Melting of two-dimensional Wigner Solids
Department of Physics and Astronomy, Wayne State University, Detroit, MI 48201, USA
A Wigner Crystal (WC) is an exciting platform for learning fundamental concepts of long-range orders that is key to magnetism, superconductivity, superfluidity, and topological matters. This solid phase of electrons is expected to emerge only when the inter-particle Coulomb energy dominates the zero-point energy. Even though such a condition can be intuitively assumed in ultra-dilute two-dimensional (2D) systems where the ratio of the Coulomb energy Eee and the Fermi energy EF, rs=Eee/EF, reaches values close to the anticipated onset point, the experimental realization of it has proven to be an outstanding challenge. The tininess of Eee and EF makes a WC fragile even to moderate random disorders that usually overwhelm the interaction effect by driving an Anderson localization or a glass. Moreover, random disorder and quantum fluctuations are important factors that reduces the melting point even below what can be reached with modern low temperature (T) experimental capabilities. Consequently, a WC has not been rigorously demonstrated. Important questions in relations to Mermin-Wagner theorem and the Kosterlitz-Thouless (KT) transition , with possible intermediate phases such as hexatic  or microemulsions  including bubble and stripe phases, remain to be explored.
We utilize ultra-high purity p-GaAs 2D systems in which the carrier density can be continuously tuned from 5x1010 cm-2 down to 7x108 cm-2. Effective sample cooling down to 10 mK is achieved via a state-of-the-art helium-3 immersion cell technique. Delicate dc and dc+ac transport study reveals benchmark signatures in terms of both many-body pinning and melting transitions. Rigorously pinned WCs are characterized by enormous (GΩ) pinning strength, even stronger than those found in charge density waves (CDWs), consistent with a macroscopic correlation length. Melting occurs under both pressure and temperature variations both of which confirm two-stage characteristics: 1st stage-discontinuous WC-intermediate phase transition; 2nd stage-smooth intermediate phase-liquid transition. Remarkably, the 1st stage transition, in contrast to the KT model, exhibits first-order characteristics according to the steep discontinuous drop in the pinning strength and it occurs at T below a melting temperature (Tm) of 30 mK. The 2nd stage is a smooth crossover occurring around 120mK, consistent with numerous previous results. Tm, being only ¼ of the classical estimate, is likely a confirmation of the disorder effect influencing melting.
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Jian Huang grew up in Beijing, China. After graduating from Peking University with a Bachelor degree in physics in 1992, he worked for AT&T (Bell Labs) for one and a half years. Then in 1994 he started his graduate training under Professor J. Knight in theoretical physics (superconductivity and quantum theory) at University of South Carolina. Obtaining a MS degree, he transferred to Michigan State University and focused on experimental study of the quantum phase coherence effect in mesoscopic systems. Upon receiving his Ph.D. in 2001, he worked as a postdoc at the Electromagnetic Division at NIST where he concentrated on developing Josephson junction-based voltage standard. Starting 2003, he joined Dan Tsui's group at Princeton University as a postdoc, working on understanding electron-electron interaction in many-body system. Two years later, he was promoted to Research Staff. In 2007, he joined Taylor University as an assistant professor. In fall 2010, he joined the faculty at the Physics Department at Wayne State University. His research concerns charge and spin phenomena in strongly correlated low dimensional systems. The work during the past 10 years has been primarily supported through NSF, DOE, and a NASA Early Career Award.