Browsing by Author "Li, ZQ"

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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.
Electrical resistivities and thermopowers of transparent Sn-doped indium oxide films
(2004-11-15)
Li, ZQ
;
Lin, JJ
;
電子物理學系
;
物理研究所
;
Department of Electrophysics
We have systematically measured the electrical resistivities and thermopowers of transparent tin-doped indium oxide films. We found that the resistivities obey the Bloch-Gruneisen law between 25 and 300 K, whereas below 25 K, the resistivities slightly increase logarithmically with the decreasing temperature due to the weak-localization and electron-electron interaction effects. The thermopowers are negative and decrease linearly with temperature from 300 K down to 1.8 K. Our results strongly indicate that the tin-doped indium oxide films behave as a good, free-electron-like conductor while being transparent. (C) 2004 American Institute of Physics.
Electronic transport studies on Sb1-x(SiO2)(x) films
(2005-05-04)
Du, J
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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.
Optical and electrical transport properties of facing-target sputtered Al doped ZnO transparent film
(2006-06-15)
Li, ZQ
;
Zhang, DX
;
Lin, JJ
;
電子物理學系
;
物理研究所
Al doped zinc oxide thin film was prepared by dc facing-target sputtering method and its structural, optical, and electrical transport properties have been investigated. The average transmittance of the films is greater than 90% in the wavelength region of 450-700 nm while the resistivity is as high as 3x10(-3) Omega cm. The band gap energy derived from the transmission data is 3.76 eV, which is higher than that of pure ZnO thin film. This band gap growth phenomenon cannot be explained in terms of the Burstein-Moss effect. The resistivity and Hall effect measurements suggest that the interaction between the charge carriers and phonons plays a key role in the electrical transport properties of the film between 60 and 300 K. The film exhibits negative magnetoresistance at low temperatures, which can be well described by a semiempirical expression that takes into account the third order s-d exchange Hamiltonians describing a negative part and a two-band model for positive contribution.