Many electrons moving but strongly avoiding each other

T V Ramakrishnan (Indian Institute of Science, Bangalore and Banaras Hindu University, Varanasi, India)
Condensed matter consists of atoms nearly touching each other, so that the outer electrons of each atom are strongly influenced by other atoms. A daring but surprisingly successful, more than a century old hypothesis of Drude, that the outer electrons can be regarded as free, ie moving independently of each other, is the basis of our present understanding of metals, semiconductors and insulators. In the last several decades, we have come across many many families of systems in which electrons do move, but need to very strongly avoid each other when close by (eg because of inevitable coulomb repulsion). They are home to many strange phenomena and seem qualitatively diffferent from conventional systems. In this talk, I will touch on the happenings in some of these strongly correlated electronic materials and our attempts to make physical sense of them. This is a large part of the immediate past of quantum condensed matter physics; perhaps its future will be greatly influenced by a better understanding of this large class of things.