Unveiling Quantum Critical Phenomena in Selected Rare Earth Intermetallic Compounds

The quantum critical phenomenon of CeCu_6-xAu_x (x =0.1) [gold-doped cerium copper six] presents a host of intriguing puzzles: In particular, the dynamic susceptibility showing E/T-scaling with a fractional exponent of 0.75 is a surprise and is clearly inconsistent with the established approach developed by Hertz, Millis, and Moriya (HMM). Interestingly, the phase diagram of CeCu_6-xAu_x [gold-doped cerium copper six] also suggests a zero temperature structural phase transition, raising the possibility of a structural quantum critical point (QCP). To provide a further insight into the unconventional quantum criticality and to investigate the possibility of the structural QCP, related materials including CeCu_6-xT_x (T = Ag, Pd) [palladium/silver-doped cerium copper six] and LaCu_6-xAu_x [gold-doped lanthanum copper six] have been studied.

This comprehensive study, largely based on neutron scattering measurements, shows that the series CeCu_6-xT_x (T = Ag, Pd) [palladium/silver-doped cerium copper six] are magnetically identical to the CeCu_6-xAu_x [gold-doped cerium copper six] despite considerable differences in the doping dependence of the structural properties. Furthermore, in CeCu_6-xAg_x [silver-doped cerium copper six], the dynamic susceptibility fits well with a single phenomenological equation that yields E/T scaling with a similar anomalous exponent, [alpha] = 0.72(1). The key result is rather than a single fluctuation there two competing magnetic fluctuations that correspond to an incommensurate and a commensurate wave-vector. Only the incommensurate fluctuations appear to go critical at the QCP. While the total dynamic susceptibility displays an unconventional E/T scaling, the critical part of the dynamic susceptibility is consistent with the framework of the conventional HMM model indicating that the exotic physical behavior is the consequence of a competing noncritical term in the spectrum.

In LaCu_6-xAu_x [gold-doped lanthanum copper six], the orthorhombic-monoclinic structural phase transition temperature is suppressed linearly with Au-doping, suggesting the presence of a zero-temperature structural phase transition. Consistent with this observation, the heat capacity is enhanced at the critical composition. First principles calculations indicate that the structural instability is elastic in nature. This suggests a new type of QCP in LaCu_6-xAu_x [gold-doped lanthanum copper six] where quantum zero point motion of atoms suppresses an elastically ordered phase of matter.