Session F70: Light-Induced Structural Control of Electronic Phases I

Ultrafast control of magnetic interactions by light driven phonons

Resonant ultrafast excitation of infrared-active phonons is a powerful technique to control the electronic properties of materials, leading to remarkable phenomena such as light-induced enhancement of superconductivity, switching of ferroelectric polarization and ultrafast insulator to metal transitions. Here we show that light-driven lattice vibrations can be utilized to coherently manipulate macroscopic magnetic states. Intense mid-infrared electric field pulses, tuned to resonance with a vibrational normal mode of the archetypical antiferromagnet DyFeO₃, induce ultrafast and long-living changes of the fundamental exchange interaction between rare-earth orbitals and transition metal spins. Non-thermal lattice control of the magnetic exchange, defining the very stability of the macroscopic magnetic state, allows us to perform a coherent switching between competing antiferromagnetic and weakly ferromagnetic spin orders on the fastest possible timescale. Our discovery outlines the broad potential of resonant lattice excitation for the manipulation of ferroic order on ultrafast timescales.   

Stabilizing fluctuating high-temperature ferromagnetism in YTiO3 by optically driving the lattice

In complex oxides, the coupling between spin, orbital, and lattice degrees of freedom leads to competing ground states and, often, large fluctuations of the order parameter up to high temperatures. The Mott insulating rare-earth titanates provide a prime example of such physics, where structural distortions dictate the t_2g orbital order and the low-temperature magnetism. Here, we explore the effect of selective optical phonon excitation in ferromagnetic YTiO₃ which is known to exhibit magnetic fluctuations well in excess of T_c (= 27 K). We discover an ultrafast, phonon-dependent magnetization change below T_c induced by the pump and a corresponding modified ferromagnetic onset temperature. The strongest effect is found when driving the 9 THz B_2u mode, for which the enhanced magnetization at low temperatures saturates at the ideal spin-½ limit and non-equilibrium ferromagnetism persists up to ~100 K, more than 3 equilibrium T_c. Our findings are explained with a coupled spin-orbital model, in which the optically driven lattice vibrations modify the occupied t_2g orbital states and reduce competing antiferromagnetic fluctuations. These results highlight the ability to optically engineer crystal structures to realize non-equilibrium functional properties in complex oxides.   

Giant phonon-induced effective magnetic fields in 4f paramagnets

Ultrafast control of magnetic order can conventionally be achieved through opto-magnetic effects, such as the inverse Faraday effect. Here, we theoretically describe a vibrational analog of this mechanism, in which coherently excited infrared-active phonons replace photons in the scattering process, and which is mediated through spin-phonon coupling. In particular, we show that circularly driven phonon modes in the rare-earth trihalide CeCl3 generate giant effective magnetic fields acting on the paramagnetic 4f spins. We compute the coherent phonon dynamics in response to the excitation by a terahertz pulse using a combination of phenomenological modeling and density functional theory calculations. We find that effective magnetic fields of up to 100 tesla can possibly be generated that polarize the spins for experimentally accessible pulse energies. The direction of induced magnetization can be inverted by reversing the polarization of the laser pulse. This phono-magnetic effect offers a new route to achieving control over and switching of magnetic order at terahertz frequencies.   

Keldysh space control of charge dynamics in a strongly driven Mott insulator

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 reflectivity spectroscopy and second-harmonic generation rotational anisotropy experiments, we measured the transient out-of-equilibrium doublon-holon and electronic structure dynamics of an off resonantly pumped Mott insulator Ca₂RuO₄. We observed coherent bandwidth renormalization and nonlinear doublon-holon pair production occurring in rapid succession within a sub-100 femtosecond pump pulse duration. By sweeping the off-resonant electric field amplitude, we demonstrate continuous bandwidth tuning and a Keldysh cross-over from a multi-photon absorption to quantum tunneling dominated pair production regime. The time dynamics of antiferromagnetic order under the impact of nonlinear pair production was investigated. 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.   

Phonon-induced disorder in dynamics of optically pumped metals from nonlinear electron-phonon coupling

The nonequilibrium dynamics of matter excited by light may produce electronic phases that do not exist in equilibrium, such as laser-induced high-T_c superconductivity. Here we simulate the dynamics of a metal driven at initial time by a spatially uniform pump that excites dipole-active vibrational modes which couple quadratically to electrons. We study the evolution of electronic and vibrational observables and their coherences.  We provide evidence for enhancement of local electronic correlations, including double occupancy, accompanied by rapid loss of spatial structure, which we interpret as a signature of emergent effective disorder in the dynamics.  This effective disorder, which arises in absence of quenched randomness, dominates the electronic dynamics as the system evolves towards a correlated electron-phonon long-time state, possibly explaining why transient superconductivity is not observed. The pumped electron-phonon systems studied here, which are governed by nonlinear coupling, exhibit a much more substantial dynamical response than linearly coupled models relevant in equilibrium, thus presenting a pathway to new modalities for out-of-equilibrium phases. Our results provide a basis within which to understand correlation dynamics in pump-probe experiments.   

Ultrafast terahertz spectroscopy of optically excited iron-based superconductors

Iron-based superconductors display varied interaction amidst spin, charge, and structural orders leading to diverse phase diagrams. An A1g phonon mode, coherently excited in these materials by an ultrafast 800nm pump, offers a pathway to optical enhancement and suppression of material properties [1]. Short pump pulses have resulted in light-induced superconductivity above the critical temperature [2] and increase in superfluid density [3], suggesting optically controlled superconductivity. Using broadband terahertz time domain spectroscopy and ultrafast 800nm pump, we explore the light-induced non-equilibrium phenomena above and below the electronic, structural, and superconducting transitions in various iron-based superconductors.

[1] K. Kim, et al., Nature Mater. 11, 497–501 (2012).

[2] M. Mitrano, et al., Nature 530, 461–464 (2016).

[3] K. Isoyama, et al., Commun. Phys. 4, 160 (2021).   

THz-induced optical activity in the non-collinear antiferromagnet metal Fe1/3NbS2

The intercalated transition metal dichalcogenide Fe_1/3NbS₂ has received considerable attention due to its suitability for antiferromagnetic (AFM) spintronic devices. Recently, it has been demonstrated that the AFM order in Fe_1/3NbS₂ can be switched by applying DC pulses with remarkably low current densities. In this work, we use ultrafast THz-pump, near-infrared-probe spectroscopy to observe a transient change in the Kerr ellipticity in response to a strong THz pump pulse. We find that this change is peaked at the Nèel temperature (50K) and persists at even higher temperatures. We will discuss possible reasons for this behavior in terms of competing magnetic phases present in Fe_1/3NbS₂ and critical dynamics near the phase transition. Our results highlight the possibility of manipulating AFM ordering in metals at THz timescales.    

Withdrawn

Characterizing and controlling entanglement in quantum materials is crucial to facilitate the progress of quantum technologies. However, defining a quantifiable measure of multipartite entanglement within solids of many-body quantum systems is a challenging task both in theory and experiments. Motivated by the recent progress in equilibrium entanglement witnesses, we propose a protocol to extract the time-resolved QFI far from equilibrium via the time-resolved resonant inelastic X-ray scattering (trRIXS). We then apply our method to a material-relevant model and predicted light-induced enhancement of entanglement. Such an enhancement is amplified by the non-local interactions in the many-body system. With concrete spectral predictions and material-relevant models, our simulations provide guidance for further light-control of entanglement in materials.   

Time-Resolved Resonant Inelastic X-Ray Scattering as a Probe for Light-Driven Spin Fluctuations

Manipulating spin fluctuations with ultrafast laser pulses is a promising route to dynamically control collective phenomena in strongly correlated materials. However, understanding how photoexcited spin degrees of freedom evolve at a microscopic level requires a momentum- and energy-resolved characterization of their nonequilibrium dynamics. Here, we study the photoinduced dynamics of finite-momentum spin excitations in two-dimensional Mott insulators on a square lattice. By calculating the time-resolved resonant inelastic x-ray scattering cross-section, we show that an ultrafast pump above the Mott gap induces a prompt softening of the spin excitation energy, compatible with a transient renormalization of the exchange interaction. While spin fluctuations in a hole-doped system (paramagnons) are well described by Floquet theory, magnons at half filling are found to deviate from this picture. Furthermore, we show that the paramagnon softening is accompanied by an ultrafast suppression of $d$-wave pairing correlations, indicating a link between the transient spin excitation dynamics and superconducting pairing far from equilibrium.   

Ultrafast renormalization of the onsite Coulomb repulsion in a cuprate superconductor

Intense ultrafast electromagnetic fields represent an increasingly important tool to stabilize and control emergent phases in quantum materials through the direct light-engineering of effective many-body interactions, such as electron hopping and electron-electron repulsion. Here, we employ time-resolved x-ray absorption spectroscopy (trXAS) to demonstrate the ultrafast renormalization of the Hubbard U parameter in the underdoped cuprate La_1.905Ba_0.095CuO₄ (LBCO, x=9.5%). Our element-specific measurements reveal that intense femtosecond optical pulses (1.55 eV, 50 fs) induce a pronounced shift of the x-ray absorption maxima associated with transitions to the upper Hubbard bands, while the transition energy into Zhang-Rice singlet states near the Fermi level remains unaffected. Based on exact-diagonalization calculations of the time-dependent spectrum within the single- as well as the three-band Hubbard models, we assign these effects to a pump-induced suppression of the Hubbard U on the Cu sites. Our results represent a first demonstration of dynamically-renormalized Hubbard U in strongly correlated oxides and have significant implications for the on-demand engineering of their magnetic interaction spectrum.   

Structural dynamics in atomic wire systems at surfaces studied by ultrafast-electron diffraction: Excitation, metastable states and relaxation

The (4×1) reconstructed In atomic wires on Si(111) under-go a Peierls like symmetry breaking at T_c = 130 K. During this 1^st order structural transition the system doubles the periodicity along and normal to the wires resulting in a (8×2) ground state [PRB89,121107(2014)]. The structural dynamics upon excitation by fs-IR laser pulses is probed by ultra-fast electron diffraction at a resolution of 350 fs [RSI78,013906(2007), RSI90,045119(2019)]. The excitation results in an accelerated displacive structural transition of the atomic wires to the (4×1) excited state in only 700 fs [Nature 544,207(2017)].

Transient heating of the In atoms from 30 to 60 K occurs delayed at 6 ps [Struct.Dyn.5,025101(2018)]: transition is driven by electronic entropy and not thermally. An energy barrier for the atoms collective motion from the (4×1) to the (8×2) hinders the immediate recovery of the ground state: the system remains for ns in a super cooled metastable (4×1) state inaccessible under equilibrium conditions. Relaxation into the (8×2) ground state happens through nucleation of the (8×2) at pre-existing defects like adsorbates [PRL109,186101(2012), PRL111,149602(2013)] or step edges triggering a recrystallization front propagating at a speed of ~100 m/s [Struct.Dyn.6,045101(2019)].   

Exploring nonequilibrium phases of photo-doped Mott insulators with Generalized Gibbs ensembles

Photo-excited strongly correlated systems can exhibit intriguing non-thermal phases, but the theoretical investigation of them poses significant challenges. In this work, we introduce a generalized Gibbs ensemble type description for long-lived photo-doped states in Mott insulators. This framework enables systematic studies of photo-induced phases based on equilibrium methods, as demonstrated here for the one-dimensional extended Hubbard model. We determine the nonequi- librium phase diagram, which features η-pairing and charge density wave phases in a wide doping range, and reveal physical properties of these phases. We show that the peculiar kinematics of photo-doped carriers, and the interaction between them, play an essential role in the formation of the non-thermal phases, and we clarify the differences between photo-doped Mott insulators, chemically-doped Mott insulators and photo-doped semiconductors. Our results demonstrate a new path for the systematic exploration of nonequilibrium strongly correlated systems and show that photo-doped Mott insulators host different phases than conventional semiconductors.