M. Isobe

CaCrO3: an anomalous antiferromagnetic metallic oxide.

Combining infrared reflectivity, transport, susceptibility, and several diffraction techniques, we find compelling evidence that CaCrO3 is a rare case of a metallic and antiferromagnetic transition-metal oxide with a three-dimensional electronic structure. Local spin density approximation calculations correctly describe the metallic behavior as well as the anisotropic magnetic ordering pattern of C type: The high Cr valence state induces via sizable pd hybridization remarkably strong next-nearest-neighbor interactions stabilizing this ordering. The subtle balance of magnetic interactions gives rise to magnetoelastic coupling, explaining pronounced structural anomalies observed at the magnetic ordering transition.

Peierls mechanism of the metal-insulator transition in ferromagnetic hollandite K2Cr8O16.

Synchrotron x-ray diffraction experiment shows that the metal-insulator transition occurring in a ferromagnetic state of a hollandite K(2)Cr(8)O(16) is accompanied by a structural distortion from the tetragonal I4/m to monoclinic P112(1)/a phase with a √2×√2×1 supercell. Detailed electronic structure calculations demonstrate that the metal-insulator transition is caused by a Peierls instability in the quasi-one-dimensional column structure made of four coupled Cr-O chains running in the c direction, leading to the formation of tetramers of Cr ions below the transition temperature. This provides a rare example of the Peierls transition of fully spin-polarized electron systems.

High-frequency dielectric and magnetic anomaly at the phase transition in NaV2O5

We found anomalies in the temperature dependence of the dielectric and the magnetic susceptibiliy of NaV_2O_5 in the microwave and far infrared frequency ranges. The anomalies occur at the phase transition temperature T_c, at which the spin gap opens. The real parts of the dielectric constants epsilon_a and epsilon_c decrease below T_c. The decrease of epsilon_a (except for the narrow region close to T_c) is proportional to the intensity of the x-ray reflection appearing at T_c. The dielectric constant anomaly can be explained by the zigzag charge ordering in the ab-plane appearing below T_c. The anomaly of the microwave magnetic losses is probably related to the coupling between the spin and charge degrees of freedom in vanadium ladders.

NMR study of local magnetic and electronic properties of vanadium bronzes

Abstract 51 V NMR experiments have been performed to study the local magnetic and electronic properties of the vanadium oxide bronzes β-AV 6 O 15 (A=Na and Ca). We observed a drastic change in the 51 V NMR spectrum due to a charge-ordering transition in both oxides. The ground state of NaV 6 O 15 is found to be an antiferromagnetically ordered state, whereas that of CaV 6 O 15 is nonmagnetic. This is closely related with the number density of V 4+ ions and the structure of the charge ordering.

Raman scattering from magnetic excitations in the spin-ladder compoundsCaV2O5andMgV2O5

We present the Raman-scattering spectra of CaV2O5 and MgV2O5. The magnetic contribution in the Raman spectra of CaV2O5 is found in the form of a strong asymmetric line, centered at 2Delta, with a tail on the high-energy side. Our analysis of its spectral shape shows that the magnetic ordering in CaV2O5 can be described using a S=1/2 two-leg ladder Heisenberg antiferromagnetic model with Jparallel/Jperp = 0.1, and a small interladder exchange. The spin gap and exchange constant are estimated to be Delta = 400 cm-1 (570 K) and Jperp = 640 K. No magnon bound states are found. In contrast to CaV2O5 the existence of the spin-gap is not confirmed in MgV2O5, since we found no feature in the spectra which could be associated with the onset of the two-magnon continuum. Instead, we observe two-magnon excitation at 340 cm-1, presumably related to the top of the two-magnon brunch.

Low-energy excitations in NaV2O5

In the (ab)-polarized Raman scattering spectra of NaV 2 O 5 single crystals, measured with a 647.1 nm laser line at T<T c , we found two modes at 86 and 126 cm - 1 not previously reported. These two modes, together with 66 and 106 cm - 1 modes, make an array of four low-energy equidistant modes below the energy onset of the continuum at about 132 cm - 1 All four modes are strongly suppressed by increasing Na deficiency, suggesting a nonvibrational origin of the modes and the existence of a quantum phase transition at the critical Na deficiency between 3% and 4%. These results question our current understanding of NaV 2 O 5 as a quasi-one-dimensional Heisenberg antiferromagnet.

Thermal and magnetic properties of defects in the spin-gap compound NaV 2 O 5

The specific heat, magnetic susceptibility, and ESR signals of a Na-deficient vanadate

First evidence for charge ordering in NaV2O5 from Raman spectroscopy

{\mathrm{Na}}_{x}{\mathrm{V}}_{2}{\mathrm{O}}_{5}

Long-range magnetic order in quasi-one-dimensional NaCrSi2O6 and NaCrGe2O6 metal oxides

(x=1.00\char21{}0.90)