Observation of two critical points linked to the high field phase B in CeCu2Si2

Abstract

We present thermal expansion and magnetostriction measurements on a CeCu2Si2 single crystal of A/S-type up to 17.9T magnetic field applied along the crystallographic a-direction (Delta L ll a ll H) and down to 0.015K temperature. We identify clear thermodynamic anomalies at the superconducting transition T_c and at two second order transitions T_A,B into ordered phases A and B. Our measurements establish for the first time the boundary of phase B at high field and low temperature. No evidence for additional high field phases above B is found up to the maximum field. We speculate based on our experimental results that i) phase B is similar to phase A of spin-density wave type and ii) the first order phase transition between A and B is caused by Fermi surface reconstruction. We furthermore identify a new quantum critical point at H_c ~ 17T, where T_B is suppresssed to zero, and a bicritical point at (0.35K, 7.0T), where phase lines T_A(H) and T_B(H) meet.