Frustrated spin-1/2 square lattice in the layered perovskite PbVO(3)
  We report on the magnetic properties of the layered perovskite PbVO(3). The results of magnetic susceptibility and specific heat measurements as well as band structure calculations consistently suggest that the S=1/2 square lattice of vanadium atoms in PbVO(3) is strongly frustrated due to next-nearest-neighbor antiferromagnetic interactions. The ratio of next-nearest-neighbor (J(2)) to nearest-neighbor (J(1)) exchange integrals is estimated to be J(2)/J(1)\approx 0.2-0.4. Thus, PbVO(3) is within or close to the critical region of the J(1)-J(2) frustrated square lattice. Supporting this, no sign of long-range magnetic ordering was found down to 1.8 K. 
Alexander A. Tsirlin, Alexei A. Belik, Roman V. Shpanchenko, Evgeny V.   Antipov, Eiji Takayama-Muromachi, Helge Rosner