Tsai-Yu Huang

Electrically detected and microwave-modulated Shubnikov–de Haas oscillations in an Al0.4Ga0.6N/GaN heterostructure

We report the drastic enhancement pattern of Shubnikov–de Haas (SdH) oscillations observed in an AlGaN/GaN heterostructure by microwave modulation. The dependence of the SdH pattern on microwave power and temperature is investigated. The underlying mechanism is attributed to the effect of carrier heating. This technique helps study the transport properties of two-dimensional electrons in many wide-band-gap heterostructures, in which moderate mobilities and heavier electron effective mass (rapidly damping SdH amplitudes) are frequently encountered. In addition, this method has the advantage of keeping the carrier concentration fixed and not requiring expensive high-energy laser facilities compared with carrier-modulated SdH measurements.

Exchange-enhanced g‐factors in an Al0.25Ga0.75N∕GaN two-dimensional electron system

Low-temperature magnetotransport measurements were performed on an Al0.25Ga0.75N∕GaN two-dimensional electron system. In this system, we observe Shubnikov-de Haas (SdH) oscillations in a perpendicular magnetic field B. By measuring the positions of a pair of spin-split SdH maxima, we are able to estimate the g‐factors at different Landau level (LL) indices. We find the g‐factor is enhanced over its bulk value in GaN (≅2) due to many-body exchange interactions. Moreover, the measured g‐factor increases with decreasing LL index, indicating that many-body electron–electron interactions become stronger as the number of occupied LLs decreases. Our results suggest that the exchange energy Eex shows an approximately linear B dependence.

On the low-field insulator-quantum Hall conductor transitions

Abstract We studied the insulator-quantum Hall conductor transition which separates the low-field insulator from the quantum Hall state of the filling factor ν=4 on a gated two-dimensional GaAs electron system containing self-assembled InAs quantum dots. To enter the ν=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.

From insulator to quantum Hall liquid at low magnetic fields

We have performed low-temperature transport measurements on a GaAs two-dimensional electron system at low magnetic fields. Multiple temperature-independent points and accompanying oscillations are observed in the longitudinal resistivity between the low-field insulator and the quantum Hall (QH) liquid. Our results support the existence of an intermediate regime, where the amplitudes of magneto-oscillations can be well described by conventional Shubnikov\char21{}de Haas theory, between the low-field insulator and QH liquid.

Experimental studies of low-field landau quantization in two-dimensional electron systems in GaAs/AlGaAs heterostructures

By applying a magnetic field perpendicular to GaAs/AlGaAs two-dimensional electron systems, we study the low-field Landau quantization when the thermal damping is reduced with decreasing the temperature. Magneto-oscillations following Shubnikov-de Haas (SdH) formula are observed even when their amplitudes are so large that the deviation to such a formula is expected. Our experimental results show the importance of the positive magneto-resistance to the extension of SdH formula under the damping induced by the disorder.

Direct measurement of the spin gaps in a gated GaAs two-dimensional electron gas

We have performed magnetotransport measurements on gated GaAs two-dimensional electron gases in which electrons are confined in a layer of the nanoscale. From the slopes of a pair of spin-split Landau levels (LLs) in the energy-magnetic field plane, we can perform direct measurements of the spin gap for different LLs. The measured g-factor g is greatly enhanced over its bulk value in GaAs (0.44) due to electron–electron (e-e) interactions. Our results suggest that both the spin gap and g determined from conventional activation energy studies can be very different from those obtained by direct measurements.

'mobility gap' of a spin-split GaAs two-dimensional electron gas

Abstract We have performed magnetotransport measurements of the electron g-factor in a two-dimensional GaAs electron gas. In order to obtain the spin gap Δs, we measure the spin-split longitudinal resistivity minimum which shows an activated behavior. From the spin gaps at different odd filling factors, we can obtain the effective g-factor which is greatly enhanced over its bare value (0.44) in GaAs. This enhancement is due to many-body electron–electron interactions. Our experimental results provide compelling evidence that conventional activation energy studies yield a ‘mobility gap’ which can be very different from the real spin gap in the energy spectrum.

Experimental studies of composite fermion conductivity: dependence on carrier density

We present an experimental study of the carrier density dependence of the composite fermion (CF) conductivity sigma(xx)(CF) at both Landau level filling factors v = 1/2 and v = 3/2 and in high-quality front-gated GaAs/Al0.33Ga0.67As heterostructures. Extracting alpha from the power law ln(sigma(xx)(CF)) proportional to ln(n(e))(alpha) shows that alpha approximate to 1, in qualitative agreement with theoretical predictions based on a random magnetic field approximation in the electron density distribution

Exchange-enhanced Landé g-factor, effective disorder and collapse of spin-splitting in a two-dimensional GaAs electron system

Abstract We have measured the low-temperature transport properties of front-gated GaAs/Al 0.33 Ga 0.67 As heterostructures. Collapse of spin-splitting and an enhanced Lande | g |-factor at Landau level filling factors both ν =3 and 1 are observed. Our experimental results show direct evidence that the electron–electron interactions are stronger at ν =3 than those at ν =1 over approximately the same perpendicular magnetic field range.

Spin-dependent transport in a two-dimensional GaAs electron gas in a parallel magnetic field

Abstract We report low-temperature magnetoresistivity measurements of high-quality gated two-dimensional (2D) electron systems. In the dilute electron density limit, we show evidence for spin polarisation in an in-plane magnetic field. Using a simple model, we estimate the Lande g-factor in this dilute two-dimensional electron gas to be about 3.32. The enhanced Lande g-factor compared with that of a bulk GaAs 2D electron system (0.44) is ascribed to electron–electron interaction effects at ultra-low electron densities.