We consider the role of magnetic fields on the broken inversion superconductor ${\mathrm{CePt}}_3\mathrm{Si}$. We show that the upper critical field for a field along the $c$ axis exhibits a much weaker paramagnetic effect than for a field applied perpendicular to the $c$ axis. The in-plane paramagnetic effect is strongly reduced by the appearance of helical structure in the order parameter. We find that, to get good agreement between theory and recent experimental measurements of $H_{c2}$, this helical structure is required. We propose a Josephson junction experiment that can be used to detect this helical order. In particular, we predict that the Josephson current will exhibit a magnetic interference pattern for a magnetic field applied perpendicular to the junction normal. We also discuss unusual magnetic effects associated with the helical order.

• Received 4 August 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.137002