Arthur Foster Hebard - US grants

w:\awards\awards96\num.doc 9705224 Hebard This experimental project focuses on the properties of interfaces involving thin films of C-60, including alkali-metal-doped C-60 solids, and configurations which might be of interest in electronic devices. In the work, C-60 will be deposited sequentially or simultaneously with various metals, insulators or semiconductors. A variety of deposition techniques will be used including thermal sublimation, ion-beam sputtering and electron beam heating. In-situ characterization will primarily be accomplished by electrical transport measurements during and after deposition. These measurements involve not only in-plane transport but also tunneling and electric field effect measurements. For air-stable or passivated samples, additional characterization by x-rays, AFM, STM, magnetotransport, and optical and infrared absorption will be made. The high electron affinity and the relative ease of intercalation of C-60 will be aspects central to the research projects. One project will involve electron tunneling in trilayer structures to further understanding of superconductivity in the alkali-doped compounds and to determine the interface-related vibrational and electronic states and the expected relatively large Coulomb gap of molecules isolated in the tunnel barrier. This research project is interdisciplinary in nature and will involve graduate and undergraduate students who will be excellently trained for careers in industry, government and academia. %%% This experimental project is based on novel and potentially very useful electronic conducting films based on carbon-sixty molecules. Such molecules are like hollow spheres comprised of sixty chemically bonded carbon atoms, resembling tiny soccer balls. These molecules are relatively new and unexplored, especially in terms of their potential for forming insulating, electrically conducting, and even superconducting thin films. It has been possible to add alkali metals such as sodium or potassium to carbon-sixty to provide "electrical doping" which fosters electrical conduction and even superconductivity to temperatures up to about 30 Kelvin degrees. The work in this experimental project will, in part, involve "sandwich structures" which could perform functions which might be useful in electronic devices. This work will lead to a much better understanding of the electrical behavior of interfaces between conventional metals, insulators, and semiconductors, with films of the carbon- sixty molecules, and will possibly lead to applications for such interfaces. This research project is interdisciplinary in nature and will involve undergraduate and graduate students who will be excellently trained for positions in industry, government and academia. ***

9975833
Rinzler

This award will partially support the construction of a carbon nanotube tip-mounting workbench to be used for the routine attachment of carbon nanotubes to commercially available atomic force microscopy cantilevers and other electrodes. The workbench will consist of a dark-field-capable, inverted optical microscope on a vibration isolation table. The microscope platform will be retooled to accept three high precision XYZ translation stages equipped with electrostrictive drives for fine motion control. A laser light source will permit enhanced viewing of the nanotubes as well as the capability to heat these for certain proposed applications. A CCD camera and image capture system will permit remote observation of the microscope field of view both for training purposes and for documentation. A specially constructed, windowed, evacuable chamber, designed to fit on the platform to permit microscopic observation will allow the mounted nanotubes to be further processed in an inert atmosphere.

As part of their training in physics, a graduate student and two undergraduate students (provided for in University matching support for two years) will be involved in all phases of the design and assembly of the Workbench and its associated hardware. These students will learn the present techniques for mounting nanotube tips and be responsible, under the direction of Professors Rinzler and Hebard, for the further tip developments outlined above. Once trained these students will also share the responsibility for training other users. The advantages afforded by the nanotube tips generated with the Workbench are anticipated to be sufficiently useful in probe microscopy and other applications that we expect it to be used by a host of other graduate and undergraduate students and postdocs, to produce tips, for use in their own research over years to come.
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This is an unusual opportunity for students to gain invaluable experience in the design and use of a unique system.
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INT 9902050
Tanner


This U.S.-Hungary research project between David Tanner of the University of Florida and Katalin Kamaras of the Hungarian Institute for Solid State Physics and Optics features sample preparation and use of microspectroscopy for in-situ study of air-sensitive materials in an inert gas atmosphere. The researchers intend to examine the optical properties of fullerenes and fullerene derivatives in crystal and thin film forms. Crystalline samples will be prepared in Budapest and the thin film samples in Gainesville. The intent is to determine the optical functions of several phases of fulleride crystals from the far infrared through ultraviolet frequency ranges. Results are expected to improve our understanding of: 1) the symmetry of fullerene balls, 2) their electronic structure and chemical bonding, and 3) the dynamical properties of mobile charge carriers at metal/fullerene interfaces. This new information should help define a new class of superconducting materials.

This materials research project fulfills the program objective of advancing scientific knowledge by enabling experts in the United States and Central Europe to combine complementary talents and share research resources in areas of strong mutual interest and competence.

The focus of this project is on the fabrication and in situ characterization of air sensitive ultra-thin films and thin-film interfaces. The capability to perform these investigations relies on a recently constructed Sample-Handling-In-VAcuum (SHIVA) chamber in which thin-film samples can be reliably connected/disconnected at the deposition station (where optical and transport measurements can be made) and then transferred, without exposure to air, to the bottom of a vacuum-compatible cryostat for low temperature (1.3K) and high field (7T) magnetotransport measurements. The rationale for this approach centers on observations that air sensitivity can be a primary impediment to unraveling intrinsic behavior in a variety of systems. The scope of work will be concentrated in four overlapping areas: percolation and the metal-insulating transition, metal/C60 bilayers and composites, magnetism in reduced dimensions, and the determination of screening lengths at metal-dielectric interfaces. Specific systems to be studied include Ag films near the percolation threshold (exhibits pure geometric behavior and a negative classical magnetoresistance), magnetic (Ni, Fe, Co, Gd) metal/C-60 bilayers and composites (exhibits charge transfer and anomalous positive magnetoresistance), thin disordered magnetic and magnetic semiconductor (e.g., Fe, GdxSi1-x) films, and tunnel junction capacitor structures with magnetic electrodes (exhibits magnetocapacitance due to dependence of screening length on magnetic field). We expect that the SHIVA system will greatly facilitate a comprehensive understanding of novel physical phenomena occurring in air sensitive systems. This research is carried out with students who will acquire training skills that will prepare them for future employment in the scientific/technological sector of our economy.

As the thickness of a metallic film decreases, its electrical properties become increasingly sensitive to its interface with the underlying substrate and its interface with the ambient environment. For example, many freshly deposited thin films (e.g., Al, Fe) will rapidly oxidize when exposed to air and, if sufficiently thin, will in a short time become totally oxidized. To measure the intrinsic electrical properties of such air-sensitive thin films, it therefore becomes necessary to utilize specialized techniques in which air is prevented from reaching the sample in the period between its fabrication and electrical characterization. The capability to perform these investigations relies on a recently constructed Sample-Handling-In-VAcuum (SHIVA) chamber in which thin-film samples can be reliably connected/disconnected at the deposition station (where optical and transport measurements can be made) and then transferred, without exposure to air, to the bottom of a separate vacuum-compatible chamber for electrical measurements at low temperatures and high magnetic fields. Specific systems to be studied include metal films near the conduction threshold, magnetic metal/Carbon-60 bilayers and composites, thin disordered magnetic and magnetic semiconductor films, and tunnel junction capacitor structures with magnetic electrodes. We expect that the SHIVA vacuum deposition and characterization system will greatly facilitate a comprehensive understanding of novel physical phenomena occurring in air sensitive systems. Students will participate in this research. They will thereby acquire skills and training in preparation for employment in scientific/technological sectors of industry, academe, and government.

A physical properties measurement system for characterizing transport and complex impedance properties at elevated temperatures will be developed. The primary function of this instrumentation is to elucidate the magnetic, electronic, and thermal properties of novel spin- and charge-functional materials through temperature and magnetic field-dependent measurements of transport and electromagnetic response. The proposed system will include a "micro-heater platen" technology to provide for the extended temperature range. Capabilities of the instrumentation will include AC susceptibility, dc magnetization, I-V, Hall measurements, thermal conductivity, and complex impedance in magnetic fields up to 7 Tesla over a temperature range of 4 to 700 K. Together with parallel efforts in materials and devices, this instrumentation will serve as a vehicle to educate students and post-graduates regarding transport and electromagnetic response in these materials

In recent years, there has emerged a significant interest in the understanding and manipulation of charge and spin in a variety of electronic materials. In many cases, the spin/charge functionality extends well above room temperature. The instrumentation developed under this proposal will enable the magnetic, electronic, and thermal properties of novel spin- and charge-functional materials to be probed through an extensive temperature range. Together with parallel efforts in materials and devices, it will serve as a vehicle to educate students and post-graduates regarding transport and electromagnetic response in these materials.

This condensed matter physics project utilizes two recently developed techniques, which are separate but complementary, to study and characterize magnetism in ultra thin films and at thin-film inter-faces. The first employs a specialized vacuum deposition capability for in situ characterization of magnetotransport in freshly deposited thin films without exposure to air. The second utilizes the deposition of high quality pinhole-free dielectrics to fabricate capacitors with dielectric spacing thin enough (<50 A) to assure that the contribution from magnetically aligned spins in the conducting magnetic electrodes will dominate the measured capacitance. These techniques, which are uniquely sensitive to the presence of magnetism within a few angstroms of the surface, will be used to study both elemental (e.g., Mn, Fe, Cr) and novel (e.g., dilute magnetic semiconductors, manganites, ruthenates) materials. If successful, this project will solidify the prospects of adding in situ thin-film studies and magnetocapacitance as new tools not only to in-crease our understanding of magnetism at interfaces and surfaces but also to facilitate the engineering of interfaces for magneotelectronic and nanoscience applications. In pursuing these objectives, undergraduate students, graduate students and postdocs will acquire valuable technical and analytical skills for careers in academic, industrial or government settings. This training will be enhanced by the publication and dissemination of research results in scholarly journals, par-ticipation in scientific conferences, and participation in interdisciplinary collaborations.

Magnetism in thin-film nanostructures is strongly dominated by surfaces and interfaces. As the thickness of a thin film decreases, the influence of the interfaces at the film's two surfaces begins to dominate, and magnetic phenomena become intimately affected. This individual investigator award utilizes two recently developed techniques, which are separate but complementary, to study and characterize magnetism in ultra thin films and at thin-film interfaces. The first of these techniques employs a specialized vacuum deposition capability for in situ characterization of the magnetic properties of freshly deposited thin films without exposure to air. The second utilizes the deposition of high quality dielectrics to fabricate capacitors that are sensitive to the presence of magnetically aligned spins at the surface of conducting magnetic electrodes. Novel materials such as ferromagnetic semiconductors will be studied. If successful, this project will solidify the prospects of adding in situ thin-film studies and magnetocapacitance as new tools not only to in-crease our understanding of magnetism at interfaces and surfaces but also to facilitate the engineering of interfaces for magneotelectronic and nanoscience applications. In pursuing these objectives, undergraduate students, graduate students and postdocs will acquire valuable technical and analytical skills for careers in academic, industrial or government settings. This training will be enhanced by participation in interdisciplinary collaborations.

Technical Abstract:
In this individual investigator grant we utilize complex impedance measurement techniques to characterize the effect of magnetic fields on electrical transport in thin-film structures. The particular topics under study are: (1) the collapse of itinerant ferromagnetism in the extreme disorder limit as monitored by in situ anomalous Hall measurements,
(2) the sensitivity of the interfacial magnetocapacitance contributions to the surface proximity of ultrathin buried Fe layers in host Pd electrodes,
(3) a heretofore unrecognized 'giant' magnetocapacitance contribution in lightly doped MOS and Schottky barrier structures fabricated using both standard and magnetic semiconductors, and (4) the acquisition and analysis of the frequency response of the complex dielectric constant in anisotropic or layered correlated electron systems where insulating phases compete with metallic phases that are either magnetic (e.g.,
manganites) or superconducting (underdoped high Tc). Results are expected to give considerable insight into understanding magnetic properties at the interfaces of complex materials that show promise for use in magnetoelectronics and nanotechnology. Students working on this project will disseminate their results within the scientific community and learn thin-film growth and characterization skills that are highly marketable in industrial, government or academic environments.

Non-Technical Abstract:
This proposed work will focus on understanding the physics of magnetism at planar interfaces using electrical measurement techniques that can simultaneous sense the effect of magnetic field on the mobility of electrons and on the number of electrons present. For our purposes an interface-dominated magnetic system can be an ultrathin magnetic film, a planar interface between two dissimilar materials, a tunnel junction with magnetic electrodes, or a bulk system in which there are coexisting and competing phases, at least one of which is magnetic. Answers to scientific questions addressed in this proposal, such as "How does magnetism survive at interfaces?", "How small can a system be made before ferromagnetic ordering disappears?" and "How efficiently can spin-oriented electrons be transferred across an interface?", are expected to help provide an understanding of the potential of magnetoelectronics in future technology. Students working on this project will disseminate their results within the scientific community and learn thin-film growth and characterization skills that are highly marketable in industrial, government or academic environments.

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This project will pursue experiments to fabricate and characterize materials in which metals and insulators are in close proximity. The proximity of metallic and insulating phases in a given material arises in two ways: both phases simultaneously coexist and compete with each other, or one dominant phase can be transformed to the other. In both cases the change of an external parameter such as temperature or magnetic field can induce metal-insulator transitions accompanied by pronounced changes in electrical properties. The materials chosen for study include ultra thin disordered ferromagnetic films which can be tuned through a metal-insulator transition, complex oxides which comprise competing ferromagnetic metal and insulating phases, doped graphite which exhibits highly metallic/insulating behavior parallel/perpendicular to the carbon planes, and "topological insulators", which by virtue of their unusual electronic structure have conducting edge and surface states coexisting with insulating interiors. Understanding the results of these studies promises to unveil important new physics that will drive the design of novel composite metal-insulator materials that may be important for new technologies for the benefit of society. The research and education/outreach activities supported by this proposal will lead to the placement of graduate and undergraduate students in satisfying and productive professional careers within academic, industrial or government settings.


**** TECHNICAL ABSTRACT ****
This project will pursue experiments to fabricate and characterize materials in which metallic and insulating phases are in close proximity, thereby giving rise to complex and novel phenomenology. The materials chosen for study focus on four classes: (1) ultra thin ferromagnetic films that can be tuned through a metal-insulator transition by varying the disorder strength; (2) mixed phase manganites in which there is a temperature, strain and field-dependent competition between coexisting metallic and insulating phases near an insulator-metal percolation transition; (3) graphite in which there is an intercalation-induced in-plane "supermetallic" conductivity simultaneously appearing with an out-of-plane insulating conductivity; and (4) topological insulators, which by virtue of their unusual electronic band structure have conducting edge and surface states coexisting with insulating interiors. Understanding the results of these studies promises to unveil important new physics that will drive the design of novel composite metal-insulator materials at nanoscale dimensions under the unifying theme of proximate metallic and insulating phases. The associated research and education/outreach activities of this research will lead to the placement of graduate and undergraduate students in satisfying and productive professional careers within academic, industrial or government settings.

****Technical Abstract****
For the traditional transition-metal band ferromagnets (Fe, Co and Ni) where the aligned moments are successfully described by unequally populated majority (spin-up) and minority (spin-down) bands, the ultimate fate of magnetism, when the itinerancy is compromised by disorder to such an extent that the conductivity drops to zero at critical disorder, is unknown. This project addresses the consequences of systematically reducing the thickness of two-dimensional (2D) magnetic thin films (thereby increasing disorder strength) until the insulating magnetic state is attained. A custom high-vacuum deposition system with in situ electronic/magnetic characterization capability prevents sample deterioration due to air exposure and is essential for this work. The project's focus on tuning a variety of magnetic systems through critical disorder is expected to provide valuable insight into the properties of thin-film magnetic insulators and help answer questions about (1) the disorder induced high rate of inelastic scattering of electrons off of spin waves, (2) the ordering of local moments, (3) the role of granularity, and (4) the emergence of new phases. This project will support the education of PhD students in advanced vacuum deposition and electronic/magnetic characterization techniques, which have already proven to be excellent training for productive scientific careers in academic and technology settings. Pursuit of these studies will improve prediction of the magnetic properties of ultrathin magnetic materials and extend knowledge of the behavior of bulk magnetism in the very different environments experienced at surfaces and interfaces.

****Non-Technical Abstract****
Magnetism in the transition metal elements such as iron is not fully understood. In the itinerant (traveling) electron scenario there are more spin-up (north pole up) than spin down (north pole down) electrons and the net difference gives rise to the magnetic properties that, for example, cause an iron compass needle to align with the earth's magnetic field. The situation rapidly becomes more complicated when the itinerant electrons scatter off impurities and/or defects, losing their itinerancy and eventually becoming localized (fixed in place) when the disorder strength, as characterized by the density of scattering sites, is at a critical value. This project will pursue experimental studies of magnetic thin films in which disorder can be systematically increased and the effect on magnetism studied. At critical disorder, itinerancy is lost and the magnetic metal becomes an insulator with localized electrons accompanied by the likely appearance of new magnetic phases with unusual spin alignments. This project will support the education of PhD students in advanced vacuum deposition and electronic/magnetic characterization techniques, which have already proven to be excellent training for productive scientific careers in academic and technology settings. The expected increased physical understanding of magnetism in thin films from these studies will be relevant to technological applications which incorporate ultrathin magnetic films into multilayer configurations for magnetic recording, spin generation, spin manipulation and/or spin detection.