III-2. Atomic & Solid State Physics (Muon)

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Magnetism of novel heavy fermion compound YbCu4 Ni investigated byµSR
T. Taniguchi,∗1 K. Osato,∗1,∗2 M. Fujita,∗3 D. P. Sari,∗3,∗4 and I. Watanabe∗3,∗5
The f -electron systems show many examples of quantum critical phenomena.1) In previous works, highquality crystals and advanced experimental methods
provided many instances of quantum critical phenomena
originating from antiferromagnetism. Recently, quantum critical phenomena that cannot be explained by
the self-consistent renormalization (SCR) theory haveattracted much attention. However, to study such quantum critical phenomena, actual candidate materials andadequate experimental methods are required.
We focused on YbCu4 T (T = transition metal) because this family exhibits exotic physical phenomena
such as the valence transition at zero ﬁeld and ambient pressure in YbCu4 In.2,3) In YbCu4 Au, valence andmagnetic transitions are induced by applying a magneticﬁeld.4) Since the Yb site in this family has three-foldsymmetry, this system may show an ordered phase originating from geometrical frustration.
The temperature dependence of the speciﬁc heat
(C/T ) of YbCu4 Ni shows a power-law behavior. Thisbehavior is consistent with the quantum critical phenomena, but the SCR theory cannot explain the temperature dependence of C/T .5) The purpose of our research is to understand the origin of the power-law behavior of C/T in YbCu4 Ni. Magnetism usually plays
the vital role of quantum criticality. Thus, we performed
muon spin relaxation (µSR) measurements at RIKENRAL because µSR is a powerful tool to obtain information on static and dynamic spin correlations.
Figure 1 shows the temperature dependence of theµSR time spectra. To derive the magnetic ﬂuctuation of
the f -electron, we applied a longitudinal magnetic ﬁeldof 100 G. Rapid relaxation was observed at low temperatures below 2 K, indicating the appearance of magnetic
ﬂuctuation. However muon spin precession was not observed down to the lowest temperature of 0.4 K. We
determined the asymmetry of the spectrum at 0.4 K forthe time after ∼4 µs as the baseline (∼16%). Theseresults suggest the inhomogeneous magnetic ﬁeld at themuon stopping site even at the lowest measurement temperature.
Here, we discuss the origin of the power-law behaviorof C/T from the viewpoint of magnetic ﬂuctuation. As
in the case of antiferromagnetism, it is highly possiblethat the magnetic ﬂuctuation is the cause of the temperature dependence of C/T . There are two possible originsof the magnetic ﬂuctuations: (1) the novel quantum crit∗1∗2∗3∗4∗5

Institute for Materials Research, Tohoku University
Department of Physics, Tohoku University
RIKEN Nishina CenterCollege of Engineering, Shibaura Institute of Technology
ISIS Pulsed Neutron and Muon Facility, STFC Rutherford
Appleton Laboratory

Fig. 1. Temperature dependence of the µSR time spectra ofYbCu4 Ni in a longitudinal field of 100 G.

icality and (2) magnetic inhomogeneity. For the former
case, it was theoretically aspect pointed out that a dramatic increase in eﬀective mass should be observed near
the quantum critical point, other than that from the anitferromagnetic phase.6) For example, the temperaturedependence of C/T shows logarithmic behavior in βYbAlB4 even when the valence ﬂuctuation contributed
to the physical properties.7) For the latter case, owing
to the distribution of the Kondo temperature originatingfrom the magnetic inhomogeneity, C/T at low temperatures may show a behavior similar to quantum criticality.8) Since inhomogeneous states such as the spin-glass
state may appear because of the geometrical frustration,
the latter case is also a candidate scenario. µSR measurements below 0.4 K may provide evidence to clarify
the above two possibilities.
In conclusion, we performed the µSR measurementsof YbCu4 Ni. At low temperature, the magnetic ﬂuctuation increased. It is highly possible that the power-law
behavior of C/T is caused by this ﬂuctuation.
References

1) G. R. Stewart, Rev. Mod. Phys. 56, 755 (1984).
2) H. Nakamura et al., J. Phys. Soc. Jpn. 59, 28 (1990).
3) M. Hedo et al., Acta. Phys. Pol. B. 34, 1193 (2003).
4) S. Wada et al., J. Phys. Condens. Matt. 20, 175201 (2008).
5) J. G. Sereni et al., Phys. Rev. B. 98, 094420 (2018).
6) S. Watanabe, K. Miyake, Phys. Rev. Lett. 105, 186403(2010).
7) S. Nakatsuji et al., Nat. Phys. 4, 603 (2008).
8) E. Miranda et al., J. Phys. Condens. Matt. 8, 9871 (1996).

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