Coulomb Effects and Exotic Charge Transport in Nanostructured Materials
The effects of strong Coulomb interactions between charge carriers as they delocalize can lead to exotic phases of matter and remarkable behaviours (Basov et al. 2011, Dagotto 2005, Georges et al. 1996, Imada et al. 1998, Wirth and Steglich 2016). For example, in transition metal oxides (such as cuprates which can exhibit high Tc superconductivity), the spatially confined nature of d or f orbitals results in strong electron-electron Coulomb repulsion if multiple electrons occupy the same orbital. These materials also afford means, e.g. via doping or application of high pressure, for tuning the degree to which electron delocalizes. When electron-electron interaction dominates, the materials are insulating and when delocalization, i.e. kinetic energy, dominates, the materials are metallic. In the regime where both interactions and delocalization effects are significant, the materials exhibit an exotic “correlated” phase in which charges move in such a manner so as to accommodate their strong mutual repulsion. It is in this remarkable regime that high Tc superconductivity is observed (Lee et al. 2006). Although the exact mechanism underlying this phenomenon remains an open question, it is widely believed that strong, Coulomb-driven electron-electron interactions combined with electron delo-calization play critical roles. Another well-known example of a remarkable phenomenon arising from strong electron-electron interactions is the Kondo effect. This effect arises when an unpaired localized electron interacts with delo-calized electrons (Kondo 1964, Gruner and Zawadowski 1974). A delocalized electron cannot pair with the localized electron due to strong Coulomb repulsion, but through virtual processes, the electrons can interact strongly, form a singlet and significantly modify the resistance of the system. An example of a material exhibiting this effect is gold doped with iron atoms (Gruner and Zawadowski 1974, Costi et al. 2009).