The concept of quantum materials has become a unifying term that embraces a vast portfolio of compounds with emergent phenomena rooted in quantum mechanics. These quantum phenomena originate from fundamental interactions occurring at the atomic or molecular level but evolve in space and time in a cooperative manner due to the high degree of collectivity ensured by the crystal lattice. The net result is the creation of intricate energy landscapes that lead to a variety of coexisting or competing quantum phases. Notable examples include unconventional superconductivity, different forms of electronic liquid crystals, and quantum-entangled topological phases. Many of these phases are also spatially inhomogeneous on the mesoscale and bear similarities to complex matter where nonlinearities dominate, such as soft materials and biological systems. Domain walls, glassy behavior, phase separation, and charge-ordered textures are only some of the intriguing phenomena routinely found. In our group, we explore this quantum complexity with sensitive spectroscopy and microscopy tools based on advanced laser systems.
For example, we search for emergent electronic phases in thermodynamic equilibrium by scrutinizing the low-energy degrees of freedom of quantum materials. To this end, we develop state-of-the-art electronic spectroscopic methods that can visualize how the electrons distribute in energy and momentum in low-dimensional solids; we also take advantage of the low photon energy of extreme infrared radiation to search for exotic excitations that propagate collectively in space and time. A suite of linear and nonlinear optical spectroscopy methods finally allows us to map the symmetries broken by the emergent electronic orders across a phase transition.
We then extend these investigations far away from thermodynamic equilibrium, driving quantum materials with intense laser pulses. When these ultrashort light bursts are accurately engineered, they can break the delicate balance among different microscopic interactions and trigger non-adiabatic phase transitions along pathways that are often unpredictable. This in turn leads to the creation of transient or metastable phases of matter that do not have any counterpart in equilibrium. Our research aims to discover these hidden states, construct nonequilibrium phase diagrams of advanced materials, and unravel novel functionalities for energy sciences and quantum technologies.