Absence of the O17 Knight-shift changes across the first-order phase transition line in Sr2RuO4

Abstract

We performed17 O nuclear magnetic resonance measurements on superconducting (SC) Sr2RuO4 under in-plane magnetic fields. We found that no new signal appears in the SC state and that the 17 O Knight shifts obtained from the double-site measurements remain constant across the first-order phase-transition line, as well as across the second-order phase-transition line as already reported. The present results indicate that the SC spin susceptibility does not decrease in the high-field region, although a magnetization jump in the SC state was reported at low temperatures. Because the spin susceptibility is unchanged in the SC state in Sr2RuO4, we suggest that the first-order phase transition across the upper critical field should be interpreted as a depairing mechanism other than the conventional Pauli-paramagnetic effect.The layered perovskite Sr 2 RuO 4 has attracted special attention, because it has been suggested that Sr 2 RuO 4 may be a chiral p-wave spin-triplet superconductor.1 The chiral state is shown from the broken time-reversal symmetry probed by µSR 2 and Kerr-effect 3 measurements. The existence of spin-triplet equal-spin pairing is based on experimental results that show the spin susceptibility is unchanged on passing through the superconducting (SC) transition temperature T c , as revealed by nuclear magnetic resonance (NMR) Knight-shift measurements at the Ru and O sites 4-8 and polarized neutron scattering measurements.9 The chiral p-wave spin-triplet state would be an SC state analogous to the superfluid 3 He A-phase with two dimensionality.However, several recent experimental results are difficult to interpret with the above SC state. The firstorder (FO) SC-normal (S-N) transition 10 accompanied by a clear magnetization jump 11 is observed in a lowtemperature region near the upper critical field H c2 for fields parallel to the ab plane. This abrupt S-N transition suggests that Sr 2 RuO 4 is a spin-singlet superconductor, because this cannot be interpreted by the conventional orbital depairing effect but seems to be explained consistently by the conventional Pauli-paramagnetic effect. Indeed, the experimental µ 0 H c2 for T → 0 nearly matches the Pauli-limiting field µ 0 H Pauli estimated using the well-1/2 ∼ 1.4 T with χ sc = 0, where E cond is the SC condensation energy and χ n and χ sc are the spin susceptibilities in the normal and SC states, respectively. Here, χ sc = 0 means that the spin susceptibility totally vanishes in the SC state, which contradicts the above spin-susceptibility results showing χ n = χ sc .Recently, we performed 99 Ru Knight-shift ( 99 K) measurements again to re-examine the previous results, and found a new phenomenon that the spin susceptibility slightly increases in the SC state at lower magnetic fields.12 We reported that this experimental result further suggests the spin-triplet equal-spin pairing state. 13Because the hyperfine coupling constant A hf at the 99 Ru site is largest among the nuclei that are feasible for NMR in Sr 2 RuO 4 ( 99 A hf ∼ −25 T/µ B ), 6 the s...

Even odder after twenty-three years: the superconducting order parameter puzzle of Sr2RuO4

“…4. There is no experimental inconsistency in the reported signals: no drop of the extracted spin susceptibility has been seen in any NMR or neutron scattering measurement on Sr2RuO4 [70][71][72][73][74][75] , and indeed a small rise has been reported in the most precise measurements to date 72 . All of the available data have been interpreted in terms of odd parity order parameters, though the lack of a dependence of the results on field orientation has necessitated the slightly worrying postulate that the vector order parameter can be rotated in extremely small applied fields of order 20 mT.…”

Multicomponent order parameter superconductivity of Sr2RuO4 revealed by topological junctions

“…Spin susceptibility measurements by the nuclear magnetic resonance (NMR) 7 and the polarized neutron scattering 8 below T c supports the spin-triplet scenarios. Recently, invariant spinsusceptibility is re-confirmed using Ru and Sr nuclei and O as well 9,10 . The muon spin rotation and magnetooptical Kerr effect evidence the time-reversal symmetry breaking in the superconducting order parameter of SRO 11 .…”

Magnetization jump in one dimensional J − Q 2 model with anisotropic exchange

“…Most of these model systems involve anisotropy or frustration. Experimentally, the magnetization jump was first confirmed in hydrated copper compound [20], and then was found in many kinds of magnetic materials [21][22][23][24][25][26][27]. However, understanding the mechanism of the magnetization jump in an intuitive way is still in exploration.…”