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.

We observed de Haas-van Alphen (dHvA) oscillation in both the normal and superconducting mixed states of a heavy-fermion superconductor CeCoIn5. The Fermi surfaces are found to consist of nearly cylindrical Fermi surfaces and small ellipsoidal ones, reflecting the unique tetragonal crystal structure. The detected cyclotron masses of 5-87 m0 for these Fermi surfaces are extremely large, and correspond to a large electronic specific heat coefficient of about 1000 mJ K-2 mol-1. The cyclotron masses are also found to be field dependent in both the normal and mixed states.

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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.

dc magnetization measurements on CeCoIn 5 reveal a first-order phase transition at H c2 for both Hʈa and c axes in the isothermal magnetization M (H) below 0.7 K, indicating a strong Pauli paramagnetic suppression in the even-parity pairing. M (T) in the normal state above H c2 exhibits non-Fermi-liquid behavior down to 150 mK, implying the existence of antiferromagnetic fluctuations behind the unconventional superconductivity. We observed an unusual peak effect for Hʈc in fields 5-30 kOe below 150 mK(ϭ0.06T c ), whose anomalous temperature dependence cannot be simply explained by ordinary mechanisms.Since in 1979, 1 HF superconductivity has been attracting interest in the field of strongly correlated electron systems. Recent experimental and theoretical progress indicates that most of the HF superconductors are likely to be of an unconventional type. Until recently, to our best knowledge, CeCu 2 Si 2 was the only Ce-based HF superconductor at ambient pressure. The superconductivity in CeCu 2 Si 2 , however, is rather difficult to understand because of the complicated magnetic phase diagram. 2 Quite recently, two tetragonal Ce-based HF compounds have been discovered by Petrovic et al. to become superconducting at ambient pressure: CeXIn 5 ͓XϭIr ͑Ref. 3͒ and Co ͑Ref. 4͔͒. To our best knowledge, CeCoIn 5 has the highest T c (ϭ2.3 K) among the HF superconductors known at present. The specific heat, 4 thermal conductivity, 6 and NMR relaxation rate 8 of CeCoIn 5 show power-law temperature dependencies below T c , suggesting an unconventional superconductivity with anisotropic energy gap. Very recently, the NMR Knight-shift measurement 7,8 has revealed even-parity pairing in the superconducting state, and the angle-dependent thermal-conductivity measurement 9 has identified that the gap symmetry is k x 2 Ϫk y 2 , pointing to the fact that the pairing interaction is mediated by magnetic fluctuations. An interesting observation with respect to this point is the non-Fermiliquid ͑NFL͒ behavior in the specific heat divided by temperature C/T, showing a remarkable upturn on cooling when the superconductivity is suppressed by magnetic field. 4,5 To our best knowledge the origin of the NFL behavior has not been clarified yet.One of the features of the HF superconductors is that the orbital limiting field is relatively high despite low T c , because of the small Fermi velocity of the carriers. In addition, the HF superconductors possess quite large normal-state paramagnetic susceptibility at sufficiently low temperature, reflecting the high density of states. These facts lead to an interesting situation in which the paramagnetic energy near H c2 becomes a significant fraction of the superconducting condensation energy. 10,11 It was theoretically pointed out that a second-order transition at H c2 changes into a first-order one below ϳ0.56 T c for the singlet pairing, provided that the normal-state spin susceptibility is large enough. 12,13 Subsequent theoretical studies predicted that in the case of a clean limit (lӷ 0 ) a firs...

We confirmed bulk-superconductivity of a ferromagnet UGe 2 by the specific heat measurement, together with the measurements of the electrical resistivity and ac susceptibility, in a pressure range from p = 1.0 to 1.5 GPa, where the Curie temperature T C (= 22-36 K) is still high, but another characteristic temperature T * is close to zero. In this pressure range, the heavy fermion state is found to be formed at low temperatures.Cerium and uranium compounds indicate a variety of phenomena including magnetic and quadrupolar ordering, heavy fermion and anisotropic superconductivity [1]. In these compounds, the RKKY interaction and the Kondo effect compete with each other. The former interaction enhances the long-range magnetic order, while the latter effect quenches the magnetic moments of localized f electrons. Most of the cerium and uranium compounds order magnetically, where the former interaction overcomes the latter effect. When the magnetic ordering temperature is low enough or close to zero, the heavy fermion state is formed at low temperatures. The conduction electrons in the heavy fermion state are highly different from bare electrons. They are interacting electrons, moving slowly in the crystal, which correspond to a large effective mass m * or a large electronic specific heat coefficient γ .When pressure p is applied to the cerium compounds with antiferromagnetic ordering such as CeIn 3 and CePd 2 Si 2 , the Néel tempereture T N shifts to lower temperatures, and the magnetic quantum critical point corresponding to the extrapolation T N → 0 is reached at p = p c [2]. Superconductivity appears around p c . Correspondingly, the heavy fermion state is formed as p approaches p c . This seems to be a general feature, although the sample quality is essentially important for the appearance of superconductivity. This is because superconductivity is most likely to be magnetically-mediated or of a non-s-wave type and then the breaking of Cooperpairs is mainly due to impurities and crystal defects.

We report the discovery of pressure-induced superconductivity in ferromagnetic UIr, which lacks inversion symmetry in the crystal structure. The Curie temperature T C1 = 46 K at ambient pressure decreases with increasing pressure, reaching a value of 11 K at 1.5 GPa. It presumably decreases further up to about P c1 = 1.7 GPa. The ferromagnetic region named 'F1' exists up to P c1 . A second magnetic phase named 'F2' with a low ferromagnetic moment appears in the pressure range from 1.9 to 2.4 GPa. In the 'F2' phase, the magnetic transition temperature T C2 decreases with pressure, from 18 K at 1.9 GPa to approximately zero at P C2 = 2.6-2.7 GPa. In this critical pressure region, superconductivity appears below T sc = 0.14 K.