Interrelations of atomic structures, electronic structures, electron transport, and magnetic properties across the metal - insulator transition for amorphous alloys

Abstract

The interrelations of the atomic structures, electronic structures, electron transport, and magnetic properties for the amorphous V x Si 100−x alloy system have been studied over a wide composition range, 7 x 74, with particular attention paid to their changes across the metal-insulator transition. By analysing the temperature dependence of the conductivity, we concluded that the metal-insulator transition occurs in the composition range 15 < x < 20. Structural studies revealed that the V atom is substituted for the Si atom in the tetrahedrally bonded Si network in the range where x < 10, whereas the local atomic structure resembles that of the VSi 2 intermetallic compound in the range 20 < x < 40. These two local structures are apparently competing with each other in the critical composition range 10 < x < 20. Both XPS valence band spectra and electronic specific heat measurements proved that the density of states at the Fermi level is definitely finite even in the insulating regime, i.e., for x < 15. Both V Lα and Si Kβ SXS measurements showed that the V 3d states appear just below the Fermi level, and hybridize with the Si 3p states. It is also found that the V atom in the insulating regime possesses a localized magnetic moment, and that the magnetic susceptibility gives rise to a Curie-Weiss-like temperature dependence at low temperatures. Finally, the uniqueness of the electron transport properties for the amorphous V x Si 100−x alloys is emphasized by the ρ-γ diagram, in which the metal-insulator transition is shown to occur while the density of states at the Fermi level remains finite.

Structure, energetics, and bonding of amorphous Au–Si alloys

“…Our value is consistent with 0.31 eV ͑=30 kJ/ mol͒ per Au-Si pair as estimated for the Au 50 Si 50 alloy based on Miedema's model of Takeuchi and Inoue. 14,34 As shown in Fig. 32,33 It is also worth noting that the mixing enthalpy becomes slightly positive when the Si con-tent is small, which might indicate the presence of a barrier for incorporation of Si into pure Au.…”

Hopping conductivity and specific heat in insulating amorphousTixSi100−xalloys

“…7͔͒ and the amorphous alloys. 8,9 If the electron-electron interaction ͑EEI͒ is unimportant, a finite ␥ value may arise from a continuum of localized electronic states at the Fermi level. 10 On the other hand, Lakner et al suggested that a finite ␥ may be ascribed to metallic regions that may exist below n c because of local fluctuations in concentration.…”

Electronic and vibrational density of states through the metal-insulator transition in amorphous yttrium-silicon alloy thin films

“…Nonzero values of ␥ are on the same order of magnitude seen in systems such as Ti-and V-doped a-Si. [14][15][16] We note that the behavior of ␥ in crystalline Si is not simply monotonic with doping but drops sharply at a value slightly below the critical concentration. 5,6 Within our accuracy, the current results are similar to this behavior.…”

“…15,16 a-Mo x Ge 1−x shows an additional ␦T 5 contribution and an excess specific heat on the insulating side and has somewhat smaller ␥. 13 In all these materials, ␥ varies smoothly through the MIT, with no apparent sharp reduction on the insulating side.…”

“…10 These many-body interactions can also lead to time-dependent electronic properties, including the electronic specific heat, though a timedependent specific heat has not been observed experimentally to our knowledge. 11,12 Previously reported specific-heat measurements on the transition-metal-doped amorphous semiconductors, which include a-Mo x Ge 1−x , 13 a-Ti x Si 1−x , 14,15 a-V x Si 1−x , 16 and a-Au x Si 1−x , 17 also indicate finite values of ␥. The metalinsulator transition in these materials is the same as in Si:P in most respects, with the most substantive differences arising from the large electron concentrations ͑10 22 cm −3 ͒.…”