Browsing by Author "Rosenbaum, R"

Now showing 1 - 3 of 3

Electronic transport in insulating AlPdRe quasicrystals
(2004-02-18)
Rosenbaum, R
;
Murphy, T
;
Brandt, B
;
Wang, CR
;
Zhong, YL
Resistivity and magnetoresistance measurements have been performed on insulating icosahedral AlPdRe quasicrystal (QC) bar samples. At temperatures in the range 300 K greater than or equal to T greater than or equal to 50 K, the resistivities follow a simple inverse temperature law: rho(T) rho(0)/T(1.0+/-0.1). Below 1 K, the resistivity of a weakly insulating sample exhibited a simple inverse temperature law where rho(T) = rho0/T-0.33 and not an activated variable-range hopping (VRH) law. Strongly insulating samples exhibit saturation of their resistivities to finite values as T --> 0 K. These saturation resistivity values are believed to arise from the presence of a second metallic phase located within the quasicrystal's structure. By extrapolating the measured resistivities at 22 mK to absolute zero, the saturation conductivity values were estimated at T = 0 K and subtracted from the conductivity data points. These 'corrected' data, corresponding only to the QC phase, were found to follow activated VRH laws, having hopping exponents y that vary in the range 0.18 less than or equal to y less than or equal to 0.43. The activated VRH behaviours are observed only below 1 K. The magnetoresistances (MRs) of these samples are also anomalous. The MRs can be explained by including contributions from both the saturation conductivity values and from the QC MR ratios, estimated using the wavefunction shrinkage model.
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 %.
Superconducting fluctuation magnetoconductance in a tungsten carbide film
(2001-11-05)
Rosenbaum, R
;
Hsu, SY
;
Chen, JY
;
Lin, YH
;
Lin, JJ
Magnetoconductance (MC) measurements have been performed on a 2140 Angstrom thick tungsten carbide film at temperatures very close to the superconducting transition temperature T-c of the film. The data are dominated by superconducting fluctuations. A novel three-dimensional phenomenological model is proposed to explain the MC data, yielding good fits. The Larkin beta factor, beta (Larkin), appeared as a fitting parameter. An expression proposed by Larkin for beta (Larkin) failed badly for temperatures extremely close to T-c. But at intermediate and high temperatures compared to T-c, Larkin's expression gave very good agreement in fits to the MC data. At temperatures very close to T-c, a crossover from three dimensions to two dimensions was observed in the behaviour of the MC of the film.