Microscopic characterization of “nematic” precursor state of unconventional
superconductivity in URu2Si2
S. Kambe1), Y. Tokunaga1), H. Sakai1), T.D. Matsuda2)*, Y. Haga2), Z. Fisk2,3) and R.E. Walstedt4)
1) R.G. for Condensed matter physics of heavy element systems, JAEA 2) R.G. for Actinide
Materials Science, JAEA 3) Univ. of California Irvine 4) The Univ. of Michigan

In the superconducting state, two electrons form
superconducting pairs. For the formation of pair electrons, anattractive interaction between electrons is necessary. Inconventional superconductors, the origin of the attractive
interaction is lattice vibration. In contrast, magnetic fluctuationis considered to be the origin in actinide superconducting
compounds. Since high superconducting transition temperature
Tc can be expected for a magnetic-fluctuation induced
superconductor, it is important to clarify the mechanism of it.
The heavy fermion superconductor URu2Si2 undergoes asecond order phase transition at T0 ＝17.5 K. Since the orderparameter of the transition has not ever been clearly identified,such order has been termed “hidden order”. As thesuperconducting transition occurs at lower Tc=1.5 K, the hidden
order is precursor state of superconductivity. Owing to thetetragonal crystal structure of URu2Si2 (space group I/4mmm),
the physical properties of the paramagnetic state of thiscompound exhibit 4-fold rotational symmetry in the basal: (001)plane. In recent in-plane anisotropy measurements of the static
magnetic susceptibility, the 4-fold symmetry was reported to be
spontaneously broken below T0, and a 2-fold rotationallysymmetric state was found to appear [1]. Since the symmetry
breaking was observed without identifying its microscopic
origin, this ordered phase is termed “nematic” for the moment; it
has been interpreted with several models.
As a result of domain formation, the measurable 2-foldsymmetric susceptibility depends strongly on sample size andcan only be detected in very small (nearly mono domain)
samples [1]. On the other hand, NMR results to be presented
here do not depend on sample and domain size, since NMR
measurements probe the microscopic state at each nuclear site
individually. The present NMR results are consistent withdomain formation. While domains render the susceptibility and
the NMR shift isotropic in the basal plane, in this note we showthat a detailed analysis of Si NMR linewidth data offers aquantitative measure of the 2-fold susceptibility effect. The
intrinsic (mono–domain) 2-fold susceptibility in a macroscopic
single crystal sample has been precisely determined in thepresent study.
The low natural abundance (4.7%) of the NMR isotope 29Si
(I=1/2, gyromagnetic ratio: 845.77 Hz/Oe) has prevented highly
accurate Si NMR measurements in URu2Si2 up to now. For thepresent study a single crystal sample has been prepared with the29Si isotope enriched to 53%, improving the NMR sensitivity bya factor 11. This makes it possible for the first time to resolve
the in-plane magnetic anisotropy by means of NMR
measurements.
Figure 1 shows the angle  dependence of linewidth L(). In
the paramagnetic state above T0, L(shows a simple sinusoidal
curve, on the other hands, in the ordered state below T0, L(shows a particular peak minimum at 45 degrees which indicatesa development of nematicity as decreasing temperature [2].
Analysis of our linewidth data leads to the estimate 0.4% for the
2-fold intrinsic (mono–domain) susceptibility anisotropy in thebasal plane. This result is consistent with recent thermal

expansion measurements under uniaxial pressure [3]. It isremarkable that the unconventional superconductivity occurs inthis nematic precursor state, thus nematically magnetic
fluctuation is considered to induce the unconventional
superconducting state.
A future new superconductor with higher Tc is considered tobe realized in magnetic fluctuation mediated superconducting
system, since Tc of phonon-mediated superconductors alreadyhas reached its peak. If a new superconductor with room
temperature Tc is found, it will be quite useful for an
improvement of the energy efficiency.
We are going to investigate actinide compounds to clarify
the mechanism of superconductivity in detail.
References[1] R. Okazaki et al., Science 331, 439 (2011).
[2] S. Kambe et al., Phys. Rev. Lett. 110, 246406 (2013).
[3] S. Kambe et al., Phys. Rev. B 87, 115123 (2013).
* Present address: Tokyo Metropolitan University.

Fig. 1 Angle  dependence of 29Si-NMR spectra linewidth L() inURu2Si2 at applied field H=5.9 T in the (001) plane. The angle is angle between H and the [110] crystal direction. Below
T0=17.5 K, a particular peak minimum appears at =45 degrees,
which becomes prominent at low temperatures. The appearanceof the peak minimum indicates the nematic hidden ordering.