Superconducting Property in CePt₃Si under Pressure

Abstract

We have studied the superconducting property by measuring the electrical resistivity under the magnetic field and pressure for CePt 3 Si, which is the first antiferromagnetic heavy fermion superconductor without inversion symmetry in its tetragonal crystal structure. The upper critical field H c2 ð0Þ is found to be approximately isotropic: H c2 ð0Þ ¼ 2:7 T for H k ½100 and 3.2 T for H k ½001. The Néel temperature T N ¼ 2:3 K and the zero resistivity superconducting transition temperature T sc ' 0:65 K decrease markedly with increasing pressure, suggesting that the present superconductivity is correlated to the antiferromagnetic state.A recently discovered superconductor CePt 3 Si is unique, possessing a few characteristics. 1) Superconductivity with the transition temperature T sc ¼ 0:75 K is realized in the long-range antiferromagnetic state with the Néel temperature T N ¼ 2:2 K. Both transitions were observed clearly in the specific heat measurement. The electronic specific heat coefficient is large, 300-400 mJ/K 2 Ámol. In the previous cerium-based heavy fermion superconductors including a prototype superconductor CeCu 2 Si 2 and a quasi-two-dimensional superconductor CeCoIn 5 with a large value of 1000 mJ/K 2 Ámol, superconductivity occurs in the nonmagnetic state or the antiferromagnetically spin-fluctuating state. 2-4) Another example is CeIn 3 under pressure. 4-6) At ambient pressure, the Néel temperature in CeIn 3 is T N ¼ 10 K and the ordered moment is 0.5 B /Ce. With increasing pressure P, T N decreases smoothly as a function of pressure and becomes zero at around a critical pressure P c ' 2:7 GPa where superconductivity sets in below 0.2 K.Moreover, it is noted that CePt 3 Si is the first heavy fermion superconductor lacking a center of symmetry. The crystal structure of CePt 3 Si is tetragonal, space group P4mm (No. 99) with a ¼ 4:072 # A and c ¼ 5:442 # A. The existence of inversion symmetry in the crystal structure is believed to be a favorable factor for superconductivity. The absence of superconductivity in, for example, ferromagnet MnSi was tentatively attributed to the lack of inversion symmetry in its crystal structure. 7) Theoretically it was, however, argued that spin-triplet pairing is not entirely excluded in MnSi and CePt 3 Si because the lack of inversion symmetry reduces the effect of paramagnetic limiting for spin-singlet pairing. 8) In fact we have very recently observed superconductivity in the vicinity of a critical pressure for ferromagnet UIr without inversion symmetry. 9) The relationship between superconductivity and lack of inversion symmetry is the most crucial issue to be clarified at present.We have studied an effect of magnetic field and pressure on the superconducting state for a single crystal with high quality. The electrical resistivity has been measured to investigate the anisotropy of the upper critical field and the relationship between antiferromagnetic ordering and superconductivity.Single crystals of CePt 3 Si and non-4f reference compound LaPt 3 Si were grown by t...

Effects of impurities on superconductivity in noncentrosymmetric compounds

“…As far as experiment is concerned, the upper critical field measurements in CePt 3 Si, both in polycrystals 1 and in single crystals, 54 give the zero-temperature values of H c2 exceeding the Clogston-Chandrasekhar expression for the paramagnetic limit, H P ≃ 1.24T c /µ B . This can be understood as being due to the large residual spin paramagnetic susceptibility, see Refs.…”

Non-centrosymmetric Heavy-Fermion Superconductors

“…Measurements on high-quality single crystals of CePt 3 Si show a weak anisotropy for the upper critical field H c2 (0) with a value around 3 T (Fig. 5) [10] that exceeds the standard BCS weak-coupling paramagnetic limit H P (0) ≈ 1 T. From Eq. (3) and the predictions for χ s /χ n in the non-centrosymmetric superconductors, no limiting behavior is expected for fields parallel to the c axis, whereas H P (0) = ∆ 0 /µ B ≈ 1.4 T for fields in the ab plane.…”

“…However, CePt 3 Si becomes a non-Fermi-liquid under pressure, as indicated by the linear temperature behavior of the resistivity above 0.4 GPa [10]. Most of the unusual superconducting properties found initially in CePt 3 Si have been clarified by now, but others have appeared.…”

“…CePt 3 Si is also located near the H BCS orb line. Considering that H c2 is almost isotropic in CePt 3 Si [10], it may be also mainly constrained by the orbital limit rather than by the paramagnetic one.…”

Effects of Impurities on a Noncentrosymmetric Superconductor: Application to CePt3Si

“…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 (Frigeri et al, 2004;. This mixing of the pairing channels with different parity may result in unusual properties of experimentally observed quantities such as a very high upper critical field 2 c H which exceeds the paramagnetic limit (Bauer et al, 2004;Bauer et al, 2005aBauer et al, , 2005bYasuda et al, 2004), and the simultaneous appearance of a coherence peak feature in the NMR relaxation rate 1 1 T  and low-temperature power-law behavior suggesting line nodes in the quasiparticle gap (Bauer et al, 2005a(Bauer et al, , 2005bYogi et al, 2004). The presence of line nodes in the gap of CePt 3 Si is also indicated by measurements of the thermal conductivity (Izawa et al, 2005) and the London penetration depth Bonalde et al, 2005).…”