Second-order charge-density-wave transition in single crystals of ${\mathrm{La}}_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$

We present the temperature dependence of the electrical resistivity, the magnetic susceptibility, and the specific heat of a high-quality single crystal of ${\mathrm{La}}_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$. As opposed to earlier reports on this system, these bulk properties exhibit clear anomalies at the phase transition at $T^{*}=151\left(1\right)$ K, while the present data confirm the second-order character of this transition. X-ray diffraction with synchrotron radiation is used to solve the fourfold superstructure in space group $I{2}_{1}3$ as it exists below $T^{*}$ in the charge-density-wave (CDW) state of ${\mathrm{La}}_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$. Unlike conventional CDW systems, we have observed hysteresis between the zero-field-cooled and field-cooled magnetization below the CDW transition. This discrepancy can be attributed to a possible magnetic instability arising out of correlations of Co in the lattice, developing at the CDW transition. The crystal structure shows that any modifications of the electronic state of Co might be due to modified binding characteristics of the Sn atoms comprising the trigonal prismatic coordination of Co, while the coordination of Co itself is hardly changed at the phase transition. The superconducting transition is observed at $T_{\text{sc}}=2.85\left(2\right)$ K. The superconducting energy gap is estimated as 5 K on the basis of the specific heat measured down to 0.1 K. These results suggest that ${\mathrm{La}}_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$ is a conventional weak-electron-phonon-coupling superconductor.

Figure 1

Temperature dependence of the electrical resistivity $\rho \left(T\right)$ of ${\mathrm{La}}_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$ as measured upon heating from 2 to 300 K. An anomaly without thermal hysteresis is apparent around $T^{*}=151\left(1\right)$ K. The dotted line is a fit to the data for 3–20 K, resulting in ${\rho }_0=38.17\left(1\right)\mu \mathrm{\Omega }$cm and $A=0.022\left(2\right)\mu \mathrm{\Omega }\mathrm{cm}/{\mathrm{K}}^2$ (see the discussion in the text). The crystal becomes superconducting below 2.85(2) K. The inset shows the magnetic field ($B$) dependence of $\rho \left(T\right)$ around this transition.

Figure 2

Temperature dependence of the magnetic susceptibility $\chi \left(T\right)$ of ${\mathrm{La}}_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$, as measured upon heating after ZFC and FC conditions in a field of 5 T. The most prominent feature is the hysteresis between ZFC and FC conditions, starting at the temperature of transition $T^{*}=150\left(2\right)$ K. The inset displays the magnetic susceptibility at low temperatures around the superconducting transition at 2.85 K and measured in a field of 10 mT.

Figure 3

Specific heat $C_p\left(T\right)$ of a single crystal of ${\mathrm{La}}_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$ for 0.1–270 K. Data were measured with the PPMS upon heating from 2 to 270 K. The top inset provides an expanded view around the phase transition $T^{*}=150\left(2\right)$ K. The bottom inset shows $C_p/T$ vs $T^2$ for 0.1–9 K as measured with the custom-built setup. The solid line is a fit discussed in the text. An anomaly due to the superconducting phase transition is clearly visible at $T_{\text{sc}}=2.8\left(1\right)$ K.

Figure 4

Temperature dependence of the change in specific heat of ${\mathrm{La}}_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$, as derived from DSC measured on heating (red) and on cooling (blue). The inset shows the change in entropy near the phase transition.

Figure 5

Perspective along $\mathbf{a}$ of one unit cell of the crystal structure of ${\mathrm{La}}_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$ at 180 K. Small blue circles represent Co atoms, large green circles are La, light gray circles of intermediate sizes are Sn1 atoms located at the origin (corners of the unit cell) and exactly in the middle of the unit cell, and small circles of darker gray are Sn2.