September 16, 2002
The Curse of Original Antigenic Sin:
Localization
Prof. Michael Deem, Rice University
The immune system is a realtime 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 yeartoyear
mutation rates within a critical window, as for example with the influenza
virus.

September 30, 2002
NonConventional Metals
Dr. Eugene Pivovarov, Rice University
Several kinds of metallic systems exhibit nonFermiliquid 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 oddfrequencydependent order parameter, which has a conventional Fermi surface, but also has unusual thermodynamic and kinetic properties. The second model is the density wave with dwave 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 highT_{c} cuprates. 
October 31, 2002 (Thursday, joint CM/AMO)
Quantum Condensates And Mott Insulating States of Spin
One Bosons
Dr. Fei Zhou, Institute for Theoretical Physics, University of Utrecht, The Netherlands
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
bilinearbiquadratic 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)
OneDimensional Metals in Theory and periment
Prof. Bertrand Halperin, Harvard University
Theoretical analysis, since the 1970’s, has predicted that interacting
onedimensional electron systems should differ in important ways from their
threedimensional counterparts. For a conventional threedimensional metal,
the lowtemperature lowenergy behavior is described by Landau’s FermiLiquid
theory, which can be understood by treating the interaction as a weak
perturbation to the noninteracting behavior. Predictions for onedimensional
systems, often described as “Luttinger Liquids,” differ more radically from a
noninteracting system. In recent years, experimental realizations of
onedimensional metals, including singlewalled 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
electrontunneling experiments, including recent work on tunneling between two
parallel quantum wires, and evidence for the occurrence of “spincharge”
separation.

November 18, 2002
RealTime Electron Dynamics in a Quantum Dot
Prof. Alex Rimberg, Rice University
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 quasidc 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 radiofrequency
singleelectron transistor (RFSET), and have used the RFSET 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 RFSET and its circuitry) changes its state in response to
a quantum object (an extra electron on the quantum dot). It also
demonstrates that the RFSET can be used for singleshot measurement of
the charge state of a nanostructure, with important implications for
solidstate quantum computation.

December 2, 2002
News from the HighT_{c} Front:
Superconductivity as Bose Condensation
Prof. Kathryn Levin, University of Chicago
In this talk I will give a brief update of the solved and unsolved
problems in the field of highT_{c} 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 longrange order — at T_{c}. 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 NonFermi
Liquid?
Prof. Andrey V. Chubukov, University of Wisconsin at Madison
I will discuss recent works on the properties of 2D itinerant
fermions near antiferromagnetic quantumcritical 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 lowenergy, universal quantumcritical behavior. In the
quantumcritical regime, the fermionic spectral function interpolates
between marginal behavior at intermediate frequencies, and ω^{1/2},
T^{1/2} behavior at high frequencies. I then use the
normal state results as an input and discuss the pairing problem. I show
that quantumcritical, nonFermi liquid fermions give rise to a pairing
instability at a temperature T_{ins}, which is large and
comparable to the upper boundary of the quantumcritical 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 T_{ins} such that
fermions only form singlet pairs, but do not superconduct. The coherence
and superconductivity are recovered only at a much smaller temperature
T_{c}, 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
Prof. Andrew J. Fisher, University College London
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 solidstate 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
Compounds
Prof. Elbio Dagotto, Florida State University
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 highTc
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 LutherEmery
Model
Dr. Alexei Tsvelik, Brookhaven National Laboratory
I discuss a 2D model of electrons with a flat Fermi surface and
longrange attractive interaction. The model admits a nonperturbative
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 nontrivial 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
Dr. Karl Berggren, Lincoln Laboratory
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 behaviorsa quantum computercould solve some
problems much faster than a conventional computer. But fabrication of
atomicscale objects is difficult, while large objects appear to obey the
wellestablished laws of classical physics: these opposing constraints make
realizing such a computer challenging. In a superconductor, however, the
quantummechanical 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 entanglementthe bizarre influence that measurements
performed on one object can have on another objectin 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
in Spintronics
Prof. Sankar Das Sarma, University of Maryland
A potentially important recent experimental discovery is that a large number of semiconductors become ferromagnetic when doped carefully with 110% 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, GeMnthere 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 twocomponent 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); condmat/0211496, and more recent unpublished work.
