Light-induced bistability in spin transition solids leading to thermal and optical hysteresis

Abstract

We investigate the behaviour of photo-excitable, bistable systems, under permanent light irradiation, in presence of relaxation towards the non-excited state. Cooperativity causes bistability of the steady state, leading to light-induced thermal and optical hysteresis (LITH and LIOH). The light-induced instability is expected to induce demixtion, i.e. the coexistence of domains of the two stable steady states. Such effects are evidenced by magnetic and reflectivity measurements on the spin-crossover solid solution: [Fex Co1−x (btr)2(NCS)2] · H2O, with x = 0.3, 0.5, 0.85. Experimental data are in quantitative agreement with a simple macroscopic model which includes a non-linear relaxation term in the master equation. PACS. 64.60.My Metastable phases -70.60.Es Relaxation effects -75.90.+w Other topics in magnetic properties and materials -78.20.Nv Thermooptical and photothermal effects

Examples of molecular switching in inorganic solids, due to temperature, light, pressure, and magnetic field

“…4, we show the first example of LITH, best observed by reflectivity technique, which only probes the top layers of the absorbing sample [26]. An important consequence of these nonlinear effects is the occurrence of an intensity threshold [33], which may severely hinder the photoswitchability of absorbing materials, such as the PBAs, for instance.…”

“…Therefore, under permanent light, the steady state exhibits an HS → LS conversion at T 1/2 *. Due to cooperativity, an instability can occur at T 1/2 * [32,33]. The mechanism was described through a mean-field macroscopic master equation [33,34], which accounts for linear photoexcitation (i.e., with a constant quantum yield), and nonlinear, self-accelerated relaxation [35].…”

“…Due to cooperativity, an instability can occur at T 1/2 * [32,33]. The mechanism was described through a mean-field macroscopic master equation [33,34], which accounts for linear photoexcitation (i.e., with a constant quantum yield), and nonlinear, self-accelerated relaxation [35]. The mean-field macroscopic master equation writes, neglecting the spontaneous LS → HS relaxation (for low temperature): (8) where E A and α respectively are the activation energy barrier and the Hauser self-acceleration factor (proportional to the interaction parameter J).…”

Thermal‐ and Photoinduced Spin‐State Switching in an Unprecedented Three‐Dimensional Bimetallic Coordination Polymer

“…This photoexcitation together with the cooperative relaxation leads to the so-called Light Induced Thermal Hysteresis (LITH), [32,33] where a fraction of the complexes is maintained in the HS state in a pseudo steady-state under continuous irradiation while the temperature is slowly scanned typically from 10 up to Figure 11, was recorded at a constant temperature scan rate of 4 K per hour, by using the 647 nm line of a Kr + laser with an intensity at the sample of 4 mW mm À2 . As for the thermal spin-transition, the LITH cycle shows two steps.…”

Quasi-static nature of the light induced thermal hysteresis in [Fe(ptz)6](BF4)2 spin-transition solid

“…For this compound the LIESST is incomplete at 20 K, with around 30% of the highspin population, which makes a quantitative interpretation of LITH in this case quite difficult. Varret O] compound, where the LIESST effect is quantitative at 20 K, and the population of the metastable high-spin state is achieved to 100% [9]. Raising the temperature results in an accelerated kinetics of the high-spin low-spin relaxation and thus a depopulation of the metastable high-spin state [10].…”