I will discuss several topics in the new area of molecular electronics, focusing in particular on the electrical conductance of small molecules and atomic wires. I will consider such topics as the spatial distribution of the voltage drop in an atomic wire, the oscillatory conductance of certain atomic wires as a function of length, the conductance of parallel atomic wires as a function of wire separation, and the conductance and density of electron states for a number of di-substituted benzenes. I will also discuss the use of small molecules as field-effect transistors.

All the everyday physical properties of materials (except density) are controlled by the electrons and the atomic vibrations, and the interplay between them. A fundamental understanding therefore requires carefully controlled measurements of dynamics, or energy partition and flow. An ideal tool for such properties is high brightness tunable light, since it can couple to specific electronic and vibrational excitations. In this colloquium we will describe the production of very bright light from synchrotron radiation sources and Free Electron Lasers. We will then give several examples of studies of physical, biological, and chemical systems of high fundamental interest. Most of the studies also have important economic implications in areas of interest to the Department of Energy such energy efficiency and conversion, examples being studies of friction, superconductivity, semiconductivity, protein and combustion dynamics.

The pion and the muon, the lightest unstable particles, were discovered over fifty years ago, and have been well studied since. However, over time the Standard Model (SM) of elementary particles and interactions has become so successful that for several key pion and muon properties its predictions are far less uncertain than the best available measurements, primarily those having to do with the particles' rare decay modes. More importantly, slight deviations from the SM predictions can provide valuable clues to new physics outside of the current SM. The PIBETA experiment at the Paul Scherrer Institute (PSI) in Switzerland aims to measure accurately several such rare decays, primarily the pion beta decay, as well as π → e ν, and radiative decays of both the pion and the muon. This talk will focus on the motivation, experimental apparatus, method, and the first results of the PIBETA measurements.

Candy wrappers, magnets, earthquake faults, and many other systems respond to slowly changing external conditions with crackling noise, due to discrete, impulsive events that span a huge range of sizes (Barkhausen noise or avalanches in the case of magnets, and earthquakes in the case of the Earth). We discuss Barkhausen noise in disordered magnets as a representative of these systems and compute predictions for the universal aspects of the behavior on long length scales as a function of disorder, using ideas from phase transitions and disordered systems theory. Similar ideas can also be applied to the interpretation of the Gutenberg-Richter scaling law in the statistics of earthquakes.

The NA48 Experiment at CERN helped determine a non-zero value for the direct CP-violation parameter in neutral kaon decays. Its high-intensity neutral beam has also produced results on rare decays. In the future, beamline changes will permit even more sensitive searches for such rare decays. A description of the NA48 apparatus and a discussion of recent results and future goals will be presented.

Founded over a century ago, statistical mechanics (SM) for systems in thermal equilibrium has been so successful that, nowadays, it forms part of our physics core curriculum. On the other hand, most of "real life" phenomena occur under non-equilibrium conditions. Unfortunately, statistical mechanics for such systems is far from being well established. The goal of understanding complex collective behavior from simple microscopic rules (of evolution, say) remains elusive. As an example of the difficulties we face, consider predicting the existence of a tree from an appropriate collection of H,O,C,N,... atoms!

Over the last two decades, an increasing number of condensed matter theorists are devoting their efforts to this frontier. After a brief summary of the crucial differences between text-book equilibrium SM and non-equilibrium SM, I will give a bird's-eye view of some key issues, ranging from the "fundamental" to (a small set of) the "applied." The methods used also span a wide spectrum, from "easy" computer simulations to sophisticated field theoretic techniques. These will be illustrated in the context of an overview of the projects here at VT.

The Two Micron All Sky Survey (2MASS) has collected a 20-terabyte nearinfrared high resolution image of the entire celestial sphere in three colors. A processing pipeline has extracted more than a billion point sources from this image database with accurate photometry and positions. I will describe the construction of an external view of the disk of our Milky Way galaxy using 30,000 carbon star candidates selected on the basis of their unique infrared colors. These candidates can be selected with high reliability using a technique which distinguishes the intrinsically red colors of carbon stars from reddened stars in the Galactic plane using 2MASS photometry alone. The extracted sources serve as crude standard candles with a dispersion of ~0.3 mag. The complete stellar bar and the far edge of the Galactic disk are evident in this analysis.

Self-assembled nanoarrays fabricated in porous alumina templates have recently attracted a great deal of attention. The nanoarrays have many potential applications, for example, in magnetic recording, computing, nanoelectronics, etc. They are also scientifically interesting because they are model systems to study fundamental phenomena at the nanoscale. In this talk, I will describe the method by which porous alumina can be prepared in the laboratory with good control over the pore size. I will discuss the many ways in which the templates can be used to prepare nanoarrays of various materials: metals, alloys, compounds, multilayers, etc. These nanoarrays can be characterized very easily using AFM, SEM and TEM techniques. One can also carry out a whole range of measurements on these systems including magnetic, optical, electrical, etc. Some of my results on magnetic, semiconducting and superconducting nanoarrays will be presented in this talk and their possible applications in devices will be discussed.

Healthy and sick individuals (A and B particles) diffuse freely in a world of dimension d. Sick individuals spontaneously recover (at a rate ), but infect healthy ones (at a rate k) upon encounter. The epidemic therefore tends to propagate [to become extinct] in regions with high [low] population density r. Below a critical density r_c, global extinction will occur, and an important question is how the epidemic evolves near this threshold, where strong local fluctuations are expected. The evolution of the epidemic corresponds to a difusion-reactiondecay process of two chemical species:

A + B → 2B,   B → A.

This is but one instance of a large class of such processes whose theoretical study has considerably advanced over the last decade. On this example we illustrate some of the basic questions and methods of analysis, as well as the type of answers obtained. New results are presented: they concern the case in which the diffusion constants DA and DB of the A and B particles are unequal. A comparison with Monte Carlo simulations is made and some unresolved questions are indicated.

Quantum computers potentially can be much more efficient than conventional computers for certain applications. Quantum bits or qubits, which are the basic building block of a quantum computer, have been demonstrated in different systems. And in some systems quantum computation involving a few qubits have been performed. However,most of these systems are not suitable for scaling up to provide large number of qubits required for useful computation. Solid state qubits, in particular those based on superconducting devices employing Josephson junctions, are promising candidates for large scale implementation. These devices are made with nanofabrication technology and are much easier to scale up. There have been some promising progress in developing superconducting qubits. In this talk recent results will be reviewed.

Novel oxide thin films, such as highly conductive SrRuO₃ and La_1-xSr_xCoO₃, superconductive YBa₂Cu₃O_7-??, highly dielectric CaCu₃Ti₄O₁₂, ferroelectric Ba_1-xSr_xTiO₃ (BSTO), and ferromagnetic La_1-xCa_xMnO₃, have been epitaxially grown for various device applications, such as field effect transistors, MEMS, tunable microwave elements, quantum cascade systems, and solid state fuel cell applications. For instance, ferroelectric (Ba,Sr)TiO₃ thin films have been fabricated for tunable microwave phase shifters and exhibited excellent room temperature dielectric properties with high dielectric constant, low loss tangent, and large tunability. Microwave property measurements indicated that the room temperature coupled microwave phase shifter has achieved a phase shift over 275° at 24 GHz and a figure of Merit of near 70°/dB (best record). Furthermore, microstructural studies suggest that the conservative and non-conservative antiphase boundaries can form in the thin films due surface steps and terraces induced antiphase domain formation. Artificial Domain Structured Optoelectronic (ADSOM) is proposed from this epitaxial model for optoelectronic device applications such as quasi-phase modulators, quantum cascade (QC) lasers, and new optoelectronic device systems. Giant dielectric CaCu₃Ti₄O₁₂ thin films (?~10⁵) have been epitaxially grown to investigate the unusual physical properties of this newly discovered material. Many interesting phenomena have been found from the as-grown films. In addition, pulsed laser induced self-assembly (PLISA) is a unique technique that we have developed for synthesizing nano-oxide particles and particle arrays with modulated structures. By controlling the irradiation energy, irradiation pulse period, and film thickness, various nanoparticle patterns with artificial periodical structures have been achieved. Details will be presented in the seminar.

The retina is an outpost of the brain which collects light and converts it, via a three-layer neural network, into discrete electrical pulses called spikes. The resulting digital signal is transmitted to the brain along the optic nerve and contains all the information that we have about the visual world. In this talk, I will describe the process by which light is converted into electrical pulses and the structure of used by the retina to describe the visual world. I will argue that information communicated in this way is energetically expensive and that energy efficiency is therefore an important organizing principle shaping the structure of neural codes. As an example, I will show that the theory of energy-efficient codes predicts patterns of spiking that are reproduced in the salamander retina. Finally, I will argue that despite the bewildering complexity of the central nervous system, there are general principles like energy efficiency, that can explain many aspects of the structure and function of brains.

We will explore various concepts, techniques and technologies for producing ultrashort pulses of electrons and photons of all energies and colors from the femtosecond to the attosecond duration and beyond for breakthrough research in physics, chemistry, life and information sciences.

I would like to give you an overview on the status of neutrino physics, with an emphasis on the search for neutrino oscillations. The recent history of neutrino physics offers spectacular results:

1. The first evidence of neutrino oscillations, observed for atmospheric neutrinos with the Super-Kamiokande detector. This implies the existence of neutrino masses, which has been one of the big questions of particle physics since neutrinos were invented in 1930 by Wolfgang Pauli.
2. The first evidence that solar neutrinos indeed change their flavor from electron to muon or tau neutrinos, claimed last year by comparing the results of the experiments SNO (Canada) and Super-Kamiokande (Japan).

But it is also full of drama: 7000 of the 12000 glass Photomultipliers of the Superkamiokande detector imploded recently in a catastrophic chain reaction and we will have to wait for some years until everything is rebuild. During the next 1-3 years the experiments KAMLAND and BOREXINO have the potential to pinpoint the parameters (neutrino mass and mixing) responsible for solar neutrino oscillations. Mini-BOONE will give an answer to the question: Is there a sterile neutrino?

I will conclude with an outlook to the future: What are the open questions? What experiments are needed to solve them?

As device size is reduced to dimensions below the coherence length of electrons, quantum phenomena dominate. Without special considerations, most of the effects of phase coherence are actually bad. For example, the on/off condition of a MOSFET is now more complicated when the channel is represented by a discrete quantum level. The drastic reduction in the dielectric constant when the particle size is reduced to a couple of nanometers prevents extrinsic conduction with chemical doping. Even atomic physics needs to be drastically modified to include the dielectric mismatch between the quantum dot and its environment. I shall give you my rather subjective view of what systems are overrated as potential devices, and what problems are most difficult to overcome.

Holger Meyer, a current graduate student who works with Professor Marvin Blecher, is quite close to finishing his thesis.

Holger will talk about his work at the LEGS facility at Brookhaven National Lab where he studies the production of charged pions using polarized gamma rays colliding with neutrons and protons. He will discuss the experimental setup, and what can be learned about nucleon structure from these experiments.

Black holes exhibit thermal properties when quantum theory is taken into account. The microscopic structure responsible for this behavior is quite mysterious but recent advances in string theory has shed some light on the problem. Similar ideas may apply to quantum cosmology. The colloquium is an account of this development intended for non-experts.

Molecular motors are fascinating devices, playing key roles in areas ranging from biological transport to emerging nanotechnology. Motors produce current (of particles, cars, etc.) as a result of transfer of energy (but not of momentum) from a source; many molecular motor scenarios are based on the translational Brownian ratchet mechanism proposed by Astumian [1] and Prost [2]. We consider a class of light-driven molecular motors, which consist of dichroic dye molecules in an anisotropic liquid crystal environment. Here, via an orientational ratchet mechanism, photoexcitation of the dye in the molecular field of the liquid crystal gives rise to a net orientational current of the dye and a concomitant torque on the liquid crystal; this torque appears essentially without the transfer of angular momentum from light. We consider the case where azo-dye molecules, functionalized into a polymer aligment layer, reorient the bulk liquid crystal against an elastic restoring torque when irradiated by polarized light. We present a detailed Fokker-Planck description of this system, and compare the results of numerical simulations with the experimentally observed dynamics. Finally, we inquire we inquire whether it is possible to realize a fully deterministic version of such a light driven motor, and discuss J.Prost, J-F. Chauwin, L Peliti, A. Ajdari, Phys.Rev. Lett. 72, 2652 (1994).

Most current electronics is based on semiconductor devices that rely on the manipulation of the flow of electrons with electric fields through the Coulomb interaction. The motion of electrons in these devices can, with few exceptions, be described adequately with a semiclassical theory. Hence, except for niche devices, current electronics does not rely on quantum mechanical coherence to function. Attempts to make devices which rely on this coherence in electronic motion have been disappointing due largely to the rapid decoherence rates seen at all but the lowest temperatures.

Over the past few years attention has focused on the spin-1/2 that each electron carries with it. These spins do not couple directly to electric fields, hence they can remain coherent for device-relevant times at room temperature. Recent advances in the generation, transport, and detection of coherent populations of spins in semiconductors have demonstrated unprecedented optical and electrical control of quantum mechanical coherence. The degree of success might be sufficient to permit the construction of a scalable "quantum computer".

I will describe some of these advances, as well as our own work clarifying the origin of electron spin decoherence in semiconductor quantum wells, the role of electron-electron interactions in the motion of a spin-polarized packet of carriers, and how one can manipulate the spin orientation of electrons in quantum dots without a physical magnetic field.

Mesoscale films of complex fluids, such as, proteins, DNA, and block copolymers are attractive due to their properties such as, self-assembly to form template materials for nanostructure and device fabrication. In the mesoscale, interfacial effects such as, surface tension and surface influenced mobility have profound effect on the structure and dynamics of the film. In this talk, I will describe two phenomena in < 50 nm thin films of polymers where interfacial effect has significantly impacted the material properties. In the first phenomena, we describe an observation where ~10 nm diameter polystyrene cylinders sink in a < 40 nm polystyrene-polybutadiene block copolymer film. Remarkably, this spontaneous planarization occurs, even though the cylinders are covalently stitched to the polybutadiene matrix. In the second phenomena, we report a ~10³ fold increase in chain mobility compared to bulk due to surface influenced mobility. The enhanced mobility may lead to faster piezoelectric polymer devices with significantly larger sensitivity for sensor applications.