Spin polarization enhanced by spin-triplet pairing inSr2RuO4probed by NMR

Abstract

We report a novel phenomenon intimately related to the spin-triplet superconductivity. It is well known that the spin susceptibility decreases below the superconducting transition temperature in almost all superconductors because of spin-singlet pair formation, while it may remain unchanged in a handful of spin-triplet exceptions. Here we report the observation in Sr2RuO4 with nuclear magnetic resonance (NMR) that the spin susceptibility originating from the Ru-4d electron slightly increases by ∼ 2% of total and becomes inhomogeneous in the superconducting state. These are reasonably explained if the electron pairs form the equal-spin-pairing (ESP) in the mixed state. A similar phenomenon was predicted for superfluid 3 He forty years ago, but had never been demonstrated in any superconductor. Ru [5,6] nuclei are unchanged in the SC state. The unchanged spin susceptibility, which was supported also from the polarized neutron scattering measurements [7], strongly suggests that the equal-spin-pairing (ESP) state of the spin-triplet superconductivity is realized in SRO, in which the SC pairs consist of the up-up (| ↑↑ ) or down-down (| ↓↓ ) pairs. In addition, the µSR [8] and Kerr-effect [9] measurements in the SC state suggest the broken time-reversal symmetry. Unlike heavy-fermion systems, the effect of spin-orbit coupling should be relatively weak, the pairing state in SRO would be closely analogous to the case of superfluid 3 He. By taking the experimental results into account, the chiral p-wave spin-triplet state, which is analogous to the A-phase of superfluid 3 He [10], has been considered as the most promising candidate for the SC pairing state [11,12].However, toward the establishment of this pairing state, there still remains a controversy since the superconductivity is strongly suppressed with a first-order transition under the in-plane magnetic fields near the SC critical field H c2 as shown in Fig. 1 (a) [13,14]. In addition, sharp magnetization jump with the hysteresis at H c2 was observed when the magnetic field is exactly parallel to the ab plane at 0.1 K [15]. These are phenomena usually expected in a spin-singlet superconductor [16,17]. Furthermore, non-detection of some of the behaviors expected for the chiral p-wave state (e.g. chiral edge current [18][19][20] and splitting of T c by in-plane magnetic fields of any magnitude [21]) casts some doubts on this pairing state. Therefore, convincing evidence for establishing the SC pairing state in SRO, particularly finding a new phenomenon specific to the pairing state, has been desired.In this paper, we report, from the "double-site" Knight-shift measurement in the SC state, which is the reliable method to subtract the Meissner effect from the observed Knight shift, that the spin susceptibility of SRO becomes inhomogeneous and its average slightly increases in the SC state. The small increase of spin susceptibility cannot be explained with a singlet-pairing state but consistently interpreted with the ESP state in triplet superconductivity. As far as ...

Anisotropic spin fluctuations in Sr2RuO4 : Role of spin-orbit coupling and induced strain

“…Many experimental results provide indirect evidence for a triplet state with a broken time-reversal symmetry and odd-parity Cooper pairs, although the 'smoking gun' experiment is still missing. Among these are the Knight shift measurements 3,4 that are in agreement with polarized neutron scattering experiments 5 . There are also indications of the broken time-reversal symmetry by polar Kerr Effect measurements 6 .…”

The Role of Odd‐Frequency Pairing in Multiband Superconductors

“…While it possesses a fairly low critical temperature, K, the superconducting phase of Sr 2 RuO 4 has attracted a great deal of attention since its discovery in 1994 due to its highly unusual properties. Both Knight shift and neutron scattering measurements have indicated the possibility of spin‐triplet pairing . Additionally, it has been observed that the superconducting phase exhibits spontaneous time‐reversal symmetry breaking using muon spin‐relaxation measurements, as well as measurements of the Kerr effect .…”

Strong peak in T c of Sr 2 RuO 4 under uniaxial pressure

“…Knight shift measurements [12,13] and, recently, proximity-induced superconductivity in epitaxial ferromagnetic SrRuO 6 layers [14] provide strong evidence for triplet pairing. Muon spin rotation [15] and Kerr rotation [16] experiments point to time reversal symmetry breaking at…”