Condensed Matter Physics Graduate Program

Much of the explosive growth in Condensed Matter Physics in the last decade has come at the mutual boundaries of physics, chemistry, and materials science. Recently in ever more dramatic ways, this intellectual cross-fertilization is producing key discoveries at the convergence of these fields and biology. The Penn Condensed Matter Physics group helped to create this fruitful trend, and many of our projects are highly interdisciplinary in nature.

Penn has a long and successful history of forefront research in condensed matter physics with seminal contributions to superconductivity, conducting polymers, liquid crystals, colloidal physics, and many other fields. There are currently 7 experimentalists and 7 theorists pursuing research in many areas of condensed matter physics.

Condensed Matter Experiment

• 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.
  
  

Polystyrene spheres arranged on a glue grid. Can be used as a template to form non-random close packed structures. Prepared by Keng Hui-Lin of the Yodh Group.

A molecular circuit created and studied in the Johnson Lab.

Professor Charlie Johnson and graduate student David Bergeron use a scanning tunneling microscope to examine single-atom steps on a specially prepared gold substrate.

Condensed Matter Theory

• Biophysics and Biomaterials ( Kamien, Liu, Lubensky, Nelson)
  There is a cultural gap between physics, which seeks to understand extremely simple systems, and biology, which is obliged to study the overall behavior of extremely complex systems. One area where simplicity and life intersect is the study of biomembranes, both in isolation and as active elements of cells. In recent years amazing new experimental techniques, including optical tweezers, have opened up the study of both the equilibrium conformations and dynamic response of membranes to quantitative analysis. Similarly the DNA molecule, while architecturally complex, has remarkably simple elastic behavior governed by just a few parameters. Apart from their potential relevance to biology, the study of these systems has revealed new physical phenomena characteristic of the highly-fluctuating micron world. At UPenn we currently have groups developing the theory of lipid bilayers, the physics of single DNA molecules, as well as the structure of condensed DNA in vitro and in vivo.
  
  
  
  Photomicrograph of a shape transition induced in an artificial bilayer vesicle by laser tweezers, studied by Nelson
  
  
  
• Carbon Tubules and Fullerides ( Harris, Kamien, Kane, Mele)
  Over the last few years a spectrum of new solid phases of carbon have been discovered which are derived from the carbon molecule C₆₀, and more recently from carbon nanotubules which are formed by wrapping graphitic sheets into compact cylindrical forms. These solids span an impressive range of electronic behavior: by moderately varying the degree of doping in the fullerides one finds phases with insulating, magnetic, and even superconducting ground states. New methods have been developed for controlling the growth of solid phases of single wall carbon nanotubes, and these structures hold great promise for new applications which exploit their unique electronic and elastic properties. We are actively investigating the electronic properties in these molecular solids, with a focus on understanding the phase equilibrium in these systems, and the effects of the underlying molecular symmetry, the orientational registry in the solid phase, and the quenched disorder in the condensed phases on their macroscopic electronic properties. The work is carried out in close collaboration with experimental work in the Department of Physics and Astronomy and the Department of Materials Science and Engineering on these systems.
  
  
• Liquid Crystals and Molecular Crystals ( Harris, Kamien, Lubensky)
  The theory of liquid crystals and liquid crystalline polymers runs the gamut from the rheology of complex fluids to the esoterica of homotopy and topological defects. All aspects of theory are intimately influenced and connected with experiment. This places liquid crystal physics in an exciting position: it is driven by both theory and experiment and by both the abstract and the applied. Specifically, chiral molecules are ubiquitous in nature and exhibit many liquid crystal phases. They are extensively studied by biologists, chemists and biophysicists. There is still much theoretical work to be done on the wealth of experimental data. We are actively pursuing the theory and prediction of new liquid crystalline phases, in particular equilibrium defect phases such as the twist-grain-boundary (TGB) phase of smectic liquids crystals, the analog of the Abrikosov flux lattice phase of a superconductor. Many of these defected phases are chiral and we are also actively engaged in understanding the origin of chiral interactions in crystal and liquid crystal phases. The theory group actively collaborates with local experimentalists and is part of the UPenn soft condensed matter physics effort.
  
  
• Polymer Physics ( Kamien, Liu, Lubensky, Nelson)
  Polymers are an important component and fuel in the modern world and, as a result, are carefully studied. Typically, the theory of liquids treats the constituents as featureless. However, because of their large lengths, polymers can not be treated this way. Recent progress in experimental technique has allowed direct observation of and interaction with these macromolecules. We are pursuing the physics of both single polymer molecules as well as ensembles of many polymers, studying both the static, equilibrium behavior, as well as dynamical behavior. Because of their long lengths, the dynamics of polymers is topologically obstructed through the linking and entanglement of the polymers. Part of our program is to study this "topological dynamics". We work closely with groups at Penn and nearby at the National Institutes of Health.
  
  
• Strongly Correlated Electrons in Low Dimensions( Kane, Mele)
  There are broad classes of degenerate electronic systems to which the usual Fermi liquid theory of the interacting electron fluid is inapplicable. Remarkably, these are not at all rare situations and the consequences of its failure are often dramatic. The discovery in the late 1980's of high Tc superconductivity in a family of cuprates which are doped away from a strongly correlated insulating state is perhaps the most celebrated example, and this discovery has renewed theoretical interest in understanding this long standing problem. Unfortunately theoretical methods for analyzing the degenerate electron fluid in the presence of strong interactions remain poorly developed in three dimensions. The situation is much more amenable to careful study in low dimensional systems, and in particular a solvable theoretical model, the Luttinger model, exists and describes an interacting electron problem in one dimension. We are undertaking theoretical work to further understand the predictions of the Luttinger model for the measured transport coefficients as functions of temperature, frequency and applied fields. This theory has direct applications in recent experiments studying transport in semiconductor heterostructures which effectively confine the electron motion to one dimension, to edge state transport in the 2D electron gas in the quantum Hall regime, and potentially to new structures which are derived from carbon nanotubes.
  
  
• Strongly Fluctuating Systems ( Harris)
  Fluctuations can become dominant in several situations which we have studied recently. Near a continuous phase transition even weak randomness can lead to modifications both to critical exponents (as has been known for a long time) and to the nature of the distribution function describing observables in the limit of large system size. Another example is an insulating magnetic systems in which the magnetic structure is not stable at the level of mean field theory, but is stable when fluctuations are taken into account, an for an antiferromagnet on an fcc lattice, as typified by the insulator MnO. This phenomenon also occurs in lamellar copper oxide family of antiferromagnets (which, when doped, lead to high-temperature superconductors). Here the mean field at a spin at (or directly over) the center of a square plaquette of antiferromagnetic spins vanishes if isotropic interactions are assumed. The resulting structural degeneracy can be removed by either fluctuations (ignored in mean field theory) or by pseudo-dipolar interactions arisiong from the anisotropy of the exchange interaction. A proper treament of these interactions allowed us to understand the otherwise mysterious sequence of spin reorientation transitions in the noncollinear magnet Nd₂CuO₄ (as well as the absence of such transitions in isostrucural Pr₂CuO₄) and the complex spin-wave behavior of Nd₂CuO₄ and the antiferrmagnet Sr₂Cu₃O₄Cl₂ which also displays weak ferromagnetism. This work involves a collaboration with Tel Aviv University and MIT.
  
  
• Surfaces and Interfaces ( Mele)
  At the surface of a crystalline solid the symmetry of the ordered bulk phase is broken, and one frequently encounters new structural, elastic and electronic phenomena which are not possible in the perfectly ordered bulk environment. These are important for providing a microscopic understanding of the surface effects which determine growth modes for epitaxial thin films, the equilibrium height profile of a growing interface and the pathways by which surface chemical reactions can occur. We are studying electronic, structural and elastic effects at solid surfaces by using quantum mechanical first principles electronic structure methods to determine the interatomic forces, equilibrium relaxed structures, and microscopic elastic properties at the surface. These are incorporated into appropriate long wavelength elastic theories which are then used to study stress distributions, and to explore the spectra of elementary elastic waves at solid surfaces. Presently, we are applying these methods to study the dynamics of vicinal surfaces which are miscut slightly away from a low index direction; here one finds particularly large atomic relaxations and new physical effects. We are extending these methods to investigate the possibility of surface vibrational spectrosopy as a probe of elastic properties of buried interfaces 1-5 nm below the surface, and which are difficult to access using traditional surface sensitive spectroscopies. This work is carried out in collaboration with theorists in the Department of Chemistry, and with an experimental group in the Department of Materials Science and Engineering.
  
  
  Model for interactions between chiral molecules, studied by Harris, Kamien and Lubensky.
  
  Gene Mele, students, postdocs and faculty meet in the electronic structure group meeting.
  
  The reflection of electron waves on nanotubes can lead to STM images like these, as predicted by Kane and Mele.
  Graduate Brochure / Our address