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Register a new userEffects of a Parallel Magnetic Field on the Metal-Insulator Transition in a Dilute Two-Dimensional Electron System
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The temperature dependence of conductivity $sigma (T)$ of a two-dimensional electron system in silicon has been studied in parallel magnetic fields B. At B=0, the system displays a metal-insulator transition at a critical electron density $n_c(0)$, and $dsigma/dT >0$ in the metallic phase. At low fields ($Blesssim 2$ T), $n_c$ increases as $n_c(B) - n_c(0) propto B^{beta}$ ($betasim 1$), and the zero-temperature conductivity scales as $sigma (n_s,B,T=0)/sigma (n_s,0,0)=f(B^{beta}/delta_n)$ (where $delta_n=(n_s-n_c(0))/n_c(0)$, and $n_s$ is electron density) as expected for a quantum phase transition. The metallic phase persists in fields of up to 18 T, consistent with the saturation of $n_c$ at high fields.
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Magnetoconductance (MC) in a parallel magnetic field B has been measured in a two-dimensional electron system in Si, in the regime where the conductivity decreases as sigma (n_s,T,B=0)=sigma (n_s,T=0) + A(n_s)T^2 (n_s -- carrier density) to a non-zero value as temperature T->0. Very near the B=0 metal-insulator transition, there is a large initial drop in sigma with increasing B, followed by a much weaker sigma (B). At higher n_s, the initial drop of MC is less pronounced.
Studies of low-frequency resistance noise show that the glassy freezing of the two-dimensional electron system (2DES) in Si in the vicinity of the metal-insulator transition (MIT) persists in parallel magnetic fields B of up to 9 T. At low B, both the glass transition density $n_g$ and $n_c$, the critical density for the MIT, increase with B such that the width of the metallic glass phase ($n_c<n_s<n_g$) increases with B. At higher B, where the 2DES is spin polarized, $n_c$ and $n_g$ no longer depend on B. Our results demonstrate that charge, as opposed to spin, degrees of freedom are responsible for glassy ordering of the 2DES near the MIT.
The time-dependent fluctuations of conductivity sigma have been studied in a two-dimensional electron system in low-mobility, small-size Si inversion layers. The noise power spectrum is ~1/f^{alpha} with alpha exhibiting a sharp jump at a certain electron density n_s=n_g. An enormous increase in the relative variance of sigma is observed as n_s is reduced below n_g, reflecting a dramatic slowing down of the electron dynamics. This is attributed to the freezing of the electron glass. The data strongly suggest that glassy dynamics persists in the metallic phase.
We report the observation of a metal-insulator transition in a two-dimensional electron gas in silicon. By applying substrate bias, we have varied the mobility of our samples, and observed the creation of the metallic phase when the mobility was high enough ($mu ~> 1 m^2/Vs$), consistent with the assertion that this transition is driven by electron-electron interactions. In a perpendicular magnetic field, the magnetoconductance is positive in the vicinity of the transition, but negative elsewhere. Our experiment suggests that such behavior results from a decrease of the spin-dependent part of the interaction in the vicinity of the transition.
Experimental results on the metal-insulator transition and related phenomena in strongly interacting two-dimensional electron systems are discussed. Special attention is given to recent results for the strongly enhanced spin susceptibility, effective mass, and thermopower in low-disordered silicon MOSFETs.
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