Magnetic phase diagram of Li${}_2$(Pd${}_{1-x}$Pt${}_x$)${}_3$B by ac susceptometry

The $H$-$T$ phase diagram and several superconducting parameters for the noncentrosymmetric system Li${}_{\text{2}}$(Pd${}_{\text{1}-x}$Pt${}_x$)${}_{\text{3}}$B have been determined as a function of cation substitution $x$. Notably, the coherence length may be linear in platinum concentration. Despite the superconducting pairing state and band structure apparently changing fundamentally, the $H$-$T$ phase diagram is essentially unchanged. Unusual aspects of the shape of the $H$-$T$ phase diagram are discussed. The upper critical field $H_{\text{c}2}$(0) is not anomalously high for any Pt content, likely due to an absence of high carrier masses—in such a case, the value of $H_{\text{c}2}$(0) would not serve as a probe for novel physics.

Figure 1

Temperature and field dependence of the diamagnetic shielding ${\chi }_{\mathrm{ac}}^{\prime }$ (negative) and loss ${\chi }_{\mathrm{ac}}^{\prime \prime }$ (positive) for Li${}_{\text{2}}$(Pd${}_{\text{1}-x}$Pt${}_x$)${}_{\text{3}}$B with $x=0.20$.

Figure 2

Example of a resulting $H_{\text{c}2}$ curve for the Pt end member ($x=1$) and for $x=0.20$, including $c_{\mathrm{el}}$ points taken on the same $x=1$ sample and a larger $x=0.20$ from the same ingot—the difference in slope may be due to having the sample in a different orientation relative to the field, and should not be taken as strong evidence for a vortex melting transition. The curve predicted by dirty-limit WHH (with the $T_{\text{c}}$ and initial slope taken from the linear fit) is included for comparison, and the inset shows the strong upward curvature detected in ${\chi }_{\mathrm{ac}}$ near $T_{\text{c}}$, for $x=0.20$.

Figure 3

Combined phase diagram for Li${}_{\text{2}}$(Pd${}_{\text{1}-x}$Pt${}_x$)${}_{\text{3}}$B across the substitution range studied. There is little variation in shape with cation substitution.

Figure 4

$H$-$T$ phase diagrams for all Pt concentrations studied, scaled by $T_{\text{c}}$ and $H_{\text{c}2}$(0). There is no strong evidence for a systematic evolution with cation substitution.

Figure 5

Transition in ${\chi }_{\mathrm{ac}}^{\prime \prime }$ for very low dc fields, $x=0.20$ [${\mu }_0{H}_{\text{c}2}\left(0\right)\approx 3.24$ T; $H_{\mathrm{ac}}=0.1$ mT rms]. The peak grows and broadens, developing a tail to low temperature.

Figure 6

Dependence of $T_{\text{c}}$, $H_{\text{c}2}$(0), and ${\xi }_0$ on cation substitution. Diamonds are $T_{\text{c}}$ data from Ref.22.