Controllable spin-orbit coupling and its influence on the upper critical field in the chemically doped quasi-one-dimensional ${\mathrm{Nb}}_2{\mathrm{PdS}}_5$ superconductor

By systematic chemical substitution of Pt and Ni in the newly discovered superconductor ${\mathrm{Nb}}_2{\mathrm{PdS}}_5$ ($T_c\sim 6$ K), we study the evolution of its superconducting properties with doping, focusing on the behavior of the upper critical field $H_{c2}$. In contrast to the previous results of Se doping on S sites, superconductivity is found to be rather robust against the Pt and Ni dopants on the one-dimensional Pd chains. Most strikingly, the reduced $H_{c2}$, i.e., the ratio of $H_{c2}/T_c$, is seen to be significantly enhanced by the heavier Pt doping but suppressed in the Ni-doped counterparts, distinct from the nearly constant value in the Se-doped samples. Our findings therefore suggest that the upper critical field of this system can be modified in a tunable fashion by chemical doping on the Pd chains with elements of varying mass numbers. The spin-orbit coupling on the Pd sites, by inference, should play an important role in the observed superconductivity and on the large upper critical field beyond the Pauli pair-breaking field.

Figure 1

(a) The crystallographic structure of ${\mathrm{Nb}}_2$(${\mathrm{Pd}}_{1-x}{\mathrm{Pt}}_x$)${\mathrm{S}}_5$. The one-dimensional Pd chains are extended along the $b$ axis. The unit cell is indicated by the thin dotted line. (b) The powder XRD patterns for Pt- and Ni-doped samples studied here ($x=0.05$ and 0.3 of Pt-doped samples are not included for clarity), with the asterisks marking the possible impurity phases. (c) The resultant lattice parameters extracted from the Rietveld refinement, along with those for the Se-doped samples for comparison.

Figure 2

(a) and (b) The zero-field resistivity for Pt-doped and Ni-doped samples studied in this work, respectively. Note all curves are renormalized to their room temperature values for the purpose of clarity. The insets blow up the superconducting transitions. (c) The temperature sweeps at some fixed fields for $x=0.2$ Pt-doped sample as an example.

Figure 3

The magnetizations below $T_c$ with ZFC and FC modes. The data were taken under a 10 Oe magnetic field.

Figure 4

The specific heat data for all samples in zero magnetic field, plotted as $C/T$ vs $T^2$, to demonstrate the anomalies associated with the superconducting transition.

Figure 5

(a) and (c) The temperature dependence of $H_{c2}$ for ${\mathrm{Nb}}_2$(${\mathrm{Pd}}_{1-x}{\mathrm{Pt}}_x$)${\mathrm{S}}_5$ and ${\mathrm{Nb}}_2$(${\mathrm{Pd}}_{1-x}{\mathrm{Ni}}_x$)${\mathrm{S}}_5$, respectively, extracted from the temperature sweeps at constant fields, determined by $90\%$ of the normal state resistivity. The solid lines represent the WHH fittings. $T_c$ and $H_{c2}$(0) (zero-temperature upper critical field from the fit) are plotted in (b) and (d) accordingly.

Figure 6

Top panel: The doping dependence of $T_c$ and $T_{\min }$ as a function of doping. Data for Se-doped samples are also included. Note the log-scale for $T_{\min }$. Bottom panel: The doping dependence of the ratio $H_{c2}/T_c$ for Pt and Ni substituted samples, plotted together with those for Se-doped ones.