First-order superconducting resistive transition in Ba$_{0.07}$K$_{0.93}$Fe$_2$As$_2$
The superconducting resistive transition in high-quality single crystals of Ba$_{0.07}$K$_{0.93}$Fe$_2$As$_2$ ($T_c\sim$8 K) is hysteretic below about 1 K for in-plane fields. The hysteresis is not observed as the field is tilted away from the $ab$ plane by 20$^{\circ}$ or more. The temperature and angle dependences of the upper critical field suggest a strong paramagnetic effect for in-plane fields. We ascribe the hysteresis to a first-order superconducting transition due to the paramagnetic effect.