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Sudy of strained HgTe (Mercury telluride) topological insulator – SemiTopo

Surface states studies of the topological insulator HgTe (Mercury Telluride)

Because of its topological nature, this material acquires at its surface a conducting layer with relativistic and chiral properties. The material is grown epitaxially from a CdTe substrate, which opens a gap in this semimetal. The material obtained has a very small surface roughness – at the atomic level – and its surface states could be studied by photoemission (ARPES) to reveal their dispersion and position with respect to the volume bands.

Epitaxial growth, ARPES spectra, dispersion relations, magneto-transport, quantum Hall effect

The epitaxial techniques used for infrared detectors did require further developments in order to obtain high quality strained films of HgTe on CdTe substrates of the required thicknesses (between 15 and 100 nm ). These layers have an atomic level surface roughness which is required for ARPES studies in order to measure the surface states spectra with a sufficiently high resolution.

The interest in other families of topological material led the theory group in an analysis of a dynamical control of topological phases. The idea is to induce nontrivial topological properties by driving the system with an AC electromagnetic field. Under the proper conditions, the out-of-equilibrium topological surface states manifest themselves in the differential conductance in a way similar to the surface states of equilibrium topological phase.

The epitaxial techniques used for infrared detectors did require further developments in order to obtain high quality strained films of HgTe on CdTe substrates of the required thicknesses (between 15 and 100 nm ). These layers have an atomic level surface roughness which is required for ARPES studies in order to measure the surface states spectra with a sufficiently high resolution. On 35 and 100 nm thick layers the spectra have an intense Dirac cone, with an apex very close to the top of the bulk G8,HH band. For thinner layers (between 23 and 9.5 nm thick), relativistic subbands quantified by the thickness of the sample are also observed. They coincide with a high transmission between the lower and upper sample surfaces via resonant states at the edges. These states can in fact be considered as covering all the surfaces of the sample. For transport studies, an electrostatic gate was added in order to shift the chemical potential of the electron gas to (towards) the «electronic« and «hole« sides with respect to the Dirac point. Hall effect studies on 15 nm thin films show distinct Hall quantization on each side of the Dirac point. On the electronic side, the observed plateaus correspond to a quantification sxy=(N_top+N_bottom+1) e^2/hcorresponding to the addition of independent contributions from the top and bottom faces perpendicular to the applied magnetic field. On the hole side, the quantification observed is sxy=(2N+1) e^2/h,, with identical quantum numbers on both sides. This difference is consistent with a strong coupling between opposite faces for hole states.

The results associated to this project have all been written and submitted. It is a fundamental research project involving 3 aspects: the development of HgTe growth in thin films, the study of its physical properties and the theoretical analysis of the topological phases. It was carried out by the CEA-LETI, the Institut Néel and the Laboratoire de Physique of the Ecole Normale de Lyon. The CASSIOPEE group of the Synchrotron Soleil He was also associated to the project, for the ARPES measurements.

- Revealing Dirac fermions in strained three-dimensional HgTe topological insulators via Quantum Hall spectroscopy.C. Thomas1, O. Crauste2,3, C. Bäuerle2,3, L.P. Lévy2,3, E. Orignac4, D. Carpentier4, P. Ballet1, and T. Meunier2,3, (submitted) - Probing (topological) Floquet states through DC transport Michel Fruchart, Pierre Delplace, Joseph Weston, Xavier Waintal, David Carpentier Physica E 75 (2016) 287-294 - Minimal conductivity, topological Berry winding and duality in three-band semimetals, Thibaud Louvet, Pierre Delplace, Andrei A. Fedorenko, David Carpentier, Phys. Rev. B 92, 155116 (2015) - Topological surface states of strained Mercury-Telluride probed by ARPES O. Crauste, Y. Ohtsubo, P. Ballet, P. Delplace, D. Carpentier, C. Bouvier, T. Meunier, A. Taleb-Ibrahimi, L. Lévy, [arXiv:1307.2008] - C. Thomas, X. Baudry, JP. Barnes, M. Veillerot, Ph. Jouneau, S. Pouget, O. Crauste, T. Meunier, LP. Levy and P. Ballet, « MBE growth and interface characterization of strained HgTe/CdTe topological insulators », Journal of Crystal Growth 425, 195 (2015). DOI: 10.1016/j.jcrysgro.2015.02.046. - P. Ballet, C. Thomas, X. Baudry, C. Bouvier, O. Crauste, T. Meunier, G. Badano, M. Veillerot, JP. Barnes, PH. Jouneau and LP. Levy, “MBE growth of strained HgTe/CdTe topological insulator structures”, Journal of Electronic Materials 43, 2955 (2014). DOI: 10.1007/s11664-014-3160-z. - “Un nouvel état de la matière”, Laurent P. Lévy et David Carpentier, Pour la science (2013)

When strained is applied to Mercury Telluride, a semiconducting gap is opened. When the chemical potential is placed within this gap by an external gate, the electrical conduction takes place at the material surface by charge Dirac carriers (with zero effective mass): this is an excellent (the best) topological insulator with very little residual bulk conduction. In this project, this material will be developed in order to control the energy and properties of the surface states (the Dirac carriers) in order to reach the best electronic transport properties (mobility, interface rugosity etc.). The band structure and the energies and the dispersion of the surface states will be studied by ARPES photoemission at the SOLEIL synchrotron and compared to the theoretical models appropriate to this system. The magneto-transport properties will be studied, with a particular interest on the quantum Hall phases which appear at high magnetic field. Specific experiment will be carried out to understand and characterize the bilayer quantum Hall states, we have recently discovered. At very high magnetic field, we will study an insulating phase which looks like a Hall insulator state (for which the longitudinal resistivity is infinite while the transverse Hall resistivity remains finite). Hg1-xCdxTe is a non centro-symmetric material which gap changes sign and in magnitude by varying x. Between the insulating (CdTe) and topological (HgTe) phases, a semi-metallic phase has been predicted to exist on symmetry arguments which nature is presently unknown. Its theoretical analysis should specify under which circumstances it appears and what experiment will reveal its presence.
At the device level, we will design and fabricate hybrid structures where the Dirac carriers are in proximity of a superconductor. These structures will be experimentally studied, and the spectroscopy of the Andreev state measured, with the detection of Majorana fermions which have been predicted in such device as an ultimate objective. Finally we will fabricate simple structures capable of selecting and manipulating the spins of the Dirac carriers in the 2D spin-Hall limit (quantum wells) as well as in the topological insulator (3D) limit.

Project coordination

Laurent LÉVY (Institut Néel) – laurent.levy@grenoble.cnrs.fr

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

ENS-Lyon CNRS Laboratoire de Physique
CNRS Institut Néel
CEA/LETI Commissariat à l'énergie atomique et aux énergies alternatives/Laboratoire d'électronique de technologie de l'information-Minatec

Help of the ANR 553,892 euros
Beginning and duration of the scientific project: October 2012 - 48 Months

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