We report magnetotransport measurements and scaling analysis on a series of center-doped GaAs quantum wells. Sharp phase transitions were observed in the magnetic field sweep and it was found that, depending on the doping concentration in the quantum wells, the insulating phase can make transitions to quantum Hall phase with Landau-level filling factors ͑͒ of 2, 6, and 8. The critical exponents vary from sample to sample and are mobility dependent. The longitudinal resistivities of these samples at the phase transition points decrease with increasing , and for a sample with higher mobility, its value is close to h/e 2 .

Women of climacteric age in Taiwan utilized TCM more often than other age groups. To deal with multiple symptoms and/or diseases among climacteric women, new prescription patterns of combining two or more herbal formulae have evolved. Studies on safety issues and drug-herb interactions are warranted for future research.

Insulator-quantum Hall conductor transitions at low magnetic field B were studied with a gated GaAs-AlGaAs heterostructure. A low field disorder-magnetic field phase diagram was constructed based on the experimental results. This phase diagram shows no floating up of the extended state and allows transitions from the insulating state directly to any Landau level states. The critical filling factor can change from 16 to 6 as the disorder in the sample increases. By inspecting the raw data from this and the other samples and analyzing the scaling behaviors near the transition points, we found that the observed transition has the properties of a genuine phase transition.

We present magnetotransport measurements on GaAs/Al x Ga 1Ϫx As multiple quantum wells doped with impurities at the centers of GaAs layers. Magnetic-field-induced insulator-quantum Hall conductor-insulator transitions with a well defined scaling behavior were observed. The critical exponents as well as the resistivities at the two critical magnetic fields are close to each other. Their values are, however, different from the values obtained from similar studies performed on modulation-doped GaAs/Al x Ga 1Ϫx As heterostructures.

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