563 research outputs found
A Bosonic Model of Hole Pairs
We numerically investigate a bosonic representation for hole pairs on a
two-leg t-J ladder where hard core bosons on a chain represent the hole pairs
on the ladder. The interaction between hole pairs is obtained by fitting the
density profile obtained with the effective model to the one obtained with the
\tj model, taking into account the inner structure of the hole pair given by
the hole-hole correlation function. For these interactions we calculate the
Luttinger liquid parameter, which takes the universal value as
half filling is approached, for values of the rung exchange between strong
coupling and the isotropic case. The long distance behavior of the hole-hole
correlation function is also investigated. Starting from large , the
correlation length first increases as expected, but diminishes significantly as
is reduced and bound holes sit mainly on adjacent rungs. As the isotropic
case is approached, the correlation length increases again. This effect is
related to the different kind of bonds in the region between the two holes of a
hole pair when they move apart.Comment: 11 page
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Hydrothermal oxidation of Navy shipboard excess hazardous materials
This study demonstrated effective destruction, using a novel supercritical water oxidation reactor, of oil, jet fuel, and hydraulic fluid, common excess hazardous materials found on-board Navy vessels. This reactor uses an advanced injector design to mix the hazardous compounds with water, oxidizer, and a supplementary fuel and it uses a transpiring wall to protect the surface of the reactor from corrosion and salt deposition. Our program was divided into four parts. First, basic chemical kinetic data were generated in a simple, tubular-configured reactor for short reaction times (5 seconds) as a function of temperature. Second, using the data, an engineering model was developed for the more complicated industrial reactor mentioned above. Third, the three hazardous materials were destroyed in a quarter-scale version of the industrial reactor. Finally, the test data were compared with the model. The model and the experimental results for the quarter-scale reactor are described and compared in this report. A companion report discusses the first part of the program to generate basic chemical kinetic data. The injector and reactor worked as expected. The oxidation reaction with the supplementary fuel was initiated between 400 {degrees}C and 450 {degrees}C. The released energy raised the reactor temperature to greater than 600 {degrees}C. At that temperature, the hazardous materials were efficiently destroyed in less than five seconds. The model shows good agreement with the test data and has proven to be a useful tool in designing the system and understanding the test results. 16 refs., 17 figs., 11 tabs
Two-Hole and Four-Hole Bound States in a t-J Ladder at half-filling
The two-hole excitation spectrum of the t-J ladder at half-filling is studied
using linked-cluster series expansion methods. A rich spectrum of bound states
emerges, particularly at small . Their dispersion relations and coherence
lengths are computed, along with the threshold behaviour as the bound states
merge into the continuum. A class of 4-hole bound states is also studied,
leading to the conclusion that phase separation occurs for ,
in agreement with other studies.Comment: revtex
Inhomogeneously doped two-leg ladder systems
A chemical potential difference between the legs of a two-leg ladder is found
to be harmful for Cooper pairing. The instability of superconductivity in such
systems is analyzed by compairing results of various analytical and numerical
methods. Within a strong coupling approach for the t-J model, supplemented by
exact numerical diagonalization, hole binding is found unstable beyond a
finite, critical chemical potential difference. The spinon-holon mean field
theory for the t-J model shows a clear reduction of the the BCS gaps upon
increasing the chemical potential difference leading to a breakdown of
superconductivity. Based on a renormalization group approach and Abelian
bosonization, the doping dependent phase diagram for the weakly interacting
Hubbard model with different chemical potentials was determined.Comment: Revtex4, 11 pages, 7 figure
Arithmetical properties of Multiple Ramanujan sums
In the present paper, we introduce a multiple Ramanujan sum for arithmetic
functions, which gives a multivariable extension of the generalized Ramanujan
sum studied by D. R. Anderson and T. M. Apostol. We then find fundamental
arithmetic properties of the multiple Ramanujan sum and study several types of
Dirichlet series involving the multiple Ramanujan sum. As an application, we
evaluate higher-dimensional determinants of higher-dimensional matrices, the
entries of which are given by values of the multiple Ramanujan sum.Comment: 19 page
From Majorana theory of atomic autoionization to Feshbach resonances in high temperature superconductors
The Ettore Majorana paper - Theory of incomplete P triplets- published in
1931, focuses on the role of selection rules for the non-radiative decay of two
electron excitations in atomic spectra, involving the configuration interaction
between discrete and continuum channels. This work is a key step for
understanding the 1935 work of Ugo Fano on the asymmetric lineshape of two
electron excitations and the 1958 Herman Feshbach paper on the shape resonances
in nuclear scattering arising from configuration interaction between many
different scattering channels. The Feshbach resonances are today of high
scientific interest in many different fields and in particular for ultracold
gases and high Tc superconductivity.Comment: 13 pages, 7 figures. Journal of Superconductivity and Novel Magnetism
to be publishe
Structural and functional studies of histidine biosynthesis in Acanthamoeba spp. demonstrates a novel molecular arrangement and target for antimicrobials
Acanthamoeba is normally free-living, but sometimes facultative and occasionally opportunistic parasites. Current therapies are, by necessity, arduous and yet poorly effective due to their inabilities to kill cyst stages or in some cases to actually induce encystation. Acanthamoeba can therefore survive as cysts and cause disease recurrence. Herein, in pursuit of better therapies and to understand the biochemistry of this understudied organism, we characterize its histidine biosynthesis pathway and explore the potential of targeting this with antimicrobials. We demonstrate that Acanthamoeba is a histidine autotroph, but with the ability to scavenge preformed histidine. It is able to grow in defined media lacking this amino acid, but is inhibited by 3-amino-1,2,4-triazole (3AT) that targets Imidazoleglycerol-Phosphate Dehydratase (IGPD) the rate limiting step of histidine biosynthesis. The structure of Acanthamoeba IGPD has also been determined in complex with 2-hydroxy-3-(1,2,4-triazol-1-yl) propylphosphonate [(R)-C348], a recently described novel inhibitor of Arabidopsis thaliana IGPD. This compound inhibited the growth of four Acanthamoeba species, having a 50% inhibitory concentration (IC50) ranging from 250-526 nM. This effect could be ablated by the addition of 1 mM exogenous free histidine, but importantly not by physiological concentrations found in mammalian tissues. The ability of 3AT and (R)-C348 to restrict the growth of four strains of Acanthamoeba spp. including a recently isolated clinical strain, while not inducing encystment, demonstrates the potential therapeutic utility of targeting the histidine biosynthesis pathway in Acanthamoeba
Excitation Spectra and Thermodynamic Response of Segmented Heisenberg Spin Chains
The spectral and thermodynamic response of segmented quantum spin chains is
analyzed using a combination of numerical techniques and finite-size scaling
arguments. Various distributions of segment lengths are considered, including
the two extreme cases of quenched and annealed averages. As the impurity
concentration is increased, it is found that (i) the integrated spectral weight
is rapidly reduced, (ii) a pseudo-gap feature opens up at small frequencies,
and (iii) at larger frequencies a discrete peak structure emerges, dominated by
the contributions of the smallest cluster segments. The corresponding
low-temperature thermodynamic response has a divergent contribution due to the
odd-site clusters and a sub-dominant exponentially activated component due to
the even-site segments whose finite-size gap is responsible for the spectral
weight suppression at small frequencies. Based on simple scaling arguments,
approximate low-temperature expressions are derived for the uniform
susceptibility and the heat capacity. These are shown to be in good agreement
with numerical solutions of the Bethe ansatz equations for ensembles of
open-end chains.Comment: RevTex, 9 pages with 6 figure
Paramagnetic effect in YBaCuO grain boundary junctions
A detailed investigation of the magnetic response of YBaCuO grain boundary
Josephson junctions has been carried out using both radio-frequency
measurements and Scanning SQUID Microscopy. In a nominally zero-field-cooled
regime we observed a paramagnetic response at low external fields for 45 degree
asymmetric grain boundaries. We argue that the observed phenomenology results
from the d-wave order parameter symmetry and depends on Andreev bound states.Comment: To be published in Phys. Rev.
A quantum Monte Carlo study of the one-dimensional ionic Hubbard model
Quantum Monte Carlo methods are used to study a quantum phase transition in a
1D Hubbard model with a staggered ionic potential (D). Using recently
formulated methods, the electronic polarization and localization are determined
directly from the correlated ground state wavefunction and compared to results
of previous work using exact diagonalization and Hartree-Fock. We find that the
model undergoes a thermodynamic transition from a band insulator (BI) to a
broken-symmetry bond ordered (BO) phase as the ratio of U/D is increased. Since
it is known that at D = 0 the usual Hubbard model is a Mott insulator (MI) with
no long-range order, we have searched for a second transition to this state by
(i) increasing U at fixed ionic potential (D) and (ii) decreasing D at fixed U.
We find no transition from the BO to MI state, and we propose that the MI state
in 1D is unstable to bond ordering under the addition of any finite ionic
potential. In real 1D systems the symmetric MI phase is never stable and the
transition is from a symmetric BI phase to a dimerized BO phase, with a
metallic point at the transition
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