Muon Spin Relaxation Measurements ofNaxCoO2·yH2O

Abstract

Using the transverse field muon spin relaxation technique we measure the temperature dependence of the magnetic field penetration depth λ, in the NaxCoO2 · yH2O system. We find that λ, which is determined by superfluid density ns and the effective mass m * , is very small and on the edge of the TF-µSR sensitivity. Nevertheless, the results indicate that the order parameter in this system has nodes and that it obeys the Uemura relation. By comparing λ with the normal state electron density we conclude that m * of the superconductivity carrier is 70 times larger than the mass of bare electrons. [4] found no coherence peak, questioning the previous result. Therefore, an additional and different experimental approach is required. A possible approach is to measure the temperature dependence of the magnetic field penetration depth λ. At low temperatures, λ is sensitive to low-lying excitations, and, in the case of a complete gap λ(T ) − λ(0) should vary exponentially as a function of T . On the other hand, nodes in the gap lead to a power-law dependence of this penetration depth difference. A study of λ can help addressing the other two questions as well. For the third question, one of the most universal correlations among the unconventional superconductors is the relation between the transition temperature T c and the width of the transverse field muon spin rotation (TF-µSR) line at low temperatures, σ(0) ∝ λ −2 . Uemura et. al.[5] were able to show that the same relation holds for the underdoped cuprates, the bismuthates, Chevrel-phase and the organic superconductors. This relation has no explanation in the frame-work of the BCS theory, and it is usually explained in terms of phase coherence establishment in a theory of local fluctuations of the order parameter [6]. It is interesting to know if Na x CoO 2 · yH 2 O also obeys this relation. The second question would be addressed by the absolute value of σ(0).The aim of this work is to measure the temperature dependence of λ with TF-µSR in Na x CoO 2 · yH 2 O. TF-µSR is a very useful way to study superconductors in the mixed state. In this method 100% spin polarized muons are implanted in the sample, which is cooled in a field perpendicular to initial muon spin. Above T c , where the external field penetrates the sample uniformly, the second moment of the field distribution at the muon stopping site ∆B 2 is relatively small and determined only by fields produced by nuclear moments. Consequently the muon spins rotate in a coherent way and weak depolarization of the muon ensemble is observed. When the sample is cooled below T c a flux lattice (FLL) is formed in the sample resulting in an inhomogeneous field and a therefore a larger second moment at the muon site. This increase in ∆B 2 leads, in turn, to a high muon spin depolarization rate in the sample. The penetration depth is related to the field distribution width bywhere λ ⊥ is the in-plane penetration depth, Φ 0 is the flux quanta, and F ∼ 0.44 for anisotropic compounds [7].Polycrystalline samples of Na 0.7 CoO 2 ...

Evidence of nodal superconductivity inNa0.35CoO2·1.3H2Yang1, Lin2, Sun3 et al. 2005Phys. Rev. B637530View full textAdd to dashboardCite

“…This deviation is due to the power law behavior of the data in T in contrast to the exponential T dependence in the s-wave scenario. This result of line nodal superconductivity in Na 0.35 CoO 2 ·1.3H 2 O is also consistent with the recent muon spin relaxation measurements [27]. However, the analysis does not support the scenario of the order parameter with point nodes as suggested in Ref.…”

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Nesting, Spin Fluctuations, and Odd-Gap Superconductivity inNaxCoO2·yJohannes1, Mazin2, Singh3 et al. 2004Phys. Rev. Lett.812131View full textAdd to dashboardCite

“…Third, specifically the d ẑ triplet s-state, similar to the chiral p-state in Sr 2 RuO 4 , implies no change in the in-plane susceptibility, as measured by the Knight shift. [13,14,15]. Finally, non-magnetic impurities are not pair-breaking for an oT s state, in agreement with observation [29].…”

Chirald-wave superconductivity in doped graphene

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