Temperature dependent photoemission spectroscopy on lightly-doped sodium tungsten bronze

Vacancy-induced in-gap states in sodium tungsten bronzes: Density functional investigations

“…For oxygen-deficient WO 3−δ and NaWO 3−δ , the defect state traps the electrons and thereby, promotes electron-lattice coupling, which leads to the formation of polaron-like bound states. This is evident from the outcomes of the ARPES results of low-doped Na x WO 3 [10,12] where the intensity of the 2.5 eV peak is maximum at low temperature and decreases as temperature increases indicating the breakdown of polaron formation. This gives the evidence of the oxygen-vacancy-related polaron-like bound state in NaWO 3 (in the insulating regime).…”

“…The polaron in the low-doped Na x WO 3 (in the insulating regime) has been investigated by high-resolution ARPES measurements [10,12] where the intensity variation of the band-A gives the signature of the polaron formation. In transition metal oxides, polaron formations are facilitated by the significant electron-phonon interactions [27,28].…”

“…The evolution of the induced states occurs from the unpaired electrons donated by the oxygen vacancy, which has a character contributed from W 5d orbitals. We strongly believe that the experimentally observed band-A in this system arises from the oxygen vacancy defects and its intensity variation with temperature [10,12] is associated with the formation of polarons at the defect sites in the low-sodium-doped tungsten bronzes.…”

“…Oxygen-vacancy-induced polaron formation has also been studied in systems like TiO 2 and SrTiO 3 [8,9]. In earlier works [10][11][12][13][14][15], both experimental and theoretical electronic structures, Fermi surfaces of Na-doped WO 3 have been reported. Here we show the ARPES spectral intensity plot of Na x WO 3 along the high-symmetry directions of the Brillouin zone for two compositions: x = 0.025, which is in the insulating regime (see fig.…”

“…Earlier work carried out on other transition metal systems shows that oxygen vacancies influence the electronic properties and lead to the formation of in-gap states. From the photoemission studies [10,12], it was shown that the band-A in low-Na-doped WO 3 varies with temperature and gives the signature of polaron formation. We believe that this polaronic feature arises from the electrons influenced by the potential generated by the oxygen vacancies leading to observed in-gap states.…”