Gergely T. Zimanyi - US grants

The physics of disordered, correlated Bose sytems constitutes a major challenge. Recently a density controlled superfluid transition was observed in helium-four absorbed in porous materials with a superfluid density exponent contrdicting earlier theories; new types of scaling behavior of the resistivity have been seen in disordered superconducting films and wires; and the observation of a possible universal conductance in granular superconducting films has attracted considerable excitement. A related problem of primary importance is the vortex state of the high temperature superconductors, where an unconventional, vortex-glass phase has been observed in some materials. At the same time, the discovery of heavy fermion and oxide superconductors has driven a substantial improvement in numerical techniques and also led to genuinely new analytical approaches to strongly correlated fermions. These advanceshave yet to be applied to the interacting boson problem. The research will apply Quantum Monte Carlo, the Schwinger boson approach and the perturbative scaling methids of localization theory to explore the quantum phase transitions taking place in helium-four absorbed in porous media and granular superconductors, as modeled by a disordered, interacting boson Hamiltonian. The disorder will be handled by methods which were successful in the spin-glass problem, among others finite size scaling of appropriately chosen dimensionless quantities, and a suitable version of the maximum entropy method. %%% Theoretical research will be conducted using numerical and analytical techniques to study the behavior of disordered, interacting many particle systems which obey Bose statistics. New methods will be applied to describe such diverse physical systems as properties of helium absorbed in porous materials and the behavior of high temperature superconductors.

9528535 Scalettar This grant contains a two pronged (analytical and numerical) study of interplay between disorder and strong correlations in quantum and classical phase transitions. The research covers a broad range of topics, including boson and fermion Hubbard models, the dynamics of vortices in high temperature superconductors, quantum spin glasses, possible "supersolid" phases in Fermi and Bose systems and whether a Bose system can remain normal at zero temperature due to quantum fluctuations.. In the course of this work, there is also a plan to develop new computational tools, e.g. . applications of the maximum entropy method to the dynamical response of random systems. %%% The project is a combined effort by two researchers with complementary talents, one has a strong background in the mathematical analysis and the other is well known for his work using high performance computers. They have chosen to apply their combined talents to a wide range of problems in Materials Theory: phase transition between a superconductor and an insulator, Current-voltage characteristics of superconductors and properties of Bose condensation as influenced by a variety of perturbations. Our understanding of superfluids and superconductivity in copper oxides will be main beneficiaries of this research. ***

This award supports Dr. Gergely Zimanyi and a graduate student from the University of California-Davis in a collaboration with Gerd Sch`n of the Department of Solid State Physics at the University of Karlsruhe. The collaboration will study three topics: 1) electron tunneling through junctions and quantum dots with high tunneling conductance, 2) quantum phase slips and transport in ultra-thin superconducting wires, and 3) collective properties of arrays of Josephson junctions and granular materials. Each group brings separate and complementary expertise in the topics to be studied, and exchange of students is a high priority during the two-year award period. All three topics are of great interest to both the theoretical and experimental physics communities, and may have implications for future technological advances in high-speed electronic devices.

9985978
Zimanyi

This grant continues the NSF support of the collaborative work of two mid-career PI's from UC Davis. Their work, on collective behavior of strongly interacting many-body systems in the presence of disorder, combines analytical and numerical approaches to a variety of problems of current interest. There are 3 related projects: (1) Properties of vortices in high Tc Oxide superconductors, especially those materials where the layering effects are most strongly present. (2) A search will be carried out for a Bose metal phase in disordered superconductors. This project will also involve a study of the interplay between Coulomb interaction and disorder in 2-d metal-insulator transition and explore the role of percolation effects in non-self-averaging systems. Finally project (3) consists of an extensive study of the hysteretic phenomena in recording media. Early results indicate persistence of self-organized criticality in realistic versions of Sherrington-Kirkpatrick models. It may be that in these models, such effects as aging, domain nucleation, domain wall propagation and avalanche formation play an essential role.
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This grant continues the NSF support of the collaborative work of two mid-career PI's from UC Davis. These PI's work on a variety of problems of current interest, ranging from high Tc superconductors to properties of magnetic recording media. In the former, they are continuing with a detailed exploration of subtle effects of magnetic field on the superconducting properties. They are, in particular interested in the enhancement of these effects in systems which are more layer-like. In the magnetic recording media, they plan to further explore their early results from model calculations which show the presence of "self-organized criticality".
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TECHNICAL SUMMARY

The UC Davis & Santa Cruz Solar Team will investigate a transformative new paradigm of solar energy conversion: the high efficiency Multiple Exciton Generation (MEG) pathway and the corresponding challenge of charge extraction in all-inorganic nanostructured solar cells. MEG was recently observed in nanoparticles (NPs) and is not subject to the 31% theoretical limit of solar energy conversion. The Solar Team will synthesize pure, doped and alloyed Si and Ge core-shell NPs to analyze their chemistry, quantum states and energetics in a wide range of sizes, dopings, and structures, using PbS NPs as reference. The impact of complex factors such as the relaxation of the NP surface, the various core-shell structures, the exciton-exciton interaction and the NP-NP interaction on the chemistry and spectra of the NPs as well as on the MEG will be analyzed. The tools of the analysis will include photoluminescence and transient absorption studies with femtosecond resolution; and forming fully functional NP based solar cells, complete with embedding charge transport layers. These solar cells will be developed by optimizing the competing design principles of maintaining quantum confinement to preserve the efficiency of the MEG while embedding the NPs into suitably conducting layers for efficient charge extraction and transport. A strong theoretical effort will complement the Team?s experimental work. Density functional theories (DFT) will be used to capture the surface reconstruction and the energetics of NPs; time dependent DFT and Bethe-Salpeter methods to describe the exciton-exciton interaction; and non-equilibrium rate equations to determine the full rate of MEG. Mathematical projects will assist these efforts by developing a Lanczos coefficient extrapolation method, dramatically reducing the computational workload by replacing direct matrix manipulations with matrix by vector products; and by developing global statistical methods to qualitatively improve the analysis and extraction of the hidden dynamics from the noisy, ultra-high dimensional spectrotemporal dataset, obtained by the photoluminescence and transient absorption.

NON-TECHNICAL SUMMARY

Even in theory, the efficiency of solar cells is limited to a disappointing 31%. However, this limit was based on the traditional operation of solar cells, where an incoming solar photon excites only a single electron. A recent breakthrough showed that in nanoparticles one photon may excite several electrons, thus opening a new energy conversion paradigm not constrained by the above limit. The Davis Solar Team will synthesize a wide variety of nanoparticles; perform ultra-fast optical experiments to characterize the energy conversion process in these particles; and construct fully functional solar cells by embedding the nanoparticles into charge transport layers. Path-breaking mathematical work will be performed to accelerate the computational techniques to unprecedented speeds to simulate the energy conversion process with great accuracy. Further, qualitatively new statistical analyses will be developed to uncover the complex factors embedded in the vast amount of data produced by the optical experiments. The improvement of the solar energy conversion efficiency expected to emerge from this project can considerably increase the role solar technologies will play in the US transitioning towards renewable energy sources. The Davis Solar Team will not only develop these new nanoparticle based solar cells, but also plans to chaperon this technology towards the marketplace. This will be pursued through working with the Solar Collaborative of the California Energy Commission (SC-CEC), where the Team played an early leadership role. The Team's industrial collaboration will be developed through one of the PIs who is on the advisory board of a solar company. Besides working toward a wide acceptance of nanoparticle solar technologies, the Team will reach out and serve the solar community at large by analyzing and disseminating the latest academic research to the solar stakeholders: the PV manufacturers, utilities and the regulatory bodies through the SC-CEC. The Team will also develop a "Solarwiki" as a platform for a broad electronic outreach to the interested public. The Team will integrate its work with its activity in the ACS SEED program. Graduate students and postdoctoral fellows will work jointly with the groups of the Solar Team to foster interdisciplinary thinking and to prepare them to join the solar revolution.

Quantum dots are very small particles whose properties can be changed by changing the size, shape, or composition of the dot. This research is about understanding the interactions between these quantum dots that have been arranged into ordered solids Once the quantum dots are organized into ordered solids, called a super-lattice, then the solids exhibit new optical and electronic properties that arise from the interaction between the quantum dots. The properties of the quantum dot super-lattices are controllable by changing the coupling between the quantum dots. The electronic coupling is changed by controlling the distance between particles, by connecting the quantum dots with bridges, or by filling in the spaces between the dots with another material. This research seeks to fabricate more ordered quantum dot super-lattices to explore materials properties with utilization in devices like solar cells, photodetectors, and thermoelectrics. However, it is hard to investigate structures that one cannot see. To overcome this roadblock, the use of high-resolution scanning transmission electron tomography with near-atomic direct-space imaging will be developed. This new high-resolution tomographic data will provide sufficient detail to provide feedback between sample fabrication and resulting superlattice order to enable the fabrication of more perfect samples with larger super-lattice domains, more evenly distributed bridges, and fewer defects. The new high-resolution data will also enable new theoretical approaches to model the interaction between quantum dots in the solid so that increases in super-lattice order can be tied to specific changes in the optical and electronic properties. The long-term goal is to develop solids from quantum dots that are perfect enough to increase the charge mobility by about ten times. This research will be shared with the public by publishing the scanning transmission electron tomography data on a publicly downloadable forum and creating non-technical educational videos about the materials to be published on the internet. Outreach and education to underserved communities will provide hands-on STEM training.

Colloidal quantum-dots (QDs), organized in a super-lattice, have demonstrated collective electronic and excitonic behavior across mesoscale dimensions. The specifics of how small degrees of spatial disorder, surface chemical defects, and epitaxial defects affect this collective behavior or how to fabricate more perfect super-lattice structures are not understood. This project will use tomographic imaging with a resolution of 4-5 Å over 1000s of QDs to measure these small degrees of structural disorder in real space. This research has a strong emphasis on improving the imaging technique to enable higher resolution and to improve the reconstruction technique to increase the image volume. These improvements to the image quality will enable near atomic mapping of all QDs, necks, and defects, driving improvement in fabrication, structural control, and understanding of electronic structure/property relationships. The feedback of near atomic resolution imaging will enable improved fabrication with the goals of 100% neck connectivity and uniformity with super-lattice grain sizes of at least 10 µm and charge mobility approaching 50 cm2 V-1 s-1. The improved sample quality and high-resolution 3D real-space imaging will facilitate theoretical approaches that can study hopping vs. charge transport through delocalized ?mini-bands? and will be validated by variable-temperature Hall-effect measurements. The proposed tomography pushes the limits of resolution/volume achieving reconstructions of large mesoscale samples with high spatial resolution. The expected outcome is multiple ultra-high-resolution tomograms that inform the structure formation mechanism, improved fabrication, mass transport to form QD-QD necks, and spatial resolution to inform realistic electronic modeling based on data. The research goals are multi-pronged with focus on fabrication design rules that can be applied to other QD super-lattices, improved scanning transmission electron tomography techniques to enhance tomogram spatial resolution and data interpretation, and mesoscale modeling of delocalized transport using real spatial data. By combining these approaches this project connects between nanoscale structure, mesoscale order, and bulk materials properties.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.