Suppression in critical temperature, Tc,onset, in GdBa2Cu3O6.9 superconductors by nonmagnetic Zn doping in Cu sites is observed. Nonlinear current–voltage (IV) characteristics below Tc,onset are used to determine an exponent, η(T), related to the superfluid phase stiffness, Js(T). Tc,onset of GdBa2Cu3O6.9 decreases by approximately 10-26 K as the doping concentration of Zn at the Cu site increases from 0.019 to 0.076. Transport grain critical current density, Jc(T), at several constant temperatures has been extracted in the phase transition region. Depairing current density, jdp(0) has been extracted. At a reduced temperature of 0.88, Jc decreases by approximately 10-13 times with Zn doping (x = 0.019-0.057), whereas jdp(0) is reduced by only about a factor of two at x=0.057 relative to pure GdBa2Cu3O6.9. The transport grain Jc(T) is found to be within less than 1.0% of the extracted jdp(0) of GdBa2Cu3O6.9 superconductor.

We present a numerical study on the band structure (BS), density of states (DOS) and Fermi surface (FS) of the layered Y 3 Ba 5 Cu 8 O 18 (Y-358) superconductor using two methods namely (i) generalized gradient approximation (GGA) and (ii) GGA+U within the framework of the density functional theory (DFT). Projected density of states (PDOS) corresponding to the total number of 2D CuO 2 planes, 1D CuO chains and BaO planes with apical oxygens are investigated. Shifts of peaks in PDOS contributed by Cu and O for U = 0, 2.0, 4.0, 6.0, 8.0 eV are investigated to understand pairing mechanisms in Y-358. The probability of the formation of Fermi pocket near the M high symmetry point of the Brillouin zone has been explored.

We have investigated transport properties of polycrystalline YBa2Cu3O7-(YBCO) superconductors containing tin (Sn) nanoparticles in the intergranular region. The normal state resistivity of YBCO with 2.0 wt.% of Sn at 300 K increases 2.35 times in comparison to that of the pure YBCO. The addition of Sn nanoparticles at several weight percentages has significantly influenced the transport critical current density (c) in YBCO. Using the Ambegaokar-Halperin-Nelson-Siggia (AHNS) theory, we have extracted superfluid phase stiffness (Js) as a function of temperature (T). The variation of Jc(T) in relation to Js(T) has also been examined within the vicinity of the phase transition region. Additionally, the behavior of Jc(T) has been discussed by considering the delta Tc and delta l flux pinning mechanisms.

We have investigated how the low concentrations of ferromagnetic Co-nanoparticles in intergranular networks of multi-layered Y3Ba5Cu8O18-δ (Y358) superconductor affect the coherence length (ξc) at absolute zero temperature and critical current density, Jc, near the superconducting phase transition region. Fluctuation-induced conductivity (FIC) is used to extract ξc (0). An increasing trend of ξc (0) with increasing concentration of Co is observed, attributed to the deterioration of the superconducting properties. We have extracted Jc from current–voltage (IV) characteristics in the phase transition region. Reduction in Jc has also been observed with increasing the concentration of Co-nanoparticles.

Numerically, the variation of voltage drop (V) and phase fluctuations Δθ in a Josephson junction ladder (JJL) has been studied as a result of an external modulated currents I(i, t) using random field XY model (RFXY). A disordered random magnetic field is introduced in a JJL. Asymmetry in the non-Fraunhofer current–voltage curve is obtained for different sizes of system and fields. Phase fluctuation, Δθ , increases with the increase of amplitude of external current, I_0 . Increasing the pinning strength, W, a saturation in V is possible for several constant I_0 .

We describe here the design, synthesis, and X-ray structural studies of a new class of HIV-1 protease inhibitors containing 8-oxabicyclo[3.2.1]octanol-derived P2 ligands. We investigated the functional effect of these stereochemically defined fused-poly cyclic ligands on enzyme inhibition and antiviral activity in MT-2 cells. The tricyclic core of 8-oxabicyclo[3.2.1]octan-6-ol is designed to interact with the residues in the S2 subsite of HIV-1 protease. The syntheses of the ligands were carried out using the [5+2]-cycloaddition as the key step. Several inhibitors exhibited potent enzyme inhibitory activity. High resolution room-temperature X-ray structures of inhibitor-bound HIV-1 protease were determined. These structures provided important molecular insights for further design and optimization of inhibitor potency.

Addition of magnetic nanoparticles of cobalt in the inter-granular region of polycrystalline Y _3 Ba _5 Cu _8 O _18-δ (Y-358) affects both the superfluid phase stiffness ( J _s ) and the transport critical current density ( J _c ). We have studied the current-voltage (IV) characteristics of (i) the pure Y-358 and (ii) Co-mixed Y-358 superconductors. J_s via an exponent related to nonlinearity of the IV curves is obtained within the framework of Berezinskii-Kosterlitz-Thouless (BKT) transition. A distinct indication of the BKT transition has been observed in the Y-358+0.75 wt. J_c (T) has also been extracted to understand the effectiveness of Co-nanoparticles. A suppression in the current induced activation energy (U) in zero magnetic field leads to the degradation in transport J _c in the composite Y-358 superconductors.

Superfluid phase stiffness (SPS) in the presence of two co-existing and mutually exclusive superconducting and magnetic ordering has been studied in (i) the pure EBCO and (ii) a composite system of nanoparticles of Sn and EBCO superconductors. SPS has been extracted in two different methods by using (i) nonlinear current–voltage (IV) characteristics and (ii) magnetization (M) as a function of the applied magnetic field (H). Variations of the SPS with temperature (T) are found to be different in transport and magnetic methods. A possible reason of the difference has been discussed. The nature of the variation of the SPS in the presence of nanoparticles of Sn has been explored in terms of the distribution of the bound vortex–anti-vortex pairs within the framework of the BKT transition.

Substituted tetrahydrofuran derivatives were designed and synthesized to serve as the P2 ligand for a series of potent HIV-1 protease inhibitors. Both enantiomers of the tetrahydrofuran derivatives were synthesized stereoselectivity in optically active forms using lipase-PS catalyzed enzymatic resolution as the key step. These tetrahydrofuran derivatives are designed to promote hydrogen bonding and van der Waals interactions with the backbone atoms in the S2 subsite of the HIV-1 protease active site. Several inhibitors displayed very potent HIV-1 protease inhibitory activity. A high-resolution X-ray crystal structure of an inhibitor-bound HIV-1 protease provided important insight into the ligand binding site interactions in the active site.

In this paper, we have extracted transport critical current density ([Formula: see text]) of BaZrO3 (BZO) added YBa2Cu3O[Formula: see text] (YBCO) by using several low electric field (E)-based criteria. By varying E-based criteria in order of magnitude, we have extracted [Formula: see text](T) and granular critical current density, [Formula: see text](T) by using JE (IV) characteristics. The temperature (T) dependence of [Formula: see text] and [Formula: see text] is found to be affected strongly as a result of the choice of E as criterion. The sensitiveness of [Formula: see text] and [Formula: see text] becomes prominent with the lowering of T. The Ambegaokar–Baratoff (AB) and the Ginzburg–Landau (GL) descriptions of [Formula: see text](T) and [Formula: see text](T) have been studied to understand how varying criteria may change the extraction of several associated coefficients. Extrapolated [Formula: see text]([Formula: see text]) and [Formula: see text]([Formula: see text]) is also highly sensitive to criteria used.

Magnetization (M) of HgTe, a non-superconducting and topological insulator at ambient pressure, has been calculated by using the mean field theory (MFT) and three different levels of perturbation in the exchange interaction. The spin distribution of Te solely controls the total magnetic contribution which exhibits no oscillating behavior as a function of 1/H even at a temperature of 200.0 mK and magnetic field of 25 T. Nonlinearity in M (H) arises for a suitable perturbation and lower T. The asymmetric nature of M (H) reveals that magnetic fluctuations are unequal to the polarity of the field. Uneven pinning of spins in HgTe in H may suppress the quantum fluctuations. Variations of the spin susceptibility with T exhibit peaks associated with magnetic phase transitions. However, peaks are not related to the quantum oscillations even in the presence of the perturbation. Spin distribution in a nanosized system of HgTe does now exhibit quantum oscillations down to 200 mK and 25 T within the framework of the MFT.

Inter-granular Sn particles affect transport and magnetic properties both in the normal and superconducting states of polycrystalline GdBa2Cu3O7−δ (GdBCO) superconductors drastically. Normal state resistivity at 300 K increases 18.0 times as the concentration of Sn is increased. An unusual upturn in resistivity as a function of temperature curve with a dip is observed in GdBCO with 3.0 wt

Insulating phase included in the inter-granular region of a granular superconducting YBCO affects both the superfluid density and critical current density at the zero magnetic field. Even a highly dense population of about 40 % (weight%) of the BZO insulating particles is found to be unable to destroy the superconducting nature completely. We have studied current-voltage (IV) characteristics of several BZO of high percentage added YBCO bulk samples to extract superfluid density and critical current density. Change in the nonlinear exponent has been observed. Critical current density JcG is found to be very sensitive to inter-granular high density insulating BZO particles. The variation of the activation energy (U) with T is also investigated.

We have studied how substitutions of magnetic Ni and nonmagnetic Zn in copper sites of Y 3 Ba 5 Cu 8 O 18– δ (Y-358) affect critical temperature and the nonlinear behavior of current–voltage ( IV ) characteristics in the zero magnetic field. An exponent as a function of temperature ( T ) has been extracted to understand nonlinearity of IV within the framework of the BKT transition. We have extracted superfluid phase stiffness as a function of T of both groups of samples using the nonlinear exponent and the Ambegaokar–Halperin–Nelson–Siggia (AHNS) theory. The linear nature of the suppression of the critical temperature by Zn is found to be stronger than that of Ni. A strong suppression of the superfluid phase stiffness (SPS) by both dopants has also been observed. However, the suppression of the SPS with T by the nonmagnetic Zn is entirely different in comparison with the nonlinear suppression by the magnetic Ni.

Impacts of two different types of pinning centres on magnetization (M) and magnetic critical current density (Jcm) of YBCO superconductors have been studied. An enormous increase in M below critical temperature is observed to be caused by inclusions of inter-granular magnetic pinning particles of Y2CoMnO6 (YCMO). However, irreversible magnetization (∆M) and Jcm are decreased by the magnetic pinning profile created by the inclusion of YCMO. Hirr(T) is differently sensitive to both pinning profiles. A pinning profile created by the oxygen disorder (δ) in the presence of a magnetic pinning is found to be effective in controlling M, ∆M, and Jcm. Magnetic pinning landscape is found to be even more effective in controlling Jcm. An exponent to explain Jcm as a function of H in the intermediate field H has been extracted at several T.

A spin subsystem of a nano-sized NdNi _8 of a nickelate superconductor is chosen to calculate its contribution in controlling magnetic phase transition. The role of the spin distribution of NdNi _8 in controlling magnetization (M) is studied within the framework of the mean field theory (MFT). Two different levels of perturbation in the exchange interaction between spins of Ni and Nd have been used. M (H, T) has been calculated numerically for perturbations of 0, 0.15 and 0.30 with respect to an unperturbed J_0 . Changes in M (H) in the range of T = 10.0–300.0 K have been found to be very sensitive to perturbation. Possibility of a magnetic phase transition has been found which is sensitive to the perturbation of the exchange integral. Sensitivity of the broadening of the phase transition is weaker for the level of perturbation in comparison with that for the magnetic field. Susceptibility is also calculated and analysed to understand how perturbed exchange integral affects magnetic phase transition in spin distribution of NdNi _8 .

An exponent ( η ) revealing the Berezinskii-Kosterlitz-Thouless (BKT) transition has been extracted from the nonlinear IV characteristics in Nd _x Ca _0.3 Ce _y Ba _2 Cu _3 O _7-δ (NCCBCO) superconductors with (x, y) = (0.6, 0.1), (0.55, 0.15) and (0.5, 0.2) in the zero magnetic field. The onset of the BKT phase transition temperature, T_BKT , has been identified from the nonlinear to linear transformation of current-voltage (IV) characteristics. Superfluid phase stiffness (SPS), J_S , is extracted from the BKT exponent. Sharp discontinuous jump of SPS, J_S at T_BKT , disappears in the bulk 3D cuprate system. SPS, J_S , vanishes at T_BKT corresponding to η = 1.0. The intersection between J_S (T) and the BKT line is not visible in 3D cuprate superconductors. However, an incomplete BKT phase transition observed in 3D cuprate systems is found to be a second-order transition induced by coupling between 2D superconducting layers in which BKT vortices are formed.

Resistances of grain junctions of bulk polycrystalline YBa2Cu3O7-o (YBCO) and DyBa2Cu3O7-o (DyBCO) superconductors have been extracted following (i) Ambegaokar- Baratoff (AB) and (ii) de Gennes (dG) equations. Current-voltage (IV) below the critical temperature (Tc) has been used to extract transport critical current density (Jc). The variations of the junction resistances, (RN) with temperature (T) exhibit that below a critical value of the normalised superfluid density (NSD), junctions become very low resistive and exhibit metallicity. Dependence of this feature of RN on the energy gaps has also been explored. Weak scattering limit is found to be compatible with the maximum of RN (T) as is observed from the corresponding NSD.

Spin disorders in NiO6 octahedra of several nickelate systems have been modeled by varying an inter-spin separation (rik) between spins of O and Ni. Using the mean field theory (MFT) we have calculated magnetization as a function of (i) temperature and (ii) rik in a wide range of H for a small ratio of D/T, where D is single ion anisotropy. Even at very low T, the saturation region in M(T) gets affected for rik = a/2. The curvature of M(T) is highly reduced with the increasing H including an almost linear variation at higher fields. With the change in single ion anisotropy M(rik) is found to be strongly affected. Dependencies of susceptibility (χ) on both (i) temperature (T) and (ii) rik have also been calculated. Possible magnetic phase transitions are identified to be sensitive to the positional spin disorder. Effect of D on susceptibility is found to be very different in nature for rik ≤ 0.2.

Structure-based design, synthesis, X-ray structural studies, and biological evaluation of a new series of potent HIV-1 protease inhibitors are described. These inhibitors contain various pyridyl-pyrimidine, aryl thiazole or alkylthiazole derivatives as the P2 ligands in combination with darunavir-like hydroxyethylamine sulfonamide isosteres. These heterocyclic ligands are inherent to kinase inhibitor drugs, such as nilotinib and imatinib. These ligands are designed to make hydrogen bonding interactions with the backbone atoms in the S2 subsite of HIV-1 protease. Various benzoic acid derivatives have been synthesized and incorporation of these ligands provided potent inhibitors that exhibited subnanomolar level protease inhibitory activity and low nanomolar level antiviral activity. Two high resolution X-ray structures of inhibitor-bound HIV-1 protease were determined. These structures provided important ligand-binding site interactions for further optimization of this class of protease inhibitors.