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Abstract

We present thermal expansion and magnetostriction measurements on a CeCu2Si2 single crystal of A/S type up to 17.9 T magnetic field applied along the crystallographic a direction (Delta L parallel to a parallel to H) and down to 0.015 K temperature. We identify clear thermodynamic anomalies at the superconducting transition T-c and at two second-order transitions T-A,T-B into ordered phases A and B. Our measurements establish 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 quantum critical point at H-c similar or equal to 17 T, where T-B is suppresssed to zero, and a bicritical point at (0.35 K, 7.0 T), where phase lines T-A(H) and T-B(H) meet.