Strongly correlated electron systems often exhibit very strong interactions between structural and electronic degrees of freedom that lead to complex and interesting phase diagrams. For technological applications of these materials it is important to learn how to drive transitions from one phase to another. A key question here is the ultimate speed of such phase transitions, and to understand how a phase transition evolves in the time domain. Here we apply time-resolved X-ray diffraction to directly measure the changes in long-range order during ultrafast melting of the charge and orbitally ordered phase in a perovskite manganite. We find that although the actual change in crystal symmetry associated with this transition occurs over different timescales characteristic of the many electronic and vibrational coordinates of the system, the dynamics of the phase transformation can be well described using a single time-dependent 'order parameter' that depends exclusively on the electronic excitation.

Using femtosecond time-resolved x-ray diffraction, we directly monitor the coherent lattice dynamics through an ultrafast charge-density-wave-to-metal transition in the prototypical Peierls system K(0.3)MoO(3) over a wide range of relevant excitation fluences. While in the low fluence regime we directly follow the structural dynamics associated with the collective amplitude mode; for fluences above the melting threshold of the electronic density modulation we observe a transient recovery of the periodic lattice distortion. We can describe these structural dynamics as a motion along the coordinate of the Peierls distortion triggered by the prompt collapse of electronic order after photoexcitation. The results indicate that the dynamics of a structural symmetry-breaking transition are determined by a high-symmetry excited state potential energy surface distinct from that of the initial low-temperature state.

Femtosecond time-resolved X-ray diffraction is used to study a photo-induced phase transition between two charge density wave (CDW) states in 1T-TaS2, namely the nearly commensurate (NC) and the incommensurate (I) CDW states. Structural modulations associated with the NC-CDW order are found to disappear within 400 fs. The photo-induced I-CDW phase then develops through a nucleation/growth process which ends 100 ps after laser excitation. We demonstrate that the newly formed I-CDW phase is fragmented into several nanometric domains that are growing through a coarsening process. The coarsening dynamics is found to follow the universal Lifshitz-Allen-Cahn growth law, which describes the ordering kinetics in systems exhibiting a non-conservative order parameter.Among strongly correlated electron systems, superconductors and materials exhibiting metal-insulator transitions are usually characterized by strong electronelectron and electron-phonon couplings [1][2][3]. At thermodynamic equilibrium, the corresponding many-body interactions lead to rich phase diagrams as a function of temperature, pressure or doping. Such compounds also display fascinating out-of-equilibrium physics, in the form of ultra-fast symmetry changes known as photoinduced phase transitions [4][5][6], and occurrence of new, transient states [6][7][8].Charge density wave (CDW) states are broken symmetry states of metals arising from electron-phonon interactions. They are characterized by a periodic modulation of both atomic positions and electron density. The metal-to-CDW phase transition is characterized by the growth of a complex-valued order parameter p = A exp iΦ , which reflects both the amplitude A and the phase Φ of the periodic modulation [3]. A number of photo-induced phase transitions that have been achieved in CDW compounds correspond to a suppression of the CDW order, i.e. a transition between a CDW state and a metallic state free of any structural modulation [5,[9][10][11][12][13][14][15]. Among those, the photo-induced suppression of the CDW state in blue bronze was shown to involve a coherent motion of atoms along the normal coordinates of the CDW amplitude mode [5]. In this case, the amplitude mode allows continuous variations of the modulus of the order parameter |p|, the metallic state corresponding to |p|=0. In the present work, we focus on the photo-induced phase transition between the nearly commensurate (NC) and the incommensurate (I) CDW states in 1T-TaS 2 , which exhibit two distinct order parameters. When thermallyinduced, this first-order phase transition involves a discontinuous change of atomic positions, and a coexistence of NC and I phase domains over a 3 K range [16,17]. It is thus expected that the photo-induced I phase appears through non-coherent atomic motions, by a nucleation/growth process. We report that the photo-induced NC → I phase is completed within 100 ps after laser excitation. At this 100 ps delay, the photo-induced I-CDW phase is found divided into domains with a typical size of 150Å. Its ordering kinet...

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During the COVID-19 pandemic, many educators have been required to offer their courses online. A particular challenge is the implementation of practical laboratory experiments in the field of materials science. The central questions are the following: How can students carry out laboratory experiments at home? How are applications on mobile devices helpful in this context? How can experiments be organized to make students approach the topic with self-motivation and excitement? The concept presented in this paper combines the idea of practical work and the use of an augmented reality app. Guided by a well-structured online learning platform, video tutorials, and lab handouts, students were able to carry out their experiments from home. The results of the evaluation of this new laboratory experiment concept suggest that students very positively welcomed this form of education.