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Transient phases of matter – between metal and insulator

All materials are composed of a lattice of positively charged ions and negatively charged electrons. As a consequence, due to screening effects, electrons can be affected by the movement of the ions and of the other electrons. In a conventional metal, the interactions among electrons are negligible. In “bad metals”, where the density of conducting electrons is significantly lower, the Coulomb interaction among electrons becomes instead highly relevant. Electrons “feel” each other and their behavior is referred to as “correlated”. In certain cases, electron correlations can be so strong that the metal can become an insulator, thanks to a phenomenon called “Mott-Hubbard” transition. Unlike band insulators, that simply don’t have free conducting electrons, Mott-Hubbard insulators present a high resistivity because electrons are localized due to reciprocal Coulomb repulsion. The Mott-Hubbard transition can occur by changing the material temperature, pressure, on other macroscopic parameters.

A team composed of scientist from the LPS, SISSA (Trieste, Italy), LOA, LSI, SOLEIL and from the Universities of Berkeley and Stanford has studied the behavior of electrons in a prototype Mott-Hubbard compound (vanadium sesquioxide, V2O3) after excitation with an ultrafast laser, i.e. light pulses as short as few femtoseconds (a few millionths of billionths of a second). With these ultrashort light pulses, for a short time one can heat up only the electrons, while the ion lattice remains cold. The experiments performed on out-of-equilibrium V2O3 have shown a surprising hardening of the ion lattice while the electrons are still in an excited state, revealing a novel state of matter whose lifetime is only of few picoseconds. This transient state plays a crucial role in the relaxation process following photoexcitation, and cannot be achieved by modifying conventional thermodynamic parameters such as temperature or pressure. The temporal resolution necessary for this study was obtained thanks to the use of the most recent and advanced ultrafast laser sources, like for instance the pulses of LCLS, the Stanford X-ray Free Electron Laser.

The use of femtosecond laser pulses, combined with the unique properties of out-of-equilibrium Mott compounds, makes these strongly correlated materials very promising candidates for a new generation of ultrafast switches, that will be able to be operated at speeds which cannot be achieved with the current technology based on semiconductors.

Figure : Pump-probe experimental techniques employed for this study : time resolved X-Ray Diffraction (trXRD), time resolved photoemission (trPES) and time resolved reflectivity (TRR).


Ultrafast evolution and transient phases of a prototype out-of-equilibrium Mott-Hubbard material
G. Lantz, B. Mansart, D. Grieger, D. Boschetto, N. Nilforoushan, E. Papalazarou, N. Moisan, L. Perfetti, V. L. R. Jacques, D. Le Bolloch, C. Laulhe, S. Ravy, J.-P. Rue, T.E. Glover, M.P. Hertlein, Z. Hussain, S. Song, M. Chollet, M. Fabrizio & M. Marsi
Nature Communications 8, 13917 (2017)

Article by the Institut de Physique (FR)


Marino Marsi