Crystallographic and superconducting properties of the fully gapped noncentrosymmetric 5$d$-electron superconductors Ca$M$Si${}_3$ ($M$ =  Ir, Pt)

We report crystallographic, specific heat, transport, and magnetic properties of the recently discovered noncentrosymmetric 5$d$-electron superconductors CaIrSi${}_3$ ($T_{\mathrm{c}}=3.6$ K) and CaPtSi${}_3$ ($T_{\mathrm{c}}=2.3$ K). The specific heat suggests that these superconductors are fully gapped. The upper critical fields are less than 1 T, consistent with limitation by conventional orbital depairing. High, non-Pauli-limited ${\mu }_0{H}_{\mathrm{c}2}$ values, often taken as a key signature of novel noncentrosymmetric physics, are not observed in these materials because the high carrier masses required to suppress orbital depairing and reveal the violated Pauli limit are not present.

Figure 1

Powder x-ray-diffraction results for CaIrSi${}_3$ and CaPtSi${}_3$. The crosses indicate impurity peaks. The inset displays a scanning electron micrograph of CaIrSi${}_3$. The sample contained an impurity phase. The EDX result reveals the bright area to be CaIrSi${}_3$ and the dark area to have the approximate cation ratio of Ca:Ir:Si = 1:0:2.28.

Figure 2

X-ray-diffraction results on CaIrSi${}_3$: (a) Temperature dependence of several powder XRD peaks of CaIrSi${}_3$ from 90 K (bottom) to 300 K (top) with an incident x-ray wavelength of 0.357 47(1) Å. (b) Temperature dependence of both lattice parameters. A slight slope change is visible for the $c$ axis at 200 K. (c) Noncentrosymmetric crystal structure of CaIrSi${}_3$ as determined by powder x-ray diffraction.

Figure 3

The Rietveld structure refinement of CaIrSi${}_3$ performed on the higher-statistics synchrotron x-ray data at 100 K.

Figure 4

Temperature dependence of the resistivity $\rho$ for (a) CaIrSi${}_3$ and (b) CaPtSi${}_3$ up to 300 K and low-temperature dc susceptibility ${\chi }_{\mathrm{dc}}$ and resistivity for (c) CaIrSi${}_3$ and (d) CaPtSi${}_3$. Both zero-field-cooled (ZFC) and field-cooled (FC) dc susceptibility under 1 mT are displayed. The onset temperatures of the superconducting transition are approximately 3.7 K for CaIrSi${}_3$ and 2.3 K for CaPtSi${}_3$ in both measurements. Strong diamagnetism indicates that the majority of each sample was superconducting.

Figure 5

Temperature dependence of total specific heat $c_P$ for (a) CaIrSi${}_3$ and (b) CaPtSi${}_3$, and electronic specific heat $c_{\mathrm{el}}$ for (c) CaIrSi${}_3$ and (d) CaPtSi${}_3$. $T_{\mathrm{c}}$ onsets in 0 T are 3.6 K for CaIrSi${}_3$ and 2.3 K for CaPtSi${}_3$, and in 0.1 T are 2.5 K for CaIrSi${}_3$ and 1.7 K for CaPtSi${}_3$. Solid lines are BCS fits. The shaded regions have the same area on either side of the specific heat jump, corresponding to entropy conservation. The $T_{\mathrm{c}}$ values from this procedure match with those from the BCS fits.

Figure 6

Temperature dependence of ac susceptibility (the imaginary part ${\chi }_{\mathrm{ac}}^{″}$ and the real part ${\chi }_{\mathrm{ac}}^{\prime }$) and resistivity $\rho$ of CaIrSi${}_3$ and CaPtSi${}_3$ in selected magnetic fields. The zero resistivity temperature matches or exceeds the ac susceptibility onset in magnetic fields. The diamagnetic shielding in ${\chi }_{\mathrm{ac}}$ is fully suppressed below 0.3 T for CaIrSi${}_3$ and 0.2 T for CaPtSi${}_3$, whereas $\rho$ onsets were observed up to 0.6 T.

Figure 7

$H$–$T$ phase diagram of CaIrSi${}_3$ and CaPtSi${}_3$. Points represent the onset temperatures in ${\chi }_{\mathrm{ac}}^{\prime }$, $\rho$, and $c_P$, as defined in the text. For both compounds the $H$–$T$ phase lines as extracted from $\rho$ and ${\chi }_{\mathrm{ac}}^{\prime }$ differ. Possible reasons for this difference are discussed in Sec. 4.

Figure 8

Crystal structure of CaIr${}_3$Si${}_7$ as determined by single-crystal x-ray diffraction.