Speaker
Description
Previous studies [1] of $Cu(tn)Cl₂$ $(tn=C_3 H_{10} N_2)$ proved that the system is an excellent realization of a spin-1/2 two-dimensional (2D) quantum magnet. While previous work was unable to specify the type of 2D magnetic lattice, re-analysis of the thermodynamics data within the current theories suggests realization of a spatially anisotropic square lattice (SASL) with $R=J_1/J_2≈0.5$ and $J_1/k_B=4.5 K$. However, a clear quadratic temperature dependence of specific heat observed far below 0.4 K indicates the presence of 2D magnetic correlations. Magnetic two-dimensionality of $Cu(tn)Cl₂$ can be also demonstrated by the response on the applied magnetic field, typical for the field-induced Berezinskii-Kosterlitz-Thouless (BKT) phase transition theoretically predicted for HAF on the square lattice.
The existence of long-range magnetic order (LRO) is another important issue. Previous powder studies in zero field (B=0) were not able to indicate LRO down to 50 mK. The existence of LRO in B=0 was further examined on powdered sample in the muon spin relaxation (μSR) experiment performed down to 50 mK. The absence of oscillations typical for magnetic ordered phase seems to indicate the absence of conventional LRO. It could, however, be as well due to the weakness of the local fields in the vicinity of the Cl ions, at the sites where muons are expected to stop. The fact that a sharp increase of the relaxation rate is observed at about 0.5-0.6 K may suggest the onset of LRO. It should be noted that the field induced specific heat anomalies start to form just around these temperatures. What is more, recent single crystal specific heat measurements indicated nearly negligible round hump in this temperature region [1].
We believe that elastic neutron scattering in zero magnetic field should definitely decide about the character of a magnetic long-range order which should differ from a conventional Néel order due to the presence of structural modulation introducing potential splitting of magnetic system into a few different competing 2D domains. On the other hand, the potential elimination of LRO via weakened interlayer interactions would make the compound attractive for the studies of the interplay of magnetic field and 2D quantum states, investigated within the bulk material.
References
[1] R. Tarasenko et al, PRB 108, 214432 (2023). DOI: 10.1103/PhysRevB.108.214432
