M. O. Tjia

Dynamics of photo-excited electrons in magnetically ordered TbMnO3

Time resolved optical spectroscopy is used to elucidate the dynamics of photodoped spin-aligned carriers in the presence of an underlying magnetic lattice in the multiferroic compound TbMnO(3). The transient responses while probing d-d intersite transitions show marked differences along different crystallographic directions, which are discussed in terms of the interplay between the processes of hopping of the photo-injected electrons and the magnetic order in the material.

Single-Frequency Refractive Index Sensor Based on a Finite One-Dimensional Photonic Crystals with Two Defects

A numerical analysis by means of transfer matrix method has been performed on finite one-dimensional photonic crystals consisting of two-layer repeated cells and two non-identical defect cells for the normal incident transverse electric (TE) wave. The study reveals a remarkable new feature showing that the variation of a photonic pass-band transmittance can be induced by varying the refractive index of one of the defect layer at practically the same peak frequency, which offers the potential application for single frequency sensing. The result further demonstrates the flexibility of tailoring the system parameters for application in the desired range of refractive index at the required sensitivity. It is also shown that the photonic pass-band (PPB) peak transmittance is generally less than unity in the index range considered, except for the case with the grating segment lengths (M,N,L) satisfying the condition N=M+L. This peculiar feature is explained qualitatively in this work.

Analytic study of stationary solitons in deep nonlinear Bragg grating

A study of nonlinear Bragg grating has been carried out using a modified scheme of approximation originally proposed by Iizuka and de Sterke. A complete classification of the solitonic solutions in the system was given. We further demonstrated in this work the existence of in-gap dark and antidark soliton, in addition to the out-gap solutions reported previously. We also found at the boundaries in the bifurcation diagram, the large-amplitude out-gap antidark soliton and broad in-gap dark soliton.

μSR study of electron radical dynamics in regio-regular polythiophene

We have carried out longitudinal field (LF) muon-spin-relaxation (μSR) measurements in polythiophene based polymers of Poly(3-hexylthiophene-2,5-diyl) with regio-regular structure to elucidate the intra- and inter-chain hopping mechanisms. The LF dependent muon-spin depolarization rate indicates the occurrence of dimensional crossover from 1 dimensional intra-chain spin diffusion to 3 dimensional inter-chain spin diffusion at 25 K.

Fishtail effect and the superconducting phase diagram of La1.6−xNd0.4SrxCuO4 single crystal

Abstract Measurements of isothermal magnetic hysteresis loop in c -aligned external field have been performed on superconducting La 1.6− x Nd 0.4 Sr x CuO 4 ( x =0.10, 0.20) single crystals, resulting in the H – T phase diagrams for the respective samples. The isothermal magnetization data of Sr-0.10 reveal the existence of fishtail effect in the certain temperature range of measurement (2.5⩽ T ⩽5.5 K), terminated at temperature well below the superconducting critical transition temperature ( T c on ≈8 K). Analysis of the irreversibility line shows a sign reversal of its slope at around the crossover field, H 2D , indicating 2D melting at higher field. On the other hand, a single H irr (T) function of 3D character was found to fit the irreversibility data for the Sr-0.20 sample, indicating the lack of vortex line decoupling effect in the same range. These remarkably different behaviors are suggested to have their origin in the different degrees of anisotropy ( γ ≈96 for Sr-0.10 vs. γ ≈25 for Sr-0.20) as a consequence of different levels of Sr doping.

Performance changes of a grated waveguide at resonance wavelengths next to its band-edges due to modified edge sections

An efficient numerical scheme developed on the basis of Green’s function method is applied to the investigation of structural effects on the performance of planar grated waveguide at the first resonance wavelengths next to the band-edges. Restricting ourselves to the transverse-electric waves, this study is focused on the effects induced by variations of the grating cell number and the depths of its four outer grooves on both sides. The different patterns of groove depth gradation or apodization considered in this study are all characterized by decreasing depth toward the ends while retaining the longitudinal grating symmetry. The effects of the modifications are expressed in terms of changes in the modal transmittance, reflectance, and out-of-plane scattering loss as well as the group velocity and resonant field enhancement. The most favorable result characterized by 15% transmittance enhancement and 85% loss reduction is achieved for the case with the most gradual changes in the groove depth. It is further shown that, for the investigated range of parameters, both the group velocity and field enhancement can best be improved by increasing the length of the uniform grating, without introducing any modification.

OPTICAL SENSING AND SWITCHING DEVICE BASED ON A FINITE DEEP NONLINEAR BRAGG GRATING WITH A MIRROR

We investigate the detailed transition of the dark to antidark soliton-like states in a system of finite deep nonlinear Bragg grating equipped with a movable metallic mirror and illuminated by a continuous laser source. As reported previously, the transition can be induced mechanically by moving the mirror as well as optically by changing the light source intensity.

Different roles of anisotropy and disorder on the vortex matter of Bi2Sr2CaCu2O8+δ single crystal

Abstract The oxygen doped and Pb-substituted single crystalline Bi 2 Sr 2 CaCu 2 O 8+ δ samples having T c,on and anisotropy ( γ 2 ) ranging respectively from 65 to 93 K and from 773 to 49,715 have been studied in the basis of magneto-resistance and magnetization data. It is found that the lowering of anisotropy due to higher oxygen content or Pb substitution is responsible for the reduction of magnetic field-induced transitional broadening and a shift of H irr ( T ) and H g ( T ) to higher temperatures and magnetic field. Meanwhile, the increasing disorder manifested by decreasing T c also leads to the extended range of appearance for the second magnetization peak (SMP) and enlarged critical glassy region. It is shown that decreasing anisotropy does not necessarily lead to the lowering of T c in contrast to the monotonic dependence of T c on the disorder measured in the temperature range of SMP effect. Additionally, a remarkable monotonic correlation between T c and the glassy exponent s has been established.

Variation of vortex-glass dynamics and critical region with oxygen content in Bi2Sr2CaCu2O8+δ single crystal

Abstract A systematic analysis of disorder and anisotropy effects has been performed on the transport data of three oxygen doped Bi 2 Sr 2 CaCu 2 O 8+ δ (BSCCO-2212) single crystals obtained in external magnetic field aligned with the crystal c -axis, ranging from 100 Oe to 40 kOe. The result confirms the existence of finite critical region in all cases and ostensible crossover behavior associated with 3-D vortex-glass transition. The maximum widths of the regions ([ T * − T g ]/ T g =0.66, 0.53 and 0.38) as well as the critical exponents ( s =5.3±1.3, 3.8±0.7 and 2.8±0.6) are shown to decrease consistently with lower anisotropy ( γ 2 =19,500, 8300 and 3000) corresponding to samples with increasing doping. While being responsible for the suppression of first-order melting, the effect of stronger disorder also contributes to larger temperature and field ranges of the critical region.

Single crystal growth of T*-phase SmLa0.8Sr0.2CuO4

Abstract A single crystal of SmLa 0.8 Sr 0.2 CuO 4 superconductor has been grown by means of the travelling solvent floating zone method. The growth was performed in a 1:20 mixture of oxygen and argon with a total pressure of 2 bar. The highest bulk superconducting transition temperature of 24 K was achieved after annealing the as grown crystal in 200 bar oxygen at 600°C for 7 days and 300°C for 3 days followed by slow cooling to room temperature. The structural refinement analysis of the XRD pattern for the as-grown and as-annealed crystal reveals an enlargement of the unit cell in all crystallographic directions, with the T-type and T ′ -type block layers indicating respectively c -direction expansion and shrinkage due to the annealing process. This result further shows an increase in oxygen filling at the apical site of the T-block, which explains the occurrence of bulk superconductivity in our crystal.