Ultrafast reversal of the excitonic order in Ta2NiSe5
Excitonic insulator is a novel strongly correlated insulating phase where bound electron-hole pairs, dubbed as excitons, naturally condense without external assistance. In the presence of electron-phonon coupling, an excitonic insulator harbors two degenerate ground states described by an Ising-type order parameters of equal magnitude but opposite phase. There is currently no experimental method to switch nor directly measure the phase of an EI order parameter. Starting from a microscopic Hamiltonian, we derive the equations of motion for the Ising order parameter in the phonon coupled excitonic insulator candidate Ta2NiSe5 and show that the order parameter can be controllably reversed on ultrashort timescales using appropriate laser pulse sequences. Using a combination of theory and time-resolved optical reflectivity measurements, we report evidence of such order parameter reversal in Ta2NiSe5 based on the anomalous behavior of its coherently excited order-parameter-coupled phonons. This work expands the field of ultrafast order parameter control beyond conventional spin and charge ordered materials.
Transient switch of the spin-nematic order in Ca2RuO4
The competition and coexistence of electronic correlation, Jahn-Teller effect (orbital-lattice coupling), and spin-orbital coupling can engender a variety of exotic phases. Transition metal oxides with intertwined orbital, spin, and lattice degrees of freedom provide an arena to explore such physics. The interplay of different mechanisms governs the low-energy properties of the 4d Mott insulator Ca2RuO4 and renders a spin-nematic phase featuring a pseudospin quadrupolar order. Since this phase harbors no dipolar magnetic order and preserves time-reversal symmetry, it is hard to directly couple to and widely known as a hidden order. Both experimental investigation and manipulation of such phase are missing. We harness the intense midinfrared optical pumping to induce a transient quadrupolar order switch and probed such ultrafast transition via broadband coherent phonon spectroscopy. As the order is switched, we observe that the amplitude of the 3.7 THz phonon exhibits a temperature- and probe-energy-dependent nonlinearity over the pump fluence, which can be perfectly simulated with a microscopic model that captures all the entangled degrees of freedom. Our work not only reveals a strong pseudospin-lattice coupling in Ca2RuO4, but opens a pathway to probe and manipulate the hidden electronic orders out of equilibrium.
Nonthermal quenching of magnetism in Sr2IrO4
Dynamically driven interacting quantum many-body systems have the potential to exhibit properties that defy the laws of equilibrium statistical mechanics. A widely studied model is the impulsively driven antiferromagnetic Mott insulator, which is predicted to realize exotic transient phenomena including highly non-thermal magnon distributions and dynamical phase transitions into thermally forbidden states. The former induces an unusual far-from-equilibrium critical regime in which the divergences of the magnetic correlation length and relaxation time are decoupled, which was recently observed in the n-type photo-doped Mott insulator Sr2IrO4 using time-resolved second harmonic optical polarimetry, while the latter remains elusive. By tuning the carrier excitation from n-type doping to doublon-holon generation, we discover a nonthermal antiferromagnetic phase. Our findings, embodied in a nonequilibrium phase diagram, provide a blueprint for engineering the out-of-equilibrium properties of quantum matter, with potential applications to terahertz spintronics technologies.