1,301 research outputs found
Effects of semiclassical spiral fluctuations on hole dynamics
We investigate the dynamics of a single hole coupled to the spiral
fluctuations related to the magnetic ground states of the antiferromagnetic
J_1-J_2-J_3 Heisenberg model on a square lattice. Using exact diagonalization
on finite size clusters and the self consistent Born approximation in the
thermodynamic limit we find, as a general feature, a strong reduction of the
quasiparticle weight along the spiral phases of the magnetic phase diagram. For
an important region of the Brillouin Zone the hole spectral functions are
completely incoherent, whereas at low energies the spectral weight is
redistributed on several irregular peaks. We find a characteristic value of the
spiral pitch, Q=(0.7,0.7)\pi, for which the available phase space for hole
scattering is maximum. We argue that this behavior is due to the non trivial
interference of the magnon assisted and the free hopping mechanism for hole
motion, characteristic of a hole coupled to semiclassical spiral fluctuations.Comment: 6 pages, 5 figure
Transport properties of a two impurity system: a theoretical approach
A system of two interacting cobalt atoms, at varying distances, was studied
in a recent scanning tunneling microscope experiment by Bork et. al.[Nature
Phys. 7, 901 (2011)]. We propose a microscopic model that explains, for all
experimentally analyzed interatomic distances, the physics observed in these
experiments. Our proposal is based on the two-impurity Anderson model, with the
inclusion of a two-path geometry for charge transport. This many-body system is
treated in the finite-U slave boson mean-field approximation and the
logarithmic-discretization embedded-cluster approximation. We physically
characterize the different charge transport regimes of this system at various
interatomic distances and show that, as in the experiments, the features
observed in the transport properties depend on the presence of two impurities
but also on the existence of two conducting channels for electron transport. We
interpret the splitting observed in the conductance as the result of the
hybridization of the two Kondo resonances associated with each impurity.Comment: 5 pages, 5 figure
Selfconsistent hybridization expansions for static properties of the Anderson impurity model
By means of a projector-operator formalism we derive an approximation based
on a self consistent hybridization expansion to study the ground state
properties of the Anderson Impurity model. We applied the approximation to the
general case of finite Coulomb repulsion , extending previous work with the
same formalism in the infinite- case. The treatment provides a very accurate
calculation of the ground state energy and their related zero temperature
properties in the case in which is large enough, but still finite, as
compared with the rest of energy scales involved in the model. The results for
the valence of the impurity are compared with exact results that we obtain from
equations derived using the Bethe ansatz and with a perturbative approach. The
magnetization and magnetic susceptibility is also compared with Bethe ansatz
results. In order to do this comparison, we also show how to regularize the
Bethe ansatz integral equations necessary to calculate the impurity valence,
for arbitrary values of the parameters.Comment: 8 pages, 5 figure
Cl electrosorption on Ag(100): Lateral interactions and electrosorption valency from comparison of Monte Carlo simulations with chronocoulometry experiments
We present Monte Carlo Simulations using an equilibrium lattice-gas model for
the electrosorption of Cl on Ag(100) single-crystal surfaces. Fitting the
simulated isotherms to chronocoulometry experiments, we extract parameters such
as the electrosorption valency gamma and the next-nearest-neighbor lateral
interaction energy phi_nnn. Both coverage-dependent and coverage independent
gamma were previously studied assuming a constant phi_nnn [I. Abou Hamad, Th.
Wandlowski, G. Brown, P.A. Rikvold, J. Electroanal. Chem. 554-555 (2003) 211].
Here, a self-consistent, entirely electrostatic picture of the lateral
interactions with a coverage-dependent phi_nnn is developed, and a relationship
between phi_nnn and gamma is investigated for Cl on Ag(100).Comment: Accepted for publication in Electrochimica Acta, 10 pages, 7 figures,
2 tables and an appendi
Spin polaron in the J1-J2 Heisenberg model
We have studied the validity of the spin polaron picture in the frustrated
J1-J2 Heisenberg model. For this purpose, we have computed the hole spectral
functions for the Neel, collinear, and disordered phases of this model, by
means of the self-consistent Born approximation and Lanczos exact
diagonalization on finite-size clusters. We have found that the spin polaron
quasiparticle excitation is always well defined for the magnetically ordered
Neel and collinear phases, even in the vicinity of the magnetic quantum
critical points, where the local magnetization vanishes. As a general feature,
the effect of frustration is to increase the amplitude of the multimagnon
states that build up the spin polaron wave function, leading to the reduction
of the quasiparticle coherence. Based on Lanczos results, we discuss the
validity of the spin polaron picture in the disordered phase.Comment: 9 pages, 12 figure
Effects of lateral diffusion on morphology and dynamics of a microscopic lattice-gas model of pulsed electrodeposition
The influence of nearest-neighbor diffusion on the decay of a metastable
low-coverage phase (monolayer adsorption) in a square lattice-gas model of
electrochemical metal deposition is investigated by kinetic Monte Carlo
simulations. The phase-transformation dynamics are compared to the
well-established Kolmogorov-Johnson-Mehl-Avrami theory. The phase
transformation is accelerated by diffusion, but remains in accord with the
theory for continuous nucleation up to moderate diffusion rates. At very high
diffusion rates the phase-transformation kinetic shows a crossover to
instantaneous nucleation. Then, the probability of medium-sized clusters is
reduced in favor of large clusters. Upon reversal of the supersaturation, the
adsorbate desorbs, but large clusters still tend to grow during the initial
stages of desorption. Calculation of the free energy of subcritical clusters by
enumeration of lattice animals yields a quasi-equilibrium distribution which is
in reasonable agreement with the simulation results. This is an improvement
relative to classical droplet theory, which fails to describe the
distributions, since the macroscopic surface tension is a bad approximation for
small clusters.Comment: Minor corrections and modifications. 15 pages with 10 figures.
Accepted for publication in the Journal of Chemical Physics, see
http://jcp.aip.org/jcp
A prospective randomized trial of tacrolimus and prednisone versus tacrolimus, prednisone, and mycophenolate mofetil in primary adult liver transplant recipients: An interim report
Background. Tacrolimus (Tac) and mycophenolate mofetil (MMF) are newly approved immunosuppressive agents. However, the safety and efficacy of the combination of MMF and Tac in primary liver transplantation has not been determined. Methods. An Institutional Review Board-approved, open-label prospective randomized protocol was initiated to study the efficacy and toxicity of Tac and steroids (double-drug therapy) versus Tac, steroids, and MMF (triple-drug therapy) in primary adult liver transplant recipients. Both groups of patients began on the same doses of Tac and steroids. Patients randomized to triple-drug therapy also received 1 g of MMF twice a day. Results. Between August 1995 and January 1997, 200 patients were enrolled, 99 in double-drug therapy and 101 in triple-drug therapy. All patients were followed until May 1997, with a mean follow-up of 12.7 months. During the study period, 28 of 99 patients in double-drug therapy received MMF to control ongoing acute rejection, nephrotoxicity, and/or neurotoxicity. On the other hand, 61 patients in triple-drug therapy discontinued MMF for infection, myelosuppression, and/or gastrointestinal disturbances. By an 'intention-to-treat analysis,' the actuarial 1-year patient survival rate was 85.1% in double-drug therapy and 83.1% in triple-drug therapy (P=0.77). The actuarial 1-year graft survival rate was 80.2% for double-drug therapy and 79.2% for triple-drug therapy (P=0.77). Forty-one patients (41.4%) in double- drug therapy and 32 (31.7%) in triple-drug therapy had at least one episode of rejection, but this was not statistically significant (P=0.15). The mean maintenance dose of corticosteroids was slightly lower in triple-drug compared with double-drug therapy. Conclusion. Patient and graft survival rates were similar in both groups. There was a trend to a lower incidence of rejection, reduced nephrotoxicity, and a lesser amount of maintenance corticosteroids in triple-drug therapy compared with double-drug therapy
1/T_1 nuclear relaxation time of \kappa-(BEDT-TTF)_ 2 Cu [N(CN)_2] Cl : effects of magnetic frustration
We study the role played by the magnetic frustration in the antiferromagnetic
phase of the organic salt \kappa-(BEDT-TTF)_ 2 Cu [N(CN)_2] Cl. Using the
spatially anisotropic triangular Heisenberg model we analyze previous and new
performed NMR experiments. We compute the 1/T_1 relaxation time by means of the
modified spin wave theory. The strong suppression of the nuclear relaxation
time observed experimentally under varying pressure and magnetic field is
qualitatively well reproduced by the model. Our results suggest the existence
of a close relation between the effects of pressure and magnetic frustration.Comment: 10 pages, 9 figures, to appear in Journal of Phys.: Condens Matte
Predictors of Literacy and Attitudes Toward Reading Among Syrian Refugee Children in Jordan
Refugee children often face disruptions to their education before and during displacement. However, little is known about either levels or predictors of refugee childrenâs literacy or about their attitudes toward reading in low- or middle-income countries. To address this, we conducted in-home literacy assessments using the Holistic Assessment of Learning and Development Outcomes with 322 Syrian refugee motherâchild dyads who lived in Jordan (child age range 4â8Â years, Mâ=â6.32Â years, 50% female). Overall, the children had quite low levels of literacy, although they indicated a strong enthusiasm for reading. Child age, maternal education, and maternal ability to read all predicted child literacy, although maternal literacy predicted it only among children enrolled in school. Among those enrolled in school (64.9% of the total sample, 88.7% of those agedââ„â6), students attending hybrid classes had better literacy than those attending either solely in-person or solely online, although the frequency of school attendance did not predict literacy. A less consistent pattern emerged for predicting childrenâs attitudes toward reading. Our results suggest an urgent need to improve literacy skills among refugee children in Jordan, as well as a need for validated measures of attitudes toward reading for use with Arabic-speaking youth. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s13158-022-00334-x
Coagulation factor XII protease domain crystal structure
Background: Coagulation factor XII is a serine protease that is important for kinin generation and blood coagulation, cleaving the substrates plasma kallikrein and FXI.Objective: To investigate FXII zymogen activation and substrate recognition by determining the crystal structure of the FXII protease domain.Methods and results: A series of recombinant FXII protease constructs were characterized by measurement of cleavage of chromogenic peptide and plasma kallikrein protein substrates. This revealed that the FXII protease construct spanning the light chain has unexpectedly weak proteolytic activity compared to ÎČâFXIIa, which has an additional nine amino acid remnant of the heavy chain present. Consistent with these data, the crystal structure of the light chain protease reveals a zymogen conformation for active site residues Gly193 and Ser195, where the oxyanion hole is absent. The Asp194 side chain salt bridge to Arg73 constitutes an atypical conformation of the 70âloop. In one crystal form, the S1 pocket loops are partially flexible, which is typical of a zymogen. In a second crystal form of the deglycosylated light chain, the S1 pocket loops are ordered, and a short αâhelix in the 180âloop of the structure results in an enlarged and distorted S1 pocket with a buried conformation of Asp189, which is critical for P1 Arg substrate recognition. The FXII structures define patches of negative charge surrounding the active site cleft that may be critical for interactions with inhibitors and substrates.Conclusions: These data provide the first structural basis for understanding FXII substrate recognition and zymogen activation
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