Bistable Terahertz Functionality of a Near Room-Temperature Spin Crossover Material relevant for Next Generation Communication Technologies

The light-matter interaction at terahertz (THz) frequencies is poised to advance the frontiers of information processing and communication technologies, especially as its scope expands through the integration of emerging materials. In this context, a particular class of materials known for their spin-crossover (SCO) properties remains underexplored in the THz spectral region. This gap not only restricts our understanding of how structural and magnetic changes in these systems are controlled with low-energy optical stimulus, but it also leaves their potential unrealized for promising technological applications. In this work, we designed a novel Hofmann-type SCO framework, [Fe(mpi)₂Pt(CN)₄], exhibiting an abrupt SCO with ~20 K hysteresis centered around 245 K. THz spectroscopy revealed thermal bistability in both THz amplitude and dielectric function in heating and cooling protocols, mirroring the SCO profile. This bistability originates from modifications in phonon mode dynamics linked to structural transitions between triclinic P-1 and monoclinic C2/m symmetries, as confirmed by X-ray diffraction. Additionally, leveraging the bistable dielectric response of this complex has demonstrated its potential as a THz optical switch which unveils novel optical response of SCO-THz light interactions as well as a potential ultrafast switching functionality relevant in the development of futuristic communication technologies, including 6G and beyond.