Spin-singlet dimer phase in a frustrated square lattice under a magnetic field

Abstract

We investigated the isotropic spin-1/2 Heisenberg model on an anisotropic square lattice with competing exchange interactions, motivated by the unconventional magnetic behavior observed in the verdazyl-based compound (o-MePy-V)PF6. Using a cluster mean-field approach, we explore a field-induced phase stabilized by the interplay between frustration and quantum fluctuations, focusing on the role of exchange interactions. We identify: (i) the formation of spin singlet pairs, signaled by enhanced spin-spin correlations in specific field regimes; and (ii) a one-half magnetization plateau, emerging from a subtle balance between competing exchange couplings and field-enhanced quantum fluctuations. Our results reveal that an enhancement of frustration, achieved by tuning small variations in the spatially anisotropic exchange interactions of the compound (o-MePy-V)PF6, can stabilize a field-induced quantum phase where ferromagnetism coexists with antiferromagnetic dimers. Our results provide microscopic insight into the mechanisms driving these nontrivial phases and offer theoretical support for interpreting experimental observations in this class of low-dimensional quantum magnets.