We report thermodynamic measurements in a magnetic-field-driven quantum critical point of a heavy fermion metal, YbRh2Si2. The data provide evidence for an energy scale in the equilibrium excitation spectrum that is in addition to the one expected from the slow fluctuations of the order parameter. Both energy scales approach zero as the quantum critical point is reached, thereby providing evidence for a new class of quantum criticality.

Superconductivity without phonons has been proposed for strongly correlated electron materials that are tuned close to a zero-temperature magnetic instability of itinerant charge carriers. Near this boundary, quantum fluctuations of magnetic degrees of freedom assume the role of phonons in conventional superconductors, creating an attractive interaction that 'glues' electrons into superconducting pairs. Here we show that superconductivity can arise from a very different spectrum of fluctuations associated with a local (or Kondo-breakdown) quantum critical point that is revealed in isotropic scattering of charge carriers and a sublinear, temperature-dependent electrical resistivity. At this critical point, accessed by applying pressure to the strongly correlated, local-moment antiferromagnet CeRhIn(5), magnetic and charge fluctuations coexist and produce electronic scattering that is maximal at the optimal pressure for superconductivity. This previously unanticipated source of pairing glue opens possibilities for understanding and discovering new unconventional forms of superconductivity.

We studied the magnetic properties in a non-magnetic heavy-fermion compound CeIrIn 5 and an antiferromagnetic compound CeRhIn 5 with the tetragonal structure. High-field magnetization of CeIrIn 5 shows a weak metamagnetic transition around 420 kOe for the field along the [001] direction, while the magnetization gradually increases up to 500 kOe for [110]. On the other hand, we observed a two-step metamagnetic transition for the field perpendicular to [001] at 20 and 500 kOe in CeRhIn 5 . The magnetization is of the x-y type. The former transition indicates a 1/ cos θ dependence, where the θ means a tilted angle of the field direction from the (001) plane. From the magnetic susceptibility and high-field magnetization measurements, we formed a magnetic phase diagram in CeRhIn 5 . The temperature dependences of the magnetic susceptibility and thermal expansion coefficient of both compounds were analyzed on the basis of the crystalline electric field model.