R. Bansil
Polymer gels are unique materials in which the molecules are connected to make an infinite network. As a consequence, such materials have unusual elastic and flow properties in the sense that they behave neither like solids nor like liquids. Gels have many practical and technological applications, e.g., they are used to separate biomacromolecules, in all kinds of gel-based products, and in enhanced oil recovery. Many living tissues are in the form of gels, and the formation of gels plays a crucial role in problems such as the clotting of blood and aggregation of mucus. From a fundamental viewpoint, the physics of gels provides information of general relevance to the problems of random, porous media.
Laser light-scattering techniques are being used to study the structure of gels, the mechanism of gelation, and the diffusive movement of particles and polymers in gels. Our work has shown that diffusion in gels is qualitatively different than in solutions. Similarly, the kinetics of phase-separation processes in polymeric systems are examined using light-scattering techniques. The experiments are complemented by computer simulations of model gels and diffusion in gels. Physical Properties of Exotic Materials J. Brooks
Some examples of novel materials being studied include heavy electron superconductors, organic conductors, intercalates of graphite, high-transition temperature superconductors, small single-domain magnetic particles, and gallium arsenide heterostructures. In these materials, details of the electronic ground state, anisotropy, charge transfer, and lower dimensionality can profoundly alter the electronic, magnetic, optical and thermal properties. Measurements of these properties involve low temperatures (down to 25 mK), high magnetic fields (up to 45 Tesla) and high pressures (up to 15 kBar).
M.F. Crommie
A new field is rising between the more established areas of surface science and device physics. The goal here is to understand the physics of artificial structures fabricated at the atomic length scale. Boston University researchers will explore this area by building and investigating atomic-scale structures on metal and semiconducting surfaces. A scanning tunneling microscope (STM) will be used to build precise structures atom by atom.
To gain this ability, the group will construct a liquid-helium- cooled ultra-high vacuum (UHV) scanning tunneling microscope at Boston University. This device will have standard surface science tools for preparing and characterizing clean surfaces in UHV. The building blocks that will be used to fabricate atomic scale structures are adsorbates (atoms and molecules) that will be deposited onto liquid-helium cooled surfaces. One subject that will be studied is the interaction between atomic scale structures and the 2-dimensional electron gas known to exist on certain metallic surfaces. By confining these "surface state" electrons within atomically constructed boundaries, we will be able to study the quantum-mechanical eigenstates of various mesoscopic geometries. We will have the potential of actually mapping out, in real space, the quantum-mechanical state density of atomic scale waveguides, superlattices, Aharonov-Bohm rings, and classically chaotic structures. We also hope to explore the quantum-mechanical tunneling of whole atoms, the electromigration of adsorbates, transport in atomic scale wires, and the local electronic structure of magnetic impurities and clusters. The ability to manipulate matter at so fundamental a level allows us to realistically consider experiments that just five years ago would have seemed absolutely impossible.
M. El-Batanouny
Elastic and inelastic scattering of neutral thermal beams of helium atoms are used to study the dynamical properties of surfaces and the initial growth of ultra-thin metallic composite films. Boston University has been one of the pioneers in neutral helium atom scattering from solid surfaces and our facility is one of seven world-wide. The technique is the surface equivalent of thermal neutron scattering from bulk crystals, which has provided valuable information about bulk dynamics and structural-phase transitions over the past three decades. Our investigations have significant impact on both the understanding of basic physical phenomena, especially non-linear physics, and potential technological applications. For instance, most cutting-edge technologies involve thin-film growth processes in the synthesis of artificial materials, such as the surface modification processes in the fabrication of electronic chips and devices. Recent studies of the dynamics of hydrogen atoms on metallic surfaces have led to the discovery of the surface quantum motion of hydrogen -- thus shattering the conventional perception of localized hydrogen atomic bonds.
The surface physics group has recently pioneered a novel technique that employs spin-polarized metastable helium atoms to investigate, for the first time, the long-range magnetic (or spin) order of the surfaces of a class of crystalline materials known as antiferromagnets. Many of the new high Tc materials belong to this class. The studies provide valuable information about how changes in the atomic environment at the surface modify the long-range order of the spin structure. The technique probes whether all the surface electronic spins point in a single direction (ferromagnetic), are randomly oriented (non-magnetic), or exhibit a more complex spin ordering than observed in the crystalline bulk. The experimental effort is backed by large-scale computer simulations used to model dynamical and magnetic properties of surfaces, and thus provide detailed information about the microscopic mechanisms.
B. Goldberg
Near field scanning optical microscopy and spectroscopy (NSOM) is a recent technique that affords both imaging and spectroscopy of materials at resolutions far below the diffraction limit. An NSOM consists of a tapered optical fiber with typical apertures of 20 - 50 nm mounted to an atomic resolution piezo-electric x-y-z motion controller. The tapered optical fiber probe is placed within the near optical field of a sample and scanned over the surface. Because both the tip to sample separation and the tip aperture is a small fraction of the wavelength, the spatial resolution is not limited by the usual far field Rayleigh criteria to ~/2. In NSOM, the electric and magnetic fields at the sample are effectively confined to the tip diameter, and therefore can yield resolution as high as ~20nm, or ~/40 for visible wavelengths. An NSOM has been built and operated in the laboratory of Prof. Goldberg. Only the second such device in the world to operate at low temperatures, this NSOM has obtained images and spectra of individual quantum wires and dots with a spatial resolution of ~40nm. The current research emphasis is on examining the electronic states and spatial extent of wavefunctions in single quantum dots and wires. Near-term research will be extended to spectroscopically map the optical modes, doping profile, and strain related defects in high power semiconductor electronic and optoelectronic devices. In addition to studying semiconductors, the NSOM will play a large role in biomaterial optical recording, imaging live biological systems, magnetic domain recording media, and other photonic systems.
B. Goldberg
PbTe is a narrow-gap semiconducting compound that approaches a structural phase transition at low-temperatures. Due to this, the material has an anomalous dielectric constant, and recent measurements have pointed to a novel high field magneto-resistance transition, thought to be a kind of 1-dimensional Anderson transition. Projects are underway to utilize the high dielectric constant, low carrier mass, and ease of magnetically doping the material to examine many-body phenomena unattainable in standard III-V materials. Recent data have displayed the integral quantum Hall effect, and measurements of 1D quantum wires are in progress.
B. Goldberg
Low-temperature simultaneous transport and optical probes of 2-dimensional electrons confined at a GaAs-AlGaAs interface are revealing new physics about the way electrons interact. Luminescence and transmission spectroscopies are combined with resistivity measurements to elucidate a number of many-body electron effects. The optical response of 2D electrons to the presence of a photoexcited valence hole has been directly linked to transport phenomena. In high magnetic fields, the 2D electron states are split into a series of Landau levels. When the Fermi energy is in a localized state region between magnetically quantized levels and transport exhibits the well- known zero-resistance state, optical emission shows a strong, non-linear reduction of the electron screening. At higher magnetic fields and lower temperatures, the electrons form a liquid-like quantum state called the fractional quantum Hall fluid, and at the highest fields and lowest temperatures condense into a solid state know as the Wigner crystal. We have identified energy shifts, line splittings, and intensity changes in optical recombination, which are providing information about the collective state. Future experiments are planned to examine transmission in single quantum wells and to study the detailed interaction between carrier localization and changes in the electron screening in optical phenomena.
B. Goldberg, W. Skocpol
The main intellectual focus of this area is "mesoscopic phenomena," a generic term for quantum transport and quantum coherence phenomena in ultra-small structures. These phenomena are most clearly observable at low temperatures and in high magnetic fields. The general idea uses, among other things, recent advances in molecular lithography together with modern surface science techniques to make small electronic structures and devices. Ideally, these devices could display novel quantum properties and be tailored to probe specific quantum physics problems.
The Mesoscopic effort takes direct advantage of the Boston University Materials Research Facility (MRF), which has the capability of patterning materials down to 10nm (100Ao)size scales, using a new electron-beam scanning transmission electron microscope. One student is exploring the interaction between physical nanostructures and biological membranes. Another is building quantum point contacts and one-dimensional wires to examine the one-dimensional state transitions using far-infrared photoconductivity. Future students may study quantum effects in submicron electronic devices, drawing in part upon Skocpol's prior experience in this field at AT&T Bell Laboratories. Professor Skocpol has recently taught a graduate course based on the principle that recent progress in mesoscopic physics is laying the basis for a new perspective on solid-state physics, starting from quantum effects in the smallest structures and working up to larger size scales.
K. Ludwig
While many of the equilibrium properties of phase transitions are well understood, a basic understanding of the kinetics of these transitions remains elusive. The early stages of the transformation process are particularly challenging, as the relatively fast time scales make experiments difficult and the strong non-linearities make theory arduous. While x-ray scattering is traditionally the preferred tool for the study of structural phase transitions, it has generally been too slow to study the early-stage kinetics. Using time-resolved x-ray scattering at the National Synchrotron Light Source at Brookhaven National Laboratory, however, we have developed the ability to study kinetics with millisecond resolution, an improvement of over two orders of magnitude. We are using this capability to closely examine early-stage kinetics in metallic alloys undergoing ordering and spinodal decomposition. This work is in close collaboration with the theory group of Prof. W. Klein. We are now beginning to study the kinetics of spinodal decomposition and gelation in polymer systems as well. These systems exhibit a rich variety of dynamics and are ideal for studying the effect of interaction range on kinetics. Light scattering experiments parallel to our x-ray studies are being performed by Prof. R. Bansil's group.
T. Moustakas
Using the modern thin film growth techniques of Molecular Beam Epitaxy, Sputtering, and Chemical Vapor Deposition, we are pursuing a program in the growth and characterization of thin films as well as device fabrication. We are studying the properties of refractory semiconductors employing a novel Electron Cyclotron Resonance assisted MBE growth technique. These are large energy gap materials such as nitrides and metastable materials in the diamond family, which hold great promise for high temperature electronics and optical devices in the UV part of the spectrum. Recently we have succeeded in growing p-n junctions in wide gap GaN, a first which lays the groundwork for future optoelectronic devices. Other work includes the mechanical properties of metallic superlattices fabricated between metals or metal-ceramics and III-V compound semiconductors based on GaAs and AlGaAs fabricated with the intellectual focus on growth and the optoelectronic properties of nanodevices.
W. Skocpol
A major research effort is underway in High-Tc Superconducting Thin Films, relying heavily upon Boston University's unique Materials Research Facility. The High-Tc effort involves undergraduates as well as graduate students; students make, pattern, and measure device structures in high-quality epitaxial thin films of the recently discovered High Temperature Superconductors. A large number of questions are addressed, ranging from fundamental phenomena such as flux creep and proximity effect, to practical limits such as critical currents and rf losses in microwave devices. As a result of their efforts, Boston University is a Participating Member of the Consortium for Superconducting Electronics; this involves close collaborations and joint projects with AT&T, IBM, MIT, and MIT Lincoln Laboratories.
K. Smith
The goal of this research program is to understand experimentally how electronic structure determines the optical, electrical, structural, and chemical properties of novel materials. A powerful array of electron and photon spectroscopic probes is used to measure the electronic structure of a selection of unusual compounds. Some probes are well established, while the development of others is an exciting and important component of the program. Overall, this is an ambitious interdisciplinary program that combines elements of physics, chemistry, and materials science, and may ultimately address issues of fundamental scientific and technical importance.
Presently under study are an unusual class of transition metal oxides that display a plethora of unusual physical phenomena, including quasi-low dimensional electrical conductivity, metal-insulator transitions, periodic lattice distortions, charge density waves, and superconductivity.
Among the sophisticated spectroscopic probes utilized in this program are the following: (1) soft x-ray fluorescence spectroscopy, (2) ultraviolet spectroscopy, (3) x-ray photoemission spectroscopy, (4) inverse photoemission spectroscopy, (5) Auger electron spectroscopy, and (6) low-energy electron diffraction. These probes reveal complementary aspects of the electronic structure of solids and their surfaces. Photoemission is a powerful and well established probe of electronic structure; soft x-ray fluorescence spectroscopy, however, is an exciting new tool that is in the early stages of development.
The design and construction of a state-of-the-art compact grating-based soft x-ray fluorescence spectrometer is a major goal of this program. Experiments are being undertaken both at Boston University using conventional radiation sources, and at the National Synchrotron Light Source, Brookhaven National Laboratory, using synchrotron radiation. Specific topics being studied include: (1) band dispersion, spatial localization, and hybridization of d-electrons, (2) the nature of band gaps formed during temperature-induced metal-insulator transitions, (3) the structure of defects in the bulk and at the surface of these oxides, and (4) the low-dimensional Fermi surfaces.
G. Zimmerman
Research in superconductivity is concentrated on the high power applications of high temperature superconducting materials. Research is conducted on contacts between high Tc materials, and normal and superconducting metals. In this investigation, one finds analogues to semiconducting systems. Another problem in the utilization of high Tc superconductors is the rapid deterioration of the current carrying capacity with the application of a magnetic field. Magnetic shielding is being developed. In order to understand this problem, we are performing computer simulations of the behavior of arrays of superconducting islands connected by Josephson junctions.
G. Zimmerman
Research is being conducted on the difference between two and three dimensional behavior of magnetic transitions. Examples of materials studied are cerium magnesium nitrate, FeC3 graphite intercalation compounds (GIC), and others. Cerium magnesium nitrate undergoes a phase transition at about 1mK, while the magnetic transition temperature of the GIC is above 1K. The properties measured are specific heat, magnetic susceptibility, and magnetization measured by means of SQUIDS. Electronic properties are investigated by means of Schubnikov-deHaas and deHaas-vanAlphen effects at high magnetic fields. The data can be compared to various models that are simulated by computer calculations.