Light control of orbital domains: case of the prototypical manganite La_0.5Sr_1.5MnO₄

Abstract

Control of electronic and structural ordering in correlated materials on the ultrafast timescale with light is a new and emerging approach to disentangle the complex interplay of the charge, spin, orbital and structural degree of freedom. In this paper we present an overview of how orbital order and orbital domains can be controlled by near IR and THz radiation in the layered manganite La 0.5 Sr 1.5 MnO 4 . We show how near-IR pumping can efficiently and rapidly melt orbital ordering. However, the nanoscale domain structure recovers unchanged demonstrating the importance of structural defects for the orbital domain formation. On the contrary, we show that pulsed THz fields can be used to effectively orientate the domains. In this case the alignment depends on the in-plane electric field polarization and is induced by an energy penalty that arises from THz field induced hopping of the localized charges.

Ultrafast X-ray imaging of the light-induced phase transition in VO2

“…A key challenge to understanding these phases is therefore to isolate the photo-induced state and to directly probe its properties at the nanoscale. Resonant coherent diffraction from electronically ordered states has been used to infer the statistical properties of a domain [16][17][18] , but real-space images have not been produced. Here we use time-and energy-resolved coherent resonant soft X-ray imaging to observe the ultrafast insulator-metal phase transition in the prototypical quantum material vanadium dioxide (VO 2 ), over a macroscopic area with sub-50 nm spatial resolution and 150 fs time resolution, returning full spectroscopic information on transient states at the nanoscale.…”

Multi-mode excitation drives disorder during the ultrafast melting of a C4-symmetry-broken phase

“…The phase transition involves the condensation of multiple phonon modes, primarily of oxygen character, which break the equivalence (charge ordering, CO) and isotropy (orbital ordering, OO) of the Mn environment, changing the space group from I4/mmm to Cmnm and quadrupling the unit cell 17 . The reduction to C2 symmetry manifests in an electronic anisotropy and optical birefringence 18 . Two coefficients 𝑟 1 and 𝑟 2 , the reflectivity parallel and perpendicular to the anisotropy axis, fully describe the ab plane reflectivity.…”

Inhomogeneity and disorder in ultrafast phase transitions

“…In direct contrast, the normalized response of the order parameter shown in Figure 4.6a and 4.9c shows that the order parameter actually speeds up as the system approaches the phase transition. We note that the reflectivity dynamics do slow down when pumped across the phase transition, and has been interpreted as a signature of critical slowdown [20] or a bottle-neck timescale [65,87,90] in other materials, but here is found to be unrelated to the order parameter. Notably the isotropic reflectivity shows significant probe wavelength dependence, while the anisotropic dynamics do not (Figure 4.11), further indicating the reflectivity measures fundamentally different effects.…”

“…This makes in-plane optical anisotropy, birefringence in this case, a suitable probe of the phase transition in LSMO as it directly measures the change in symmetry of the material. Indeed, optical birefringence has been used to successfully observe the phase transition in LSMO [65][66][67], however this possibility has not been exploited as extensively as diffraction techniques and the current available results offer wide room for experimental improvements and a wider scope. The experimental technique used in [53] is based on detection of in-plane THz birefringence in thin films and the authors note the value of trying similar measurements with single crystals and near-infrared light, which is in direct connection with the experiments presented in this thesis.…”

“…In order to acquire further insight on the transition and perhaps establish a hierarchy among the seemingly concomitant changes in the different degrees of freedom, the possibility of taking the system out of equilibrium was explored and, as a result, today there are a wide variety of theoretical and experimental studies on transient phenomena in the manganites. To take advantage of the ever-improving ultrafast pump-probe techniques most of the experiments have focused on the photoexcitation of the low-temperature CO-OO phase [11,13,[65][66][67][87][88][89][90][91][92][93]. In this section we will briefly mention some of the key results obtained in the last couple of decades from ultrafast photoexcitation experiments in the manganites, that are particularly relevant for this thesis.…”