Superconductivity and equation of state of lanthanum at megabar pressures

Lanthanum (La) is the first member of the lanthanoid that has recently raised considerable interest because of its unique La superhydride ${\mathrm{LaH}}_{10}$ and its own superconducting properties. There has been a lack of experimental evidence for the equation of state (EOS) and superconductivity of La at pressures exceeding one megabar. Here, we extend the pressure range up to 140 GPa to explore the EOS and superconductivity of La via the electrical resistance and x-ray diffraction measurements. We identified the phase transition sequences by the La Bail refinements of the experimental XRD patterns and discovered a distorted fcc-La phase (space group Fmmm) above 78 GPa with nonhydrostatic pressure transmitting media. All the experimental pressure-volume data were fitted by the third-order Birch-Murnaghan equation: $V_0=37.5{\AA }^3, B_0=14.5\left(1\right)\mathrm{GPa}$, and $B_0^{\prime }=5$. Superconductivity still existed in the distorted fcc-La with an onset critical temperature $T_c$ of 9.6 K at 78 GPa, which decreases to 2.2 K at 140 GPa. We calculated the superconducting parameters of La at several pressure points, and discussed the difference from experimetal $T_c$.

Figure 1

(a), (b) Rietveld refinement of the experimental XRD pattern of La sample for two experimental runs. Positions of the Bragg reflections from La, MgO, and W gasket are marked with black, yellow, and blue vertical ticks, respectively. (c) The lattice parameters as a function of pressure. The straight dashed lines represent the phase boundaries. (d) Pressure-volume relations for La sample. The gray solid curve shows the third-order Birch-Murnaghan equation fitting to the experimental data (squares and circles). Triangles and cyan dashed curve represent the data from Ref. [20].

Figure 2

(a) Schematic of the assembly used for the electrical resistance measurements. The sample chamber consisted of a tungsten outer gasket (W) with an insulating ${\mathrm{Al}}_2{\mathrm{O}}_3$ and epoxy. (b) Microphotograph of a sample at 140 GPa illuminated from the top. An inset shows the sample illuminated from the bottom.

Figure 3

(a) Resistance of the La sample as a function of temperature at various pressures during cooling in the experimental run 1. The gray characters represent the magnification of $R$. The lines with arrows show the definition of the superconducting transition temperature $T_c$. (b) The phase diagram and superconducting transition temperature $T_c$ of La as a function of pressure. The red hexagon, green triangle, and blue pentagon symbols denote our experimental superconducting $T_c$ data from runs 1–3. The purple asterisks and gray squares represent the data from Wittig et al. [15] and Tissen et al. [18]. The black dashed line marks the phase-transition boundaries determined by the present study.

Figure 4

(a) The experimentally measured superconducting transition of the La sample at 140 GPa with a magnetic field applied. In the inset, the dots show the measured values of $T_c$ and magnetic field, while the solid lines represent fittings by the WHH and LG equations. (b), (c) The calculated superconducting parameters of the distorted fcc-La phase at 100 and 130 GPa.