Helical Fermi liquids and their breakdown

The surface of 3D strong topological insulators such as Bi₂Se₃ harbors a new gapless state of matter, the 2D helical Fermi liquid, where a novel phenomenology distinct from that of the conventional 2D Fermi liquid arises out of the interplay of Coulomb interactions and a spin-momentum locked Fermi surface, the latter originating from strong spin-orbit coupling in the topological bandstructure. Aspects of this predicted phenomenology, such as novel spin-orbit coupled collective modes, have recently been observed experimentally. In this talk I will present an extension of Landau’s phenomenological Fermi liquid theory to the case of the helical Fermi liquid, and derive from it the renormalization of equilibrium properties, criteria for Fermi surface instabilities, and collective mode dispersions. I will then discuss the quadrupolar Pomeranchuk instability in greater detail and its observable signatures, such as anisotropies in charge and spin response functions, the partial breakdown of spin-momentum locking, collective modes and induced spin fluctuations, and non-Fermi liquid behavior.