Magnetic field- and pressure-induced quantum phase transitions in NH ₄CuCl₃

Quantum antiferromagnet NH₄CuCl₃ shows magnetization plateaus at 1/4 and 3/4 of the saturation magnetization. Specific heat measurements at ambient pressure and magnetization measurements under hydrostatic pressure were performed on NH₄CuCl₃ in magnetic fields. From specific heat measurements, we obtained the detailed magnetic phase diagram below 9 T, which includes the lower edge field H _c1 of 1/4 plateau. The phase boundary just below H_c1 can be described by a power low, H_c1 - H_N(T) ∝ T ^φ with φ = 2, where H_N(T) is the transition field at temperature T. Magnetic susceptibility measurements under pressure revealed reentrant quantum phase transitions, such that the gapless ground state at zero field changes into gapped state with increasing pressure, and further into gapless state again.