Raman Scattering from Magnetic Excitations in the Spin Ladder Compounds CaV 2 O 5 and MgV

We present the Raman scattering spectra of CaV 2 O 5 and MgV 2 O 5. Our analysis of the magnetic modes shows that the magnetic ordering in CaV 2 O 5 can be described using two-leg ladder Heisenberg antiferromagnetic model with J /J ⊥ ∼ 0.1, and a small interladder exchange. The spin-gap and exchange constant are estimated to be ∆ s ∼ 400 cm −1 (570 K) and J ⊥ ∼ 640 K. No magnon bound states are found. In contrast to CaV 2 O 5 the existence of the spin-gap is not confirmed in MgV 2 O 5 and the magnetic ordering is shown to be quasi two-dimensional. The spin-1/2 ladder systems, which are the intermediate situation between one-dimensional (1D) and two-dimensional (2D) Heisenberg antiferromagnet, show fascinating quantum effects such as dramatic dependence of the ground states on the width of the ladder (the width of the ladder is defined as a number of coupled chains) 1. Namely, ladders made of even number of legs have spin-liquid ground state with a exponential decay of the spin-spin correlations produced by the finite spin-gap. Among them, the two-leg ladder is the first member of that family, with a largest spin-gap of about 0.5J (assuming the same coupling constant along the legs and rungs). Realization of the ladders is recognized in Sr-Cu-O systems 2 , and recently in AV 2 O 5 , A=Ca, Mg (two-leg ladder) 3. According the their crystal structures, vanadium oxides are quasi-2D layered materials, with spin-1/2 vanadium ions which are forming two-leg ladders coupled in the trellis lattice. The spin-gap is observed in CaV 2 O 5 to be around 600 K 4,5 , and the magnetic susceptibility measurements 5 indicated that the magnetic ordering in this compound can be described using spin-1/2 two-leg ladder Heisenber antiferromagnetic model. On the other hand there is only contradicting evidence for the existence of the spin-gap in MgV 2 O 5 6,7. Excitation spectrum of the ladder has been analyzed by variety of the techniques, but it was quite recently shown that the two-magnon bound states (we will use word magnon although classical magnons do not exist in quantum systems), besides usual triplet (gap mode) and two-magnon continuum, may appear in the spectra 8. Such a bound states were also predicted for the dimerized quantum spin chains 9 and indeed observed in CuGeO 3 10 1