Nascent Research Groups A key element of the ICAM research plan is catalyzing new transdisciplinary multi-institutional research groups to work on specific problems in the study of complex adaptive matter. Thus ICAM views a successful workshop as one that gives rise to one or more ultimately successful grant proposals by multi-institutional research teams who wish to work together on problems arising out of discussions at the workshop. Because of their transdisciplinary and multi-institutional character, such proposals cannot be written overnight. Rather, a period of time, which might be as long as two years, is required for the lead principal investigators to visit one another and, in many cases, arrange for the exchange of graduate students and postdocs who wish to work in the proposed new area of study. One of the novel features of the ICAM research plan, discussed later in this proposal, is the provision of glue money for such nascent research collaborations. During the course of each exploratory workshop, the issue of follow-on research collaborations is raised, while following the workshop a call goes out requesting proposals for bridging grants for prospective multi-institutional research collaborations arising out of the workshop. These proposals are then reviewed by the ICAM Science Steering Committee, which has the responsibility of developing a priority list for their funding. The first two ICAM workshops have given rise to the following proposals for such bridging support. As funds become available to support these nascent research groupings, the ICAM SSC will develop a rank-ordered list for their funding.
Adaptive Atoms in Physics, Chemistry, Biology, and the Environment
1. Study of Mixed Valence (MV) Systems in Solid State, Chemical and Biological Condensed Matter: from Heterogeneous to Homogeneous MV Bob Heffner, Heinrich Roder, John Sarrao, Andy Shreve, A. Taylor, S. Trugman, and Dave Whitten (Los Alamos); Dan Cox, and Rajiv Singh (UC, Davis); and Zach Fisk (Florida State University) Mixed-valence materials, which include molecules, metalloenzymes and extended solid-state systems, contain ions whose valence or oxidation state can occur in a (fluctuating) range of integral or fractional values. In addition, mixed-valence systems are often characterized by very strong interactions between the electronic states of the ions and the vibrational motion and structure of the environment (e.g., lattice, protein or solvent). These complex and strong environmental interactions generally tend to favor localization of the electronic states, and compete with the electronic coupling between the ions, which generally favors the formation of delocalized states. This competition can generate coupled electronic, magnetic and structural dynamics that are difficult to understand. Although the importance and the cross-disciplinary nature of mixed valence is often recognized, the development of a unified, quantitative, multidisciplinary approach has not occurred. Thus, largely independent descriptions of mixed valence have arisen in chemistry and physics, each with its own strengths and weaknesses and each with its own language. (Biological mixed-valence systems have usually been treated in the context of the chemical descriptions.) This has hindered cross fertilization of ideas and advances across disciplines. Before a general treatment of mixed valence in all of its contexts can be provided, bridges between existing disciplinary treatments of the problem need to be established. We propose to examine the possibilities of studying some specific mixed-valence systems using a appropriate combinations of theoretical and experimental techniques derived from physics and chemistry, in order to bring the strengths of each to bear on outstanding fundamental cross-disciplinary problems. We divide the problem into three areas, each related to the other: 1) Heterogeneous mixed valence (usually found in chemical and biological systems), 2) Intermediate mixed valence (such as is found in the metallic state of the manganites), and 3) Homogeneous mixed valence (such as found in heavy fermion and Kondo systems). Brief examples of these systems are now given. 1) Heterogeneous MV. In chemistry, a long-standing issue that continues to plague understanding of even very extensively studied mixed-valence systems is how to describe the dynamics that follow electronic or vibrational excitation. For example, in the system [(NC)5 Ru II (CN)Ru III (NH 3 ) 5 ] -, which is one example taken from a class of mixed-valence molecules that have been investigated via synthetic and spectroscopic means, recent experiments have shown that, following photoexcitation, recovery of the ground electronic state occurs within 80 fs. Furthermore, the dynamics are influenced by solvent isotope changes and result in the transient excitation of highly energetic vibrational states in the ground electronic state. Using the traditional approaches developed within chemistry and molecular spectroscopy to describe charge transfer processes, there has been only limited success at explaining these observations. In fact, in this and related systems, the strong coupling between vibrational, electronic and spin dynamics greatly hampers use of standard approaches. 2) Intermediate mixed valence. The colossal magnetoresistive manganites possess octahedrally coordinated manganese ions having a valence varying between 3+ and 4+. Strong spin, charge and lattice coupling leads to 1) an extremely rich phase diagram possessing charge-ordering transitions (with "stripes"), antiferromagnetism, ferromagnetism, and metal/insulator transitions, and 2) great sensitivity to very small external perturbations (pressure, magnetic field, chemical substitutions, etc.). Many biological or biomimetic molecules of interest possess similar transition metal complexes on small a scale. For example, at the core of photosystem II, the molecule responsible for catalyzing production of oxygen in photosynthesis, is a cluster of manganese ions about which charge is shuttled giving a valence varying between 2+ and 3+. Recently, a molecule with a Mn(2+)-Mn(3+) dimer has been developed which presents an excellent model for the production of oxygen by photosystem II. Such mixed valence transition metal clusters are present in other biologically important molecules as well; cytochrome c oxidase, for example, has iron and copper clusters which are active participants in its operation. It is clear that the local physics and chemistry of the Mn ions in photosystem II, its analogues, and the manganites may have similarities worth exploring. 3) Homogeneous Mixed Valence and Kondo Phenomena. The study of homogeneous mixed valence at single ions in solids and the often concomitant Kondo effect has been a rich area of study. In materials such as CeSn3 and the heavy electron superconductors CeCu2Si2 and UBe13 the rare earth or actinide ions (Ce or U) possess a single average valence in the ground state, rather than the well defined ionic valence anticipated for such relatively localized f-shell systems. This quantum averaging of the valence can wipe out the local moment if the fluctuations are between a spin-full and spin-less configuration, as is the case for Ce. Recently, it has been proposed theoretically that precisely this kind of single ion mixed valence might be present for Ce in the molecule cerocene, which is formally tetravalent. Also, it has been proposed theoretically that single ion mixed valence may play a role in anomalously long-ranged electron transfer in quasi-one-dimensional biological molecules. We recognize that the classification of intermediate valence materials along a continuum from homogeneous mixed valence to heterogeneous mixed valence, although illuminating, is somewhat artificial. For example, this categorization for the materials of principal interest to us changes as a function of the length and time scale of the experimental probe being employed. This is a consequence, not of the inferiority of the probe, but rather a manifestation of the complex behavior exhibited by these materials. Nevertheless, the concepts invoked are useful to frame future discussions. We have formed a core group of physicists and chemists whose objective for this proposal is to frame a few important questions concerning mixed valence in the types of materials addressed above. The answers to these questions will require a cross-disciplinary experimental and theoretical approach. Once a clear set of questions are framed, we will pursue funding to implement an integrated research program. Some questions which might be addressed: Motivated by studies of quantum dots, we have now results for the dynamics of the formation of magnetic screening clouds on such mixed valence ions. Can the fast probes of biochemistry be brought to bear in the metallic/solid situation to examine how long it takes the homogeneous mixed valence state to form and what role structural degrees of freedom play in its formation? Can model biomimetic molecules be developed to study the strong possibility of homogeneous mixed valence in the molecular context? For example, can studies of charge transfer between cerocene molecules or Ce complexes in water be used as a probe of this state in molecules? The homogeneous mixed valence concept has been useful in the context of transition metal systems, where it plays a role in the understanding of the metal insulator transition. To what extent are these ideas transferable to transition-metal-based molecules with their stronger coupling to the surrounding environment? Surface and finite size effects are important for biological molecules; do they play a corresponding role in the manganites? In particular, how are the properties affected by the surface and domain boundaries, and are there analogies in the surface dynamics to the function of the biological molecules? The fast dynamics probes important in the biochemistry studies could be of value here. Physicists studying manganites have emphasized the importance of orbital order, Jahn-Teller polaron formation, and double exchange (hopping assisted by local spin alignment). To what extent do these effects play a role in the biological molecules? Of considerable importance in molecules will be non-adiabaticity, due to the mixing of electronic states via vibronic coupling. Non Born-Oppenheimer effects are especially important in the conformal relaxation of biomolecules, and their investigation requires the development of new theoretical techniques.
2). Macroporous Ceramics for Photonic Applications Vicki L. Colvin (Rice University), David E. Morris and Carol J. Burns (Los Alamos) Over the past few years there has been a resurgence of interest in the use of artificial opals to structure porous materials. The opals, which are comprised of colloidal silica or polystyrene (d=50-1000 nm) crystals, can be interpenetrated with inorganic materials. Removal of the colloids leads to a macroporous structure with an ordered array of spherical voids. These methods have been widely applied to polymers, ceramics and even metals. Such samples provide monolithic high surface area samples well suited for a number of applications, including catalysis, sensors and chromatography. In addition, because of the long range ordering of the voids, the materials exhibit diffractive properties making them suitable for numerous optical applications. The goal of this collaboration would be to combine the expertise at LANL in actinide chemistry with Colvin's research in this area to create macroporous actinide ceramics. While potentially valuable for a number of applications, actinide ceramics are particularly well-suited for optics. Their high refractive index, in particular, suggests that if made macroporous, the materials would exhibit much stronger diffractive properties and hence higher performance optical filtering than the corresponding transition metal macroporous ceramics. Our specific plan would be to adapt current methods for forming dense solid ceramics from sol-gel type reactions to the actinide reactions. Colvin's group has found that simple hydrolysis of metal alkoxides, completed using a simple dip-coat technique, leads to optical materials which are non-uniform due to thick non-porous crusts that form on the top and bottom of the opal templates. To circumvent this, alkoxide condensation needs to occur not homogeneously in solution, but heterogeneously at the surfaces of the colloids. By lowering alkoxide concentrations and using alcohol as a solvent, it is possible to deposit condensed ceramics only at surfaces and as a result homogeneous macroporous ceramics can be produced with titania and zirconia. The first goal of our work would be to explore whether similar conditions can be used to condense urania only at surfaces. Then, we will use this chemistry to deposit ceramics in the colloidal interstitial areas. Colvin's group will supply the opal templates, LANL will work with her group on the alkoxide chemistry. Optical characterization is routine and can be performed at both locations. The impact of this work would be twofold. First, it would illuminate the strategies for chemical processing of actinide alkoxides into optical materials. While much is known about the solution phase chemistry, there is relatively less known about their consolidation into solid ceramics. This project would require that this issue be addressed directly. Second, it would have the potential to create new materials of relevance to a number of emerging technologies. The diffractive properties of chemically assembled optics find application in any process that requires large format and lightweight diffractive filters. Perhaps the most suitable would be the use of macroporous urania as optical filters in thermophotovoltaic energy conversion (TPV). This technology is proposed for use in nuclear power plants to convert waste heat into usable electricity; formation of high quality and cost-effective band-pass filters is the central problem, and macroporous urania films could potentially offer the ideal solution. Relevant Literature P. Jiang, J. F. Bertone, K, S. Hwang, and V. L. Colvin. Single Crystal Colloidal Multilayers with Controlled Thickness. Chemistry of Materials 11(8), p.2132-2140, 1999. Bertone JF. Jiang P. Hwang KS. Mittleman DM. Colvin VL. Thickness dependence of the optical properties of ordered silica-air and air-polymer photonic crystals. Physical Review Letters 83(2):300-303, 1999 Jul 12 Mittleman DM. Bertone JF. Jiang P. Hwang KS. Colvin VL. Optical properties of planar colloidal crystals: Dynamical diffraction and the scalar wave approximation. Journal of Chemical Physics 111(1):345-354, 1999 Jul 1. Jiang, P., Colvin, V. L. et al., Macroporous Metals Made from Colloidal Crystal Templates. J. Am. Chem. Soc., 1999, to appear in September, 1999 issue. Jiang, P., et al., Colvin, V. L. Macroporous Polymers: Preparation and Diffractive Properties. J. Am. Chem. Soc., 1999. In review. Asher, S. A., Crystalline Colloidal Bragg Diffraction Devices: the Basis for a New Generation of Raman Instrumentation. Applied Spectroscopy 1989. 1(12): p. 26. Kazmerski, L.L. Measurements and Characterization of Photovoltaics: Lessons Learned for TPV in The Third NREL Conference on Thermophotovoltaic Generation of Electricity. 1997. Copper Mountain: AIP. Iles, P.A. Photovoltainc Principles Used in Thermophotovoltaic Generators in the First NREL Conference on Thermophotovoltaic Generation of Electricity. 1994. Copper Mountain: AIP. Horne, W.E., M.D. Morgan, and V.S. Sundaram. IR Filters for TPV Converter Modules in The Second NREL Conference on Thermophotovoltaic Generation of Electricity. 1997. Copper Mountain: AIP. Asher, S. A.; Patent: Crystalline colloidal narrow band radiation filter. 1986. Proposed Budget Travel funds are requested to initiate this ICAM collaboration between Los Alamos and Rice University. These funds will cover travel, lodging, and per diem for three one-week student trips to conduct scoping experiments at Los Alamos and Rice and two one-week trips for the principal investigators to consult and develop intra- and extramural research proposals.
3. Theoretical Approaches to Lanthanide Ions in the Solid State and the Environment D.L. Cox (UC Davis), and R.L. Martin, Jeff Hay, and Lawrence Pratt (Los Alamos)
The study of actinide, lanthanide, and transition metal ions in aqueous solution under varying chemical conditions is of considerable importance to design of toxic and nuclear waste storage facilities and cleanup of existing waste spills. The complexity of studying these ions in solution is already difficult by virtue of their rich orbital structure, and in the case of light actinide ions and transition metals, their highly variable valence. Hence, the theoretical study of ground state and thermodynamic energies relevant to equilibrium constants using ab initio methods such as hybrid density functional theory together with classical dielectric theory for the water is already a forefront research area. To make matters more complicated however, these ions have strong, local electronic interactions which can strongly affect the excitation spectrum. The effects of strong local interactions on excitation spectra are well known to be inadequately treated in traditional density functional methods. Hence, while properties sensitive to total energy differences might be reliably examined using existing ab initio schemes, properties transfer processes may not be. Recently, in the context of solid state physics, considerable progress has been made with an ad hoc scheme for combining ab initio electronic structure calculations with many body theory. Essentially, the electronic structure calculation is used to provide a starting point one particle reference and model parameters (such as the Hubbard $U$) for input to methods of many body solution of local "quantum impurity" models. These many body methods range from highly accurate and computational intensive, such as quantum Monte Carlo, to approximate yet very reliable and fast, such as the so called "non-crossing approximation." This hybridization of ab initio electronic structure with many body theory goes under the whimsical name of "LDA++"1, and among the recent applications are:
There is reason to believe that the strong interaction effects important for these ions in solids also manifest in molecules and aqua ions. For example, the cerocene molecule Ce[(CH)8]2 which has formally tetravalent Ce, has been argued to have a many body singlet ground state6 with a quantum mixed valence on the Ce site of about 3.5. Such singlet ground states are responsible for the formation of heavy quasiparticle excitations in Ce intermetallics (heavy electron materials) and close to the Mott-Hubbard transition of transition metal oxides. The f-electron addition and removal energies for elemental lanthanide metals7 are in almost every case very close in value to the corresponding redox potentials for the corresponding ions in solution,8 suggesting that the local physics is very similar. In particular, for Ce in solution, the local oxygen coordination is that of CeO2, in which the Ce ion is known to have a mixed valence of average value approx 3.5. We propose to combine the expertise of the Los Alamos group in studying f ions and molecules in solution using hybrid density functional theory together with dielectric theory with the expertise of the theory group at UC Davis in modeling quantum impurity problems. Our goal will be to examine the excitation spectra and many body effects on wave functions and ground states of various aqua ions and molecules. Of particular importance as a model system will be Ce both in solution and in the cerocene molecule (can the "LDA++'" reproduce the quantum chemistry results of Ref. 6), and light actinide molecules such as uranocene together with aqua ions. We will first study individual ions and molecules and then turn to a study of reactions between ions and molecules. We believe that these calculations will be of considerable importance for obtaining quantitative and predictive understanding of the redox/charge transfer reaction chemistry of these important ions. Support under ICAM will allow participants to travel between UC Davis and Los Alamos, and to partially support graduate students and/or postdoctoral researchers working in this area.
4. Nanomaterial Applications to Problems in Actinide Environmental Geochemistry Paul Alivisatos (UC Berkeley), Vicki Colvin (Rice University) and John P. Kaszuba (Los Alamos) The Department of Energy’s (DOE) historic mission with nuclear weapons produced large volumes of high-level waste and widespread radioactive accumulations in tank farms, soil, and groundwater within the DOE complex. Radionuclides in these wastes include toxic, highly-soluble uranium, neptunium, plutonium, and technetium that pose threats to human health and environment. If unchecked, these potential environmental hazards threaten the viability of nuclear energy in the United States. Traditional approaches of environmental aqueous geochemistry include using natural materials as reactive barriers or encapsulating precursor phases. Examples include phosphate, silica, iron, and cement. Several problems, however, remain intractable. Technetium, for example, occurs in most aqueous systems as the large pertechnetate anion. This ion does not sorb to naturally-occurring materials, nor does it easily fit into most lattice sites. Scientifically new and technologically important materials are emerging that may address some of these problems. Examples include anionic sponges tailored to encapsulate large radionuclides and hollow ceramic spheres as traps for waste containment. New developments in the area of porous materials have now made it possible to create monolithic ceramic sponges. These systems not only possess large internal surface areas, but contain spherical voids interconnected throughout the entire sample. Such accessibility suggests these samples, if engineered with the appropriate ceramic material, may offer a high capacity sequestering agent for hazardous waste. Alternatively, hollow ceramic spheres which can disperse in liquids may offer similar containment in the solution phase. These spheres, typically made with diameters ranging from 100 to 400 nanometers, can be made from a variety of ceramics and present high surface areas for sequestering. Since they are hollow, they do not settle out of aqueous environments and could serve to remove hazardous waste in solution. The applications of these new developments in nanoscale materials to problems in actinide environmental geochemistry are potentially quite extensive. We seek a bridging grant to make it possible for us to 1) obtain travel funds for 3-4 total trips over the period of one year to begin to develop this collaboration, 2) determine interest from graduate students and/or postdocs and fund 1-2 visits to LANL for them, and 3) define the project, identify funding for the work, and work on grants. We request a sum of $16,000 to accomplish these tasks. This total includes $9,000 for travel for students and for Drs. Kaszuba and Colvin. It also includes $7,000 for costs for Drs. Kaszuba and Colvin and for preliminary scoping experiments or feasibility checks. Pertinent References "The aqueous geochemistry of neptunium: Dynamic control of soluble concentrations with applications to nuclear waste disposal", J.P. Kaszuba and W.H. Runde, ES&T in press. "Synthesis of TiO2 nanocrystals by nonhydrolytic solution-based reactions", Trentler TJ. Denler TE. Bertone JF. Agrawal A. Colvin VL J. Am. Chem. Soc., 121(7), 1613-1614, 1999. "Macroporous Metals Made from Colloidal Crystal Templates", P. Jiang, J. Cizeron, J. F. Bertone and V. L. Colvin J. Am. Chem. Soc., 1999, to appear in September, 1999 issue. "Macroporous Polymers: Preparation and Diffractive Properties" P. Jiang, K. S. Hwang, J. F. Bertone, D. M. Mittleman and V. L. Colvin, J. Am. Chem. Soc., 1999. In review. "Neptunium and plutonium solubilities in a Yucca Mountain groundwater", D.W. Efurd, W. Runde, J.C. Banar, D.R. Janecky, J.P. Kaszuba, P.D. Palmer, F.R. Roensch, and C.D. Tait ES&T 32, 3893-3900, 1998.
5. Thermodynamic Applications to Problems in the Geochemistry of Plutonium Colloid Complexes John P. Kaszuba (Los Alamos) and Alexandra Navrotsky (UC Davis) The historic production and testing of nuclear weapons in the DOE complex contaminated numerous sites with radioactive materials. Most current research in environmental geochemistry addresses the equilibrium thermodynamics and phase equilibria of aqueous species and solids containing radionuclides. A recently-publicized problem is the transport of radionuclides on colloids. Plutonium at the Nevada Test Site, for example, has migrated a much greater distance than was predicted because of plutonium complexation with colloids. The Waste Isolation Pilot Plant in New Mexico is evaluating reactive barriers that inhibit colloid formation in aqueous systems with high ionic strength. Despite these known problems with colloids, the stability and behavior of plutonium colloid complexes in the aqueous environment is not understood. Calorimetry is one of the most powerful techniques, sometimes the only technique, for providing the fundamental thermodynamic data needed to understand and predict the behavior of the mineral materials in the environment. Application of this technique to plutonium colloids may provide the insight needed to understand and predict the bonding characteristics and stabilities of these materials as well as their behavior in the environment. Using oxide melt reaction calorimetry, Navrotsky's group is currently pursuing three projects of relevance to Pu-colloids. (1) We are determining the enthalpies of formation of a series of rare earth, uranium, and thorium pyrochlores and fluorite based materials at UC Davis. This is complemented by the construction of a similar calorimeter at Los Alamos, with a recent Ph.D. of ours, Robert Putnam, having just joined Mark Williamson's group to work directly on Pu-containing ceramic materials. (2) We are working at UC Davis on calorimetric studies of heats of formation and surface energies of nanophase oxides and oxyhydroxides, especially containing Fe, Al, and Mn. (3) With Norwegian colleagues we have been studying the energetics of some lanthanide oxycarbonates. Combining the methodology of these three projects could lay the groundwork for energetic study of lanthanide and actinide oxyhydroxides and oxy- and hydroxy - carbonates, small particles and coatings of which are implicated in the aqueous transport of uranium and plutonium. We seek a bridging grant to make it possible for us to 1) obtain travel funds for 3-4 total trips over the period of one year to begin to develop this collaboration, 2) determine interest from graduate students and/or postdocs and fund 1-2 visits to LANL for them, and 3) define more precisely the collaborative project, identify funding sources for the work, and submit grant proposals. References "The aqueous geochemistry of neptunium: Dynamic control of soluble concentrations with applications to nuclear waste disposal", J.P. Kaszuba and W.H. Runde, ES&T (in press). "Energetics of Stable and Metastable Low Temperature Iron Oxides and Oxyhydroxides", C. Laberty and A. Navrotsky, Geochim. Cosmochim. Acta 62, 2905-2913 (1998). "Thermodynamics of Formation of Zirconolite (CaZrTi2O7) from T = 298.15 K to T = 1500 K", R. L. Putnam, A. Navrotsky, B. F. Woodfield, J. Boerio-Goates, and J. L. Shapiro, J. Chem. Thermo. 31, 229-243 (1999). "Energetics of Kaolin Polymorphs", D. de Ligny and A. Navrotsky, Amer. Miner. 84, 506-516 (1999). "Thermochemistry of Crystalline and Amorphous Phases Related to Radioactive Waste", A. Navrotsky, in Actinides and the Environment, P. A. Sterne et al., Eds., Kluwer Academic Pub., The Netherlands, 267-97 (1998). "Neptunium and plutonium solubilities in a Yucca Mountain groundwater", D.W. Efurd, W. Runde, J.C. Banar, D.R. Janecky, J.P. Kaszuba, P.D. Palmer, F.R. Roensch, and C.D. Tait ES&T 32, 3893-3900 (1998).
6. Anomalous Elasticity and Electronic Structure in Actinide Compounds Gabriel Kotliar (Rutgers University) and Joe Thompson (Los Alamos) The interplay of structural properties and electronic structure is a central theme in materials science. Actinide materials and plutonium in particular, challenge us with anomalously large volume changes in response to small perturbations (slight changes in composition or temperature, intentional doping with impurities). We propose to develop the physics concepts and the computational techniques for understanding and predicting the interplay of elasticity and electronic structure (as measured in photoemssion and thermodynamic properties) in strongly correlated systems in general, and, in particular, in the actinides where these effects are manifest in a most dramatic form. These systems have one of the most {complex} phase diagrams, featuring large volume changes, and anomalous elastic distortions. The understanding of these materials requires the extension of the existing conceptual framework and computational techniques far beyond our current capabilities. Disorder is fundamental to understanding the characteristics of these systems. For instance, small percentages of Ga or Al alters the sign of the thermal expansion in a high temperature phase of plutonium metal. Furthermore, these systems are generally far from equilibrium, given their radioactivity, phase multiplicity and complex preparation conditions. For example the electrical resistivity of plutonium is known to be a strong function of time. Ageing, and long time dynamics are characteristics of non-equilibrium systems adjusting slowly to a time-dependent environment. The dynamical mean field technique, to which Kotliar has made substantial contributions, has been instrumental in unravelling {equilibrium} properties of complex materials. It has been formulated as a first-principle theory, capable describing in detail the complicated chemistry of d and f electron systems. It has succeeded in predicting the equilibrium volume of delta plutonium, where more conventional techniques such as density functional theory have failed. It has provided a first principles description of the Mott transition in three-dimensional transition metal oxides. A central concept emerges from this approach, namely the crucial importance of the one-electron spectral function which plays the role of a Weiss field, in systems where well-defined quasiparticles do not exist. These techniques and notions will play an even more important role in systems far from equilibrium. We propose to extend concepts and techniques from dynamical mean field theory to treat situations far from equilibrium. This is required by the need to describe actinide-based compounds in particular and complex correlated systems in general. The goal is to provide a unified picture of thermodynamics, spectral properties and structural properties of materials containing f electrons. J. Thompson in Condensed Matter and Thermal Physics at Los Alamos and G. Kotliar in the Center for Materials Theory at Rutgers would closely collaborate in developing the concepts and techniques required for experimental and theoretical developments required for the quantitative description of these strongly-correlated systems. (Support under ICAM would allow Kotliar to travel between Rutgers University and Los Alamos, and to partially support graduate students and/or postdoctoral researchers working in this area.)
7.Actinide Structure Issues Peter Nordlander and Gus Scuseria (Rice University) and Richard Martin, Jeff Hay, and Matt Challacombe, (Los Alamos) We wish to initiate a collaboration on theoretical issues relevant to actinide electronic structure.
1. Pattern Formation in Langmuir Films David Oxtoby (University of Chicago), and Atul Parikh and Branko P. Stojkovic (Los Alamos) Experiments performed on Langmuir films of binary mixtures show dynamic pattern (stripe or droplet) formation, as a result of competition of long-range and short-range interactions. While the (quasi) equilibrium properties of such systems are well known, the dynamics of pattern formation is not well understood. Moreover, similar, but much more complicated pattern formation occurs also in dewetting problems. We propose to study the pattern formation in 2D mixtures by developing accurate free energy functionals for these systems and obtain a number of analytical and numerical results for the appropriate order parameters and correlation functions. This method will enable us to generalize the obtained results into general principles of organization of matter in such systems. Furthermore, we will seek the existence of similar principles in the existing results for systems where free energy landscape studies show slow dynamics, such as glasses.
2. The Influence of Protein Substrates on the Chemical Function of Heme Proteins Peter Wolynes (UIUC), Benjamin H. McMahon and Branko P. Stojkovic (Los Alamos) Proteins are biological systems of mesoscopic size, which are responsible for essentially all of the biochemistry of life. Hundreds of proteins have been isolated and extensively characterized both structurally and functionally. Crucial roles of electronic, vibrational, and conformational motions in determining the rate at which proteins carry out their function has been well established. However, general principles of organization of proteins which are presumably responsible for their particular amino acid sequences (so called "mesoscopic protectorates in biological matter") are not yet available. We propose to apply theoretical methods from both soft and hard condensed matter physics, including quantum chemistry calculations to address local structural energetics, and monte carlo and other techniques commonly used in statistical physics (through, e.g., the mapping of protein function onto reduced models of the confirmational states of a folded and partially folded protein) to quantitatively explain how protein substates influence the chemical function of heme proteins. We believe that models, much like the defect picture of ordinary crystalline materials, can be used to account for the influence of distant side chains on the local chemical dynamics and can establish the basis for a "protein function protectorate." We will focus on CO binding to myoglobin for which there exists an enormous body of experimental data, still not well understood and then extend our work to other ligands, such as O2 or NO and to other proteins in a predictive fashion. This collaboration of analytic and computational theorists with experimentalists is crucial for a successful completion of this project.
3. Stability of Quantum Glasses J. Schmalian (Iowa State and Ames National Lab) and P. G. Wolynes (UIUC) Broken ergodicity and anomalous long time relaxation belong to the hallmarks of classical glasses. In order to describe this behavior a scaling theory for the dynamics close to an ideal structural glass transition was developed (T. R. Kirkpatrick, D. Thirumalai, and P. G. Wolynes, Phys. Rev A 40, 1045 (1989)), where the supercooled-liquid phase is composed of glassy clusters separated by interfaces or domain walls. The driving force for the formation and disappearance of droplets is the configurational entropy. In the quantum regime the new question arises of whether quantum tunneling between states, normally separated by large energy barriers, leads to an ergodic behavior of the system. The main idea is that non-ergodic behavior would manifest itself as a quantum localization (D. Logan and P. Wolynes, J. Chem. Phys. 93, 4994 (1990)) in the space of states which are classically pure states of the glass but are connected by tunneling processes in the quantum regime. Two states which are connected by a tunneling process are assumed to have a slightly distinct droplet distribution. Thus, one needs to determine the tunneling rate, T(R), and the density of states, D(E,R), of states with characteristic droplet radius, R. These quantities will be determined by extending the scaling picture of classical glasses to the quantum case and enable us to decide whether localization or delocalization in configuration space, i.e. nonergodicity or ergodicity, occurs. Thus, depending on the droplet scaling behavior and the number of states with characteristic droplet size we can analyze the stability of a quantum glass. While we believe that the question of quantum stability is of obvious conceptual relevance to the low temperature structural glasses, it may also be essential to disordered electronic systems. Puzzling glassy behavior of electrons has been observed in doped semiconductors and in part of the phase diagram of high-temperature superconductors.
4. Inelastic Neutron Experiments on the Mesoscopic Organization of Spin and Charge in Underdoped Cuprate Superconductors G. Aeppli (NEC), T. Mason , P. Dai and , H. Mook (ORNL), R. Birgenau (MIT), G. Shirane, (BNL) and Dirk Morr and David Pines (Los Alamos) We seek to understand the mesoscopic organization of the spin and charge degrees of freedom in the high temperature superconductors. Of particular interest is the question of the microscopic origin of the incommensurate spin response, observed in YBa2Cu3O6+x, La2-xSxxCuO4 and Bi-2212 in the low frequency regime. Some first steps in this direction have been taken in two recent papers by Morr and Pines and by Morr, Schmalian and Pines. The first paper investigates the interplay between spin and charge excitations in the superconducting state and presents a connection between inelastic neutron scattering (INS) and angle-resolved photoemission (ARPES) experiments. The second paper presents a theoretical scenario in which one can simultaneously describe INS and nuclear magnetic resonance (NMR) experiments. By combining the results obtained from these two experimental probes, in a theoretical analysis Morr, Schmalian and Pines have shown that the incommensurate magnetic response in the high temperature cuprates very likely originates from a mesoscopic organization of the cuprates into magnetic clusters. Initial steps have been taken to develop joint theory/experiment collaborations with Aeppli, Mason, and their colleagues and with the group of Birgenau, Shirane, et al. In particular, Morr and Pines are currently studying to what extent the experimental data, e.g., intensity, line width, etc., constrain the theoretical scenarios for the mesoscopic organization of the spin and charge degrees of freedom. To advance our understanding of the nature of the magnetic structure, the experimental side of this collaboration proposes to extend the scope of their experiments to a wider range of cuprates, while the theorists intend to continue their study of the dynamical behavior.
5. Electrically Active Domain Walls in Manganites Andy Millis (Rutgers Univ.) and Avadh Saxena (Los Alamos) A crucial theme in materials science is the formation of small-scale structures with interesting or useful properties. One mechanism for the formation of nanostructures within a bulk sample involves the coupling of long-ranged elastic forces to the local charge and spin degrees of freedom of complex materials. We propose to investigate this in the context of the "colossal" magnetoresistance rare earth manganites. These materials exhibit a wide range of interesting and potentially useful properties, all related to an unusually sensitive dependence of physical properties on external perturbations including magnetic or electric field, applied strain, and chemical composition. The sensitive dependence has been empirically related to the existence of a particular kind of nanoscale structure, namely small metallic "rivers" running through an intrinsically insulating background. We will explore the possibility that these metallic regions are domain walls between differently ordered insulating regions. The work will require linking expertise in atomic-scale many-body physics (Rutgers) with issues of mesoscopic ordering and its coupling to strain fields (LANL). Our central point is that long-range elastic forces have texture-inducing tendencies which can be realized through strong coupling in complex materials. Thus effective, strain-mediated long-range interactions between charges (and spins) provide a generic (and non-Coulombic) elastic mechanism for cooperative, mutual, multiscale patterning of all the fields (i.e. strain, charge and spin). The manganites are an ideal system in which to investigate this issue because they are the subject of extensive experimental study and are known to display a wide range of spin and charge ordering phenomena which couple strongly to strain. The theoretical analysis of intrinisically inhomogeneous states requires a combination of atomic-scale `strong correlation' physics and a continuum theory to describe texture/domain formation. One natural approach is a Ginzburg-Landau formalism with charge/spin considered as primary order parameter and the strain as a secondary order parameter and with the atomic scale physics expressed via coefficients determined from more fundamental calculations and structural data. The proposed research is a natural combination of the expertise at Rutgers in fundamental theory of correlated materials and at Los Alamos in elasticity theory, time dependent Ginzburg-Landau simulations and large scale computing. The mechanism for transferring expertise will be to send a Rutgers graduate student to work at Los Alamos. The student will gain experience in large scale computation and time-dependent Ginzburg-Landau theory, and in addition will benefit by spending time in a different and very interactive environment.
6. Shape Deformation, Phase Separation and Hydrodynamics in Vesicles and Related Soft Condensed Matter Systems Jack Douglas (NIST), Evan Evans (Univ. British Columbia), Greg Smith, T. Lookman, A. Saxena, Y. Jiang, Alan Bishop, and A. N. Parikh (Los Alamos) Despite their differing functions, all biological membranes have a common structure: a very thin film of lipid and protein molecules, which are able to move about in the plane of the membrane. Artificial membranes can be synthesized from certain amphiphilic molecules, such as phospholipids. These molecules assemble in water to build bilayer membranes and vesicles. In many ways, these lipid bilayers and vesicles are simple models of biological membrane and cells, especially for studying physical properties such as shape transformations, elasticity, transport. They show an amazing variety of shapes of different symmetry and topology, under different temperature, osmotic pressure, chemical concentration, etc. Spherical and tubular vesicles are common in nature. Toroidal vesicles, including those with higher genus, have also been observed. It has recently been recognized that internal degrees of freedom of membranes can crucially influence their shapes. Experiments indicate that a phase separation occurs on a two-component artificial membrane, where phase separated domains prefer increase or decrease of local curvature depending on local composition of the membrane. Moreover, it has been observed that in a two-component system the line tension of domain boundaries can cause budding. Recently the dynamics of a two-component vesicle have been simulated using a purely dissipative model in which hydrodynamic interactions have been neglected. In order to study shape deformations when the vesicle is immersed in a low viscosity fluid or in different hydrodynamic regimes, coupling to a hydrodynamic model is essential. We propose to study the free energy that includes curvature terms and hydrodynamics and understand how the local coupling of the curvature, the composition of the membrane and the line tension at domain boundaries influence the formation of protrusions on the membrane and how these affect growth kinetics. The physics underlying these shape transformations involves a subtle interplay of fluid and elastic properties plus highly nonlinear geometric and hydrodynamics effects. Most theoretical work of shape transformations of vesicles has treated membranes as a laterally homogeneous elastic layer with area and volume constraints. However, other factors play a crucial role. For example, the transition from a biconcave shape of discocytes to a crenated echinocyte shape of a human red blood cell can be induced by an asymmetric adsorption of certain drugs, i.e., a local asymmetry in the composition plays an important role in the crenated shape. As molecules are free to move, in plane phase separation are constantly observed in lipid membranes. A coupling between the local curvature and the order-parameter field can result in shape deformation in vesicles. Other systems of technological interest, with interaction between the shape and internal degrees of freedom of the surface, include crystal growth on curved surfaces and thin film deposition. In these cases the coupling initiates, modifies or eliminates chemical or physical intra-membrane domain ordering processes. This is a joint experiment-theory collaboration with active student and postdoc participation. Jack Douglas (NIST), Evan Evans (Univ. British Columbia), Greg Smith (LANSCE) and Atul Parikh (CST-1) will carry out the necessary synthesis and characterization.
7. Glassy Behavior in Unconventional Superconductors Joerg Schmalian ( Iowa State and Ames National Laboratory), Chris Hammel, Nick Curro, Branko Stojkovic, Joe Thompson, Alan Bishop, and John Sarrao (Los Alamos) Recent NMR measurements on doped La-based cuprate antiferromagnets show activated behavior of the La-spin relaxation with a Gaussian distribution of (small) activation energies (average barrier height and the width of the distribution both of order 5 meV). This behavior is seen regardless of the hole doping level or the presence of extrinsic disorder. Such behavior suggests a complex energy landscape governing the low frequency modes, similar to that seen in glasses. We propose a study aimed at the understanding of the mechanisms underlying the slow magnetic dynamics seen in experiment. We will use models which include magnetic domains, interacting with in-plane inhomogeneities, present due to competing interactions (short-range magnetic and long-range Coulomb between doped holes), independent of extrinsic disorder. These inhomogeneities involve holes in either stripe ordered states or holes grouped in "stripe segments," which may yield small activation energies, seen experimentally. In particular we will seek to understand the dynamics of the charged stripes themselves and the dynamics of an ordered incommensurate antiferromagnet interacting with disordered antiphase domain walls. We will also seek to examine whether similar evidence for mesoscopic organization can be found in those effectively two-dimensional organic superconductors which have been found to display pseudogap behavior in the normal state before making a transition to an unconventional superconducting pairing state.
8). Complexity in Correlated Electronic Materials B. Maple (UCSD) and A.V. Balatsky and J. Sarrao (Los Alamos) The observation of multiple, relevant length and time scales and, frequently, associated frustration and glassy dynamics in correlated electron materials is beyond any conventional independent-particle approximation. In spite of this complexity, materials as seemingly diverse as quasi-1D organics, cuprate superconductors, and correlated f-electron systems appear to exhibit a remarkable commonality in their phase diagrams of temperature versus control parameter, e.g., pressure, extrinsic disorder/doping or magnetic field. Understanding such "organizing" universality in the presence of complexity requires entirely new levels of scientific inquiry and new scientific principles. To address these questions, we propose a program in complex electronic materials that integrates synthesis with multiscale characterization and modeling. Our starting point is realization that competing interactions simultaneously couple, non-linearly, spin, charge, and lattice degrees-of-freedom to produce intrinsic inhomogeneity on multiple energy, length, and time scales. Large-scale sensitivity to small perturbations comes from tipping the delicate balance among interactions. This sensitivity is most pronounced near the boundary between broken-symmetry ground states or between broken-symmetry and complementary disordered states. As transition temperatures of the states are tuned toward zero temperature, quantum fluctuations become non-negligible and invalidate conventional theories of phase transitions and the thermodynamic notion that entropy must approach zero with decreasing temperature. The scientific issues presented by these materials are clear: What are the fundamental similarities in the nature of microscopic interactions in these complex materials that lead to intrinsic inhomogeneity and common phase diagrams? What are the relevant spatial and temporal measures of complexity controlling macroscopic function? How are quantum and thermal fluctuations affected by coexisting intrinsic and extrinsic disorder? We propose to investigate the similarities of the generalized phase diagram and quantum critical behavior in low-dimensional electronic materials that demonstrate competition between distinct ordered and disordered broken-symmetry states. We will focus on the phase diagram and quantum critical point description of low-dimensional organic materials, inorganic ladder compounds and quasi-two-dimensional materials, such as cuprates. We propose a unifying approach to these classes of materials where sensitivity to control parameters (temperature, pressure, doping, and magnetic field) results from competition between different ordered and disordered states on multiple length and time scales.
9. The Emergence of the Phylotypic Stage of Biological Development Andrei Ruckenstein (Rutgers University) and Michel Kerszberg (Pasteur Institute) Sometime ago Michel Kerszberg and Zvia Agur have proposed an abstract cellular automata model of biological development [American Naturalist, vol 129, No. 6, June 1987, 862]. They introduced a number of simple hierarchical mappings from the space of all possible gene compositions (the "genotype") to the space of possible expression of these genes. The deepest level of the hierarchy, is supposed to represent the collection of characters of interest associated with the starting population, known as the "phenotype". For example, the starting point might be two different forms of a particular gene (two different "alleles") one of which is a mutant form of the other -- the two alleles constitute our "genotypical space". We might be interested, for example, in whether the mutation will affect a particular trait (say, the resistance to some pesticide). The degree of resistance of the emerging organism is what defines the "phenotypical space". The development of the phenotype from the starting genotype is a non-linear dynamical process in the course of which the system visits all stages of development. In the two-allele example the intermediate levels of development may correspond to some other genes being activated in the process of synthesizing one or the other form of a protein coded by the two forms of our gene. The level of pesticide resistance, may depend in an essential way on the interactions with the activated genes which must then be incorporated as part of the rules of the dynamical system. The "genotype-phenotype" mapping proposed by Agur and Kerszberg is many-to-one and one-to-many, accounting for a number of well known biological situations: (i) many genes may be involved in determining a particular trait ("polygeny") and (ii) a gene usually has many effects - for example, by being expressed in different tissues of an organism ("pleiotropy"). Moreover, these models display "error correction" allowing only certain mutations to modify the ultimate phenotype (i.e., the model also displays "canalization", the fact that mutations of certain genes do not affect the ultimate phenotype). In the past few months, Ruckenstein and Kerszberg have begun considering applying hierarchical models of the type mentioned above to a very explicit biological context, that of the so-called "phylotypic stage" of biological development. Living beings, however varied, fall into distinct categories ("phyla"). There are about 35 distinct "phyla" in the Animal Kingdom alone, each characterized by a distinct "body plan". The unity of the body plan within a phylum is often not apparent at the earliest embryonic stage, nor is it evident in the adult individuals. For example, the early embryos of vertebrates differ greatly in form and shape as a result of the different ways in which the egg is fertilized, and the different ways in which embryos utilize nutrition sources [see, for example, L. Wolpert, Principles of Development, Oxford University Press, 1998]; and, of course, the adult mouse differs in obvious ways from a zebrafish! Remarkably, most investigated species display an intermediate stage of development -- the "phylotypic stage" -- during which all embryos of the phyla more or less resemble one another and thus best display the fundamental body plan. It is the emergence of this stage that we propose to investigate as part of the Proposal. At a more molecular level, the phylotypic stage can be defined in terms of the spatial and temporal expression of a self-sustaining network of a number of selector genes, most notably the Hox genes [Slack, Holland and Graham, Nature 361, p.490, 1993] which differentiate regions of the embryo from one another. It is then natural to attempt to understand the emergence of the phylotypic stage as an "unstable fixed point" of a non-linear dynamical network model. The model should be detailed enough and biologically realistic enough to also describe some of the main features of the pre-phylotypic stage of embryonic development, as well as the diversity of the phenotypes emerging in adulthood. More generally, patterns of expression of developmentally important genes are similar at a certain stage even across phyla, for example in insects and vertebrates. We also hope to eventually understand this more general but related issue. We are looking for funding for one month summer salary for AER and a number of collaborative visits (three to four per year) across the ocean. |