Low-temperature physical properties of new orthorhombic compounds RE2Au3Sn6 (RE = Ce, La)

We have prepared polycrystalline samples of RE2Au3Sn6 (RE = Ce, La) and investigated their magnetic, transport and thermal properties. We found that Ce2Au3Sn6 shows antiferromagnetic transition at T N = 2.54 K, while La2Au3Sn6 shows no phase transition above 0.4 K. The electronic specific heat coefficient of Ce2Au3Sn6 is γ = 350 mJ Ce-mol K2, which indicates that Ce2Au3Sn6 is a heavy fermion compound. The magnetic entropy of Ce2Au3Sn6 at T N is estimated to be 70% of Rln2, which can be attributed to the shielding of the magnetic moment by the Kondo effect.


Introduction
Compounds containing Ce show a variety of interesting phenomena, such as anomalous magnetic ordering and heavy fermion state due to the interaction between 4f and conduction electrons. These phenomena are due to the interplay between the Ruderman-Kittel-Kasuya-Yosida interaction and the Kondo effect [1]. It is well known that Ce compounds with their magnetic states being at the vicinity of the quantum critical point exhibit some anomalous phenomena such as non-Fermi liquid behavior and unconventional superconductivity. One possible way to search for new compounds with quantum criticality is to search for compounds without magnetic order down to cryogenic temperatures. From this view point, we have focused on newly synthesized compounds RE2Au3Sn6 (RE = La, Ce, Pr, Nd, Sm) [2]. RE2Au3Sn6 crystallize with the orthorhombic La2Zn3Ge6 type structure (space group Cmcm, D2h 17 , No. 63) as shown in the figure 1 [2]. This structure has two independent RE sites. The magnetic susceptibility measurement down to 2.5 K has revealed that Ce2Au3Sn6 has trivalent Ce ions and no phase transition above 2.5 K [2]. In this study, we have prepared polycrystalline samples of RE2Au3Sn6 (RE = Ce, La) and investigated their magnetic, transport and thermal properties down to 0.4 K.

Experimental Methods
The polycrystalline samples of RE2Au3Sn6 (RE = Ce, La) have been prepared as the following procedure. First, stoichiometric amount of RE (99.9%), Au (99.99%) and Sn (99.999%) were melted by using arc furnace in an argon atmosphere. After that, the samples wrapped in Ta foils were sealed in the evacuated quartz tubes and annealed at 600°C for one week. The resulting polycrystalline samples were analyzed by powder X-ray diffraction. For both Ce2Au3Sn6 and La2Au3Sn6, all Bragg peaks in the diffraction patterns are indexed on the basis of the reported structure. The lattice parameters determined from Xray diffraction agree with those reported in the Ref. [2]. We then measured the following three properties. The magnetization M was measured from 1.8 K to 300 K and from 0 T to 5 T using a superconducting quantum interference device magnetometer (Quantum Design, MPMS). The electrical resistivity ρ was measured from 0.4 K to 300 K using a 3 He cryostat with the DC four-terminal method. The specific heat C was measured from 0.6 K to 10 K by the thermal relaxation method using a 3 He cryostat. The temperature dependence of the electrical resistivity ρ is also shown in the figure 2 and the inset. The ρ decreases with decreasing temperature and shows a broad shoulder around 70 K. The steep decrease at TN is ascribable to the decrease in scattering of conduction electrons by Ce magnetic moments.

Results and Discussion
The magnetic-field dependence of the magnetization M of Ce2Au3Sn6 is shown in the figure 3. At 5 K (> TN), the M increases with increasing magnetic field and shows no anomalies. On the other hand, the M at 1.8 K (< TN) shows the metamagnetic transition due to a spin flop at 2 T. The transition field   Figure 4 shows the temperature dependence of the specific heat C of Ce2Au3Sn6 and La2Au3Sn6. The C of Ce2Au3Sn6 shows a l-type anomaly at TN, which indicates that the cusp of M/H and the decrease in ρ are due to the intrinsic second-order antiferromagnetic transition. On the other hand, La2Au3Sn6 does not show any anomaly down to 0.4 K. The electronic specific heat coefficient of Ce2Au3Sn6 is estimated to be g = 350 mJ/Ce-mol K 2 , which is about 600 times larger than that of La2Au3Sn6, g = 0.59 mJ/La-mol K 2 . This indicates that Ce2Au3Sn6 is a heavy fermion compound. Here, g and the phonon specific heat coefficient b were determined by fitting the formula C/T = g + β T 2 on the experimental C/T from 6 K to 10 K for Ce2Au3Sn6 and from 0.4 K to 4 K for La2Au3Sn6. Figure 5 shows the temperature dependence of the magnetic entropy Smag calculated by integrating Cmag/T in temperature. Cmag is the magnetic specific heat obtained by subtracting the C of La2Au3Sn6 from that of Ce2Au3Sn6 under the assumption that the nonmagnetic contribution to the C of these compounds is identical. The Smag at TN is estimated to be 4.0 J/Ce-mol K which is 70% of Rln2 (R: gas constant). The reduction of Smag value from Rln2 can be attributed to the shielding of the magnetic moment by the Kondo effect.

Conclusion
We have prepared polycrystalline samples of RE2Au3Sn6 (RE = Ce, La) and investigated their magnetic, transport, and thermal properties by measuring the magnetization, the electrical resistivity, and the specific heat. Ce2Au3Sn6 is an antiferromagnet with the transition temperature at TN = 2.54 K. The large electronic specific heat coefficient g = 350 mJ/Ce-mol K 2 indicates that Ce2Au3Sn6 is a heavy fermion compound. The reduced magnetic entropy of Ce2Au3Sn6 (70% of Rln2) at TN can be attributed to the Kondo effect. The nonmagnetic reference compound La2Au3Sn6 shows a nonmagnetic metallic behavior and does not show any phase transition down to 0.4 K.

Acknowledgment
This work was partly supported by JSPS KAKENHI Grant Number 20K03861.