Metal-insulator transition (MIT) is commonly used to study strong electron–electron correlations in two-dimensional (2D) electron systems. The existence of MIT and metallic states in strongly interacting 2D electronic systems at temperatures close to absolute zero was predicted in the beginning of 1980s, contrarily to the popular belief that only insulating states can be realized in non-interacting 2D systems. With extensive research, the phenomenon has been experimentally discovered in silicon MOSFETs and subsequently observed in many strongly interacting 2D systems like p-type GaAs/AlGaAs, AlAs and ZnO-related heterostructures and SiGe/Si/SiGe quantum wells. Today, the strong correlations between carriers have been widely accepted as the core driving force behind the existence of MIT.
MIT and related phenomena have been extensively studied in the literature. Herein, Professor Alexander Shashkin from the Institute of Solid State Physics together with Professor Sergey Kravchenko from Northeastern University provided expert opinion of the research progress on MIT and low-density phases in strongly-interacting and low-disordered silicon-based 2D electron systems. Two groups of samples were utilized. The first group comprised ultra-clean CVD-grown (001)-SiGe/Si/SiGe quantum wells with maximum electron mobility of up to 240m2/Vs, which is the highest mobility reported in this system. The second group consisted of clean (100)-silicon MOSFETs with electron mobility of about 3m2/Vs.
The MIT in ultra-clean SiGe/Si/SiGe quantum wells is qualitatively different from the one in Si MOSFETs in that it occurs at the critical electron density nc below the electron density nm at which the effective electron mass at the Fermi level tends to diverge, i.e., it occurs in an unconventional Fermi liquid state, where the Fermi surface breaks into several separate surfaces. This effect leads to strengthening the metallic temperature dependence of the resistance. In contrast, the MIT in clean Si MOSFETs occurs in a conventional Fermi liquid state at nc slightly above nm.
The authors also showed that the metallic state in clean two-valley electron system in SiGe/Si/SiGe quantum wells survives even when the electron spins are completely polarized.
The non-monotonic temperature dependences of the resistance on the metallic side near the MIT in SiGe/Si/SiGe quantum wells were quantitatively described by the dynamical mean-field theory (DMFT). Interestingly, similar non-monotonic temperature dependences of the resistance have been observed in quasi-2D organic charge-transfer salts and 2D transition metal dichalcogenides, indicating potential application of DMFT in studying a wide range of strongly correlated systems.
Special consideration was given to the characteristics of the metallic state in ultra-clean SiGe/Si/SiGe quantum wells and existing evidence supporting the possible existence of flat bands at the Fermi level in the aforementioned quantum wells. An increase in the electron-electron interaction strength led to the flattening of the single-particle spectrum at the Fermi level, which was interpreted within the concept of the fermion condensation.
The transport evidence for the quantum electron solid formation in silicon MOSFETs and the underlying mechanism behind this phenomenon were discussed. The physics of pinned elastic/periodic objects was shown to be relevant for the low-density state in 2D electron system based on Si MOSFETs. In a statement to Advances in Engineering, the authors noted that even though the proposed model can effectively describe the experimental results, more comprehensive studies are still necessary.
Their work is currently published in the research journal, Annals of Physics. In summary, a review of recent experimental findings on MIT and low-density phases in strongly interacting 2D electron systems was presented.
Shashkin, A., & Kravchenko, S. (2021). Metal–insulator transition and low-density phases in a strongly-interacting two-dimensional electron system. Annals of Physics, 435, 168542.
Melnikov, M. Yu. et al. (2015). Ultra-high mobility two-dimensional electron gas in a SiGe/Si/SiGe quantum well. Applied Physics Letters, 106, 092102.
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