Novel $H_{\rm c2}$ suppression mechanism in a spin triplet superconductor -- Application to UTe$_2$--

A novel $H_{\rm c2}$ suppression mechanism is theoretically proposed in a spin triplet superconductor (SC) with equal spin pairs. We show that the upper critical field $H_{\rm c2}$ can be reduced from the orbital depairing limit $H^{\rm orb}_{\rm c2}$ to arbitrarily small value, keeping the second order phase transition nature. This mechanism is sharply different from the known Pauli-Clogston limit for a spin singlet SC where the reduction is limited to $\sim$0.3$H^{\rm orb}_{\rm c2}$ with the first order transition when the Maki parameter goes infinity. This novel $H_{\rm c2}$ suppression mechanism is applied to UTe$_2$, which is a prime candidate for a spin triplet SC, to successfully analyze the $H_{\rm c2}$ data for various crystalline orientations both under ambient and applied pressure, and to identify the pairing symmetry. It is concluded that the non-unitary spin triplet state with equal spin pairs is realized in UTe$_2$, namely $(\hat b+i\hat c)k_a$ in $^3$B$_{\rm 3u}$ which is classified under finite spin orbit coupling scheme.