Fig. 2: C4 symmetry breaking and reflection anisotropy in La0.5Sr1.5MnO4.
From: Multi-mode excitation drives disorder during the ultrafast melting of a C4-symmetry-broken phase
a Schematic representation of the potential energy surface of the symmetry-broken phase. The four minima (±ƞ, ±iƞ) correspond to the directions along which the low temperature phase can form, relative to the high-symmetry structure, labelled ƞ0. Inserts show the reflection anisotropy signal for each domain structure, which can point along the [110] or [1–10] crystallographic axis. Domains of opposite parity have the same anisotropy and are thus indistinguishable. b. Expected signals for ƞ2 based on the two pathways shown in a. An amplitude-mode-like motion, in which the order is restored along a direct pathway, but overshoots, resulting in a “rebound” in the signal, and a phase-like motion, which flips the anisotropy axis (Line colour corresponds to the domain state). c Experimental setup for measurement of time-dependent anisotropy. A 60 fs 1500 nm probe pulse passes through a linear polarizer and rotating a halfwave plate. The probe is focused onto the LSMO sample, where the polarization component parallel to the input state is attenuated and a transverse component with a complex phase delay is introduced. The reflected beam is collected by the same lens and propagates back through the waveplate, where the initial polarization rotation is undone. The beam then passes back through the linear polarizer, removing the transverse polarization component introduced by the LSMO, and the modulated parallel component is collected on a photodiode. The real time-modulation of the signal is processed to yield the angular anisotropy signal (inset, top left). d ƞ2 and r as a function of temperature in our single-crystal LSMO sample. The polar plot radial axes run from 1.42 to 1.8 with divisions marked every 0.1 (arb. units). ƞ2 shows a clear discontinuity at TCO while r is less sensitive. The small non-zero value of ƞ2 above TCO is due to strain.
