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Kuang Yao Chen, Y H Chang,
C. -T. Liang,
N Aoki,
Y. Ochiai,
C F Huang,
Li-Hung Lin,
K. A. Cheng,
H. H. Cheng,
H H Lin,
Jau-Yang Wu,
Sheng-Di Lin
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ABSTRACT: Magneto-transport measurements are performed on the two-dimensional electron system (2DES) in an AlGaAs/GaAs heterostructure. By increasing the magnetic field perpendicular to the 2DES, magnetoresistivity oscillations due to Landau quantisation can be identified just near the direct insulator-quantum Hall (I-QH) transition. However, different mobilities are obtained from the oscillations and transition point. Our study shows that the direct I-QH transition does not always correspond to the onset of strong localisation.
03/2008;
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ABSTRACT: Magnetic-field-induced phase transitions were studied with a two-dimensional electron AlGaAs/GaAs system. The temperature-driven flow diagram shows the features of the $\Gamma$(2) modular symmetry, which includes distorted flowlines and shiftted critical point. The deviation of the critical conductivities is attributed to a small but resolved spin splitting, which reduces the symmetry in Landau quantization. [B. P. Dolan, Phys. Rev. B 62, 10278.] Universal scaling is found under the reduction of the modular symmetry. It is also shown that the Hall conductivity could still be governed by the scaling law when the semicircle law and the scaling on the longitudinal conductivity are invalid. *corresponding author:yhchang@phys.ntu.edu.tw
10/2006;
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ABSTRACT: Magnetic-field-induced phase transitions in the integer quantum Hall effect are studied under the formation of paired Landau bands arising from Zeeman spin splitting. By investigating features of modular symmetry, we showed that modifications to the particle-hole transformation should be considered under the coupling between the paired Landau bands. Our study indicates that such a transformation should be modified either when the Zeeman gap is much smaller than the cyclotron gap, or when these two gaps are comparable.
09/2005;
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ABSTRACT: We studied the insulator-quantum Hall conductor transition which separates the low-field insulator from the quantum Hall state of the filling factor $\nu=4$ on a gated two-dimensional GaAs electron system containing self-assembled InAs quantum dots. To enter the $\nu=4$ quantum Hall state directly from the low-field insulator, the two-dimensional system undergoes a crossover from the low-field localization to Landau quantization. The crossover, in fact, covers a wide range with respect to the magnetic field rather than only a small region near the critical point of the insulator-quantum Hall conductor transition. Comment: Our experimental results are inconsistent with Huckestein's argument [PRL 84, 3141 (2000)]
03/2005;
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ABSTRACT: Plateau-plateau (P-P) and insulator-quantum Hall conductor (I-QH) transitions are observed in the two-dimensional electron system in an AlGaAs/GaAs heterostructure. At high fields, the critical conductivities are not of the expected universal values and the temperature-dependence of the width of the P-P transition does not follow the universal scaling. However, the semicircle law still holds, and universal scaling behavior was found in the P-P transition after mapping it to the I-QH transition by the Landau-level addition transformation. We pointed out that in order to get a correct critical exponent, it is essential that the scaling analysis must be performed near the critical point. And with proper analysis, we found that the P-P transition and the insulator quantum Hall conductor transitions are of the same universal class. Comment: 11 pages, 3 figures
04/2004;
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ABSTRACT: We have performed low-temperature transport experiments on a Si/SiGe hole system. The measured transverse and longitudinal condductivities $\sigma_{xx}$ and $\sigma_{xy}$ allow us to study the magnetic-field-induced transitions in the system. In particular, we present the first study of the temperature-driven flow lines in the "anomalous Hall insulator" regime near a Landau level filling factor $\nu=1.5$. The "anomalous" temperature-driven flow lines could be due to the unusual energy level scheme in a Si/SiGe hole system. Moreover, for $3<\nu<5$, there is a temperature-independent point in $\rho_{xx} (B)$, $\rho_{xy} (B)$, $\sigma_{xx}$, and $\sigma_{xy}$ which corresponds to a boundary of the quantum phase transition.
02/2002;
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ABSTRACT: We present magneto-photoconductivity studies on GaAs/Al0.3Ga0.7As multiple quantum wells selectively doped with Si donors in the center of the wells and in the barrier layers. Under extra illumination of an He–Ne laser, dramatic results of the D− centers were observed as a function of the electron density. A band-bending model associated with a “barrier D− center” configuration was used to analyze the evolution of the binding energy of the spin-singlet D− transition and magnetic vaporization.
Solid State Communications.
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ABSTRACT: We have measured the low-temperature transport properties of two-dimensional (2D) GaAs electron gases and 2D SiGe hole gases. Our experimental results fall into three categories. (i) Collapse of spin-splitting and an enhanced Landé g-factor at Landau level filling factors both ν=3 and ν=1 in a 2D GaAs electron gas are observed. Our experimental results show direct evidence that the effective disorder is stronger at ν=1 than that at ν=3 over approximately the same perpendicular magnetic field range. (ii) We present evidence for spin-polarisation of a dilute 2D GaAs electron gas. The Lande g-factor of the system is estimated to be 1.66. This enhanced g value is ascribed to electron–electron interactions at ultra low carrier density limit. (iii) In a high-quality SiGe hole gas, there is a temperature-independent point in the magnetoresistivity ρxx and ρxy which is ascribed to experimental evidence for a quantum phase transition between ν=3 and ν=5. We also present a study on the temperature(T)-driven flow lines in our system.
Journal of Physics and Chemistry of Solids.
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ABSTRACT: We have studied insulator–quantum Hall–insulator (I-QH-I) transitions in a gated two-dimensional GaAs electron gas containing InAs quantum dots. In this system Shubnikov–de Haas oscillations are observed in both the low-field and high-field insulating regimes, showing that Landau quantization and localization can coexist. A phase diagram is constructed based on our experimental results, and we see that the critical points of the I-QH-I transitions do not correspond to crossover from localization to Landau quantization. Moreover, good scaling behavior is observed on both sides of low- and high-field I-QH transitions.
Phys. Rev. B. 69(7).
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ABSTRACT: We report a novel negative magnetophoconductivity response occuring at cyclotron resonance in As quantum wells under the illumination of the photon energy larger than the energy gap of the barrier. The experiment can be explained by the creation of the asymmetry of the confined potential due to the charge transfer of the photoexcited electrons and holes. The prediction of the model that the negative photoconductivity disappears for the sample under the illumination of the photon energy between the energy gaps of the barrier and the well is further confirmed by the experiment.
Solid State Communications.
