Charge ordering and opening of spin gap in NaV2O5

We argue that the origin of the phase transition in quasi-one-dimensional antiferromagnet NaV2O5 is not the spin-Peierls (SP) instability, but a charge ordering. The opening of the spin gap and the lattice dimerization, characteristic for the spin-Peierls systems, in NaV2O5 result from the interplay between the charge, lattice, and spin degrees of freedom

Orbital ordering and frustrations

Orbital ordering occurs in many transition metal compounds with Jahn–Teller ions (Cu2+, Mn3+, low-spin Ni3+, Ti3+ etc). It plays an important role in these materials. At the same time, exchange interactions in orbitally degenerate systems are inherently frustrated, even in materials with simple crystal lattices. We discuss the origin of this frustration, considering in detail materials with cubic and triangular lattices of transition metal ions. We also discuss possible types of ground states of such systems, e.g., disordered orbital liquids and ordering due to the order-from-disorder mechanism

The Measurement of the Asymmetry of Tensor-Polarized Deuteron Electrodisintegration at 180 MeV Electron Energy

The nucleon emission asymmetry in d(e, pn)e' reaction was measured using the tensor-polarized deuterium jet target in the VEPP-2 electron storage ring. At the present experimental accuracy, the results for the proton energy interval Ep= 12-100 MeV do not contradict the nonrelativistic calculations.

Temperature dependence of optical spectral weights in quarter-filled ladder systems

The temperature dependence of the integrated optical conductivity I(T) reflects the changes of the kinetic energy as spin and charge correlations develop. It provides a unique way to explore experimentally the kinetic properties of strongly correlated systems. We calculated I(T) in the frame of a t-J-V model at quarter-filling for ladder systems, like NaV_2O_5, and show that the measured strong T dependence of I(T) for NaV_2O_5 can be explained by the destruction of short range antiferromagnetic correlations. Thus I(T) provides detailed information about super-exchange and magnetic energy scales.Comment: 4 pages, 5 figure

Infrared optical properties of the spin-1/2 quantum magnet TiOClTiOCl

We report results on the electrodynamic response of $TiOCl$, a low-dimensional spin-1/2 quantum magnet that shows a spin gap formation for T$<T_{c1}$= 67 $K$. The Fano-like shape of a few selected infrared active phonons suggests an interaction between lattice vibrations and a continuum of low frequency (spin) excitations. The temperature dependence of the phonon mode parameters extends over a broad temperature range well above $T_{c1}$, indicating the presence of an extended fluctuation regime. In the temperature interval between 200 $K$ and $T_{c1}$ there is a progressive dimensionality crossover (from two to one), as well as a spectral weight shift from low towards high frequencies. This allows us to identify a characteristic energy scale of about 430 $K$, ascribed to a pseudo spin-gap

Investigation of thermal and magnetic properties of defects in a spin-gap compound NaV2O5

The specific heat, magnetic susceptibility and ESR signals of a Na-deficient vanadate Na_xV_2O_5 (x=1.00 - 0.90) were studied in the temperature range 0.07 - 10 K, well below the transition point to a spin-gap state. The contribution of defects provided by sodium vacancies to the specific heat was observed. It has a low temperature part which does not tend to zero till at least 0.3 K and a high temperature power-like tail appears above 2 K. Such dependence may correspond to the existence of local modes and correlations between defects in V-O layers. The magnetic measurements and ESR data reveal S=1/2 degrees of freedom for the defects, with their effective number increasing in temperature and under magnetic field. The latter results in the nonsaturating magnetization at low temperature. No long-range magnetic ordering in the system of defects was found. A model for the defects based on electron jumps near vacancies is proposed to explain the observed effects. The concept of a frustrated two-dimensional correlated magnet induced by the defects is considered to be responsible for the absence of magnetic ordering.Comment: 6 pages, 8 figure

X-ray anomalous scattering investigations on the charge order in α′\alpha^\prime-NaV2_2O5_5

Anomalous x-ray diffraction studies show that the charge ordering in $\alpha^\prime$-NaV$_2$O$_5$ is of zig-zag type in all vanadium ladders. We have found that there are two models of the stacking of layers along \emph{c-}direction, each of them consisting of 2 degenerated patterns, and that the experimental data is well reproduced if the 2 patterns appears simultaneously. We believe that the low temperature structure contains stacking faults separating regions corresponding to the four possible patterns.Comment: Submitted to Phys. Rev. Lett., 4 pages, 4 eps figures inserted in the tex