Tunneling injection of doublon-like excitations and their coupling to the charge density wave in 1T-TaS₂

Christopher Butler^1*, Masaro Yoshida¹, Tetsuo Hanaguri¹, Yoshihiro Iwasa^1,2

¹Center for Emergent Matter Science (CEMS), RIKEN, Wako Saitama, Japan
²Quantum-Phase Electronics Center and Department of Applied Physics, The University of Tokyo, Tokyo, Japan

* Presenter:Christopher Butler, email:christopher.butler@riken.jp

In some materials, the overall system energy is minimized when the electrons and ions agree to form matching static density wave patterns. In 1T-TaS₂, such modulations reduce the electrons’ kinetic energy such that they can no longer overcome their mutual repulsion to flow amongst each other freely. They are thus stuck in place, forming a so-called ‘Mott insulator’. But 1T-TaS₂ is unusual in that its Mott insulating state can be manipulated and even destroyed through even mild perturbations. How do the electrons and ionic lattice coordinate to form this interesting state? Some clues may be found by inducing an abrupt disturbance in the electronic density wave, to observe the dynamic response of the coupled ionic density wave. We achieve this using a scanning tunneling microscope, observing the spectroscopic markers of the response upon injecting excess electrons into the Mott state. The abrupt addition of an electron upsets the balance between the ionic and electronic density waves, exciting one or more phonons - specifically, oscillations of the ionic density wave around the newly arrived electron. More interestingly, the exceptionally small bandwidth of the spectroscopic features, only a few percent of the bandwidth of the Hubbard bands, hints at unusual excitations which seem to defy understanding in current theories. These observations extend our microscopic understanding of systems in which mutual electronic repulsion and electron-lattice coupling are both significant, and should spur a new theoretical push to understand the possible new excitations which can arise.

Keywords: Charge density waves, Electron-phonon coupling, Strongly correlated electrons, Scanning tunneling spectroscopy