Unconventional superconductivity and an ambient-pressure magnetic quantum critical point in single-crystal LaNiC2

Abstract

Superconductivity in noncentrosymmetric LaNiC 2 is expected to be induced by electron-phonon interactions due to its lack of magnetic instabilities. The non-Bardeen-Cooper-Schrieffer (BCS) behaviors found in this material call into question the long-standing idea that relates unconventional superconductivity with magnetic interactions. Here we report magnetic penetration-depth measurements in a high-purity single crystal of LaNiC 2 at pressures up to 2.5 GPa and temperatures down to 0.04 K. At ambient pressure and below 0.5T c the penetration depth goes as T 4 for the in-plane and T 2 for the out-of-plane component, firmly implying the existence of point nodes in the energy gap and the unconventional character of this superconductor. The present study also provides first evidence of magnetism in LaNiC 2 by unraveling a pressure-induced antiferromagnetic phase inside the superconducting state at temperatures below 0.5 K, with a quantum critical point around ambient pressure. The results presented here maintain a solid base for the notion that unconventional superconductivity only arises near magnetic order or fluctuations. 1 arXiv:1806.03818v1 [cond-mat.supr-con]

Nonunitary triplet pairing in the noncentrosymmetric superconductor LaNiC2

“…However, this does not provide clear evidence for a ferromagnetic order, since the proper way to determine the magnetic order of the ground state is to perform ab-initio spin dynamics at zero temperature [54]. Recently, an experimental hint of a magnetic phase was also found in reference [19] even for small pressure which seems to agree with our results.…”

“…Although evidence for nodal gap structure was found from some measurements [17][18][19], recent specific heat [20,21], nuclear quadrupole relaxation [22], magnetic impurity [23], and penetration depth measurements [24] indicate fully gapped behavior in LaNiC 2 . It was also measured that the magnetization points in the direction of the z-axis indicating close relationship with the crystal structure, which lacks inversion symmetry along the z-axis [25].…”

Structure and superconductivity in the binary Re1−xMox alloys

“…For instance, some NCSCs, such as CePt 3 Si [3], CeIrSi 3 [4], Li 2 Pt 3 B [5,6], K 2 Cr 3 As 3 [7,8], and YBiPt [9] exhibit line nodes in the gap, while others, such as LaNiC 2 [10] and (La,Y) 2 C 3 [11], show multiple-gap superconductivity. The external pressure drives CeIrSi 3 into a gapless superconductor [12], while a nodal behavior has been observed in LaNiC 2 and Y 2 C 3 [13][14][15]. Recently, many NCSCs [2,9,16,[16][17][18][19][20][21], in particular YPtBi [9], BiPd [20], and PbTaSe 2 [21], have been closely investigated as possible models of topological superconductors.…”

Simultaneous Nodal Superconductivity and Time-Reversal Symmetry Breaking in the Noncentrosymmetric Superconductor CaPtAs

“…A notable feature of most of the weakly correlated NCSCs with broken TRS is the presence of fully opened superconducting gaps. In the case of LaNiC 2 , inconsistent results, including both fully opened and nodal gap structures, have been found from measurements of the order parameter [39][40][41][42][43][44]. The nodeless superconductivity of weakly correlated NCSCs is not only in contrast to the general expectations for strong singlettriplet mixing, but also sets these systems apart from the strongly correlated superconductors Sr 2 RuO 4 and UPt 3 [3,4], where the presence of unconventional pairing mechanisms is more unambiguously determined.…”

“…In LaNiC 2 , the low symmetry of its orthorhombic crystal structure means that the breaking of TRS at T c necessarily implies nonunitary triplet pairing, and rules out the mixed singlet-triplet state described below [7,45]. However, measurements of the gap symmetry have yielded inconsistent results, where both fully-opened and nodal gap structures have been reported [39,40,[40][41][42][43]. Compared to the above cases, CaPtAs represents a new remarkable NCSC, which accommodates both broken TRS and nodal SC.…”