Probing two-dimensional metallic-like and localization effects at low magnetic fields (original) (raw)
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Journal of Physics: Condensed Matter, 2008
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. * ctliang@phys.ntu.edu.tw and ochiai@faculty.chiba-u.jp PACS numbers: 72.15.Rn, 71.70.Di, The insulator to quantum Hall (I-QH) transition in a two-dimensional electron system (2DES) at low perpendicular magnetic fields B has attracted much attention [1, 2,. Theoretically, the direct I-QH transition from the insulator to an integer QH state of ν = 1 is forbidden in an infinite, non-interacting 2DES with arbitrary amount of disorder, where ν is the Landau level filling factor [1, 2, 3]. In such a system, the only allowed state at B = 0 is the insulating one, and the 2DES undergoes the I-QH transition to enter the ν = 1 QH state . Realistically, however, only systems of finite sizes are available, and the effects of the electron-electron (e-e) interaction are significant in some 2DESs . As a result, the 2DESs may experience the direct I-QH transition from the low-field insulator to QH states of higher filling factors [2,. Such a transition can be related to the zero-field metal-insulator transition, to which e-e interaction cannot be ignored . Given that most 2DESs show metallic behavior at B=0, the investigation of the direct I-QH transition at low B should be conducted in low-mobility 2DESs [1,.
Physica E: Low-dimensional Systems and Nanostructures, 2002
We investigate the transport properties of two-dimensional electron gases (2DEG) formed in a GaAs=AlGaAs quantum well, where self-assembled InAs quantum dots were grown at the center of the GaAs well. Due to the resulting strain ÿelds repulsive short-range scattering is experienced by the conduction electrons in the 2DEG. In a perpendicular magnetic ÿeld, there are transitions between quantum Hall liquids at ÿlling factors = 1 and 2 and the insulating phase. We show that the boundary of insulator-Quantum Hall transitions can be identiÿed either by analysing the temperature-independent points in xx or from the peaks in xx at low temperatures and both methods give similar results. ?
Physical Review B, 2004
We have studied insulator-quantum Hall-insulator ͑I-QH-I͒ 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.
On the low-field insulator-quantum Hall conductor transitions
Physica E: Low-dimensional Systems and Nanostructures, 2004
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.
Hopping conductivity in heavily doped n-type GaAs layers in the quantum Hall effect regime
We investigate the magnetoresistance of epitaxially grown, heavily doped n-type GaAs layers with thickness (40-50 nm) larger than the electronic mean free path (23 nm). The temperature dependence of the dissipative resistance Rxx in the quantum Hall effect regime can be well described by a hopping law (Rxx ∝ exp {−(T0/T ) p }) with p ≈ 0.6. We discuss this result in terms of variable range hopping in a Coulomb gap together with a dependence of the electron localization length on the energy in the gap. The value of the exponent p 0.5 shows that electron-electron interactions have to be taken into account in order to explain the occurrence of the quantum Hall effect in these samples, which have a three-dimensional single electron density of states.
Physical Review Letters, 1999
A metal-insulator transition in two-dimensional electron gases at B = 0 is found in Ga[Al]As heterostructures, where a high density of self-assembled InAs quantum dots is incorporated just 3 nm below the heterointerface. The transition occurs at resistances around h/e 2 and critical carrier densities of 1.2 • 10 11 cm −2. Effects of electron-electron interactions are expected to be rather weak in our samples, while disorder plays a crucial role.
Transition to the Quantum Hall regime in InAs nanowire cross-junctions
Semiconductor Science and Technology, 2019
We present a low-temperature electrical transport study on four-terminal ballistic InAs nanowire cross-junctions in magnetic fields aligned perpendicular to the cross-plane. Two-terminal longitudinal conductance measurements between opposing contact terminals reveal typical 1D conductance quantization at zero magnetic field. As the magnetic field is applied, the 1D bands evolve into hybrid magneto-electric sub-levels that eventually transform into Landau levels for the widest nanowire devices investigated (width = 100 nm). Hall measurements in a four-terminal configuration on these devices show plateaus in the transverse Hall resistance at high magnetic fields that scale with (ve 2 /h)-1. e is the elementary charge, h denotes Planck's constant and v is an integer that coincides with the Landau level index determined from the longitudinal conductance measurements. While the 1D conductance quantization in zero magnetic field is fragile against disorder at the NW surface, the plateaus in the Hall resistance at high fields remain robust as expected for a topologically protected Quantum Hall phase.
Journal of Physics: Condensed Matter, 1994
We present low-temperature uanspon measurements on the two-dimensional electron gas in 8-doped GaAs which undergoes an insulatorquantum Hall-insulator vansition as the magnetic field is increased. Both low-and high-field vansitions are marked by pe& in ox, and Lhe temperahlre-independent critical value of oxy of 0.5ez/h per spin. We map out the phase diagram versus disorder and magnetic field and study the t e m p e m dependence of ox* throughout. In the quantum Hall region we observe Molt variable range hopping and, around the high-field transitions, s d i g via a single parameter: z = (S -B')T-"'. The functional dependence on L above this transition is fitted by recent network percolation calculations.