## 4 Discussion and Outlook
In addition to the mean-field models by [PERSON] and [PERSON], a strong interaction and Wigner-Mott transition based theoretical studies, led by [PERSON] and collaborators [32; 33], may also be relevant to the experimental findings. Modern DMFT was applied to study the MIT in 2D carriers with a high \(r_{\rm s}\) and the early approaches of [PERSON] and [PERSON] were reconciled. Based on this 'Wigner-Mott' transition scenario, DMFT calculations are able to explain many detailed behaviors in the electrical transport and charge ordered intermediate phases similar to a charge density wave (CDW) predicted to form before the system enters a WC. It is worth noting that in the DMFT theory, both metallic CDW and insulating CDW are found [33]. It will be very interesting to see whether such CDW states exist in experiments. On the experimental side, besides the previously mentioned compressibility and inductance measurements that require further advancements and understanding, other striking effects found in the zero-field 2D MIT are very worth exploring in relation to the RIPs. One particular case is the effect of an in-plane magnetic field. In the zero-field MIT, it was established that an in-plane magnetic field causes large positive magneto-resistance and drives the system toward the insulating phase [8; 9; 43; 44]. Recent in-plane field magneto-transport experiments have revealed that the resistivity of dilute 2D electrons in Si-MOSFET in the insulating state is the same for the zero-field and in the presence of an in-plane field that polarizes the spins [45], a behavior different from the metallic phase of the 2D MIT [8; 9; 43]. Thus, t would be very insightful to study the effect of in-plane magnetic field induced spin-polarization on the RIPs and examine how the in-plane field affects the transition between the RIPs and a metallic liquid. In addition to the spin-polarization effect, understanding the physics of correlated 2D systems at 'high temperatures' comparable with the Fermi temperature is also an interesting topic. In this'semi-quantum' regime, the correlated electron fluid is expected to show unique behavior in viscosity [27]. While many experiments have been carried out at low temperatures, transport studies in this'semi-quantum' regime where \(T\)-\(T_{\rm F}\) are limited and worth exploring further [46]. In addition to transport, new techniques such as thermopower measurements are strongly desired to shed new light on the RIPs. For instance, diverging thermopower observed for strongly interacting 2D electrons in Si-MOSFET [47] revealed the critical nature of the zero-field MIT, complementing the views from critical behavior in resistance or effective mass [48]. Such critical behavior in thermopower or effective mass has generated theoretical interest and analogy to other strongly correlated materials like high T\({}_{\rm c}\) cuprate may be made [49]. Finally, although this review is focused on strongly interacting 2D systems with an ultra-high purity/mobility and the effect of the impurities or disorder is not discussed much, understanding the effect of disorder on a 2D MIT has elicited rich physics [50] and thus we expect the disorder effect on the RIPs and RIP to liquid transition to be an interesting outstanding issue. In summary, the recent observations of RIPs in low magnetic fields where \(\
u>1\) and the connections between low field RIPs and high field RIPs and HFIFPs with \(\
u<1\) point to the formation of a WC as the origin of the low field RIP and the 2D MIT being driven by the liquid-WC transition. There is also transport evidence for possible new intermediate phases when the insulating phase (either the insulator in a zero-field or the RIPs at low magnetic fields) is destroyed by an increasing voltage, temperature or carrier density. Further experimental and theoretical progress in these fronts is needed and is expected to yield many more exciting new insights on the long-standing problems of the 2D WC-liquid transition and MIT.