Quantum Hall Effect near the Charge Neutrality Point in a Two-Dimensional Electron-Hole System

Abstract

We study the transport properties of HgT e-based quantum wells containing simultaneously electrons and holes in magnetic field B. At the charge neutrality point (CNP) with nearly equal electron and hole densities, the resistance is found to increase very strongly with B while the Hall resistivity turns to zero. This behavior results in a wide plateau in the Hall conductivity σxy ≈ 0 and in a minimum of diagonal conductivity σxx at ν = νp − νn = 0, where νn and νp are the electron and hole Landau filling factors. We suggest that the transport at the CNP point is determined by electron-hole "snake states" propagating along the ν = 0 lines. Our observations are qualitatively similar to the quantum Hall effect in graphene as well as to the transport in random magnetic field with zero mean value. The quantum Hall effect (QHE) of a two-dimensional (2D) electron gas in a strong magnetic field is one of the most fascinating quantum phenomena discovered in the condensed matter physics. Its basic experimental manifestation is a vanishing longitudinal conductivity σ xx ≈ 0 and a quantization of the Hall conductivity σ xy = ν e 2 h , where ν is the Landau filling factor [1]. The discovery of a 2D electron-hole system in graphene at a finite magnetic field has put a beginning to a series of studies of the properties of the special state realized when the densities of the electrons and holes are equal, the charge neutrality point (CNP) [4][5][6]. In a strong magnetic field the QHE near the CNP reveals a plateau in σ xy with ν = 0 currently associated with the resolution of the spin or the valley splitting of the lowest Landau level (LL) [5,6]. However, the longitudinal resistivity ρ xx at ν = 0 demonstrates different behavior in various but otherwise quite similar samples: in some samples ρ xx shows a rapid divergence at a critical field and, with the temperature decreasing, saturates at a value much larger than the quantum of resistance [6], while in the others it decreases with lowering the temperature [5]. The QHE behavior near the CNP has attracted much theoretical interest and several microscopic mechanisms that might be responsible for these phenomena have been proposed [5,7], however no final quantitative conclusion has been drawn up yet. Graphene remained a unique 2D system with described properties when recently a new 2D system showing similar properties has been discovered. It has been shown [8] that a two-dimensional semimetal exists in undoped HgTe-based quantum wells with an inverse band structure and a (013) surface orientation. In this paper we report the results of our study of the transport properties of the HgTe-based quantum wells near the CNP. At filling factor ν = 0 (CNP) our system goes into a high resistivity state with a moderate temperature dependence markedly different from a thermally activated behavior expected when there is an opening of the Landau gap in the density of states. We suggest that at the CNP the 2D electron-hole gas in our HgTe quantum wells is not homogeneous due to the random potent...

Unconventional Hall effect near charge neutrality point in a two-dimensional electron-hole system

“…Very likely, the same mechanism describes the σ xy = 0 quantum Hall state observed previously in 2D semimetals with higher carrier densities. 12 …”

“…The densities of carriers in these wells are considerably smaller those those for the [013]-, [112]-, and slightly wider [001]-grown wells examined previously in our experiments. 7,10,12,13 For this reason, we see quantum features in transport at smaller magnetic fields. Apart from the existence of the σ xy = 0 plateau in the dependence of Hall conductivity on the gate voltage, we have found an unusual nonmonotonic dependence of the Hall resistivity ρ yx on the magnetic field.…”

“…11 Recently, it has been demonstrated that the Hall conductivity of CdHgTe/HgTe/CdHgTe quantum wells of 20 nm width and [013] interface direction exhibits a quantized plateau σ xy = 0 in the magnetic fields of a few tesla when the gate voltage is varied in the vicinity of the charge neutrality point (CNP). 12 The universality of the edge-state transport picture suggests the existence of a pair of counterpropagating edge states in such 2D systems, which is equivalent to QHE near the Dirac point in graphene within a spin-first scenario. Despite this similarity, one should point out several differences.…”

“…This behavior, observed in the fields above 3.5 T, agrees with our previous study of the quantum Hall effect near CNP in wide HgTe quantum wells in samples with [013] surface orientation. 12 Now we turn our attention to magnetoresistance measured as a function of the gate voltage with increasing magnetic field, shown in Fig. 3.…”

Insulating State and Giant Nonlocal Response in anInAs/GaSbQuantum Well in the Quantum Hall Regime

“…Beyond the topological insulator properties, that manifest themselves, the fate of topological edge states at finite magnetic field has not been investigated so far. Similarly to other semi-metals like graphene [9,10] or CdHgTe/HgTe quantum wells [11,12], electron and hole Landau levels (LLs) can coexist close to the CNP [13,14]. A detailed understanding of the expected hybridization of LLs [15] and its manifestation in a transport experiment is still missing.…”