Strong correlation between superconductivity
ferromagnetic fluctuations in UGe2

andpressure-induced

N. Tateiwa, Y. Haga, and E. Yamamoto
R. G. for Materials Physics for Heavy Element Systems, ASRC, JAEA

The coexistence of superconductivity and ferromagnetism
carried by the same electrons was thought to be a fantastic
theoretical possibility after its prediction by Ginzburg[1].
Recently, the discovery of the superconductivity in uranium
ferromagnets UGe2[2], URhGe[3], and UCoGe[4] has attracted
much attention since the same 5f electrons of uranium atoms areresponsible for the two states. As unconventional
superconductivity has been typically found around theboundaries of magnetic ordered phases in strongly correlated
electron systems, it is important to study a relation between themagnetism and the superconductivity. Neutron scattering studies
have shown correlations between the superconductivity andmagnetic excitations in high-Tc cuprate, iron arsenide, and heavy
fermion superconductors. Here, we report a clear correlation
between the superconductivity and pressure-induced
ferromagnetic fluctuations in UGe2.
Figure 1 shows the temperature-pressure phase diagram inUGe2[5]. Open circles and closed triangles represent the Curie
temperature TCurie determined by the resistivity and present
magnetic measurements. TCurie decreases with increasing pressure
from 53 K at ambient pressure. The ferromagnetic state
disappears above Pc ~ 1.5 GPa. There is an additional boundary
Tx that splits the ferromagnetic phase into FM2 and FM1. Openand closed diamonds represent Tx determined by the resistivity
and the magnetic measurements. The critical pressure of Tx is Px~ 1.2 GPa. The superconductivity (SC) appears fromapproximately 1.0 GPa to Pc. The superconducting transition
temperature Tsc becomes highest near the phase boundary of FM1and FM2 at Px. The microscopic origin of the transition fromFM2 to FM1 has not been understood yet. The spontaneous
magnetic moment ps, the coefficient of the T2-term in theresistivity A and the linear specific heat coefficient γ show drasticchanges at Px[2].
We measured dc magnetization at high pressure in UGe2 with aminiature ceramic-anvil high-pressure cell (mCAC) designed byus for use in a commercial SQUID magnetometer[6]. A highquality single crystal of UGe2 with residual resistivity ratio RRR= 600 was used. The magnetic data were analyzed using
Takahashi's spin fluctuation theory[7]. We determined thepressure dependencies of the widths of the spin fluctuation
spectrum T0 and TA in the energy and momentum spaces,respectively.
Figure 2(a) shows the pressure dependencies of T0 and TA. Bothquantities show anomalous enhancements from 1.0 to Pc where
the superconductivity appears. This suggests the change of thespin fluctuation spectrum. The pressure dependence of T0 can be
expressed as T0(P) = T*0+ ∆T0(P) where T*0 = 95 K is a pressureindependent constant. Figure 2(b) shows the pressure
dependencies of ∆T0(P) (right axis) and Tsc determined by theresistivity measurement (left axis). The pressure dependence of∆T0(P) scales with that of Tsc (P). This suggests a clear correlation
between the superconductivity and pressure-induced
ferromagnetic fluctuations with characteristic energy of 300 K
developing around Px, the phase boundary of FM1 and FM2.
Theoretically, it has been suggested that the p-waveferromagnetic superconductivity is mediated by critical

ferromagnetic fluctuations around a ferromagnetic quantum
critical point[8]. Meanwhile, this study suggests the importanceof the phase boundary of FM1 and FM2 for the superconductivity
in UGe2.
This work was supported by JSPS KAKENHI Grant No.
JP16K05463.

Fig. 1 Temperature-pressure phase diagram in UGe2[5].

Fig. 2 Pressure dependencies of (a) the widths of the spin
fluctuation spectrum T0 and TA, and (b) Tsc (left axis) and∆T0(P) [=T0(P)-95 K] (right axis) in UGe2[5].

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[3] D. Aoki et al., Nature 413, 613 (2001).
[4] N. T. Huy et al., Phys. Rev. Lett. 99, 067006 (2007).
[5] N. Tateiwa et al., Phys. Rev. Lett. 121, 237001 (2018).
[6] N. Tateiwa et al., Rev. Sci. Instrum. 82, 053906 (2011).
[7] Y. Takahashi, J. Phys. Soc. Jpn. 55, 3553 (1986).
[8] D. Fay and J. Appel, Phys. Rev. B 22, 3173 (1980).