Recently it was demonstrated that Sr intercalation provides a new route to induce superconductivity in the topological insulator Bi₂Se₃. Topological superconductors are predicted to be unconventional with an odd-parity pairing symmetry. An adequate probe to test for unconventional superconductivity is the upper critical field,B_c2. For a standard BCS layered superconductorB_c2shows an anisotropy when the magnetic field is applied parallel and perpendicular to the layers, but is isotropic when the field is rotated in the plane of the layers. Here we report measurements of the upper critical field of superconducting Sr_xBi₂Se₃crystals (T_c = 3.0 K). Surprisingly, field-angle dependent magnetotransport measurements reveal a large anisotropy ofB_c2when the magnet field is rotated in the basal plane. The large two-fold anisotropy, while six-fold is anticipated, cannot be explained with the Ginzburg-Landau anisotropic effective mass model or flux flow induced by the Lorentz force. The rotational symmetry breaking ofB_c2indicates unconventional superconductivity with odd-parity spin-triplet Cooper pairs (Δ₄-pairing) recently proposed for rhombohedral topological superconductors, or might have a structural nature, such as self-organized stripe ordering of Sr atoms.

Epitaxial growth of ε-Ga₂O₃ is demonstrated for the first time. The ε-Ga₂O₃ films are grown on GaN (0001), AlN (0001), and β-Ga₂O₃ ($\overline{2}01$ ) by halide vapor phase epitaxy at 550 °C using gallium chloride and O₂ as precursors. X-ray ω-2θ and pole figure measurements prove that phase-pure ε-Ga₂O₃ (0001) films are epitaxially grown on the three kinds of substrates, although some minor misoriented domains are observed. High temperature X-ray diffraction measurements reveal that the ε-Ga₂O₃ is thermally stable up to approximately 700 °C. The optical bandgap of ε-Ga₂O₃ is determined for the first time to be 4.9 eV.

Magnetic refrigeration (MR) is a key technique for hydrogen liquefaction. Although the MR has ideally higher performance than the conventional gas compression technique around the hydrogen liquefaction temperature, the lack of MR materials with high magnetic entropy change in a wide temperature range required for the hydrogen liquefaction is a bottle-neck for practical applications of MR cooling systems. Here, we show a series of materials with a giant magnetocaloric effect (MCE) in magnetic entropy change (-∆S_m > 0.2 J cm⁻³K⁻¹) in the Er(Ho)Co₂-based compounds, suitable for operation in the full temperature range required for hydrogen liquefaction (20-77 K). We also demonstrate that the giant MCE becomes reversible, enabling sustainable use of the MR materials, by eliminating the magneto-structural phase transition that leads to deterioration of the MCE. This discovery can lead to the application of Er(Ho)Co₂-based alloys for the hydrogen liquefaction using MR cooling technology for the future green fuel society.