Multiband Superconductivity of Heavy Electrons in a TlNi2Se2 Single Crystal

We have made the first observation of superconductivity in TlNi₂Se₂ at T_C=3.7K, and it appears to involve heavy electrons with an effective mass m^*=(14-20)m_b, as inferred from the normal-state electronic specific heat and the upper critical field, H_C2(T). We found that the zero-field electronic specific-heat data, C_es(T) (0.5K≤T<3.7K) in the superconducting state can be fitted with a two-gap BCS model, indicating that TlNi₂Se₂ seems to be a multiband superconductor, which is consistent with the band calculation for the isostructural KNi₂S₂. It is also found that the electronic specific-heat coefficient in the mixed state γ_N(H) exhibits a H^1/2 behavior, which is considered as a common feature of the d-wave superconductors. TlNi₂Se₂, as a d-electron system with heavy electron superconductivity, may be a bridge between cuprate- or iron-based and conventional heavy-fermion superconductors.