Low-dimensional electronic materials with layered or chain-like crystal structures are at the core of some of the most exciting discoveries in modern materials research. Nobel prize winning discoveries of novel quantum-mechanical effects in artificially structured semiconducting materials (1985, 1998), giant magneto-resistance (the change in electrical resistance due to the presence of a magnetic field) in stacked metal layers (2007), and the existence of electrical conductivity with no energy loss ("high-temperature superconductivity") in low-dimensional compounds made from copper and oxygen (1987) are just a few examples of transformational discoveries that define the frontiers of condensed matter science. In particular, the fascinating properties of technologically important oxide materials are rooted in the correlated motion of electrons, arising from the delicate interplay between the electron's charge, its magnetic property known as "spin" or "magnetic moment", and atomic vibrations. This project aims to study these delicate interactions in low-dimensional model systems, such as atom chains or single-atom layers on surfaces. These extreme low-dimensional systems also exhibit the rich physics associated with correlated electron motion but they are easier to control and analyze than bulk oxide materials. The key aspect of this project will be the education and training of two PhD students and a postdoctoral research associate in the use of advanced scientific instrumentation and analytical problem solving. These are qualities and experience that provide an excellent preparation for careers in academia and high-tech industry. The project both nurtures and expands the fundamental knowledge base and workforce for materials innovations that may ultimately drive technological and economic development.
Surfaces and interfaces function as ideal platforms for studying low-dimensional electron systems and phase transitions. This project will focus on the triangular surface phases of group IV elements on Si(111) and Ge(111) surfaces with odd electron counts; and on quasi one-dimensional atom chains on silicon surfaces with nested Fermi surfaces and strong spin-orbit coupling. These surface phases exhibit the rich physics arising from competing electron-electron interactions, electron-phonon coupling, magnetic interactions, broken symmetries and geometrical frustration, and are amenable to first principles calculations and theoretical modeling of the many-body interactions. The nature and driving forces of the electronic phase transitions in these systems will be studied using the unique combination of high-resolution angle-resolved photoemission and helium atom scattering. The strong coupling between the electronic and phonon excitations will be disentangled using a novel pump-probe scheme for time-resolved photoemission. Finally, the nature and strength of the many-body interactions and electronic phase diagram will be explored using chemical doping and pressure tuning via epitaxial strain. This program provides an excellent setting for the education and training of two PhD students and a postdoc at the frontiers of condensed matter physics. These learning opportunities will be enhanced by direct access to sophisticated instrumentation and an international network of theory collaborators. Successful implementation not only advances the frontiers of modern condensed matter science but also fosters fundamental knowledge that will eventually facilitate the creation of novel functional materials by design.
Chemical doping, i.e., the introduction of a foreign elemental species into a host material, is the main avenue for creating materials with tunable electrical and magnetic properties. For instance, the silicon-based microelectronics industry exists by virtue of the fact that silicon can be made electrically conductive by substituting Si atoms in the crystal lattice with trace amounts of boron or phosphorous. Likewise, high-temperature superconductivity was discovered when researchers created doped antiferromagnetic ceramic compounds. The role of the dopant atoms is to introduce free charge carriers into the crystal, which give rise to electrical conductivity or even superconductivity. Intellectual merit: In this project, we aim to establish novel electronic phases in monatomic tin layers on silicon. These systems are of great interest because they may exhibit emergent properties, including unconventional superconductivity, that are related to strong electron-electron correlations in the surface layer. The underlying mechanism for superconductivity would be similar to that of the ceramic high-temperature superconductors, but the surface system itself is much simpler in structure and composition and, consequently, surface layers may be an ideal platform for helping us understand the unsolved mystery of high temperature superconductivity in the ceramic cuprate compounds. In order to accomplish this, one must be able to dope a surface layer without introducing foreign atoms into the surface layer. In this project, this was accomplished by 'delta-doping', a doping scheme in which the dopant atoms are located several atomic distances below the monatomic tin layer, while the dopant charges are transferred to the tin layer where they can roam freely. This project produced the first successful surface 'modulation doping' experiment. In these studies, we discovered two new surface phases. One phase can be characterized as a conventional metal that undergoes a metal-insulator transition at low temperature, due to the spatial ordering of the charge. The other phase appear to be superconducting with a fairly high critical temperature, according to tunneling spectroscopy experiments. If confirmed with different measurement techniques, this will be a very important breakthrough in superconductivity research. Broader impact: Nanoscience is a very important driver for technological innovation and this work may have very important ramifications in the development of novel nanophase materials with tunable properties. The demonstrated feasibility of doping a strictly two-dimensional surface layer may find use in other applications where one aspires to engineer the electronic properties of other two-dimensional systems, think for instance about graphene sheets and related materials that are by many believed to be the materials of the future. The project furthermore provided excellent training for two postdocs and three students in surface- and nanoscience. As they move on to take positions at national laboratories, industry and/or academia, they represent the future of science and technology. Ultimately, the education and training of young scientists are important drivers for technological innovation and economic development, resulting in job creation and advancing human society as a whole.
