Quantum size effect on charges and phonons ultrafast dynamics in atomically controlled nanolayers of topological insulators Bi2Te3

M Weis,B Arnaud,P Ruello,R Rapacz,B Wilk,A Bulou,J Szade,K Balin,G Vaudel

doi:10.1038/s41598-017-12920-4

Abstract

Heralded as one of the key elements for next generation spintronics devices, topological insulators (TIs) are now step by step envisioned as nanodevices like charge-to-spin current conversion or as Dirac fermions based nanometer Schottky diode for example. However, reduced to few nanometers, TIs layers exhibit a profound modification of the electronic structure and the consequence of this quantum size effect on the fundamental carriers and phonons ultrafast dynamics has been poorly investigated so far. Here, thanks to a complete study of a set of high quality molecular beam epitaxy grown nanolayers, we report the existence of a critical thickness of around ~6 nm, below which a spectacular reduction of the carrier relaxation time by a factor of ten is found in comparison to bulk Bi2 Te3 In addition, we also evidence an A1g optical phonon mode softening together with the appearance of a thickness dependence of the photoinduced coherent acoustic phonons signals. This drastic evolution of the carriers and phonons dynamics might be due an important electron-phonon coupling evolution due to the quantum confinement. These properties have to be taken into account for future TIs-based spintronic devices.

Highlights

Topological Insulators (TIs) provide new perspectives for generation spintronics devices thanks to a natural spin polarized surface current that is topologically protected[1,2]
We first confirm the appearance of the evolution of the electronic structure for very thin layers with X-ray Photoemission Spectroscopy (XPS) in accordance with what was observed in BS10,11 and BT12 compounds
Low Energy Electron Diffraction (LEED) image was realized for the step sample as a function of the BT layer thickness (Fig. 1b confirms the films grow along the c-axis having a six-fold symmetry)

Summary

Introduction

Topological Insulators (TIs) provide new perspectives for generation spintronics devices thanks to a natural spin polarized surface current that is topologically protected[1,2]. The band gap opening has been reproduced theoretically and it has been reported that additional quantum size effects should appear leading to topological quantum phase transitions that depend on the film thickness[14,15,16] While this drastic evolution of the electronic structure submitted to a strong confinement has been well described in TIs at the thermodynamic equilibrium with photoemission spectroscopy[10,11,12] and with transport properties measurements[17,18], the consequence of this confinement on the ultrafast dynamics of carriers and phonons has been debated only recently in BS19,20. These carriers and phonons dynamics size dependence suggests the existence of a transition in the mechanism of the electron-phonon coupling when reducing the TIs layer which has to be taken into account in the perspectives of the development of TIs nanodevices

Methods

Results

Conclusion