Surface and interface physics driven by quantum materials

Electronic states at the boundaries of crystals, such as surfaces, interfaces, edges, hinges, corners, and extremities, play crucial roles in emerging quantum materials, such as graphene and similar monatomic-layer materials, van der Waals crystals, and topological insulators. Electronic states at such boundaries are different from those inside the three- or two-dimensional crystals, not only because of the truncation of crystal lattices but also because of space-inversion-symmetry breaking and difference in topology in band structures across the boundaries. Such quantum materials are expected to advance energy-saving/-harvesting technology as well as quantum computing/information technology because of exotic phenomena, such as spin–momentum locking of an electron, pure spin current, dissipation-less charge current, nonreciprocal current, and possible Majorana fermions. In this review, their fundamental concepts are introduced from the viewpoint of surface physics, in which atomic and electronic structures, as well as charge/spin transport properties, are directly probed using state-of-the-art techniques.