Charge gap and magnetism across the metal-insulator transition in (Na,Sr)_xV₆O₁₅

Yannick Fagot-Revurat

Laboratoire de physique des matériaux - LPM- UMR7556, Université Henri Poincaré,
Bd. des Aiguillettes - BP 239, 54506 Nancy, France

Some ARPES data devoted to the study of the metal-insulator transition occuring in the quasi-one dimensionnal compounds (Na,Sr)xV6O15 will be presented. This vanadium bronzes family has been first studied a long time ago. Nevertheless, the recent discovery in stoichiometric samples (x=1) of a complex phase transition associating a metal-insulator transition (MIT), a structural transition and a charge ordering [2] and later the observation of a superconducting behavior below 9 K under high pressure in NaV6O15 (being observed for the first time in a vanadium-based compound) [2] have renewed the interest for these mixed-valent V4+-V5+ compounds. ARPES technics is a powerfull tool to study high and low energy electronic properties across a metal-insulator transition, especially if electronic correlations play an important role. In the high temperature phase of SrxV6O15 (insulating for x=0.5 to 0.8, and quasi-metallic for x=1), UPS spectra show a broad incoherent peak (d spectral weight) around 1 eV, reminiscent of the lower Hubbard band showing that the correlations are not negligible in these compounds even far from the half-filling (0.33 electron/site). In addition, we will discuss the possible origin of the absence of spectral weight at the Fermi level (surface effects, quasi-1D typical spectral weight or strong electron-phonon coupling ?). Comparaison with LDA band-structure calculations will be made. The temperature dependence of ARPES spectra observed in SrV6O15 evidences the opening of a charge gap of 200 meV below the MIT transition (occuring at 170 K in this compound). Starting from the strong measured 2delat/kTc=30 BCS ratio and taking into account the magnetic properties observed below the MIT, we will try to discuss the origin(s) of the MIT (2kF or 4kF charge density waves...), the possible charge ordering pattern, focusing on the (rare) complexity of such transition.