Low-energy quasiparticle excitations in superconducting doped Topological Insulator Sr_xBi₂Se₃ studied by Penetration depth measurements

Superconducting doped bismuth selenide (Bi₂Se₃) is a promising candidate of topological superconductors. Recent studies in the superconducting state reveal that this material shows spontaneous rotational symmetry breaking in the gap function, which is termed as nematic superconductivity. Among pairing symmetries proposed for superconducting doped Bi₂Se₃, only the odd-parity pairings with E_u representation generate nematicity in the superconducting gap. E_u pairing states allow two kinds of gap structures, Δ_4x state with point nodes along k_y direction and Δ_4y state with gap minima along k_x direction. These two states can be distinguished from the measurements which is sensitive to low-energy quasiparticle excitations. Here we report the magnetic penetration depth in Sr-doped Bi₂Se₃ (T_c~2.8K) down to 50 mK measured by tunnel-diode oscillator technique. All samples show power-law behavior λ(T)∝Tⁿ (n=1~2) down to 0.05T_c, indicating strong momentum dependence of the gap. However, the data give much higher exponent n>3 at the lowest temperature region below 0.05T_c. Our data can be interpreted as tiny but finite gap minima exists on Fermi surfaces, which is consistent with Δ_4y state.