Compositional disorder and transport peculiarities in the amorphous indium oxides

Abstract

We present results of the disorder-induced metal-insulator-transition (MIT) in three-dimensional amorphous indium-oxide films. The amorphous version studied here differs from the one reported earlier [Phys. Rev. B 46, 10917 (1992)] in that it has a much lower carrier concentration. As a measure of the static disorder we use the dimensionless parameter kF ℓ. Thermal annealing is employed as the experimental handle to tune the disorder. On the metallic side of the transition, the low temperature transport exhibits weak-localization and electron-electron correlation effects characteristic of disordered electronic systems. These include a fractional power-law conductivity versus temperature behavior anticipated to occur at the critical regime of the transition. The MIT occurs at a kF ℓ ≈0.3 for both versions of the amorphous material. However, in contrast with the results obtained on the electron-rich version of this system, no sign of superconductivity is seen down to ≈0.3K even for the most metallic sample used in the current study. This demonstrates that using kF ℓ as a disorder parameter for the superconductor-insulator-transition (SIT) is an ill defined procedure. A microstructural study of the films, employing high resolution chemical analysis, gives evidence for spatial fluctuations of the stoichiometry. This brings to light that, while the films are amorphous and show excellent uniformity in transport measurements of macroscopic samples, they contain compositional fluctuations that extend over mesoscopic scales. These, in turn, reflect prominent variations of carrier concentrations thus introducing an unusual type of disorder. It is argued that this compositional disorder may be the reason for the apparent violation of the Ioffe-Regel criterion in the two versions of the amorphous indium-oxide. However, more dramatic effects due to this disorder are expected when superconductivity sets in, which are in fact consistent with the prominent transport anomalies observed in the electron-rich version of indium-oxide. The relevance of compositional disorder (or other agents that are effective in spatially modulating the BCS potential) to other systems near their SIT is discussed. PACS numbers: 72.80.Ng 72.15.Rn 68.55.Ln 68.35.Dv Typeset by REVT E X

Absorption of ac fields in amorphous indium-oxide films

“…B) Many-body localization effects [13,16] begin to manifest themselves in this 3D sample which, unlike in lower dimensionality systems, have a mobility-edge. Note that the Coulomb interaction E C on a scale of the transport-derived-and using for the dielectric-constant 10 [14] is E C 200K, still smaller than the level-spacing in a localization volume. However, the combination of potential fluctuations, higher-order excitations (many-body effects), and extended states lying close to the Fermi energy may enhance the occurrence of metallic puddles.…”

“…To assess how close these samples are to the metal-insulator transition it is illuminating to compare their k F with their localization-length. The 'disorder-parameter' k F is evaluated as in [14]:…”

Coexistence of electron-glass phase and persistent photoconductivity in GeSbTe compounds

“…As pointed out by Givan and Ovadyahu [42], the energy cost of forming charge-gradients in a conducting matter may be mitigated by variation in chemical composition. Inhomogeneous carrier-concentration is especially important when a many-body phenomenon like superconductivity is involved [42]. The sensitivity to carrier-concentration [26,38] makes it also important for the electron-glass phenomenon.…”

The metal-insulator transition in disordered solids: How theoretical prejudices influence its characterization A critical review of analyses of experimental data

“…[7][8][9][10] Many of these solids are or may become application relevant; therefore they are often considered to be among the materials. 3 In various experiments, the MIT has been triggered by diverse control parameters: composition / doping, stress, magnetic field, light, as well as structure; [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] in part, these publications substantially contradict each other.…”

“…(2) was claimed to be more appropriate for describing the experimental data than Eq. (2) with p D 1/2, see Waffenschmidt et al, 23 Givan and Ovadyahu, 33 and Maliepaard et al 70 In practice, the choice of the value of p seems not to influence the conclusion about the qualitative character of lim T!0 s(T, x) in the vicinity of the MIT. 50 Nevertheless, a modification of p as well as a variation of the T range taken into account in the fit usually cause a small shift of the resulting MIT point; an example will be given in Subsection 4.2.…”