Results 1 - 10 of 37
Results 1 - 10 of 37. Search took: 0.02 seconds
|Sort by: date | relevance|
[en] The ultrafast electron relaxation dynamics of anionic and neutral Au25(SR)18 nanoclusters are investigated using broad-band time-resolved optical spectroscopy. From an analysis of the wavelength-dependent transient absorption kinetics, we have obtained valuable information on the spectral features that originate from excitation of 'core' and 'core-shell' states. In both clusters, photoexcitation occurs into two nondegenerate states near the HOMO-LUMO gap that are derived from the core orbitals. A large difference in the lifetime of the core excitations is observed, with (Au25(SR)18)- exhibiting a decay rate more than 1000 times slower than the neutral cluster. Both clusters show strong coupling to two different coherent phonon modes, which are observed at 2.4 and 1.2 THz. The electron-phonon coupling is analyzed in terms of the spectral distribution and damping of the coherent modes.
[en] LaB6 and other hexaborides are inclusion compounds in which the rare earth or other metal ion is weakly bound and sits in an oversized ''cage'' of boron ions. Here we show that a simple model that treats the La ions as independent harmonic (Einstein) oscillators embedded in a Debye framework of boron ions successfully accounts for the anomalies in the specific heat and resistivity of LaB6. One of the nice features of the model is that the Einstein temperature of the La atoms and the Debye temperature of the boron framework are derived from room-temperature x-ray crystallography data. This feature makes the model easy to apply to other hexaborides and other materials that can be treated as inclusion compounds. The results from this work imply that local modes are likely to be important for understanding the physical properties of all the hexaborides
[en] We report a femtosecond spectroscopic investigation on the electronic structure and relaxation dynamics of a rod-shaped, 25-atom (Au25) nanocluster capped by organic ligands. Broadband femtosecond transient absorption spectra of the cluster show overlapped excited state absorption and ground state bleach signals. Two lifetimes (i.e., 0.8 ps fast component and a 2.4 (micro)s long component) are identified, with the 0.8 ps component attributed to the fast internal conversion process from LUMO+n to LUMO and the long component to electron relaxation to the ground state. The rod shape of the cluster induces a strong anisotropic response in the transient absorption spectra, from which we deduce that the transition moment is oriented with the long axis of the prolate-shaped cluster. In addition, coherent phonon emission at 26 cm-1 was observed and results in the modulation of the excited state absorption transition energy.
[en] The tunneling spectrum of the superconducting phase with Tc≅3.0 K has been measured in the Ru-embedded region of Sr2RuO 4 using cleaved junctions. A sharp zero-bias conductance peak (ZBCP) has been observed below 3K. All characteristics of this ZBCP suggest that it originates from Andreev surface bound states, indicating that the pairing in the 3-K phase is also non-s -wave. Below the bulk Tc of Sr2RuO 4 (≅1.5 K) , a crossover from sharp to bell-shaped ZBCP was found. This supports the theory that there is a phase transition in the 3-K phase region near the bulk Tc
[en] Structural, magnetic, electrical and thermal transport, and heat-capacity measurements are reported on single crystals of Eu8Ga16Ge30, Sr8Ga16Ge30, and Ba8Ga16Ge30. These compounds all crystallize in a cubic type-I ice clathrate structure, and are of interest as potential thermoelectric materials. Neutron-diffraction measurements were made on a single crystal of Eu8Ga16Ge30 that was grown using isotopically pure Eu153. Nuclear density maps clearly show that Eu atoms at the 6d sites 1/4,0 can move away from the cage center to one of four nearby positions. Ferromagnetism is observed in Eu8Ga16Ge30 for temperatures below 32 K, with the preferred direction of the Eu spins along the (100) axis. Ferromagnetism in these heavily doped semiconductors (∼1021 electrons/cm3) is likely due to a Rudermann-Kittel-Kasuya-Yoshida-type interaction. A large (∼10% at 8 T) negative magnetoresistance was measured near the Curie temperature of Eu8Ga16Ge30. The lattice thermal conductivities of Eu8Ga16Ge30 and Sr8Ga16Ge30 single crystals show all of the characteristics of a structural glass. The thermal conductivity of Ba8Ga16Ge30 is low at room temperature (1.3 W/m K), but exhibits a temperature dependence characteristic of a crystal. A magnetic field has no significant effect on the thermal conductivity of any of the crystals for temperatures between 2 and 300 K. Heat-capacity measurements indicated Einstein contributions from each of the rattlers, with characteristic temperatures of 60, 53, and 30 K for Ba, Sr, and Eu atoms respectively. No superconductivity was observed in heavily doped single crystals of Ba8Ga16Ge30 for temperatures above 2 K, contrary to a previous report
[en] LiVO2 undergoes an imperfectly understood orbital ordering transition near 500 K resulting in a loss of magnetic moment below the transition. Studies of the transition have been hampered by a lack of high-quality stoichiometric single crystals. Here we report the growth and basic characterization of large, nearly stoichiometric LiVO2 single crystals. The crystals were characterized by magnetic susceptibility, electrical resistivity, differential scanning calorimetry, and specific heat measurements over a temperature range from 2 to 650 K. A first-order phase transition with large hysteresis near Tt∼500 K was observed in all measurements. An anisotropy of the order of 100 was observed in the in-plane versus out-of-plane resistivity, and the inferred semiconducting energy gap was 0.18 eV for T< Tt and 0.14 eV for T< Tt. Electron diffraction experiments were performed on LiVO2 single crystals at temperatures below and above Tt. Superlattice reflections were observed below Tt and disappeared upon heating above the phase transition temperature. Upon cooling below Tt, the supperlattice spots reappeared. Bright field electron micrographs indicate that the crystals develop a roughly hexagonal network of cracks
[en] The Hall effect of single-crystalline Bi2Sr2CaCu2O8+δ and Bi1.95Sr1.65La0.4CuO6+δ has been measured above and below the superconducting transition temperature Tc0 by applying magnetic fields H parallel c and currents I perpendicular c, where c denotes the direction perpendicular to the Cu-O planes of these materials. For both compounds, the Hall resistivity ρxy(T,H) reveals similar temperature and field dependences. As T approaches Tc0 from higher temperatures, the ρxy(T,H) versus H curves gradually deviate from linearity and exhibit an unusual sign reversal in the low-field regime. The total Hall resistivity ρxy(T,H) may be decomposed into three distinctly different terms, i.e., ρxy(T,H)=ρxy0(T)+RH'(T) HH+ρxyx(T,H). The contribution ρxxy(T,H), representing the deviation of ρxy(T,H) from H linearity, exhibits a universal scaling behavior as a function of H/H0(T) for fields between 0 and H0(T), the field below which ρxy(T,H) is no longer linear in H. copyright 1996 The American Physical Society
[en] The meshless method is applied to the response analysis of a geometrically nonlinear beam. The corresponding nonlinear strain operator matrix and tangent rigidity matrix are given. The computational results agree well with Holden's analytic solution. It is found that the meshless method is more efficient for large displacement
[en] We report the electrical transport, magnetic, and thermodynamic properties of polycrystalline PdTe which exhibits superconductivity below 4.5 K. Using the measured values for the lower (Hc1) and upper (Hc2) critical fields, and the specific heat Cp, we estimate the thermodynamic critical field Hc(0), coherence length ξ(0), penetration depth λ(0), and the Ginzburg-Landau parameter κ. Compared with band structure calculations, the density of states at the Fermi level is enhanced due to electron-phonon coupling with λep = 1.4. Furthermore, the large values of ΔCp/γnTc and 2Δ(0)/kBTc suggest that PdTe is a strongly coupled superconductor. (paper)
[en] Single crystals of Bi2M3Co2Oy phases with M=Ca, Sr, and Ba were synthesized by the flux method. The undoped Ca and Sr analogs are semiconducting while the Ba analog has a metal to semiconductor transition at ∼80 K. Pb-substituted single crystals in which Pb partially substitutes for Bi were prepared by a similar method. Transmission electron microscopy carried out on single crystals of the undoped phases reveals a superstructure similar to that of superconducting bismuth-2212. This superstructure disappears when the Sr and Ba compounds are Pb doped. Transport measurements show that Pb doping induces a clear increase in the metallic character of the samples for Sr and Ba analogs. The metal to semiconductor transition in Bi2Ba3Co2Oy is suppressed by Pb doping, and (Bi,Pb)2Ba3Co2Oy is metallic down to 30 mK. The magnetic-susceptibility data do not show any evidence of ordering, and the magnetic moment Co atom was found to be ∼1μB. Resistivity measurements carried out up to 20 GPa show that the samples become more semiconducting with increasing pressure