Resolving the Kagome Origin of the Strange Metallicity in Ni3In

When electronic interactions become comparable to the band width the stability of quasipartlicles is compromised and the Fermi-liquid theory fails. A strange metal may form instead. Understanding the mechanism for such emerging universality is an outstanding challenge, given that the underlying degrees of freedom can be complex and varied. Progress may be made in flat band systems, especially kagome and other frustrated-lattice metals with active flat bands. These systems show strange metal behavior that bears a striking resemblance to what happens in heavy-fermion metals. Here, we study the kagome metal Ni$_3$In whose kagome flat band is close to the Fermi energy using scanning tunneling spectroscopy [1]. We find a zero-bias peak-dip structure whose variation with magnetic field and temperature tracks the evolution of the strange metal properties. We identify the origin of the peak as compact molecular orbitals formed by destructive interference over the kagome sites, resulting in emergent $f$-shell-like localized moments [2]. We visualize scattering among the Dirac bands degenerate with the kagome flat band through quasi-particle interference. We find a suppression of intensity at the flat band energy, signifying the destabilization of quasiparticles within the strange metal phase. We thus unveil the essential microscopic ingredients of the $d$-electron-based kagome metals that, while distinct from the atomic orbitals of the $f$-electron-based heavy fermion materials, are responsible for a shared phenomenology between the two types of systems. Our findings provide a new window to uncover and interconnect the essential and yet diverse microscopic building blocks in disparate families of quantum materials that drive a convergence towards a universal understanding in the regime of amplified quantum fluctuations.