Metamagnetic
Quantum Criticality and the Special Case CeRu2Si2
F. Weickert, FZD
Recently,
quantum criticality close to metamagnetic (MM) transitions has become a new and
exciting area of investigations. While the existence of antiferromagnetic (AF)
quantum critical points is without controversy, the final proof of
ferromagnetic (FM) ones is still outstanding. In all known ferromagnetic
systems either the 2nd order phase transition changes to 1st
order, the ferromagnetism is replaced by AF order, or spin glass behaviour appears
on lowering the transition temperature by an external tuning parameter. Another
route to get FM quantum criticality is the suppression of a critical end-point
of a 1st order MM transition to T
= 0. The so called quantum critical end-point is defined by divergent susceptibility and
Ising symmetry and causes distinct signatures in the thermodynamic quantities
irrespective of the internal structure of the material. An example, where this
description works successfully is e.g. the itinerant metamagnet Sr3Ru2O7.
Another prototype of a MM system intensive investigated in the past is CeRu2Si2. Here the metamagnetism at Hc @ 8 T is not of first order type but a crossover with very pronounced features in thermodynamic as well as transport properties. First interpretations of dHvA studies claimed the metamagnetism to be a change from itinerant to localized behavior of the heavy 4f-electrons, while later it was considered to be a continuous shrinking of one heavy sheet of the Fermi surface.
In the presented talk we use the theoretical concept of the quantum critical end-point to explain the anomalies occuring in CeRu2Si2. We show new measurements of the thermal expansion a, magnetostriction l, and specific heat C inside a narrow field range around Hc, combined with known results of the susceptibility c(T). We find a correspondence between c(T) and l(T), a minimum in C/T(H) and a sign change of a(H) at Hc. These results agree qualitatively with theory and indicate that the system CeRu2Si2 is not at, but very close to the QCEP in parameter space.