Quantum phase transition in ultrahigh mobility SiGe/Si/SiGe two-dimensional electron system

The metal-insulator transition (MIT) is an exceptional test bed for studying strong electron correlations in two dimensions in the presence of disorder. In the present study, it is found that in contrast to previous experiments on lower-mobility samples, in ultra-high mobility SiGe/Si/SiGe quantum wells the critical electron density, $n_{\text{c}}$, of the MIT becomes smaller than the density, $n_{\text{m}}$, where the effective mass at the Fermi level tends to diverge. Near the topological phase transition expected at $n_{\text{m}}$, the metallic temperature dependence of the resistance should be strengthened, which is consistent with the experimental observation of more than an order of magnitude resistance drop with decreasing temperature below $\sim1$ K.Comment: Misprints corrected. As publishe

Metallic behavior and related phenomena in two dimensions

For about twenty years, it has been the prevailing view that there can be no metallic state or metal-insulator transition in two dimensions in zero magnetic field. In the last several years, however, unusual behavior suggestive of such a transition has been reported in a variety of dilute two-dimensional electron and hole systems. The physics behind these observations is presently not understood. We review and discuss the main experimental findings and suggested theoretical models.Comment: To be published in Rev. Mod. Phy

Complex sentence as a structure for representing knowledge

Structural variations involving both morphological and syntactic features of the complex sentence of the type “When S, S” and their relevance for the interpretation of sentence meaning are analyzed. It is hypothesized that the constraints on certain sequences intuitively felt by native speakers are due to semantic contradictions that arise between the indexical content of the verbal tense and aspect and the syntactic structure of the sentence which iconically reflects the cognitive processing of perceptual data. The cognitive value of different syntactically acceptable sequences is assessed from the point of view of the relationship between the morphosyntactic categories of tense and as-pect and sentence iconicity