C.-T. Liang

Transport behavior and negative magnetoresistance in chemically reduced graphene oxide nanofilms

The electron transport behavior in chemically reduced graphene oxide (rGO) sheets with different thicknesses of 2, 3, and 5xa0nm was investigated. The four-probe method for the sheet resistance (R(S)) measurement on the intensively reduced graphene oxide samples indicates an Arrhenius characteristic of the electron transport at zero magnetic field B = 0, consistent with previous experimental results on well-reduced GO samples. The anticipated variable range hopping (VRH) transport of electrons in a two-dimensional electron system at low temperatures was not observed. The measured R(S) of the rGO samples are below 52xa0kΩ/square at room temperature. With the application of a magnetic field up to 4xa0T, negative magnetoresistance in the Mott VRH regime was observed. The magnetotransport features support a model based on the spin-coupling effect from the vacancy-induced midgap states that facilitate the Mott VRH conduction in the presence of an external magnetic field.

Zero-field spin splitting in modulation-doped AlxGa1−xN∕GaN two-dimensional electron systems

Low-temperature magnetotransport measurements were performed on AlxGa1−xN∕GaN two-dimensional electron systems. By studying the beating pattern in the Shubnikov–de Haas oscillations in a perpendicular magnetic field, we are able to measure the zero-field spin-splitting energies in our systems. Our experimental results demonstrate that the Rashba term due to structural inversion asymmetry is the dominant mechanism which gives rise to the measured zero-field spin splitting in our wurzite AlGaN∕GaN structures. By utilizing the persistent photoconductivity (PPC) effect, we are able to increase the carrier density n in our AlGaN∕GaN two-dimensional electron system. It is found that the Rashba spin-orbit splitting parameter α decreases with increasing n. We suggest that the formation of long-lived electron-hole pairs induced by the PPC effect decreases the large electric field near the AlGaN∕GaN interface, causing α to decrease with increasing n.

Probing the onset of strong localization and electron–electron interactions with the presence of a direct insulator–quantum Hall transition

Abstract We have performed low-temperature transport measurements on a disordered two-dimensional electron system (2DES). Features of the strong localization leading to the quantum Hall effect are observed after the 2DES undergoes a direct insulator–quantum Hall transition on increasing the perpendicular magnetic field. However, such a transition does not correspond to the onset of strong localization. The temperature dependences of the Hall resistivity and Hall conductivity reveal the importance of the electron–electron interaction effects for the observed transition in our study.

Insulator, semiclassical oscillations and quantum Hall liquids at low magnetic fields

Magneto-transport measurements are performed on two-dimensional GaAs electron systems to probe the quantum Hall (QH) effect at low magnetic fields. Oscillations following the Shubnikov-de Haas (SdH) formula are observed in the transition from the insulator to QH liquid when the observed almost temperature-independent Hall slope indicates insignificant interaction correction. Our study shows that the existence of SdH oscillations in such a transition can be understood based on the non-interacting model.

Demonstration of rashba spin splitting in an Al0,25Ga0,75N/GaN heterostructure by microwave-modulated shubnikov-de Haas oscillations

We report zero-field spin splitting using both conventional and microwave-modulated Shubnikov–de Hass (SdH) oscillations in an Al0.25Ga0.75N/GaN heterostructure. By studying the apparent beating pattern in SdH oscillations, we are able to measure the spin splitting parameter α. In addition, the persistent photoconductivity effect allows us to increase the carrier concentration in our AlGaN/GaN two-dimensional electron system and vary α. With our microwave-modulation technique, we suggest that a sizeable Rashba type of spin splitting is present in AlGaN/GaN heterostructures.

Crossover from Efros-Shklovskii to Mott Variable Range Hopping in Amorphous Thin NixSi(subscript 1-x) Films

The temperature dependence of the electrical conductivity of insulating amorphous Ni(subscript x)Si(subscript 1-x) alloys is studied in the temperature range 1-160 K. At low temperatures, Efros-Shklovskii (ES) variable range hopping (VRH) is observed. This is assumed to occur because of the creation of the Coulomb gap (CG) in the vicinity of the Fermi level. With increasing temperature, the CG vanishes and the measured conductivity can be described by the Mott VRH model, where the density of states is constant. The criterion of the crossover ES from to Mott VRH is assessed by extracting the related parameters.

Spin polarization in a two-dimensional electron gas in GaAs

In this paper, positive magnetoresistance of a dilute two-dimensional electron gas in GaAs is studied in a parallel magnetic field B. It is found that the normalized resistivity curves, ρ(B)/ρ(0), merge together when we scale the field according to B/Bχ, where Bχ is assumed to be the field in which full spin polarization of electrons is reached. It is also shown that the crossing field, Bcross, determined by the crossover of the B2 dependence of the resistivity, becomes lower than Bχ with increasing electron density, which cannot be explained in terms of filling of the upper electron subbands in the fully spin-polarized regime. The spin susceptibility, χ, is assessed by extracting the product g*m*, where g* and m* are the effective Lande factor and electron mass, respectively. The behavior of χ with increasing electron density, however, deserves further theoretical and experimental study.

Hot Carriers in CVD-Grown Graphene Device with a Top h-BN Layer

We investigate the energy relaxation of hot carriers in a CVD-grown graphene device with a top h-BN layer by driving the devices into the nonequilibrium regime. By using the magnetic field dependent conductance fluctuations of our graphene device as a self-thermometer, we can determine the effective carrier temperature at various driving currents while keeping the lattice temperature fixed. Interestingly, it is found that is proportional to I, indicating little electron-phonon scattering in our device. Furthermore the average rate of energy loss per carrier is proportional to ( ), suggesting the heat diffusion rather than acoustic phonon processes in our system. The long energy relaxation times due to the weak electron-phonon coupling in CVD graphene capped with h-BN layer as well as in exfoliated multilayer graphene can find applications in hot carrier graphene-based devices.

Low temperature hopping in granular oxides La0.67Ca0.33MnO3 and La0.5Sr0.5CoO3

We studied electrical transport in La0.67Sr0.33MnO3 (LCMO) and La0.5Sr0.5CoO3 (LSCO) nanocrystals over a temperature range of 0.4 K ≤ T ≤ 5 K. In both samples, two distinct behaviors of the conductivity are observed. For T>1 K, the variable range hopping regime is observed in LCMO sample with enhancement of the relative permittivity. In contrast, this behavior is not consistent with the extracted parameters in LSCO sample. For T<1 K, the transport is believed to occur through the metallic droplets connected to the intergranular regions where the negative magnetoresistance is due to spin-polarized tunneling phenomenon.We studied electrical transport in La{sub 0.67}Sr{sub 0.33}MnO{sub 3} (LCMO) and La{sub 0.5}Sr{sub 0.5}CoO{sub 3} (LSCO) nanocrystals over a temperature range of 0.4 K ≤ T ≤ 5 K. In both samples, two distinct behaviors of the conductivity are observed. For T>1 K, the variable range hopping regime is observed in LCMO sample with enhancement of the relative permittivity. In contrast, this behavior is not consistent with the extracted parameters in LSCO sample. For T<1 K, the transport is believed to occur through the metallic droplets connected to the intergranular regions where the negative magnetoresistance is due to spin-polarized tunneling phenomenon.