Xiuai Lu

Modification of the lattice thermal conductivity in semiconductor rectangular nanowires

A model for lattice thermal conductivity in a semiconductor nanowire with a rectangular cross section is investigated. It is based on solving the equations of phonon radiative transfer taking into account interface scattering and modification of the acoustic phonon dispersion. From the numerical calculations, we predict a decrease by an order of magnitude of the lattice thermal conductivity of a 10-nm-thick and 20-nm-wide rectangular nanowire. The interface scattering and phonon confinement play important roles in the reduction of lattice thermal conductivity. The nanowire lattice thermal conductivity is found to decrease with increasing temperature at moderate and high temperatures. It is shown that the phonon confinement becomes more important than interface scattering in the highly specular scattering case. The effect of interface roughness scattering and phonon confinement on the lattice thermal conductance in a rectangular wire at low temperature is also examined. We calculate the contribution to the...

Size effect on the thermal conductivity of nanowires

The size effect on thermal conductivity of a free-standing wire with rectangular cross-section is investigated. The electronic thermal conductivity is calculated within a Boltzmann transport equation approach. A simple expression for the reduction in conductivity due to the increase of boundary scattering is presented. The values of thermal conductivity deduced from our calculation are in good agreement with the experimental data for sodium and gold nanowires. The lattice thermal conductivities of silicon (Si) and cadmium telluride (CdTe) nanowires are examined on the basis of the equations of phonon radiative transfer. The calculated thermal conductivities of Si nanowires with square cross-section are found to be in good agreement with molecular dynamics results. The Umklapp and boundary scattering limited lattice thermal conductivities of CdTe nanowires are investigated theoretically by taking into account acoustic phonon dispersion. It is found that the thermal conductivity of nanowires decreases with ...

Spectroscopic properties of Er3+ transitions in SrWO4 crystal

Er3+:SrWO4 crystal of good optical quality was grown by Czochralski technique method. Polarized absorption and emission spectra of Er3+ ions in SrWO4 crystal have been investigated. The Judd-Ofelt (JO) theory was applied to the polarized absorption spectra of Er3+:SrWO4 to obtain the three JO parameters: Ω2=7.41×10−20cm2, Ω4=0.25×10−20cm2, and Ω6=1.71×10−20cm2. The radiative probabilities, radiative lifetimes, and branch ratios of Er3+:SrWO4 were calculated. The stimulated emission cross sections of the potential laser transitions at 1535nm were determined using the reciprocity method and the Fuchtbauer-Ladenberg formula, and the potential laser gain versus wavelength for the I13∕24→I15∕24 transition has been estimated and discussed.

Spectroscopic analysis of Ho3+ in an NaY(MoO4)2 single crystal

A systematic investigation of room temperature absorption and emission spectra were presented for trivalent holmium in a crystal of NaY(MoO4)2. Oscillator strengths of the transitions to all levels up to 361 nm were calculated from the absorption spectra. Judd–Ofelt intensity parameters of Ho3+ were used to calculate the radiative transition rates, branching ratios and radiative lifetimes. The emission cross-sections for the 5S2 → 5I8 and 5I7 → 5I8 transitions of special interest for laser application were determined.

Optical spectroscopy of Ca3Gd2(BO3)4:Nd 3+ laser crystal

Ca3Gd2(BO3)4:Nd3+ crystals of high optical quality with a size up to 20 × 50 mm were grown by the Czochralski method. The absorption spectra of the crystal in three mutually perpendicular arbitrary directions were measured. Based on Judd–Ofelt and TPM methods, the intensity parameters Ω t (t = 2, 4, 6), the experimental and theoretical oscillator strengths, the radiative transition rates, radiative lifetime and fluorescent branching ratios of Nd3+ from 4F3/2 to 4IJ, were obtained. The fluorescence spectrum and decay curve under excitation at 808 nm were also measured. The stimulated emission cross-section and radiative quantum efficiency were also calculated.

Optical properties of Er3+ doped Ca3(BO3)2 crystal

The polarized absorption, emission spectra, and the fluorescence decay curve of Er3+ ions in the anisotropic Ca3(BO3)2 crystal were investigated at room temperature. The experimental oscillator strengths for the transitions from the ground state to excited states were compared with the theoretical electric dipole oscillator strengths calculated from the Judd-Ofelt [Phys. Rev. 127, 750 (1962); J. Chem. Phys. 37, 511 (1962)] theory. The emission cross sections and gain cross sections of the π and σ spectra were determined for the H13∕24→H15∕24 transition of special interest for laser application.

Optical and thermal properties of Co2+:ZnWO4 crystal

The crystal of ZnWO4 doped with Co2+ has been grown by the Czochralski technique. The orientation procedure was presented. Along the crystallographic axes, the principal coefficients of thermal expansion and absorption spectra were measured and discussed. Based on the data, the crystal field parameter 10Dq, Racah parameter B, as well as the absorption cross sections were calculated. The nonlinear dependence of optical absorption on concentration of cobalt ions was found and discussed in single crystal Co2+:ZnWO4. The results indicate that inner defects such as natural cleavage or slip faces in crystal are important in the study of optical and thermal properties.

Spectroscopic assessment of Tm3+:GdAl3(BO3)4 crystal as a potential diode pumped laser near 1.9μm

The spectroscopic properties of GdAl3(BO3)4:Tm3+ single crystal for the Tm3+ transition near 1.9μm are presented. The absorption cross section, the emission cross section, and the potential laser gain near 1.9μm are investigated for both σ and π polarizations. The broad absorption band at about 797nm and high value of the above cross section are promising features for obtaining a laser diode pumped Tm3+ laser. The maximum emission cross sections are 1.29×10−20 and 6.60×10−20cm2 at 1.85μm for the σ and π polarizations, respectively. The potential laser gain curves indicate that more than 200nm tunability range is expected for this material.

Buffer-layer-enhanced magnetic field effect in La0.5Ca0.5MnO3/LaMnO3/SrTiO3:Nb heterojunctions

The transport behaviors of La0.5Ca0.5MnO3/LaMnO3/SrTiO3:Nb heterojunctions with a LaMnO3 layer thickness of between 0 and 12 nm have been systematically studied. The effect of the magnetic field on the junction without the buffer layer is weak. The influence of the magnetic field on the junction is maximized when the layer thickness is ∼3 nm, demonstrated by a significant field-induced increase in current when the bias voltage is fixed. The corresponding magnetoresistance of the junction is negative, and its maximal value is ∼−32% for a field change of 5 T at 80 K. Based on a quantitative analysis of the current-voltage characteristics, the interfacial barrier can be derived, and it shows a complex variation with an increase in layer thickness, first decreasing and then increasing. This is the apparent reason for the change in buffer-layer-enhanced negative magnetoresistance of the junctions for different layer thicknesses.