Magnetochiral anisotropy in quasi-2D materials

Abstract

Condensed matter system is considered to be the most prominent example of quantum many-body systems. Under thermodynamic limit (macroscopic limit), the dissipative process occurs commonly in these systems and determines the direction of the time due to its irreversible nature. However, unidirectional transport and propagation of quantum particle, such as electron, photon, spin, and phonon, are known to occur in the materials with broken inversion symmetry, as exemplified by the diode in semiconductor p–n junction and the natural optical activity in chiral materials. In this thesis, we first review the general unidirectional responses in condensed matter materials, then investigate one specific phenomenon of this kind, the magnetochiral anisotropy (MCA) effect. We primarily focus the underlying symmetry restriction mechanisms responsible for MCA in general quasi-2D materials combining with the semi-classical theory. The quasi-2D materials are a class of materials that exhibit unique electronic and magnetic properties due to their reduced imensionality. They have high electron mobility, tunable bandgap, and strong spin-orbit coupling, which make them ideal candidates for studying various manybody correlated and band topological states.

Magnetochiral anisotropy (MCA) is a kind of nonreciprocal transport effect induced by an external magnetic field in lattice systems without inversion symmetry. Normally, it is considered to have a relativistic origin and ubiquitously found to be very small compared to the common Ohm resistivity. However, recent experiments had showed gigantic MCA effect in tellurium (Te) and ZrTe5, which may relate to the chiral anomaly in Weyl semimetals. MCA effect also possesses unique features such as non-Hall transverse response and applied-field angle dependence. In this thesis, we’ll utilize both theoretical framework and numerical calculation method to study the MCA effect, including symmetry analysis, semi-classical theory, tight binding model and perturbation theory. These methods are used to justify MCA’s unique properties and reveal how it behaves like in quasi-2D materials with different point group symmetries. We also develop an effective model to take account of the interlayer coupling in quasi-2D materials, and ascribe its contribution to MCA effect as layer coherent magnetic moment.

Recently, moiré superlattices of semiconducting transition metal dichalcogenides (TMD) have attracted interest as a playground for the topological band effects and the strongly correlation phenomena, as exemplified by the recent discovery of fractional Chern insulator states, quantum Hall effects, and the unconventional superconductivity. In this thesis, we present various numerical results of the band geometric quantities in TMD twisted bilayer MoTe2, obtained using the continuum model for moiré system and the aforementioned generalized semi-classical theory. Our results reveal the presence of layer coherent magnetic moment and the MCA effect originated from it in quasi-2D materials, including the multilayer TMD materials and the graphenebased heterostructures. The numerical results are also compared with the predictions from the symmetry analysis, demonstrating good agreement between theory and calculation. Our results also explore potential applications in the fields of spintronics, nanotechnology, and quantum computing.

In conclusion, this master thesis provides a specific study of the MCA effect in quasi-2D materials, elucidating the fundamental point group symmetries responsible for this phenomenon and paving the way for future research in this emerging field. The results presented herein have significant implications for the design and development of advanced electronic devices that harness the unique properties of quasi-2D materials.