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.

Selective amplification of collective excitations in excitonic insulator We have established a microscopic model to understand the dynamical behavior of excitonic insulator Ta2NiSe5 and a reversal of excitonic was successfully predicted and later demonstrated by coherent phonon spectroscopy. We adopted this model and tuned the electronic damping such that an coherent oscillation of the amplitude and phase of the order parameter emerges, which correspond to the celebrated Higgs and Goldstone mode. Due to the electron phonon coupling, Goldstone mode gains mass. By tuning the pump wavelength, a strong amplification of both the Higgs and the Goldstone modes was identified when pumping at half of the Higgs mode frequency, accompanied by a concomitant drop of the fluence where the order reverses. This matches the second-order Raman-active symmetry of the Higgs mode. The simultaneous amplification of the Goldstone mode arises from the possible decay from Higgs modes to two Goldstones modes at opposite momenta with energies identical to half of the Higgs frequency. A comprehensive model considering the interplay of different degrees of freedom in transition metal oxides The low-energy physics governing the electronic properties of complex transition metal oxides are very complex. Entangled charge, orbital, spin, and lattice degrees of freedom in the d shell all play a role with nonnegligible interaction, engendering a complex phase diagram with a plethora of exotic orders. We consider a microscopic model counting for (1) the Kanamori-type electronic interaction parameterized by onsite (U) and interorbital (U’) electro-electron interaction and Hund’s coupling (JH), (2) Jahn-Teller coupling (g) which captures the orbital-lattice interaction, (3) spin-orbital coupling (λ), (4) lattice geometric splitting (Δ) and elastic energy (B). Surprisingly, the eigenvalue of the Hamiltonian can be calculated analytically, and the potential energy surface can be deduced with respect to the lattice order parameters. By tuning the aforementioned various parameters, we are able to explore the diverse ground states and phase transitions. By further considering either an impulsive or displacive excitation, the dynamics of the system can be fully simulated.

Ultrafast nonthermal restoration of higher crystalline symmetry in topological candidate elemental tellurium One route to the control of topological properties in materials is through the manipulation of crystalline structure. We demonstrate an ultrafast structural phase transition in elemental tellurium, a prototypical Peierls-distorted topological material candidate. Our static and time-dependent density functional theory simulations predict dynamical concomitant topological and structural transitions upon optical excitation. This transition is probed experimentally through correlated time-resolved second harmonic generation polarimetry and coherent phonon spectroscopy measurements. Upon irradiation with light, we observe a dramatic drop of the second harmonic intensity followed by a coherent A1 phonon mode. Further, as a function of fluence, an inflection point is observed in both the drop of the second harmonic intensity and the phonon dynamics, which is quantitatively reproduced by our theory. Our work not only reveals a metastable structural transition, but also points towards a possible topological switch that has no equilibrium analog. Structural characterization of magnetic Weyl semimetal candidates CeAlGe and PrAlSi The crystallographic structure of the magnetic Weyl semimetal candidate PrAlSi is presently unclear. Specifically, it is difficult to distinguish between a non-centrosymmetric tetragonal point group 4mm versus a centrosymmetric group 4/mmm with current diffraction-based techniques. However, the presence or absence of inversion symmetry has a profound effect on optical second harmonic generation (SHG). We employed optical second harmonic generation (SHG) polarimetry and established that even well above the magnetic transition the SHG response from PrAlSi is of predominantly bulk electrical dipolar origin from 4mm group, same as its analog CeAlGe. This result confirms the lattice structure where nontrivial topological electronic band structure can emerge.

Nonlinear doublon-holon production and nonthermal dynamics in a strongly driven Mott insulator Ca2RuO4 The fate of a Mott insulator under strong low frequency optical driving conditions is a fundamental problem in quantum many-body dynamics. Using ultrafast broadband optical spectroscopy, we measured the out-of equilibrium doublon-holon and electronic structure dynamics of a Mott insulator Ca2RuO4 upon intense midinfrared sub-gap pump. We observe coherent bandwidth renormalization and nonlinear doublon-holon pair production occurring within the pump pulse duration. By sweeping the off-resonant electric field amplitude, we demonstrate continuous bandwidth tuning and a Keldysh crossover from a multi-photon absorption to quantum tunneling dominated pair production regime, distinguished by their pump field scaling relations and pair distribution functions. Our results provide a procedure to control coherent and nonlinear heating processes in Mott insulators, facilitating the discovery of novel out-of-equilibrium phenomena in strongly correlated systems.

The excitons are traditionally bound via the long-range Coulomb interaction, akin to an electron bound to a proton in a hydrogen. To date, however, most discoveries related to excitons have been limited to weakly correlated rigid band III-V semiconductor and transition metal dichalcogenides, where electron-electron correlations are not significant. In contrast, far less is understood about excitons in strongly correlated materials. As an archetypal example, excitons in the Hubbard model are predicted bind via the spin exchange interaction. Thus far, there has been no smoking-gun experimental signatures for these so-called Hubbard excitons and their spin-based binding mechanism. Using THz intra-excitonic spectroscopy, we demonstrate the unambiguous existence of a photo-induced Hubbard excitonic gas in the relativistic antiferromagnetic Mott insulator Sr2IrO4. By tracking the central energy of the excitonic mode as a function of the antiferromagnetic correlation strength, we establish the presence of the spin-exchange binding mechanism. Our results demonstrate an excitonic mode that is bound by an interaction other than the Coulomb interaction, paving the path to engineer excitonic states not accessible in traditional rigid band insulators.

Construction of time- and angle-resolved photoemission spectroscopy Angle-resolved photoemission spectroscopy (ARPES) is able to directly measure the energy and momentum of an electron, dubbed as band structure, in solid-state materials. The principle is based on the celebrated Einstein photoelectric effect, in which a photon with sufficient energy can eject electrons from the material. Using femtosecond lasers, the method can be extended to a time-resolved version in which not only can unoccupied excited electronic states be accessed but also the time evolution of the electrons accompanied by potential collective modes can be traced. Honglie has been working on a laser-based ARPES which can operate at an unprecedentedly low temperature (0.7 K) with high energy resolution (5 meV) in the first three years of his Ph.D. Meanwhile, he also designed and constructed a 4th harmonic generation setup to generate 6.2 eV probe based on Ti:sapphire laser (1.55 eV) so that time-resolved ARPES function is realized. Further upgrade is undergoing. Construction of time-resolved THz reflection and emission spectroscopy and intense THz pump Time-domain THz spectroscopy (TDTS) allows for a phase-sensitive measurement of both real and imaginary optical conductivity as low as 1-24meV. Honglie has been working on developing a reflection-based TDTS setup and further extending to a time-resolved version by incorporating a near-infrared pump. A ZnTe/GaP crystal is used to generate the THz probe through optical rectification of the 800 nm pulses from the Ti:sapphire laser, and another ZnTe/GaP crystal detects the reflected THz pulse via electro-optic sampling. Removing the generation crystal the setup naturally transforms into a THz emission setup, which can be served as a DC-limit second harmonic generation setup. Employing a rotation stage, we are able to conduct THz emission polarimetry measurement to clarify the symmetry of the electron near the Fermi levels. Currently, Honglie is working on upgrading the setup to accommodate an intense THz pump generated by organic crystal. This can extensively expand the pump energy down to 1-10 meV where abundant charge, magnetic, orbital, and lattice excitations reside and can thus be resonantly excited.

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