Multiple fluctuations near an unconventional quantum critical point

A quantum critical point (QCP) is a nexus instability where competing and frustrated interactions can drive the formation of new states of matter. Elucidating the underlying critical behavior is key to a fundamental understanding of these phenomena. In the case of metallic systems, the situation is particularly unsettled with mounting violations of the canonical Hertz-Millis-Moriya (HMM) approach based on long wavelength fluctuations of the order parameter. Here, we discuss the critical behavior of the model QCP material CeCu_5.8Ag_0.2 which appears to fall into the class of materials violating the HMM paradigm. Using state of the art inelastic neutron scattering measurements, we show that the critical dynamics comprises fluctuations at distinct wave vectors indicating intense competition between nearly degenerate ground states. A scaling analysis of the fluctuation spectrum yields energy over temperature (E/T) scaling with an anomalous critical exponent. However, the component of the spectrum corresponding to the magnetically ordered side of the phase diagram is three dimensional and scales as E/T^3/2, demonstrating that at least a fraction of the spectrum is consistent with the conventional description of a QCP proposed by HMM. The presence of other low energy fluctuations suggests that a complete description of the QCP requires a coupling between multiple order parameters in order to explain the critical dynamics and associated thermodynamic properties of the class of materials exemplified by CeCu_5.8Ag_0.2.