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

Abstract

The dynamics of an order parameter's 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 parameter's amplitude fluctuations, and thus limits charge order recovery. This new aspect of phase fluctuations provides a more holistic view of the phase's importance in ordering phenomena of quantum matter.

Direct observation of dynamic charge stripes in La2–xSrxNiO4

“…They also provide an explanation for the fact that spin fluctuations observed above T co remain incommensurate 20,21,24 : the charge and spin stripes can maintain an instantaneous correlation even when static order is absent. Similar conclusions regarding the coupling of spin and charge stripes and their survival as dynamic entities have been inferred from recent time-resolved pump-probe studies, involving an optical pump and a probe by resonant X-ray diffraction [35][36][37] . In a THz spectroscopy study of La 1.67 Sr 0.33 NiO 4 (ref.…”

Speed limit of the insulator–metal transition in magnetite

“…This behaviour of negligible change in correlation length despite strong reduction in intensity has also been observed in nickelates [26], where it is associated with small fluctuations in amplitude and phase of charge order. In contrast, in magnetite, large phase fluctuations in the electronic order result in the observed ps decrease of the correlation length in Fig.…”

Photoinduced melting of magnetic order in the correlated electron insulator NdNiO3

“…Here, prompt perturbation in the orbital occupancy is expected to affect the pattern of exchange interactions 8 in the spirit of GoodenoughKanamori-Anderson rules. 9 Recent time-resolved experiments have combined ultrafast optical excitation with resonant soft x-ray diffraction [10][11][12][13][14] and have received particular impetus by the availability of x-ray free-electron lasers (XFELs). However, to date, direct experimental proof of the correlation between magnetic and charge/orbital dynamics still needs to be obtained.…”