Abstract
Low-dimensional spin systems are one of the actively studied subjects insolid state physics due to the possibility to observe numerous quantumphenomena and to interpret these phenomena within relatively simple models(e.g., Ising or Heisenberg models for different lattice types). Aninteresting phenomenon in spin physics is the formation of a spin liquid –a strongly correlated ground state lacking long-range magnetic order. Spinliquids originate from quantum fluctuations that are particularly strongin systems with reduced dimensionality and low spin value. Thefluctuations can be further enhanced by introducing magnetic frustrationwhich impedes long-range ordering of the system. This talk mainly focuseson frustrated two-dimensional square lattice systems.The spin-1/2 frustrated square lattice (FSL) is one of the simplest modelsgiving rise to a spin liquid ground state. In this model (also known asthe J1 − J2 model), magnetic moments on a square-lattice aresubjected to nearest-neighbor interaction J1 along the side of the squareand next-nearest-neighbor interaction J2 along the diagonal of the square.Extensive theoretical research on the FSL model has been done in the pastand a rich phase diagram has been proposed. Despite numerous theoreticalinvestigations, experimental realizations of the J1 − J2 modelcompounds are scarce. Recently we synthesized and investigated thephysical properties of two new compounds Pb2VO(PO4)2 and BaCdVO(PO4)2 viamagnetization, heat capacity, and nuclear magnetic resonance (NMR)measurements. These two compounds were found to be strongly frustrated J1− J2 square lattice compounds with ferromagnetic J1 andantiferromagnetic J2. Based on the frustration ratio J2/J1, BaCdVO(PO4)2(J2/J1 = 0.9) is placed more close to the quantum spin liquidregime of the phase diagram than the Pb2VO(PO4)2 (J2/J1 = 1.76).Both compounds undergo magnetic ordering at TN ≈ 1 K and 3.6 K,respectively, likely towards a columnar antiferromagnetic state. Using 31PNMR on a large single crystal we investigated the static and dynamicproperties of Pb2VO(PO4)2. From the NMR spectral measurements below TN, weobtained direct evidence for a columnar antiferromagnetic ground state.This ground state is consistent with the phase diagram, expected based onthe observed J2/J1 ratio.
