Comment: 7 pages, REVTEX file (3 Postscript figures). To appear in J. Phys.:
Condens. Mattte
Generalised Shastry-Sutherland Models in three and higher dimensions
We construct Heisenberg anti-ferromagnetic models in arbitrary dimensions
that have isotropic valence bond crystals (VBC) as their exact ground states.
The d=2 model is the Shastry-Sutherland model. In the 3-d case we show that it
is possible to have a lattice structure, analogous to that of SrCu_2(BO_3)_2,
where the stronger bonds are associated with shorter bond lengths. A dimer mean
field theory becomes exact at d -> infinity and a systematic 1/d expansion can
be developed about it. We study the Neel-VBC transition at large d and find
that the transition is first order in even but second order in odd dimensions.Comment: Published version; slightly expande
NASA's Evolutionary Xenon Thruster (NEXT) Long-Duration Test as of 736 kg of Propellant Throughput
The NASA s Evolutionary Xenon Thruster (NEXT) program is developing the next-generation solar-electric ion propulsion system with significant enhancements beyond the state-of-the-art NASA Solar Electric Propulsion Technology Application Readiness (NSTAR) ion propulsion system to provide future NASA science missions with enhanced mission capabilities. A Long-Duration Test (LDT) was initiated in June 2005 to validate the thruster service life modeling and to qualify the thruster propellant throughput capability. The thruster has set electric propulsion records for the longest operating duration, highest propellant throughput, and most total impulse demonstrated. At the time of this publication, the NEXT LDT has surpassed 42,100 h of operation, processed more than 736 kg of xenon propellant, and demonstrated greater than 28.1 MN s total impulse. Thruster performance has been steady with negligible degradation. The NEXT thruster design has mitigated several lifetime limiting mechanisms encountered in the NSTAR design, including the NSTAR first failure mode, thereby drastically improving thruster capabilities. Component erosion rates and the progression of the predicted life-limiting erosion mechanism for the thruster compare favorably to pretest predictions based upon semi-empirical ion thruster models used in the thruster service life assessment. Service life model validation has been accomplished by the NEXT LDT. Assuming full-power operation until test article failure, the models and extrapolated erosion data predict penetration of the accelerator grid grooves after more than 45,000 hours of operation while processing over 800 kg of xenon propellant. Thruster failure due to degradation of the accelerator grid structural integrity is expected after groove penetration
Near-equivalence of the role of structural unpinning number, basicity and reciprocal average electronegativity in determining the conductivity of glasses
The chemical approach made to investigate the origin of fast ion conduction in Agl-based fast ion conducting (FIC) glasses has been extended to various ionically conducting systems containing Na+ ion. An index known as structural unpinning number (SUN), S, has been defined for this purpose based on the unscreened nuclear charge on the cation and the average electronegativity of all the anions. Variation of the log(conductivity), at a given temperature, as a function of structural unpinning number, optical basicity, λ, and the reciprocal average electronegativity of all the anions, l/χa, has been examined for a number of Na+-ion conducting glasses and a nearly identical variation has been noticed in all the cases. The equivalence of these chemical parameters as determinants of the conductivity behavior of glasses has thus been established and the origin of this equivalence has been discussed
Fermionic R-Operator and Integrability of the One-Dimensional Hubbard Model
We propose a new type of the Yang-Baxter equation (YBE) and the decorated
Yang-Baxter equation (DYBE). Those relations for the fermionic R-operator were
introduced recently as a tool to treat the integrability of the fermion models.
Using the YBE and the DYBE for the XX fermion model, we construct the fermionic
R-operator for the one-dimensional (1D) Hubbard model. It gives another proof
of the integrability of the 1D Hubbard model. Furthermore a new approach to the
SO(4) symmetry of the 1D Hubbard model is discussed.Comment: 25 page
Status of NASA's Evolutionary Xenon Thruster (NEXT) Long-Duration Test as of 50,000 h and 900 kg Throughput
The NASA's Evolutionary Xenon Thruster (NEXT) project is developing the next-generation solar electric propulsion ion propulsion system with significant enhancements beyond the state-of-the-art NASA Solar Electric Propulsion Technology Application Readiness (NSTAR) ion propulsion system in order to provide future NASA science missions with enhanced propulsion capabilities. As part of a comprehensive thruster service life assessment, the NEXT Long-Duration Test (LDT) was initiated in June 2005 to demonstrate throughput capability and validate thruster service life modeling. The NEXT LDT exceeded its original qualification throughput requirement of 450 kg in December 2009. To date, the NEXT LDT has set records for electric propulsion lifetime and has demonstrated 50,170 h of operation, processed 902 kg of propellant, and delivered 34.9 MN-s of total impulse. The NEXT thruster design mitigated several life-limiting mechanisms encountered in the NSTAR design, dramatically increasing service life capability. Various component erosion rates compare favorably to the pretest predictions based upon semi-empirical ion thruster models. The NEXT LDT either met or exceeded all of its original goals regarding lifetime demonstration, performance and wear characterization, and modeling validation. In light of recent budget constraints and to focus on development of other components of the NEXT ion propulsion system, a voluntary termination procedure for the NEXT LDT began in April 2013. As part of this termination procedure, a comprehensive post-test performance characterization was conducted across all operating conditions of the NEXT throttle table. These measurements were found to be consistent with prior data that show minimal degradation of performance over the thruster's 50 kh lifetime. Repair of various diagnostics within the test facility is presently planned while keeping the thruster under high vacuum conditions. These diagnostics will provide additional critical information on the current state of the thruster, in regards to performance and wear, prior to destructive post-test analyses performed on the thruster under atmosphere conditions
Status of NASA's Evolutionary Xenon Thruster (NEXT) Long-Duration Test as of 50,000 h and 900 kg Throughput
The NASA's Evolutionary Xenon Thruster (NEXT) project is developing the next-generation solar electric propulsion ion propulsion system with significant enhancements beyond the state-of-the-art NASA Solar Electric Propulsion Technology Application Readiness (NSTAR) ion propulsion system in order to provide future NASA science missions with enhanced propulsion capabilities. As part of a comprehensive thruster service life assessment, the NEXT Long-Duration Test (LDT) was initiated in June 2005 to demonstrate throughput capability and validate thruster service life modeling. The NEXT LDT exceeded its original qualification throughput requirement of 450 kg in December 2009. To date, the NEXT LDT has set records for electric propulsion lifetime and has demonstrated 50,170 hours of operation, processed 902 kg of propellant, and delivered 34.9 MN-s of total impulse. The NEXT thruster design mitigated several life-limiting mechanisms encountered in the NSTAR design, dramatically increasing service life capability. Various component erosion rates compare favorably to the pretest predictions based upon semi-empirical ion thruster models. The NEXT LDT either met or exceeded all of its original goals regarding lifetime demonstration, performance and wear characterization, and modeling validation. In light of recent budget constraints and to focus on development of other components of the NEXT ion propulsion system, a voluntary termination procedure for the NEXT LDT began in April 2013. As part of this termination procedure, a comprehensive post-test performance characterization was conducted across all operating conditions of the NEXT throttle table. These measurements were found to be consistent with prior data that show minimal degradation of performance over the thruster's 50 kh lifetime. Repair of various diagnostics within the test facility is presently planned while keeping the thruster under high vacuum conditions. These diagnostics will provide additional critical information on the current state of the thruster, in regards to performance and wear, prior to destructive post-test analyses performed on the thruster under atmosphere conditions
Direct observation of the multiple spin gap excitations in two-dimensional dimer system SrCu2(BO3)2
Various spin gap excitations have been observed in the two-dimensional dimer
system SrCu_2(BO_3)_2 by means of submillimeter wave ESR. The zero-field energy
gap of the lowest spin gap excitation shows a splitting into two triplet modes
and the energy splitting clearly depends on the magnetic field orientation when
a field is rotated in the {\mib {ac}}-plane. A zero-field splitting is also
found between the S(_z)=+1 and S(_z)=-1 branches of each triplet. These
behaviors are qualitatively explained by considering the anisotropic exchange
coupling of inter-dimer and intra-dimer, respectively. The averaged value of
the lowest spin gap energy is determined to be 722 \pm 2 GHz(34.7 K). We have
also found the second spin gap excitation at 1140 GHz(54.7 K), which indicates
that the inter-dimer coupling is significantly strong. Besides these modes, a
number of gapped ESR absorption are found and we propose that these multiple
magnetic excitations are caused by the localized nature of the excited state in
the present system.Comment: 4pages 4figure
- …