Y.W. Li

Temperature dependent phonon evolutions and optical properties of highly c-axis oriented CuGaO2 semiconductor films grown by the sol-gel method

Transparent conductive CuGaO2 oxide films were prepared on sapphire substrates by the sol-gel method. The highly c-axis orientation and optical transparency (60%-80%) in the visible region were obtained. It indicates that the A 1 g phonon mode shifts about 20 cm−1 with the temperature due to the thermal expansion of the lattice and anharmonic phonon coupling. Moreover, temperature-dependent dielectric function has been investigated and three electronic transitions located at about 1.05, 2.67, and 3.99 eV can be uniquely assigned. It was found that the optical band gap of the CuGaO2film decreases with the temperature, which mainly originated from the electron-phonon interactions.

Composition dependence of dielectric function in ferroelectric BaCoxTi1−xO3 films grown on quartz substrates by transmittance spectra

Near-infrared-ultraviolet optical properties of BaCoxTi1−xO3 (BCT) (x from 1.0% to 10%) films have been investigated by the transmittance spectra. The dispersion functions in the photon energy range of 1.24–6.2eV have been extracted by fitting the experimental data with Adachi’s model. It was found that the oscillator and dispersion energies linearly increase with the Co composition and the maximum optical transition occurs near the energy range of 4.3–5.0eV for the BCT materials. The absorption coefficient at the visible region linearly increases with the composition due to grain boundaries and disorder induced band tail into the forbidden gap.

Electronic properties of nanocrystalline LaNiO3 and La0.5Sr0.5CoO3 conductive films grown on silicon substrates determined by infrared to ultraviolet reflectance spectra

Electronic band structures of nanostructured LaNiO3 (LNO) and La0.5Sr0.5CoO3 (LSCO) films have been investigated by near-normal incident optical reflectance at room temperature. Dielectric constants of the conductive films in the photon energy range of 0.47–6.5 eV have been extracted with the Drude–Lorentz function. It is found that four interband electronic transitions can be uniquely assigned for the perovskite-type metallic oxides. Moreover, optical conductivity is approximately varied from 100 to 450u2002Ω−1u2009cm−1 and shows a different variation trend for the LNO and LSCO layers. The discrepancy could be ascribed to diverse electronic structure, grain size, and crystalline formation.

Light-scattering detection of quantum phases of ultracold atoms in optical lattices

Ultracold atoms loaded on optical lattices can provide unprecedented experimental systems for the quantum simulations and manipulations of many quantum phases. However, so far, how to detect these quantum phases effectively remains an outstanding challenge. Here, we show that the optical Bragg scattering of cold atoms loaded on optical lattices can be used to detect many quantum phases, which include not only the conventional superfluid and Mott insulating phases, but also other important phases, such as various kinds of charge density wave (CDW), valence bond solid (VBS), CDW supersolid (CDW-SS) and Valence bond supersolid (VB-SS).

Optical Bragg, atomic Bragg and cavity QED detections of quantum phases and excitation spectra of ultracold atoms in bipartite and frustrated optical lattices

Abstract Ultracold atoms loaded on optical lattices can provide unprecedented experimental systems for the quantum simulations and manipulations of many quantum phases and quantum phase transitions between these phases. However, so far, how to detect these quantum phases and phase transitions effectively remains an outstanding challenge. In this paper, we will develop a systematic and unified theory of using the optical Bragg scattering, atomic Bragg scattering or cavity QED to detect the ground state and the excitation spectrum of many quantum phases of interacting bosons loaded in bipartite and frustrated optical lattices. The physically measurable quantities of the three experiments are the light scattering cross sections, the atom scattered clouds and the cavity leaking photons respectively. We show that the two photon Raman transition processes in the three detection methods not only couple to the density order parameter, but also the valence bond order parameter due to the hopping of the bosons on the lattice. This valence bond order coupling is very sensitive to any superfluid order or any valence bond (VB) order in the quantum phases to be probed. These quantum phases include not only the well-known superfluid and Mott insulating phases, but also other important phases such as various kinds of charge density waves (CDW), valence bond solids (VBS), and CDW-VBS phases with both CDW and VBS orders unique to frustrated lattices, and also various kinds of supersolids. We analyze respectively the experimental conditions of the three detection methods to probe these various quantum phases and their corresponding excitation spectra. We also address the effects of a finite temperature and a harmonic trap. We contrast the three scattering methods with recent in situ measurements inside a harmonic trap and argue that the two kinds of measurements are complementary to each other. The combination of both kinds of detection methods could be used to match the combination of Scanning tunneling microscopy (STM), the Angle Resolved Photo Emission spectroscopy (ARPES) and neutron scattering in condensed matter systems, therefore achieve the putative goals of quantum simulations

Effects of applied electrical field on electronic structures in LaNiO3 conductive metallic oxide film: An optical spectroscopic study

The reflectance spectra of LaNiO 3 film on silicon have been investigated in the wavelength range of 190–2650 nm (0.47–6.5 eV) under different external direct-current voltage. The Drude–Lorentz dispersion model is used to extract the optical function. The O 2 p to Ni 3 d electronic transition can be uniquely assigned to the energy of about 1.96 eV and decreases with decreasing applied voltage. The discrepancy from the real part of dielectric function with the applied voltage has a strong spectral dependence. The optical conductivity variation under different external voltage indicates that the electrical field can induce the modification of the carrier transport.

Structural, optical and electrical properties of delafossite CuGaO2 films grown by sol-gel method

Transparent p-type conductive CuGaO2 films have been fabricated on sapphire substrates by sol–gel method. The stable sol solution for CuGaO2 were formed by copper(II) acetate monohydrate and gallium(III) nitrate hydrate, and the c-axis orientation of CuGaO2 films were strengthened with increasing annealing temperature. The pure phase CuGaO2 film was obtained at 900°C for 30 min in N2 atmosphere, and its microstructure, compositions, optical and electrical properties were analyzed. It was found that the sol-gel derived CuGaO2 films show a high optical transparency (60-80%) in the visible region, the direct and indirect band gaps were approximately 3.56 and 3.24 eV, respectively. It shows a crossover from the thermal activation behavior to that of three-dimensional variable range hopping from the temperature-dependent electrical conductivity at about 160 K.

Synthesis and optical properties of the (La, Mn)-codoped BiFeO3 films on n+-Si(100) substrates

La-doped BiFe0.92Mn0.08O3 films with the composition from 0 to 20% (BLFMx) have been deposited on Si(100) substrates by a sol-gel route. X-ray diffraction analysis shows that the films are polycrystalline and exhibit the pure perovskite phase structure (R3c). The La dopant effects on the surface morphology, dielectric function, and optical transition of the films have been investigated by atomic force microscopy and spectroscopic ellipsometry at room temperature. The dielectric functions of the films have been uniquely extracted by fitting the measured ellipsometric spectra with a three phase layered model (air/film/Si) and the Tauc-Lorentz dielectric function model in the photon energy range of 0.5-3.5 eV. It is found that the optical transitions decrease with increasing La composition, which is crucial for future photoelectric device.

Temperature dependence of photoluminescence, Raman scattering, and transmittance spectra of anatase Ti1-xFexO2 nanocrystalline films

Anatase Ti1-xFexO2 (x=0, 1%, 2%) nanocrystalline films were prepared on quartz substrates by a facile nonhydrolytic sol-gel route. The structure and optical properties have been studied by X-ray diffraction (XRD), Raman scattering, transmittance spectra and temperature dependent photoluminescence (PL). The B1g, Eg and (A1g+B1g) modes of anatase phase TiO2 can be observed in Raman spectra. Dielectric functions have been extracted by fitting the transmittance spectra in the photon energy range of 0.5-6.5 eV with Adachis model. The pure TiO2 film displays a strong broadening visible luminescence band; however, Fe-doped samples exhibit a very weak luminescence due to the increase of oxygen vacancy concentration in TiO2. With the temperature increases, the PL intensity decreases monotonously and there are five emission peaks for the pure sample in low temperatures, which could be attributed to oxygen vacancies, surface states and F+ center.