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Driving strontium titanate ferroelectricHidden phases are metastable collective states of matter that are typically not accessible on equilibrium phase diagrams. Nova et al. used infrared pulses to excite higher-frequency lattice modes that drive the crystal into a metastable ferroelectric phase, a phase that can persist for many hours. X. Li et al. used terahertz fields to drive the soft mode that moves the ions in the crystal into the positions they occupy in the new phase. The ferroelectric phase in this case was transient, lasting on the order of 10 picoseconds. Because these hidden phases can host exotic properties in otherwise conventional materials, the accessibility to and control of such hidden phases may broaden potential functionality and applications.Science, this issue p. 1075, p. 1079Fluctuating orders in solids are generally considered high-temperature precursors of broken symmetry phases. However, in some cases, these fluctuations persist to zero temperature and prevent the emergence of long-range order. Strontium titanate (SrTiO3) is a quantum paraelectric in which dipolar fluctuations grow upon cooling, although a long-range ferroelectric order never sets in. Here, we show that optical excitation of lattice vibrations can induce polar order. This metastable polar phase, observed up to temperatures exceeding 290 kelvin, persists for hours after the optical pump is interrupted. Furthermore, hardening of a low-frequency vibration points to a photoinduced ferroelectric phase transition, with a spatial domain distribution suggestive of a photoflexoelectric coupling.

Photo-excitation at terahertz and mid-infrared frequencies has emerged as an effective way to manipulate functionalities in quantum materials, in some cases creating non-equilibrium phases that have no equilibrium analogue. In K3C60, a metastable zero-resistance phase was observed that has optical properties, nonlinear electrical transport and pressure dependencies compatible with non-equilibrium high-temperature superconductivity. Here we demonstrate a two-orders-of-magnitude increase in photo-susceptibility near 10 THz excitation frequency. At these drive frequencies, a metastable superconducting-like phase is observed up to room temperature. The discovery of a dominant frequency scale sheds light on the microscopic mechanism underlying photo-induced superconductivity. It also indicates a path towards steady-state operation, limited at present by the availability of a suitable high-repetition-rate optical source at these frequencies.There is evidence that K3C60 can host a photo-induced superconducting state. Now, resonant excitation at low frequencies allows this phenomenon at room temperature and low pumping fluence.

In quantum materials, degeneracies and frustrated interactions can have a profound impact on the emergence of long-range order, often driving strong fluctuations that suppress functionally relevant electronic or magnetic phases 1 – 7 . Engineering the atomic structure in the bulk or at heterointerfaces has been an important research strategy to lift these degeneracies, but these equilibrium methods are limited by thermodynamic, elastic and chemical constraints 8 . Here we show that all-optical, mode-selective manipulation of the crystal lattice can be used to enhance and stabilize high-temperature ferromagnetism in YTiO 3 , a material that shows only partial orbital polarization, an unsaturated low-temperature magnetic moment and a suppressed Curie temperature, T c  = 27 K (refs. 9 – 13 ). The enhancement is largest when exciting a 9 THz oxygen rotation mode, for which complete magnetic saturation is achieved at low temperatures and transient ferromagnetism is realized up to T neq  > 80 K, nearly three times the thermodynamic transition temperature. We interpret these effects as a consequence of the light-induced dynamical changes to the quasi-degenerate Ti t 2g orbitals, which affect the magnetic phase competition and fluctuations found in the equilibrium state 14 – 20 . Notably, the light-induced high-temperature ferromagnetism discovered in our work is metastable over many nanoseconds, underscoring the ability to dynamically engineer practically useful non-equilibrium functionalities. All-optical, mode-selective manipulation of the crystal lattice can be used to enhance and stabilize ferromagnetism in YTiO 3 well above its equilibrium ordering temperature and for many nanoseconds, enabling dynamic engineering of practically useful non-equilibrium functionalities in fluctuating electronic systems.

We use coherent midinfrared optical pulses to resonantly excite large-amplitude oscillations of the Si–C stretching mode in silicon carbide. When probing the sample with a second pulse, we observe parametric optical gain at all wavelengths throughout the reststrahlen band. This effect reflects the amplification of light by phonon-mediated four-wave mixing and, by extension, of optical-phonon fluctuations. Density functional theory calculations clarify aspects of the microscopic mechanism for this phenomenon. The high-frequency dielectric permittivity and the phonon oscillator strength depend quadratically on the lattice coordinate; they oscillate at twice the frequency of the optical field and provide a parametric drive for the lattice mode. Parametric gain in phononic four-wave mixing is a generic mechanism that can be extended to all polar modes of solids, as a means to control the kinetics of phase transitions, to amplify many-body interactions or to control phonon-polariton waves.

Nonlinear phononics relies on the resonant optical excitation of infrared-active lattice vibrations to induce targeted structural deformations in solids. This form of dynamical crystal structure design has been applied to control the functional properties of many complex solids, including magnetic materials, superconductors and ferroelectrics. However, phononics has so far been restricted to protocols in which structural deformations occur within the optically excited volume, sometimes resulting in unwanted heating. Here, we extend nonlinear phononics to propagating polaritons, spatially separating the functional response from the optical drive. We use mid-infrared optical pulses to resonantly drive a phonon at the surface of ferroelectric LiNbO 3 . Time-resolved stimulated Raman scattering reveals that the ferroelectric polarization is reduced over the entire 50 µm depth of the sample, far beyond the micrometre depth of the evanescent phonon field. We attribute this effect to the anharmonic coupling between the driven mode and a polariton that propagates into the material. For high excitation amplitudes, we reach a regime in which the ferroelectric polarization is reversed, as revealed by a sign change in the Raman tensor coefficients of all the polar modes. Nonlinear phononics is a method for creating transient structural changes in solids, but its effect is limited to the region of optical excitation. Now, coupling to a propagating polariton allows nonlinear phononics to drive a nonlocal response.

Crystal lattice fluctuations, which are known to influence phase transitions of quantum materials in equilibrium, are also expected to determine the dynamics of light-induced phase changes. However, they have only rarely been explored in these dynamical settings. Here we study the time evolution of lattice fluctuations in the quantum paraelectric SrTiO 3 , in which mid-infrared drives have been shown to induce a metastable ferroelectric state. Crucial in these physics is the competition between polar instabilities and antiferrodistortive rotations, which in equilibrium frustrate the formation of long-range ferroelectricity. We make use of high-intensity mid-infrared optical pulses to resonantly drive the Ti–O-stretching mode at 17 THz, and we measure the resulting change in lattice fluctuations using time-resolved X-ray diffuse scattering at a free-electron laser. After a prompt increase, we observe a long-lived quench in R-point antiferrodistortive lattice fluctuations. Their enhancement and reduction are theoretically explained by considering the fourth-order nonlinear phononic interactions to the driven optical phonon and third-order coupling to lattice strain, respectively. These observations provide a number of testable hypotheses for the physics of light-induced ferroelectricity. Intense light pulses can induce symmetry breaking, as for the generation of ferroelectricity in SrTiO 3 . Using ultrafast X-ray diffuse scattering at a free-electron laser, nonlinear phonon interactions that occur on such mid-IR excitation are observed, with a theory for the dynamics presented.

Femtosecond X-ray diffraction and ab initio density functional theory calculations are used to determine the crystal structure of YBa 2 Cu 3 O 6.5 undergoing optically driven, nonlinear lattice excitation above the transition temperature of 52 kelvin, under which conditions the electronic structure of the material changes in such a way as to favour superconductivity. Structure of superconducting YBa 2 Cu 3 O 6+ x Andrea Cavalleri and colleagues use femtosecond X-ray diffraction measurements and ab initio density functional theory calculations to determine the crystal structure of YBa 2 Cu 3 O 6+ x undergoing optically driven, nonlinear lattice excitation at 100 kelvin. In this exotic non-equilibrium state, the electronic structure of the material changes in such a way as to favour superconductivity. The results reveal that in the driven state the superconducting planes are displaced closer and away from one another in a staggered manner, explaining how superconducting coupling can be enhanced or reduced, inside and between the bilayers. Terahertz-frequency optical pulses can resonantly drive selected vibrational modes in solids and deform their crystal structures 1 , 2 , 3 . In complex oxides, this method has been used to melt electronic order 4 , 5 , 6 , drive insulator-to-metal transitions 7 and induce superconductivity 8 . Strikingly, coherent interlayer transport strongly reminiscent of superconductivity can be transiently induced up to room temperature (300 kelvin) in YBa 2 Cu 3 O 6+ x (refs 9 , 10 ). Here we report the crystal structure of this exotic non-equilibrium state, determined by femtosecond X-ray diffraction and ab initio density functional theory calculations. We find that nonlinear lattice excitation in normal-state YBa 2 Cu 3 O 6+ x at above the transition temperature of 52 kelvin causes a simultaneous increase and decrease in the Cu–O 2 intra-bilayer and, respectively, inter-bilayer distances, accompanied by anisotropic changes in the in-plane O–Cu–O bond buckling. Density functional theory calculations indicate that these motions cause drastic changes in the electronic structure. Among these, the enhancement in the character of the in-plane electronic structure is likely to favour superconductivity.

The properties of organic conductors are often tuned by the application of chemical or external pressure, which change orbital overlaps and electronic bandwidths while leaving the molecular building blocks virtually unperturbed. Here, we show that, unlike any other method, light can be used to manipulate the local electronic properties at the molecular sites, giving rise to new emergent properties. Targeted molecular excitations in the charge-transfer saltκ−(BEDT−TTF)2Cu[N(CN)2]Brinduce a colossal increase in carrier mobility and the opening of a superconducting optical gap. Both features track the density of quasiparticles of the equilibrium metal and can be observed up to a characteristic coherence temperatureT*≃50K, far higher than the equilibrium transition temperatureTC=12.5K. Notably, the large optical gap achieved by photoexcitation is not observed in the equilibrium superconductor, pointing to a light-induced state that is different from that obtained by cooling. First-principles calculations and model Hamiltonian dynamics predict a transient state with long-range pairing correlations, providing a possible physical scenario for photomolecular superconductivity.

In many theoretical models of high-temperature superconductors, remnants of superconductivity persist to temperatures higher than the transition temperature, T C . Rajasekaran et al. used nonlinear terahertz spectroscopy to probe this region of the phase diagram of a cuprate superconductor that is well known for a stripe phase that appears for certain doping levels (see the Perspective by Ergeçen and Gedik). For a sample deep in the stripe phase, a large nonlinear signal persisted from the superconducting region up to temperatures much higher than T C . The findings suggest the formation of a peculiar spatially modulated superconducting state called the pair-density wave. Science , this issue p. 575 ; see also p. 519 Nonlinear terahertz spectroscopy data on La 2− x Ba x CuO 4 are consistent with the presence of a pair density wave state. Unconventional superconductivity in the cuprates coexists with other types of electronic order. However, some of these orders are invisible to most experimental probes because of their symmetry. For example, the possible existence of superfluid stripes is not easily validated with linear optics, because the stripe alignment causes interlayer superconducting tunneling to vanish on average. Here we show that this frustration is removed in the nonlinear optical response. A giant terahertz third harmonic, characteristic of nonlinear Josephson tunneling, is observed in La 1.885 Ba 0.115 CuO 4 above the transition temperature T c = 13 kelvin and up to the charge-ordering temperature T co = 55 kelvin. We model these results by hypothesizing the presence of a pair density wave condensate, in which nonlinear mixing of optically silent tunneling modes drives large dipole-carrying supercurrents.

Ultrashort mid-infrared laser pulses can drive atoms far from their equilibrium positions in LiNbO 3 , exciting high phonon harmonics and providing a way to map the interatomic potential. Solid display of phonon harmonics High-harmonic generation of electromagnetic radiation is a well-known example of a nonlinear process. It occurs when photons—usually from a strong laser pulse—interact with nonlinear systems, such as a gas, plasma or solid, in a way that generates new photons with energies that are multiples of the original. This idea can also be applied to phonons—the quasiparticles associated with lattice vibrations—but high-order phonon modes are much harder to generate. Andrea Cavalleri and colleagues now show that ultrashort mid-infrared laser pulses can induce field strengths in lithium niobate (LiNbO 3 ) that are large enough to drive atoms far away from their equilibrium positions. Such strong fields can excite up to five phonon harmonics and provide a way to map the interatomic potential, which can be used to benchmark ab initio calculations. Nonlinear optical techniques at visible frequencies have long been applied to condensed matter spectroscopy 1 . However, because many important excitations of solids are found at low energies, much can be gained from the extension of nonlinear optics to mid-infrared and terahertz frequencies 2 , 3 . For example, the nonlinear excitation of lattice vibrations has enabled the dynamic control of material functions 4 , 5 , 6 , 7 , 8 . So far it has only been possible to exploit second-order phonon nonlinearities 9 at terahertz field strengths near one million volts per centimetre. Here we achieve an order-of-magnitude increase in field strength and explore higher-order phonon nonlinearities. We excite up to five harmonics of the A 1 (transverse optical) phonon mode in the ferroelectric material lithium niobate. By using ultrashort mid-infrared laser pulses to drive the atoms far from their equilibrium positions, and measuring the large-amplitude atomic trajectories, we can sample the interatomic potential of lithium niobate, providing a benchmark for ab initio calculations for the material. Tomography of the energy surface by high-order nonlinear phononics could benefit many aspects of materials research, including the study of classical and quantum phase transitions.