Unconventional Correlation between Quantum Hall Transport Quantization and Bulk State Filling in Gated Graphene Devices

Cui, Yong‐Tao; Wen, Bo; Eric, Y.; Diankov, Georgi; Han, Zheng; Amet, François; Taniguchi, Takashi; Watanabe, Kenji; Goldhaber‐Gordon, David; Dean, Cory; Shen, Zhi‐Xun

doi:10.1103/physrevlett.117.186601

Cited by 45 publications

(55 citation statements)

References 42 publications

(56 reference statements)

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“…The edge reconstruction elucidates previously reported discrepancies in the QH state of graphene [8] and other 2DEG systems [21,22]. Although several mechanism were proposed [23,24], edge accumulation was mainly ascribed to electrostatic gating [7,8] which should lead to symmetric accumulation for p and n doping. We find that at charge neutrality and for both dopings, the accumulation remains hole-type (SI10 and Fig.…”

supporting

confidence: 79%

Imaging work and dissipation in the quantum Hall state in graphene

Marguerite

Birkbeck

Aharon-Steinberg

et al. 2019

Nature

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Topology is a powerful recent concept asserting that quantum states could be globally protected against local perturbations [1,2]. Dissipationless topologically protected states are thus of major fundamental interest as well as of practical importance in metrology and quantum information technology. Although topological protection can be robust theoretically, in realistic devices it is often fragile against various dissipative mechanisms, which are difficult to probe directly because of their microscopic origins. By utilizing scanning nanothermometry [3], we visualize and investigate microscopic mechanisms undermining the apparent topological protection in the quantum Hall state in graphene. Our simultaneous nanoscale thermal and scanning gate microscopy shows that the dissipation is governed by crosstalk between counterpropagating pairs of downstream and upstream channels that appear at graphene boundaries because of edge reconstruction. Instead of local Joule heating, however, the dissipation mechanism comprises two distinct and spatially separated processes. The work generating process that we image directly and which involves elastic tunneling of charge carriers between the quantum channels, determines the transport properties but does not generate local heat. The independently visualized heat and entropy generation process, in contrast, occurs nonlocally upon inelastic resonant scattering off single atomic defects at graphene edges, while not affecting the transport. Our findings offer a crucial insight into the mechanisms concealing the true topological protection and suggest venues for engineering more robust quantum states for device applications. (E.Z.) 2 In recent years, major progress has been made in identifying new topological states of matter [1,2] but the extent to which the topological protection is manifested in realistic systems and the microscopic mechanisms leading to its apparent breakdown remain poorly understood. The quantum Hall (QH) effect is a prime example of a topologically protected state exhibiting quantized dissipationless electron transport. While an extremely high degree of conductance quantization has been achieved in engineered systems in GaAs and in graphene [4], QH devices commonly exhibit small but fundamentally important deviations from the ideal quantized conductance. Various mechanisms undermining the topological protection were explored, including imperfect contacts [5], current-induced breakdown [4], absence of edge equilibration [6], and edge reconstruction [7,8]. Nonetheless, how exactly the dissipation in the QH regime occurs on a microscopic level has eluded direct identification. Here we provide nanoscale imaging of the dissipation processes in the QH state in graphene and reveal the intricate mechanisms compromising the apparent global topological protection.A superconducting quantum interference device, SQUID-on-tip (SOT) [9] acting as nanothermometer (tSOT) with µK sensitivity [3] and effective diameter of ~50 nm was scanned ~50 nm above high-mobility hBN-encapsu...

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supporting

confidence: 79%

Imaging work and dissipation in the quantum Hall state in graphene

Marguerite

Birkbeck

Aharon-Steinberg

et al. 2019

Nature

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“…1), where additional sets of edge channels running in the forward and counterflow directions form [27]. As recently revealed in graphene [28], a similar situation can also occur in a gated device due to electric-field focusing near the edge [29].…”

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confidence: 64%

“…Notably, density varies in a stepwise manner due to the formation of compressible and incompressible strips [19]. As the charge equilibration between adjacent edge channels occurs via scattering across the incompressible strip between them [28,35], its width is the important parameter determining the scattering rate. The width is determined by the density gradient at B = 0 and the Landau-level energy separation at the strip [19], the latter being the cyclotron and Zeeman energy for even and odd local filling (ν local ), respectively.…”

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confidence: 99%

Counterflowing edge current and its equilibration in quantum Hall devices with sharp edge potential: Roles of incompressible strips and contact configuration

et al. 2019

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We report the observation of counterflowing edge current in InAs quantum wells which leads to the breakdown of quantum Hall (QH) effects at high magnetic fields. Counterflowing edge channels arise from the Fermi-level pinning of InAs and the resultant sharp edge potential with downward bending. By measuring the counterflow conductance for varying edge lengths, we determine the effective number N C of counterflowing modes and their equilibration length λ eq at bulk integer filling factor ν = 1-4. λ eq increased exponentially with magnetic field B, reaching 200 µm for ν = 4 at B ≥ 7.6 T. Our data reveal important roles of the innermost incompressible strip with even filling in determining N C and λ eq and the impact of the contact configuration on the QH effect breakdown. Our results show that counterflowing edge channels manifest as transport anomalies only at high fields and in short edges. This in turn suggests that, even in the integer QH regime, the actual microscopic structure of edge states can differ from that anticipated from macroscopic transport measurements, which is relevant to various systems including atomic-layer materials.

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“…sMIM was proposed by Lai et al and developing quickly these years [10,11,23,24]. It can detect many kinds of electrical properties of various samples (including conductors, semiconductors, insulators and other functional materials) at the micro/nano scale [14,[25][26][27][28][29][30][31][32][33][34][35]. Figure 1 illustrates the microwave electronics.…”

Section: Basic Principles and Experimental Setupmentioning

confidence: 99%

Local characterization of mobile charge carriers by two electrical AFM modes: multi-harmonic EFM versus sMIM

Lei

et al. 2018

J. Phys. Commun.

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The characterization of mobile charge carriers of semiconductor materials has spurred the development of numerous two dimensional carrier profiling tools. Here, we investigate the mobile charge carriers of several samples by multi-harmonic electrostatic force microscopy (MH-EFM) and scanning microwave impedance microscopy (sMIM). We present the basic principles and experiment setups of these two methods. And then several typical samples, i.e. a standard n-type doped Si sample, mechanical exfoliation and chemical vapor deposition grown molybdenum disulfide (MoS 2 ) layers are systemically investigated by sMIM and MH-EFM. The difference and (dis)advantages of these two modes are discussed. Both modes can provide carrier concentration profiles and have sub-surface sensitivity. They also have advantages in sample preparation in which contact electrodes are not required and insulating or electrically isolated samples can readily be studied. The basic mode, physics quantities extracted, dielectric response form and parasitic charges in scanning environment result in difference in experiment results for these two kinds of methods. The techniques described in this study will effectively promote research on basic science and semiconductor applications.

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Unconventional Correlation between Quantum Hall Transport Quantization and Bulk State Filling in Gated Graphene Devices

Cited by 45 publications

References 42 publications

Imaging work and dissipation in the quantum Hall state in graphene

Imaging work and dissipation in the quantum Hall state in graphene

Counterflowing edge current and its equilibration in quantum Hall devices with sharp edge potential: Roles of incompressible strips and contact configuration

Local characterization of mobile charge carriers by two electrical AFM modes: multi-harmonic EFM versus sMIM

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