Structural and magnetic properties of (CsCl)Cu(VO₃)₂: Observation of unusual magnetism in a copper-based square lattice

In low-dimensional spin systems, the S = ½ copper-based square lattices are of particular interest due to their possible connection with superconductivity. We have synthesized and investigated the structural, magnetic, and thermodynamic properties of single-crystalline (CsCl)₂Cu(VO₃)₂, a material featuring Cu-Cl square sheets interlinked by non-magnetic vanadate chains. Our investigations, employing x-ray diffraction, bulk susceptibility, specific heat, ESR, and ¹³³Cs NMR, revealed a structural transition from tetragonal to orthorhombic symmetry at ~140 K and magnetic ordering transitions at 2.7 K and 1.5 K, which are suppressed in an external field of about 2 kOe. ¹³³Cs NMR and ESR measurements suggest short-ranged correlations emerging around 8 K. The ¹³³Cs spin-lattice relaxation rate 1/T₁ follows a gapped behavior with an activation energy of ~10 K, and this gap increases with the applied magnetic field. These findings reveal unusual magnetic behavior in (CsCl)₂Cu(VO₃)₂, potentially offering new insights into quantum magnetic phenomena in copper-based systems.