Pressure-Tuned Classical–Quantum Crossover in Magnetic Field-Induced Quantum Phase Transitions of a Triangular-Lattice Antiferromagnet

The high-pressure application is an experimental means to significantly alter the microscopic physical parameters of a material. At ambient pressure, these physical parameters generally do not change significantly, regardless of the temperature. Recently, the effects of high pressure have been studied across a broad area of condensed matter physics, including pressure-driven high-temperature superconductivity and topological phases. Frustrated quantum magnets are expected to be significantly affected by pressure because frustration due to competing interactions gives rise to many low-energy states with small energy differences. Additionally, small quantum fluctuations play an important role in manifesting unconventional physical phenomena, such as unconventional quantum phases. Therefore, applying external pressure to frustrated quantum materials enables the manipulation of quantum correlations across the classical and quantum mechanical regimes, facilitating the exploration of exotic phenomena along the crossover.

This study explores an exciting example of a frustrated antiferromagnet—triangular-lattice compound CsCuCl₃. To generate the phase diagram of magnetic field vs. pressure for this compound, we utilized our newly developed proximity detector oscillator system and high-pressure cell to measure magnetic susceptibility in pulsed magnetic fields exceeding the saturation field up to 55 T, under pressures of up to 2.08 GPa. We observed the field-induced quantum phase transitions from umbrella to up-up-down (UUD) and Y-coplanar phase immediately below the UUD phase above 0.90 and 1.7 GPa, respectively. Moreover, we calculated the pressure dependence of the transition fields, including the saturation field, and reliably determined the exchange interaction and easy-plane anisotropy parameters under pressure. With increasing pressure, the magnitude of the inter-chain antiferromagnetic exchange interaction increased linearly, whereas the magnitude of the intrachain ferromagnetic exchange interaction decreased significantly. Consequently, the ratio of the intra- to inter-chain exchange interactions decreased substantially with increasing pressure, indicating that the largely coupled ferromagnetic spins, regarded as semi-classical spins, became quantum spins. This suggests that the occurrence of the UUD and Y-coplanar phases is accompanied by a crossover from semiclassical to quantum spins in CsCuCl₃.

(Written by Masayuki Hagiwara on behalf of all authors)