Evolution of a complex electronic system to an ordered hidden state: optical quench in TaS2 – theory versus experiment

Contact: S. Brazovskii, brazov@lptms.u-psud.fr

http://lptms.u-psud.fr/membres/brazov/

Femto-second optical techniques addressing pump-induced phase transitions (PIPT) put an ambitious goal to reach “hidden” states of matter – those which are not accessible and not known under equilibrium conditions or more conventional treatments. While there was a case of success in magnetic materials, there has been no success yet in transforming cooperative electronic systems. For the first time, a group from Jozef Stefan Institute in Ljubljana, Slovenia has achieved [1] a bistable switching to a hidden spontaneously ordered conducting state in 1T-TaS2. The hidden state is stable until a laser pulse, electrical current or thermal erase procedure reverts it to the thermodynamic ground state. The theoretical part of this work was developed at the LPTMS, Orsay, France. The theoretical concept and the modeling provided understanding and detailed description of the time evolution under protocols of optical treatments.
The developed theory [1] focuses upon dynamic evolution of electrons and holes as mobile charge carriers, crystallized electrons modifiable by intrinsic defects (interstitials and voids), and the crystal bath. Mutual transformations among the three reservoirs of electrons, together with the heat production, are dictated by imbalances of three partial chemical potentials. The exchange rate among any two reservoirs vanishes when the corresponding chemical potentials become equal. The theory fits and explains all major observations for switching to and from the hidden state. The phenomenological approach sheds a light on very complicated and not yet resolved physics of this material including interplaying effects like CDW, Wigner crystal, polarons, Mott state.
This is already the second fruitful example of cooperation among the LPTMS and the JSI in the field of the PIPT by means of the femtosecond optics [2]. Another branch of theoretical work on this subject is performed also at Orsay, between LPTMS and LPS [3]; it is motivated by experiments performed at the University of Tokyo.

[1] L. Stojchevska, I. Vaskivskyi, T. Mertelj, P. Kusar, D. Svetin, S. Brazovskii, and D. Mihailovic. “Ultrafast switching to a stable hidden quantum state in an electronic crystal”, Science, 11 April 2014.
[2] R. Yusupov, T. Mertelj, V.V. Kabanov, S. Brazovskii, J.-H. Chu, I. R. Fisher, and D. Mihailovic, “Coherent dynamics of macroscopic electronic order through a symmetry breaking transition”, Nature Physics, 6, 681-684 (2010).
[3] S. Brazovskii and N. Kirova, “Excitonic Mechanism of Local Phase Transformations by Optical Pumping”, J. Supercond. Nov. Magn., 27, 1009–1013 (2014).