Strongly anisotropic spin response as a signature of the helical regime in Rashba nanowires

Rashba nanowires in a magnetic field exhibit a helical regime when the spin-orbit momentum is close to the Fermi momentum, k(F) approximate to k(SO). We show that this regime is characterized by a strongly anisotropic electron-spin susceptibility, with an exponentially suppressed signal along one direction in spin space, and that there are no low-frequency spin fluctuations along this direction. Since the spin response in the gapless regime k(F) not approximate to k(SO) has a power-law behavior in all three directions, spin measurements provide a signature of the helical regime that complements spin-insensitive conductance measurements.