Photo-induced phase-transitions in complex solids

Abstract

Photo-induced phase-transitions (PIPTs) driven by highly cooperative interactions are of fundamental interest as they offer a way to tune and control material properties on ultrafast timescales. Due to strong correlations and interactions, complex quantum materials host several fascinating PIPTs such as light-induced charge density waves and ferroelectricity and have become a desirable setting for studying these PIPTs. A central issue in this field is the proper understanding of the underlying mechanisms driving the PIPTs. As these PIPTs are highly nonlinear processes and often involve multiple time and length scales, different theoretical approaches are often needed to understand the underlying mechanisms. In this review, we present a brief overview of PIPTs realized in complex materials, followed by a discussion of the available theoretical methods with selected examples of recent progress in understanding of the nonequilibrium pathways of PIPTs.

Fig. 1. Illustration of changes in the potential energy surface (PES) during phase-transitions.

Fig. 2. (Top) Optical resistive switching in 1T-TaS₂. The graph in center shows change in resistivity with temperature is shown in blue. The photoexcitation in low-temperature ‘SD’ phase reduces the resistivity, indicated in red arrow, by orders of magnitude. This figure has been adapted/reproduced from ref. with permission from AAAS, copyright 2014. (Bottom) The graph indicates step-like rise in conductivity in strain-engineered thin film of La_2/3Ca_1/3MnO₃ on photoexcitation at 80 K corresponding to IMT. The CO, SO and OO pattern of La_2/3Ca_1/3MnO₃ is shown in the left. Spin up and down Mn sites are indicated in blue and red. This figure has been adapted/reproduced from ref. with permission from Springer, copyright 2016.

Fig. 3. Timescales of elementary excitations and decay processes in complex quantum materials. The blue dashed lines indicate the time-period of single-cycle of optical pulse and THz electric field which excites electrons and phonons, respectively. This figure has been adapted/reproduced from ref. and with permissions from Elsevier and John Wiley and Sons, copyrights 1997 and 2012.

Fig. 4. (Top) Long-lived local excitations in a dimerized chain induced by optical excitation, simulated with t-DMRG. (Top-left) Unit-cell with four-sites. (Top-right) Evolution of electron density after local excitation for different values of Δ/t_hop, where t_hop and Δ is hopping and onsite Hund's splitting between opposite spin–orbitals, in a chain with 40 lattice sites. The local excitations are long lived at small Δ/t_hop but spread with a “light-cone effect” at large Δ/t_hop. This figure has been adapted/reproduced from ref. with permission from APS, copyright 2018. (Bottom) t-DMFT study showing photo-induced hidden-phase with new OO polarization in KCuF₃. (Bottom-left) SO and OO in the equilibrium state. Linear combinations of eg-states |θ〉. (Bottom-right) Evolution of the total spin S_z component and OO, defined as the occupation difference X₃ = 1/2(n₂ − n₁) between two eg-orbitals, in the long-time limit under three different non-equilibrium protocols, that is electric-field pulse (solid-pink), photo-doping electrons in (solid-yellow) and out (solid-blue). The inset shows the Z₁–Z₃ plane in the orbital pseudospin space. The X₃, Y₃, Z₃ directions and their corresponding orbitals are marked. This figure has been adapted/reproduced from ref. with permission from Springer, copyright 2018.

Fig. 5. rt-TDDFT study of photo-induced melting of ‘Star-of-Davidson’ (SD) pattern in TaS₂. (Top-left) Root mean square distance (RMSD) under three laser intensities, η = 0.64% (black), η = 1.28% (pink) and η = 1.92% (red). The system retains original SD structure at η = 0.64%, but exhibits photo-induced metallic state ‘T’ at η = 1.28%. At high intensity η = 1.92%, the periodic oscillations suggests a new photo-induced transient metallic state ‘M’ with a new spatially-ordered atomic distortions. (Top-right) Light-induced charge-density redistribution at t = 25 fs where yellow region shows increased density. (Bottom) Nature of ‘SD’, ‘T’ and ‘M’ states. This figure has been adapted/reproduced from ref. with permission from ACS, copyright 2019.

Fig. 6. Tight-binding model study of photo-induced FM-metallic phase in CO and OO antiferromagnetic Pr_1/2Ca_1/2MnO₃. (Right-top) CO, SO, and OO in equilibrium state. (Left-top) Evolution of CO and OO peak as at high and low (inset) fluence value. For higher fluences, CO and OO melt. (Left-bottom) Melting of the original SO (corresponding magnetic peaks are in green and red) and photo-induced FM order. (Right-bottom) Experimentally study showing melting of CO (left) and OO (right) at high fluences. This figure has been adapted/reproduced from ref. and with permissions from Springer and APS, copyrights 2014 and 2020.