P2: Role of the tuning parameter at a magnetic quantum phase transition

V. Fritsch, O. Stockert, H. v. Löhneysen

Magnetic quantum phase transitions (QPT) can have very different physical origins. In the simplest case, magnetic correlations develop out of interactions in the Fermi liquid, leading to spin-density wave (SDW) magnetic order at a quantum critical point (QCP) which is reached by pressure or composition. The destruction of the antiferromagnetic order by magnetic field is a distinctly different route to a zero temperature phase transition, which for strongly anisotropic systems may be of first order or may be tuned by some further control parameter to a quantum critical and second-order end point.

We have started a comparative study for the system CeCu6-xAux, where the different behavior in thermodynamic and transport properties for x = 0.2 can be traced back to differences in the magnetic fluctuation spectrum. This will be investigated by inelastic neutron scattering. Further, we propose to study stoichiometric antiferromagnets which can be both pressure- and field-tuned across a QCP. A possible candidate is CeNiGa2 where TN = 4 K is suppressed to zero at a moderate pressure p = 3.5 kbar. Our experimental techniques include crystal growth and thermodynamic and transport measurements, i.e. specific heat and magnetic susceptibility, electrical resistivity and Hall effect.

By investigating the field-tuned vs. pressure-tuned QCP by inelastic neutron scattering, we intend to shed light on the microscopics and to identify different universality classes of magnetic quantum phase transitions.