Observation of Edge Transport in the Disordered Regime of Topologically InsulatingInAs/GaSbQuantum Wells

Abstract

We observe edge transport in the topologically insulating InAs=GaSb system in the disordered regime. Using asymmetric current paths we show that conduction occurs exclusively along the device edge, exhibiting a large Hall signal at zero magnetic fields, while for symmetric current paths, the conductance between the two mesoscopicly separated probes is quantized to 2e 2 =h. Both quantized and self-averaged transport show resilience to magnetic fields, and are temperature independent for temperatures between 20 mK and 1 K. DOI: 10.1103/PhysRevLett.112.026602 PACS numbers: 72.25.Dc, 73.23.-b, 73.63.Hs Two-dimensional (2D) topological insulators (TI) are a novel class of materials that are insulating in the bulk but which display uniquely conductive edge channels [1][2][3][4]. These one-dimensional (1D) edge modes are helical, with the spin direction tied to the electron direction of motion, and are protected from backscattering by the time reversal symmetry (TRS) [5,6]. Applying magnetic fields breaks the TRS, removing the topological protection of the 1D helical liquid (HL) from single particle backscattering, resulting in a gap opening in the edge spectrum. Such HL channels were first observed in transport measurements in HgTe=CdTe quantum wells, and much of the HL phenomenology has been confirmed and elucidated in those first experiments [7,8]. Recently, Du et al. reported quantized transport in the inverted regime of Si-doped InAs=GaSb quantum wells in mesoscopic samples [9], where the existence of helical edge states was proposed in Ref.[10] and the first experimental evidence provided in Ref.[11] through scaling arguments due to the presence of residual bulk carriers. Unlike that observed in HgTe=CdTe [7], quantized transport in InAs=GaSb persists to magnetic fields of several Tesla [9,11], challenging the common understanding of 2D TIs in terms of TRS protected edge states and associated Z 2 topological invariant.Much remains to be learned about the nature and robustness of HL, in particular, to TRS breaking and disorder. In addition, edge transport in InAs=GaSb has so far only been indirectly assessed in ballistic samples [9,11]. In this Letter we study TI InAs=GaSb quantum wells in the disordered regime [12], where the total device size is much larger than the ballistic length of the HL, and show that transport in the topological regime manifestly occurs along the sample perimeter and is quantized to values consistent with the existence of a HL. Similar to ballistic regime studies [9,11], the conduction is also seen to be only weakly dependent on externally applied magnetic fields of up to 1 T. We argue that this behavior is due to the reduced effective g factor of the edge states originating from their small Fermi velocity v F . In addition, the edge states do not exhibit significant variation in transport properties for temperatures between 20 mK and 1 K measured. This is in contrast to theoretical studies, which have predicted power law corrections to the edge conductance as a function of temperatur...

Shot noise in the edge states of 2D topological insulators

“…The common observation for all the experiments was that the suppression of the conductance was weakly temperature dependent. A similar behavior of the conductance was observed in InAs/GaSb/AlSb heterostructures [6,7]. These facts had no satisfactory explanation so far despite a large number of theoretical papers proposing different mechanisms of electron backscattering in the edge states.…”

Noise and current correlations in tunnel junctions of quantum spin Hall edge states

“…For InAs/GaSb quantum wells, the conditions are even more favorable, since the smaller Fermi velocity v F 2 × 10 4 m/s leads to shorter values for ξ 0 . 14, 16 The value of the spin-flipping magnitude |Γ f | is typically smaller than |Γ p | by a factor 3 to 4 only. 28,29,35,37,39 Then, within the suitable voltage bias configurations discussed above, the two partition noises P p and P f can be tuned with varying the charge and spin gate voltages V p and V f in the meV regime.…”

“…5 In contrast, in the recently discovered Quantum Spin Hall (QSH) systems, [6][7][8][9] noise measurements are lacking, so far. These 2D topological systems, realized in HgTe/CdTe 10-13 or in InAs/GaSb quantum wells [14][15][16][17] , exhibit helical edge states, where the group velocity is locked to the spin orientation, so that the two counterpropagating modes along a given edge exhibit opposite spin orientations. As far as the theoretical analysis of noise in QSH systems is concerned, most studies have focussed on the effect of electron interaction on the currentcurrent correlations, within the helical Luttinger liquid model in the presence of a point-like constriction in the QSH bar.…”

Вертикальный транспорт в гетеропереходах II типа с композитными квантовыми ямами InAs/GaSb/AlSb в сильном магнитном поле

“…В последние десятилетия активно исследуются маг-нитотранспортные и спин-зависимые явления в разъ-единенных гетеропереходах II типа InAs/GaSb, вклю-чая квантовые ямы и сверхрешетки с такими перехо-дами [1][2][3][4][5][6][7][8][9][10][11]. Благодаря уникальной зонной структуре, в которой край зоны проводимости InAs на гетеро-границе лежит на = 150 мэВ ниже дна валентной зоны GaSb [1,2,4], появляется возможность управлять смещением и гибридизацией электронно-дырочных со-стояний с помощью электрического [1][2][3][4][5] и магнит-ного полей [3][4][5][6][7][8][9][10].…”

“…Благодаря уникальной зонной структуре, в которой край зоны проводимости InAs на гетеро-границе лежит на = 150 мэВ ниже дна валентной зоны GaSb [1,2,4], появляется возможность управлять смещением и гибридизацией электронно-дырочных со-стояний с помощью электрического [1][2][3][4][5] и магнит-ного полей [3][4][5][6][7][8][9][10]. В композитных квантовых ямах (ККЯ) InAs/GaSb/AlSb с инвертированной зонной струк-турой на гетерогранице II типа, выращенных методом молекулярно-пучковой эпитаксии (МПЭ) на подложках GaAs, исследованы низкотемпературные топологические свойства и спиновый эффект Холла [6,9] при располо-жении уровня Ферми в пределах щели гибридизирован-ных электронно-дырочных состояний [7], изучена роль краевых состояний [9] и показана возможность создания гибридных структур со сверхпроводящими слоями [11].…”