IMX talk - Two-dimensional coherent spectroscopy for composite collective excitations

Unlocking the nonlinear response of quantum materials is key to technological advances in high-frequency and low-power electronics, spintronics and quantum information processing. Massive advances in the technology of shaping ultrafast and ultra-intense laser pulses over wide frequency ranges have enabled the creation of many nonequilibrium phenomena whose origins lie in nonlinear processes. One of the most promising approaches to the ultrafast control of collective excitations in quantum materials is magnetophononics, which combines coherent driving of lattice excitations by ultrashort THz pulses with the intrinsic spin-lattice coupling to create a nonequilibrium magnetic response. To separate and analyze all of the high-order response features in a magnetophononic system, we apply two-dimensional coherent spectroscopy (2DCS) to the minimal model of an alternating spin chain coupled to one Einstein phonon by performing pump-pump-probe calculations at all time delays. The resulting complete map of the intrinsic nonlinear dynamics includes at first order composite phonon-bitriplon hybrid excitations, second-order peaks at the sum and difference frequencies of these mutually repelling hybrids and multiple third-order response peaks corresponding to the characteristic 2Q, NR, PP and R processes of 2DCS. The composite nature of magnetophononic excitations is reflected in a selection of streak features, most notably strong antidiagonal streaks at the rephasing peaks, whose origin lies in the availability of elementary magnetic excitations over a continuum of energies. To stress the ubiquity of streaks in 2DCS as the fingerprint of intrinsic continuum dynamics, which goes far beyond conventional disorder interpretations, we compute 2DCS spectra for the strongly spin-phonon-coupled insulating quantum magnet CuGeO$_3$ and demonstrate extensive streaking at the R and NR peaks for multiple driven phonon modes.