Nicole A. Benedek

There is significant technological interest in developing ever faster switching between different electronic and magnetic states of matter. Manipulating properties at terahertz rates requires accessing the intrinsic timescales of both electrons and associated phonons, which is possible with short-pulse photoexcitation. However, in many Mott insulators, the electronic transition is accompanied by the nucleation and growth of percolating domains of the changed lattice structure, leading to empirical timescales dominated by slowly coarsening dynamics. Here we use time-resolved X-ray diffraction and reflectivity measurements to show that the photoinduced insulator-to-metal transition in an epitaxially strained Mott insulating thin film occurs without observable domain formation and coarsening effects, allowing the study of the intrinsic electronic and lattice dynamics. Above a fluence threshold, the initial electronic excitation …

We use theory and first-principles calculations to explore mechanisms for control of the translational and point group symmetries of crystals in ultrafast optical experiments. We focus, in particular, on mechanisms that exploit anharmonic (biquadratic) lattice couplings between a driven infrared-active phonon mode and other modes at arbitrary wave vector, which are always allowed by symmetry in any space group. We use Floquet theory to develop a general phase diagram depicting the various dynamical regimes accessible to materials, with simulated dynamics to illustrate how the biquadratic coupling changes materials structures depending on both extrinsic factors (light pulse characteristics) and intrinsic materials parameters (phonon frequencies and phonon coupling strengths). We use our phase diagram, in conjunction with density functional theory calculations, both to suggest experiments to reveal hidden …

Advancements of high-power laser sources in the THz frequency range have opened up opportunities for coherent intense excitation of infrared (IR)-active phonons in crystalline materials. The nonlinear phononics mechanism utilizes anharmonic coupling between driven IR-active modes and other lattice modes to induce changes in crystal structure and functional properties on picosecond timescales. Recent experimental and theoretical works have noted the potential for phonon-induced strains on these short time scales [npj Quantum Mater. 5, 95 (2020), PRL 129, 167401 (2022)]. A natural way to measure the response of the lattice due to these THz pulses is by employing X-ray diffraction to observe changes to the position and intensities of Bragg peaks.We show, using theory and first-principle calculations, that these lattice anharmonicities are responsible for the expansion of the c-axis in a biaxially strained thin …

Recent experiments have demonstrated the potential for modifying the properties of materials using ultrafast optical pulses to selectively and coherently excite particular phonon modes. One such mechanism involves optical excitation of an IR-active phonon Q IR, which produces a displacement of a Raman-active mode Q R due to a special anharmonic coupling between the modes, Q 2 IR Q R. This nonlinear phononic effect, and the subtle structural changes it induces, has been invoked to interpret, for example, observations of a five-orders-of-magnitude decrease in resistivity in Pr 0.7 Ca 0.3 MnO 3 (a material that is insulating at equilibrium at all measured temperatures), and the observation of room temperature superconductivity in YBa 2 Cu 3 O 6.5. The nonlinear phononics mechanism essentially exploits the strong coupling between an optically excited IR-active mode and some order parameter of the system …

Materials that exhibit structural ferroaxial order hold potential for novel multiferroic applications. However, in pure ferroaxials, domains are not directly coupled to stress or static electric field due to their symmetry, limiting the ability to pole the crystal and switch between domains. Here we propose a general optical approach to transiently excite single-domain ferroaxial order from a high-symmetry non-ferroaxial phase. We show that circularly polarized light pulses on resonance with selected infrared-active phonons dynamically induce helicity-controlled single-domain ferroaxial order. Nonlinear contributions to the polarizability play an essential role in this phenomenon. We illustrate the feasibility of our approach using the prototypical ferroaxial material RbFe (MoO 4) 2. Our first-principle calculations and dynamical simulations show a fluence threshold, beyond which noticeable single-domain ferroaxial order is …

The development of high-power laser sources in the THz frequency range has opened up opportunities for large-amplitude excitation of infrared (IR)-active phonon modes in materials. The nonlinear phononics mechanism relies on the anharmonic coupling between driven IR-active modes and other lattice modes to induce changes in crystal structure and functional properties on picosecond timescales. Most nonlinear phononics experiments to date have focused on the coupling between phonon modes at the center of the Brillouin zone. We show, using theory and first-principle calculations, that coupling between IR modes at the zone center and other modes at nonzero wavevector can be exploited to dynamically induce materials phases with translational symmetry that differs from that of the equilibrium crystal structure. We focus on KTaO 3, a cubic perovskite with no known structural phase transitions as a …

Advances in mid and far-infrared THz sources have created a new paradigm in condensed matter physics: ultrafast structural and functional control through direct lattice excitation. Striking changes in magnetism, metallicity, ferroelectricity, and superconductivity, observed experimentally on ultrafast timescales, have been tied to the anharmonic coupling between pumped infrared-active (IR) phonons and Raman-active phonons via the nonlinear phononics effect. This nonlinear phononics pathway elevates coupling between two zero-wavevector phonons-the IR and Raman phonons-above the vast scattering phase space allowed in the Brillouin zone of crystalline materials. In our theoretical exploration of energy transfer from large amplitude excitations of IR phonons we have found that strong coupling to non-zero wavevector phonons-those that are not optically active-is common and can dominate the lattice …

The nonlinear phononics mechanism involves the coherent optical excitation of infrared-active phonons that, via anharmonic coupling, induce large quasistatic, unidirectional displacement of Raman-active phonons. This process occurs on sub-picosecond timescales. On longer timescales, the motion of the IR phonon is damped and the system reaches a higher temperature via several energy and momentum transferring processes: electron-electron, phonon-phonon, and defect scattering. What are the pathways by which an excited IR-active phonon transfers energy to the other lattice degrees of freedom? We use first-principles theory together with symmetry considerations to explore phonon-phonon scattering in LaAlO3, an experimentally important nonlinear phononics material. We explore ways to quantify the contribution of low frequency modes in energy transfer, and attempt to better understand the time scales …

Raman scattering via ionic dipoles is a forgotten pathway that produces giant optical nonlinearities in crystals when a field is IR resonant, offering ultrafast control of optical symmetry and constants from THz to visible frequencies.

Recent experiments have shown a transient ferroelectric phase is accessible through optical excitation of infrared active (IR) phonons in bulk, paraelectric SrTiO 3 [1]. 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 [2]. We further explore a complementary pathway of phonon-phonon coupling for stabilization of the ferroelectric phase using theory and first-principles calculations (see also [3]). Our preliminary results suggest that epitaxial strain provides a handle for tuning the optically induced ferroelectric phase transition in SrTiO 3.

We predict a new pathway for ultrafast coherent control of fully symmetric Raman phonons, allowing manipulation of sign, magnitude, and temporal response, enabled by the nonlinear lattice polarizability and a two-terahertz-pulse excitation sequence.

Driving the lattice structure with resonant phononic excitations presents a novel way for controlling quantum materials. We present the results of an ultrafast diffraction experiment using THz resonant excitation to drive an IR mode in a thin, strained LaAlO 3 epitaxial film. Our results explore predicted and measured ultrafast, IR light induced structural processes in the film. We do not observe the prediction that nonlinear phonon coupling excites a giant ultrafast response in the lowest frequency Raman phonon. The ultrafast structural phase transition associated with this Raman phonon is not observed. However, our results show that the IR pump induces the growth of structural domains on the timescale of several picoseconds. Moreover, we find that a longitudinal acoustic phonon forms in the films, the period of which is several picoseconds and is determined by the film thickness. Finally, we observe that the film …

We review the theoretical, computational, and synthetic literature on hybrid improper ferroelectricity in layered perovskite oxides. Different ferroelectric mechanisms are described and compared, and their elucidation using theory and first-principles calculations is discussed. We also highlight the connections between crystal chemistry and the physical mechanisms of ferroelectricity. The experimental literature on hybrid improper ferroelectrics is surveyed, with a particular emphasis on cation-ordered double perovskites, Ruddlesden–Popper and Dion–Jacobson phases. We discuss preparative routes for synthesizing hybrid improper ferroelectrics in all three families and the conditions under which different phases can be stabilized. Finally, we survey some synthetic opportunities for expanding the family of hybrid improper ferroelectrics.

PbTiO 3 is an important technological material due to its high-temperature ferroelectricity, piezoelectricity, and negative thermal expansion. Previous theoretical work has predicted an anomalously large elastic compliance in PbTiO 3 that can be induced by negative pressure [1], stress [2, 3], or strain [3], and could potentially be exploited in applications requiring large tailored piezoelectric and elastic responses. The microscopic mechanism of this anomalous behavior remains unknown but has been hypothesized to involve breaking of the Ti-O bond along the polarization direction.[1] In this work, we use a newly developed first-principles-based energy partitioning procedure to show that the microscopic origin of the elastic anomaly in PbTiO 3 is a competition between the ionic lattice response and collective electronic interactions, and is surprisingly dominated by the Pb-O bond, rather than the Ti-O bond. Our insights …

When driven at infrared resonances, crystals possess giant Raman scattering susceptibilities due to nonlinear lattice coupling in the polarizability. A first-principles study of SrTiO3 reveals Stokes and anti-Stokes peaks in the cubic susceptibility with values >10^(-15) m^2/V^2, and large values of chi^(3) representing cross-phase and cross-amplitude modulation that persist from THz to visible frequencies. This implies a new route for achieving strong nonlinear frequency conversion in the infrared over short length scales, and a route for strong light-driven control of material optical properties on ultrafast timescales covering a hyperspectral range. We additionally discuss implications for non-centrosymmetric materials such as LiNbO3.

We use theory and first-principles calculations to investigate how structural changes induced by ultrafast optical excitation of infrared-active phonons change with hydrostatic pressure in LaAlO 3. Our calculations show that the observed structural changes are sensitive to pressure, with the largest changes occurring at pressures near the boundary between the cubic perovskite and rhombohedral phases. We rationalize our findings by defining a figure of merit that depends only on intrinsic materials quantities, and show that the peak response near the phase boundary is dictated by different microscopic materials properties depending on the particular phonon mode being excited. Our work demonstrates how it is possible to systematically identify materials that may exhibit particularly large changes in structure and properties due to optical excitation of infrared-active phonons.

Recent experiments have demonstrated the potential for ultrafast changes in the functional properties of materials upon selective optical excitation of particular phonon modes. The chemical diversity of complex oxides, and their strong lattice-properties coupling, have made them ideal test systems for new experimental approaches that exploit anharmonic phonon couplings to induce and modify magnetism, superconductivity and ferroelectricity with light. In this talk, I will describe our recent theoretical efforts exploring ultrafast optical control of the functional properties of perovskite oxides. I will focus on two examples in particular: dynamical stabilization of a non-equilibrium magnetic phase in GdTiO 3, and transient switching of ferroelastic domains in LaAlO 3 under realistic experimental conditions. Our work highlights the importance of understanding the contributions of small structural distortions to the optical …

An unconventional Raman scattering pathway via the nonlinear lattice polarizability can dominate the optical response of crystals driven resonantly with IR phonons, allowing giant optical nonlinearities and ultrafast dynamical control of material optical properties.

Nonlinear phononics provides an avenue for ultrafast control of crystal structure and properties. Anharmonic coupling between IR and Raman phonons in the lattice potential opens pathways for light to induce sizable unidirectional excitations of Raman phonons, altering the crystal structure and therefore its properties. The magnitude of the change in structure and properties is controlled by a combination of key intrinsic microscopic quantities. Strategies for tuning these microscopic quantities, and for engineering an enhanced response, are currently unknown. Using theory and first-principles calculations, we investigate how the nonlinear phononics response evolves with pressure and strain in SrTiO 3 and LaAlO 3. Specifically, we track how the strength of the anharmonic coupling between, and force constants of, the IR and Raman modes, and the strength of the coupling between the light pulse and the excited IR …

Spin-phonon interaction is an important channel for spin and energy relaxation in magnetic insulators. Understanding this interaction is critical for developing magnetic insulator-based spintronic devices. Quantifying this interaction in yttrium iron garnet (YIG), one of the most extensively investigated magnetic insulators, remains challenging because of the large number of atoms in a unit cell. Here, we report temperature-dependent and polarization-resolved Raman measurements in a YIG bulk crystal. We first classify the phonon modes based on their symmetry. We then develop a modified mean-field theory and define a symmetry-adapted parameter to quantify spin-phonon interaction in a phonon-mode specific way in YIG. Based on this improved mean-field theory, we discover a positive correlation between the spin-phonon interaction strength and the phonon frequency.