Department of Physics & Astronomy
Where: HZ 116
When: Mondays at 4:00 p.m.
|September 16, 2002
The Curse of Original Antigenic Sin:
The immune system is a real-time example of an evolving system that navigates the essentially infinite complexity of protein sequence space. How this system responds to disease and vaccination will be discussed. The phenomenon of original antigenic sin, wherein vaccination can increase susceptibility to future exposures to the same disease, is explained as resulting from localization of the immune system response in antibody sequence space. The localization is observed for diseases with year-to-year mutation rates within a critical window, as for example with the influenza virus.
|September 30, 2002
Several kinds of metallic systems exhibit non-Fermi-liquid behavior. In this talk we will focus on two theoretical models, in which the system is not a Fermi liquid due to the presence of a “hidden” order. The first model is the density wave with odd-frequency-dependent order parameter, which has a conventional Fermi surface, but also has unusual thermodynamic and kinetic properties. The second model is the density wave with d-wave symmetry, in which the gap vanishes at the nodes. We will discuss the effects of the competing orders on the phase diagram and the implications for the high-Tc cuprates.
|October 31, 2002 (Thursday, joint CM/AMO)
Quantum Condensates And Mott Insulating States of Spin
Motivated by recent experiments on spin one cold atoms, we have investigated quantum condensates and Mott insulating states of spin one atoms. Besides the conventional BECs, we find quantum condensates of fractionalized atoms where only a fraction of each spin one boson condenses. These novel condensates have remarkably distinct macroscopic properties and can be distinguished in experiments.
In Mott insulating limits, we demonstrate that ground states are Gutzwiller projected quantum condensates. The quantum numbers of excitations depend on numbers of atoms per lattice site. These results are supported by the mappings from the problem of interacting spin one bosons to the bilinear-biquadratic model, to the dilute instanton gas model of Ising gauge fields and to the quantum dimer model. We expect the properties summarized in this talk to be observed in future experiments on optical lattices. Finally we will briefly discuss some potential applications to quantum information storage or processing.
|November 4, 2002 (in HZ210)
One-Dimensional Metals in Theory and periment
Theoretical analysis, since the 1970’s, has predicted that interacting one-dimensional electron systems should differ in important ways from their three-dimensional counterparts. For a conventional three-dimensional metal, the low-temperature low-energy behavior is described by Landau’s Fermi-Liquid theory, which can be understood by treating the interaction as a weak perturbation to the non-interacting behavior. Predictions for one-dimensional systems, often described as “Luttinger Liquids,” differ more radically from a non-interacting system. In recent years, experimental realizations of one-dimensional metals, including single-walled carbon nanotubes, the edges of quantized Hall systems, and “quantum wires” in GaAs heterostructures, have led to direct experimental tests of some of the predictions of Luttinger liquid theory. We shall discuss some of these results, with emphasis on electron-tunneling experiments, including recent work on tunneling between two parallel quantum wires, and evidence for the occurrence of “spin-charge” separation.
|November 18, 2002
Real-Time Electron Dynamics in a Quantum Dot
It has long been known that temporal correlations exist between tunneling electrons in Coulomb blockade nanostructures. Still, electron dynamics in qauntum dots and other nanostructures have usually been inferred from dc or quasi-dc measurements rather than observed directly. Being able to detect the motion of individual electrons on a quantum dot would allow access to dynamical information not available by other means, and would allow the development of electron counting experiments analogous to the photon counting experiments of quantum optics. We have addressed this problem by fabricating a quantum dot with an integrated radio-frequency single-electron transistor (RF-SET), and have used the RF-SET to observe electron tunneling events on the quantum dot in real time. This is a paradigmatic quantum measurement in which the state of a macroscopic detector (the RF-SET and its circuitry) changes its state in response to a quantum object (an extra electron on the quantum dot). It also demonstrates that the RF-SET can be used for single-shot measurement of the charge state of a nanostructure, with important implications for solid-state quantum computation.
|December 2, 2002
News from the High-Tc Front:
Superconductivity as Bose Condensation
In this talk I will give a brief update of the solved and unsolved problems in the field of high-Tc superconductivity. Among the most important unsolved problems is what causes these materials to behave so differently from BCS superconductors? BCS theory has never failed so notably in our past experience. In particular, precursor superconductivity effects (associated with what is now called the “pseudogap” phase) appear at temperatures much higher than the onset of true long-range order — at Tc. We review BCS theory here and argue that these cuprate superconductors exhibit the most generic form of Bose condensation (of fermion pairs), of which BCS theory is a very special case: pairs form at a much higher temperature than that at which they Bose condense. A variety of experiments, including electrodynamical data, are discussed to support this viewpoint.
|December 9, 2002 (at 1:30 p.m.)
Can Superconductivity Emerge Out of a Non-Fermi
I will discuss recent works on the properties of 2D itinerant fermions near antiferromagnetic quantum-critical point. This problem is relevant both to cuprates and to heavy fermion materials. I first show that, as the system moves towards quantum criticality, the width of the Fermi liquid region progressively shrinks, and there appears a wide range of low-energy, universal quantum-critical behavior. In the quantum-critical regime, the fermionic spectral function interpolates between marginal behavior at intermediate frequencies, and ω1/2, T1/2 behavior at high frequencies. I then use the normal state results as an input and discuss the pairing problem. I show that quantum-critical, non-Fermi liquid fermions give rise to a pairing instability at a temperature Tins, which is large and comparable to the upper boundary of the quantum-critical behavior. However, this pairing is qualitatively different from the BCS theory as it is produced by the exchange of real rather than virtual bosons. This implies that the magnitude of the superconducting order parameter remains random immediately below Tins such that fermions only form singlet pairs, but do not superconduct. The coherence and superconductivity are recovered only at a much smaller temperature Tc, which scales inversely with the magnetic correlation length.
|February 28, 2003 (Friday, at 11:00 a.m.)
Quantum Computing in the Solid State:
the Challenge of Decoherence
It seems everyone now wants to build a quantum computer. The inherent parallelism available within quantum mechanics would allow such a device to perform certain operations, including the factoring of large integers, exponentially faster than any known classical algorithm. For reasons of scalability and integration, the solid state seems an attractive system to construct a quantum computer; however, one then has to confront the problem of decoherence caused by the interaction of the quantum bits (qubits) with their environment. In this talk I will give a brief survey of some proposals for solid-state quantum computing, and explain the origin of a fundamental limitation we have discovered on the unavoidable decoherence which is introduced when manipulating the qubits.
|March 17, 2003
Prof. Subir Schdev, Yale University
|March 31, 2003
Clustered States in CMR Manganites and Other
In this talk, recent developments in the context of theory and experiments for manganites will be discussed. It will be argued that the presence of nanoscale phase separation is at the heart of the CMR phenomenon. Simulation results support this view. These effects are not limited to manganites, but they appear in other compounds as well, such as the high-Tc cuprates. In addition, physics somewhat analogous to that of Mn oxides is present in diluted magnetic semiconductors, and results will be shown. The overall picture is that “clustered” states appear to be a new paradigm for the understanding of many compounds.
|April 14, 2003
Exact Solution of Tomographic Luther-Emery
I discuss a 2D model of electrons with a flat Fermi surface and long-range attractive interaction. The model admits a non-perturbative solution under rather general conditions on the interaction. The spectrum consists of the gapless charge mode, a spin mode with a gap and neutral singlet modes. The gap for the latter modes has nodes on the Fermi surface. This is the first non-trivial 2D theory solved by Bethe ansatz.
|April 21, 2003 (in Space Science 337,
at 2:30 p.m., joint CM/AMO)
Superconductive Quantum Computation:
Moore’s Law Meets Schrödinger’s Cat
At the level of single atoms and electrons, matter exhibits strange quantum behaviors that we do not encounter in our everyday world. If it can be built, a computer using these behaviors-a quantum computer-could solve some problems much faster than a conventional computer. But fabrication of atomic-scale objects is difficult, while large objects appear to obey the well-established laws of classical physics: these opposing constraints make realizing such a computer challenging. In a superconductor, however, the quantum-mechanical wavefunction of the electron pairs extends for a macroscopic distance, so some of the stranger aspects of quantum mechanics remain observable at a macroscopic scale. Superconductors are, therefore, well suited to quantum computation because they allow manipulation of quantum effects using devices with a length scale accessible with modern microfabrication techniques. Indeed, experiments around the world have already observed quantum superpositions of macroscopic states and seen strong evidence for entanglement-the bizarre influence that measurements performed on one object can have on another object-in superconductors. This seminar will discuss our effort to build an elementary quantum computer using superconductive electronics.
|April 28, 2003
How To Make Semiconductors Ferromagnetic: A First Course
A potentially important recent experimental discovery is that a large number of semiconductors become ferromagnetic when doped carefully with 1-10% of magnetic impurities (mostly Mn although Co has been used in some cases too). Some examples of such ‘diluted magnetic semiconductors’ (DMS) are GaMnAs, InMnAs, GaMnN, GaMnP, GeMn-there is recent speculation that even the reported ferromagnetism in hexaborides is due to unintentional doping by Fe impurities. Ferromagnetic semiconductors, where magnetic and semiconducting properties can be controlled and tuned at will, are projected to form the basic ingredients in the emerging field of spintronics. I will critically discuss in this talk the physical mechanisms underlying DMS ferromagnetism within a minimal model where magnetic coupling between the impurity local moments is mediated by the semiconductor carriers (mostly valence band holes). Ferromagnetism occurs irrespective of whether the system is metallic or insulating, leading to our recent polaron percolation theory of DMS ferromagnetism. I will discuss the very unusual concave magnetization and related strange behavior shown by DMS systems using a two-component mean field theory, the dynamical mean field theory, and the polaron percolation theory. Extensive comparison between theory and experiment will be presented. The talk will be based on our recent work as appearing in PRL 87, 227202 (2001); PRL 88, 247202 (2002); cond-mat/0211496, and more recent unpublished work.
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