Interaction-driven metal-insulator transitions in disordered fermion systems

We study the effects of electron-electron interactions in disordered metals in and close to two dimensions (2D). We consider physical situations in which localization effects are suppressed. The field-theoretical renormalization-group (RG) calculation performed recently by Finkelstein is interpreted and rederived in terms of perturbative results. Surprisingly, except for the density of states, the scaling behavior is independent of the interaction range. We further extend the model to several new universality classes. In the presence of a strong magnetic field the metal is unstable in 2D and undergoes a metal-insulator transition in $d=2+\epsilon$. The conductivity exponent, defined by $\sigma \sim {(n-n_c)}^{\mu }$, is universal with $\mu =1+O\left(\epsilon \right)$ but $N\left(E_F\right)$ depends not only on the range of the interaction but also on its strength for short-ranged interactions. In 2D the conductivity has a universal temperature dependence [$\delta \sigma \mathrm{}\left(T\right)={\sigma }_N(2-2\mathrm{}\ln 2)\ln \left(T\tau \right)$, ${\sigma }_N=\frac{e^2}{2{\pi }^2\hslash }$] if the interaction is Coulombic. If magnetic impurities (or strong spin-orbit scattering with a weak magnetic field) are present instead, the noninteracting fixed point is stable for short-ranged interactions ($\mu =\frac{1}{2}$). For the Coulomb interaction the interaction is relevant and drives a metal-insulator transition in $d=2+\epsilon$ with universal critical properties ($\mu =1\mathrm{}$). In 2D the conductivity also has a universal temperature dependence [$\delta \sigma \mathrm{}\left(T\right)={\sigma }_N\ln \left(T\tau \right)$]. We also discuss the behavior of the dielectric constant on the insulator side and the frequency (temperature) dependence of the conductivity at criticality. Remarks are made on the relationship of the above to experiments.