Abhay Pasupathy - US grants

This PIRE project forms an international consortium of leading superconductivity researchers from the U.S., Japan, Canada, UK, and China to investigate novel superconductors to clarify superconducting mechanisms and properties and develop novel superconducting materials. In conventional electrical systems heat is generated by friction as electrons collide with atoms and impurities in the wire, a property that is ideal for appliances such as toasters or irons but not for most other electrical applications. Superconductivity can be thought of as "frictionless" electricity whereby electrons glide unimpeded between atoms, thus vastly improving the conductor's energy efficiency. To date this has only been achieved at extremely low temperatures; the challenge is to harness this phenomenon at or near room temperature and at high electrical currents. This project will fill gaps in our current understanding of superconductivity, reconcile current theories, and advance the development of better materials for fast-performing devices and cost-saving electric motors, generators, and power transmission lines.

The project links leading materials experimentalists and eminent theorists in a study of FeAs, CuO, CeCoIn5, and URu2Si2 superconductors using powerful experimental probing techniques including neutron scattering, muon spin relaxation, X-ray scattering, Raman spectroscopy, and scanning tunneling microscopy. These advanced methods allow elucidation of the phase diagrams of these important new materials of which some significant aspects are currently unknown. The PIRE team will explore the parameters affecting the highest temperature at which a certain material is superconducting and ways of increasing that temperature so that superconductivity will not require such expensive refrigeration. Some anomalies in the superfluid density and specific heat discontinuities, inconsistent with the standard theory of superconductivity, will also be investigated both experimentally and theoretically.

International collaboration is essential for this work because it will provide U.S. scientists and students with access to critical world-class accelerator-based facilities available in the UK and Canada but not in the U.S., to high quality specimens fabricated in China and Japan, and to first-rate scientific expertise from all countries. Combining and comparing the results of multiple probes on the same high-quality specimens will significantly improve the accuracy of data. Face to face collaboration of theorists and experimentalists focused on key concepts will facilitate the translation of mathematical theory into realistic and effective models and materials. The project places great emphasis on training students and early career scientists. Students and postdoctoral researchers will undertake 3-6 month research visits to work on superconducting mechanisms at foreign sites, where they will also receive language and cultural training. The project will actively recruit minority students into the sciences via workshops for high-school students and teachers from disadvantaged schools in New York and via an outreach program on superconductivity and scanning tunneling microscopy. High school and undergraduate students will gain valuable beam-time experience through the project, and female students, who are as a group underrepresented in the physical sciences, will be provided valuable mentoring from four female leading scientists on the team. The PIRE team will also develop a contemporary, internet-based set of solid state physics lectures and a text book on introductory solid state physics that reflect current knowledge in condensed matter physics and related experimental techniques.

The project will strengthen and internationalize materials research programs at the U.S. institutions and engage more U.S. students in international research collaborations. It will place Columbia University and its students and faculty at the core of a research and education partnership with extensive research collaborations, teaching cooperation, and frequent reciprocal research visits for participating faculty and students. Impacts extend beyond the PI and his institution, including providing U.S. students with research opportunities at two Department of Energy U.S. National Laboratories (Oak Ridge and Los Alamos) and training of early career scientists at the UK's ISIS and Canada's TRIUMF facilities, both of which will build the core workforce for new probing facilities currently under construction in the U.S. and Japan. This PIRE project will build upon an existing Inter American materials science network (CIAM) and forge a foundation for long-term research and educational collaborations among scientists and institutions in the five participating nations, all advancing the state-of-the-art in superconductivity and its applications.

Participating U.S. institutions include Columbia University (NY), University of Tennessee at Knoxville, and the Department of Energy's Oak Ridge (TN) and Los Alamos (NM) National Laboratories. Foreign institutions include Institute of Physics - Chinese Academy of Sciences, University of Bristol (UK), the UK Science and Technology Facilities Council's ISIS facility, McMaster University (Canada), TRIUMF Canada's National Laboratory for Particle and Nuclear Physics, Tokyo University (Japan), Osaka University (Japan), Tohoku University (Japan), and the National Institute of Advanced Industrial Science and Technology (AIST) (Japan).

This award is co-funded by the Office of International Science and Engineering and the Division of Materials Research.

****NON-TECHNICAL ABSTRACT****
A simple metal such as gold or copper can be imagined as an empty box with electrons bouncing around freely inside. In some solids, however, the electrons form waves in space with alternating regions of higher and lower charge. This state of matter is known as a "charge density wave" (CDW). In such solids, the formation of the charge density wave happens at a critical temperature, above which the electrons are once again free to move around. Why does this happen? How exactly do these waves of electrons form in space as the sample goes through the critical temperature? This project aims to answer this question by performing temperature-dependent scanning tunneling microscopy (STM) measurements of CDW materials to directly visualize the onset of charge density waves at the critical temperature. An STM is an instrument with which we can probe the electrons at the surface of a material with sub-atomic precision. These advanced instruments will be custom-built for this project, and the new STM measurements will give us vital information on how electrons with different energies behave in these materials as they go through the CDW transition. This project will support the education of undergraduate and graduate students in the advanced technologies required to perform STM experiments including electronics, computer-aided design, vacuum technology and cryogenics. This project seeks to answer questions about the collective motion of electrons in solids, one of the fundamental challenges in modern physics research.

****TECHNICAL ABSTRACT****
The aim of this project is to visualize the onset of the charge density wave (CDW) phase in real space using scanning tunneling microscopy (STM). In a simple second-order phase transition in a uniform system, the amplitude of the order parameter goes to zero at the phase transition temperature. When defects or other spatial inhomogeneity is present, the situation can be dramatically different.
Using variable-temperature atomic resolution STM, recent experiments have shown that nanoscale CDW order can be stabilized above the bulk transition temperature in the transition metal dichalcogenides. How do these nanoscale patches of CDW transition to bulk CDW order? What is the local electronic spectrum in a nanoscale patch? What is the electronic spectroscopic difference between the CDW state and the normal state in these materials? What is the nature of scattering from defects and CDW patches above the transition temperature? During this project, state of the art, homebuilt STM instruments will be used to answer these questions. Graduate and undergraduate students will learn how to build and operate these instruments, and new designs for improved stability and cryogenic efficiency will be implemented. The nature of spatially ordered collective electronic phases is a topical question that arises in many modern materials, and the dichalcogenides present a clean material system where the onset of such a phase can be measured with atomic spatial precision and millivolt energy resolution.

This project is awarded under the Nanoelectronics for 2020 and Beyond competition, with support by multiple Directorates and Divisions at the National Science Foundation as well as by the Nanoelectronics Research Initiative of the Semiconductor Research Corporation.

TECHNICAL: The search for high-performance electronic switches operating at low power dissipation has generated many concepts that go beyond the control of charge flow in traditional semiconductor device structures. These novel devices are based on the use of alternative state variables, including the characteristics of single-particle quantum systems, such as spin, pseudospin, and carrier wave-function phase, and the characteristics of correlated many-body quantum systems, such as excitons and exciton condensates. The goal of this project is to develop the basis for transformative technology that would be made possible by the availability of high-performance electronic devices employing such quantum state variables, rather than traditional semi-classical transport of charge. To this end, a team of investigators at Columbia University and University of Florida is devoted to the fabrication, characterization, and theoretical analysis of such quantum switches. The research exploits recent technological advances in the synthesis of atomically thin layers of van der Waals solids and heterostructures formed from combinations of such layered materials. The potential of this approach is exemplified by the excellent electrical characteristics exhibited by heterostructures of atomically thin layers of graphene and hexagonal boron nitride. In this project, devices are built from such well-developed material systems, where the primary fabrication challenges involve precise control over geometry and interface cleanliness. The key research components of the project are as follows: (i) The assembly and fabrication of atomically thin heterostructure devices based on the co-lamination of van der Waals materials, atomic-layer deposition processes, and advanced patterning techniques; (ii) the analysis of distinctive quantum coherent transport processes in weakly coupled layered heterojunction device structures by electrical and optical measurements; (iii) the establishment of new state variables based on quantum coherence; and (iv) the demonstration, characterization, and theoretical modeling of switching devices based on novel state variables. Devices resulting from this research effort promise performance with respect to switching speed and energy dissipation that significantly exceeds the limits imposed by conventional semiconductor device technology.

NON-TECHNICAL: The development of the new electronic devices based on low-dimensional functional material platforms opens important directions in both fundamental and applied research. The availability of practical high-performance, low-energy switching devices is of great significance for the continued advancement of electronics and the associated information technology industry. Thus, the demonstration of devices based on new switching principles has the potential for broad technological impact. The diverse capabilities of the team also significantly enhance the educational opportunities for students and postdocs at Columbia and at collaborating institutions. The highly interdisciplinary research carried out in this project provides cutting-edge training for graduate students and postdocs, as well as for undergraduate students. The team integrates research activities with educational efforts by offering new lecture and laboratory courses, as well as modifying existing ones. The team also undertakes broader educational outreach through sponsorship of summer research projects for high school students. Significant efforts are made toward K-12 outreach by training of highly motivated high school students, and by enhancing interactions with local K-12 educators to introduce front-line research to students.

Microscopy is a pillar of modern science, which enables us to understand, inspect and improve on nature. While the technology of modern microscopes has progressed by leaps and bounds in the past decades, the methods used by microscopists to analyze data remain primitive. Common to new and emerging modalities of microscopy is the generation of massive, multi-dimensional data sets. This project develops fundamental analysis tools to extract basic motifs from these datasets; in particular, from data produced by scanning tunneling microscopes. These analysis tools will transform microscopy imaging by improving the quality and statistical significance of atomic-scale observations of materials. Key analysis goals that will be addressed include guarantees that algorithms produce models which accurately reflect the physics of the material of interest, and that algorithms perform reliably on practical data which may contain noise and errors. Key experimental goals include the generation of large scale data sets from multiple microscopy modalities which will be used to test and extend the analysis tools.

The project leverages recent advances in high-dimensional nonconvex optimization to address fundamental challenges in convolutional data modeling, the problem of modeling data as superpositions of translated motifs. Because the goal is to produce accurate information about novel materials whose properties are not yet understood, the investigators seek algorithms which exhibit (i) guaranteed performance,(ii) robustness to gross errors and (iii) scalability to massive, high-dimensional datasets. Building on recent progress in dictionary learning, the investigators study the properties of efficient methods for recovering models with one or more motifs. They seek highly scalable algorithms for these problems, using Riemannian optimization and active set methods. They study variants which are robust to commonly occurring gross errors, including pixel and scanline corruption, and contrast variations. The algorithms are applied to study materials for which previous analysis methodologies fail, including materials with multiple types of defects, quasiparticle interference, and high temperature superconductors. For each of these materials, high resolution scanning tunneling microscopy and spectroscopic imaging will be performed to produce large-scale, multidimensional data sets. Data sets on well-studied materials will be used to test and verify analysis algorithms, and the application of these algorithms to data sets on novel materials will be used to transform our understanding of the electronic structure of complex materials. Data sets on other microscopy modalities will also be obtained to generalize analysis tools to multiple scales in space, time and energy.

Non-technical Abstract:
When a material is mechanically stretched, its properties can undergo dramatic changes. For example, we are familiar with how the elasticity of rubber changes as it is stretched. Less familiar but equally important are changes to the electronic properties of materials. When stretched, materials can change their electrical resistance, light absorption and a host of other electronic properties. The causes for such changes in the electronic properties are often poorly understood at the microscopic level. In this project, the research team will use scanning tunneling microscopy (STM) to study the electronic properties of materials as they are stretched. STM is a technique to measure the electronic properties of materials with the precision of single atoms. In this research, a specially designed apparatus is used to mount and stretch crystal materials in the STM while their response is monitored. This equipment is designed, manufactured, assembled and run by the principal investigator's research team. It is used to impart training to students at the high school, undergraduate and graduate levels, and is used to liaison with industrial partners. The goal of the project will be to develop a microscopic understanding of the changes brought about by strain in materials, especially those belonging to the iron pnictide class of superconductors.

Technical abstract:
External strain when applied to materials can cause electronic changes via modifying band structure, interaction strengths and even changes in phase. Understanding the microscopic changes brought about by the application of strain is the key scientific problem of interest in this project. The main scope of the research is to study the electronic nematic phase of the iron pnictides. In this phase, electronic properties display nematicity, the spontaneous breaking of the underlying discrete rotational symmetry of the lattice. Developing an understanding of the nematic electronic phase and its coupling to the other phases of these materials is the key scientific goal of this project. The main experimental technique used will be atomic-resolution, cryogenic STM measurements. The project is enabled by a technical breakthrough that allows for the very first time the application of uniaxial or biaxial in-plane strains to a sample, while continuously performing microscopic measurements on the same atomically resolved area of the surface. Using this apparatus, STM is used to measure (a) gaps from spectroscopy (b) domain structure from topography and (c) scattering rates from quasiparticle interference, all under the influence of strain. The apparatus is used across the phase diagram in three different pnictide families of NaFe(Co)As, Fe(Te)Se and LiFe(Co)As systems that display very different nematic behaviors. The final goal of the project is to determine the driving forces for nematicity and its connection to superconductivity in the pnictides.

This Grant Opportunity for Academic Liaison with Industry (GOALI) project brings together academic leadership of Columbia University with the technical capabilities of the IBM T. J. Watson Research Center. Two-dimensional (2D) semiconductors are materials just a single atomic layer thick. These materials are of great interest to the electronics community for potential applications in a whole range of logic, memory and sensor applications. In the past few years, new techniques have emerged by which these materials can be grown over several inches, making them easily usable in standard semiconductor processes. However, the quality of these grown materials is at present far inferior to the classic semiconductors used by the electronics industry such as silicon. In this project, the research team will use a four-probe scanning tunneling microscope to characterize the electronic quality of 2D films at the atomic scale. The team will identify the major barriers to electron flow in these materials by directly flowing electron currents through the materials at the nanoscale while simultaneously imaging the regions of high resistance. During the project, specialized laboratory modules for teaching advanced electronics concepts at the undergraduate level will be developed, and undergraduate and graduate students will be trained in the laboratory techniques of direct relevance to the electronics industry. The proposal has significant interactions between the faculty and students from Columbia University and the IBM partner where the academic partners will gain significant experience in an industrial setting. Finally, the industrial partner, IBM, will benefit from the fundamental nature of the academic research joining their perspective and integrative skills.

This project involves interdisciplinary university-industry teams that will conduct the collaborative research in transition-metal dichalcogenides(TMD_-based materials, in which the industry research participant (IBM) provides critical research expertise that is crucial for the success of the project. The large-area growth of 2D materials have made them attractive for industrial applications in a variety of optoelectronic devices, but a key issue is understanding and controlling the transport and contact properties of wafer-scale materials. In this project, scanning tunneling potentiometry will be used to directly measure the nanoscale electronic transport properties of 2D materials including semiconducting TMD and graphene. First, scanning tunneling potentiometry (STP) will be used to measure transport on wafer-scale TMD materials. These measurements will quantify the scattering processes associated with point and line defects in these materials. Second, STP will be used to measure the microscopic band diagram in TMD field effect transistors. These measurements will elucidate the physics of electrical contact to 2D semiconductors. Third, STP will be used to measure current flow patterns in graphene. These measurements will investigate the possibility of viscous hydrodynamic flow in graphene and measure quantitatively the electron viscosity in this regime. Finally, graphene films on silicon carbide substrates will be used to grow single crystal metal films. The quality of these films will be investigated by STP and traditional transport. The use of these films in surface polariton plasmon applications will then be investigated.

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.

Nontechnical Description: The Columbia Materials Science and Engineering Center (MRSEC) - Center for Precision-Assembled Quantum Materials P(AQM) partners with faculty at Minority Serving Institutions and explores new materials systems that will enable future quantum technologies, and educates a diverse new generation of scientists and engineers who reach across disciplines to advance the frontiers of knowledge and technology. The PAQM research program comprises two interdisciplinary research groups (IRGs), both of which study materials assembled from lower-dimensional building blocks: the first group creates layered structures by stacking atomically thin sheets, and the second group uses chemically synthesized molecular clusters to create bulk materials. In both systems, the emergent properties can be controlled both by choosing different building blocks and controlling how they are assembled. PAQM seeks to harness this design freedom to create a next generation of quantum materials which provide new ways to manipulate the flow of charge, spin, and energy, and host quantum states such as superconductivity. These new properties will in turn enable future quantum technologies in computing, sensing, and communications like digital memory, switchable absorbers, and new photodetectors. PAQM trains researchers at the high school, community college, undergraduate, and graduate levels in an environment that brings together researchers from multiple science and engineering disciplines. The center engages students and teachers at the elementary and middle school levels to build interest in science. The educational and research activities of the Columbia MRSEC are designed to increase diversity at all levels, particularly in fields related to Materials Science and Engineering.

Technical Description: The Columbia MRSEC - PAQM consists of two IRGs focused on materials created by precise assembly of low-dimensional building blocks. IRG1 combines two-dimensional materials into van der Waals heterostructures (vdWH) hosting emergent quantum phenomena. Three classes of quantum phenomena ? tunable superfluids, non-equilibrium states, and topological quantum states ? motivate this work. IRG1 focuses on foundational materials issues by synthesizing high-purity crystals, creating ultraclean heterostructures, and performing detailed characterization to fully understand structure/property relationships in vdWH. These advances will propel the field and enable harnessing of quantum phenomena underpinning future quantum information, sensing and computing technologies. IRG2 designs and synthesizes atomically precise, functional materials from chemically synthesized molecular clusters (superatoms). Using a closed-loop approach that combines synthesis, theory, and characterization, the IRG2 team develops methods to control the coupling between superatoms. Tuning the superatoms? electronic, magnetic, vibrational, and symmetry characteristics allows the team to design reconfigurable phase change materials; control directional transport of energy, charge and spin; and achieve emergent quantum phenomena, properties that underpin future technologies including electronics, digital memory, switchable absorbers, and new photodetectors. Investments in new research tools and shared facilities supports this work. These research goals are propelled by collaborations, with major partners including Brookhaven National Laboratory, the Flatiron Institute, and the Max Planck Society. Industrial partnerships and an entrepreneurial seed program support translational efforts toward applications. PAQM education and outreach activities support STEM and materials education at all levels and train the next generation of interdisciplinary materials researchers in the cutting-edge area of quantum materials. Reflecting the diversity of the Columbia MRSEC faculty and its urban location, research and education are integrated with a diversity strategic plan aimed at increasing participation of underrepresented groups in materials science and related fields.

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.