SPEAKER: Dr. Lekh Poudel, NIST Center for Neutron Research
TITLE: Quantum criticality in partially substituted RCu6 (R = Ce, La)
ABSTRACT: The quantum critical point (QCP) in CeCu6-xAux is one of the most interesting QCPs yet uncovered. The behavior of QCP in CeCu6-xAux is almost singular and is inconsistent with the approach provided by Hertz-Millis-Moriya (HMM). The defining signature of this exotic behavior is unusual temperature dependence of the critical spin fluctuations resulting in a fractional exponent in the energy over temperature (E/T) scaling of the dynamic susceptibility. This experimental observation has recently motivated several theoretical proposals including local criticality, the coupling between quasiparticles and order parameter fluctuations, and topological excitations.
Using neutron and x-ray scattering measurements, we have studied structural and magnetic properties of related series CeCu6-xAgx and LaCu6-xAux. The influence of Ag-doping in CeCu6 results in a magnetic QCP similar to that found in CeCu6-xAux . Studies of the nonmagnetic analogue LaCu6-xAux reveal a new type of QCP, an elastic quantum critical point, is present in this family of materials . Inelastic neutron scattering measurements of the critical composition, CeCu5.8Ag0.2, show that the spin excitation spectrum results in E/T scaling with a fractional exponent, indicating identical unconventional behavior to CeCu6-xAux . The key result of this study is to identify the presence of multiple fluctuations that are intricately overlapped in the reciprocal space . When the components of the spectrum corresponding to the magnetically ordered side of the phase diagram are analyzed, they are found to be three dimensional and are consistent with the scaling expected for a conventional spin density wave QCP, indicating that the unusual E/T scaling stems from the superposition of multiple order parameters .
 L. Poudel, et al., Phys. Rev. B 92, 214421 (2015).
 L. Poudel et al., Phys. Rev. Lett. 117, 235701 (2016)
 L. Poudel et al., arXiv: 1705.05913
HOST: Johnpierre Paglione