Clare C. YuProfessor, Physics & Astronomy |
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Research Interests |
Condensed Matter Theory | |
| URLs | www.physics.uci.edu/faculty/yu.html | |
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Academic Distinctions | Alfred P. Sloan Fellow | |
| Appointments |
University of Illinois at Urbana-Champaign; Los Alamos National Laboratory |
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Research Abstract |
Professor Yu has a broad range of research interests which include disordered systems, biophysics, noise, and quantum computing. BIOPHYSICS OF INTRACELLULAR TRANSPORT: A cell is like a city. It has all the basic infrastructure that a city has. For example, it has power plants (mitochondria), workers (proteins), a library (genome), recycling centers (lysosomes), etc. A cell also has a transportation system that works like container shipping. There are interstate highways (microtubules) and local streets (actin filaments) as well as trucks or motors (kinesin, dynein, and myosin) that pull large cargo vesicles along the roads. We are working with Steven Gross' group (UCI Dept. of Cell and Developmental Biology) to understand how the motors and roads conspire to get cargo vesicles where they need to go. The motion of the vesicles does not proceed smoothly in one direction. Rather it can frequently reverse direction along one road or switch roads as it diffuses through the cell. So how does the cargo get to where it needs to go? To answer this question, we are using a variety of theoretical techniques including computer simulations, graph theory, time series analysis, noise analysis, and chaos theory to analyze the motion. DISORDERED SYSTEMS: Disordered systems such as glasses and spin glasses. The physics of glassy systems is one of the most interesting and least understood problems in condensed matter physics. In the field of disordered systems, the Yu group is investigating several topics including the glass transition, Coulomb glasses, dipolar glasses, and the low temperature properties of glasses. The Glass Transition: Is a glass is a liquid or a solid? Is the glass transition a thermodynamic phase transition or a kinetic slowing down of the molecules? No one knows the answers. Professor Yu's group is currently studying the glass transition using molecular dynamics simulations. They recently showed that the glass transition as signaled by a peak in the specific heat can be the result of insufficient sampling of the energy landscape. Another result of insufficient sampling is that the specific heat displays frequency dependence even in a system that shows no signs of aging. Coulomb Glasses: A Coulomb glass is an insulator in which the electrons occupy randomly placed sites, and interact with one another via a long-range Coulomb potential. A practical realization of a Coulomb glass is found in doped, compensated semiconductors where the disorder is produced by the random placement of donor and acceptor impurities. We have used simulations to show for the first time that as the temperature is lowered, the electrons undergoes a second order phase transition into a Coulomb glass phase. We are currently exploring how this transition changes with the amount of disorder. The competition between interactions and disorder result in glassy dynamics that are often associated with very long relaxation times extending over many decades. One might not expect the same to be true in an electronic system since electrons typically respond very quickly. However, in the presence of strong disorder such as that found in a Coulomb glass, electrons can indeed have very long relaxation times. In particular I have done a calculation where the Coulomb interaction between electrons is suddenly turned on and I follow the subsequent time development of the Coulomb gap (see above) in the density of states and show that it can occur over many decades of time due to slow electron hopping and rearrangement. (In order for the ground state to be stable to small perturbations such as single electron hops, Pollak, and Efros and Shlovskii showed that the zero temperature density of single-particle states must vanish at zero energy, i.e., at the Fermi energy. This is the origin of the Coulomb gap.) This is consistent with experiments. Because of the competition between randomness and interactions, the electrons exhibit glassy behavior at low temperatures with electron hopping occurring over a broad range of times scales. We have shown that these long relaxation times produce low frequency 1/f noise at low temperatures. Even though 1/f noise is ubiquitous in conducting devices, the microscopic mechanisms are not well understood. Our approach was new and works much better than previous theories of 1/f noise in Coulomb glasses. We used a microscopic model in which the conduction electrons travel through a percolating network. The noise is produced by electrons which occasionally hop between isolated clusters and the extended network. The Low Temperatures Properties of Glasses: Glasses at low temperatures also present a challenge. A bunch of molecules in a disordered jumble behaves very differently from an ordered crystalline array of those same molecules. This can be seen in low temperature thermodynamic properties that tend to be universal, independent of the particular material and its chemistry. What is it about the nature of disorder that gives rise to such universal properties? This is the basic problem of disordered systems. Professor Yu has studied this question with models in which defects or tunneling centers interact with one another. More recently her group has been concerned with the influence of these two level systems on qubits. QUANTUM COMPUTING QUBITS: The basic unit of information for any computer is the bit. For a quantum computer the quantum bit, or "qubit", is a wavefunction describing a coherent superposition of the |0> and |1> state. Decoherence of the wavefunction is one of the great obstacles facing the realization of quantum computers. Josephson junction (JJ) qubits are a leading candidate for making a quantum computer. A major obstacle to the realization of quantum computers with Josephson junction qubits is decoherence. The goal of our research is to elucidate the microscopic sources of this decoherence and to suggest ways to eliminate or reduce these culprits. We are working closely with experimentalists. A Josephson junction consists of 2 superconducting electrodes separated by a tunnel barrier that is often an insulator. The current passing through a Josephson junction is superconducting. We are investigating how fluctuating two level systems in the barrier produce noise and can lead to decoherence of the qubit. |
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| Publications |
Frequency Dependence and Equilibration of the Specific Heat of Glass Forming Liquids (with H. M. Carruzzo), condmat/0209221. |
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1/f Noise in Electron Glasses (with K. Shtengel), Phys. Rev. B 67, 165106-1-8 (2003). |
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Structural Probe of a Glass Forming Liquid: Generalized Compressibility, (with H. M. Carruzzo), Phys. Rev. E 66, 021204 (2002). |
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Generalized Compressibility in a Glass Forming Liquid (with H. M. Carruzzo), Phil. Mag. B 82, 125 (2002). |
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| Slow Dynamics in Glassy Systems, Phil. Mag. B 81, 1209 (2001). | ||
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Viscoelasticity and Surface Tension at the Defect--Induced First--Order Melting Transition of a Vortex Lattice (with H. M. Carruzzo), Phys. Rev. B 61, 1521 (2000). |
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Time Dependent Development of the Coulomb Gap, Phys. Rev. Lett. 82, 4074 (1999). |
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Absence of a Magnetic Field Induced Metal-Insulator Transition in Kondo Insulators (with H. M. Carruzzo), Phys. Rev. B. 53, 15377 (1996). |
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Kondo Insulators Modeled by the One Dimensional Anderson Lattice: A Numerical Renormalization Group Study, (with M. Guerrero), Phys. Rev. B 51, 10301 (1995). |
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Non-EquilibriumDielectric Behavior in Glasses at Low Temperatures Evidence for Interacting Defects (with H. M. Carruzzo and E. R. Grannan), Phys. Rev. B 50, 6685 (1994). |
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A Numerical Renormalization Group Study of the One Dimensional Kondo Insulator, (with S. R. White), Phys. Rev. Lett. 71, 3866 (1993). |
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Critical Behavior of the Coulomb Glass, (with E. R. Grannan), Phys. Rev. Lett. 71, 3335 (1993). |
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Phase Transitions of Interacting Elastic Defects, Phys. Rev. Lett. 69, 2787 (1992). |
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Low Temperature Properties of Amorphous Materials: Through a Glass Darkly, (with A. J. Leggett) Comments on Condensed Matter Physics, 14, 231 (1988). |
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Professional Society |
American Physical Society |
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| Link to this profile | http://www.faculty.uci.edu/profile.cfm?faculty_id=2196 | |
| Last updated | 07/22/2008 | |