Session G70: Light-Induced Structural Control of Electronic Phases II

Nonlocal Nonlinear Phononics

Nonlinear phononics in solids involves the resonant optical excitation of infrared-active lattice vibrations to a regime in which they can drive other modes through anharmonic coupling. This unique form of selective lattice control has led to the ultrafast manipulation of superconductivity, magnetism, and ferroelectricity in complex materials. However, studies in nonlinear phononics have thus far focused on the response of the material within the few-micron depth of the optically excited volume. In my talk, I will show that functional control through nonlinear phononics extends to polarization waves which propagate well-beyond the excited region. In our experiments, mid-infrared optical pulses were used to resonantly excite an 18 THz phonon below the surface of ferroelectric LiNbO3. A time-resolved stimulated Raman scattering probe revealed that the ferroelectric polarization was manipulated over the entire 50 μm depth of the sample, far beyond the few μm depth of the optically-driven phonon. At the highest excitation fluence, we observed a transient reversal of the ferroelectric polarization. These results build upon a paradigm for nonlocal material control in which the functional response of a material is altered in a region that is untraversed by the optical driving field.   

Exploring light-induced ferroelectricity in SrTiO3: phonon-phonon coupling as a function of epitaxial strain

Recent experiments have shown a transient ferroelectric phase is accessible through optical excitation of infrared active (IR) phonons in bulk, paraelectric SrTiO₃ [Science Vol 364, Issue 6445 pp. 1075-1079 and 1079-1082 (2016)]. One explanation for this behavior is that when the lowest frequency IR phonon is excited, a transient lattice strain induced by the displaced IR phonon stabilizes the ferroelectric phase [arXiv:2106.03957]. We further explore a complementary pathway of phonon-phonon coupling for stabilization of the ferroelectric phase using theory and first-principles calculations (see also [PRB 95, 134113 (2017)]). Our preliminary results suggest that epitaxial strain provides a handle for tuning the optically induced ferroelectric phase transition in SrTiO₃.   

Photo-induced strain in ferroelectric BiFeO3 by resonant phonon excitation

Strong-field mid-infrared or terahertz laser pulses tuned to specific infrared-active phonons can directly modify structural parameters which are crucial to the physical properties of quantum materials. It provides a unique platform to engineer desired phases which may not exist at thermal equilibrium. Here we use time-resolved optical pump-probe spectroscopy to study the lattice dynamics in ferroelectric BiFeO₃. With resonant excitation of a mid-infrared optical phonon, an oscillatory response in the ellipticity change of the probe polarization is observed, indicating coherent excitation of an acoustic phonon. This response is absent when the pump frequency is tuned away from the optical phonon.   

THz electric field-driven dynamical multiferroicity in paraelectric SrTiO3

The emergence of collective order in matter is among the most fundamental and intriguing phenomena in physics. In recent years, the ultrafast dynamical control and creation of novel ordered states of matter not accessible in thermodynamic equilibrium is receiving much attention. Among those, the theoretical concept of dynamical multiferroicity has been introduced to describe the emergence of magnetization by means of a time-dependent electric polarization in non-ferromagnetic materials^1,2. In simple terms, a large amplitude coherent rotating motion of the ions in a crystal induces a magnetic moment along the axis of rotation. However, the experimental verification of this effect is still lacking. Here, we provide the first evidence of room temperature magnetization in the archetypal paraelectric perovskite SrTiO3 due to this mechanism. To achieve it, we resonantly drive the infrared-active soft phonon mode with an intense circularly polarized terahertz electric field and detect a large magneto-optical Kerr effect. A simple model, which includes two coupled nonlinear oscillators whose forces and couplings are derived with ab-initio calculations using self-consistent phonon theory at a finite temperature³, reproduces qualitatively our experimental observations on the temporal and frequency domains. A quantitatively correct magnitude of the effect is obtained when one also considers the phonon analog of the reciprocal of the Einstein de Haas effect, also called the Barnett effect, where the total angular momentum is transferred from the coherent phonon motion to the electrons. Our findings show a new path for designing ultrafast magnetic switches by means of coherent control of lattice vibrations with light. 

¹ Phys. Rev. Mat. 1, 014401 (2017)

² Phys. Rev. Mat. 3, 064405 (2019)

³ Phys. Rev. Mat. 3, 033601 (2019)

    

Electron Correlation Effects and High-Order Harmonic Spectrum of Perovskite BiFeO3

We present the results of our analysis of the role of electron-electron correlations in the high-order harmonic generation (HHG) spectrum generated in BiFeO₃. The results were obtained by using the combined dynamical mean-field theory and time-dependent density-functional theory (DMFT-TDDFT) approach. Namely, the electronic spectral properties of the system were analyzed by using DFT+DMFT that takes into account the effects of strong electron correlations. Then, the TDDFT exchange-correlation kernel was calculated from the DMFT charge susceptibility and was used in the TDDFT analysis of the response of the perturbed system, with a focus on the HHG. We found that correlation (in addition to memory) effects increase the number of harmonics and the inter-band dynamics contribute more to the correlation-induced HHG compared to the intra-band one. Furthermore, correlation effects suppress emission when the pulse is directed along certain crystal axes, which can be explained by a longer free path of electrons and hence weaker dynamical correlations. The results may help to shed light on the role of correlation effects in the optical properties of strongly correlated materials like BiFeO₃, one of the most important perovskites.    

Expansive Open Fermi Arcs and Connectivity Changes Induced by Infrared Phonons in ZrTe5

Expansive open Fermi arcs covering most of the surface Brillouin zone (SBZ) are desirable for detection and control of many topological phenomena, but so far has been only reported for Kramers-Weyl points, pinned at time-reversal invariant momentum in chiral materials. Here from first-principles calculations, the conventional Weyl points in ZrTe₅ with the chirality of +1/–1 near the BZ center at general momentum are shown to also form expansive open Fermi arcs across the SBZ boundary to occupy most of the SBZ, which is induced by one of the infrared phonons, the second lowest B_1u mode for breaking inversion symmetry. Such expansive open Fermi arcs are revealed to evolve from the topological surface states that connect multiple surface Dirac points on the (001) surface of the topological insulator phase without lattice distortion in ZrTe₅. Furthermore, the connectivity of the induced open Fermi arcs can be changed by the magnitude of the lattice distortion of this infrared phonon mode. Thus, coherent optical phonon can be used to modulate lattice distortion to induce novel topological features including expansive open Fermi arcs and also dynamically control Fermi arcs connectivity in ZrTe₅.   

Influence of local symmetry on lattice dynamics of topological surface states

We investigate coupled electron-lattice dynamics in the topological insulator Bi₂Te₃ with time- and angle-resolved photoemission spectroscopy (trARPES). It is well established that coherent phonons can be launched by optical excitation, but selection rules typically restrict these modes to specific wavevectors (q~0) and symmetries (Raman-active).  We find that the topological surface state in Bi₂Te₃ couples to coherent modes that are forbidden in the bulk, including infrared active phonons as well as those with non-zero wavevectors. Our calculations show that these behaviors naturally arise as a consequence of the translational and inversion symmetries broken at the surface. These effects are important to consider when interpreting ultrafast experiments, and may expand the phase space for tailoring material properties on-demand.   

Ultrafast light-induced strain and symmetry breaking in multiferroic BiFeO3

The understanding of the lattice dynamics in ferroic compounds driven by an ultrashort light pulse is an exciting research direction due to the exceptional non-linear properties (optical, elastic, electric and magnetic) of ferroic and multiferroic materials [1-5]. Photo-induced strain in ferroic materials is driven by a complex interplay between charge, phonon and spin dynamics with microscopic mechanisms that still need to be determined [5-10]. We present recent experiments where ultrafast photoinduced strain is evaluated in BiFeO3-based multiferroic materials, with a focus on the description of the ultrafast symmetry change of the unit-cell that appears after an ultrashort laser pulse. A combination of optical and X-ray time-resolved techniques enables to clearly demonstrate how the light excitation can lead to a modulation of the symmetry in ferroic materials [11]. By studying two asymmetric Bragg reflections (i.e. h0l and -h0l for instance) of a (001)c single BiFeO3 crystal, we show how it is possible to disentangle at the picosecond time scale the light-induced longitudinal and shear strain in the unit cell [11]. Due to a difference between the value of the longitudinal and shear velocities, the strain develops within a two-step mechanism. This temporal evolution of the strain within the unit cell highlights the transient symmetry breaking of the pseudocubic unit cell of BiFeO3. Moreover, recent experiments with thin films of multiferroic nanostructures having different ferroelectric domain organizations and different elastic and electrostatic boundary conditions will be presented. In these systems, we discuss the different physical mechanisms at play in the light-induced strain and in the ferroelectric polarization modulation. These results provide insights for the understanding of the physics of photo-induced strain including the modulatuon of the ferroelectricity with the light.   

Photodynamics of optical excitations in Mott insulators via differential optical conductivity

We examine the nonequilibrium optical response of the one-dimensional half-filled extended Hubbard model to a pump field pulse. We derive, compute, and analyze the differential optical conductivity, which is related to the time-resolved optical conductivity: a key quantity in pump and probe spectroscopy experiments. Using time-dependent density matrix renormalization group, we calculate the differential optical conductivity and detect a concurrent photoexcitation of doublons and holons along with a reduction of the local magnetic moment and antiferromagnetic correlations. The differential optical conductivity exhibits two main features: (1) photogeneration of mid-gap spectral weight associated to parity-forbidden optical states, and (2) melting of the excitonic peak and emergence of a Fano-like optical resonance due to quantum interference of the excitons and the absorption band. The resulting nonequilibrium optical excitations correspond to renormalized excitonic strings, excitons, or unbound doublon-holon pairs, upon decreasing of intersite Coulomb repulsion, respectively. Our results have direct relevance to pump and probe spectroscopy experiments in the THz domain performed on organic salts such as ET-F₂TCNQ, where quantum interference between excitons and absorption-band states give rise to nonequilibrium optical excitations and photometallization.   

Mode-selective control over the structural phase transition in atomic indium wires on silicon

Driving a phase transition along a nonequilibrium pathway via optical control of coherent phonons has proven powerful in controlling electronic and structural phases on ultrafast time scales [1]. In this regard, charge density wave systems have emerged as natural targets as their amplitude modes are directly linked with the associated structural phase transition. Coherent manipulation of these modes thus allows to guide the system from one state to another and provides access to the complex geometry of the underlying potential energy landscape. Here, using ultrafast low-energy electron diffraction and tailored pulse sequences, we demonstrate mode-selective control over the phase transition of In/Si(111) [2,3]. We track key vibrational modes along transient trajectories and reveal characteristics of the underlying potential landscape, corroborated by DFT calculations. The manipulation of vibrational coherences provides a playground for exploring nonequilibrium dynamics in strongly correlated systems and promises the active selection of their properties.

[1] Nova, T. F et al., Science 364, 1075–1079 (2019); [2] Horstmann, J. G. et al., Nature 583, 232-236 (2020); [3] Böckmann, H. et al., arXiv:2108.13966   

Nonlinear Phononics in Hybrid Lead Halide Perovskites

The microscopic origin of the surprising performance of hybrid lead halide perovskite (LHP) semiconductors for optoelectronic devices is still under debate. It has been suggested that their highly polarizable and anharmonic lattice might beneficially govern their optoelectronic properties in the form of dynamic charge carrier screening. To study the LHP’s ultrafast lattice response when subjected to a transient electric field, we employ intense, close to single-cycle, THz fields exceeding 1.5 MV/cm. By probing the THz induced Kerr effect, we witness the nonlinear excitation of coherent phonons of the inorganic cage in CsPbBr₃ and in the hybrid MAPbBr₃ in their low-temperature orthorhombic phase. We discuss different possible nonlinear photonic and phononic excitation pathways and their implications on dynamic charge carrier screening. Identifying the dominating polarizable and/or anharmonic phonon modes might unveil a fundamental mechanism how lattice dynamics enable charge carrier protection, even in more exotic electronic phases.