Line Nodes in the Superconducting Gap Function of NoncentrosymmetricCePt3Si

Abstract

The superconducting gap structure of recently discovered heavy fermion CePt3Si without spatial inversion symmetry was investigated by thermal transport measurements down to 40 mK. In zero field a residual T -linear term was clearly resolved as T → 0, with a magnitude in good agreement with the value expected for a residual normal fluid with a nodal gap structure, together with a T 2 -dependence at high temperatures. With an applied magnetic fields, the thermal conductivity grows rapidly, in dramatic contrast to fully gapped superconductors, and exhibits one-parameter scaling with T / √ H. These results place an important constraint on the order parameter symmetry, that is CePt3Si is most likely to have line nodes.PACS numbers: 74.20. Rp, 74.25.Bt, 74.25.Fy, 74.70.Tx @ In heavy fermion (HF) compounds containing Ce, Pr and U atoms, the strong Coulomb repulsion within the atomic f -shells leads to a notable many-body effect and often gives rise to unconventional superconductivity, in which the superconducting gap function has line or point nodes along certain directions in the Brillouin zone. Since the gap structure is closely related to the pairing mechanism of the electrons, the gap structure of HF superconductors is very important in understanding the physics of unconventional superconductivity in strongly correlated systems [1,2].Very recently a new HF superconductor CePt 3 Si with T c =0.75 K has been discovered [3]. CePt 3 Si has aroused great interest because it possesses quite unique properties. The most noticeable feature is the absence of spatial inversion symmetry, in contrast to most unconventional superconductors. It is well known that in the presence of strong spin-orbit interaction, the absence of inversion symmetry gives rise to a splitting of the two spin degenerate bands. It has been shown that the lifted spin degeneracy strongly influences the pairing symmetry of Cooper pairs [3,4,5,6,7,8,9,10,11,12]. For example, Rashba-type spin-orbit interaction dramatically changes the paramagnetic effect, which breaks up Cooper pairs [5,6,7,8,9]. According to a subsequent band structure calculation, the band splitting energy near the Fermi level in CePt 3 Si is more than a thousand times larger than k B T c , indicating strong spin-orbit coupling [10]. The special interest in the symmetry of CePt 3 Si has arisen with the observation that the upper critical field H c2 ∼ 4 T drastically exceeds the Pauli paramagnetic limit H P ∼ 1.3k B T c /µ B ∼1 T [3]. Moreover, recent NMR measurements have reported no change of the Knight shift below T c for H c [13]. These results indicate that the paramagnetic effect on Cooper pairs is absent or is strongly reduced in CePt 3 Si. Generally in the system without inversion symmetry the gap function is a mixture of spin-singlet and -triplet channels in the pres-

Enhanced triplet superconductivity in noncentrosymmetric systems

“…1 This is because this material does not have inversion center, which has stimulated further studies. 2,3,4,5,6,7,8,9,10,11,12,13,14,15 Because of the broken inversion symmetry, Rashba type spin-orbit coupling 16 (RSOC) is induced, and hence different parities, spinsinglet pairing and spin-triplet pairing, can be mixed in superconducting state. 17 From a lot of experimental and theoretical studies, it is believed that the most possible candidate of superconducting state in CePt 3 Si is s+p-wave pairing.…”

“…17 From a lot of experimental and theoretical studies, it is believed that the most possible candidate of superconducting state in CePt 3 Si is s+p-wave pairing. 4,5,6,7,8,9,10,11,12 In general, d+f -wave pairing or other mixtures are allowed in noncentrosymmetric superconductors depending on material parameters. 11 One important and interesting point about noncentrosymmetric superconductors is how relative magnitude of the singlet and triplet components is determined by microscopic parameters, which has been unclear up to now.…”

Non-centrosymmetric Heavy-Fermion Superconductors

“…Temperature dependence of (a) thermal conductivity [29] and (b) specific heat [30] of CePt 3 Si. The linear temperature response suggests line nodes in the energy gap.…”

Effects of impurities on superconductivity in noncentrosymmetric compounds

“…54,55,56,57,58 The lines of nodes are required by symmetry for all nontrivial one-dimensional representations of C 4v (A 2 , B 1 , and B 2 ), so that the superconductivity in CePt 3 Si is most likely unconventional. This can be verified using the measurements of the dependence of T c on the impurity concentration: For all types of unconventional pairing, the suppression of the critical temperature is described by the universal Abrikosov-Gor'kov function, see Eq.…”