Optical Spectroscopy of Complex Materials

Richard Averitt, Mengkun Liu, Andrew Strikwerda, Jingdi Zhang

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Complex materials may be defined as having no dominant energy scale the implication being that the charge, lattice, orbital, and spin degrees of freedom conspire to determine their functional, and often emergent, properties. This leads to rich macroscopic and mesoscopic behavior with examples including colossal magnetoresistance, superconductivity, multiferroicity, and electronic phase separation. Furthermore, advances in the synthesis, growth, and integration of nanomaterials make possible the design of nanoscale complex materials inspired by their bulk counterparts. Optical spectroscopy is an important tool to interrogate complexity in materials, naturally complementing techniques such as angle-resolved photoemission or inelastic neutron scattering.

In particular, the beauty of optical studies of condensed phases is the breadth of applicability. This is depicted in the Figure which displays the spectral range and timescales of different phenomena occurring in materials. Spectral coverage from approximately 0.001 – 4.0 eV is especially important since many relevant excitations lie in this range. This includes, as examples, gapped excitations related to superconductivity, charge ordering, and hybridization phenomena; polaron, exciton, and plasmon dynamics; or the coherent Drude response so intimately related to metal-insulator transitions. For these reasons, optical spectroscopy plays an important role in many areas of applied and fundamental condensed matter physics. Examples include spintronics, Bose-Einstein exciton condensation, plasmonics, dynamics in DNA, and semiconductor heterostructures.

Importantly, ultrafast optical spectroscopy probes dynamics at the fundamental timescales of electronic and atomic motion thereby providing an important approach to investigate dynamical phenomena in complex materials. We utilize time-resolved optical spectroscopy spanning from the far-infrared through the visible to gain insights into the functional response complex materials. This includes superconductors, Heavy Fermions, manganites, and multiferroics. Our current work focuses dynamical investigations of a host of transition metal oxides including photoinduced phase transitions in vanadates and dynamical spin-phonon coupling in multiferroics such as barium hexaferrite.