Abstract. The Hubbard Hamiltonian is widely studied and applied to important problems in condensed matter physics. With only one parameter, it essays to explain the qualitative origin of (i) magnets and insulators in transition metal oxides and (ii) superconductivity and density inhomogeneities in high temperature superconductors. Now the Hubbard Hamiltonian seeks to model phase transitions in ultracold, optically trapped atoms. The two-fold challenge is to map the system onto the Hubbard Hamiltonian, and to explicitly calculate its implications for the system's properties.
Abstract. Elemental plutonium (Pu) assumes more crystal structures than other elements, plausibly due to bonding f electrons becoming non-bonding. Complex geometries hamper understanding of the transition in Pu, but calculations predict this transition in a system with simpler geometry: alternating layers of plutonium and indium. Here the transition occurs via a pairing-up of planes within Pu layers. Calculations stepping through this pairing-up reveal details of the transition, for example that the transition from bonding to non-bonding proceeds smoothly.
Abstract. Macroscopic physical phenomena play a role in the physiology and hence in the daily life of social hornets. Question: How do hornets (and honeybees) manage to construct combs with precise lateral honeycomb symmetry for breeding? Hornets even do this in totally dark underground cavern! Possible answer: They exploit ultrasonic acoustic resonance in the comb cells.
Abstract. What does spectroscopy tell us about the many-body state of a cloud of cold atoms? Can it diagnose Bose-Einstein condensation? Distinguish superfluids and Mott insulators? Detect pairs in Fermi glass? In a talk accessible to graduate students and non-experts, I will describe what we know and remaining puzzles.
Abstract. Strong interactions can affect the tunneling between metals. These effects are heightened in reduced dimensionality -- such as in quantum wires. In one dimension, repulsive interactions between electrons usually suppress the tunneling at low energies. However, if the repulsion is very strong, an opposite effect may occur, resulting in a peak in the density of states. This phenomenon results from a subtle coupling of the charge and spin excitations.
Abstract. Fractional quantized Hall systems have a highly-correlated electronic ground state with very peculiar properties. Elementary excitations possess fractional charge and obey fractional statistics, intermediate between the properties of fermions and bosons. It has been proposed that certain observed quantized Hall states may be even more peculiar, with excitations that obey "non-abelian statistics," but so far there has not been experimental confirmation of this. I will try to explain the meaning of these concepts and discuss prospects for experimental verification.
Abstract. Efforts to control single spins in semiconductors focus on the design, construction, and properties of semiconductor quantum dots. Dilute magnetic ions with room-temperature electronic bound states share many interesting and desirable properties with quantum dots, including long-range (> 3 nanometer) couplings to other magnetic ions, sensitivity to applied electric fields, and electronic transitions with large optical coupling strengths. Magnetic ions, unlike quantum dots, are identical. Recent advances in dilute magnetic ions -- theory, observation and control -- reveal advantages of these "natural quantum dots" over the traditional sort.
Abstract. Despite enormous advances in computer capacity, many important problems in condensed matter physics requires new theoretical approaches to connect between first principles fundamentals and the properties of real materials. Two examples are: 1. Highly excited electronic systems involved in problems ranging from warm dense matter to plasma etching of materials; 2. The anomalous transport and physical phenomena involved in high temperature cuprate superconductors.
Abstract. Advances in theoretical and computational chemistry are making it practical to consider fully first principles predictions of important systems and processes in the chemical, biological, and materials sciences. Our approach to applying first principles to such systems is to build a hierarchy of models each based on the results of more fundamental methods but coarsened to make practical the consideration of much larger length and time scales. Connecting this hierarchy back to quantum mechanics enables the application of first principles to the coarse levels essential for practical simulations of complex systems. We will highlight some recent advances in methodology and will illustrate them with recent applications to materials problems involving Catalysis, Nanoelectronics, Fuel Cells and pharma.
Abstract. Classical Heisenberg model on geometrically frustrated lattices possesses macroscopically degenerate classical ground states. Thermal and quantum fluctuations on the degenerate classical ground states lift the degeneracy and lead to various magnetically ordered and/or quantum spin liquid phases. Applications to volborthites, Na4Ir3O8, and Zn-paratacamite are discussed.
Abstract. Neel's concept of anti-ferromagnetic order gradually attracted wide acceptance. Indeed 30+ years of experimental search for a different phase have only recently revealed two promising examples. These, if confirmed, represent a new state of matter, called quantum spin liquid, with novel properties such as low-energy excitations that are fermionic instead of bosonic spin waves and optical absorption inside the Mott gap.
Abstract. Graphene is a single layer of carbon atoms arranged in a honeycomb lattice. A recently discovered novel technique for fabricating individual graphene layers is driving the tremendous growth in experimental and theoretical activity. The linear 2-dimensional electronic dispersion of graphene spawns surprising electronic and many-body properties and the possibility of electronic applications. Theory essays several important electronic properties of graphene, including mobility, compressibility, inelastic scattering lifetime and many-body phonon renormalization.
Abstract. [unedited, except for removing fifty `waste' words] The first step in protein synthesis is transcription of the genetic information in DNA into RNA via a complex molecular motor: RNA polymerase (RNAP). The Brownian ratchet model of transcription incorporates RNAP's internal structural degrees of freedom and its kinetic barriers to backtracking resulting from steric clashes with co-transcriptionally folded RNA. This model can (a) study the behavior of a number of mutants of RNAP, with different elongation behaviors, believed to involve specific internal motions of the enzyme; (b) predict sequence-dependent positions where RNAP pauses during the elongation process; and (c) elucidate some inconsistencies in the interpretations of single-molecule transcription elongation experiments. The same model can characterize the stability of the elongation complex at specific termination sequences, specific regions of DNA where, with high probability, RNAP releases the RNA transcript and disengages from the template. Recent experiments on termination reinforce a picture of the elongation complex as a flexible structure, not a rigid body. More generally, the modeling raises fundamental issues related to model comparison and model selection, the problem of identifying and characterizing quantitative models on the basis of limited sets of experimental data. In other works. well-known physics concerns arise in biology.
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