Article

On the direct insulator-quantum Hall transition in two-dimensional electron systems in the vicinity of nanoscaled scatterers

Department of Physics, National Taiwan University, Taipei 106, Taiwan. .
Nanoscale Research Letters (Impact Factor: 2.48). 02/2011; 6(1):131. DOI: 10.1186/1556-276X-6-131
Source: PubMed

ABSTRACT A direct insulator-quantum Hall (I-QH) transition corresponds to a crossover/transition from the insulating regime to a high Landau level filling factor ν > 2 QH state. Such a transition has been attracting a great deal of both experimental and theoretical interests. In this study, we present three different two-dimensional electron systems (2DESs) which are in the vicinity of nanoscaled scatterers. All these three devices exhibit a direct I-QH transition, and the transport properties under different nanaoscaled scatterers are discussed.

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    • "Insulator-quantum Hall (I-QH) transition [12-15] is an interesting physical phenomenon in the field of 2D physics. Especially, a direct transition from an insulator to a high Landau level filling factor ν ≥ 3 QH state which is normally described as the direct I-QH transition continues to attract interest [16-18]. Very recently, experimental evidence for direct I-QH transition in epitaxial monolayer graphene [19] and in mechanically exfoliated multilayer graphene [20] has been reported. "
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    ABSTRACT: We have performed magnetotransport measurements on multilayer epitaxial graphene. By increasing the driving current I through our graphene devices while keeping the bath temperature fixed, we are able to study Dirac fermion heating and current scaling in such devices. Using zero-field resistivity as a self thermometer, we are able to determine the effective Dirac fermion temperature (TDF) at various driving currents. At zero field, it is found that TDF [proportional to] I[almost equal to]1/2. Such results are consistent with electron heating in conventional two-dimensional systems in the plateau-plateau transition regime. With increasing magnetic field B, we observe an I-independent point in the measured longitudinal resistivity rhoxx which is equivalent to the direct insulator-quantum Hall (I-QH) transition characterized by a temperature-independent point in rhoxx. Together with recent experimental evidence for direct I-QH transition, our new data suggest that such a transition is a universal effect in graphene, albeit further studies are required to obtain a thorough understanding of such an effect.
    Nanoscale Research Letters 08/2013; 8(1):360. DOI:10.1186/1556-276X-8-360 · 2.52 Impact Factor
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    • "In particular, a direct transition from an insulator to a high Landau-level filling factor ν > 2 QH state which is normally dubbed as the direct I-QH transition continues to attract interest [14]. The direct I-QH transition has been observed in various systems such as SiGe hole gas [14], GaAs multiple quantum well devices [15], GaAs two-dimensional electron gases (2DEGs) containing InAs quantum dots [16-18], a delta-doped GaAs quantum well with additional modulation doping [19,20], GaN-based 2DEGs grown on sapphire [21] and on Si [22], InAs-based 2DEGs [23], and even some conventional GaAs-based 2DEGs [24], suggesting that it is a universal effect. Although some quantum phase transitions, such as plateau-plateau transitions [25] and metal-to-insulator transitions [26-29], have been observed in single-layer graphene and insulating behavior has been observed in disordered graphene such as hydrogenated graphene [30-33], graphene exposed to ozone [34], reduced graphene oxide [35], and fluorinated graphene [36,37], the direct I-QH transition has not been observed in a graphene-based system. "
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    ABSTRACT: We have performed magnetotransport measurements on a multi-layer graphene flake. At the crossing magnetic field Bc, an approximately temperature-independent point in the measured longitudinal resistivity rhoxx, which is ascribed to the direct insulator-quantum Hall (I-QH) transition, is observed. By analyzing the amplitudes of the magnetoresistivity oscillations, we are able to measure the quantum mobility muq of our device. It is found that at the direct I-QH transition, muqBc [almost equal to] 0.37 which is considerably smaller than 1. In contrast, at Bc, rhoxx is close to the Hall resistivity rhoxy, i.e., the classical mobility muBc is [almost equal to] 1. Therefore, our results suggest that different mobilities need to be introduced for the direct I-QH transition observed in multi-layered graphene. Combined with existing experimental results obtained in various material systems, our data obtained on graphene suggest that the direct I-QH transition is a universal effect in 2D.
    Nanoscale Research Letters 05/2013; 8(1):214. DOI:10.1186/1556-276X-8-214 · 2.52 Impact Factor
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    • "For sample B, at Vg = 0 the carrier concentration of the 2DEG was 9.1 × 1010 cm-2 with a mobility of 2.0 × 106 cm2/Vs in the dark. The self-assembled InAs dots act as scattering centers in the GaAs 2DEG [12,14]; thus, the 2DEG has a mobility much lower than those for samples A and B. For sample C, at Vg = 0 the carrier concentration of the 2DEG was 1.48 × 1011 cm-2 with a mobility of 1.86 × 104 cm2/Vs in the dark. Experiments were performed in a He3 cryostat and the four-terminal magnetoresistance was measured with standard phase-sensitive lock-in techniques. "
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    ABSTRACT: 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.
    Nanoscale Research Letters 03/2013; 8(1):138. DOI:10.1186/1556-276X-8-138 · 2.52 Impact Factor
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