Coupling between electronic
superconductor URu2Si2

andlatticesymmetry

inunconventional

S. Kambe1,2), D. Aoki2,3), B. Salce2), F. Bourdarot2), D. Braithewait2), J. Flouquet2) and J.-P. Brison2)
1) R.G. for Condensed matter physics of heavy element systems, JAEA
2) SPSMS/INAC, CEA-Grenoble 3) IMR, Tohoku Univ.

So far many theoretical models have been proposed in order
to interpret the hidden order state. As the proper model should
reproduce the observed relation, the number of possible models
will be considerably reduced. In addtion, uniaxial pressure isfound to be useful to controle the electronic state in the presentstudy.

Fig. 1 The uniaxial pressure cell. The sample (black) is locatedbetween two clamps (red).

1918.5T0 (K)The heavy fermion compound URu2Si2 undergoes a second
order phase transition at T0~17.5 K under hydrostatic ambientpressure. Since the order parameter of the transition has not everbeen clearly identified, such order has been termed “hiddenorder”.
Owing to the tetragonal crystal structure of URu2Si2 (spacegroup I4/mmm), the physical properties of the paramagnetic
state of this compound exhibit 4-fold rotational symmetry in thebasal (001) plane. Thus, uniaxial pressure in the basal plane
lowers the lattice symmetry and can induce a significant
modification of the phase diagram.
In recent in-plane 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]. In this case, in-plane
2-fold electronic symmetry appears spontaneously below T0.
In the present study [2], the phase diagram of URu2Si2 under
uniaxial pressure [100] along the [100] and [110] along [110]directions in the basal plane is investigated based on the thermal
expansion coefficient. Both the phase diagram and the thermal
expansion anomaly at T0 with hydrostatic pressure P are foundto be different for uniaxial pressuresalong the [100] and [110]directions.
Single crystals of URu2Si2 were grown using the Czochralski
method in a tetra-arc furnace under an argon gas atmosphere.
The high quality of the samples was confirmed by the resistivity
ratio between 300 K and 2 K (RRR), which is over 50. In orderto realize homogeneous uniaxial pressure conditions, the sample
shape is important. For a rectangular geometry, the sample
length along the direction of uniaxial pressure should be at least
twice as great as the sample length perpendicular to the uniaxialpressure. In the present study, single crystals with the shape of arectangular parallelepiped are prepared.
Figure 1 shows the uniaxial pressure cell. Using a bellows
system pressurized with helium gas at low temperatures at CEAGrenoble, uniaxial pressure has been applied at low temperature(5 K) while monitoring pressure with a piezoelectric device. The
thermal expansion was measured using strain gauges by theactive dummy method with a Wheatstone bridge.
It appears that the observed value dT0/d[110] is larger thanthat of dT0/d[100] and dT0/dP as shown in Fig. 2. As discussed
previously [3], [110] and [100] induce symmetry-breaking
strains xy and (xx-yy)√2, respectively, in addition tosymmetry-invariant strains (xx+yy+zz)√3 and (zz0.5(yy+xx))√(2/3), which are also induced in the hydrostaticcase. The observed difference between - and P-dependences ofT0: dT0/d[110]>dT0/d[100]>dT0/dP indicates that the onset ofhidden order is prompted by symmetry-breaking strains,particularly by the xy strain. This fact is consistent with theappearance of 2-fold ordering with a [110] principal axis that iscoupled with an xy strain, i.e., the 2-fold electronic state iscoupled and stabilized with 2-fold lattice distortion.

18s // [100]s // [110]T0 under hydrostatic P
Expected slope at s=0

17.5

170.0

0.2

0.40.6s i or P (GPa)0.8

1.0Fig. 2 - and P-dependences of the hidden order transition
temperature T0. The solid lines are obtained using linear
least-squares fits. dT0/d is larger for [110]. The
expected dT0/dslope at =0 from the Ehrenfest
relation is presented as a green dashed line.
References[1] R. Okazaki et al., Science. 331, 439 (2011).
[2] S. Kambe et al., Phys. Rev. B87, 115123 (2013).
[3] M. Yokoyama et al., Phys. Rev. B72, 214419 (2005).