Bosonic metal states in crystalline iron-based superconductors at the two-dimensional limit

The nature of the anomalous metal, one of the quantum ground states of two-dimensional (2D) bosonic systems, remains a major puzzle even after several decades of study. Here, we report a systematic investigation on the transport properties of ultrathin crystalline FeSe films grown on SrTiO3 (STO) as well as the nanopatterned FeSe/STO, where the 2D high-temperature superconductivity is confined at the interface. Remarkably, the bosonic anomalous metal state emerges around 20 K, an exceptionally high temperature compared to all previous observations. Furthermore, a linear-in-temperature (T-linear) resistance with suppressed Hall coefficient below onset temperature for superconductivity is observed, indicating a bosonic strange metal. We give quantitative analysis for the bosonic anomalous metal state, based on the quantum dynamical property of vortices influenced by ohmic dissipation. This microscopic model pins down the origin of the intriguing anomalous state to the superconducting phase dynamics in both spatial and temporal domain. Our findings shed new light on the bosonic metal states in crystalline superconductors at the 2D limit.