Browsing by Author "Liu, H"

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Competition between the charge ordered and ferromagnetic states in (La,Nd)(0.75)Na0.25MnO3 manganites
(2004-05-24)
Li, ZQ
;
Zhang, XH
;
Yu, JS
;
Liu, XJ
;
Liu, XD
Structural, magnetic, and transport properties of (La1-xNdx)(0.75)Na0.25MnO3 (0 less than or equal to x less than or equal to 1) are studied. The system exhibits a rhombohedrally distorted perovskite structure for x less than or equal to 0.2 and it convert to an orthorhombic structure for x greater than or equal to 0.4. Re-entrant ferromagnetic type charge ordering transitions are observed in the narrow bandwidth samples (x greater than or equal to 0.6), while charge ordering transition is not observed for these samples with rhombohedral structure. Combining with the published results related to the charge ordering transition, we argue that the rhombohedral structure likely favor double exchange interaction and suppress charge ordering interaction. The compounds show metal to insulator transitions except Nd0.75Na0.25MnO3. The resistivity data above the metal to insulator transition temperature for the charge ordered sample are discussed in the frame work of variable-range hopping model. (C) 2004 Elsevier B.V. All rights reserved.
Design and analysis of VCSEL based two-dimension wavelength converter
(2003-07-14)
Liu, H
;
Shum, P
;
Kao, MS
;
電信工程研究所
;
Institute of Communications Engineering
A novel two-dimensional vertical cavity surface emission laser (VCSEL) based wavelength converter is proposed. We developed a two-dimensional transmission line laser model (TLLM) to analyze the proposed wavelength converter. This model takes into account Bragg reflectors by using the modified connecting matrix. Therefore, accurate and efficient modeling of the VCSEL structure is achieved. Extinction ratio of the output signal is investigated by considering input signal power, wavelength, facet reflectivity and cavity diameter. (C) 2003 Optical Society of America.
Electronic transport studies on Sb1-x(SiO2)(x) films
(2005-05-04)
Du, J
;
Li, ZQ
;
Lin, JJ
;
Liu, H
;
Zheng, RK
Sb1-x(SiO2)(x) granular films were prepared by the co-sputtering method with the volume fraction of SiO2, x, ranging from 0 (i.e. pure Sb) to about 30 %. Systematic electronic transport studies, including resistivity, magnetoresistance, Hall effect and Seebeck effect, were carried out against the temperature, magnetic field and volume fraction x of SiO2. With the gradual increase of the SiO2 content, the mean grain size of the Sb decreases, and eventually the film becomes amorphous, as illustrated by the changes of the x-ray diffraction patterns. The temperature coefficient of resistivity also changes its sign from positive to negative, indicating a semimetal-semiconductor or insulator transition. Magnetoresistance studies using the weak localization theory revealed that the electron dephasing time follows approximately a T-2 law. This behaviour indicates that the electron-phonon (e-ph) scattering still dominates the electron dephasing processes in these granular systems with a fair number of SiO2 inclusions. The Hall coefficient decreases monotonically with temperature and with the volume fraction of SiO2. The giant Hall effect is absent in these granular films. Finally, an interesting but rather complicated behaviour of the Seebeck coefficient versus temperature was observed when the volume content of the SiO2 exceeded 18 %.
Giant Hall effect in nonmagnetic granular metal films
(2001-06-11)
Zhang, XX
;
Wan, CC
;
Liu, H
;
Li, ZQ
;
Sheng, P
Nearly 3 orders of magnitude enhancement in the Hall coefficient is observed in Cu-x -(SiO2)(1-x) granular films. This large enhancement of the Hall coefficient not only is significantly larger than the prediction of the classical percolation theory, but also occurs at a metal concentration identified to be the quantum percolation threshold. Measurements of the electron dephasing length and magnetoresistance, plus the TEM characterization of microstructures, yield a physical picture consistent with the mechanism of the local quantum interference effect.
Low temperature electrical transport properties of RuO2 and IrO2 single crystals
(2004-11-17)
Lin, JJ
;
Huang, SM
;
Lin, YH
;
Lee, TC
;
Liu, H
We have systematically measured the electrical transport properties of several RuO2 and IrO2 single crystals over a wide temperature range from 300 K down to 0.3 K to study the conduction mechanisms in these oxides. Our measured resistivities are in close agreement with the recent band-theory calculations for these materials. The characteristic temperatures for the acoustic-mode and optical-mode phonons are determined. Our measured magnetoresistances are positive and follow the Kohler rule, indicating that the transport properties of these oxides exhibit normal behaviour as described by the Boltzmann equation. In contrast, we do not find any signature of superconductivity down to 0.3 K, though the band-theory calculations predict a superconducting transition temperature of similar to5 K. Magnetization measurements suggest a very low level of paramagnetic impurities in our crystals.
Theoretical study of extinction ratio and frequency chirping of VCSEL-based two-dimensional wavelength converter
(2004-02-01)
Liu, H
;
Shum, P
;
Kao, MS
;
電信工程研究所
;
Institute of Communications Engineering
A novel two-dimensional (2-D) vertical-cavity surface-emission laser (VCSEL)-based wavelength converter was reported, in which the VCSEL was used both as the laser source for the probe wave and as the semiconductor amplifier for performing cross-gain modulation. We have developed a complete 2-D transmission line laser model to analyze the proposed structure. The optimization of the wavelength location, input power level, as well as the reflectivity of the VCSEL facet have been done with respect to the, extinction ratio and peak chirp frequency.