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HgTe based topological insulators

Seminar über Theoretische Festkörperphysik


Z. D. Kvon


23.07.2018 14:00


Room 10.01, 10th Floor, Bldg. 30.23, KIT Campus South


Novosibirsk State University; A.V.Rzhanov Institute of Semiconductor Physics, Novosibirsk, Russia


Dr. Igor Gornyi


HgTe is the semiconductor of the highest quality with an inverted band structure. Just for this reason only on the basis of HgTe the both kinds of topological insulators (2D and 3D) have been realized. I am going to tell about the most interesting (mainly transport) properties of these systems. The first part of my talk is devoted to 2D topological insulators. In the beginning I will describe field effect transistor made of HgTe quantum wells (QW) or HgTe films. This FET is the key device to study both 2DTIs and 3DTIs because it gives the possibility to change the Fermi level position placing it into al-lowed bands and inside of the gap. Then study of transport response of 2DTI in (8-9) nm QW will be presented and analyzed. It is shown that the edge current states in this TI are robust, and the ballistic and diffusive regimes are observed dependently on the size and quality of experimental samples. Ap-plying of a normal magnetic field up to 5 T does not destroy the edge transport but in the magnetic fields above 6 T transition to insulator state occurs. Applying of in-plane magnetic field destroys the edge transport in magnetic fields larger than 10 T because of a magnetic breakdown of the bulk gap. Study of 2DTI edge resistance in a diffusive regime at different temperatures up to 50 mK shows no significant temperature dependence of the resistance value. It means the absence of any signatures of the 1D localization. So one can conclude that in real samples of HgTe 2DTI edge current states are not ideally 1D because of the strong interaction with the bulk states. At the end of the first part realized in the wide (14 nm) QW 2DTI is described and the suggestion that this 2DTI is the Anderson-like one is done.
3DTI on the basis of the strained HgTe film with the thicknesses of (80 – 200) nm is the subject of the second part. The origin of the strain is the lattice mismatch between HgTe and CdTe, on which surface HgTe film is grown. This strain creates the gap in HgTe and gapless HgTe transformed into 3DTI. The Dirac point of this TI places into a bulk valence band. For this reason inside of the bulk gap only surface Dirac electrons with a significant Fermi energy are detected. These electrons have very high mobility (500 thousands), that almost two orders higher than the mobility of the similar electrons in the most popular 3DTIs on the basis of bismuth compounds. Just due to this high mobility the whole information about TI main parameters (bulk gap, Dirac fermions densities on the both surfaces and their mobility) was obtained. Measurements of SdH oscillations in transport and capacitance response clearly demonstrate their anomalous phase shift that indicates that the Dirac electrons from the top sur-face of TI are the spin-polarized system with spin-momentum locking. High electron mobility of sur-face Dirac electrons has allowed to perform antidot lattice experiment that has shown how completely classical effect (caused by commensurability of a cyclotron radius and an antidot lattice period the magnetoresistance peak) reflects one of the most fundamental law of the quantum mechanics – the Pauli exclusion principle.