Bernhard Huber

Phase fluctuations and the absence of topological defects in a photo-excited charge-ordered nickelate

The dynamics of an order parameters amplitude and phase determines the collective behaviour of novel states emerging in complex materials. Time- and momentum-resolved pump-probe spectroscopy, by virtue of measuring material properties at atomic and electronic time scales out of equilibrium, can decouple entangled degrees of freedom by visualizing their corresponding dynamics in the time domain. Here we combine time-resolved femotosecond optical and resonant X-ray diffraction measurements on charge ordered La(1.75)Sr(0.25)NiO(4) to reveal unforeseen photoinduced phase fluctuations of the charge order parameter. Such fluctuations preserve long-range order without creating topological defects, distinct from thermal phase fluctuations near the critical temperature in equilibrium. Importantly, relaxation of the phase fluctuations is found to be an order of magnitude slower than that of the order parameters amplitude fluctuations, and thus limits charge order recovery. This new aspect of phase fluctuations provides a more holistic view of the phases importance in ordering phenomena of quantum matter.

Ultrafast charge localization in a stripe-phase nickelate

Self-organized electronically ordered phases are a recurring feature in correlated materials, resulting in, for example, fluctuating charge stripes whose role in high-TC superconductivity is under debate. However, the relevant cause–effect relations between real-space charge correlations and low-energy excitations remain hidden in time-averaged studies. Here we reveal ultrafast charge localization and lattice vibrational coupling as dynamic precursors of stripe formation in the model compound La1.75Sr0.25NiO4, using ultrafast and equilibrium mid-infrared spectroscopy. The opening of a pseudogap at a crossover temperature T* far above long-range stripe formation establishes the onset of electronic localization, which is accompanied by an enhanced Fano asymmetry of Ni-O stretch vibrations. Ultrafast excitation triggers a sub-picosecond dynamics exposing the synchronous modulation of electron–phonon coupling and charge localization. These results illuminate the role of localization in forming the pseudogap in nickelates, opening a path to understanding this mysterious phase in a broad class of complex oxides.

Ultrafast dynamics of vibrational symmetry breaking in a charge-ordered nickelate

Terahertz pulses reveal the multiscale dynamics of the crystal symmetry upon melting and formation of atomic-scale stripes. The ability to probe symmetry-breaking transitions on their natural time scales is one of the key challenges in nonequilibrium physics. Stripe ordering represents an intriguing type of broken symmetry, where complex interactions result in atomic-scale lines of charge and spin density. Although phonon anomalies and periodic distortions attest the importance of electron-phonon coupling in the formation of stripe phases, a direct time-domain view of vibrational symmetry breaking is lacking. We report experiments that track the transient multi-terahertz response of the model stripe compound La1.75Sr0.25NiO4, yielding novel insight into its electronic and structural dynamics following an ultrafast optical quench. We find that although electronic carriers are immediately delocalized, the crystal symmetry remains initially frozen—as witnessed by time-delayed suppression of zone-folded Ni–O bending modes acting as a fingerprint of lattice symmetry. Longitudinal and transverse vibrations react with different speeds, indicating a strong directionality and an important role of polar interactions. The hidden complexity of electronic and structural coupling during stripe melting and formation, captured here within a single terahertz spectrum, opens new paths to understanding symmetry-breaking dynamics in solids.

Ultrafast mid-infrared spectroscopy of the charge- and spin-ordered nickelates

We discuss the mid-infrared optical response of a charge and spin-ordered nickelate in the ultrafast time domain. A strong photo-induced modulation of the optical reflectivity is observed on the sub-picosecond timescale, indicating the transient filling and recovery of the pseudogap in the mid-infrared charge transport. A variational Kramers-Kronig analysis of equilibrium reflectivity data is extended to time-resolved experiments, allowing us to extract the optical conductivity despite a comparatively limited frequency range of tunable femtosecond parametric sources. The fast dynamics of the spectral weight transfer supports an electronic origin of the mid-infrared pseudogap in nickelates.

Ultrafast multi-terahertz probes of symmetry breaking in a stripe-phase correlated oxide

The application of transient terahertz (THz) pulses to excite and probe low-energy quantum and collective excitations in materials represents a powerful tool to study both intrinsic interactions and non-equilibrium phases. In the following, we discuss ultrafast multi-THz studies that resolve the dynamics of electronic itineracy and vibrational symmetries in a strongly-correlated nickelate. Many transition-metal oxides exhibit the emergence of “stripes,” corresponding to quasione- dimensional charge, spin and lattice modulations as a manifestation of strong correlations. In our experiments, optical excitation of a stripe-phase nickel oxide triggers the rapid melting of its atomic-scale charge order and results in dynamics that yields insight into the couplings underlying the stripes. The transient optical conductivity is sensitive to both charges and in-plane vibrations and reveals a succession of ultrafast processes, ranging from rapid delocalization and localization of charges, via a time-delayed reaction of vibrational distortions to the electronic quench, up to the multi-picosecond re-establishment of the symmetry-broken phase.

Nanoscale charge-order dynamics in stripe-phase nickelates probed via ultrafast THz spectroscopy

We discuss equilibrium and ultrafast optical pump-THz probe spectroscopy of the model stripe-ordered system La1.75Sr0.25NiO4. We present a multi-oscillator analysis of the phonon bending mode splitting observed at low temperatures in equilibrium, along with a variational model for the transient THz reflectivity variations. The low temperature splitting is directly related to the formation of the long-range stripe-order, while the background conductivity is reminiscent of the opening of the mid-IR pseudogap. Ultrafast experiments in the multi-THz spectral range show strong THz reflectivity variations around the phonon bending mode frequency (≈11 THz).

Nonlinear Optics with Rydberg Atoms at Room Temperature

Rydberg atoms have exaggerated properties, which is especially manifest in the optical excitation blockade. Recent experiments show that this interaction is also effective at room temperatures, opening a pathway to room temperature single photon sources.

Tracking charge localization via transient electron-phonon coupling in a stripe-ordered nickelate

We study the low-energy conductivity dynamics after femtosecond perturbation of the stripe-ordered phase in a strongly-correlated nickelate. The experiments reveal ultrafast suppression and recovery of electron-phonon coupling that tracks the atomic-scale localization of correlated charges.

Ultrafast dynamics of the mid-infrared pseudogap in stripe-phase La 1.75 Sr 0.25 NiO 4

We present the first ultrafast mid-infrared study of charge and spin-ordered nickelates. A multi-component dynamics is observed, evidencing the femtosecond decay and formation of the low-energy pseudogap in the optical conductivity.