Speaker
Description
Low-dimensional spin-1/2 Heisenberg antiferromagnets provide an ideal platform for studying topological magnetic phases, including a magnetic Berezinskii–Kosterlitz–Thouless (BKT) transition. The previously well-studied metal-organic complex Cu(pm)(ea)$_2$ (pm = pyromellitic acid anion, [C$_6$H$_2$(COO)$_4$]$^{4-}$; ea = ethylamine cation, [C$_2$H$_5$NH$_3$]$^+$) represents a quasi-two-dimensional antiferromagnet with anisotropic interactions forming a rectangular lattice with dominant exchange J$_1$/k$_B$ ≈ 10 K, and anisotropy parameter R = J$_2$/J$_1$ ≈ 0.7, while no long-range magnetic ordering has been observed down to 1.8 K [1].
We extended heat capacity measurements down to 0.4 K in magnetic fields up to 9 T. While no magnetic phase transition was detected in zero field, the field behavior of the heat-capacity anomaly enabled construction of the magnetic field–temperature phase diagram. The resulting phase boundary is characterized by a small initial increase followed by the expected suppression of the BKT transition temperature with increasing magnetic field. Furthermore, the magnetic entropy released across the transition was analyzed and compared with theoretical predictions for a two-dimensional Heisenberg antiferromagnet on a isotropic lattice.
Based on the established microscopic magnetic model, the experimental phase diagram is compared with Quantum Monte Carlo simulations performed for different values of the exchange anisotropy parameter R. The resulting phase diagram provides a roadmap for future neutron scattering experiments aimed at probing short-range magnetic correlations across the field-induced BKT magnetic phase in Cu(pm)(ea)$_2$.
Acknowledgements
The work was supported by the projects vvgs-2026-3893, APVV-23-0006 and APVV-22-0172.
References
[1] R. Nath, M. Padmanabhan, S. Baby, A. Thirumurugan, D. Ehlers, M. Hemmida, H.-A. Krug von Nidda, and A. A. Tsirlin, “Quasi-two-dimensional S = ½ magnetism of Cu[C$_6$H$_2$(COO)$_4$][C$_2$H$_5$NH$_3$]$_2$,” Physical Review B, vol. 91, no. 5. American Physical Society (APS), Art. no. 054409, Feb. 2015. doi: 10.1103/PhysRevB.91.054409.
