Honglong Shi

Electron energy-loss spectroscopy and ab initio electronic structure of the LaOFeP superconductor

The electronic band structures of the LaOFeP superconductor have been calculated theoretically by the first-principles method and measured experimentally by electron energy loss spectroscopy (EELS). The calculations indicate that the Fe atom in a LaOFeP crystal shows a weak magnetic moment (0.14 mu(B)/atom) and does not form a long-range magnetic ordering. Band structure, Fermi surfaces, and fluorine-doping effects are also analyzed based on the data of the density functional theory. The fine structures of the EELS data have been carefully examined in both the low loss energy region (<60 eV) and the core losses region (O K, Fe L(2,3), and La M(4,5)). A slight bump edge at similar to 44 eV shows notable orientation dependence: it can be observed in the low loss EELS spectra with q parallel to c but becomes almost invisible in the q perpendicular to c spectra. Annealing experiments indicate that low oxygen pressure favors the appearance of superconductivity in LaOFeP: this fact is also confirmed by the changes of Fe L(2,3) and O K excitation edges in the experimental EELS data.

Growth of nearly one nanometer large silicon particles in silicon carbide and their quantum-confined photoluminescence features

Silicon particles approaching the size of 1 nm were grown along with the confining SiC films by employing a low-temperature chemical vapor deposition procedure. The resulting amorphous composite structure enables an experimental study of the quantum confinement effect in extremely narrow potential wells, as exemplified here by photoluminescence measurement. Owing to the enhanced energy fluctuation for such small particles, strong photoluminescence centered at 450–540 nm, and of comparable profiles, was measured in one single sample with an excitation wavelength selectable within 360–420 nm. Moreover, the typical decay time was found to be below 3.0 ns. These properties hold promise for the fabrication of wide-spectrum photoreceptors, ultraviolet-light detectors, and other optoelectronic devices.

Strong coupling of the iron-quadrupole and anion-dipole polarizations in Ba(Fe(1-x)Co(x))2As2.

We use a quantitative convergent beam electron diffraction based method to image the valence electron density distribution in Ba(Fe1-xCox)2As2. We show a remarkable increase in both the charge quadrupole of the Fe cations and the charge dipole of the arsenic anions upon Co doping from x=0 (Tc=0  K) to x=0.1 (Tc=22.5  K). Our data suggest that an unexpected electronic correlation effect, namely strong coupling of Fe orbital fluctuation and anion electronic polarization, is present in iron-based superconductors.

Two-coupled structural modulations in charge-density-wave state of SrPt2As2 superconductor

Transmission electron microscopy (TEM) study of SrPt2As2 reveals two incommensurate modulations appearing in the charge-density-wave (CDW) state below TCDW ≈ 470 K. These two structural modulations can be well explained in terms of condensations of two-coupled phonon modes with wave vectors of q1 = 0.62a* on the a*−b* plane and q2 = 0.23a* on the a*−c* plane. The atomic displacements occur along the b-axis direction for q1 and along the c-axis direction for q2, respectively. Moreover, the correlation between q1 and q1 can be generally written as q1 = (q2 + a*)/2 in the CDW state, suggesting the presence of essential coupling between q1 and q2. A small fraction of Ir doping on the Pt site in Sr(Pt1–xIrx)2As2 (x ≤ 0.06) could moderately change these CDW modulations and also affect their superconductivities.

Contribution of interlayer hybridization to the electronic structure in iron pnictides: a study of EELS and first-principles calculations

Using electron energy loss spectroscopy (EELS) measurements and first-principles electronic structure calculations, the significant interlayer hybridization between the insulating layers (ReO or Ba) and the conducting FeAs layers was investigated in the layered iron pnictides, which is quite different from the case in the cuprate superconductors. This interlayer hybridization would result in an increase in the bandwidth near the Fermi level and interorbital charge transfer in the Fe 3d orbitals, which subsequently leads to a decrease in the Fe local moment and the modification of the Fermi surface topology. Therefore, a three-dimensional character of the electronic structure due to the interlayer hybridization is expected, as observed in previous experiments. These findings indicate that reduced dimensionality is no longer a necessary condition in the search for high-T(c) superconductors in iron pnictides.

Resistivity and Field Electron Emission of Nanowires Formed by Electron Beam Induced Chemical Vapor Deposition

Self-standing iron nanowires were fabricated at the apex of a tungsten needle tip by electron beam induced deposition. This sharp needle tip which adhered to the nanowire can be moved with a stepping motor and piezo-driving device, and was attached inside a specially designed transmission electron microscope specimen holder. A copper conductor substrate, with which the approaching nanowires will build up a closed electric circuit, was set on the holder. The tungsten needle tip accompanied with the EBICVD nanowires made contact with the substrate and then a voltage was applied between the two electrodes. Resistivity values of the examined nanowires, by a devised Lock-in-Amplifier circuit, range from 0.1 Ωm to 10−3 Ωm. Our investigation might have implications in the fabrication and characterization of nano-electronics device. Precursor with phenanthrene (C4H10) was used and the deposition experiment was done using a scanning electron microscope at room temperature. It was found that the surface structure at the top of the nanorod, such as a small protrusion within only several nanometers scale, has significant influence on the field emission property. An emission current of several tens of nano-ampere flowing through this nanorod could induce resistance heating. In several minutes, this thermal energy could transform the original amorphous carbon into a graphite-like structure embedded with fullerenes. The turn-on voltage of the graphite-like nanorod was about 11 V less than that of the original amorphous case.

Structure Characterization of CuCl2-FeCl3-H2SO4 Graphite Intercalation Compounds

Graphite intercalation compounds with CuCl2-FeCl3-H2SO4 were synthesized via a hydrothermal treatment at 150C and exfoliation method. The structure and composition of these graphite intercalation compounds were analyzed by means of X-ray diffraction, energy dispersive X-ray and high-resolution transmission electron microscopy. The results demonstrate that the CuCl2-FeCl3-H2SO4 molecules were successfully intercalated into the interlayer of the graphite sheets. The temperature dependence of magnetization was measured from 5 K to 300 K. Two antiferromagnetic transitions of the graphite intercalation compounds were observed at low temperatures. The critical transition temperatures are estimated to be about 50 and 102 K. The related magnetic properties are discussed briefly.