Electron spin resonance in three spin- 1/2 dimer systems: VO(HPO4)0.5H2O , KZn(H2O)(VO)2(PO4)2(H2PO4) , and CsV_{2}O_{5}

Using X-band electron spin resonance (ESR) spectroscopy, we investigate the magnetic properties of the low-dimensional quantum-spin systems, VO(HPO4)⋅0.5H2O, KZn(H2O)(VO)2(PO4)2(H2PO4), and CsV2O5, where the magnetic V4+(S=1/2) ions are arranged in pairs. By studying the temperature dependence as well as the angular dependence of the ESR spectra, we find that these systems, which may be well described as spin-1/2 dimer systems, show a change in behavior as a function of temperature. At room temperature, in accordance with the Kubo-Tomita prediction, an exchange narrowing phenomenon takes place, whereas linewidth broadening and then the appearance of a fine structure are instead observed at helium temperatures where spins are strongly correlated.