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Magnetic Phase Diagram of Alternating Chain Compound Pb2V3O9

Pb2V3O9 was found to have antiferromagnetic alternating chains of S=1/2 and show the antiferromagnetic long range ordered (AFLRO) state under the magnetic field between 4 and 38 T. We succeeded in growing single crystals of Pb2V3O9 by floating zone method and determined the asymmetric H-T phase diagram of Pb2V3O9 from measurements of the magnetization and the heat capacity. Previous reports focused on the AFLRO state in the low magnetic field region and that in high magnetic field region was not discussed so much. In this paper, we examined the AFLRO state in terms of the mean field theory and the picture of Bose-Einstein condensation of magnons to discuss the whole phase diagram. Consequently, the asymmetry of the phase boundary in Pb2V3O9 is explained by the mixing contribution of excited states and by interchain interactions.