USC Physics Seminars

                                   Quantum Information – Condensed Matter – Biophysics

These seminars are scheduled on Fridays at 2:00pm, and the location is SSL 150, unless otherwise noted. Some of the seminars will be held jointly with UCLA and CALTECH.
For more information contact Lorenzo Campos Venuti (condensed matter) or Ben Reichardt (quantum information). Biophysics seminars are held the first Friday of each month and are indicated in blue.
UPC seminar location: Ahmanson Center for Biological Research, ACB 238. UPC location for remote viewing of HSC seminars: To be determined.
HSC seminar location: Herklotz seminar room, Zilkha Neurogenetic Institute HSC.  Location for remote viewing of UPC seminars: Herklotz

August 28, 2pm SSL150
Andreia Seguia (Universidade Federal Fluminense Brazil
Quantum phase transitions in a chain with two- and four-spin interactions in a transverse field

We use entanglement entropy to investigate the ground-state properties of a spin-1/2 Ising chain with two-spin (J2) and four-spin (J4) interactions in a transverse magnetic field (B). We concentrate our study on the unexplored critical region B=1 and obtain the phase diagram of the model in the (J4-J2) plane. The phases found include ferromagnetic (F), antiferromagnetic (AF), as well as more complex phases involving spin configurations with multiple periodicity. The system presents both first- and second-order transitions separated by tricritical points. We find an unusual phase boundary on the semi-infinite segment (J4<1,J2=0) separating the F and AF phases.

September 4, 2pm HSC
James Boedicker (PHYS)
Predicting and Controlling the Activity of Microbial Ecosystems

September 18, 2pm SSL150
Jie Yuan (
RWTH Aachen, Germany)

Triplet pairing driven by Hund's coupling in doped monolayer MoS2

We investigate superconducting pairing driven by electron-electron interactions in a theoretical model for monolayer MoS2 with the temperature-flow functional renormalization group (fRG). At low doping, the dominant instability is toward odd-parity pairing with f-wave Mo-nearest-neighbor structure. We compute the fRG phase diagram versus electron doping below the van Hove filling of the conduction band. In the superconducting regime, the critical temperature grows with doping, comparable to the experiments. Near van Hove filling the system favors a ferromagnetic state. We demonstrate that the triplet pairing is driven by ferromagnetic fluctuations and that the multiorbital nature of the conduction band as well as the Hund’s coupling appear crucial in making the physics of MoS2 different from e.g. doped graphene.

Emily Liman (BIOL)

October 9, 2pm SSL150
Paul Marjoram (Biostatistics Division, Keck School of Medicine, USC)
Statistical Analysis of High-Dimensional Data

Modern datasets are growing increasingly large and complex. This leads to lack of tractability for traditional analysis methods, often resulting in the use of approaches such as optimization that are designed to find a single, best-fitting model. In this talk we argue for the advantages of a full, statistical treatment of such data, which allows for principled inference regarding the relative fit of differing models or parameter values. We then outline the way that existing statistical methods have been adapted in order to restore tractability of such analyses in the modern, high-dimensional context, primarily through increasing exploitation of simulation-based methods. We illustrate with a number of examples from the field of biology, including agent-based models. Finally, we discuss connections between those biological systems and interactive particle systems, arguing for the potential for physics to bring some theoretical rigor to the often somewhat ad-hoc predictions of biological models.

October 23, 2pm SSL150
Ariane Briegel (Caltech)
New insights into structure, assembly and function of chemoreceptor arrays from electron cryotomography

Nearly all motile prokaryotic cells utilize a highly sensitive and adaptable sensory system to detect changes in nutrient concentrations in the environment and guide their movements towards attractants and away from repellents. This chemosensory system allows the cells to selectively colonize preferential environments and is also involved in host infection by some pathogenic bacteria.
The bacterial chemoreceptor array is a polar, highly organized sensory patch composed of thousands of transmembrane receptor proteins. Attractants and repellents bind to the sensory domains of these receptors, thereby regulating activity of the histidine kinase CheA, which phosphorylates a soluble messenger protein. This messenger protein in turn diffuses through the cytoplasm to the flagellar basal body, where it modulates the direction of flagellar rotation.
By combining 3D data from electron cryotomography (ECT) with high resolution structures derived from crystallography, we have determined that native chemoreceptor arrays are composed of trimers of receptor-dimers that are connected by rings of the histidine kinase and a linking protein, CheW. Analyses of receptor complexes assembled both in vitro and in vivo have yielded new insights into de novo array formation. Following commonly used in vitro protocols and comparing these assemblies with in vivo arrays, we have proposed a model for the formation of chemoreceptor arrays in which CheA and CheW cross-link the receptors into an extended hexagonal lattice.
To gain insight into how the activity of the kinase CheA is controlled in the native array, we used ECT to characterize a set of receptor mutants that lock the kinase in specific activation states. These studies revealed that kinase activity relies on the flexibility of two of the five kinase domains, and that inactivation occurs by the unproductive binding of these domains.
While the best-studied bacterial chemoreceptor arrays are membrane-bound, many motile bacteria and archaea contain one or more additional, purely cytoplasmic chemoreceptor systems. We have recently reported the architecture of the cytoplasmic chemoreceptor arrays and are currently investigating a novel, third chemoreceptor array that is membrane bound but lacks any detectable membrane binding domain.

October 30, 2pm SSL150
Micah McCauley (Northeastern)
Energy Landscapes Determined from Single Molecule Non-Equilibrium Experiments

HIV-1 nucleocapsid (NC) proteins facilitate the rearrangement of nucleic acid secondary structure, allowing the transactivation response (TAR) RNA hairpin to be transiently destabilized during reverse transcription. Single molecule optical tweezers measurements were used to probe the stability of RNA hairpins as NC was introduced. Unfortunately, these experiments produce forces that drive hairpin unfolding/folding far from equilibrium. To overcome this limitation, the methods of Jarzynski, Crooks, Bennett and Dudko have been developed to deduce equilibrium and transition state energies of a reaction during non-equilibrium experiments. Combining these results with a quantitative mfold-based model, we characterize the equilibrium TAR stability and unfolding barrier for TAR RNA. We find that a subset of preferential protein binding sites is responsible for the observed changes in the unfolding landscape, including an unusual shift in the transition state, and results in the dramatic destabilization of this specific structure that is required for reverse transcription.

November 6, 2pm HSC
Osman Kahraman (USC)
Multiscale theory of membrane protein organization and function

Many cellular processes rely crucially on the concerted functions of integral and peripheral membrane proteins. Recent experimental breakthroughs have revealed that cell membranes are not passive envelopes with membrane proteins functioning in isolation. Instead, many key aspects of cell membrane function emerge from the collective properties of protein structure, interactions between proteins and the surrounding lipid bilayer, membrane shape, and the supramolecular organization of proteins into membrane protein lattices. In the Haselwandter group we develop novel theoretical models to capture the physical mechanisms underlying the supramolecular organization and collective function of membrane proteins. I will illustrate our work by discussing the self-assembly of membrane protein polyhedral nanoparticles, the multimerization of N-BAR proteins, and the organization of synaptic receptor domains.

November 13, 2pm SSL150

Michael Peterson (California State University Long Beach)

Non-abelian anyons in the fractional quantum Hall effect

In strongly correlated systems in two-dimensions, new quantum topologically ordered phases of matter can emerge with the fractional quantum Hall effect being the best example.  These fascinating phases can have (quasi-)particle excitations with fractional charge and fractional (braiding) statistics, i.e., anyons.  So-called non-abelian anyons have potential applications in the construction of a fault-tolerant topological quantum computer.  I will discuss numerical studies of  non-abelian anyon states in the second Landau level of the fractional quantum Hall effect.

November 20, 2pm SSL150
Richard Ross (HRL laboratories)

Electrically Controlled Qubits in Silicon

Quantum information processing aims to leverage the properties of quantum mechanics to manipulate information in ways that are not otherwise possible.  This would enable, for example, quantum computers that could solve certain problems exponentially faster than a conventional supercomputer.  One promising approach for building such a machine is to use gated silicon quantum dots.  In the approach taken at HRL Laboratories, individual electrons are trapped in a gated potential well at the barrier of a Si/SiGe heterostructure.  Spins on these electrons are compelling candidates for qubits due to their long coherence time, all-electrical control, and compatibility with conventional fabrication techniques.  In this talk I will discuss the recent demonstration of all-electrical control of silicon-based qubits made from triple quantum dots in isotopically purified material, including methods to mitigate charge noise.  The results indicate a strong future for silicon-based quantum technology.

December 4, 2pm UPC
Ralf Langen (BIOC)

December 11, 2pm SSL150
Lea F. Santos (Yeshiva University, New York)

Nonequilibrium dynamics and thermalization of isolated many-body quantum systems

We study the evolution of isolated systems with two-body interactions after an abrupt perturbation. Two aspects are addressed: the conditions for the system to reach thermal equilibrium and the description of the relaxation process. Both depend on the interplay between the initial state and the Hamiltonian after the perturbation, rather than only on the regime of the system. Thermalization may not occur in the chaotic regime if the energy of the initial state is close to the edges of the spectrum and it may take place in integrable systems provided the initial state be sufficiently delocalized in the energy eigenbasis. In the latter case, the dynamics is very fast. The decay may be exponential, Gaussian, and even faster than Gaussian. We show how the limit imposed by the energy-time uncertainty relation can be reached. In contrast, the time evolution slows down significantly when the system undergoes an excited state quantum phase transition or when disorder is added to the Hamiltonian.


January-August 2015

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