Heterogeneous Rare-Gas Quantum Solids - A New Class of Condensed Matter

About This Grant

Nontechnical Abstract The goal of this project is to create a new class of matter by freezing a heterogeneous mixture of rare gases: helium (He), neon (Ne), and argon (Ar), into multilayer solids at low temperatures. The new kind of matter, called heterogeneous rare-gas quantum solids, is analogous to heterogeneous semiconductors but can exhibit more unusual electronic properties on the surface, at the interface, or in the bulk. These quantum solids, defined by the fact that the atomic positions are strongly vibrating even at the lowest temperatures, are exceptionally inert and pristine. Their surfaces and interfaces therefore provide an ideal platform for studying the quantum properties of electrons confined to moving in two dimensions, including tendencies to form exotic phases of matter. In addition, these quantum solids can serve as ultraclean host materials for quantum bits – the fundamental building blocks of quantum computers. The project will provide comprehensive training to undergraduate and graduate students across interdisciplinary fields and prepare the next generation of quantum scientists and engineers. Technical Abstract We will create and characterize a new class of condensed matter – heterogeneous rare-gas quantum solids, which consist of multilayer thin films of solid neon (Ne), solid argon (Ar), and (optionally) a mono-atomic layer of solid helium (He) at near-zero temperature and pressure. We target revealing unique electronic structures on the surface, at the interface, or in the bulk of these systems. Experimentally, we will grow lattice-matched heterogeneous films on single-crystalline silicon (Si) substrate to host a high-mobility two-dimensional electron gas (2DEG). We will image the films’ surface morphology and perform transport measurements on the 2DEG upon tuning electron density, atomic disorder, magnetic field, and film thickness. Theoretically, we will begin with density-functional-theory (DFT) calculations to find the optimal lattice structures of solid Ar and Ne on Si and the single-electronic wavefunctions. Quantum Monte Carlo (QMC) calculations will then be used to study the zero-point motion of the Ne atoms and the metal-insulator transitions of the 2DEG. In this novel heterogeneous system, a widely tunable, high-mobility 2DEG will access regimes beyond usual 2D semiconductor systems and can provide a complete understanding of many important topics in condensed matter physics. This includes the crossover from Wigner-crystallization to Anderson-localization, and the interplay between Wigner crystallization and quantum Hall effect. The proposed research will also enrich our understanding of the decoherence mechanics of single electrons on a quantum solid surface, a critically important topic for quantum information science. The rare-gas quantum liquids and solids may also find applications beyond condensed matter physics, from isolating chemical reactions to dark matter sensing. Our work will introduce a novel platform for these purposes. Finally, the variety of subjects, from cryogenic engineering to information science, involved in our proposed research provides comprehensive training for graduate and undergraduate students. By mentorship and by sharing our results in public outreach, we will educate the future generation of scientists and engineers. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.