Search Results (10 total)

The band-edge characteristic of germanium dichalcogenides Ge(Se,S)₂ has been characterized using thermoreflectance (TR) and resistivity measurements at 300  K. Two band-edge excitonic transitions denoted as E_A and E_B are found in each TR spectrum of Ge(Se_1−xS_x)₂ (0≤x≤1). A Rydberg excitonic series that contains two excitonic levels of n=1 and n=2 are simultaneously detected in the E_A and E_B transitions of GeSe₂. For the other layered crystals, only the ground-state level (n=1) is observed. Angular-dependent and polarization-dependent TR measurements of Ge(Se_1−xS_x)₂ were carried out to identify the transition origins of the band-edge transitions. The experimental TR results show that Ge(Se_1−xS_x)₂ with x=0, 0.2, 0.4, 0.6, and 0.8 are crystallized in the high-temperature crystalline phase, while the GeS₂ belongs to the high-temperature and low-temperature mixed phases. The high in-plane resistivities of the Ge(Se_1−xS_x)₂ indicate the high purity of the layer crystals.

The band structure of Ge(Se_1−xS_x)₂ (0⩽x⩽1) series solids was probed experimentally by thermoreflectance (TR) measurements in the energy range of 2–6.5 eV. The TR measurements were done with the layered samples put in air and operated at room temperature. The TR spectra of the Ge(Se_1−xS_x)₂ series layers exhibit derivative-like spectral features in the vicinity of band edge as well as higher-lying interband transitions. The energies and broadening parameters of the interband transition features of Ge(Se_1−xS_x)₂ are analyzed by detailed line-shape fits to the TR spectra. Compositional dependences of the transition energies of the Ge(Se_1−xS_x)₂ are analyzed. The presence of higher-lying interband transitions as well as the analysis of line-width broadening of the direct band gap indicates that the Ge(Se_1−xS_x)₂ series should not be amorphous but belong to the polycrystalline solids with medium-range order. Based on the experimental results of TR measurements together with referring to previous band-structure calculations of crystalline and glassy GeX₂ (X=S,Se), a probable experimental band structure near the fundamental edge of Ge(Se_1−xS_x)₂ is proposed.

Excitonic transitions of the ReS₂ triclinic layer semiconductor are studied using polarized photoreflectance (PPR) spectroscopy with optical polarizations along and perpendicular to the b axis in the temperature range of 25–300 K. The low-temperature PPR spectra reveal a prominent and enlarged excitonic series positioned at the higher-energy side with respect to previously identified band-edge excitons E₁^ex and E₂^ex [Phys. Rev. B 55, 15 608 (1997)]. A detailed line-shape analysis and the photosensitive characteristic of the excitonic sequence provide conclusive evidence that the series and those of band-edge excitons E₁^ex and E₂^ex are interband excitonic transitions originated from different origins. The possible origin of the excitonic sequence originates from nonbonding Re 5d t_2g (d_xy,d_x²-y²) states to antibonding S 3p σ^* states. The sequence is shown to correspond to the Rydberg series starting with a principal quantum number n=2. The temperature dependence of transition energies of the exciton series is analyzed. The parameters that describe the temperature variations of the excitonic transitions in ReS₂ layers are evaluated and discussed.

Detailed experimental studies of the first operation of an X-band (8.547 GHz) rf photoinjector are reported. The rf characteristics of the device are first described, as well as the tuning technique used to ensure operation of the 11 / 2-cell rf gun in the balanced π-mode. The characterization of the photoelectron beam produced by the rf gun includes: measurements of the bunch charge as a function of the laser injection phase, yielding information about the quantum efficiency of the Cu photocathode ( 2×10^-5 for a surface field of 100 MV/m); measurements of the beam energy (1.5–2 MeV) and relative energy spread ( Δγ/γ₀  =  1.8±0.2%) using a magnetic spectrometer; measurements of the beam 90% normalized emittance, which is found to be ɛ_n  =  1.65π mm mrad for a charge of 25 pC; and measurements of the bunch duration ( <2 ps). Coherent synchrotron radiation experiments at Ku-band and Ka-band confirm the extremely short duration of the photoelectron bunch and a peak power scaling quadratically with the bunch charge.

A strong temperature dependence of the characteristic behavior of the interlayer exchange bias coupling was observed in a ferromagnet/nonmagnetic metal/antiferromagnet trilayer system (NiO/Cu/NiFe). The oscillation of the interlayer exchange bias coupling was found to be thermally assisted. At low temperature, the exchange bias field decreased monotonically with the Cu spacer thickness. Increasing the temperature close to the Neél temperature, the interlayer exchange bias field became oscillatory with the Cu spacer thickness. A simple picture of the temperature-dependent competition between the RKKY-like coupling and the antiferromagnetic coupling within the antiferromagnetic layer as well as the interlayer dipolar interaction is proposed to explain these findings.

The electronic structures of ReS₂ and ReSe₂ single crystals are investigated using a first-principles density-of-states (DOS) calculation, ultraviolet photoelectron spectroscopy (UPS), and electrolyte electroreflectance (EER). The total and partial DOS were calculated by the full-potential linearized-augmented-plane-wave method. From the calculations, the main contribution near the band edge of Re X₂ (X=S,Se) is determined to be dominated by the nonbonding Re d orbitals. The valence-band DOS is experimentally verified by the UPS measurements. EER measurements were performed in the energy range of 1.3–6 eV. The EER spectra exhibit sharp derivativelike structures in the vicinity of the band-edge excitonic transitions as well as higher-lying interband transitions. Transition energies are determined accurately. From the experimental and the theoretically calculated results, probable energy-band structures of ReS₂ and ReSe₂ are constructed.

Polarization-dependent absorption measurements of ReS₂ and ReSe₂ single crystals have been carried out in the temperature range between 25 and 500 K. A significant shift towards lower energies has been observed in the transmittance spectra of E∥b polarization with respect to those corresponding to E⊥b polarization. Analysis reveals that the absorption edges of ReS₂ and ReSe₂ are indirect allowed transitions. The parameters that describe the temperature dependence of the absorption edges with different polarizations in the van der Waals plane are evaluated. The results indicate that the electron-phonon coupling constants for E∥b polarization are considerably larger than those of E⊥b polarization.

The temperature dependence of the spectral features in the vicinity of the direct band edge of ReS_2-xSe_x single crystals is measured over a temperature range of 25–300 K using piezoreflectance (PzR). From a detailed line-shape fit of the PzR spectra, the temperature dependence of the energies and broadening parameters of the band-edge excitons are determined accurately. The excitonic transition energies at different temperature vary smoothly with the Se composition x, indicating that the natures of the direct band edges of ReS_2-xSe_x are similar. The parameters that describe the temperature variation of the energies and broadening function of the excitonic transitions are evaluated and discussed.

We have measured the temperature dependence of the spectral features in the vicinity of the direct gaps E_g^d of ReS₂ and ReSe₂ in the temperature range between 25 and 450 K using piezoreflectance (PzR). From a detailed line-shape fit to the PzR spectra we have been able to determine accurately the temperature dependence of the energies and broadening parameters of the band-edge excitons. The parameters that describe the temperature variation of the transition energies and broadening function have been evaluated.

The effect of a focused laser beam on the critical junction current of a one-dimensional Josephson tunnel junction is analyzed for both short and long junctions. The laser heating locally alters the critical current density and London penetration depth, giving rise to a change in the critical current which is proportional to the local current density of the irradiated region. In addition, the relative pair phase is modified in a nonlocal manner which gives rise to an additional contribution to the change in the critical current of the junction. For short junctions, this nonlocal phase modification contributes two parts to the junction critical current. One has the same spatial dependence as that of the current distribution itself, while the other is independent of the position of the laser beam. This latter contribution depends upon the applied magnetic field and is a signature of the nonlocal effect. For a long junction, the nonlocal phase contribution to the change in the junction critical current can give rise to dramatic effects when the applied magnetic field is less than a critical value H_c=ħc/(2edλ_J). In this case the spatial dependence of the laser-induced change in the critical current can be qualitatively different from the unperturbed current distribution.