We investigate the magnetotransport properties of strained 80 nm thick HgTe layers featuring a high mobility of μ ∼ 4 × 10 5 cm 2 =V · s. By means of a top gate, the Fermi energy is tuned from the valence band through the Dirac-type surface states into the conduction band. Magnetotransport measurements allow us to disentangle the different contributions of conduction band electrons, holes, and Dirac electrons to the conductivity. The results are in line with previous claims that strained HgTe is a topological insulator with a bulk gap of ≈ 15 meV and gapless surface states. DOI: 10.1103/PhysRevLett.112.196801 PACS numbers: 73.25.+i, 05.60.Gg, 73.20.At, 73.43.-f The discovery of two-(2D) and three-dimensional (3D) topological insulators (TIs), a new material class with insulating bulk and topologically protected conducting surface states, has opened an exciting research field in condensed matter physics [1][2][3][4][5][6][7][8][9][10]. Although quite a number of different, especially, Bi-based materials [11][12][13][14], belong to this category, materials which combine high charge carrier mobility and insulating bulk are still scarce. This is mostly due to the fact that Bi-based 3D TIs are heavily doped alloy films with a low mobility ≈1000 cm 2 =V · s and a high bulk carrier density of 10 17 -10 19 cm −3 . HgTe-based 2D TIs, on the other hand, are characterized by very high mobilities enabling the discovery of the quantum spin Hall effect [15]. A recent analysis of the sequence of quantum Hall plateaus suggests that also strained HgTe layers constitute a 3D TI. The strain opens a gap in the gapless semimetal HgTe so that the TI properties can be explored by tuning the Fermi energy E F into the bulk gap and probing the transport properties of the gapless surface states. Although the strained HgTe film has a much higher mobility μ ¼ ð3 − 4Þ × 10 4 cm 2 =V · s, the high bulk carrier density and the absence of a top gate have complicated the detection of 3D TIs so far [16,17].The strain in HgTe layers grown by molecular beam epitaxy stems from a 0.3% lattice mismatch between HgTe and CdTe. The corresponding critical film thickness is larger than 100 nm, meaning that thinner films adopt the substrate lattice constant. Because of this strain, a small gap of ∼15 meV opens (see below) in the bulk energy spectrum of the film. Within the bulk gap, the gapless surface states reside. The charge neutrality point of the corresponding Dirac cone is located in the valence band [16].In this Letter, we report on transport properties of highmobility, 80 nm wide, strained HgTe films equipped with a gate. The low disorder manifested in high charge carrier mobilities, together with the possibility to tune E F from the valence via the gap into the conduction band, enables us to probe the 2D Dirac surface states when E F is in the bulk energy gap. Since HgTe films grown on CdTe suffer from dislocations due to the lattice mismatch, our 80 nm thick HgTe films were separated from the CdTe substrate by a 20 nm thin Cd 0.7 Hg 0.3 Te buf...

Our experimental studies of electron transport in wide (14 nm) HgTe quantum wells confirm the persistence of a two-dimensional topological insulator state reported previously for narrower wells, where it was justified theoretically. Comparison of local and nonlocal resistance measurements indicate edge state transport in the samples of about 1 mm size at temperatures below 1 K. Temperature dependence of the resistances suggests an insulating gap of the order of a few meV. In samples with sizes smaller than 10  μm a quasiballistic transport via the edge states is observed.

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