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

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 cointercalation of THF (tetrahydrofuran) or propylene carbonate. The three-dimensional superfluid density ns/m* (superconducting carrier density/effective mass) as well as the two-dimensional superfluid density ns2D/mab* [two-dimensional (2D) 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 2D honeycomb layer of HfN/ZrN. In a plot of Tc versus ns2D/mab*, most of the results lie close to the linear relationship found for underdoped high-Tc cuprate (HTSC) and layered organic BEDT (bis(ethylenedithio)) 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 2D superfluid density ns2D/mab* 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. A 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.