Nano-Optical Spectroscopy and Microscopy of Quantum Matter
Phase coexistence in correlated matter
Strongly correlated electron systems and the diverse emergent phenomena they host are one of the cornerstones of contemporary condensed-matter physics research. The intimate interplay of lattice, orbital, spin, and charge degrees of freedom emphasized by strong electronic interactions has been found to give rise to an enormous complexity of correlated phases and physical properties. Due to the strong coupling between the aforementioned degrees of freedom, it is possible to navigate between different phases by means of easily accessible external parameters such as static or transient lattice strain, static or transient electric/magnetic fields, and temperature. In many cases the transition between phases is of first order, entailing a phase separation in real space and the occurrence of substantial hysteresis in physical properties. The latter is of substantial technological importance because it provides persistent electronic states that can be switched at will: an essential property of the next generation transistor. Such an implementation is particularly appealing as it minimizes leakage current, which limits further miniaturization and reduction in power consumption of modern electronic devices. It is, therefore, of both fundamental and technological importance to achieve a good understanding of the formation, dynamics, and interplay between various correlated phases. Given that the phase separation typically occurs on the nano-/mesoscale and that the phases often exhibit dramatically different transport properties (such as a metal-insulator transition in VO2), a spectroscopic, low-energy and -temperature, ultrafast optical probe with nanometer spatial resolution is indispensable. Our group is currently constructing such an instrument based on the unique combination of already existing and well-established techniques: scattering-type near-field optical spectroscopy affording spectroscopic investigation of quasiparticle optical response at sub-10 nm spatial resolution, coupled with ultrafast time-domain terahertz spectroscopy providing access to the static and transient lowest-energy material optical response, all at cryogenic temperatures down to 1.6 K required for state-of-the-art condensed-matter research.