Hanno H. Weitering - US grants

9705246 Weitering The primary goal of this new research program is to explore the fundamentals of electron localization at crystalline semiconductor surfaces or in ultrathin epitaxial films. The PI who is a junior faculty member at Tennessee will measure the temperature dependence of the surface state conductance and attempt to correlate these classical transport measurements with state densities and band dispersions as measured with angle-resolved photoemission, electron energy loss spectroscopy and scanning tunneling spectroscopy. In addition, we will explore the possible formation of local magnetic moments using the a.c. susceptibility technique in ultrahigh vacuum. It is expected that this multi-technique approach will ultimately bridge the gap between surface science and mesoscopic physics, thereby generating a profound impact on both condensed matter physics and semiconductor technology. %%% The primary goal of this new research program is to explore the fundamentals of electron localization at crystalline semiconductor surfaces or in ultrathin epitaxial films. The trend toward miniaturization of industrial components is leading to materials that are so small as to be nearly two-dimensional. The smaller a sample, the more important its surface properties, simply because the surface represents a higher percentage of the sample. In order to learn something about surfaces, scientists have "bombarded" surfaces with quantum particles such as electrons or photons. Unfortunately, such experiments are extremely difficult to relate to practical matters such as electrical conductance. In this research project, we are exploring new ways to measure the electrical and magnetic properties of semiconductor surfaces and thin films directly. A novel method is being implemented to electrically insulate the surface from the bulk so that we can measure the electrical resistance o f the surface. We are also constructing a very sensitive nstrument capable of detecting extremely weak magnetic signals from a surface. It is expected that these new measurements will provide unprecedented insights that will help to bridge the gap between the physicist's world of atoms and molecules and the engineer's world of device fabrication. ***

The primary objective of the proposed research is the investigation of quantum electrical transport in thin film nanostructures on silicon substrates. The project focuses on atomic-wire arrays and on atomically smooth metal films. Under certain growth conditions, individual metal atoms "self-assemble" into macroscopic domains of parallel "atom wires" with equal but tunable spacing between the wires. These arrays are ideal for studying the fundamentals of quantum electrical transport a function of system dimensionality and Fermi surface topology using nanoscopic probes such as a four-tip Scanning Tunneling Microscope, as well as macroscopic measurement probes such as electron- or laser beams. Atomically smooth metal films will be synthesized via a novel self-assembly mechanism that is driven by the quantum-size effect, which allows for a systematic investigation of the conductivity in relation to quantum interference, quantum confinement, and classical size effects. These studies will allow researchers to extract the key characteristics that are relevant for the operation of future nanoscale devices. The proposed science and supporting infrastructure at The University of Tennessee and nearby Center for Nanophase Materials Sciences provide an excellent setting for the education and training of internationally competitive students and postdocs. These young people will take their place in the highly skilled workforce that will continue to drive innovation and prosperity in today's high-tech society.

Quantum electrical transport is at the heart of nanoscience. The idea of assembling single atoms or molecules and small chemical groups into much faster and powerful electronic devices no longer belongs to the realm of science fiction. Recently, researchers wired up their first molecular-scale electronic circuits, an achievement Science Magazine selected as the "Breakthrough of 2001". Researchers now face the daunting task of taking this new technology from basic electronic components to complex integrated circuits that can rival silicon's low cost performance and reliability. Reaching that level of complexity requires several scientific breakthroughs besides the obviously needed revolution in chip fabrication and design. In this project, researchers will align individual metal atoms into "atomic wires" or arrange the atoms into a perfectly smooth film that is only a few atom layers thick. These nanostructured materials are ideal model systems that will allow researchers to explore the fundamentals and key characteristics of electrical currents in nanophase materials. Quantum transport marries the most fundamental laws of nature, namely quantum mechanics, with applied electrical engineering and emerging materials technologies. The proposed science and supporting infrastructure at The University of Tennessee and the nearby Center for Nanophase Materials Sciences of the Department of Energy provide an excellent setting for the education and training of internationally competitive students and postdocs using specialized national research facilities. These young people will take their place in the highly skilled workforce that will continue to drive innovation and prosperity in today's high-tech society.

Nearly all materials properties are determined by electrons close to the Fermi level, usually within 100 meV. In order to probe the behavior of electrons in this energy range, one needs to employ spectroscopy with ultrahigh resolution. This project entails the acquisition of an ultrahigh-resolution photoemission spectrometer of the Scienta type, which offers the best resolution that is currently achievable. This instrumentation will be used to investigate the electronic states in a wide variety of advanced electronic materials, including complex transition metal oxides, thin film nanostructures, organic superconductors, and atomic-wire arrays. Coupled with other spectroscopic methods, such as inelastic neutron scattering for probing spin and lattice excitations, and optical spectroscopy and Electron Energy Loss Spectroscopy for charge dynamics, it will enable researchers to unravel the highly complex entanglement of the spin, charge, and lattice degrees of freedom in these exotic materials. The new instrumentation will be based on campus, not at the synchrotron, so that students and faculty can have easy access and copious amounts of high quality photoemission time. The proposed science and new infrastructure will provide an excellent setting for the education and training of internationally competitive students and postdocs.


Nearly all materials properties are determined by electrons close to the Fermi level, which represents the highest occupied energy level. Examples include electrical conductivity, magneto-resistance, superconductivity, and magnetism. In order to understand these important materials properties, one should probe the electrons near the Fermi level with ultrahigh resolution spectroscopy. From the late 1980s, Angular Resolved Photo-Emission Spectroscopy has been intensively applied to unravel the origins of superconductivity in high-temperature superconductors. In recent years the resolution of photoemission experiments has improved so much that electrons within a fraction of a milli-electronvolt around Fermi level can now be distinguished. Many of these potentially prize-winning studies have been published in highly prestigious journals because the ever increasing resolution unraveled novel properties that challenged the community and triggered new discovery. This project entails the acquisition of the world's best ultrahigh-resolution photoemission apparatus for the University of Tennessee. It will be used to study a wide variety of advanced electronic materials, including complex transition-metal oxides, thin film nanostructures, organic superconductors, and atom-wire arrays. A key aspect of the proposed research activities is that the powerful capabilities of this instrument will be combined with the local, complementary expertise and capabilities in neutron scattering and materials synthesis, thus providing researchers in East Tennessee with a competitive edge. The new instrumentation will be based on the Knoxville campus, not at a national synchrotron facility, so that students and faculty can have easy access and copious amounts of high quality photoemission time. The proposed science and infrastructure will also be accessible to the African American and Hispanic minority-student population at Florida International University and provide an excellent setting for the education and training of internationally competitive students and postdocs from diverse backgrounds.

***NON_TECHNICAL***
Treating the dynamic interaction of the electrons and the atoms or molecules in a solid is difficult but essential if functionality is to be designed into advanced materials. In textbooks this interaction is handled by using the Born-Oppenheimer Approximation (BOA), which allows scientists to consider the slow-moving atoms to be frozen in space in their average positions while the more energetic electrons do their thing. But a multitude of important chemical, physical, and biological phenomena are driven by violations of the BOA. Breakdown in the BOA results from low-energy excitations of the electrons near what is known as the Fermi energy coupling with vibrational excitations of the solid. The resulting vibronic interactions are a necessary ingredient in any process that makes or breaks a covalent bond or in conventional superconductivity, which is driven by the electron-lattice interaction. Many of the emergent properties of complex materials or artificially nanostructured materials result from coupling of the electronic and lattice (atom) motion in systems that are inherently anisotropic. This project will use a newly developed data analysis procedure to extract the details of this coupling between the electrons and the lattice from high-resolution spectrocopic data. Undergraduates, graduates and post docs will be involved with state-of-the-art instrumentation and materials preparation. This award is co-funded by NSF's Division of Materials Research and the Department of Energy's Office of Basic Energy Sciences.

***TECHNICAL***
This individual investigator award supports an experimental investigation of the electron-phonon coupling (EPC) at metal surfaces. A newly developed data analysis procedure enables the spectroscopic features of this coupling to be exacted from high-resolution angle-resolved photoemssion spectroscopy (ARPES) data. For the first time in very anisotropic systems it is possible to "see" which vibrational modes couple to which electrons. This procedure will be applied to a variety of materials, ranging from layered transition-metal oxides, MBE grown thin films of MgB2, and clean and modified surfaces. ARPES measurements will be correlated with scanning tunneling spectroscopy measurements of the EPC and inelastic electron scattering measurements of the phonon dispersion at the surface. A world wide collaboration has been created to take full advantage of theoretical and experimental advances in this field. This research project offers an ideal training ground for undergraduates, graduate students, post docs and visiting scientists, exposing them the state-of-the-art instrumentation, sophisticated growth procedures and close coupling with theory. This award is co-funded by NSF's Division of Materials Research and the Department of Energy's Office of Basic Energy Sciences.

0923125
Mannella
U. of Tennessee Knoxville

Technical Summary: Besides growth and characterizion capabilities with systematic transport and structural studies, detailed investigations of the electronic structure of advanced materials are essential in order to advance our understanding of the fundamental underpinnings of their properties. Photoemission Spectroscopy (PES) is one of the most powerful techniques for characterizing the electronic structure of materials. We propose the realization of a laboratory-based PES user facility for analysis of the chemical and electronic properties of various forms of condensed matter that are at the forefront of scientific and technological innovation. These include Correlated Oxides, Physics of Low Dimensional Systems (surfaces, interfaces, and nanophase materials), Materials for Solar Energy Conversion, Thin film superconductors, Magnetic Semiconductors and Nanostructures, Hydrogen Storage, Lunar rocks and Soils, Water Hydration of Minerals, Characterizattion of Electronics (PCBs) and Biomedical Devices for Drug Delivery. The spectrometer presents truly unique characteristics such as 1) a monochromatized x-ray source with two different energies (Al K?Ñ = 1486 eV and Ag L?Ñ?n?­ 2984 eV) with micro-spot of ?l 130 ?Ým for analysis of very small or inhomogeneous samples, and 2) a state-of-the-art hemispherical electron analyzer provisioned with a mini-Mott detector for electron spin detection. The facility will be an asset for the UT system and for a broad range of UTK departments ranging from Physics, Chemistry, Geology, Biology, and Material Engineering, and it is thus expected to nucleate interdisciplinary research covered under several NSF program areas. The proposed instrument will be based on campus, so that our students and faculty can have easy access and copious amounts of time using a state-of-the-art electron spectrometer. The proposed science and supporting infrastructure will provide an excellent setting for the education and training of internationally competitive students and postdocs from several departments. The instrument will not only complement, but also enhance the productivity of investigations carried out at facilities such as the Spallation Neutron Source (SNS), the Center for Nanophase Materials Sciences (CNMS), the Joint Institute for Advanced Materials (JIAM) which render UTK a unique place in the nation for advanced materials research. The addition of the multiple-user photoemission facility hereby proposed will grant UTK an invaluable asset for becoming a leading world-wide institution for materials characterization, research and development.
Laymen Summary: Materials are the building blocks of every form of solid matter naturally existing in the universe or manufactured by humankind. The onset of new materials has always marked a major turning point in human society, with the material of choice of a given era often being its defining point. More recently, impressive advances in materials synthesis have resulted in the discovery of an ever-increasing number of complex materials exhibiting exotic properties at the forefront of promising revolutionary technological applications ranging from engineering to biotechnology. Detailed investigations of the electronic structure are essential in order to advance our understanding of the properties of advanced material. We propose the realization of a laboratory-based user facility for carrying out experiments using Photoelectron Spectroscopy, one of the most powerful techniques for measuring the electronic structure and chemical nature of materials. This instrumentation will be used to investigate the electronic properties of a wide variety of materials, including Materials for Solar Energy Conversion and Hydrogen Storage, Superconductors, Magnetic materials for data storage, Polymers, Catalysts, Biomedical Devices for Drug Delivery, Nanostructures, Lunar rocks and Soil, Biological Cells, Bacteria and Minerals. The facility will be an asset for the UT system and for a broad range of UTK departments ranging from Physics, Chemistry, Geology, Biology, and Material Engineering, and it is thus expected to nucleate interdisciplinary research. The proposed instrument will be based on campus, so that our students and faculty can have easy access and copious amounts of time using a state-of-the-art electron spectrometer. The proposed science and supporting infrastructure will provide an excellent setting for the education and training of internationally competitive students and postdocs from several departments. The instrument will not only complement, but also enhance the productivity of investigations carried out at other existing facilities which render UTK a unique place in the nation for advanced materials research. The addition of the multiple-user photoemission facility hereby proposed will grant UTK an invaluable asset for becoming a leading world-wide institution for materials characterization, research and development.

****NON-TECHNICAL ABSTRACT****

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.


****TECHNICAL ABSTRACT****

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.

Technical summary: Transition metal-oxides are highly promising materials in modern technology because they are stable at high temperatures and in corrosive environments, and because their physical and chemical properties are highly tunable. The design of metal oxides for technological applications such as electronics, photovoltaics, and catalysis necessitates a thorough understanding of the physical complexity that lies beneath the broad functionality of these materials. This project involves the acquisition of a molecular beam epitaxy apparatus with an in-situ low-temperature scanning probe microscope for the synthesis and atomic-scale characterization of novel artificially-structured oxide materials. Molecular beam epitaxy offers unique capabilities of creating atomic arrangements with atomically precise control of thickness and composition, which will be utilized to systematically tune the properties of oxide thin films and interfaces with special emphasis on clean energy applications. This special instrument for epitaxial synthesis and characterization will be an important nucleus of the educational and training activities at the Joint Institute for Advanced Materials, which is a newly-established umbrella organization at The University of Tennessee and Oak Ridge National Laboratory, fostering interdisciplinary research, education, and partnership for the development of advanced materials in East Tennessee.



Layman summary: Metal oxides are highly promising materials for electronic and clean energy applications, including photocatalysis, where light-activated catalysts are used, for example, to split water into pure oxygen and hydrogen; and photovoltaics, which convert solar radiation into direct electric current. Nearly all such applications involve processes that take place at the surfaces or interfaces of these oxide materials. Fundamental understanding and better control of these processes would greatly benefit from the capability of producing well-defined surfaces, interfaces, and thin film materials, as well as from the capability to systematically alter and characterize the structural and electronic properties of these materials with precision down to the atomic level. The project involves the acquisition of a molecular beam epitaxy apparatus for the synthesis of artificially-structured metal-oxide materials, with special emphasis on clean energy applications, along with a scanning probe microscope for imaging individual atoms and mapping the nanoscale properties of these materials. Molecular beam epitaxy offers researchers the extraordinary capability of constructing novel materials from atomic "Lego principles," guided by theoretical calculations or predictions. This special instrument for epitaxial synthesis and characterization will be an important nucleus of the educational and training activities at the Joint Institute for Advanced Materials, which is a newly-established umbrella organization at The University of Tennessee and Oak Ridge National Laboratory, fostering interdisciplinary research, education, and partnership for the development of advanced materials in East Tennessee.

Technical Abstract

The objective of this research program is to explore and understand the microscopic origin of high-transition-temperature (high-Tc) superconductivity in iron and copper based superconductors using neutron scattering as a primary tool. Specially, the project will focus on electron-doped iron and copper based high-Tc superconductors. We will investigate the nature of the interplay between magnetism and superconductivity. Neutron scattering experiments, the core part of this research program, will be performed mostly at the high-flux isotope reactor (HFIR) and Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL). However, the project will also utilize other world-class facilities in the U.S. and Europe when similar capabilities are unavailable at HFIR and SNS. The impact of this research program will include the training of the next generation of neutron scatters and elucidating the nature of the exotic properties of these high-Tc superconductors.

Non-Technical Abstract

This NSF project addresses the fundamental physical processes that give rise to novel collective phenomena such as high-transition temperature superconductivity. The materials known to exhibit these collective phenomena are the strongly correlated electron materials. The understanding of these phenomena will not only enhance our knowledge of basic science, but also gives us the ability to design materials with novel and predictable properties. Specifically, the experimental program integrates neutron scattering experiments with lab based materials efforts, aimed at the fundamental understanding of the spin and lattice excitations in electron-doped high-transition-temperature (high-Tc) superconductors based on iron and copper. The objective of the program is to explore and understand the microscopic origins of various phases of iron and copper high-Tc superconductors using neutron as a probe. Neutron scattering experiments will be performed mostly at the high-flux isotope reactor (HFIR) and Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory. However, the project will also utilize other world-class facilities in the U.S. and Europe when similar capabilities are unavailable at HFIR and SNS. The impact of this research program will include the training of the next generation of neutron scatters and elucidating the nature of the exotic properties of the correlated electron materials.

Non-Technical Abstract:
This project aims to controllably modify the electronic properties of monatomic thin film materials via chemical doping experiments. The thin film systems under investigation consist of tin or thallium atoms arranged in various triangular patterns on a silicon substrate. The monatomic tin layer is of particular interest because there is the potential to obtain novel electronic materials with possibly exotic properties, such as superconductivity via chemical doping. Exotic magnetism may be obtained through the chemical doping of monatomic thallium films. Atomic-scale understanding of the quantum mechanical mechanisms that give rise to such emergent electronic behavior is crucial in order to advance a new class of electronic materials as the possible replacement of silicon in, e.g., nano-electronic devices used in information technology. The proposed science and supporting infrastructure at The University of Tennessee provide an excellent setting for the education and training of internationally competitive graduate students. A rigorous plan for integrating sponsored research with undergraduate education is being implemented to boost enrolment in the physics major. The outreach program targets sophomore and junior high school students who are participating in the prestigious Tennessee Governor School for the Sciences and Engineering. By providing all of these young people with unique research and educational experiences, the investigators aim to entice these deep thinkers and future decision-makers to pursue an advanced degree in the physical sciences.

Technical Abstract:

A critical aspect for advancing knowledge and practical applications of complex materials is the ability to control their electronic properties via chemical doping. For a strictly two-dimensional system, chemical doping inevitably introduces structural disorder as the ionized dopant impurities become an integral part of the two-dimensional electron system. This project builds on the main accomplishments from earlier NSF sponsored research, in which hole doping of a surface layer or thin film was accomplished via subsurface- or ?modulation doping?. In analogy with studies on complex oxide systems, the investigations aim to widen the concentration range of the hole dopants and expand the modulation doping concept to n-type species, so as to establish a full electronic phase diagram. This strategy is being employed to dope triangular surface lattices with strong Mott correlations and strong spin-orbit coupling. The exciting possibilities of d-wave superconductivity in the former, and spin density wave instabilities in the latter will be explored as a function of the doping level. The project involves thin film growth in ultrahigh vacuum, scanning tunneling microscopy and spectroscopy, and angle-resolved photoemission experiments, all in conjunction with theoretical support from outside collaborators. The proposed science and supporting infrastructure at The University of Tennessee provide an excellent setting for the education and training of internationally competitive graduate students. A rigorous plan for integrating sponsored research with undergraduate education is being implemented to boost enrolment in the physics major. The outreach program targets sophomore and junior high school students who are participating in the prestigious Tennessee Governor School for the Sciences and Engineering. By providing all of these young people with unique research and educational experiences, the investigators aim to entice these deep thinkers and future decision-makers to pursue an advanced degree in the physical sciences.