Magnetic-Field Dependence of the Anomalous Noise Behavior in a Two-Dimensional Electron System in Silicon

Abstract

Studies of low-frequency resistance noise show that the dramatic change in the dynamics of the twodimensional electron system (2DES) in Si that occurs near the metal-insulator transition (MIT) persists in high parallel magnetic fields B such that the 2DES is fully spin polarized. This strongly suggests that charge, as opposed to spin, degrees of freedom are responsible for this effect. In the metallic phase, however, noise is suppressed by a parallel B, pointing to the role of spins. At low B, the temperature dependence of conductivity in the metallic phase provides evidence for a MIT. DOI: 10.1103/PhysRevLett.92.226403 PACS numbers: 71.30.+h, 64.70.Pf, 71.27.+a, 73.40.Qv The fascinating strong correlation physics exhibited by low-density two-dimensional (2D) electron and hole systems [1] has been the subject of intensive research. In particular, the precise role of the spin degrees of freedom remains the central unresolved issue in this field [2]. Recent resistance (R) noise measurements on a 2D electron system in Si [3,4] have revealed a striking change in the dynamics of the two-dimensional electron system (2DES) in the vicinity of the apparent metal-insulator transition (MIT), posing a question of whether charge or spin degrees of freedom are responsible for this effect. Since it is known [5] that a relatively weak magnetic field B is required to fully spin polarize the 2DES in the relevant range of densities n s , experimental studies in parallel B [6] should be able to distinguish between the two possibilities. Here we present such a study, which shows that the dramatic change in the electron dynamics persists even when the 2DES is spin polarized, strongly pointing to charge, as opposed to spin, degrees of freedom as the origin of this effect. At high n s , on the other hand, noise measurements in B suggest that electrons' spins may play a relevant role.The B 0 studies employed a combination of transport and low-frequency resistance noise measurements [3,4] to probe the system dynamics. By reducing n s , it was found that, at some well-defined density n g , (i) the dynamics suddenly and dramatically slowed down, and (ii) there was an abrupt change to the sort of correlated statistics characteristic of complicated multistate systems. These features were attributed to the glassy freezing of the 2DES at n g , with the data being consistent with the hierarchical picture of glassy dynamics. Hence, for brevity, this change in the noise behavior will be called the ''glass transition.'' The ''glass transition'' occurs [3] in the metallic phase, i.e., at n g > n c , where n c is the critical density for the MIT determined from the vanishing of activation energy in the insulating regime [7,8]. The intermediate metallic phase with slow electron dynamics (MSED, or ''metallic glass'') is considerably wide in strongly disordered samples [n g ÿ n c =n c 0:5 [3]], whereas in devices with low disorder n g is at most a few percent higher than n c [4]. In this work, we investigate these same high peak mobility (), i.e., ...

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