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National audience; Phase transitions give rise to different and unique modifications of physical properties in materials. Understanding these changes of state and their origin is of great interest for the further tuning of such properties, through the application of external stimuli, just as temperature, pressure, electric or magnetic fields, and light irradiation.1,2In this sense, the classical Landau theory is a useful tool for the description of different ordering phenomena, taking place during phase transitions, originally through the evolution of symmetry-breaking order parameters, which measure the deviation from the high-symmetry phase.2 Due to entropy gain, it is the disordered high symmetry phase that is stable at high temperature, in contrast to the ordered low symmetry phase, which is stable at low temperature. In addition, there are many materials, like charge-transfer, spin-crossover, and Mott insulator systems, in which the phase transition is related to an electronic instability without symmetry-breaking. In these cases, the non-symmetry-breaking phase transition should be depicted with the evolution of another order parameter, accounting for the aforementioned electronic instability.1For some materials, like in the case of spin-crossover systems, both, electronic and structural order instabilities may couple. This can result in the widening of hysteresis loops between high- and low- spin phases or in a step-wise evolution of the transition.3 In order to describe the aforementioned phenomena, the addition of the symmetry-allowed coupling terms to the Landau theory main equation is needed.In this work, we are interested in the [Fe(PM-PEA)2(NCS)2] system exhibiting a spin transition coupled to a ferroelastic symmetry-breaking. In particular, this system is unusual because it presents a high symmetry configuration (orthorhombic – Pccn) for the low-temperature low-spin state, while the high-temperature high-spin state is in a low-symmetry phase (monoclinic – P21/c).3,4 In this poster, we will show a way to rationalize this unusual behavior within the Landau theory framework, towards the reproduction and understanding of the experimentally observed asymmetric spin-transition hysteresis loop, and coupled changes of spin state and symmetry of the crystal.

International audience; Controlling material properties with light pulses represents one of the next great challenge in material science. To achieve this goal, one needs to impact the material on the relevant time scale for controlling electronic and phononic properties. Targeting this challenge becomes possible thanks to the emerging field of photo-induced phase transition when a femtosecond light pulse interacts with the system. Along this line, we demonstrate the possibility to perform femtosecond optical spectroscopy under control thermodynamical environment. This new experimental setup includes a very precise pressure control between 1 and 6 kbar. These experimental results clearly demonstrate an effect of hydrostatic pressure onto the out-of-equilibrium photo-response of V2O3. The main feature is a blue shift of the Brillouin frequency that correlates with an increasing speed of sound.

International audience; Triggering new stable macroscopic orders in materials by ultrafast optical or terahertz pump pulses is a difficult challenge, complicated by the interplay between multiscale microscopic mechanisms, and macroscopic excitation profiles in samples. In particular, the differences between the two types of excitations are still unclear. In this article, we compare the optical response on acoustic timescale of a V2O3 Paramagnetic Metallic (PM) thin film excited by a terahertz (THz) pump or an optical pump, at room temperature. We show that the penetration depth of the deposited energy has a strong influence on the shape of the optical transmission signal, consistent with the modulation of permittivity by the superposition of depth-dependent static strain, and dynamical strain waves travelling back and forth in the sample layer. In particular, the temporal modulation of the optical transmission directly reflects the excitation profile as a function of depth, as well as the sign of the acoustic reflection coefficient between the film and the substrate. The acoustic mismatch between the V2O3 layer and the substrate was also measured. The raw data were interpreted with a one-dimensional analytical model, using three fitting parameters only. These results are discussed in the context of triggering phase transitions by ultrafast pump pulses. To the best of our knowledge, this is the first report of the modulation of the optical transmission of V2O3 with a THz pump within the acoustic timescale.

International audience; In the family of spin-crossover materials, which undergo thermal conversion between low-spin (LS) and high-spin (HS) phases, it is of great interest to study vibrational modes. On the one hand, vibration modes are characteristic of the spin state, and vibrational spectroscopies are often used for monitoring spin-state switching driven by temperature, pressure, or light. On the other hand, spin-state thermal conversion is an entropy-driven process, and the vibrational entropy change represents the main contribution to the total entropy difference between LS and HS phases at solid state. However, the discussion of vibrations in spin-crossover materials is often limited at the molecular scale. Here we study vibration modes in the [Fe(phen)(2)(NCS)(2)] crystal, and we compare symmetry-resolved vibrational spectroscopy data performed on single crystal to density functional theory calculations performed in a periodic three-dimensional crystal. We discuss the complex nature of vibrational modes in crystals, including the vibration of molecules within the crystalline lattice, with different symmetries and frequencies. We also highlight the presence of many low-frequency libration modes of different symmetries. The contribution of vibrational entropy, added to the electronic entropy, provides a total entropy difference in the solid state, which is in very good agreement with calorimetric measurements.