Dimensional crossover in the superconducting intercalated layer compound2H-TaS2

The superconducting critical-field behavior of $2H$-Ta${\mathrm{S}}_2$ single crystals intercalated with pyridine (PY), methylamine (MeA), dimethylamine (DMA), and ethylenediamine (EDA) has been measured in the field range 0-267 kG. The critical fields parallel to the layers for $2H$-Ta${\mathrm{S}}_2$ intercalated with PY and MeA exhibit all of the features expected for crossover to two-dimensional behavior. A strong upward curvature in $H_{c2\mathrm{}\parallel }$ vs $T$ is present. A temperature-dependent critical-field anisotropy, $\frac{H_{c2\mathrm{}\parallel }}{H_{c2\mathrm{}\perp }}$, is observed which reaches values of ∼60 for $\frac{T}{T_c}<0.7\mathrm{}$. The coupling constants are on the order of 1 and the data are all consistent with coherence lengths perpendicular to the layers that are less than the layer spacing. The data for $H_{c2\mathrm{}\parallel }$ vs $T$ have been analyzed using a computer fit to the theory of dimensional crossover developed by Klemm, Luther, and Beasley. The parameters generated by this fit are in complete agreement with values of these parameters calculated directly from the data and fully confirm the observation of complete dimensional crossover. Crystals intercalated with DMA and EDA show a less complete dimensional crossover and the computer-generated and calculated values of the parameters are not in agreement. This intermediate behavior is characterized by an anomalous upward curvature near $T_c$ which cannot be systematically separated from the upward curvature due to dimensional crossover. The data presented on this entire group of intercalated layer compounds provide a complete analysis of the range of behavior observed and allow a critical evaluation of the case for observation of dimensional crossover. Discussion and analysis includes comparison to selected data on superconducting metallic intercalates as well as to other published data on organic intercalates.