Cooperative photoinduced metastable phase control in strained manganite films

Abstract

These results reveal that strain engineering can tune emergent functionality towards proximal macroscopic states to enable dynamic ultrafast optical phase switching and control.Precision tuning of the local environment in complex materials provides a route to control macroscopic functionality thereby offering a glimpse into how microscopic interactions conspire toward emergent behavior. Figure 1ashows resistivity measurements for a strained 30 nm LCMO film. In zero applied magnetic field, the film remains insulating at all temperatures (due to strain-enhanced orthorombicity 22 23 as depicted in Fig. 1d). For fields above 3 T the insulating phase collapses becoming a ferromagnetic metal at low temperatures. Figure 1b details the phase diagram of strained LCMO as determined from the field-dependent transport measurements 22 23 . The FM and AFM phases coexist over the range from 0-3 T, depending on temperature. To characterize the time-integrated electrodynamic response of the strained films, the optical conductivity was measured from 100 meV to 5 eV using spectroscopic ellipsometry (Fig. 1c). With decreasing temperature, the film displays spectral weight transfer from a small polaron (~1.5 eV) peak to a sharp well- Fig. 3a).As we now show, photoexcitation recovers the hidden FM phase of the strain- The photoinduced THz conductivity (PTC) is stable as long as the temperature is maintained. Further, the maximum conductivity is the same value as obtained with a strong magnetic field. This is clear from the blue dots of Fig. 1a, where the PTC from pulses, the resistivity continues to decrease by an additional order of magnitude, reaching a minimum after ~20 pulses. Figure 3b plots the photoinduced conductivity versus shot number for different fluences. The 4 mJ/cm 2 (same data as Fig. 3a) saturates at 800 Ω -1 cm -1 , while at lower fluences the conductivity saturates at a lower value. This is important, showing that the conductivity change does not simply arise from the absorbed number of photons. If there were a simple dependence on the number of absorbed photons, the data at lower fluences would saturate to the same conductivity value as the higher fluence data after a sufficient number of pulses. This is clearly not the case and indicates a cooperative process with a photon-absorption threshold. Figure 3c plots the conductivity plateaus from Measuring the conductivity dynamics of the pristine AFI state following singlepulse excitation would provide insight into the photoinduced IMT but is not experimentally feasible on a shot-to-shot basis. Instead, we employed an all-optical single-shot ultrafast spectroscopic method 28 which faithfully represents the conductivity dynamics because of the aforementioned spectral weight transfer. Figure 3d shows the results of single-shot photoinduced reflectivity dynamics (R/R) probing at the peak of the intersite transition (1.7 eV) following 1.55 eV excitation. There is a decrease in R/R consistent with dynamic spectral weight transfer to THz frequencies and the IMT dy...

Towards properties on demand in quantum materials

“…Photoinduced metastable IMTs have been observed in the CDW material 1T-TaS 2 (refs 54,115) and in the strained manganite La 0.7 Ca 0.3 MnO 3 (LCMO; ref. 56), with the results highlighted in Fig. 4c 55 .…”

“…Impulsively induced electronic phases are typically short-lived, with lifetimes on the order of the electromagnetic pulse width or the energy relaxation time of the material. Yet, in quantum materials with corrugated free-energy landscapes, impulsive stimulation can trap the system in new phases at auxiliary free-energy minima, separated from the true ground states by a significant kinetic barrier [54][55][56] . These metastable phases persist indefinitely on experimental timescales but can be controllably erased by external parameters such as temperature and magnetic field.…”

“…In TaS 2 , 1.55-eV photoexcitation at low temperatures in the commensurate CDW phase results in a decrease in the resistance by three orders of magnitude arising from collective polaron reordering 54 . Similarly, at low temperatures, strained LCMO is a charge-ordered antiferromagnetic insulator where 1.55-eV excitation results in a collapse to a ferromagnetic metallic state, with strong magneto-elastic coupling being of importance 56 . In both cases, a minimum fluence (~1 mJ cm -2 ) is required to achieve metastability suggestive of the need for a critical photoexcited volume to prevent collapse back to the initial phase.…”

Interplay of charge and spin dynamics after an interaction quench in the Hubbard model

“…This might be suggestive of a quasi-thermal pathway, though that is not generically the case, even in the same VO 2 [3]. Indeed, there are evidences of photo-induced hidden phases that are absent at equilibrium [4,5], as well as of remarkable non-thermal transient properties [6,7].At first sight it might seem unsurprising to observe non-thermal behaviour in correlated electron systems, which are complex materials with several competing phases and many actors playing a role. In reality, even the simplest among all models of correlated electrons, i.e.…”

“…This might be suggestive of a quasi-thermal pathway, though that is not generically the case, even in the same VO 2 [3]. Indeed, there are evidences of photo-induced hidden phases that are absent at equilibrium [4,5], as well as of remarkable non-thermal transient properties [6,7].…”