A second-order phase transition is characterized by spontaneous symmetry breaking. The nature of the broken symmetry in the so-called "hidden-order" phase transition in the heavy-fermion compound URu(2)Si(2), at transition temperature T(h) = 17.5 K, has posed a long-standing mystery. We report the emergence of an in-plane anisotropy of the magnetic susceptibility below T(h), which breaks the four-fold rotational symmetry of the tetragonal URu(2)Si(2). Two-fold oscillations in the magnetic torque under in-plane field rotation were sensitively detected in small pure crystals. Our findings suggest that the hidden-order phase is an electronic "nematic" phase, a translationally invariant metallic phase with spontaneous breaking of rotational symmetry.

Magnetoresistivity measurements with fine tuning of the field direction on high quality single crystals of the ferromagnetic superconductor UCoGe show anomalous anisotropy of the upper critical field Hc2. Hc2 for H b-axis (H b c2 ) in the orthorhombic crystal structure is strongly enhanced with decreasing temperature with an S-shape and reaches nearly 20 T at 0 K. The temperature dependence of H a c2 shows upward curvature with a low temperature value exceeding 30 T, while H c c2 at 0 K is very small (∼ 0.6 T). Contrary to conventional ferromagnets, the decrease of the Curie temperature with increasing field for H b-axis marked by an enhancement of the effective mass of the conduction electrons appears to be the origin of the S-shaped H b c2 curve. These results indicate that the field-induced ferromagnetic instability or magnetic quantum criticality reinforces superconductivity.The coexistence of superconductivity (SC) and ferromagnetism (FM) has attracted much attention, since the exotic SC state based on spin-triplet pairing mediated by longitudinal spin fluctuations is expected. 1 The first example was discovered in UGe 2 under pressure, where T sc is much lower than T Curie . 2 The SC phase exists only in the FM phase, and SC disappears in the paramagnetic (PM) phase above the critical pressure P c . Soon after that, SC was found at ambient pressure in the weak ferromagnet URhGe. 3 T sc (= 0.25 K) is much lower than T Curie (= 9.5 K). The upper critical field H c2 exceeds the Pauli paramagnetic limit, thus it is believed that the spin-triplet state with equal-spin pairing is realized. Recently, reentrant superconductivity (RSC) was found between 8 and 13 T when the field is applied along the b-axis of the orthorhombic TiNiSi-type structure (space group: Pnma). 4 With increasing field, the magnetic moment gradually tilts from the c-axis (easy-magnetization axis) to b-axis. Finally the moment is suddenly aligned to the b-axis at the spin reorientation field H R . The fieldinduced critical magnetic fluctuations induce RSC. Recently, we have observed the enhancement of effective mass in the RSC phase, and explained the emergence of RSC. 5,6 A newcomer, UCoGe, which crystallizes in the same structure as URhGe, was recently reported. 7 UCoGe is a weak ferromagnet with T Curie ∼ 3 K and the ordered moment µ 0 = 0.07 µ B /U. T sc (∼ 0.6 K) is larger than that in URhGe. Since T Curie is low, one can naively consider that UCoGe is close to the quantum critical point. Indeed, our previous measurement shows that T Curie is immediately suppressed by applying a small pressure (P c ∼ 1 GPa). 8 Contrary to the case of UGe 2 where SC exists only in the FM domain, SC survives even in the PM phase with the maximum T sc (≃ 0.75 K) around P c . A new theory * E-mail address: aokidai@gmail.com from symmetry considerations was developed in order to explain the temperature-pressure phase diagram. 9 H c2 at ambient pressure shows strong anisotropy. H c2 for H a (H a c2 ) and b-axis (H b c2 ) reveal almost the same temperature depend...

We show that the charge and thermal transport measurements on ultraclean crystals of URu2Si2 reveal a number of unprecedented superconducting properties. The uniqueness is best highlighted by the peculiar field dependence of thermal conductivity including the first order transition at Hc2 with a reduction of entropy flow. This is a consequence of multi-band superconductivity with compensated electronic structure in the hidden order state of this system. We provide strong evidence for a new type of unconventional superconductivity with two distinct gaps having different nodal topology.The heavy-Fermion compound URu 2 Si 2 has mystified researchers since the superconducting state (T c = 1.5 K) is embedded within the "hidden order" phase (T h = 17.5 K) [1,2,3]. Although several exotic order parameters have been proposed for the hidden order phase [4], it is not identified yet. According to several experimental observations, most of the carriers disappear below T h resulting in a density one order of magnitude smaller than in other heavy-Fermion superconductors [5,6,7]. Superconductivity with such a low density is remarkablesince the superfluid density is very low in some way reminiscent of underdoped cuprates. Moreover, pressure studies reveal that the superconductivity coexists with the hidden order but is suppressed by antiferromagnetic ordering [8].In this Letter, using ultraclean single crystals, we report various anomalous superconducting properties in URu 2 Si 2 . We show that a peculiar electronic structure appearing below the hidden order transition provides an intriguing stage on which a new type of unconventional superconducting state appears.Single crystals of URu 2 Si 2 were grown by the Czochralski pulling method in a tetra-arc furnace. The welldefined superconducting transition was confirmed by the specific heat measurements. The thermal conductivity κ was measured using a standard four-wire steady state method along the a-axis (heat current q a). The contact resistance at low temperatures is less than 10 mΩ.We first discuss the electronic structure below T h . Figure 1 shows the temperature dependence of the resistivity ρ along the a-axis and Hall coefficient R H (solid circles) defined as R H ≡ dρxy dH at H → 0 T for H c in the tetragonal crystal structure. In zero field, ρ depends on T as ρ = ρ 0 + AT 2 below 6 K down to T c . The exceptionally low residual resistivity ρ 0 = 0.48 µΩ cm and large residual resistivity ratio RRR = 670 attest the highest crystal quality currently achievable. R H exhibits an eight-fold increase below T h and attains a T -independent value at low temperatures, associated with a strong reduction of the carrier density. Most remarkably, the magnetoresistance (MR) increases with decreasing temperature and becomes extremely large at the lowest temperatures. The inset of Fig.

Since the 1985 discovery of the phase transition at T HO ¼ 17.5 K in the heavy-fermion metal URu 2 Si 2 , neither symmetry change in the crystal structure nor large magnetic moment that can account for the entropy change has been observed, which makes this hidden order enigmatic. Recent high-field experiments have suggested electronic nematicity that breaks fourfold rotational symmetry, but direct evidence has been lacking for its ground state in the absence of magnetic field. Here we report on the observation of lattice symmetry breaking from the fourfold tetragonal to twofold orthorhombic structure by high-resolution synchrotron X-ray diffraction measurements at zero field, which pins down the space symmetry of the order. Small orthorhombic symmetry-breaking distortion sets in at T HO with a jump, uncovering the weakly first-order nature of the hidden-order transition. This distortion is observed only in ultrapure samples, implying a highly unusual coupling nature between the electronic nematicity and underlying lattice.

We report on novel antiferromagnetic (AFM) and superconducting (SC) properties of noncentrosymmetric CePt3Si through measurements of the 195Pt nuclear spin-lattice relaxation rate 1/T(1). In the normal state, the temperature (T) dependence of 1/T(1) unraveled the existence of low-lying levels in crystal-electric-field multiplets and the formation of a heavy-fermion (HF) state. The coexistence of AFM and SC phases that emerge at T(N)=2.2 K and T(c)=0.75 K, respectively, takes place on a microscopic level. CePt3Si is the first HF superconductor that reveals a peak in 1/T(1) just below T(c) and, additionally, does not follow the T3 law that used to be reported for most unconventional HF superconductors. We remark that this unexpected SC characteristic may be related to the lack of an inversion center in its crystal structure.

Neutron scattering experiments have been carried out on the heavy fermion antiferromagnetic (AFM) superconductor CePt 3 Si with T N = 2.2 K and T SC = 0.75 K. We observed clear AFM Bragg reflections with Q 0 = (001/2) below and above T SC , indicating microscopic coexistence of AFM order and heavy fermion superconductivity. The AFM structure, of two interleaved ferromagnetic sublattices of local Ce 4f moments, has inversion symmetry under simultaneous space-time reversal. However, hybridization with Pt and Si breaks this degeneracy and a combination of these two competing effects may be relevant to an understanding of the simultaneous occurrence of superconductivity and AFM order. The observed magnetic moment 0.16(1) µ B /Ce is strongly reduced from the Curie-Weiss effective moment 2.54 µ B /Ce. Clear crystal field excitations at 1 and 24 meV were observed. The magnetic susceptibility can be well explained in a level scheme assuming the 7 ground state, 6 and 7 first and second excited states, respectively.

We make the first report that a metallic pyrochlore oxide, Cd2Re2O7, exhibits type II superconductivity at 1.1 K. The pyrochlore oxide is known to be a geometrically frustrated system, which includes a tetrahedral network of magnetic ions. A large number of compounds are classified in the family of pyrochlore oxides, and these compounds exhibit a wide variety of physical properties ranging from insulator through semiconductor and from bad metal to good metal. Until now, however, no superconductivity has been reported for frustrated pyrochlore oxides. The bulk superconductivity of this compound is confirmed by measurements of the resistivity and the alternating-current magnetic susceptibility. The upper critical field Hc2, which is extrapolated to 0 K, is estimated as about 0.8 T, using the resistivity measurements under an applied field. The plot of Hc2 versus T indicates that the Cooper pairs are composed of rather heavy quasiparticles. This fact suggests that frustrated heavy electrons become superconducting in this compound.

Specific heat and magnetization measurements have been performed on high-quality single crystals of filled-skutterudite PrFe4P12 in order to study the high-field heavy fermion state (HFS) and low-field ordered state (ODS). From a broad hump observed in C/T vs T in HFS for magnetic fields applied along the 100 direction, the Kondo temperature of ∼ 9 K and the existence of ferromagnetic Pr-Pr interactions are deduced. The 141 Pr nuclear Schottky contribution, which works as a highly-sensitive on-site probe for the Pr magnetic moment, sets an upper bound for the ordered moment as ∼ 0.03µB /Pr-ion. This fact strongly indicates that the primary order parameter in the ODS is nonmagnetic and most probably of quadrupolar origin, combined with other experimental facts. Significantly suppressed heavy-fermion behavior in the ODS suggests a possibility that the quadrupolar degrees of freedom is essential for the heavy quasiparticle band formation in the HFS. Possible crystalline-electric-field level schemes estimated from the anisotropy in the magnetization are consistent with this conjecture. 71.27.+a, 75.40.Cx, 71.70.Jp, 71.70.Ch