## 4 Discussion and Outlook  
Although various recent transport experiments and findings point to the existence of a 2D Wigner crystal and the relevance of a liquid-WC transition in the observed RIPs and peculiar voltage-current characteristics in strongly correlated 2D carrier systems, due to the limitations of transport data and the various challenging aspects of the experiments (ultra-low temperatures, low noise-low level measurements, and stringent sample quality requirements), information about how the 2D Wigner  
Figure 8: Voltage-current characteristics of 2D holes in GaAs in a possible WC state to show the two-stage melting of the WC. (**a**) dc _IV_ at 28 mK. (**b**) _IVs_ at different temperatures. (**c**) Temperature dependence of \(r_{d(T)}|_{V\to 0}\). (**d**) Suggested phase diagram. Figure from Reference [36]. Figure 7: Voltage-current characteristics of 2D electrons in a Si-metal-oxide-semiconductor field-effect transistor (Si-MOSFET) in a possible Wigner crystal (WC) state. V-I curves of different electron densities are shown to show the depinning of the WC. Figure taken from Reference [35]. crystal transforms into a metallic liquid and the nature of the intermediate phase are still limited and further experiments are required to obtain an in-depth understanding. On the theoretical side, from the beginning of the discovery of 2D MIT, a number of weak-interaction based models were proposed to explain the metal-insulator transition or cross-over behavior of the electrical resistivity such as screening, potential fluctuation, and percolation [37; 38; 39]. In these theories, conventional Anderson localization (weak-localization in the metal-side and strong localization in the insulator side of the MIT) is still relevant. From extensive prior studies on the Anderson insulator to 2D QH transition, it is expected that the zero-field insulator would directly transition to an integer QH state upon the application of perpendicular magnetic fields [40]. This is in stark contrast to the observation of low field RIPs between the zero-field insulator and integer QH state [10; 11; 12]. It appears that it is difficult to use weak-interaction based models to reconcile the observations of various RIP, QH and HFIP states and their connection to the liquid-WC transition when the behavior of strongly interacting high \(r_{\rm s}\) 2D systems is examined beyond the zero magnetic field to finite perpendicular magnetic fields. In contrast to weak interaction theories, many other theories emphasize the importance of strong correlations and relevance of WC physics in the systems showing a 2D MIT [9; 26; 27; 33] and therefore, may be further compared with the experimental results. These theories are based on a number of different approaches: Analytical mean-field models [26; 27], quantum Monte Carlo simulations [30], or dynamic mean field theory (DMFT) [32; 33]. In the mean-field theories by [PERSON] and [PERSON] [26; 27], the various spectacular transport behavior in the correlated 2D systems showing an MIT are attributed to the Fermi liquid to WC transition where intermediate states are unavoidable [31]. In the intermediate'micro-emulsion' states (e.g., WC bubbles in a Fermi liquid background), it is the interplay or transformation between a Fermi liquid and the WC components tuned by the temperature or a magnetic field that dictates the transport behavior and gives rise to the resistivity change of the system. It seems that the most relevant micro-emulsion phase to the experimentally observed RIPs and intermediate phases is the scenario where WC bubbles co-exist in a Fermi liquid. Whether other micro-emulsion states (Fermi liquid bubbles in WC, 1D ordered stripes) exist in experiments requires further research and more theoretical developments are desired to establish more quantitative predictions on the experimental systems. New theoretical approaches, based on hydrodynamics, seem quite promising and are currently being developed [41; 42].