Research - SOLARIS National Synchrotron Radiation Centre - Jagiellonian University

Strain-control over the valley degree of freedom

Department of the Ecole Normale Supérieure Paris, France in collaboration with scientists from URANOS SOLARIS Centre have published in Physical Review Letters the results of their research on the Valley-Polarized Quantum Hall Phase Dirac System. The different local band minima – called valleys – of a multivalley 2D topological insulator have been independently characterized using ARPES experiments. An efficient control over their electronic population using mechanical strain has been demonstrated.

In a semiconductor multivalley system, local electronic band minima occur at different momenta of the Brillouin zone. These valleys embody a new degree of freedom for the electrons and can be seen as pseudospins. Controlling the valley populations independently from each other is of prime interest to develop valleytronics – the technology of controlling the valley degree of freedom – and investigate interesting pseudospin phenomena such as skyrmions, charge density waves or quantum Hall ferromagnetism.

Using the URANOS beamline, we demonstrate a full control over the valley energy splitting as a function of in-plane biaxial strain in the IV-VI system Pb_1-xSn_xSe. The strain is determined by the lattice mismatch between the buffer and film layer, which depends on the Sn content. The band structure of several Pb_1-xSn_xSe 20 nm quantum wells have been measured near the two types of valleys in these materials, at the Γ ̅ and M ̅ points. As shown in Fig. 1, a huge amount of subband dispersions is observed, witnessing the high-resolution experiments at the URANOS beamline and the high-quality of the samples.

ARPES is one of the rare experimental techniques to independently assess band structure properties near selected momenta of the Brillouin zone. In this way, the relative energy position of the band minima near the Γ ̅-point and the M ̅-point can be measured with respect to the Fermi energy. As strain is increased, the bands at the M ̅-point are seen to shift upward in energy compared to the bands near the Γ ̅-point (see Fig. 1). The valley energy splitting, represented by Δ _l-o, is found to be very sensitive to strain and varies as 160 meV per 1 % strain, twice than the AlAs system for instance.

This work has shown the very efficient valley control using strain in the IV-VI materials, and has allowed for studying valley-polarized quantum Hall phases in tra nsport experiments, where chiral edge states of only one valley or the other are observed. This ARPES study demonstrates a key properties for the development of valleytronics devices based on Pb_1-xSn_xSe system.

Figure 1. ARPES measurements of the confined subbands near the center (Γ ̅-point; in black) and edge (M ̅-point; in red) of the 2D Brillouin zone of PbSe (a), PbSnSe (b) and PbSnSe (c) 20 nm thick quantum wells (QWs). The strain magnitude accordingly evolves from 0 to 0.3 and 0.5 %. The valley splitting Δ _l-o is indicated.

Author: Gauthier Krizman

Link to the publication: G. Krizman, J. Bermejo-Ortiz, T. Zakusylo, M. Hajlaoui, T. Takashiro, M. Rosmus, N. Olszowska, J. J. Kołodziej, G. Bauer, Y. Guldner, G. Springholz, and L.-A. de Vaulchier,Valley-Polarized Quantum Hall Phase in a Strain-Controlled Dirac System, Physical Review Letters, 132, 166601, Vol. 132, Iss. 16 — 19 April 2024 doi:10.1103/PhysRevLett.132.166601

Published date 15/05/2024 - Dagmara Chylewska-Olech

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