Fluctuation Effects And Order Parameter Symmetry In The Cuprate Superconductors

Effect of phase fluctuations on superconducting states with anisotropic order parameters is studied in a BCS like lattice model of cuprate superconductors. The degradation of the mean field transition temperature due to phase fluctuations is estimated within a Kosterlitz-Thouless scenario. Values of the interaction parameters for optimal doping, corresponding to a stable superconducting state of $S_{xy}$ symmetry, which fit the nodal structure of the superconducting order parameter in the Bi2212 compound, are obtained. The angular position of the node is found to be insensitive to the dopant concentration.Comment: Latex file, 8 output pages, 5 figures (available from Authors on request), to appear in Europhysics Letter

Sidewall Effects on Exact Reynolds-Stress Budgets in an Impinging Shock Wave/Boundary Layer Interaction

Large-eddy simulations are performed using wall-resolved mesh for a Mach 2.29 impinging shock wave/boundary-layer interaction. Flow conditions are based on an experiment and therefore entire span was simulated, including the two sidewalls. Mean flow comparison with the experimental data showed that the interaction was larger in the simulation. Time-series analysis of a rake of pressure probes immediately downstream of the mean reflected shock position showed a peak in weighted power spectral density occurred about $St_{Lint}=0.01$, owing to a larger interaction length. Budgets of Reynolds-stress transport calculated across the span and along the corner bisector showed high degree of anisotropy. Merging of the secondary flows and separation along the corner gives rise to unstablecounter rotating vortices, which straddle the corner and grow in size. This also leads to a development of new behavior in the viscous sublayer along the corner bisector, where the pressure strain andmolecular diffusion mechanisms become prominent

Josephson Coupling through a Quantum Dot

We derive, via fourth order perturbation theory, an expression for the Josephson current through a gated interacting quantum dot. We analyze our expression for two different models of the superconductor-dot-superconductor (SDS) system. When the matrix elements connecting dot and leads are featureless constants, we compute the Josephson coupling J_c as a function of the gate voltage and Coulomb interaction. In the diffusive dot limit, we compute the probability distribution P(J_c) of Josephson couplings. In both cases, pi junction behavior (J_c < 0) is possible, and is not simply dependent on the parity of the dot occupancy.Comment: 9 pages; 3 encapsulated PostScript figure

A study of the action of risperidone at 5-HT2A receptors

Risperidone is an ‘atypical’ antipsychotic and is approved by the USFDA mainly for the treatment of schizophrenia and symptoms of bipolar disorder. Risperidone (an SDA or serotonin-dopamine antagonist) has ~20-fold higher affinity at 5-HT2A receptors over dopamine D2 receptors, which makes it more efficacious against the negative symptoms of schizophrenia and less liable to causing extrapyramidal side effects than ‘typical’ antipsychotics. The major goal of the current investigation was to study the structure of risperidone and to identify the minimum structural features required for 5-HT2A receptor affinity that retain antagonist action. The structure of risperidone was systematically deconstructed, and functional activity studies using calcium imaging in HEK293 cells and a two-electrode voltage clamp (TEVC) assay in a Xenopus laevis heterologous system were coupled with radioligand binding affinity studies to achieve this goal. The biological studies showed that the entire structure of risperidone was not required for activity or affinity at the receptor, as 6-fluoro-[3-(1-methylpiperidin-4-yl)]benz[d]isoxazole was comparable to risperidone in both affinity and activity. Next, the structure of risperidone was elaborated to determine the importance of its left and right “halves” in its actions. The left and the right halves of risperidone were substituted with those of another antagonist, ketanserin, to give structural hybrids. Biological studies suggested that the right half of risperidone [i.e., the 6-fluoro-(3-piperidin-4-yl)benz[d]isoxazole moiety] might be important for affinity. In order to assess how the biologically-active compounds interact at the receptor, homology models of the human 5-HT2A receptor were developed, and docking and Hydropathic INTeraction studies were conducted. Risperidone seemed to form a bifurcated hydrogen bond with S159 (TM3), which ketanserin was unable to form. This interaction might account for high binding affinity at the receptor as it is common to both, risperidone and 3-[2-(4-(6-fluorobenz[d]isoxazol-3-yl)piperidin-1-yl)ethyl]-2,4-(1H,3H)quinazolinedione. With the data currently in hand, we can conclude that the entire structure of risperidone is not required for activity or affinity, and that the right “half” (i.e. the benzisoxazolyl portion) of risperidone might be influencing activity and affinity at 5-HT2A receptors

Reynolds-Stress Budgets in an Impinging Shock Wave/Boundary-Layer Interaction

Implicit large-eddy simulation (ILES) of a shock wave/boundary-layer interaction (SBLI) was performed. Comparisons with experimental data showed a sensitivity of the current prediction to the modeling of the sidewalls. This was found to be common among various computational studies in the literature where periodic boundary conditions were used in the spanwise direction, as was the case in the present work. Thus, although the experiment was quasi-two-dimensional, the present simulation was determined to be two-dimensional. Quantities present in the exact equation of the Reynolds-stress transport, i.e., production, molecular diffusion, turbulent transport, pressure diffusion, pressure strain, dissipation, and turbulent mass flux were calculated. Reynolds-stress budgets were compared with past large-eddy simulation and direct numerical simulation datasets in the undisturbed portion of the turbulent boundary layer to validate the current approach. The budgets in SBLI showed the growth in the production term for the primary normal stress and energy transfer mechanism was led by the pressure strain term in the secondary normal stresses. The pressure diffusion term, commonly assumed as negligible by turbulence model developers, was shown to be small but non-zero in the normal stress budgets, however it played a key role in the primary shear stress budget

A pulse size estimation method for reduced-order models

Model-Order Reduction (MOR) is an important technique that allows Reduced-Order Models (ROMs) of physical systems to be generated that can capture the dominant dynamics, but at lower cost than the full order system. One approach to MOR that has been successfully implemented in fluid dynamics is the Eigensystem Realization Algorithm (ERA). This method requires only minimal changes to the inputs and outputs of a CFD code so that the linear responses of the system to unit impulses on each input channel can be extracted. One of the challenges with the method is to specify the size of the input pulse. An inappropriate size may cause a failure of the code to converge due to non-physical behaviour arising during the solution process. This paper addresses this issue by using piston theory to estimate the appropriate input pulse size

Raman scattering in high temperature superconductors : An integrated view

The common features in the Raman data of high temperature superconductors: (the cuprates, bismathates, alkali doped fullerides and some organic superconductors), are analyzed. It was shown that qualitative understanding of the data can be achieved in terms of non-Fermi liquid models for their normal state, with appropiate bag mechanisms for the superconducting state.Comment: To appear in Physica B (1996). Invited talk presented by S. N. Behera, Latex file in revtex style, six figures available on request to first author (e-mail : [email protected]

Phase Diagram of the Half-Filled Extended Hubbard Model in Two Dimensions

We consider an extended Hubbard model of interacting fermions on a lattice. The fermion kinetic energy corresponds to a tight binding Hamiltonian with nearest neighbour (-t) and next nearest neighbour (t') hopping matrix elements. In addition to the onsite Hubbard interaction (U) we also consider a nearest neighbour repulsion (V). We obtain the zero temperature phase diagram of our model within the Hartree-Fock approximation. We consider ground states having charge and spin density wave ordering as well as states with orbital antiferromagnetism or spin nematic order. The latter two states correspond to particle-hole binding with $d_{x^2-y^2}$ symmetry in the charge and spin channels respectively. For $t' = 0$, only the charge density wave and spin density wave states are energetically stable. For non-zero t', we find that orbital antiferromagnetism (or spin nematic) order is stable over a finite portion of the phase diagram at weak coupling. This region of stability is seen to grow with increasing values of t'.Comment: Latex file, 10 output pages, 3 Figures (available on request to [email protected]), to appear in Phys. Rev. B (BR