Triggering a Phase Transition by a Spatially Localized Laser Pulse: Role of Strain

Abstract

We report here the optical microscopic imaging of a first-order phase transition induced by a nanosecond laser pulse (532 nm) in a single crystal of the molecular spin-crossover complex [FeðbapbpyÞðNCSÞ 2 ]. The transition starts with the formation of a high spin domain in the region irradiated by the focused laser beam, followed by the subsequent growth or contraction of the initial domain. Remarkably, in otherwise identical experimental conditions one can observe either the irreversible transition of the whole crystal or merely the formation of a transient domain-depending on which region of the crystal is excited. This observation as well as the rather slow dynamics suggest that the main control parameter is the inhomogeneous accommodation strain, which destabilizes the photoinduced domain. DOI: 10.1103/PhysRevLett.109.135702 PACS numbers: 64.70.KÀ, 07.60.Pb, 64.60.My, 75.30.Wx Photoinduced phase transition (PIPT) phenomena have received much attention in various types of solids such as organic charge transfer compounds, metal oxides, inorganic complexes, and even in metals [1][2][3][4]. In these materials, the strong electron-lattice or spin-lattice interactions have been shown to play a key role in driving the photoinduced transformation. Experimental evidence indicating that relatively weak photoexcitation can trigger a real macroscopic phase transition has been inferred in these systems from the huge photoconversion efficiencies as well as from the occurrence of characteristic nonlinear phenomena such as a threshold behavior. Here, we report on a conceptually different investigation where the phase transition was triggered by a spatially localized laser pulse within a comparatively large single crystal. Compared to previous studies on PIPT phenomena, our work brings in two new experimental features:(1) only a small part of the crystal is photoexcited by the focused laser beam, and (2) the phase change is not averaged for the whole crystal by a point or spectroscopic array detector but followed instead by an imaging charge-coupled device.For our experiments, we have chosen the molecular spin-crossover compound [Fe II ðbapbpyÞðNCSÞ 2 ] (1) (where bapbpy ¼ N-{6-[6-(pyridin-2-ylamino)pyridin-2-yl]pyridin-2-yl}pyridin-2-amine). Spin-crossover (SCO) materials of transition metal complexes are benchmark examples for PIPT. They exhibit bistability between the so-called low spin (LS) and high spin (HS) electronic configurations, which display strikingly different physical properties, including mass density, magnetic susceptibility, optical density, etc. [5]. Switching between the two molecular spin states can be induced by changing the sample temperature, applying external pressure or an intense magnetic field, and also by light irradiation. Light-induced excited spin state trapping in the solid state was first demonstrated by Decurtins et al. [6]. This so-called ''LIESST effect'' is basically a molecular phenomenon and can be observed only at cryogenic temperatures. On the other hand, several studies aimed a...

Spatiotemporal dynamics of the spin transition in[Fe(HB(tz)3)2]single crystals

“…The spatiotemporal characteristics of these phase transitions are indeed essential to interpret the ensuing phenomena as they are directly related to the mechanistic details of the switching of their physical properties. In the case of strongly cooperative SCO systems, singular phenomena and dynamics come into play during the transition within the thermal hysteresis region including nucleation and growth processes [4][5][6][7], reversible photo-control of the LS/HS phase boundary motion [8][9][10], (bidirectional) photo-switching induced by pulsed laser excitations [11][12][13] and subsequent "cascade" phenomena [14]. In all these processes, the non-equilibrium structural domain evolution turns out to be one of the key aspects of these phase transitions.…”

“…In the last decade, many studies have been devoted to the microscopic observation of the spatiotemporal aspects of the spin transition in cooperative SCO single crystals, either by optical microscopy [7][8][9][10][14][15][16][17][18][19][20][21][22][23][24][25][26][27] or Raman micro-spectroscopy [6,28]. These investigations revealed a universal phase separation mechanism with the formation of predetermined heterogeneous nuclei (induced by light irradiation or temperature change) and the existence of moving macroscopic domain walls with low propagation velocities (typically ~ 1-20 µm.s -1 [7,8,14,20,22]).…”

“…These investigations revealed a universal phase separation mechanism with the formation of predetermined heterogeneous nuclei (induced by light irradiation or temperature change) and the existence of moving macroscopic domain walls with low propagation velocities (typically ~ 1-20 µm.s -1 [7,8,14,20,22]). They also put into evidence the important role of crystal defects, strain and microstructure in the spatiotemporal development of the spin transition making this process deeply crystal dependent [21,23].…”

Multistep Relaxations in a Spin-Crossover Lattice with Defect: A Spatiotemporal Study of the Domain Propagation

“…On the other hand, the study of the relaxation properties of the lattice with defect demonstrated that the defect plays the role of the pinning site for the front interface. Indeed, the results showed that the interface propagation is significantly altered around the defect, which then can be used as a way to stabilize or to control [26,27] the dynamic HS/LS elastic interface.…”

“…This pure mechanical effect affects the spin state to which it is coupled via the elastic field (see contribution h i in Equation (3)) which results in a local increase of the effective ligand field in favor of the LS state. It is interesting to notice here that optical microscopy experiments [20,26] often lead to the observation of such accelerations in the end of the interface propagation mechanism, which usually end by a macroscopic motion of the crystal due to the release of elastic strain, generated by the presence of the HS/LS interface.…”