⁵⁹Co Nuclear Quadrupole Resonance Studies of Superconducting and Nonsuperconducting Bilayer Water Intercalated Sodium Cobalt Oxides Na_xCoO₂·yH₂O

Abstract

We report 59 Co nuclear quadrupole resonance (NQR) studies of bilayer water intercalated sodium cobalt oxides NaxCoO2·yH2O (BLH) with the superconducting transition temperatures, 2 K < T c ≤ 4.6 K, as well as a magnetic BLH sample without superconductivity. We obtained a magnetic phase diagram of T c and the magnetic ordering temperature T M against the peak frequency ν3 of the 59 Co NQR transition Iz = ±5/2 ↔ ±7/2 and found a dome shape superconducting phase. The 59 Co NQR spectrum of the non-superconducting BLH shows a broadening below T M without the critical divergence of 1/T1 and 1/T2, suggesting an unconventional magnetic ordering. The degree of the enhancement of 1/T1T at low temperatures increases with the increase of ν3 though the optimal ν3 ∼ 12.30 MHz. In the NaxCoO2·yH2O system, the optimal-T c superconductivity emerges close to the magnetic instability. T c is suppressed near the phase boundary at ν3 ∼ 12.50 MHz, which is not a conventional magnetic quantum critical point.

59Co, 23Na, and 1H NMR Studies of Double-Layer Hydrated Superconductors NaxCoO2 · yH2O

“…2, superconducting transition temperatures T c and magnetic transition temperatures T M are plotted against 59 Co NQR frequency ν Q3 at 10 K [15,19,21,22,24]. The existence of two superconducting phases were confirmed.…”

“…Although no distinct changes in X-ray diffraction patterns have been observed for the various double-layer hydrated compounds, systematic changes in the peak values of 59 Co NQR spectra were observed [15,21,22,23]. Thus, we present the results of 59 Co NQR spectrum measurments to characterize the samples and to classify the electronic states.…”

“…After the powders of Na 0.35 CoO 2 · yH 2 O were separated from the solution by filtration, they were exposed in various humidity air. For the synthesized samples, we observed various duration (keeping time in the humidity-controlled chamber in a daily basis) effects on T c [15,16,17,19,20,21].…”

“…65 MHz, a possibility of charge-density-wave (CDW) ordering is excluded by the temperature dependence of both 59 Co NQR spectra of ν Q2 and ν Q3 [21,22]. The CDW ordering at a transition temperature T M must result in…”

“…Magnetic phase diagram of Tc and TM plotted against 59 Co NQR frequency νQ3 at 10 K. We reproduced the data of Y1 from [22], H1 from [15,21], H2 from [19], and H3 from [24]. The solid and dashed curves are guides for eyes.…”