****Technical Abstract**** The study of the many-body electron systems encompasses two of the most fundamental subjects: electron-electron interaction and electron-disorder interaction. It is well known that sufficient disorder causes electron states to undergo Anderson Transition. On the other hand, whether strong electron-electron interaction also brings radical changes, such as the predicted Wigner Crystallization (electron solid), is still unknown. For a long time, experimental effort was hindered because most devices contain a high level of unwanted disorder which overwhelms the interaction effect at low electron densities. Recent breakthroughs have been made in providing ultra-high quality two-dimensional electron systems in GaAs semiconductor field-effect-transistors. This project will utilize this type of devices with record low electron densities to perform transport experiments at low temperatures. The goal is to verify whether strong electron-electron interaction drives a first-order phase transition (Wigner crystallization) or some intermediate phases. Either observation will provide insights in understanding the quantum mechanical nature of strongly interacting electrons in the form of a solid or a strongly correlated liquid. This project will support the education of one Ph.D. student in pursuing discovery and in learning most advanced technologies, which has historically shown to be excellent training for scientific careers.
Electrons are quantum mechanical objects that exist in all physical systems and most systems contain a large number of them. Understanding the states of electrons, how they interact with the environment and each other, is a vital scientific subject and has played a critical role in advancing modern science and technologies. Analogous to water, electrons manifest both gaseous states at high temperatures and liquid states at low temperatures. Another form of the states is solid which was predicted but never observed. To obtain evidence of this solid state of electrons is not only important to understand how the most basic force among the electrons can radically affect the quantum states, but also allow scientists to utilize remarkable properties in developing quantum electronics and spintronics. As nanotechnology marks the opening of the 21st century, semiconductor technologies have greatly improved. A novel type of semiconductors of ultra-high purity has become available as a result of a recent breakthrough. This project will utilize such devices to perform experiments with the most advanced scientific tools: nanofabrication and ultra-low temperature physics. The goal is to capture direct evidences that either indicate an electron solid, or some other complex states. This project will support the education of one Ph.D. student in pursuing discovery and in learning most advanced technologies, and allow the group to conduct outreach activities with local high schools.
Electrons are tiny quantum mechanical objects that exist in all physical systems and most systems contain a large number of them. Understanding how electrons interact with each other and with the environment is a vital scientific subject and has played a critical role in advancing modern science and technologies. However, to experimentally demonstrate effects driven by strong interaction has been an outstanding challenge because it requires performing experiments in systems that have very dilute charge concentrations where the unwanted disorder effect usually overpowers interaction effects. The goal for this project is to first realize ultrapure systems in which interaction effects prevail even when the charge concentration is dilute, then capture the interaction driven characteristics including the intriguing electron solid phase which is the most prominent interaction-driven phenomenon. We have succssfully established the micro- and nano-fabrication processes that allow us to routinely produce ultra-dilute electron systems in high purity field-effect-transistors in GaAs semiconductors. The average spacing between neighboring charges is up to 500 nm. We have measured the temperature dependence of the resistivity of many devices to verify whether such systems are disorder-driven or interaction-driven. A consistent nonactived behavior is observed in contrast to the activated behaviors expected for disorder-driven systems. The nonactived transport behavior is characterized by a power-law and we have found that the exponent of the power-laws scales with the interaction strength. This further confirms the interaction-driven nature of the system. Consequently, we utilized a sensitive dc technique to gently probe the dynamical response of the system. This established technique has been used for discovering the famous charge density waves, a sliding sheet of ordered the charges which results in a threshold behavior. Remarkably, we have found the same threshold behavior marked by a 20-fold resistivity change around the threshold (see attached figure). Surprisingly, the threshold limit is one million times smaller. Moreover, since an electron solid has to be pinned by residual disorder, our results show an enormous pinning strength which is expected for a genuine electron solid. Therefore, we have produced the evidence for a Wigner crystal. In addition, the low threshold limit suggests a quantum depinning mechanism that is unknown previously. This project has supported the education of one Ph.D. student and one postdoc in pursuing discovery and advanced technologies. It also opens up wonderful opportunities to interact with other research groups in and out of physics, and allow the group to conduct outreach activities through which the whole team has had the chance of sharing with college and high school students.