Possible Bose-Einstein condensation of magnons in a S = 5/2 honeycomb lattice

Quantum magnets offer a unique platform for exploring exotic quantum phases and quantum phase transitions through external magnetic fields. A prominent example is the field-induced Bose–Einstein condensation (BEC) of magnons near the saturation field. While this behavior has been observed in low-spin systems, its realization in high-spin, quasi-two-dimensional magnets – where multiple on-site excitations are possible – remains exceptionally rare. Here, we report thermodynamic and density functional theory results on single crystals of the honeycomb-lattice antiferromagnet K${}_4$MnMo${}_4$O${}_{15}$ with $S=5\mathrm{/}2$. The system undergoes a field-induced transition to a fully polarized state at the critical field ${\mu }_0{H}_{\mathrm{s}}=6.4$~T. Our results reveal possible thermodynamic signatures of magnon BEC, $T_{\mathrm{N}}\sim (H_{\mathrm{s}}-H{)}^{2\mathrm{/}d}$ ($d=3$), expanding the purview of BEC-driven quantum criticality to a high-spin, quasi-two-dimensional antiferromagnets with negligibly small anisotropy.