Fujio Minami

Temperature dependences of anisotropic resistivity in the a−b plane and Hall coefficient in a single-crystal Bi2Sr2CaCu2Oy

Abstract The anisotropic resistivity in the a − b plane and Hall coefficient have been measured in a single-crystal Bi 2 Sr 2 CaCU 2 O y (BSCCO ) and their temperature dependences have been analyzed by a two-band model involving holes and electrons. It is confirmed that the calculated values of the resistivities and Hall coefficient reproduce well the data and the carrier concentrations, mobilities and mean-free-paths obtained from the model are reasonable values in physics. It is found that the hole concentration is smaller than the electron one, but the hole mobility is larger than the electron one. The transport properties of BSCCO are discussed in terms of two types of the hole and electron carrier.

Angle-resolved inverse photoemission study of Bi2Sr2CaCu2O8: Existence of two CuO2 bands across the Fermi level

Abstract Angle-resolved inverse photoemission spectroscopy was performed on a Bi 2 Sr 2 CaCu 2 O 8 single crystal to study the electronic structure in the vicinity of the Fermi level. Inverse photoemission intensity shows two maxima at about 0.21 ΓX and 0.6ΓX from the Γ point along the ΓX direction, suggesting that there are two bands crossing the Fermi level. The present experimental result was compared with angle-resolved photoemission results and band structure calculations.

Residual strain in epitaxially grown ZnSe/GaAs studied by two‐photon absorption spectroscopy

We apply two‐photon absorption spectroscopy as a novel tool to study residual strain in ZnSe epitaxial layers grown on GaAs substrates. Taking advantage of the polarization selection rules, we find that ZnSe layers thicker than 2 μm suffer in‐plane tensile strain of the order of 10−4. Deformation potential for the ZnSe layer is determined directly from two‐photon data to high accuracy.

Electronic Raman scattering in Bi2Sr2CaCu2O8

Abstract Raman scattering has been investigated in Bi 2 Sr 2 CaCu 2 O 8 . It is found that the electronic Raman scattering is observed as a continuum spectrum and this continuum is redistributed itself due to the gap formation below T c . These indicate that the origin of the electronic continuum may be attributed to the single particle excitations of holes on the Fermi surface. Further, the intensity of the phonon Raman scattering has been measured as a function of the excitation photon energy in the visible region. A resonant enhancement of the intensity is found in the vibrational modes related to the CuO 2 SrO layers, but not observed in that of the BiO layer. On the basis of these results, the electronic (hole) states near E F are discussed.

STM study of the electronic density of states in the superconducting phase of Bi2Sr2CaCu2O8

Abstract Tunneling spectroscopic measurement was carried out in the superconducting phase of Bi 2 Sr 2 CaCu 2 O 8 with use of a low temperature scanning tunneling microscope. Both the cleaved Bi-O surface and the lateral surface of a single crystal were investigated. The tunneling conductance shows an explicit superconducting gap structure at both surfaces. For the lateral surface the conductance curve is independent of the tip distance from the sample surface and gives directly the electronic density of states in the superconducting phase. The obtained gap structure is in good agreement with the BCS prediction, although it seems that the gap is slightly anisotropic. The obtained gap-ratio 2Δ/k B T c =6.1 indicates the strong-coupling nature of this superconductor. For the Bi-O surface, on the other hand, the normalized differential conductance increases with increasing the tip distance, especially at the outside of the superconducting gap as reported by Hasegawa et al . Such a dependence on the tip distance is attributed to series barriers of Bi-O, Sr-O layers and vacuum for the tunneling between the Cu-O layer and the tip metal.

Estimation of misfit strain in epitaxially grown ZnSe/GaAs with two-photon absorption spectroscopy

Abstract Two-photon absorption spectroscopy is used as a novel tool to study residual strain in ZnSe epitaxial layers grown on GaAs substrates. Taking advantage of the polarization selection rules, we find that ZnSe layers thicker than 2 μm suffer in-plane tensile strain of the order of 10 -4 , while the epilayer thinner than 1 μm undergoes biaxial compression. The measured thickness dependence of misfit strains is compared to that obtained from X-ray diffraction.

Population and phase relaxations of quasi-two-dimensional excitons in GaSe: Femtosecond photon echoes and luminescence decay experiments

Abstract Dynamics of quasi-two-dimensional excitons in the layered compound GaSe are investigated through temperature-dependent photon echo and luminescence decay measurements. A close correlation is found between the radiative lifetime τ and the phase coherence time T 2 of the free excitons. The relation between τ and T -1 2 (or the homogeneous linewidth) is compared to the prediction of a recent theoretical model.

Scanning tunnelling spectroscopy of an oxide superconductor Bi2Sr2CaCu2O8-δ

The authors carried out a tunnelling spectroscopy measurement on a single crystal of superconducting oxide Bi2Sr2CaCu2O8- delta using a low temperature scanning tunnelling microscope. The superconducting gap parameter shows a spatial variation over a relatively small area ( approximately 10 nm). The authors obtained the energy gap parameter Delta (4.2 K)=26 meV, the most observable value, and correspondingly 2 Delta /kBTc=7, larger than the BCS value. The shape of the (dI/dV)-V curve suggests an anisotropy of the energy gap. Thermal smearing of the gap structure is observed below Tc. Temperature dependence of the gap parameter is consistent with BCS. Even above Tc the conductance curve is not flat.

Dynamics of excitons localized by disorder in semiconductors

Abstract The dynamics of excitons at temperatures down to 1.6 K in three different (stacking, interface and composition) disordered systems are investigated in the pico- and nanosecond time domains by measuring time- and energy-resolved luminescence. With the aid of theoretical calculations based on the rate equation, the observed spectral diffusion within an inhomogeneously broadened exciton band is interpreted as phonon-assisted transfer of excitons between states localized by random potential fluctuations.

Pressure dependence of band discontinuity in GaAs/AlInP quantum well structures

We have performed high-pressure photoluminescence measurements on GaAs/AlInP quantum wells of widths 10 to 100 A at liquid helium temperatures. It is found from the pressure coefficient that the X minima in the AlInP become lower than the confined Γ state in the GaAs wells with widths of less than 30 A. The zero-pressure extrapo-lation of the AlInP(X) to GaAs(Γ) transitions yields a 680 meV valence-band offset and a 770 meV conduction-band offset.