****NON-TECHNICAL ABSTRACT**** A Bose-Einstein condensate is like a cold gas, but instead of liquefying into a high-density droplet at low temperatures, the gas particles remain dilute. However, they behave collectively like a large single quantum particle. To make electrons form a Bose-Einstein condensate, one must find a way to cause them to bind together in pairs before they can collapse into this curious low temperature phase. It is the goal of this Faculty Early Career Development project at Northwestern University to study a new way of pairing electrons inside of thin layers of the semiconductor aluminum arsenide, in order to realize a Bose-Einstein condensate. Discovering and studying new realizations of quantum condensates will deepen our understanding of this exotic phenomenon, and the technology developed along the way may illuminate new concepts for making quantum computers. The educational component of this project will train both graduate and undergraduate students in the techniques of solid-state research. The outreach element will bring science into the public eye by coordinating the efforts of students and faculty in the theater, engineering, and science departments to present scientifically themed plays. The plays will be performed in the lecture halls of the science and engineering building for an audience of university students, local high school students, and community members.
A Bose-Einstein condensate is a rare example of a quantum coherent state of matter. Particles must first pair to form bosons, and then at low enough temperatures, these bosons condense into a single coherent wave-like ground state characterized collectively by a single order parameter. In aluminum arsenide, electrons have an extra label called the valley index, and it is the goal of this Faculty Early Career Development project at Northwestern University to apply high magnetic fields and low temperatures to a single quantum well of aluminum arsenide to induce the electrons from different valleys to pair and Bose-condense. Discovering and studying new realizations of quantum condensates will deepen our understanding of quantum coherence, and the technology developed along the way may illuminate new concepts for quantum information storage and manipulation. The educational component of this project will train both graduate and undergraduate students in the techniques of solid-state research. The outreach element will bring science into the public eye by coordinating the efforts of students and faculty in the theater, engineering, and science departments to present scientifically themed plays. The plays will be performed in the lecture halls of the science and engineering building for an audience of university students, local high school students, and community members.
The transistors that make up computer chips are the result of decades of basic research in the electronic properties of materials, and investigations of new electrical properties lead to novel devices that might again revolutionize electronics. The intellectual merit of this reseach is to investigate how electrons in some materials carry an extra label -- the valley index -- and demonstrate that it should be possible to sort electrons according to this index, defining the new field of electronics called "valley-tronics." A primary goal of valley-tronics is to eventually make logic switches and memory elements whose functionality is defined by the ability to sort electrons from different valleys, and this work lays the necessary foundation and achieves key milestones, described below. The outcomes of the research effort include novel advances in valley-tronics with respect to theory, materials, and electronic characterization. Regarding new theory, we developed equations for calculating how to capture such multi-valley electrons in a single layer of material, called a quantum well, and we predict how many electrons of each valley could be found there, depending on the crystal orientation of the underlying substrate. The material of immediate interest is aluminum arsenide (AlAs), though all of the valley effects studied here will also have broad impact in silicon (Si), graphene, and molybdenum disulfide (MoS2). We also developed a theory for how resistances in large magnetic fields in semicoductors can cause the transverse Hall resistance to rise and fall with increasing magnetic field -- the so-called quantum Hall overshoot effect. Regarding new materials, we collaborated with the Technical University of Munich and the ETH Zurich to grow new valley-tronic quantum wells out of AlAs that had either 2 or 3 different kinds of valleys in each quantum well. These studies required crystal growth in molecular-beam epitaxy machines, where beams of atoms land on the surface of a crystal, growing layers of different materials atom-by-atom. Regarding electronic characterization, we measured these materials at low temperatures below T = 1 K and high magnetic fields up to B = 15 T, to determine how many valleys we had in each kind of quantum well, and we characterized their quality according to the mobility of the electrons. We were able to observe the primary goal of this research: the sorting of valleys, showing that in a magnetic field, the valleys become filled one at a time, and we were able to show with resistive signatures which valley is being filled at any given moment. This demonstrates fulfillment of the principle goal of this work -- to sort valleys, in this case with control of the magnetic field. The broader outcomes of this work include two patents-pending -- one patent explaining how to make a valley-tronic transistor as a long-term memory element, and another explaining how to sense the conductivity of electrons buried inside a semoconductor without making physical contact to the sample. This award also led to the completion of 1 PhD thesis, 3 MS theses, and 4 undergraduate research projects. Several peer reviewed journal papers were also published under the support of this grant. This grant also inaugurated the "Engineering Transdisciplinary Outreach Program in the Arts (ETOPiA)" whereby scientifically themed plays were performed free to the public in a converted classroom of the Technology Institute of Northwestern University. The plays performed during the lifetime of this award were "Copenhagen" by Michael Frayn, "Manya: A Living History of Marie Curie" by Susan Marie Frontczak, "QED" about Richard Feynman by Peter Parnell, "A Number" by Caryl Churchill, "The How and the Why" by Sarah Treem, and "The Agony and the Ecstasy of Steve Jobs" by Mike Daisey. Between 700 - 1,000 community members, high schoo students, undergraduates, faculty, and staff saw each show.
