Chao-Ping Huang

Electron transport in In-rich InxGa1−xN films

We have performed electrical transport measurements on metal-organic vapor phase epitaxy grown In-rich InxGa1−xN (x=1, 0.98, and 0.92) films. Within the experimental error, the electron density in InGaN films is temperature independent over a wide temperature range (4K⩽T⩽285K). Therefore, InxGa1−xN (0.92⩽x⩽1) films can be regarded as degenerate semiconductor systems. The experimental results demonstrate that electron transport in In-rich InxGa1−xN (x=1, 0.98, and 0.92) films is metalliclike. This is supported by the temperature dependence of the density, resistivity, and mobility which is similar to that of a metal. We suggest that over the whole measuring temperature range residue imperfection scattering limits the electron mobility in In-rich InxGa1−xN (x=1, 0.98, and 0.92) films.

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.

Transport measurements on MOVPE-grown InN films

We have performed electrical transport measurements on InN films. Our results show that the electron transport in our InN films is metallic-like, that is, within the experimental error the carrier density is temperature-independent over a wide temperature range (4 K≤T≤290 K). At low temperatures, the resistivities of our InN devices appear to saturate and show gradual increase with increasing temperatures. We suggest that residue impurity scattering limits the electron mobility in InN films. We compare our results with existing theoretical models.

'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.

Transport Properties of Si+ Irradiated YBa2Cu3Oy Thin Films

Hall coefficients, magnetic hysteretic loops, and electrical resistivities in magnetic fields for YBa2Cu3Oy (YBCO) films under irradiation with Si+ were systematically examined in various ranges of temperature to investigate effects of the pinning force and disorder on the transverse Hall voltage near Tc . The incident energy of Si ions was 3 MeV and the fluence φt varied from 1×10 to 1×1014 cm-2 . Under irradiation with Si+, the critical current density of irradiated YBCO films derived from M-H curves showed enhancement and the resistive transition in magnetic fields became narrower in Si+ -irradiated YBCO films. The temperature ranges in which negative Hall coefficients occurred became narrower for a Si+–irradiated YBCO film that showed a stronger pinning force in magnetic fields; hence the negative Hall coefficient is affected by the pinning ability of YBCO films. The results are discussed.

Josephson effects in U-shaped YBa2Cu3O7−y microbridges☆

Abstract We have fabricated U-shape YBa 2 Cu 3 O y high-T c microbridges on SrTiO 3 (001) substrates. The bridge is microprocessed by means of photolithography and wet etching. The fabricated configuration of the bridge is 20 μm wide, 10 μm long and 10 – 40 nm thick. Current-voltage curves show a hysteretic behavior. The junction capacitance is 0.57 pF and the I c R n product is 0.5 mV at 35 K. Shapiro steps are observed under microwave irradiation; voltage spikes are occasionally observed in current-voltage curves and are owing to the absence of the phase locking in the weak links.