Quantum magnetism is nowadays a fashionable topic in the physics of condensed matter. This is related to the exotic magnetic properties observed in low-dimensional quantum spin systems. Initially, most of the studies were dealing with Cu2+-based materials owing to the structural and electronic relationships with high-Tc superconducting layered cuprates. Recently however, interest moved to V4+ and Ti3+, both ions with a 3d1 configuration (S=1/2 ).. In the sodium-vanadium bronze system NaxV2O5, there exist seven phases denoted by a-, b-, d-, t-, a’-, h-, and k-phases in the ascending order of x (1-3). They are mixed-valent compounds of V4+ (d1, S=1/2 ) and V5+ (d0, S=0), among which a’-NaV2O5 (x=1.0) has been studied most intensively for its anomalous phase transition like a spin- Peierls transition. The spin state of a’-NaV2O5 surely falls into spin singlet accompanied with the lattice distortion, despite various arguments on the origin of phase transition. Low-dimensional spin systems with S=1/2 and singlet ground states are of great interest because of their fundamental quantum nature. The transitions to spin singlet without lattice distortion have been observed in layered vanadates of CaV2O5, MgV2O5, and CaV4O9: the former two compounds whose intralayer structures are similar to that of a’-NaV2O5 are considered to be two-leg spin-ladder systems and the latter one to be a plaquette spin-gap system.
 
    Electrons in strongly correlated transition-metal compounds can be regarded as having separate spin, charge, and orbital degrees of freedom. It is the interplay between these, combined with their coupling to the lattice, that gives rise to a wealth of possible spin, charge, and orbital orderings, as observed for instance in many colossal magneto resistance manganites, cuprates, titanates and vanadates . Systems with orbital degeneracy are particularly interesting because orbitals couple to the lattice via the cooperative Jahn-Teller effect, on the one hand, and via superexchange interactions to the electronic spin on the other hand. Therefore, at an orbital ordering phase transition, the magnetic susceptibility and phonon properties will be affected at the same time. Experimentally, however, such an interrelation is seldom found: in general the dominant JT orbital-lattice coupling obscures the more subtle effects due to the superexchange. Recently, a new type of magnetic phase transition is observed in NaTiSi2O6 The magnetic susceptibility of NaTiSi2O6 sharply decreases below 210 K, indicating a transition to a nonmagnetic, spin-singlet state, with signs of a opening of the spin-gap, in contrast to previous studies of the pyroxene family, which showed that almost all compounds in this class exhibit long-range antiferromagnetic ordering at low temperatures.

    Raman scattering offers several unique features in the investigation of spin, charge, orbital-ordered systems. For example, by providing energy, symmetry, and lifetime information concerning lattice, spin, as well as charge excitations, Raman scattering affords unique insight into the interplay among these coupled excitations in various phases. 

1. M. J. Konstantinovic, Z. V. Popovic, A. N Vasil’ev, M. Isobe and Y. Ueda, 
   “First evidence for charge ordering in NaV2O5 from Raman spectroscopy” 
   Solid State Communications, 112, 397 (1999)

2. M. J. Konstantinovic , Z. V. Popovic, M. Isobe and Y. Ueda, 
   “Raman scattering from magnetic excitations in the spin ladder compounds CaV2O5 and MgV2O5”
   Phys. Rev. B, 61, 15185 (2000)

3. Z. V. Popovic, M. J. Konstantinovic, V. A. Ivanov, O.P. Khuong, R. Gajic, A. Vietkin, V.V. Moshchalkov 
   “Optical properties of the spin-ladder compound Sr14Cu24O41” 
   Phys. Rev. B 62, 4963 (2000)

4. Z. V. Popovic, V. A. Ivanov, M. J. Konstantinovic , A. Cantarero, J. Martinez-Pastor, D. Olguin, M. I. Alonso, M. Gariga, O. P. Khoung, A. Vietkin, V. V. Moshchalkov: 
   “Optical studies of gap, hopping energies and the Anderson-Hubbard parameter in the zigzag-chain compound SrCuO2” 
   Phys. Rev. B 63, 165105 (2001).

5. Z. V. Popovic, V. A. Ivanov, O. P. Khoung, V. V. Moshchalkov,: 
   “Raman scattering study of phonon and spin modes in (TMTSF)2PF6” 
   Synthetic. Metals, 124, (2001), 421

6. Z.V.Popovic, G. Mihaly, I. Kezsmarki, H. Berger, L. Forro, and V. V. Moshchalkov: 
   ”Phonon and spin dynamics in BaVS3”, 
   Phys. Rev. B, 65 (13): art. no. 132301 (2002).

7. Z.V.Popovic, M.J. Konstantinovic , R. Gajic, V. N. Popov, M. Isobe, Y. Ueda and V. V. Moshchalkov: 
   ”Phonon dynamics in AV2O5 (A=Na, Ca, Mg, Cs) oxides”, 
   Phys. Rev. B 65 (18): art. no. 184303 (2002). 

8. M. J. Konstantinovic, Z. V. Popovic, V. V. Moshchalkov, C. Presura, R. Gajic, M. Isobe, and Y.Ueda, 
   Optical properties of alpha’-NaV2O5, 
   Phys. Rev. B 65 245103 ( 2002). 

9. Z. V. Popovic, M. J. Konstantinovic, V. V. Moshchalkov, M Isobe and Y. Ueda, 
   Raman scattering study of charge ordering in beta-Ca0.33V2O5, 
   J PHYS-CONDENS MATTER, 15, L139 (2003).

10. G. De Marzi, Z. V. Popovic, A. Cantarero, Z. Dohcevic-Mitrovic, N. Paunovic, and F. Sapina:
    Effect of A-site and B-site substitution on the infrared reflectivity spectra of La1-yAyMn1-xBxO3 (A=Ba, Sr; B=Cu, Zn, Sc; 0<y<0.3, 0<x<0.1) manganites, 
    Phys. Rev. B 68, 064302 (2003).

11. Z. V. Popovic, G. De Marzi, M. J. Konstantinovic, A. Cantarero, Z. Dohcevic-Mitrovic, M. Isobe, and Y. Ueda: 
    Phonon properties of MgTi2O4 spinel oxide with S=1/2 Pyrochlore Lattice, 
    Phys. Rev. B 68, 224302 (2003).

12. M.J. Konstantinovic, J. van den Brink, Z. V. Popovic, V. V. Moshchalkov, M. Isobe, and Y. Ueda,
    Orbital Dimerization and Dynamic Jahn-Teller Effect in NaTiSi2O6, 
    Phys. Rev. B 69, 020409(R) (2004)

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