Magnetic field-induced XY-AFM in 2D Heisenberg Antiferromagnet [Cu(pyz)$_{2}$(pyO)$_{2}$](PF$_{6})$

Specific heat ($C_{p})$ and magnetic susceptibility (\textit{$\chi $}) measurements were performed on the two-dimensional spin-1/2 Heisenberg antiferromagnet [Cu(pyz)$_{2}$(pyO)$_{2}$](PF$_{6})_{2}$ in DC and pulsed magnetic fields up to H = 15 T and 40 T, respectively [1]. We observe no long-range magnetic order down to 500 mK in zero applied magnetic field, suggesting that [Cu(pyz)$_{2}$(pyO)$_{2}$](PF$_{6})_{2}$ is close to an ideal 2D AFM and instead undergoes a Berezinskii-Kosterlitz-Thouless (BKT) transition. However, the application of a finite magnetic field induces a clear anomaly in $C_{p}$, although not in \textit{$\chi $}. This behavior is known to be a remarkable signature of magnetic field induced XY-AFM [2]. In addition, $C_{p}$(H) measurements in pulsed fields, performed down to T = 1.5 K and up to H = 40 T, were used to map out the asymmetric Field-Temperature phase diagram which provides additional support for an ideal realization of field-induced XY-AFM in [Cu(pyz)$_{2}$(pyO)$_{2}$](PF$_{6})_{2}$.\\[0pt] [1] Y. Kohama et al., \textit{Rev. Sci. Instrum.} \textbf{81}, 104902 (2010).\\[0pt] [2] A. Cuccoli et al., \textit{Phys. Rev. }\textbf{B68}, 060402(R) (2003).