Research Highlights

Anomalous electrical resistivity associated with unconventional
superconductivity in URu2Si2
N. Tateiwa1), Y. Haga1), T. D. Matsuda1), Y. Onuki2), and Z. Fisk1, 3)
1) R.G. for Actinide Materials, ASRC, JAEA
2) Osaka University 3) University of California

Unconventional superconductivity (SC) in the uranium
compound URu 2Si 2 below T sc = 1.4 K has attracted muchattention due to its novel superconducting properties [1]. The
SC has a strong relation with the electronic state of an unknown
ordered phase whose transition temperature is T0 = 17.5 K atambient pressure. The nature of the ordered phase known as“hidden order” (HO) has not been resolved for more than
25 years. The low temperature physical properties of URu2Si2
are very sensitive to the sample quality. We have grown a highquality single crystal of URu2Si2 to eliminate impurity effects[2]. In this study, we have investigated the electrical transport
under high pressures [3].
Figure 1 shows the pressure-temperature phase diagram in
URu2Si2 [3]. The ground state is changed from the HO to theantiferromagnetic (AF) state at a first order phase transitionpressure Px. The bulk superconducting state exists only below
Px. We have measured the electrical resistivity ρ under highpressure. The electronic property of the ordered state is reflected
through the scattering process of electron. For example, theusual electron-electron scattering gives the T 2-term in theresistivity. We focus on the pressure effects on T sc and theelectrical transport. We analyse the temperature dependence of2
ρ using the expression ρ = ρ0 + α1T + α2T , assuming that theresistivity ρ consists of a T-linear resistivity from the unusualscattering process of electron and the usual T 2-term.
We find a linearity between α1/α2 and Tsc as shown in Fig. 2.
The pressure dependence of the coefficient of α2 is very weak.
The value of Tsc depends primarily on the coefficient α1. Thissuggests that the anomalous electron scattering derives theunconventional SC in the hidden order phase. This finding
provides a key for further studies on the hidden and SC states in
URu2Si2 [3].

Fig. 1 Temperature pressure phase diagram in URu2Si2.
“SC” indicates the superconducting phase.

5 ASRC

Fig. 2 Relation between α 1 /α 2 for the resistivity ρ andsuperconducting transition temperatures Tsc, where the α1and α2 are obtained by the fitting to ρ0+α1T+α2T 2.
Similar correlation between the T-linear resistivity and Tsc
has been found in the organic superconductors, the iron pnictide
superconductors and the high-Tc cuprate superconductors [4].
This correlation may be a universality in the unconventional
superconductors. In these systems, the T-linear resistivity
appearing around a magnetic phase boundary has beeninterpreted as manifestation of quantum criticality.
So far, many theoretical models have been proposed for
the HO phase. Generally, the peculiarity of the HO phaseoriginating from the multipolar degree of freedom in the 5f
electrons has been stressed but no clear experimental evidence
was found. Unidentified mysterious phases have been reported
near the SC phase such as “pseudo-gap phase” in the cuprate
superconductors. We note recent studies on the mysterious
phases from the view point of electronic nematicity [5].
Interestingly, the symmetry breaking of the electronic state
has been revealed also in the HO phase by the collaboration
work between our research group and Kyoto University [6].
The result as well as the present finding suggests that the HO
phase in URu2Si2 shares the universality inherent to the strongly
correlated electron superconductors near quantum criticality.
Our studies provide a different point of view for theories of theHO phase. Future studies on the phase will contribute to theunderstanding of all strongly correlated electron systems.
References[1] T. T. M. Palstra et al., Phys. Rev. Lett. 55, 2727 (1985).
[2] T. D. Matsuda et al., J. Phys. Soc. Jpn. 80, 114710 (2011).
[3] N. Tateiwa et al., Phys. Rev. B 85, 054516 (2012).
[4] L. Taillefer, Annu. Rev. Condensed Matter Phys. 1, 51(2010).
[5] J-H. Chu et al., Science 329, 824 (2010).
[6] R. Okazaki et al., Science 331, 439 (2011).