Department of Physics and Astronomy at the University of Pennsylvania // Condensed Matter Experiment Research

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Condensed Matter Experiment

Faculty: E. Burstein (emeritus), D. Durian, M. Drndic, M. Goulian, P.A. Heiney, A.T. Johnson, J.M. Kikkawa, A.G. Yodh

Biophysics and Biomaterials (Drndic, Goulian, Johnson, Yodh )

Physicists at Penn and elsewhere have recently turned increasing attention to the intricate and beautiful physics of biological systems. An explosion of new experimental techniques that probe and manipulate complex biological materials at the molecular level has allowed quantitative measurements of properties that were previously but the subject of speculation. Researchers in our Condensed Matter Group are particularly interested in exploring the physical properties of biological systems and biologically important molecules (e.g. DNA, proteins, lipids). In particular, we have developed a novel light scattering method to measure their elastic properties. While this new technique is being perfected, we are able to exploit it to perform novel measurements on other materials of great biological significance. This research effort involves strong interaction with our theory group.

In addition to illuminating the basic physics of the molecules and superstructures of living organisms, Penn physicists exploit modern methods of molecular biology and fluorescence microscopy to probe the networks of interacting proteins within cells. We are reengineering new networks in order to explore the limits, range of functions and design principles underlying biochemical circuits. At the same time we are constructing synthetic networks to further test these principles and to build novel biologically based devices. We are also creating entirely new classes of biologically-inspired materials that combine the wide variety of mechanical, electrochemical, and catalytic function of natural proteins with a robustness and simplicity uncharacteristic of life. Penn's NSF-funded Laboratory for Research on the Structure of Matter (LRSM) has made a major commitment to develop a set of synthetic peptides created in Penn's department of Biophysics and Biochemistry. Researchers in the Condensed Matter Group use scanned probe techniques to measure the local electrical and structural properties of self-assembled monolayers of these molecules. Their results feed back into investigations into the optical properties of these materials, and how the molecules can be engineered to create a new class of designer biomaterials. This work is a collaboration between researchers in Biophysics and Biochemistry, Chemistry, and Physics and Astronomy.
Carbon nanotubules and fullerenes ( Heiney, Johnson)

Carbon buckeyballs and nanotubes are but two of the "fullerenes", a family of beautiful, atomically perfect, and potentially useful macromolecules made of pure carbon. With the 1985 Kroto-Smalley discovery of the C60 molecule (^E"buckeyball") and subsequent methods for large-scale production of fullerenes, research into the chemistry, physics and materials science of these materials simply exploded. Solid forms of C60 can be metallic, semiconducting, insulating, or even superconducting depending on the degree of doping. In contrast, the electrical properties of single-walled carbon nanotubes (a single graphene sheet folded into a flawless cylinder) are strongly influenced by the geometric structure of the tube, allowing metallic, semiconducting or insulating ground states in the absence of doping. Penn researchers discovered a striking orientational ordering transition in crystalline C60: at temperatures above 250K, the molecular centers of mass are fixed but the molecules rotate freely, while at low temperatures the molecules lock into a three dimensional gear structure. We are measuring the astounding mechanical and electrical properties of perfect single-walled nanotubes. X-ray scattering measurements essential to this research are performed using in-house central facilities and synchrotron facilities at Brookhaven National Laboratory. We will use the Argonne Advanced Photon Source when it becomes operational. These projets involve collaboration among scientists from Physics and Astronomy, Chemistry, and Materials Science and Engineering.
Complex fluids and liquid crystals (Durian, Heiney, Yodh)

Many fascinating materials of great technological significance are easily deformed, so thermal excitations, small fluctuations, and disorder play a major role. Such systems include foams, emulsions, colloidal suspensions, liquid crystals, and polymers. Emulsions and colloidal suspensions contain objects in the micron size range, so their structure and dynamics are accessible to optical probes such as advanced optical microscopy, diffusing wave spectroscopy and laser tweezers. We measure their mechanical properties, which differ in intriguing and often useful ways from those of crystalline solids, using ultrasonic and other mechanical excitations. The systems actively being investigated include novel liquid crystal emulsions, colloidal suspensions, and two-component colloids where structural phase instabilities and self-assembly can be driven by forces of entropic orgin. We are exploring ways to create new structures with colloids by combining lithography, a tool of the semiconductor industry, with surface functionalization and colloidal self-assembly. Of particular interest are routes to creating photonic crystals, three-dimensional arrays of dielectric material that have a band structure for photons similar to the electronic band structure of atomic crystals.

Another important set of soft materials is based on liquid crystal molecules, which show order in fewer than three dimensions. Penn has played a leading role in the elucidation of the the structural phases of discotic liquid crystals and liquid crystal polymers in bulk and thin-film form. High resolution X-ray diffraction, thermodynamic measurements, and AFM characterization complement each other to provide an understanding of the local structure and long-range order in these materials. We also explore the electrical properties of these systems upon doping with electron donors and acceptors.
Nanostructure physics and quantum transport ( Burstein, Drndic, Johnson, Kikkawa )

Confinement-induced quantization profoundly alters the electronic, optical, mechanical, and magnetic properties of a nanostructure, whether it is a quantum dot formed by surface gates in a Ga[Al]As heterostructure, a carbon nanotube, or an organic macromolecule. Research projects at Penn focus on nanostructures fabricated both "from the top down" using optical and electron-beam lithography (e.g., 100nm quantum dots defined in a GaAs wafer), and "from the bottom up", where the nanostructure is a macromolecule or cluster created via a chemical reaction. We measure the electrical properties of nanostructures using low-noise transport from room temperature to the millikelvin regime, and magnetic fields up to 14 Tesla. Scanned probe technologies, including Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM), at temperatures as low as1.4 Kelvin, are used to determine local physical and electronic properties of nanostructures. Our newest facility allows the study of quantum spin transport by combining femtosecond nonlinear optics with low-temperature methods to explore the dynamics of spin-related phenomena in solids.

Extremely sensitive tests of our understanding of nanostructures can be done through experiments on single nanostructures or intentionally fabricated arrays, rather than random collections. With this idea in mind, we are developing techniques to electrically contact single nanostructures and macromolecules, using a combination of high-resolution electron beam lithography and chemically-controlled self-assembly. Penn's new Center for Advanced Imaging and Micromanipulation is developing novel instruments where individual molecules can be used as luminescence probes for scanning near-field optical microscopy or manipulated with optical tweezers and simultaneously probed with optical excitation and advanced microscopy.
Nonlinear optics and photonics ( Kikkawa, Yodh)

Penn physicists actively exploit nonlinear optical probes to gain insight into complex electronic and spin systems. For example, the microscopic physics of complicated, correlated electron systems such as chain-like and disc-like polymers have been deduced from their nonlinear optical susceptibilities. Amazingly enough, the nonlinear optical properties can be enhanced by orders of magnitude, or even change sign, when the molecule is first promoted to an excited state. The knowledge gained through these experiments enables the custom-design of nonlinear optical molecules for use in organic optoelectronic circuitry and optical fiber.

When combined with modern pulsed laser sources, optical non-linearities also provide time-resolved access to dynamical processes. Building on such methods with new resonance techniques, we explore the physics of interacting electronic spin systems and manipulate spin information in the solid state. These efforts have led to the first demonstration of room-temperature spin memory effects in a two-dimensional electron gas, the discovery of extremely slow environmental spin decoherence in doped semiconductors, and the demonstration of spin coherent transport over distances accessible to the naked eye.

We also use nonlinear optical probes to study notoriously difficult experimental systems like buried solid-solid interfaces. The basic experimental problem is that traditional optical spectroscopies lack interface sensitivity, and traditional surface diagnostics have a limited penetration depth. To solve these problems, we take advantage of the long penetration depth and intrinsic interface sensitivity of second-order nonlinear optical probes to study buried solid-solid heterojunctions. These experiments have successfully revealed striking new interfacial excitations, and structural information about the junction. For example, we discovered new quantum well states at the interface between two semiconductors, as well as defect states at metal-semiconductor interfaces. We are presently combining photomodulation spectroscopies with our nonlinear optical methods to provide information about charge traps, electric fields and Schottky barriers in this system class.
Nonlinear systems and chaos (Durian, Gollub)

Gollub's research is in the general area of Nonlinear Physics, which is concerned with the mesoscopic and macroscopic behavior of complex systems. His group has conducted experimental work on the following topics over the years: hydrodynamic instabilities and the transition to chaos and turbulence in fluids; the morphology of growing crystals; the dynamics of nonlinear waves; turbulent convection induced by thermal gradients; thin film flows; and frictional dynamics. Current projects include mixing in fluids, spatiotemporal (or space-time) chaos, and motion and frictional forces within granular flows.