A novel nanopolycrystalline structure of vanadium dioxide thin films is deposited on silicon or fused silica substrates by reactive ion sputtering and followed by an annealing. The characteristic analysis＇shows that ...A novel nanopolycrystalline structure of vanadium dioxide thin films is deposited on silicon or fused silica substrates by reactive ion sputtering and followed by an annealing. The characteristic analysis＇shows that the films have a columnar nanostructure with an average grain of 8 nm. The resistivities as a function of ambient temperatures tested by four-point probes for as-deposited films present that the transition temperature for nanostructure of vanadium dioxide films is near 35 ℃ which lowers about 33 ℃ in comparison with the transition temperature at 68 ℃ in its microstructure.展开更多
In order to study the effect of mixing dye in ferroelectric liquid crystal （FLC） materials, the phase transition temperature and electro-optical properties of azo dye doped FLC samples have been investigated. All th...In order to study the effect of mixing dye in ferroelectric liquid crystal （FLC） materials, the phase transition temperature and electro-optical properties of azo dye doped FLC samples have been investigated. All the properties have been found to be changed drastically. The results have revealed that not only the SmC^＊- SmA^＊ transition temperature decreased markedly by the addition of azo-dye, but also dye-doped FLC had lower threshold voltage and saturation voltage than the pure FLC.展开更多
Electron density differences resulting from atom displacement patterns aligned with phonon modes in MgB2 have been calculated using density functional theory (DFT). The extent of phonon anomalies, identified as indica...Electron density differences resulting from atom displacement patterns aligned with phonon modes in MgB2 have been calculated using density functional theory (DFT). The extent of phonon anomalies, identified as indicators of the superconducting transition temperature, Tc, under a range of conditions in AlB2-type structures, reduce as boron atoms are displaced from their equilibrium positions along E2g mode directions. The Fermi energy for displacements along the directions of the E2g phonon mode accounts for changes in the covalent B-B bond electronic charge density. We applied differential atom displacements to show that the shifted σ band structure associated with the light effective mass became tangential to the Fermi level and that the Fermi surface undergoes a topological transition at a critical relative displacement of ~0.6% of the boron atoms from equilibrium. The difference in Fermi energies at this critical displacement and at the equilibrium position correspond to the superconducting energy gap. The net volume between tubular σ surfaces in reciprocal space correlated with the depth of the phonon anomaly and, by inference, it is a key to an understanding of superconductivity. This ab initioapproach offers a phenomenological understanding of the factors that determine Tc based on knowledge of the crystal structure.展开更多
X-ray powder diffraction, scanning electron microscopy （SEM）, energy dispersive X-ray, electrical resistivity and AC-magnetic susceptibility measurements have been performed for polycrystalline superconducting sampl...X-ray powder diffraction, scanning electron microscopy （SEM）, energy dispersive X-ray, electrical resistivity and AC-magnetic susceptibility measurements have been performed for polycrystalline superconducting samples of type TIBa2Ca2_xSCxCU309_δ （0.0 ≤ x 〈 0.6）. The powder X-ray diffractograms indicate that the tetragonal structure of T1-1223 is not affected by Sc-substitution whereas the lattice parameters are changed. The X-ray analysis indicates that the low-contents of scandium （x） enhance the formation of T1-1223 and reduce the secondary phases. The grain-size determined by SEM decreases as x increases. The electrical resistivity measurements show suppression in the superconducting transition temperature, Tc, and an increase in both the residual resistivity and the superconducting transition width as x increases. The suppression in Tc is attributed to the hole-filling mechanism.展开更多
By comparing the data of lattice parameters of more than 50 new FeAs-based high temperature superconductors in two syetems, the effect of bondlength on superconducting transition temperature (T C ) was found that, for...By comparing the data of lattice parameters of more than 50 new FeAs-based high temperature superconductors in two syetems, the effect of bondlength on superconducting transition temperature (T C ) was found that, for both FeAs superconductor systems with similar ionic radii of cation A, the bond length L As-A between Arsenic atom and its nearest neighbor cation at the A site is in an inverse proportion to T C , i.e. the larger the bond length L As-A , the lower the T C . In addition, we also found a noticeable effect of ionic radius on T C , in which deviation from the tendency line of bond length vs. T C decreases with the increasing ionic radius. Both bond length-T C and ionic radii-T C relations indicate that the interaction of cation A and As ion have significant effect on superconductivity. The analysis on the electronic structure indicates that there exists the proximity of the secondary-outer p-orbit of the cation A and the 4s orbit of the As ion, both in energy space and in real space. Some high frequency individual vibrating modes would be established through exchange coupling on their inner orbits. It is worth of mention that the superconducting condensation is influenced by the interaction between the cation A and the As atoms.展开更多
基金This research work was supported by the National Nature Science Foundation of China under Grant No. 60477040.
文摘A novel nanopolycrystalline structure of vanadium dioxide thin films is deposited on silicon or fused silica substrates by reactive ion sputtering and followed by an annealing. The characteristic analysis＇shows that the films have a columnar nanostructure with an average grain of 8 nm. The resistivities as a function of ambient temperatures tested by four-point probes for as-deposited films present that the transition temperature for nanostructure of vanadium dioxide films is near 35 ℃ which lowers about 33 ℃ in comparison with the transition temperature at 68 ℃ in its microstructure.
基金supported by the National"973"Project of China under Grant No.2003CB314704.
文摘In order to study the effect of mixing dye in ferroelectric liquid crystal （FLC） materials, the phase transition temperature and electro-optical properties of azo dye doped FLC samples have been investigated. All the properties have been found to be changed drastically. The results have revealed that not only the SmC^＊- SmA^＊ transition temperature decreased markedly by the addition of azo-dye, but also dye-doped FLC had lower threshold voltage and saturation voltage than the pure FLC.
文摘Electron density differences resulting from atom displacement patterns aligned with phonon modes in MgB2 have been calculated using density functional theory (DFT). The extent of phonon anomalies, identified as indicators of the superconducting transition temperature, Tc, under a range of conditions in AlB2-type structures, reduce as boron atoms are displaced from their equilibrium positions along E2g mode directions. The Fermi energy for displacements along the directions of the E2g phonon mode accounts for changes in the covalent B-B bond electronic charge density. We applied differential atom displacements to show that the shifted σ band structure associated with the light effective mass became tangential to the Fermi level and that the Fermi surface undergoes a topological transition at a critical relative displacement of ~0.6% of the boron atoms from equilibrium. The difference in Fermi energies at this critical displacement and at the equilibrium position correspond to the superconducting energy gap. The net volume between tubular σ surfaces in reciprocal space correlated with the depth of the phonon anomaly and, by inference, it is a key to an understanding of superconductivity. This ab initioapproach offers a phenomenological understanding of the factors that determine Tc based on knowledge of the crystal structure.
文摘X-ray powder diffraction, scanning electron microscopy （SEM）, energy dispersive X-ray, electrical resistivity and AC-magnetic susceptibility measurements have been performed for polycrystalline superconducting samples of type TIBa2Ca2_xSCxCU309_δ （0.0 ≤ x 〈 0.6）. The powder X-ray diffractograms indicate that the tetragonal structure of T1-1223 is not affected by Sc-substitution whereas the lattice parameters are changed. The X-ray analysis indicates that the low-contents of scandium （x） enhance the formation of T1-1223 and reduce the secondary phases. The grain-size determined by SEM decreases as x increases. The electrical resistivity measurements show suppression in the superconducting transition temperature, Tc, and an increase in both the residual resistivity and the superconducting transition width as x increases. The suppression in Tc is attributed to the hole-filling mechanism.
基金Supported by the National Science Foundation of USA (Grant No.DMR-0821284)the National Natural Science Foundation of China (Grant No. 50372005)the Natural Science Foundation of Beijing (Grant No.1072007)
文摘By comparing the data of lattice parameters of more than 50 new FeAs-based high temperature superconductors in two syetems, the effect of bondlength on superconducting transition temperature (T C ) was found that, for both FeAs superconductor systems with similar ionic radii of cation A, the bond length L As-A between Arsenic atom and its nearest neighbor cation at the A site is in an inverse proportion to T C , i.e. the larger the bond length L As-A , the lower the T C . In addition, we also found a noticeable effect of ionic radius on T C , in which deviation from the tendency line of bond length vs. T C decreases with the increasing ionic radius. Both bond length-T C and ionic radii-T C relations indicate that the interaction of cation A and As ion have significant effect on superconductivity. The analysis on the electronic structure indicates that there exists the proximity of the secondary-outer p-orbit of the cation A and the 4s orbit of the As ion, both in energy space and in real space. Some high frequency individual vibrating modes would be established through exchange coupling on their inner orbits. It is worth of mention that the superconducting condensation is influenced by the interaction between the cation A and the As atoms.
