Two dimensional nature of superconductivity in intercalated layered systems LiₓHfNCl and LiₓZrNCl: muon spin relaxation and magnetization measurements

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We report muon spin relaxation (μSR) and magnetization measurements, together with synthesis and characterization, of the Li-intercalated layered superconductors LiₓHfNCl and LiₓZrNCl with/without co-intercalation of THF (tetrahydrofuran) or PC (propylene carbonate). The 3-dimensional (3-d) superfluid density nₛ/m* (superconducting carrier density / effective mass), as well as the two dimensional superfluid density nₛ₂d/m*ab (2-dimensional (2-d) area density of superconducting carriers / ab-plane effective mass), have been derived from the μSR results of the magnetic-field penetration depth λab observed with external magnetic field applied perpendicular to the 2-d honeycomb layer of HfN / ZrN. In a plot of Tc versus nₛ₂d/m*ab, most of the results lie close to the linear relationship found for underdoped high-Tc cuprate (HTSC) and layered organic BEDT superconductors. In LiₓZrNCl without THF intercalation, the superfluid density and Tc for x = 0.17 and 0.4 do not show much difference, reminiscent of μSR results for some overdoped HTSC systems. Together with the absence of dependence of Tc on average interlayer distance among ZrN / HfN layers, these results suggest that the 2-d superfluid density nₛ₂d/m*ab is a dominant determining factor for Tc in the intercalated nitride-chloride systems. We also report μSR and magnetization results on depinning of flux vortices, and the magnetization results for the upper critical field Hc₂ and the penetration depth λ. Reasonable agreement was obtained between μSR and magnetization estimates of λ. We discuss the two dimensional nature of superconductivity in the nitride-chloride systems based on these results.