Nonequilibrium Quantum Breakdown in a Strongly Correlated Electron System

During the past decades, there has been an increasing fascination and surprises with diverse quantum many-body effects. With the magical touch of interaction a simple electron system may assume insulating, metallic, magnetic or superconducting states according as the control parameters are changed. Strongly correlated electron systems, as exemplified by the high-Tc superconductors and their host materials realized in transition-metal oxides, as well as by organic metals, have provided us with an ideal playground, where various crystal structures with band-filling control and band-width control etc provide the richness in the phase diagram[39]. On the other hand, there is a long history of the interests in non-equilibrium phase transitions. Statistical mechanically, there is an intriguing problem of how we can generally define the notion of a “phase” in non-equilibrium systems, but we can still discuss individual systems in specified non-equilibrium conditions to extract more general viewpoints. Now, if we combine the above two ingredients, namely, if we consider strongly-correlated electron systems in non-equilibrium, we plunge into an even more fascinating physics. In fact, recent years have witnessed an upsurge of interests in non-equilibrium states in many-body systems with drastic changes in the electronic states in strong dc electric fields, in intense laser fields, etc. Developments in fabrication techniques such as realization of clean thin films with electrodes attached have triggered several groundbreaking experiments, e.g., non-linear transport measurements in thin films [1, 60, 19, 50, 2, 4], in layered systems [24] and observations of clean metallic states in heterostructures [3]. Non-linear phenomena in correlated electron systems now begin to attract interests in a wide range of researchers: One obvious area of application is future-generation electronic devices, where a high sensitivity of a