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Séminaire Propriétés électroniques de l’INSP

Charge density wave and Higgs-mode radiance in dichalcogenides - Marie-Aude Méasson - Jeudi 15 juin 2017 à 11 h

INSP - UPMC - 4 place Jussieu - 75005 Paris - Barre 22-32, 2e étage, salle 201

Marie-Aude Méasson - Laboratoire Matériaux et Phénomènes Quantiques à l’université Paris 7

Abstract

Despite being usually considered two competing phenomena, charge-density-wave and superconductivity coexist in few systems, notably in the transition metal dichalcogenide family of compounds TX2 (T=Nb,Ta and X=S,Se). In the most emblematic compound 2H-NbSe2, this unusual condition is responsible for specific Raman signatures across the two phase transitions [1,2]. While the appearance of a soft phonon mode is a well-established fingerprint of the charge-density-wave order, the nature of the sharp sub-gap mode emerging below the superconducting temperature is still under debate. It might be ascribed to the amplitude fluctuations of the superconducting order parameter, an analogous of the Higgs boson [3,4]. Beside, such mode’s radiance in 2H-NbSe2 remains unique among all superconductors. In this work we used the external pressure as a knob to unveil the delicate interplay between the two orders in various transition metal dichalcogenides. This allows us to study the nature of the superconducting mode in 2H-NbSe2 [5] and to discover another example of such mode in 2H-TaS2 under high pressure.

Thanks to an advanced extreme-conditions (low Temperature and high Pressure) Raman technique we are able to follow the pressure evolution and the simultaneous collapse of the two intertwined charge density wave and superconducting modes in 2H-NbSe2 up to 5GPa. The comparison with microscopic calculations in a model system supports the Higgs-type nature of the superconducting mode and suggests that charge-density-wave and superconductivity order parameter must overlap at least in part of the Fermi surface. We demonstrate that the superconducting Higgs-mode radiance, which is vanishing small in ordinary superconductors, is finite in the presence of the charge ordering, becoming its direct fingerprint at the pressure-driven critical point. In 2H-TaS2, above 6GPa, we measured an intense sub-gap superconducting mode which coexists with a charge-density-wave soft phonon mode as in 2H-NbSe2. We will discuss the differences and similarities between these superconducting modes in both compounds.

[1] P. Littlewood and C. Varma, Phys. Rev. Lett. 47 (1981) 811.
[2] M.-A. Méasson, Y. Gallais, M. Cazayous, B. Clair, P. Rodière, L. Cario, and A. Sacuto, Phys. Rev. B 89, (2014) 060503.
[3] C. Varma, Journal of Low Temperature Physics 126 (2002) 90 ; D. Pekker and C. Varma, Annual Review of Condensed Matter Physics 6 (2015) 269.
[4] T. Cea and L. Benfatto, Phys. Rev. B 90 (2014) 224515.
[5] R. Grasset, T. Cea, Y. Gallais, M. Cazayous, A. Sacuto, L. Cario, L. Benfatto, M-A. Méasson, submitted to PRX (2017)