Charge Friedel oscillations in a Mott insulator

Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. When a metal undergoes a transition to an insulator it will lose its electronic Fermi surface. Interestingly, in some situations a " ghost " Fermi surface of electrically neutral spin carrying fermions may survive into the insulator. Such a novel ghost Fermi surface has been proposed to underlie the properties of a few different materials but its direct detection has proven elusive. In this paper, we show that the ghost Fermi surface leads to slowly decaying spatial oscillations of the electron density near impurities or other defects. These and related oscillations stem from the sharpness of the ghost Fermi surface and are direct analogs of the familiar Friedel oscillations in metals. The oscillation period contains geometric information about the shape of the ghost Fermi surface, which can be potentially exploited to detect its existence. Introduction. In recent years it has become clear that the insulating side in the vicinity of the Mott metal-insulator transition may, in some frustrated lattices, provide a realization of the long sought quantum spin-liquid (SL) state. These are insulators with an odd number of electrons per unit cell that do not order magnetically or break lattice symmetries. Candidate materials for SL phases of electronic Mott insulators are the layered quasi-two dimensional organic materials κ-(ET) 2 Cu 2 (CN) 3 and EtMe 3 Sb[Pd(dmit) 2 ] 2 , 1,2 and the three-dimensional " hyperkagome " material Na 4 Ir 3 O 8. 3 These materials all become metallic under moderate pressure 4,5 and hence are appropriately thought of as weak Mott insulators. While SL behavior may well be a common feature of weak Mott insulators, it remains much more elusive in strong Mott insulators. Notable exceptions are the spin-1/2 Kagome magnets 6 ZnCu 3 (OH) 6 Cl 2 and Cu 3 V 2 O 7 (OH) 2 · 2H 2 O.