The earliest reports of ferromagnetism date to Thales of Miletus who lived and wrote around 600 BC. Thales noted the ability of natural magnetite to attract iron, and is said to have taken this as proof that matter itself was alive. Our theories of magnetism have evolved considerably since then: we now know that ferromagnetism arises from the interplay of the Coulomb repulsion between electrons and their fermionic statistics. However, in one sense our science has advanced only little: the vast majority of magnets, like magnetite, consist of ordered arrangements of the electron spins stabilized by the spin orbit interaction. In my talk, I will describe a new class of magnets based on the spontaneous alignment of electron orbitals. Such orbital ferromagnetism may be a generic phenomena, but has, to date, found its fullest expression in graphene heterostructures in which the two dimensional orbits of electrons in distinct momentum space valleys provide the underlying degree of freedom for magnetic order. These magnetic degrees of freedom arise directly from the band wavefunctions, making orbital magnets exquisitely sensitive to both the design of the electronic wavefunctions as well as in situ control parameters. For example, in systems in which interlayer lattice mismatches lead to a moire superlattice potential, the resulting superlattice band structure may feature nontrivial topology, which in conjunction with orbital magnetism can give rise to precise quantization off the Hall effect at zero magnet field. Remarkably, the conduction electrons themselves can also directly influence the magnetic order, leading to field-effect switchable magnetic moments. Finally, I will conclude with our progress towards using orbital magnetism to engineer "topologically ordered" states at zero magnetic field, in which the electron splits into fractionally charged anyons.
Zoom talk link:
