Ultrafast non-thermal laser excitation of gigahertz longitudinal and shear acoustic waves in spin-crossover molecular crystals [Fe(PM–AzA)2(NCS)2]

Abstract

We report GHz longitudinal as well as shear acoustic phonons photoexcitation and photodetection using femtosecond laser pulses in a spin-crossover molecular crystal. From our experimental observation of time domain Brillouin scattering triggered by the photoexcitation of acoustic waves across the low-spin (LS) to high-spin (HS) thermal crossover, we reveal a link between molecular spin state and photoexcitation of coherent GHz acoustic phonons. In particular, we experimentally evidence a non-thermal pathway for the laser excitation of GHz phonons. We also provide experimental insights to the optical and mechanical parameters evolving across the LS/HS spin crossover temperature range. PACS numbers:Understanding how ultrafast photoinduced molecular switching in crystals couples to the lattice in optical materials is one of the key challenges in the fields of ultrafast photo-induced phase transitions or transformations and ultrafast acoustics. Systems like spin-crossover compounds exhibit, with temperature or pressure, change of the molecular spin state in the central d 4 -d 7 transition metals of the complex. They are promising candidates for diverse applications including miniature temperature sensors, displays, data storage and photonic devices [1,2]. Moreover, they can be fabricated in a vari

Molecular Spin Crossover Materials: Review of the Lattice Dynamical Properties

“…Brillouin scattering requires usually transparent materials and the LS state does not fulfill this condition. More recently, ultrafast time‐domain Brillouin scattering experiments on single crystals of [Fe(PM‐AzA) 2 (NCS) 2 ] allowed for the photogeneration and photodetection of GHz acoustic phonons across the whole SCO temperature range . From Brillouin spectroscopy measurements, the longitudinal ( v l ) and transverse ( v t ) sound wave velocities are obtained, which can be linked to the isotropically averaged bulk modulus and shear modulus ( G ) through the material density ρ by In the case of iron complexes, which are the most frequent SCO compounds, 57 Fe Mössbauer spectroscopy was used at several instances to infer the lattice rigidity .…”

“…Therefore, the chemical engineering of elastic properties appears as a difficult task, especially if one wish to enhance the lattice stiffness. An important parameter is the Poisson's ratio v , which can be determined in the isotropic approximation as follows: This value was determined around for three different SCO samples by Brillouin scattering and by combining HP‐XRD and NIS . It is interesting to note that for , the Young's and bulk moduli are equivalent.…”

Dynamical limits by downsizing for the photo-induced switching in a molecular material revealed by time-resolved X-ray diffraction

“…It corresponds to the travel time of the volumic deformation through a microcrystal [Schick2014]. Thus, taking a sound velocity v ~ 4 * 10³ m.s -1 estimated from previous measurements on similar samples [Parpiiev2017] and L ~ 1.3 µm the average crystal dimension, the associated acoustic time scale is τ = L/v = 1.3*10 -6 / 4*10³ = 330 ps. Moreover, the initial photo-switching of 7% of the molecules causes the average volume jump of only 1.5 Å 3 (figure 1e), significantly smaller than the observed volume change of 19 Å at the maximum of volume expansion (figure 2c).…”

Ultrafast light-induced shear strain probed by time-resolved x-ray diffraction: Multiferroic BiFeO3 as a case study

“…It has been also shown that during an ultrafast light-induced demagnetization process the shear acoustic phonons can exchange angular momentum with spins through the Einstein-de Haas effect [8,9] or sometimes called the Richardson effect [10,11]. Beyond these two examples, generating shear motion with light has received a great deal of attention in general, and several demonstrations of this phenomenon have been reported in different materials including multiferroic oxides, such as BiFeO 3 (BFO) [12][13][14], piezoelectric semiconductors GaN [15], GaAs [4], metals [4,5,16,17], or spin-crossover compounds [18] for citing a few. Despite this active and continuous effort, the underlying physics of the light-induced shear strain generation remains unclear since the quantitative measurement of the shear strain amplitude is lacking.…”