Proximity to valence transition in heavy fermion superconductor CeCu₂Si₂ under pressure

G. Seyfarth^1a, A.-S. Rüetschi¹, K. Sengupta¹, A. Georges^1,2,3 and D. Jaccard¹

¹ DPMC - Université de Genève - Quai Ernest Ansermet 24, 1211 Geneva 4, Switzerland
² Centre de Physique Théorique, École Polytechnique, CNRS - 91128 Palaiseau Cedex, France, EU
³ Collège de France - 11 place Marcelin Berthelot, 75005 Paris, France, EU

^a gabriel.seyfarth@unige.ch

Received: 29 January 2012
Accepted: 14 March 2012

Abstract

The first heavy fermion superconductor CeCu₂Si₂ is still only partially understood. At high pressures, superconductivity is supposed to be mediated by a different mechanism than at ambient pressure, where spin fluctuations most likely act as pairing glue. We have obtained and analyzed new resistivity data up to 7 GPa on a good quality CeCu₂Si₂ single crystal with an unprecedented high transition temperature. For the first time, we establish quantitatively that CeCu₂Si₂ lies in proximity to a valence transition: the critical end point could be located at 4.5±0.2 GPa and a slightly negative temperature, and the corresponding valence cross-over line has been added to the p-T phase diagram. This outcome is essential for the theoretically predicted critical valence fluctuations scenario, and opens new routes for further investigations in other Ce-based compounds.