Magnetization study of superconducting Nb-intercalated topological insulator $Nb_xBi_2Se_3$*

Niobium-intercalated topological insulator $Bi_2Se_3$ is an odd-parity p-wave superconductor.Among different possible pairing mechanisms proposed for this material there is a spin-triplet pairing which would result in topologically non-trivial superconductivity with gapped bulk and gapless Andreev surface states.One of the signatures of this topological state is spontaneous time-reversal symmetry breaking and appearance of residual magnetization in the superconducing state.Here we report magnetization studies of $Nb_xBi_2Se_3$ with Nb atomic ratios x = 0.25 and 0.27.Magnetization hysteresis loops display shift of the central peak in the direction of the field scan,indicating flux trapping.No shift of the central peak in the opposite direction is observed,indicating that magnetization in the system is negligibly small.This observation is consistent with recent reports on the spatial anisotropy of the order parameter,which suggests the presence of nematic order parameter and topologically-trivial superconductivity.