The interplay between strong Coulomb interactions and randomness has been a long-standing problem in condensed matter physics. According to the scaling theory of localization in two-dimensional systems of non-interacting or weakly interacting electrons, the ever-present randomness causes the resistance to rise as the temperature is decreased, leading to an insulating ground state. However, new evidence has emerged within the past decade indicating a transition from the insulating to metallic phase in two-dimensional systems of strongly interacting electrons. We review earlier experiments that demonstrate the unexpected presence of a metallic phase in two dimensions, and present an overview of recent experiments with emphasis on the anomalous magnetic properties that have been observed in the vicinity of the transition.

https://iopscience.iop.org/article/10.1088/0034-4885/67/1/R01/pdf

Metal insulator transition in two-dimensional electron systems

The interplay between strong Coulomb interactions and randomness has been a long-standing problem in condensed matter physics. According to the scaling theory of localization, in two-dimensional systems of noninteracting or weakly interacting electrons, the ever-present randomness causes the resistance to rise as the temperature is decreased, leading to an insulating ground state. However, new evidence has emerged within the past decade indicating a transition from insulating to metallic phase in two-dimensional systems of strongly interacting electrons. We review earlier experiments that demonstrate the unexpected presence of a metallic phase in two dimensions, and present an overview of recent experiments with emphasis on the anomalous magnetic properties that have been observed in the vicinity of the transition.

https://arxiv.org/pdf/cond-mat/0309140.pdf

Metal-insulator transition in two-dimensional electron systems

Singular or Non-Fermi Liquids

An introductory survey of the theoretical ideas and calculations and the experimental results which depart from Landau Fermi-liquids is presented. Common themes and possible routes to the singularities leading to the breakdown of Landau Fermi liquids are categorized following an elementary discussion of the theory. Soluble examples of Singular Fermi liquids (often called Non-Fermi liquids) include models of impurities in metals with special symmetries and one-dimensional interacting fermions. A review of these is followed by a discussion of Singular Fermi liquids in a wide variety of experimental situations and theoretical models. These include the effects of low-energy collective fluctuations, gauge fields due either to symmetries in the hamiltonian or possible dynamically generated symmetries, fluctuations around quantum critical points, the normal state of high temperature superconductors and the two-dimensional metallic state. For the last three systems, the principal experimental results are summarized and the outstanding theoretical issues highlighted.

Singular Fermi Liquids

Experiments on a constant-density two-dimensional hole system in a gallium arsenide quantum well revealed that the metallic behavior observed in the zero-magnetic-field temperature dependence of the resistivity depends on the symmetry of the confinement potential and the resulting spin splitting of the valence band.

https://science.sciencemag.org/content/sci/283/5410/2056.full.pdf

The effect of spin splitting on the metallic behavior of a two-dimensional system

Magnetotransport studies of the unusual two-dimensional metallic phase in high-mobility Si-MOS structures are reported. It is found that a magnetic field applied in the 2D plane suppresses the metallic state, causing the resistivity to increase dramatically (by more than 30 times). The total existence range of the metallic state is found to contain three distinct types of magnetoresistance, related to the corresponding quantum corrections to the conductivity. The data suggest that the unusual metallic state is a consequence of both spin and Coulomb interaction effects.

Instability of the two-dimensional metallic phase to a parallel magnetic field

The two-dimensional metallic state is studied in a number of Si-MOS structures with peak mobilities varying by a factor of 8.5. The data show a density dependence and disorder dependence of the major features of the scaling function and thus reveal the absence of universal one-parameter scaling over wide density range in the metallic regime.

Lack of universal one-parameter scaling in the two- dimensional metallic regime

We report on magnetotransport studies of the 2D metallic phase in high-mobility Si-MOSFETs. We have observed that the magnetic field applied in the 2D plane suppresses the metallic state, causing the resistivity to increase dramatically by more than 30 times. Over the existence range of the metallic state, we have found three distinct types of magnetoresistance behaviour, related to the corresponding quantum corrections to the conductivity. Our data suggest that the unusual metallic state is a consequence of both spin- and Coulomb-interaction effects.

Breakdown of the anomalous two-dimensional metallic phase in a parallel magnetic field

Symmetrically strained Si/SiGe superlattices (25 A wells/14 A barriers) with homogeneous Sb doping concentrations of 4.5 and show low-temperature magnetoconductivity effects which can be explained by single-electron backscattering and disorder-induced electron - electron interaction for an anisotropic three-dimensional case. The insulating phase of two lower doped samples (2.0 and ) is documented by the observation of variable range hopping conductivity. In addition, the extrapolated zero-temperature conductivities for in different magnetic fields show a magnetic-field-induced metal-to-insulator transition, demonstrating a three-dimensional behaviour of this superlattice with respect to localization.

Metal - insulator transition in Sb-doped short-period Si/SiGe superlattices

The negative magnetoresistance due to weak localization is investigated in the two-dimensional metallic state of Si-MOS structures for high conductance values between 35 and 120 e2/h. The extracted phase coherence time is equal to the momentum relaxation time at 10 K but nearly 100 times longer at the lowest temperature. Nevertheless, only weak logarithmic corrections to the conductivity are present in the investigated temperature and concentration range thus proving the absence of strong quantum effects due to electron-electron interaction. From saturation effects of the phase coherence time a lower boundary for spin-orbit scattering of about 200 ps is estimated.

G. Brunthaler

Papers

Instability of the two-dimensional metallic phase to a parallel magnetic field

Lack of universal one-parameter scaling in the two- dimensional metallic regime

Breakdown of the anomalous two-dimensional metallic phase in a parallel magnetic field

Metal - insulator transition in Sb-doped short-period Si/SiGe superlattices

Weak localization in the 2D metallic regime of Si‐MOS