Topological materials such as topological insulators (TIs) and Dirac and Weyl semimetals are an emergent class of quantum materials whose properties are protected by symmetry and topology of the bulk band structures. Therefore, these properties are robust against scattering, leading to near dissipationless carrier transport. In particular, in topological insulators, the timer-reversal symmetry protected surface states exhibit spin-momentum locking where the electron spin is locked to momentum, and hence an unpolarized charge current creates a spontaneous spin polarization.
The goals of this program is to synthesize topological materials by molecular beam epitaxy (MBE), and their heterostructures with magnetic materials, to demonstrate control and manipulations of spin and charge transport through electrical and optical means. Of particular interests are the exploration of device applications such as nonvolatile magnetic topological memory that utilize the current generated spins in TIs and Weyl semimetals to switch the magnetization of a ferromagnet via spin orbit torque. Physical processes such as spin transport at heterointerfaces and spin dynamics will be investigated to optimize efficiency and demonstrate prototype devices that are relevant in next generation low power electronics and spintronics, information processing, and in-logic memory.
Extensive facilities exist including a newly installed cluster system integrating MBE (equipped with ebeam sources), sputter, and SPECS angle-resolved photoemission spectroscopy (ARPES) system, with an Omicron INFINITY scanning tunneling microscope (STM) to be added in 2024. Electrical transport, magneto-optical and structural/magnetic characterization are also available, as well as a class 100 cleanroom for nanofabrication.
Reference:
C. H. Li et al., Electrical detection of charge-current-induced spin polarization due to spin-momentum locking in Bi2Se3, Nat. Nanotech. 9, 218–224 (2014).
C. H. Li et al., Direct comparison of current-induced spin polarization in topological insulator Bi2Se3 and InAs Rashba states, Nat. Commun. 7, 13518 (2016).
C. H. Li et al., Electrical Detection of Spin-to-Charge Conversion in a Topological Insulator Bi2Te3, Sci. Rep. 8, 10265 (2018).
C. H. Li et al., Electrical detection of current generated spin in topological insulator surface states: Role of interface resistance, Sci. Rep. 9, 6906 (2019).
J. Moon et al., Magnetic Field-Induced Spin Nematic Phase Up to Room Temperature in Epitaxial Antiferromagnetic FeTe Thin Films Grown by Molecular Beam Epitaxy, ACS Nano 17, 16886 (2023).
M. Lohmann et al., Highly Efficient Spin−Orbit Torque Switching in Bi2Se3/Fe3GeTe2 van der Waals Heterostructures, ACS Nano (2023). DOI: 10.1021/acsnano.3c09041
Topological insulator; Dirac semimetal; Weyl semimetal; Spintronics; Spin-momentum locking; Spin transport; Molecular beam epitaxy (MBE); angle-resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM)