We report on a collective low temperature metal-insulator transition which develops in a dilute 2D electron system in Si at zero magnetic field, below a critical carrier density ris 10'^ cm-^. In the insulator phase, the dc conduction is thermally activated and exhibits a sharp threshold as a function of electric field. The collective insulator state at zero field shows many of the features attributed to the pinned Wigner solid. We have also observed a trend from a collective to a single-particle insulator state with decreasing electron density and/or increasing disorder. PACS numbers: 73.40.Qv, 71.55.Jv, 71.30.+h In the limit of zero temperature, the dilute twodimensional electron (2DE) system is expected to become an insulator in the presence of disorder [1]. Various theoretical models for this insulating state have been proposed ranging from Anderson's single-particle localization (SPL) [1] to collective phases like the pinned Wigner solid (WS) [2], and the pinned charge density wave (CDW) [3]. Pioneering work [4] on low mobility Si samples revealed features indicative of a collective glassy state in high magnetic fields. The search for an electron ordered state in a quantizing magnetic field has recently led to the experimental observation of metal-insulator (MI) transitions in high mobility samples: GaAs-AlGaAs [5] heterostructures and Si-MOSFET's [6][7][8]. In these samples, reentrant MI transitions in a magnetic field were observed around fractional (mainly 1/5 in GaAs) and integer (1 and 2 in Si) filling factors. However, the realization of the "true" Wigner solid at zero magnetic field in GaAs would require an unrealistically dilute electron system since the critical density for quantum melting at T -0 [9] (also known as "cold melting") is very low, i.e., Ticm ~ 3 X 10^ cm~^. In contrast, the cold melting density in (100) Si should be at least 20 x larger, due to the larger effective mass (m* = 0.197ne) and lower dielectric constant {n = 7.7) which increases the ratio of the electron-electron interaction energy [{nnsY^'^e^/K\ to the kinetic energy {nrish'^/2m*) [8]. Therefore, a zerofield collective electron solid (ES) is more likely to be observed in high mobility Si inversion layers. There has not been, to our knowledge, any observation of the collective ES (and, in particular, the pinned Wigner solid) and its quantum melting at zero magnetic field in 2D. In our earlier work [6,8], there is an indication, however, that the insulating phase does not disappear as iif -> 0 but rather shifts to lower density and has features similar to those of the reentrant MI transitions in a magnetic field.In this Letter, we present experimental evidence for the collective character of the low temperature metalinsulator transition at zero field in Si MOSFET's with weak disorder. The data show a trend from a collective to a single-particle insulator state with decreasing density and/or increasing disorder. We compare our data on nonlinear dc transport, activation energy, and threshold electric field with the available...
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