Faculty Members -- R. Bansil, J. Brooks, M.F. Crommie, M. El-Batanouny, B. Goldberg, K. Ludwig, T. Moustakas, K. Smith, W. Skocpol, G. Zimmerman
Research Faculty & Associates -- M. Kaplan, Y. Negm
Graduate Students -- E. Aifer, G. Athas, K. Breuer, M. Bunea, S. Chen, G. Liao, V. Madhaven, O. Malis, M. Manfra, M. Marynowski, G. Morales, G. Rhodes, H. Robinson, S. Roy, A. Smirnov, C. Stagarescu
Research in this area is involved with the structure and behavior of condensed states of matter where interactions between adjacent atoms, molecules, and electrons determine the physical nature of the material. Condensed matter physics occupies a unique niche in physics. Developments in this field are often of equal importance for technological applications, as well as for our fundamental understanding of the nature of matter. One need only consider the revolution caused by the development of the transistor, and a similar one likely to follow from the discovery of high-temperature superconductors. Boston University has undergone rapid growth in condensed-matter physics and has strong research efforts in scattering physics, surface physics, advanced materials physics, low-dimensional electronic systems, low-temperature physics, and structural biology and chemistry. The Physics Department is also playing a major role in the new Center for Photonics Research, recently established at the University. This effort, as well as many others in condensed-matter physics, work in close association with scientists in the College of Engineering.
Experimental Studies of Gels -- 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.
Atomic Scale Surface Structure -- 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.
The scanning tunneling microscope under construction in Prof. Crommie's laboratory.
To gain this ability, the group is constructing 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.
Atomic Scale Structure: a scanning tunneling microscope image of iron electrons on a copper substrate produced in collaboration with Prof. M. Crommie at the IBM Almaden Research Center.
Experimental Surface Physics -- 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. These 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.
Studies of IV-VI PbTe Thin Films -- 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.
Near Field Scanning Optical Microscopy and Spectroscopy -- 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 ~lambda/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 ~lambda/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.
Optical and Transport Properties of 2-Dimensional Electron Gases -- 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.
Advanced Electronic Material -- 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.
Synchrotron X-ray Scattering Studies -- 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.
Growth of Artificially Structured Materials -- T. Moustakas
Using the modern thin film growth techniques of Molecular Beam Epitaxy, Sputtering, and Chemical Vapor Deposition, the condensed-matter group pursuing a program in the growth and characterization of thin films as well as device fabrication. They 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 Moustakas has 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.
High-Tc Superconducting Thin Films -- 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.
Electronic Structure in Novel Materials -- 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 will address issues of fundamental scientific and technical importance.
Presently, three classes of materials are under investigation:
1) quasi-low dimensional transition metal oxides,
2) organic metals and superconductors, and
3) thin film wide band gap semiconductors.
Both the metal oxides and the organic conductors display a host of unusual physical phenomena, including quasi-low dimensional electrical transport, metal-insulator transitions, periodic lattice distortions, charge density waves, and superconductivity. The fundamental electronic origins of many of these phenomena are poorly understood, and current research aims to rectify this situation. While this aspect of the work deals with problems of fundamental physical significance, a third project, dealing with thin films of wide band gap semiconductors, will explore the problems of immediate technological significance. The condensed-matter group is studying the electronic and structural properties of surfaces an interfaces in GaN and related III-V nitride semiconductors. This project on the wide band gap semiconductors is performed in collaboration with Prof. Moustakas.
Numerous spectroscopies are used in the study of these systems, including soft x-ray emission, angle-resolved photoemission, angle-integrates photoemission, inverse photoemission spectroscopy, Auger electro spectroscopy, and low energy electron diffraction. Each probe reveals different aspects of electronic and geometric structure, and when used in concert, they allow a comprehensive model of the properties of these solids to be developed.
The experiments are performed both in Smith's lab at Boston University, and at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory. B.U. researchers are members of the two Participating Research Teams (PRT) at the NSLS. The first of these is centered around beamline U4A on the UV ring, where the team has 50% of the operating time. This facility is where all of our angle resolved photoemission data are obtained. Furthermore, our x-ray emission spectrometer will be stationed on the high intensity undulator beamline X1B on the x-ray ring; the group is also a member of the X1B PRT. A post-doctoral research associate from the group is stationed full time at the NSLS.
High-Tc Normal-Superconducting Interfaces
-- 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, Zimmerman and his group are performing computer simulations of the behavior of arrays of superconducting islands connected by Josephson junctions.
Magnetic Measurements in 2+epsilon Dimensions -- 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.