Enhancement of superconducting properties in the La-Ce-H system at moderate pressures

Abstract

Ternary hydrides are regarded as an important platform for exploring high-temperature superconductivity at relatively low pressures. Here, we successfully synthesized the hcp-(La,Ce)H_9-10 at 113 GPa with the initial La/Ce ratio close to 3:1. The high-temperature superconductivity was strikingly observed at 176 K and 100 GPa with the extrapolated upper critical field H_c2(0) reaching 235 T. We also studied the binary La-H system for comparison, which exhibited a T_c of 103 K at 78 GPa. The T_c and H_c2(0) of the La-Ce-H are respectively enhanced by over 80 K and 100 T with respect to the binary La-H and Ce-H components. The experimental results and theoretical calculations indicate that the formation of the solid solution contributes not only to enhanced stability but also to superior superconducting properties. These results show how better superconductors can be engineered in the new hydrides by large addition of alloy-forming elements.

Fig. 1. Characterization of the superconducting transitions using electrical resistance measurements at selected pressures for typical runs.

a Photographs of the sample after laser heating together with four electrodes. b–e The temperature dependence of the electrical resistance for the La–Ce–H sample in DACs #2, 5, 9 and La–H sample in DAC #L1.

Fig. 2. Synchrotron X-ray diffraction (0.6199 Å) analysis of the synthesized hydrides.

a Peaks indexing for the La–Ce–H samples in DACs #2, 3 and the La–H sample in DAC #S. Insets show the integrated diffraction patterns. The wide diffraction band of an impurity located on the seat surface in DAC #3 is masked by gridlines (Supplementary Fig. S10). b Pressure dependence of the unit cell volume of different polyhydrides. The experimental results for the La–Ce–H and La–H systems are shown in color and black, respectively. Gray symbols show literature data for the synthesized La–H phases^,,. Solid and dashed lines indicate the P–V relation of La–H and Ce–H phases, respectively. “VASP” marks the equation of state (EoS) calculated using the VASP code (PBE GGA), and “QE” marks the EoS calculated using the Quantum ESPRESSO code (PAW PBE).

Fig. 3. Electrical measurements of the superconducting transition in external magnetic fields.

a–c Temperature dependences of the electrical resistance in external magnetic fields for different runs. The T_cs are marked at the temperature in the middle of transition. Insets are the enlarged part of the fitting results using the simplified WHH formula. d Extrapolated upper critical magnetic fields from different runs for La–Ce–H and La–H systems. Inset shows the obtained upper critical field H_c2(0) at different pressures.

Fig. 4. The stability and superconductivity of La–Ce–H system.

a Phonon band structure and density of states of ordered hcp-La₃CeH₃₆ at 120 GPa calculated within the harmonic approximation. b Qualitative phase diagram of (La,Ce)H₉ depends on the concentration and pressure. c T_c–P relationship of the binary La–H and Ce–H, and ternary P6₃/mmc-(La,Ce)H_9-10. The gray dashed lines indicate the equivalent S values, which are marked in the white circles. d The structure model of the P6₃/mmc-(La,Ce)H_9-10.