Evidence of nodal gap structure in the noncentrosymmetric superconductorY2C3

Abstract

The magnetic penetration depth λ(T ) and the upper critical field µ0Hc2(Tc) of the noncentrosymmetric (NCS) superconductor Y2C3 have been measured using a tunnel-diode (TDO) based resonant oscillation technique. We found that the penetration depth λ(T ) and its corresponding superfluid density ρs(T ) show linear temperature dependence at very low temperatures (T ≪ Tc), indicating the existence of line nodes in the superconducting energy gap. Moreover, the upper critical field µ0Hc2(Tc) presents an upturn at low temperatures with a rather high value of µ0Hc2(0) ≃ 29T, which slightly exceeds the weak-coupling Pauli limit. We discuss the possible origins for these nontrivial superconducting properties, and argue that the nodal gap structure in Y2C3 is likely attributed to the absence of inversion symmetry, which allows the admixture of spin-singlet and spin-triplet pairing states. The symmetries of a superconductor, e.g., timereversal and spatial inversion symmetries, may impose important constraints on the pairing states. Among the previously investigated superconductors, most possess an inversion center in their crystal structures, in which the Cooper pairs are either in an even-parity spin-singlet or odd-parity spin-triplet pairing state, according to the Pauli principle and parity conservation [1,2]. However, the tie between spatial symmetry and the Cooper-pair spins might be violated in a superconductor lacking inversion symmetry [3][4][5][6][7]. In these materials, an asymmetric potential gradient yields an antisymmetric spin-orbit coupling (ASOC), which splits the Fermi surface into two spin-ordered subsurfaces, with pairing allowed both across one subsurface and between the two. ASOC splits the degeneracy of conduction electron, and allows the admixture of spin-singlet and spin-triplet pairing states within the same orbital channel. Various exotic electromagnetic properties may arise in the parity-violated materials [3][4][5][6][7].Until now, only a very few non-centrosymmetric (NCS) superconductors have been investigated. In the heavy fermion NCS superconductors, e.g., CePt 3 Si and CeMSi 3 (M=Rh, Ir), the upper critical field is greatly enhanced in comparison with that of other heavy fermion materials [8][9][10], exceeding the Pauli limit in a weak coupling BCS model. Measurements of thermodynamic properties in CePt 3 Si indicated the existence of line nodes in the superconducting gap, even though a weak Hebel-Slichter coherence peak is observed below T c [11][12][13][14]. These unconventional features were interpreted in terms of a Rashbatype spin-orbit coupling model arising from the absence of inversion symmetry [5][6][7]. However, the strong electronic correlations and the closeness to a magnetic instability might complicate the analysis of the ASOC effect on superconductivity. Recent work on Li 2 (Pd 1−x Pt x ) 3 B systems demonstrated that spin-singlet and spin-triplet order parameters can add constructively and destructively, leading to two different gap functions and the possible occurrence of acc...

Superconducting and normal-state properties of the noncentrosymmetric superconductorRe6Zr

“…Singlet-triplet mixing can lead NCS materials to display significantly different properties from conventional superconducting systems, for example, the triplet pairing seen in Li 2 (Pd,Pt) 3 Si [8][9][10][11], and upper critical fields close to or exceeding the Pauli limiting field observed in Mo 3 Al 2 C [3,12] Re 3 W [13], Ca(Ir, Pt)Si 3 [14], Li 2 (Pd,Pt) 3 Si [8][9][10][11], LaRhSi 3 [15], Nb 0. 18 Re 0.82 [16], Y 2 C 3 [17], and Mg 10 Ir 19 B 16 [18].…”

Evidence of double-gap superconductivity in noncentrosymmetricNb0.18Re0.82single crystals

“…As a consequence of inversion symmetry breaking, exotic properties have been predicted and observed, such as multigap and nodal superconductivity [8][9][10][11], large upper critical fields [12][13][14], nontrivial quantum topology in the superconducting ground state [15], as well as charge and spin currents along the edge [16]. The understanding of the nature of the pairing and the mechanisms that lead to superconductivity in materials without inversion symmetry is thus a fundamental and highly debated issue in the field of unconventional superconductors.…”

Tuning of superconductivity by Ni substitution into noncentrosymmetric ThCo1−xNixC2

“…The superconducting properties of several weakly correlated NCS systems have been extensively investigated, with notable examples such as: Li2(Pd,Pt)3B, Y2C3, Mg10Ir19B16, Ru7B3, LaIrP, Mo3Al3C, and with particular interest for this work LaNiC2. [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] A large series of rare-earth (R) carbides RNiC2 and RCoC2 crystallize in the NCS CeNiC2 prototype structure, which is in the base centered orthorhombic space group Amm2. 30 These compounds generally display magnetic order arising from the 4f electrons confined to the rare earth sites, 31 in addition to the series being near a charge density wave (CDW) instability.…”