Spin and Orbital Physics in Novel Correlated Materials
Piers Coleman, Rutgers, The State University of New Jersey, New Brunswick
(Principal Investigator)
Electrons in metals interact via their mutual Coulomb repulsion and when these interactions become large, the electrons rearrange themselves into new quantum states of order. Such “quantum materials” include magnets, spin liquids, high temperature superconductors, and materials which acquire topological order. The insights into these phenomena accrued from research into their underlying physics provides a vital framework for their future applications, as materials for new devices, as new kinds of memory, and for communication and energy applications. Of particular interest to this science, are the transformative effects of the magnetic moments and the orbital motion of the electrons.
The first focus of the proposed research concerns the physics of “magic-angle" twisted bilayer graphene, in which the development of a Moire pattern gives rise to a novel two-dimensional quantum material that can be tuned by external gate voltages to produce topological magnets and unconventional superconductivity. Recent work has shown that the physics of magic angle twisted bilayer graphene is closely analogous to three-dimensional heavy fermion materials, containing magnetic moments embedded in a conducting medium. The proposed research will apply methodologies developed for Kondo lattice materials to twisted bilayer graphene, seeking to understand the metallic, insulating, and superconducting phases of these new materials.
A second thrust will examine the effects of interactions and orbital physics on high-temperature iron-based superconductors. Strong Coulomb interactions on electrons in different atomic orbitals give rise to ferromagnetic coupling between the electrons, called Hund’s interactions. Experiments suggest that iron-based superconductivity may be related to these Hund’s interactions. We seek to test a triplet resonating valence bond mechanism for this pairing in which triplet electron pairs formed by the Hund’s interactions resonate between iron atoms and the condensate. The recently discovered “nematic” iron-based superconductor FeSe will be used as a simplified setting for examining the resonating triplet valence bond theory.
The third focus of the research will examine the effects of electron interactions on topological quantum materials. Research over the past fifteen years has revealed a class of topological quantum materials in which the wave function of the electrons acquires a topological twist that cannot be undone in a smooth fashion, and which transforms the physics of the electronic ground-state, giving rise to new kinds of properties, such as robust conducting surface states. The simplest examples of such interacting topological materials are topological Kondo insulators. In these materials, magnetic interactions amongst the atoms cause the low-temperature transformation of the material from a metal into an insulator with novel conducting surface states. The proposed research will develop a theory for the tunneling of spin polarized currents from a SmB6 nanowire. Recent discoveries suggest that under some circumstances, electron interactions can transform the bulk properties of these materials, generating some kind of internal spin liquid with neutral excitations. The proposed research will develop a theory for this novel state that builds on recent advances in our understanding of three-dimensional spin liquids and their interaction with mobile electrons.
