We present results of high-resolution thermal-expansion and specific-heat measurements on single crystalline alpha'-NaV2O5. We find clear evidence for two almost degenerate phase transitions associated with the formation of the dimerized state around 33K: A sharp first-order transition at T1=(33+-0.1)K slightly below the onset of a second-order transition at T2onset around (34+-0.1)K. The latter is accompanied by pronounced spontaneous strains. Our results are consistent with a structural transformation at T1 induced by the incipient spin-Peierls (SP) order parameter above T2=TSP.

We present results of high-resolution thermal-expansion and specific-heat measurements on single crystalline alpha'-NaV2O5. We find clear evidence for two almost degenerate phase transitions associated with the formation of the dimerized state around 33K: A sharp first-order transition at T1=(33+-0.1)K slightly below the onset of a second-order transition at T2onset around (34+-0.1)K. The latter is accompanied by pronounced spontaneous strains. Our results are consistent with a structural transformation at T1 induced by the incipient spin-Peierls (SP) order parameter above T2=TSP.

Raman scattering in the quarter-filled spin ladder system alpha'-NaV2O5 shows in the dimerized singlet ground state (T less than or equal to T-SP = 35 K) an unexpected sequence of three magnetic bound stares. Our results suggest that the recently proposed mapping onto an effective spin chain for T > T-SP has to be given up in favor of the full topology and exchange paths of a ladder in the dimerized phase for T<T-SP. As the new ground state we propose a dynamic superposition of energetically nearly degenerate dimer configurations on the ladder. [S0163-1829(98)03646-7].

We present results from magnetic-resonance measurements for 75--350 GHz in ${\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{-}{\mathrm{NaV}}_{2}{\mathrm{O}}_{5}.$ The temperature dependence of the integrated intensity indicates that we observe transitions in the excited state. A quantitative description gives resonances in the triplet state at high-symmetry points of the excitation spectrum of this spin-Peierls compound. This energy has the same temperature dependence as the spin-Peierls gap. Similarities and differences with another inorganic compound ${\mathrm{CuGeO}}_{3}$ are discussed.

By means of Raman scattering, we investigated single-crystalline samples of the new inorganic spin-Peierls compound α′-NaV2O5 = 34 with TSP = 34 K. Phonons with anomalous broad line shape indicating a strong spin-phonon coupling above TSP were detected. Below TSP several additional modes could be observed. Different temperature dependencies of these modes are interpreted as a first hint that low-energy modes are related to the opening of the spin-Peierls gap observed in neutron scattering experiments while other modes are related to the lowering of the lattice symmetry due to the spin-Peierls transition.

Neutron and X-ray diffraction experiments have shown that NaV2O5 consists of zig-zag V chains of average valence 4.5 rather than a charge ordered state at room temperature. Both these experiments revealed superlattice reflections below the phase transition temperature TSP≈34K. The modulation below TSP is due to displacements predominantly of V atoms for which we propose a model.

We present measurements on single- and polycrystalline samples of the spin-Peierls compound α′ -NaV 2 O 5 . The transition temperature T SP obtained in susceptibility measurements is strongly dependent on the Na-stoichiometry of the samples. The g -values obtained in ESR measurements are close to g ≈ 2. The crystallographic distortion due to the dimerisation at T SP is seen in thermal expansion as well as in Raman light scattering measurements.

We present results from magnetic-resonance measurements for 75-350 GHz in alpha'-NaV2O5. The temperature dependence of the integrated intensity indicates that we observe transitions in the excited state. A quantitative description gives resonances in the triplet state at high-symmetry points of the excitation spectrum of this spin-Peierls compound. This energy has the same temperature dependence as the spin-Peierls gap. Similarities and differences with another inorganic compound CuGeO3 are discussed.

The spin–Peierls (SP) transition is still one of the most challenging effects in quasi-one-dimensional magnetism. A few years ago the first inorganic spin–Peierls compound CuGeO3 with TSP=14.3K was discovered, and recently α′-NaV2O5 was found to be another inorganic SP system with the highest transition temperature so far observed: TSP=35K. Electron spin resonance (ESR) is the only direct way to probe electron spin dynamics in magnetic fields higher than 12T, where a transition to an incommensurate magnetic phase can occur. We present ESR results on single crystals of pure and Si-doped CuGeO3 and pure α′-NaV2O5. Our experiments were done in a wide frequency range 35–440GHz in magnetic fields up to 16T, covering a large temperature range 1.5–100K. The temperature dependence of the ESR absorption in the D-phase in α′-NaV2O5 points to transitions among triplet states, which are separated from the singlet ground state by an energy gap Δ≈85K for T→0. In contrast to χ(T) the ESR absorption does not stay finite for T→0. In the incommensurate phase of slightly Si-doped CuGeO3 (0.2% Si) ESR signals were observed, but their behaviour is much different from the ones in the pure compound.

The effect of Si doping on the magnetic properties of the spin-Peierls (SP) system CuGeO3 was found to differ strongly between polycrystals (PC's) and single crystals (SC's). In SC's, the SP state is suppressed much mon strongly, whereas the existence region of the antiferromagnetic (AF) state is enhanced. We investigated the origin of this difference by means of magnetic susceptibility, specific heat, thermal expansion, Raman scattering, elastic neutron scattering, and x-ray measurements on CuGe1-xSixO3 samples prepared under different conditions. The partial oxygen pressure and the temperature during the synthesis were found to have a profound influence on the magnetic properties: preparation under reduced oxygen pressure leads to a stabilization of the AF state, whereas heating above the melting point results in a strong decrease of T-SP in Si-doped samples. Therefore, both the AF stabilization and the TSP reduction observed in SC's are not an intrinsic effect of Si doping PC samples, which can be prepared at lower temperatures and more oxidizing conditions, reflect much better the intrinsic properties of CuGe1-xSixO3. We were able to prepare PC samples up to 50 at. % Si and found a continuous decrease of the one-dimensional character of the magnetic properties without pronounced changes in the structure.

We present the first measurements of magnetisation and Raman light scattering on alpha l- NaV2O5 single crystals. Below 34 K, we observe a pronounced isotropic decrease of the susceptibility indicating the opening of a spin gap. The transition temperature is slightly field dependent. Raman experiments reveal a crystallographic distortion at the transition. Our results clearly establish alpha' - NaV2O5 to be the second inorganic spin-Peierls system.

We present results from magnetic resonance measurements for 75-350 GHz in α'-NaV_2O_5. The temperature dependence of the integrated intensity indicates that we observe transitions in the excited state. A quantitative description gives resonances in the triplet state at high symmetry points of the excitation spectrum of this Spin-Peierls compound. This energy has the same temperature dependence as the Spin-Peierls gap. Similarities and differences with the other inorganic compound CuGeO_3 are discussed.