INVESTIGATIONS OF THE NATURE OF THE 0.4 K PHASE-TRANSITION IN YBBIPT

We report results of resistivity measurements at ambient and hydrostat ic pressures of up to almost-equal-to 6 kbar. Large resistivity anisot ropy sets in below the phase-transition temperature of 0.4 K. In some directions the resistivity increases below 0.4 K, which we interpret a s partial gapping of the Fermi surface. Hydrostatic pressure of less t han 1 kbar suppresses the low-temperature phase entirely.