Disorder effects in the bipolaron system Ti ₄ O ₇ studied by photoemission spectroscopy

Abstract

We have performed a photoemission study of Ti4O7 around its two transition temperatures so as to cover the metallic, high-temperature insulating (bipolaron-liquid), and low-temperature insulating (bipolaron-crystal) phases. While the spectra of the low-temperature insulating phase show a finite gap at the Fermi level, the spectra of the high-temperature insulating phase are gapless, which is interpreted as a soft Coulomb gap due to dynamical disorder. We suggest that the spectra of the high-temperature disordered phase of Fe3O4, which exhibits a charge order-disorder transition (Verwey transition), can be interpreted in terms of a Coulomb gap.PACS numbers: 71.30.+h, 72.80.Ga, Since Mott [1] proposed the idea of variable-range hopping and minimum metallic conductivity for disordered systems and Anderson [2] raised the concept of localization due to disorder, physical properties of disordered solids have been extensively studied. Influence of Coulomb interaction on the electronic density of states (DOS) near the Fermi level (E F ) of disordered systems is one of the most fundamental issues to be clarified. Efros and Shklovskii [3] proposed that in disordered insulators long-range Coulomb interaction opens a soft Coulomb gap (SCG), whose DOS is proportional to (E − E F ) 2 . So far, there have been tunneling spectroscopic confirmations of the SCG in some disordered systems such as doped semiconductors [4]. Davies and Franz [5] pointed out the possibility of an SCG opening in the photoemission spectra of Na x Ta y W 1−y O 3 [6], but the experiments did not have sufficient energy resolution to critically address this problem. Another important issue is how short-range order (SRO) affects DOS near E F , that is, how the electronic structure evolves when a charge ordering develops from disorder to SRO to long-range order.Ti 4 O 7 is a suitable system to study the above problems: It undergoes successive phase transitions with decreasing temperature from a metal to a charge-ordered insulator via an insulator with SRO [7]. It is a system with nominally 0.5 3d electron per Ti, allowing two possible valence states of Ti 3+ (3d 1 ) and Ti 4+ (3d 0 ), and attracted particular attention in 1970's as a system where bipolarons, or singlet pairs of two polarons, are formed in real space [7][8][9]. Above T MI = 154 K, the system is in the metallic (M) phase and the Ti valence is believed to be uniform 3.5+ as shown in Fig. 1 (a). That is, the electrical resistivity ρ(T ) is metallic with Pauliparamagnetic χ(T ). With decreasing temperature, singlet Ti 3+ -Ti 3+ pairs, namely bipolarons, are formed, resulting in the metal-to-insulator transition at T MI with a steep increase in ρ(T ) by three orders of magnitude. At the same time, χ(T ) almost vanishes, reflecting the formation of the singlet bipolarons. We refer to this phase as the high-temperature insulating (HI) phase. Because the bipolarons are dynamically disordered in this phase as has been established by EPR studies [10], the HI phase may be called a bipolaron liquid. Sub...

In-gap state and effect of light illumination inCuIr2S4probed by photoemission spectroscopy

“…4 and can be regarded as a kind of bipolaronic solid-to-liquid transition similar to Ti 4 O 7 . 42 This picture is consistent with the recent diffraction study of CuIr 2 S 4 at low temperature, 30 indicating that the long-range order ͑octamer order͒ is destroyed but the dimers are preserved locally after x-ray or electron irradiation. Moreover similar charge ordering as shown in Fig.…”

“…Actually, the UPS line shapes of CuIr 2 S 4 across the MIT resemble those of a bipolaronic material Ti 4 O 7 . 42 The spectral weight in the insulating phase of Ti 4 O 7 also obeys the power-law function A͑E − E 0 ͒ n with n ϳ 2. Moreover, even in the metallic phase, both the spectra of CuIr 2 S 4 and Ti 4 O 7 show weak Fermi edges and broad peaks at ϳ0.75 eV, around which most of spectral weights are distributed.…”

Thermodynamics of oxygen defective Magnéli phases in rutile: A first-principles study

“…They observed sudden changes in the O 1s XAS at the semiconductor-metal transition, and a 0.25 eV upward shift of the Ti 3d bands in the low temperature semiconducting phase. The behavior of the electronic states at the phase transitions has also been analyzed by Kobashi et al 27 Using photoemission spectroscopy, they studied the electronic structure and measured Ti 4 O 7 's band gap. The optical properties of Ti 4 O 7 have been experimentally investigated as well.…”

Evolution of the electronic structure from electron-doped to hole-doped states in the two-dimensional Mott-Hubbard systemLa1.17−xPbxIno1, Okane2, Fujimori3 et al. 2004Phys. Rev. B7040View full textAdd to dashboardCite

“…30,31 As seen from Figs. 9 and 10, the leading edge of the V 3d band is broadened and obscured for all the compositions.…”