Quantum Griffiths phase near an antiferromagnetic quantum critical point: Muon spin relaxation study of Ce(Cu_1-xCox)₂Ge₂

The antiferromagnetic order in the heavy-fermion compound CeCu2Ge2 can be suppressed by Co-doping, and at critical composition xc=0.6 (TN→0 K) a quantum critical point has been observed. We have performed zero-field (ZF) and longitudinal-field muon spin relaxation (μSR) measurements on polycrystalline samples of Ce(Cu_1-xCox)2Ge2 (x=0,0.2,0.6,1) over a temperature range of 100 mK to 10 K and in applied fields from 0 up to 3000 G. Above any ordering temperature, the muon relaxation spectra can be described by a Gaussian-Kubo-Toyabe times exponential line shape. Below the magnetic ordering temperature (i.e., for x<0.6), an additional Gaussian relaxation is observed. The zero-field muon relaxation rate suggests the presence of antiferromagnetic ordering below 4 and 0.8 K for x=0 and 0.2 samples, respectively. For x=0.6, the magnetic order is completely suppressed, and the quantum critical point is accompanied by non-Fermi-liquid behavior, manifested in the power-law divergence of exponential depolarization, i.e., λ T0.55. The relaxation rate of x=0.6 obeys the time-field scaling relation Gz(t,H)=Gz(t/Hγ), which is considered to be a characteristic feature of quantum critical magnetic fluctuations. Furthermore, for x=0.6, the exponent of isotherm magnetization, M∼Hη, and magnetization-fielderature scaling is consistent with the ZF-μSR data. These results provide strong evidence for the formation of a quantum Griffiths phase near the antiferromagnetic quantum phase transition.