Superconductivity in 122-type antimonide ${\mathrm{BaPt}}_2{\mathrm{Sb}}_2$

The crystal structure, superconducting properties, and electronic structure of the novel superconducting 122-type antimonide ${\mathrm{BaPt}}_2{\mathrm{Sb}}_2$ have been studied by measurements of powder x-ray diffraction patterns, electrical resistivity, ac magnetic susceptibility, specific heat, and by ab initio calculations. The material crystallizes in a new monoclinic variant of the ${\mathrm{CaBe}}_2{\mathrm{Ge}}_2$-type structure, in which ${\mathrm{Pt}}_2{\mathrm{Sb}}_2$ layers that consist of ${\mathrm{PtSb}}_4$ tetrahedra, and ${\mathrm{Sb}}_2{\mathrm{Pt}}_2$ layers that consist of ${\mathrm{SbPt}}_4$ tetrahedra, are stacked alternatively, with Ba atoms located between them. Measurements of electrical resistivity, ac magnetic susceptibility, and specific heat revealed that ${\mathrm{BaPt}}_2{\mathrm{Sb}}_2$ is a superconducting material with a critical temperature of 1.8 K. The electronic heat capacity coefficient and the Debye temperature were $8.6\left(2\right)\mathrm{mJ}/\mathrm{mol}{\mathrm{K}}^2$ and 146(4) K, respectively. The upper critical field and the Ginzburg-Landau coherent length were determined to be 0.27 T and 35 nm, respectively. The calculations showed that the material has two three-dimensional Fermi surfaces (FSs) and two two-dimensional FSs, leading to anisotropic transport properties. The $d$ states of the Pt atoms in the ${\mathrm{Pt}}_2{\mathrm{Sb}}_2$ layers are the main contributors to the density of states at the Fermi level. A comparison between experimental and calculated results indicates that ${\mathrm{BaPt}}_2{\mathrm{Sb}}_2$ is a superconducting material with moderate coupling.

Figure 1

Powder XRD pattern for ${\mathrm{BaPt}}_2{\mathrm{Sb}}_2$. The red points and black line represent observed and calculated intensities, respectively. The difference between the two intensities is indicated by the blue line shifted by −7000 counts. Peak positions for ${\mathrm{BaPt}}_2{\mathrm{Sb}}_2$ and the PtSb impurity are labeled by green and purple vertical bars located at −2000 and −4000 counts, respectively.

Figure 2

(a) Crystal structure of ${\mathrm{BaPt}}_2{\mathrm{Sb}}_2$, (b) ${\mathrm{PtSb}}_4$ and ${\mathrm{SbPt}}_4$ tetrahedra that form the ${\mathrm{Pt}}_2{\mathrm{Sb}}_2$ and ${\mathrm{Sb}}_2{\mathrm{Pt}}_2$ layers, (c) view along the [001] direction, (d) view along the [010] direction, (e) deformed Pt square lattice of ${\mathrm{BaPt}}_2{\mathrm{Sb}}_2$ along the [001] direction, and (f) Pt square lattice of ${\mathrm{CaBe}}_2{\mathrm{Ge}}_2$-type ${\mathrm{BaPt}}_2{\mathrm{Sb}}_2$ along the [001] direction. Red broken lines show Pt lattice. Blue solid lines show unit cell. Large purple spheres, small white spheres, and small orange spheres represent Ba, Pt, and Sb atoms, respectively.

Figure 3

(a) Electrical resistivity as a function of temperature, (b) electrical resistivity at fixed temperatures as a function of magnetic field strength. The inset in Fig. 3 is an expanded view of the resistivity at low temperatures ranging from 1.8 to 2.1 K. The symbols ${H_{\mathrm{C}2}}^{\rho \_\mathrm{on}},{H_{\mathrm{C}2}}^{\rho \_\mathrm{mid}}$, and ${H_{\mathrm{C}2}}^{\rho \_\mathrm{comp}}$ represent the magnetic field strength at which ρ starts to increase from zero, that at which ρ is half the normal resistivity value, and that at which an extrapolation of the ρ-$T$ curve during the transition reaches the normal resistivity value, respectively.

Figure 4

Real and imaginary parts of ac magnetic susceptibility ${\chi }^{\prime }$ and ${\chi }^{\prime \prime }$, respectively, measured at constant temperatures below 1.44 K as a function of magnetic field amplitude ${\mu }_{0}H$. The inset shows an expanded view of ${\chi }^{\prime }$ measured at $0.03\mathrm{K}$. ${H_{\mathrm{C}2}}^{{\chi }^{\prime }}$ indicates the magnetic field amplitude where the diamagnetic signal disappears.

Figure 5

(a) Specific heat divided by temperature $C/T$, measured at various magnetic field strengths as a function of squared temperature $T^2$, and (b) specific-heat difference between 0 and 0.2 T divided by temperature $[C\left(0\mathrm{T}\right)-C\left(0.2\mathrm{T}\right)]/T$ as a function of temperature. The inset in Fig. 5 shows the difference in entropy $S$ between the normal and superconducting states $S_{\mathrm{es}}-S_{\mathrm{en}}$ as a function of temperature.

Figure 6

(a) Magnetic field-temperature phase diagram deduced from measurements of electrical resistivity, ac magnetic susceptibility, and specific heat. The empty triangles, circles, and rhombus indicate ${T_{\mathrm{C}}}^{\rho \_\mathrm{on}},{T_{\mathrm{C}}}^{\rho \_\mathrm{mid}}$, and ${T_{\mathrm{C}}}^{\rho \_\mathrm{comp}}$, shown in Fig. 3. The solid squares and triangles represent $H_{\mathrm{C}2}$ determined from ${\chi }^{\prime }$, and $T_{\mathrm{C}}$ determined from the specific-heat data, respectively. (b) Reduced magnetic field $h^{*}$ as a function of reduced temperature $t^{*}$, where $h^{*}=H/(-dH/dT)T_{\mathrm{C}}$ and $t^{*}=T/T_{\mathrm{C}}$. The dashed-dotted and dotted lines show $h^{*}$ calculated using a pair-breaking model in the clean and dirty limits, respectively.

Figure 7

Band structure of ${\mathrm{BaPt}}_2{\mathrm{Sb}}_2$.

Figure 8

(a) Total and partial density of states (DOS) for ${\mathrm{BaPt}}_2{\mathrm{Sb}}_2$. (b) Expanded view of partial DOS for ${\mathrm{BaPt}}_2{\mathrm{Sb}}_2$ near the Fermi level $E_{\mathrm{F}}$.

Figure 9

Fermi surfaces (FSs) for ${\mathrm{BaPt}}_2{\mathrm{Sb}}_2$, formed by the (a) 37th, (b) 38th, (c) 39th, and (d) 40th bands. The FSs for the 37th and 38th bands are holelike, and FSs for the 39th and 40th bands are electronlike.