Robert Joynt - US grants

The project is a prospective, interdisciplinary, longitudinal study of senile dementia of the Alzheimer type (SDAT). The aim is the correlation of clinical and autopsy findings. The diagnosis will be established on clinical grounds after eliminating other possible causes. Extensive physical, neurologic, radiologic, neuropsychologic, and language testing will be completed at the first encounter with subsequent updating of these data at regular, usually six-month, intervals. Suitable control population will be selectively studied for comparison. Autopsy material derived from this group will have neuropathologic, immunohistochemical studies for hormones and peptides, neurotransmitter, and quantitative neuron examinations. This longitudinal study during life with subsequent autopsy study will provide information regarding the taxonomy of the disease, subgroups, clinical course, relationship of various behavioral or language disorders with regional or system involvement, compensation and decompensation mechanisms in the nervous system, and survival predictions.

A program of braodly-based theoretical research into superconductivity is proposed, ranging from the microscopic description of the superconductive mechanism in high-Tc superconductors to the investigation of flux-pinning in high- field Niobium-Titanium superconducting magnets. In the microscopic work, the approach is to construct variational wavefunctions for the ground and excited states of the two- dimensional Hubbard model. The investigator has recently found an expression for the excited states, building on earlier work for the ground state. The energy and other properties of these can be evaluated using a numerical technic previously developed by the investigator and others. They also propose to continue to develop the analytic theory of this model to finite temperatures, using a recently discovered mapping of the entropy into a well- understood statistical mechanical problem. Further work aimed at determining the microscopic phase of UPt3 using experimental neutron-scattering data as input will be pursued. Collaboration with experimental groups at Wisconsin will also continue. The model of granular superconductivity to explain properties of weakly-connected superconductors will be explored by numerical methods. Numerical solutions of the Ginsburg-Landau equations for different flux-pinning morphologies (experimentally determined) will be obtained.. These last two projects will help to optimize the properties of oxide and Niobium-Titanium magnets.

This research focuses on one of the most critical and difficult issues in the field of high temperature superconductors; the role of grain boundaries as current limiting structures in bulk and thin film materials. This research will provide a detailed study of weak links, grain boundary structure and composition as well as surface properties of high temperature superconductors. The weak link investigations focus on single grain boundaries in polycrystalline bulk materials, and will employ thinned samples for which the critical current can be probed locally and the grain boundaries to their structure and composition. Complementary surface characterization work using synchrotron radiation techniques will also be carried out. This research is potentially of great strategic importance and could provide enabling technology for commercial applications of bulk high temperature superconducting materials.

Theoretical research will be conducted on materials for which electron correlations play a fundamental role, focusing on heavy fermion and high-temperature superconductors. The heavy fermion system will be attacked using both phenomenological techniques and fundamental studies of microscopic origins with an emphasis on the coupling of the superconducting order parameter with the staggered magnetization. For high-temperature superconductors, analysis will be conducted on the theoretical basis for photoemission experiments. Variational Monte Carlo techniques will be used to study the interplay between correlation and disorder in both of these systems. %%% Theoretical research will be undertaken on fundamental issues relating to two very important systems of current interest: high- temperature superconductors and heavy fermion superconductors. Insight into both of these systems will affect basic concepts of condensed matter and materials physics, yet has the potential to have significant payoff in the commercial arena.

9704972 Joynt This grant will support the work of this mid-career theorist in three different projects in the general area of strongly correlated electrons. Continuing the PI's work on supercondictivity of heavy fermions, the first project contains a study of the expected effects in neutron scattering from a flux lattice. In the second project based on the properties high Tc superconductors, there are two sub- projects: (1) Properties of a twin boundary junction and a proximity effect junction and (2) a variational study of disorder on a "d-wave"superconductor. Finally, there are plans to study the properties of a liquid crystal like state in quantum Hall effect. %%% This grant to mid-career theorist supports work in several projects related to the properties of some curious superconductors, all discovered during the Eighties. They include alloys of Uranium and Platinum, where the PI pioneered with some early work on the unconventional superconducting properties. They also include the better known high Tc materials where again the PI made some early conjectures about some special effects and which survive a closer scrutiny in more recent years. Much of the work proposed here builds on this very creative and original PI's earlier successes. ***

9903843
Halley

This award supports initial stages of collaboration on study of two types of correlated electron systems, rare earth magnets and semiconductor quantum dots, between the Universities of Minnesota and Wisconsin, and Vietnam National University, Hanoi; the National Center for Natural Science and Technology, Vietnam; and the Technical University of Hanoi. The proposed research is an outgrowth of discussions held in Hanoi January 1998 during the Science and Technology Week, partially supported by NSF.

The Vietnamese have already established research strength in the area of rare earth permanent magnets, an important area for Vietnam, which has significant rare earth reserves. The combination of the Vietnamese experimental work in this area with the modeling support available through the collaboration is promising for a mutually beneficial research partnership. The work on quantum dots is relevant to an area of considerable interest, the failure of the simple random matrix theory model to predict the energy level structure of disordered quantum dots. Vietnamese scientists have experience in modeling that will complement the approach of the US team and will contribute to understanding this problem, which underlies much of the current fundamental experimental research in nanostructures.

0081039
Joynt
Some of the deepest problems in science have to do with the motion of electrons, the carriers of electricity. Their behavior is governed by quantum many-body theory, whose laws often lead to strange and beautiful consequences. The understanding of these particles, and the resulting control that we have over them, underlies a wide range of technology from chemical engineering to computers.

A particularly difficult challenge for solid-state physics is to understand the behavior of groups of electrons under special circumstances: when they act collectively because of their mutual interactions. This is a much deeper problem than the individual behavior of isolated electrons. The understanding of the latter is relatively well developed and forms the basis for today's electrical technology. The understanding of collective behavior will be important for the technology of tomorrow. The research in thi sgrant attacks this problem from two different directions: experimental analysis and fundamental calculations.

In order to understand collective, or correlated, behavior of electrons, we must have well-developed tools for gathering information about them. One very important such tool is the photoelectric effect, which probes electron behavior by looking at the electrons that emerge from a metal when light is shown on it. In order for this experiment to give accurate information about the metal, we must understand the various ways that the electron can slow down and lose energy before it is detected. Calculations of thi senergy loss, and the resulting experimental signatures, i sone focus of the research.

A second focus is to calculate the energies of electrons in very small structures called quantum dots. These structures are sure to be important in future computer technology, as miniaturization of chips and memory elements continues. Current theory does not furnish a good account of the motion of electrons in these structures. Their energy levels are puzzling: we even lack a rough, statistical description. We will use an algorithm based on biological ideas, the so-called gentic algorithm, to calculate these levels. This will be the first time such ideas have been applied specifically to the quantum nature of these particles.

One very important example of collective behavior of electrons is superconductivity: the ability of electrons to carry electrical current as a group. Important new classes of these materials have been discovered in recent years, and their properties are novel and, in many cases, poorly understood. The theoretical research in this area, a continuation of a long-standing effort, focuses on understanding numerous experiments and constructing models that combine the phenomena of superconductivity and magnetism.
***

EIA-0130400
Robert Joynt
University of Wisconsin-Madison

Title: Connecting the Quantum Dots; Theory of Quantum Computing in a Solid-state Implementation

A program of theoretical research on quantum computation in quantum dot structures is underway. In this type of quantum computer, the qubits are the spins of electrons trapped in a silicon-germanium semiconductor heterostructure. The error distributions and decoherence properties of these electrons are being calculated, and it appears that correlated errors and mutual decoherence are important in this dot implementation. The impact of these phenomena on quantum algorithms and quantum error correction schemes is being investigated. This is done by examining their effect in two paradigmatic quantum processes. The first is the quantum random walk, a simple example of an algorithm that can be implemented on a one- or two-dimensional array of quantum dot qubits. The second is the computation of the majority function. A particularly attractive feature of both problems is that it is interesting and feasible to do them in cases that require relatively few qubits. More general questions are also being addressed. Certain types of quantum algorithms will be more susceptible to certain types of errors. This leads to the possibility of quantum error-resistant algorithms, and the feasibility of this generalization of the similar classical concept is being determined.

The subject of this proposal is the theory of the decay of electron spin in semiconductors. It aims to understand the time that it takes for non-equilibrium spin distributions to disappear once they are created. All semiconductor materials will be considered, but there will be a particular focus on silicon and gallium arsenide, which have the most technological importance. Furthermore, the devices under consideration confine the motion of the electrons in a way that affects the spin decay. This will also be taken into account. There are multiple causes for this spin decay or relaxation. This will necessitate the development of computer programs to handle the complex interaction of different mechanisms.

Broad Impact
Most of the important technological advances in the last 50 years have been closely associated with the progress of electronics, which, in physical terms, is the manipulation of electronic charge. In computer technology in particular, this manipulation is done in semiconductors. This steady advance has shown signs of slowing in the last decade or so, and the future will depend on the manipulation of electronic spin - the field of spintronics. This proposal aims at increasing our theoretical understanding of the behavior of electron spins in semiconductors. This area is the most promising for technology in the short term, since spintronics in semiconductors would allow rapid integration with current manufacturing techniques.

Intellectual merit
Many basic mechanisms of spin relaxation in semiconductors are reasonably well understood. However, crucial difficulties remain. The effect of magnetic fields, doping ranges and particularly geometry of devices introduce complications that can change the relaxation in very significant ways. The work proposed here focuses on some novel physical mechanisms that we believe are central to understanding these unresolved issues. The most important of these novel insights is spin transfer between extended and localized states. This is not a spin relaxation in itself, since the transfer conserves spin, but the transfer accelerates spin relaxation. A second point is the unexpectedly large field dependence of the spin-phonon relaxation due to symmetry considerations and the phonon density of states. These insights will be incorporated into computer calculations of spin relaxation from all mechanisms to achieve quantitative understanding of relaxation times. This will be a major advance the theory of spin relaxation in semiconductors.

This proposal investigates quantum algorithms based on quantum physical processes.The goal
is to identify quantum processes that can be reformulated to serve as quantum algorithms,and to
identify the types of errors that can disrupt these processes.The main focus is on NP-intermediate
problems,which are problems in NP that are neither NP-complete nor in P.
Intellectual Merit .One main focus of this research is the study of graph isomorphism,an NP-
intermediate problem that is a central problem in complexity theory.A many-particle random
walk process that may be particularly well-suited to solve this problem will be investigated.Since
the quantum dynamics of many-particle systems can often be e .ciently simulated on quantum
computers but not on classical computers,understanding the performance of the many-particle
random walk may yield new insight into inherently quantum algorithms for this problem.Speci .c
graphs known as strongly regular graphs (already shown to be useful to test critically algorithms
proposed for solving graph isomorphism)will be investigated in detail numerically to test the
many-particle quantum random walk algorithm,and fully characterize its complexity.In addition,
an e .ort will be made to make progress by characterizing the algebraic graph invariants that are
accessible to quantum computers.
Other related problems and methods will also be investigated,including integer factorization,
the discrete logarithm,and .nding short vectors in lattices.It will also be investigated whether
classical algorithms that depend on the birthday paradox can be used to devise new quantum
algorithms.
A complementary research thrust is to analyze the sensitivity of the multi-particle dynamical
algorithms to errors,focusing speci .cally on the scaling of errors as the number of particles increases,
and to understand the connection of the physical processes utilized by the algorithms to the physical
processes that actually occur at the hardware level and use this to improve the e .ciency of quantum
computations.
The goal of this research is to extend the limits of computation to qualitatively new problems.
For example,at present one cannot reliably test for the isomorphism of graphs with more than a
few hundred vertices.A quantum computer could,however,accurately compute the dynamics of
quantum particles on graphs of this size.If it is possible to use the information so obtained to test
isomorphism,this would be a signi .cant step forward in conquering complex problems.
Broader Impacts .The proposed work will have broad impact because of the insight it will yield
into hard computational problems.Further broad impact will be through the training of graduate
and undergraduate students with substantial interdisciplinary experience with both computer sci-
ence and physics.In addition to web-based dissemination of research results,an interdisciplinary
workshop will be held that will help improve communication between computer scientists and
physicists working on problems of common interest.

0435632
Joynt


This is a U.S.-Vietnam cooperative research project between Dr. Robert Joynt, University of Wisconsin and Professor Bach Thanh Cong, Hanoi University to study computational materials and device physics. This study includes four separate projects related to computational device physics: magnetic structure of manganites, photochemistry at the rutile/water interface, spin relaxation in quantum dots, and optimization of device design for quantum computing. Proposed topics are modern and relevant to ongoing research in condensed and materials physics. This is a thoughtful and carefully designed study. It can help the development of computational materials science in Vietnam and can enhance the human resource development in computational physics in both countries

****NON-TECHNICAL ABSTRACT****
Modern electronic devices work by controlling the location and motion of electrons through semiconductors. Past and current devices work with relatively large numbers of electrons at a time, but there is a continuing drive towards smaller and smaller devices, including those that control and manipulate one electron at a time. In such one-electron devices, the role of quantum mechanics is especially important and potentially useful. Materials properties are important in enabling such devices, and silicon has especially useful characteristics. Fabrication of silicon devices in which individual electrons can be controlled and manipulated has recently been achieved. This project will optimize, characterize, and further develop these materials and devices with the aim of enabling potentially transformative applications in which quantum mechanics is a critical factor. One such application is the quantum computer, which promises to be much more powerful than the classical computers used today. Graduate students participating in this project will obtain valuable interdisciplinary training. In addition, the project will involve the creation of a new workshop for high school teachers, to provide the necessary background for understanding the operation and functionality of modern nanodevices, including the role of quantum mechanics. This award receives support from the Divisions of Materials Research and Physics, as well as the Office of Multidisciplinary Activities.

****TECHNICAL ABSTRACT****
Research has shown that gated quantum dots in semiconductors can be tuned to contain a controllable number of electrons, and that number can be monitored noninvasively by using integrated charge sensors. Quantum dots in silicon are of particular interest because the electron spin coherence times in silicon quantum dots containing only a few electrons are expected to be quite long ? a feature that may be useful for storing and manipulating quantum information. This project focuses on the fundamental role of materials properties on the coherence and control of spins in silicon/silicon-germanium quantum dots. In addition to research on the manipulation of spin and the measurement of spin coherence times, this project will focus on the design, simulation, fabrication, and measurement of silicon/silicon-germanium quantum devices to study the physics of the valley degree of freedom and its interaction with spin. Students participating in this project will obtain valuable interdisciplinary training. The project also involves the creation of a new workshop for high school teachers. The workshop is designed to provide the necessary context for understanding the basic operation and functionality of modern nanodevices, particularly the role of quantum mechanics. The workshops will be accomplished in conjunction with physics outreach specialists at UW-Madison. The project receives support from the Divisions of Materials Research and Physics, as well as the Office of Multidisciplinary Activities.