Experimental Condensed Matter Physics (CME)
Members of this group:
- Richard Averitt,
- Rama Bansil,
- Michael El-Batanouny,
- Bennett Goldberg,
- Karl Ludwig,
- Raj Mohanty,
- Theodore Moustakas,
- William Skocpol,
- Kevin Smith,
- Ophelia Tsui
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.
Research Descriptions
Applications of Nanomechanical Devices: Wireless Communications and Sensors
Our research in this specific area focuses on the development of ultra-high-frequency (UHF) oscillators, filters and frequency-selective elements for eventual use as high-speed sensors and communication systems. Our systems are nanomechanical oscillators, and they vibrate at speeds exceeding 2 GHz—the highest mechanical frequency ever reported. Because of their mechanical construct and the small size, they will enable a host of future applications. Currently, we are exploring specific applications, which include nanomechanical memory elements for high-speed high-density data storage and gigahertz-range nanomechanical oscillators for use as frequency-selecting device in cellular phones.
Dewetting of Polymer Films
Dewetting is a phenomenon in which a liquid film of uniform thickness on a non-wetting substrate surface breaks up and develops into liquid beads. In some sense, this process can be viewed as a phase separation process, wherein a homogeneous polymer film phase separates into regions deficient of the polymer (i.e. holes) and regions rich in the polymer (i.e. polymer beads). Though simple as this analogy may sound, for a decade opinions have been divided between spinodal instability (which is tie to an unstable system) and heterogeneous nucleation (tie to a metastable system) being the major cause of the instability. We devise experiments to be able to make the distinction unambiguously and hence delineate applicability of the classical linear theory of spinodal decomposition in regimes close to metastable regions.
Besides studying the fundamentals, we also explore possibilities to make use of this spontaneous dewetting process to make useful mesoscopic structures in large areas. Figure 1 shows one kind of pattern that can be made by using this approach. It is interesting to note that the pattern contains features of sub-micron sizes. Normally, to make features with such sizes will require sophisticated techniques such as photo- or e-beam lithography. But with our approach, it is as simple as roasting the sample on a hot plate for a few minutes.
Experimental Surface Physics
Elastic and inelastic scattering of neutral thermal beams of helium atoms or metastable 3S helium atons are used to study the dynamical and magnetic properties of surfaces.
The surface physics group at Boston University has been one of the pioneers in neutral helium atom scattering from solid surfaces in the mid 1980s, and our facility is one of seven worldwide. 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.
Helium atomic scattering was used by our group to study the structural and dynamical evolution during the initial growth of ultra-thin metallic composite films. These investigations have impact on the understanding of basic physical phenomena, especially non-linear physics, as well as 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 and magnetic devices.
Another direction of investigations using helium scattering were 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 at Boston University has pioneered, in the 1990, a novel technique that employs spin-polarized metastable helium atoms to investigate, for the first time, the long-range magnetic (or spin) ordering on surfaces of a class of crystalline materials known as antiferromagnets. In this class of materials the net magnetization is zero, but the direction of the spins alternate from one direction at some site to the opposite direction at neighboring sites. 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. It also has the capability of providing detailed information about propagating surface magnetic (or spin) waves, such as relations between their frequencies and wavewlength. The 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.
One of the interesting results obtained with this technique, revealed the unique behavior that on the crystalline surface of CoO, the degree of magnetic-ordering increases with temperature over the range 250K to 320K!
High Resolution 4Pi Microscopy
Confocal fluorescence microscopy has developed into a standard tool in cell biology research; Light can easily penetrate inside the cell and furthermore, a fluorescent dye can be made to interact with specific cellular components, for example attach to an antibody that binds to a cellular protein. The resolution of confocal microscopy is ca 0.5 um laterally and 0.75 um axially.
The axial resolution of a conventional confocal microscope can be improved by a factor of 3-5 in 4Pi microscopy. A 4Pi confocal fluorescence microscope uses two opposing, high numerical aperture objectives, shown in Fig 1. The counter propagating wave fronts of the illumination form interference fringes at the common focal point of the two objectives. Likewise, the collected light is interfered at the detector. This effectively reduces the axial focal volume compared to conventional confocal microscope, illustrated in Fig. 2. The measured point spread function for a fluorescent 100 nm bead is shown in figure 3. The improvement in axial resolution using two objectives (b) compared to standard confocal microscopy (1) can clearly be seen.
We are now combining 4Pi microscopy with an interferometric method we have developed, spectral self-interference fluorescent microscopy. The technique transforms the variation in emission intensity for different path lengths used in fluorescence interferometry to a variation in the intensity for different wavelengths in emission, encoding the high-resolution information in the emission spectrum. Using monolayers of streptavidin, we have demonstrated better than 5nm axial height determination for thin layers of fluorophores and built successful models that accurately fit the data.
Microring Resonator Biosensors
Microring resonators provide high sensitive label-free optical biosensor platforms. The light coupled into the resonator via a waveguide is confined within the microring cavity due to total internal reflections and high-Q resonant modes (Q~12000) are formed. The positions of these modes depend on the effective index of the resonant structure and thus get shifted when there is a molecular interaction on the surface. This shift can be determined with high precision using our method of detection. We have accomplished to demonstrate biosensing application of the microring resonators by investigating a well studied binding event (Avidin-Biotin complex). The high sensitivities (1.8×10-5 refractive index units) obtained with this method are comparable to commercially available surface plasmon resonance devices.
The detection methods that are currently available deliver high sensitivity and specificity; however they fail to provide a clear path to compact optical packaging that will lead to portable devices for field use. Towards this goal, and with the efforts of our interdisciplinary research team and industrial collaborators, we are currently modifying our table-top, fiber-coupled system to develop a cost- efficient, compact integrated biosensor platform.
Novel Materials Laboratory
The goal of this research program is to understand experimentally how electronic structure determines the optical, electrical, magnetic, structural, and chemical properties of new and unusual materials. A powerful array of electron and photon spectroscopic probes is used to measure electronic structure. Some of these probes are well established, while the development of others is an exciting and important component of our research efforts. This is an ambitious interdisciplinary program that combines elements of physics, chemistry, and materials science, and addresses issues of fundamental scientific and technical importance.
The spectroscopies used in our studies are soft x-ray emission, resonant inelastic x-ray scattering, soft x-ray absorption, angle-resolved photoemission, and core level x-ray photoemission. All of our experiments are performed off-campus at synchrotron radiation light sources. We presently run experiments at
i) the National Synchrotron Light Source, Brookhaven National Laboratory in New York,
ii) the Advanced Light Source at Lawrence Berkeley National Laboratory in California, and
iii) the MAXLAB synchrotron in Lund, Sweden.
Currently, five distinct classes of materials are under investigation: low-dimensional and correlated solids, organic metals and semiconductors, III-N nitride semiconductors, transparent conducting oxides, and rare-earth nitrides. Each of these independently funded projects is described below.
1. Many Body Physics in Low- Dimensional and Correlated Solids
Correlated and low dimensional solids present important challenges to modern solid-state physics. While our knowledge of the origin the physical properties of many simple solids is both comprehensive and sophisticated, this is not the case in low dimensional and correlated solids. From quasi-low dimensional conductors, through magnetoresistive oxides, to high temperature superconductors, there exists a plethora of physical phenomena displayed by these materials that remain poorly understood. Transition metal oxide bronzes are inorganic quasi-low dimensional correlated solids that are ideal prototypical systems for spectroscopic studies, due to the large size and high quality of available crystals. Among the unusual properties of these oxides are low-dimensional electron transport, metal-insulator and metal-metal transitions, periodic lattice distortions and charge-density-wave transport. Many of these phenomena are related to the coupling between vibrational modes of the lattice with the electrons at the Fermi level. Of particular importance in this coupling is the structure of the Fermi surface itself. Despite its fundamental role, the structure of the Fermi surface in low-dimensional systems is generally undetermined. Our research program addresses numerous aspects of the physical nature of these oxides and combines the information on each topic into a coherent model for their low dimensional properties. We measure quasi-low-dimensional Fermi surfaces, metal d-band dispersion, spatial localization and hybridization of the d-electrons, bonding interactions at different sites in the lattice, the creation of energy gaps at the Fermi level, and the formation and structure of point and extended defects.
2. Organic Semiconductors and Metals
Organic electronic materials are of interest due to both the technological promise of carbon-based devices and to the scientific challenge posed by these materials to our understanding of basic physical processes in complex solids. Despite the importance of semiconducting organic materials, there have been few studies of their valence band electronic structure using modern synchrotron radiation-based spectroscopic methods. In part this is due to the difficulty in preparing clean samples for study, but also due to the severe x-ray radiation beam damage suffered by these materials. We are undertaking a comprehensive synchrotron radiation-based soft x-ray spectroscopic study of a variety of thin film organic electronic materials, including semiconductors, metals, and molecular magnets. The organic films are grown in-situ, with proper attention paid to minimizing beam damage effects. The films are grown in a custom built organic molecular beam epitaxy chamber attached to the spectrometer chamber at a soft x-ray undulator beamline at the National Synchrotron Light Source. Information is obtained on the element and site specific valence band partial density of states, band dispersions, chemical state, and orbital bonding in the films. Our work on novel organic metals is undertaken in collaboration with Professor Linda Doerrer in the BU Department of Chemistry, whose group synthesizes the materials.
3. III-Nitride Semiconductors
Our third area of research concerns the surface and interface properties of nitride semiconductors. GaN and related III-V nitrides (InN, AlN) are candidates for use in high-temperature microelectronic devices and used in blue wavelength light-emitting diodes and lasers. Despite success in the growth and application of these materials, the basic physics underlying the intentional doping of the films, the nature of defects, and the formation of metal overlayers remains to be determined. All of these problems are related directly to the detailed electronic structure of the nitrides, and yet this structure has not been extensively measured. The development of these nitrides as electronic materials requires that their defect and dopant electronic structure be fully understood. In collaboration with Prof. Ted Moustakas from the BU College of Engineering, we undertake a comprehensive determination of the detailed electronic structure of thin film refractory nitrides. Specifically, the electronic structure of defects and dopants in these nitrides is being studied, as well as the electronic structure of the interface between the films and their substrates, and between the films and metal overlayers. Furthermore, the chemical reactivity of the films (in particular, when defective or doped) is being investigated in order to understand the nature of adhesion of these nitrides to both substrates and overlayers, and to understand the chemical stability of the films at elevated temperatures or in model corrosive environments.
4. Transparent Conducting Oxides
Transparent conducting oxides (TCOs) play a crucial role in thin film solar cells. Central to the operation of such cells is the ability to have solar radiation penetrate the cell and be absorbed, with the light efficiently converted to an electrical current that can be extracted. To accomplish this in thin film solar cells, the outer body of the cell needs to be optically transparent yet electrically conducting. TCOs are the enabling technology in this regard. There is an inherent competition between optical transparency and electrical conductivity, and great efforts are being expended to achieve low resistivity TCOs that can be doped reproducibly both n-type or p-type. Since the interest in TCOs lies in their electronic properties, a comprehensive study of their electronic structure is of vital importance in understanding the properties of these materials, improving their synthesis, and discovering new TCOs. We are measuring the detailed surface and bulk electronic structure of novel TCOs using our set of complimentary synchrotron radiation-based soft x-ray spectroscopies. The TCOs are synthesized by Professor Russ Egdell, from the Department of Chemistry, Oxford University. Our measurements hold the promise of significant impact in the area of TCOs for use in solar energy conversion.
5. Rare-Earth Nitrides
The rare-earth nitrides form a closely related family of magnetic semiconducting or semimetallic materials, exhibiting interesting properties because of the strongly correlated 4f electrons. We use our array of spectroscopic techniques to measure the electronic structure of rare-earth nitride thin films grown in-situ, as well as films capped with suitable protective layers grown by our collaborators in New Zealand: Professors Joe Trodahl and Ben Ruck from Victoria University. The spectroscopies provide unique and complementary information on the energy position, splitting and degree of hybridisation with other states of the occupied and empty 4f- states. Resonant inelastic x-ray scattering provides additional insights into the excited states of the 4f system and into other many-body effects related to these strongly correlated electrons. Calculations of the electronic structure are undertaken using the LSDA+U approach (local spin-density approximation complemented with orbital dependent Coulomb and exchange interactions) and implemented in the full-potential linear muffin-tin orbital method by Professor Walter Lambrecht from Case Western Reserve University. Calculations of the spectroscopic quantities are being developed to extract the maximum information from the experiments. The theoretical analysis of the spectra will identify the need for theoretical developments beyond the mean-field approximation embodied by current LSDA+U, such as integrating atomic multiplet splitting theory with band structure approaches and dynamic mean field theory. Furthermore, calculations of optical conductivity from interband transitions are carried out and correlated with measurements of the same quantity from the infrared to the near UV to be carried out by the NZ group. The calculations also tie the electronic structure to magnetic properties by calculation of the magnetic exchange parameters.
Numerical Aperture Increasing Lens Microscopy (NAIL)
Numerical Aperture Increasing Lens (NAIL) microscopy is a far-field subsurface imaging technique that simultaneously enhances the light gathering power and resolution of an optical microscope. When a NAIL is placed on the backside of a sample, its convex surface effectively transforms the NAIL and the planar sample into an integrated solid immersion lens, capable of aberration-free imaging of the structures underneath the substrate. Addition of the NAIL to a standard microscope increases the numerical aperture (NA) by a factor of the square of the optical index n. The NAIL technology has had the greatest impact in the field of optical failure analysis of Si integrated circuits. In silicon, the NA is increased by a factor of 13. Using an optimized confocal microscope, we have already demonstrated a lateral resolution of 230 nm. Recently, we have applied the technique to optical spectroscopy of single quantum dots demonstrating an 8-fold improvement in light collection from a single dot.
Optical Properties of Carbon Nanotubes
Since the accidental discovery of a new form of carbon in 1991, carbon nanotubes have attracted wide-spread interest due to their remarkable properties. It is the strongest, stiffest and toughest material known as well as the best possible conductor of heat and electricity. A nanotube can be thought of as a rolled up sheet of graphite, with typical diameters around 1-2 nanometers for single wall carbon nanotubes, and a length-to-diameter aspect ratio of up to 108. The nanotubes can be either metallic or semiconducting, depending on the chirality and diameter of the nanotube, characterized by the roll-up vectors (n,m). The variable and direct bandgap for the semiconducting tubes makes for potentially powerful photonics applications.
In our lab we specialize in optical measurements of individual carbon nanotubes so that we are able to measure properties that are not possible to extract in ensemble measurements. For example, a combination of applied strain and strain measurements using resonant micro Raman showed that previous strain measurements of CNTs in composites overestimated the strain applied to the nanotubes by a factor of 4, indicating problems with adhesion between the composite and the nanotubes.
Due to the strong optical resonances typical for a one dimensional material, we are also able to use resonant Raman to map the optical resonance energies and correlate the energies to the specific tube diameter and chirality and gain understanding of the strong environmental influence on the excitonic binding energies.
See ultra.bu.edu for further information.
Quantum Computing: Quantum Control of Coherence of the Electron Wave Function
This project involves the development of quantum control techniques to externally control coherence properties of electron wave functions, or to increase the coherence time of electron wave functions in sub-micron and nanoscale mesoscopic systems. The main thrust behind our approach is to develop enabling technologies for increasing the coherence time scales from the usual nanosecond range to microseconds or even milliseconds. The long-term direction is to perform picosecond time-domain reflectometry and a host of quantum control techniques (open-loop, closed-loop learning feedback and bang-bang).
Quantum Nanomechanics: Quantum Motion to Testing the Limit of Quantum Mechanics
Our current research efforts in this area involve the observation of quantized displacement, energy quantization and Rabi oscillations in macroscopic mechanical oscillators. We have already observed the first two phenomena. In the next few years, we will explore approaches to use these mechanical systems as quantum bits, particularly for quantum information processing. Furthermore, we are exploring foundations of quantum mechanics with these experiments, which involve the largest quantum systems ever realized in a laboratory.
Real-Time X-Ray Studies of Ion Bombardment/Plasma Processing
K. Ludwig, G. Ince-Ozaydin, Y. Wang, A. Ozcan,
Ion bombardment of surfaces is at the core of sputtering and plasma processing technologies that are vital to several of the largest manufacturing industries in the world. However, recent experimental and theoretical studies of surface morphology evolution during ion sputter erosion suggest that there is a rich variety to surface morphology evolution during ion bombardment. Our group currently focuses on the low energy Ar+ ion bombardment of Si and GaSb surfaces. We study the evolution of Si and GaSb surface morphology using real-time x-ray scattering techniques and ex-situ AFM analysis. Synchrotron-based real-time x-ray studies offer at least five important advantages in gaining fundamental atomic-level insight into such processes: penetration of ambient gaseous/liquid environments, ability to vary depth of structural sensitivity, ability to probe structures from 0.1-100 nm in length scale, subsecond temporal resolution, and ease of interpretation. To take advantage of these powerful attributes, we have constructed a new facility in the back hutch of NSLS beamline X21, funded with NSF-MRI and NSF-IMR support, which is being spearheaded by collaborators at the University of Vermont (R. Headrick), Boston University (K. Ludwig and T. Moustakas).
Resonant Cavity Imaging Biosensor (RCIB)
The Resonant Cavity Imaging Biosensor (RCIB) detects binding between target biomolecules from a sample and probe biomolecules fixed to a microarray surface with the potential for tens of thousands of simultaneous parallel observation sites. Such ability yields information about the affinity of the biomolecules under test for the molecules on the capturing surface. Information about the affinity between molecules of interest such as particular proteins or DNA strands, yields great benefit to a number of applications in biological research, medical diagnostics, and biohazard detection. Current high-throughput microarray technology requires that the target molecules be labeled with a fluorescent dye. At best, this preparation step adds an acceptably small amount of time and money, but at its worse, can be prohibitively difficult depending on the nature of the application.
RCIB operates label-free without the need to add fluorescent labels or otherwise modify the target molecules in any way. An optical IR beam couples resonantly through a cavity constructed from Bragg mirrors that contains the microarray surface; the wavelength of the IR beam is swept using a tunable IR laser source; and an IR camera monitors cavity transmittance at each pixel, creating a highly parallel signature of transmittance versus wavelength for the microarray surface. This novel technique is enabled by high quality silicon substrates with buried Bragg reflectors previously developed within our group for improved photodetectors. The technique additionally relies on the use of commercial telecommunications hardware that has become readily available in recent years. In an alternative approach for microarray detection, the reflection from the substrate is measured with the varying wavelength. When binding occurs on the surface of the wafer, reflectivity vs. wavelength curve shifts, from which the height information can be extracted. This alternative approach is less sensitive than RCIB, but it draws attention with its simplicity. RCIB improves on existing label-free methods by offering dramatically improved throughput necessary to meet the needs of the microarray user community.
Spintronics: Control of Spins with Nanomechanical Torque
Our current research efforts in this area involve a new proposal for spintronics: spin detection and control can be done mechanical torque. We have recently proposed a comprehensive technique to carry out a series of experiments for demonstrating spin current and spin transfer across a hybrid junction of nanowire, fabricated on top of a two-element suspended torsion oscillator with sub-micron features. The proposed plan of this 3-year project is to demonstrate spin detection and control. This project involves measurement of single-spin conductance and the value of Planck’s constant.
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