  A nematic topological superconductor has an order parameter symmetry, which
spontaneously breaks the crystalline symmetry in its superconducting state.
This state can be observed, for example, by thermodynamic or upper critical
field experiments in which a magnetic field is rotated with respect to the
crystalline axes. The corresponding physical quantity then directly reflects
the symmetry of the order parameter. We present a study on the superconducting
upper critical field of the Nb-doped topological insulator NbxBi2Se3 for
various magnetic field orientations parallel and perpendicular to the basal
plane of the Bi2Se3 layers. The data were obtained by two complementary
experimental techniques, magnetoresistance and DC magnetization, on three
different single crystalline samples of the same batch. Both methods and all
samples show with perfect agreement that the in-plane upper critical fields
clearly demonstrate a two-fold symmetry that breaks the three-fold crystal
symmetry. The two-fold symmetry is also found in the absolute value of the
magnetization of the initial zero-field-cooled branch of the hysteresis loop
and in the value of the thermodynamic contribution above the irreversibility
field, but also in the irreversible properties such as the value of the
characteristic irreversibility field and in the width of the hysteresis loop.
This provides strong experimental evidence that Nb-doped Bi2Se3 is a nematic
topological superconductor similar to the Cu- and Sr-doped Bi2Se3.
