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The Y1-xLixBa2Cu3O7-d polycrystalline bulk superconductors doped with Li substituting at the Y site at different concentrations (x = 0, 0.01, 0.02, 0.1) were prepared using the citrate-gel method to study the effects of doping on the superconducting temperature and critical current density. The question was whether Li addition characterized by a high Debye frequency would have any positive effects on Tc. The optimum citrate-gel and heat treatment conditions were identified as those yielding samples with a maximum grain size on the order of 50 micro.m (up to the optimum Li-doping level, x=0.01). Li substitution at the Y site was verified by structural, electrical, and magnetic measurements of the produced samples, whereas X-ray diffraction (XRD) analysis revealed the formation of a pure phase with no visible impurity phases. Moreover, AC magnetic susceptibility measurements showed no increase in the superconducting transition temperature Tc, consistent with the predicted results obtained by machine learning method, although it was theoretically expected to increase owing to the high Debye frequency of Li. This observation is consistent with magnetic coupling models for pairing mechanism in cuprates. Finally, because of the optimum conditions of the preparation procedure, nearly identical values of the critical current density (Jc) were recorded for samples with different Li doping levels (up to the optimum Li doping level). It was found that improved compound preparation conditions would have a critical and extensive effect on Jc enhancement, with nearly no Tc suppression.

The structural, electronic, and vibrational thermodynamic properties of the (ZnO)$_n$ (n=2-16) clusters are studied using density functional - full potential computations. The results show, small clusters up to $n=9$ stabilize in the 2D ring shape geometries while the larger clusters prefer the 3D cage like structures. The ring to cage structural cross over in ZnO clusters is studied by investigating the behavior of the Zn-O-Zn bond angle, the Zn-O bond strength, and the number of bonds in the systems. It is argued that 12 is the lowest magic number of ZnO clusters at ground state, while finite temperature vibrational excitations enhance the relative stability of the (ZnO)$_9$ cluster and make it a magic system at temperatures above about 170 K. The obtained electronic structure of ZnO clusters before and after applying the many-body GW corrections evidence a size induced red shift originated from the ring to cage structural cross over in these systems. The behavior of the extremal points of electron density of the clusters along with the extrapolated cluster binding energies at very large sizes may be evidences for existence of a metastable structure for large ZnO nanostructures, different with the bulk ZnO structure., Comment: 8 pages, 8 figures and 1 table

We performed detailed ac susceptibility measurements on Pr$_{1-x}$Ag$_x$MnO$_3$ ($x$=0.15, 0.20) manganites and observed unusual and interesting features, which are associated with the domain walls and the effects of grain-boundaries on their movements. It is shown that the ac field, frequency, temperature, thermal cycling and grain coupling significantly influence the real and especially imaginary parts of the ac susceptibility. We argue that the cooperative depinning of the domain walls from the grain-boundaries accompanying with a large distance movements of the walls leads to the appearance of an anomaly at low temperatures. The anomaly is observed above a threshold ac field and depends on the intergrain coupling. The results show that in the powdered form of bulk samples, the anomaly disappears and the ac field dependence of the $\chi'$ and $\chi"$ suppresses. Below the anomaly temperature, significant and unusual thermal hysteresis occurs in the real and especially imaginary parts of the ac susceptibility. Thermal hysteresis depends on the thermal cycling and indicates thermally irreversible wall pinning and depinning. The susceptibility is practically frequency-independent in the temperature range of thermal hysteresis i.e. below the anomaly. The frequency dependence of ac susceptibility between the anomaly temperature and the transition temperature is discussed by considering the temperature dependence of the relaxation process of the domain walls., Comment: 10 figures

Magnetic nanoparticles of La0.67Sr0.33MnO3 (LSMO) manganite with mean particle sizes of 13, 16, 18 and 21 nm were prepared by the sol-gel method. The crystal structure and mean particle size of the synthesized powders were estimated by X-ray diffraction (XRD) analysis using rietveld refinement and transmission electron microscopy (TEM). Fourier transform infrared (FTIR) transmission spectroscopy revealed that stretching and bending modes are influenced by calcinations temperature. Dc magnetization versus magnetic field of the samples was carried out at room temperature. Magnetic dynamics of the samples was studied by the measurement of ac magnetic susceptibility versus temperature at different frequencies and ac magnetic fields. A frequency-dependent peak was observed in ac magnetic susceptibility versus temperature which is well described by Vogel-Fulcher and critical slowing down laws, and empirical and parameters. By fitting the experimental data with Vogel-Fulcher and critical slowing down laws, the relaxation time, characteristic temperature, magnetic anisotropy energy, effective magnetic anisotropy constant and critical exponent zv have been estimated. The obtained values of c1, c2, T0 and t0 from the Vogel-Fulcher law support of the presence of strong interaction between magnetic nanoparticles. The values of zv and t0 obtained from critical slowing down fit suggest the presence of superspin glass behavior in LSMO nanoparticles of different sizes., Comment: 26 pages, 12 figures and 2 tables

X-Ray diffraction, ac susceptibility and electrical resistivity measurements were performed in order to investigate the effect of Ag substitution for Pr in the polycrystalline Pr$_{1-x}$Ag$_x$MnO$_3$ (0.0$\leq$x$\leq$0.25) manganites. The XRD results show that the samples crystallize in the O'-orthorhombic structure with a cooperative Jahn-Teller deformation. We give evidence that the Ag$^{+}$ is not substituted at the Pr$^{3+}$ site in the PrMnO$_{3}$ structure. The increase of $T_{C}$ up to 130 K is suggested to be due to further oxidation of manganite grains by the oxygen released from the metallic silver at high temperatures. No metal-insulator transition was observed in the samples., Comment: 5 Figures, 1 Table