Electronic structure of Mu-complex donor state in rutileTiO2

Abstract

The hyperfine structure of the interstitial muonium (Mu) in rutile (TiO2, weakly n-type) has been identified by means of a muon spin rotation technique. The angle-resolved hyperfine parameters exhibit a tetragonal anisotropy within the ab plane and axial anisotropy with respect to the 001 (ĉ) axis. This strongly suggests that the Mu is bound to O (forming an OH bond) at an offcenter site within a channel along theĉ axis, while the unpaired Mu electron is localized around the neighboring Ti site. The hyperfine parameters are quantitatively explained by a model that considers spin polarization of the unpaired electron at both the Ti and O sites, providing evidence for the formation of Mu as a Ti-O-Mu complex paramagnetic state. The disappearance of the Mu signal above ∼10 K suggests that the energy necessary for the promotion of the unpaired electron to the conduction band by thermal activation is of the order of 10 1 meV. These observations suggest that, while the electronic structure of Mu (and hence H) differs considerably from that of the conventional shallow level donor described by the effective mass model, Mu supplies a loosely bound electron, and thus, serves as a donor in rutile.

Influence of annealing atmosphere on formation of electrically-active defects in rutile TiO2

“…30,53 Monitoring the free carrier absorption as a function of temperature and concurrently the transformation between neutral and positively ionized H i , Herklotz et al 30 determined E A to be 10 6 1 meV for the H i donor. This value is also corroborated by a muon spectroscopy study conducted by Shimomura et al 22 To the best of our knowledge, no corresponding theoretical estimates are available in the literature. Accordingly, H i can be excluded as the origin of any of the three "general" defect levels observed in the present samples.…”

“…18 The identification of defects in rutile TiO 2 is particularly challenging as the energy scheme of the defect levels may be influenced by polaronic effects. [19][20][21][22][23][24][25][26][27][28][29] First, this makes the theoretical description of defects in rutile TiO 2 exceptionally challenging. 19,24 Second, polaronic effects can also lead to different defects having similar experimental signatures as electrons are trapped at similar Ti sites no matter which defect they actually originate from.…”

Development of General Purpose μSR Spectrometer ARTEMIS at S1 Experimental Area, MLF J-PARC

“…In Fig.4a shown are the µTC setup and vertical helium flow cryostat as installed in ARTEMIS. In order to measure the anisotropy of hyper-fine parameter of shallow muonium state, such as in ZnO [5] and rutile TiO 2 [6], an automated sample rotating rod, with the rotation axis parallel to the beam axis is prepared (Fig.4b); with the µTC and vertical helium flow cryostat, the angle dependence measurement of the hyperfine coupling parameter of the shallow muonium state is now fully automated. An example of a Fast Fourier Transform spectrum of the shallow muonium state in ZnO is shown in Fig.4c. (a) …”

Cage electron-hydroxyl complex state as electron donor in mayenite

“…The chemical formula of a unit cell for mayenite in a fully oxidized state is represented by a positively charged lattice framework and two endohedral oxygen ions, [Ca 24 Al 28 O 64 ] 4+ [ 10 2O…”

“…The polaron uses hopping motion to reach the neighboring cage by quantum tunneling, contributing to the conductivity. As electron doping proceeds as described by the formula [Ca 24 Al 28 O 64 ] 4+ [2(1 − x)O 2− + 4x(e − )], it exhibits insulator-to-metal transition [8], eventually exhibiting superconductivity with a transition temperature of ∼0.4 K in the compound with maximum doping (x ≃ 1, corresponding to an electron concentration of ∼2×10 21 cm −3 ) [11].…”