The Department of Physics and Astronomy Colloquium is held on Monday afternoons at 4:15 pm in room SLH 102 unless otherwise noted. Refreshments are served at 4:00 pm.

Spring 2016

January 11

Chalcogenide-type nanostructures: Topological insulator nature versus thermoelectric performance

Kornelius Nielsch
Director, Institute for Metallic Materials Leibniz Institute for Solid State and Materials Research
Dresden, Germany

View Abstract

In this presentation we challenge the interconnection between thermoelectric performance and topological insulator nature of chalcogenide-type materials. While topological surface states seem to play minor role in the thermoelectric transport in bulk materials [1], it will be shown that they severely contribute to the transport in nanostructures due to their high surface-to-volume ratio [2-4]. Specifically, thermoelectric and magnetotransport experiments on ALD-grown Sb2Te3 thin films as well as on VLS-grown Sb2Te3 and Bi2Te3 nanowires are presented and the results of which are interpreted using thermoelectric transport calculations [5]. In all systems investigated, the maximum TE performance converges towards the maximum TE performance of the surface states with decreasing system size into the nanometer-range, limiting their application in efficient thermoelectric devices.

January 18
No Colloquium
USC will be closed in observance of the Martin Luther King holiday

January 25

Quantum control and sensing with electron spins in semiconductors

John Nichol, PhD
Condensed Matter Physics
Department of Physics
Harvard University

View Abstract

Confined spins in semiconductors are a versatile platform for exploring quantum information processing and condensed matter physics.
Individual spins can have coherence times exceeding seconds in some cases, making them promising quantum bits, or qubits, and also highly sensitive probes of their local electric and magnetic environments. I will discuss recent work exploiting the joint spin-state of two electrons in a GaAs double quantum dot as a “singlet-triplet” qubit.
We perform high-fidelity single- and two-qubit gates with this architecture. We also use the qubit as a sensor to precisely measure its magnetic environment, which results from the statistically fluctuating nuclear spins in the semiconductor crystal. Using these measurement techniques, we extend the qubit coherence time by more than two orders of magnitude through adaptive control, and we uncover the surprisingly strong effect of spin-orbit coupling on electron-nuclear dynamics in GaAs.

February 1

Toward single atom qubits on a surface: Pump-probe spectroscopy and electrically-driven spin resonance

William Paul, PhD
Almaden Research Center
IBM Research

View Abstract

Single Fe atoms placed on a thin MgO film have exceptional magnetic properties: Their spin relaxation lifetime can extend to many milliseconds, and their quantum state can be coherently manipulated by RF electric fields. In this talk, we will discuss a scanning tunneling microscopy (STM) investigation of the dynamics of spin-relaxation and the electric-field-driven spin resonance of individual Fe atoms on a MgO/Ag(001) surface. The energy relaxation time, T1, of single spins on surfaces can be measured by spin-polarized pump-probe STM [1]. To date, the relaxation times reported for Fe-Cu dimers on Cu2N insulating films have been of the order ~100 ns [1]. A three-order-of-magnitude enhancement of lifetime, to ~200 us, was recently demonstrated for Co on a single-monolayer of MgO [2]. Here, we show that the T1 lifetime of single Fe atoms on MgO can exceed 10 ms under certain conditions, and can be tuned by adjusting the thickness of insulating MgO film grown on Ag(001). Next, we demonstrate electron spin resonance of an individual single Fe atom, driven by a gigahertz-frequency electric field applied across the tip-sample junction, and detected by a spin-polarized tunneling current [3]. The principle parameters of the spin resonance experiment, namely the phase coherence time T2 and the Rabi rate, are characterized for Fe atoms adsorbed to the monolayer MgO film. We conclude with an outlook toward quantum devices built with atomic precision on surfaces.

February 4

Bottom-up approaches for quantum many-body physics with cold trapped atoms

*Please note this is a specially scheduled seminar that will take place in SLH 102 on Thursday, February 4th from 4:00-5:00 pm*
Crystal Senko, PhD
Department of Physics
Harvard University

View Abstract

A major outstanding challenge in quantum science is the development and refinement of techniques to control interactions among quantum particles, which will be a key ingredient in quantum information processing and laboratory studies of quantum many-body physics. This talk will describe two atom-based platforms for studying artificial spin-spin interactions. Using trapped atomic ions, we implement tunable long-range spin-spin interactions mediated by optical dipole forces, which form the backbone of current quantum simulations of magnetism. This platform has enabled sophisticated manipulations of more than 20 spins, and is additionally the first platform to support quantum simulations of integer-spin chains. A separate set of experiments builds on a hybrid system in which single photons, confined to sub-wavelength dimensions with a photonic crystal cavity, are coupled to single trapped neutral atoms. Extending this architecture to multiple atoms will enable photon-induced quantum gates, and even tunable spin-spin interactions, between distant atoms.

February 8

Designing Defect Spins for Wafer Scale Quantum Technologies

William Koehl, PhD
Institute for Molecular Engineering, University of Chicago
Materials Science Division, Argonne National Lab

View Abstract

Spins bound to point defects and impurities in semiconductors are increasingly viewed as an important resource for solid-state implementations of quantum information technologies. A prime example of this is the nitrogen-vacancy (NV) center in diamond, a defect spin that can be controlled optically to function as an individually-addressable room temperature qubit. The exceptional quantum properties of the diamond NV have stimulated an active search for similar defects in other materials, since this would expand the range of scientific and technological opportunity available to defect-based quantum systems. I will describe recent experimental and theoretical efforts to systematically identify new semiconductor-based defect spin qubits, focusing in particular on silicon carbide (SiC) – a wafer-scale, wide-bandgap material common to the optoelectronics industry. The robust and varied quantum properties of the spins we identify make them promising candidates for photonic, spintronic, and quantum information applications that merge quantum degrees of freedom with classical electronic and optical technologies.

February 15
No Colloquium
USC will be closed in observance of the President's Day holiday

February 22

Big-Data Analytics for Materials Science: Concepts, Challenges, and Hype

Matthias Scheffler
Theory Department
Fritz Haber Institute of the Max Planck Society and Department of Chemistry and Biochemistry and Materials Department, UC Santa Barbara
Berlin, Germany

View Abstract

On the steady search for advanced or even novel materials with tailored properties and functions, high-throughput screening is by now an established branch of materials research. For successfully exploring the huge chemical-compound space from a computational point of view, two aspects are crucial. These are (i) reliable methodologies to accurately describe all relevant properties for all materials on the same footing, and (ii) new concepts for extracting maximal information from the big data of materials that are produced since many years with an exponential growth rate.
The talk will address both challenges. In particular, I will stress that big data of materials are structured in a way that is typically not visible by standard tools. Furthermore, with respect to a certain (desired) property, the practically infinite-dimensional space of different materials is very sparsely populated. Indeed, the key issue in data-driven materials science and machine learning of materials is to find the proper descriptive parameters (descriptors) that characterize the materials and their property.
We will show that and how compressed sensing, originally designed for representing a complex signal in the lowest possible dimensionality, can select, out of a huge-dimensional space of potential descriptors (features), a low dimensional descriptor. The talk also emphasizes the importance of causality in this learning process.

February 29


Peter Kuhn
The Bridge@USC

March 7

Unknowns of Energy Concentrating Phenomena

Seth Putterman
Physics and Astronomy

View Abstract

The path to equilibrium is not controlled by entropy production. Although entropy increases with every time step, dynamical motion can be dominated by nonlinear physical process which spontaneously concentrate energy density. In sonoluminescence a bubble concentrates the energy of a traveling sound wave by 12 orders of magnitude to create picoseconds flashes of blackbody radiation that originate in a new state of matter. When surfaces are brought into and out of contact they exchange charge: a process call tribo-electrification. This phenomenon can be so strong that the power applied to peel sticky tape is efficiently transduced into a flux of high energy electrons, and x-ray photons that can expose an image in a few seconds. For a ferroelectric crystal, instabilities in the phonon spectrum lead to a spontaneous polarization that for Lithium Niobate reaches 15.million volts per cm. The temperature dependence of this field can be used to build a neutron generator based on the fusion of deuterium nuclei. These phenomena challenge a reductionist approach to the theoretical physics of emergent phenomena. The degree to which the energy density of a continuous system can be concentrated by off-equilibrium motion has not been determined by theory. For sonoluminescence we do not know if the parameter space includes a region where an extra factor of 100 in energy density makes it possible to realize thermonuclear fusion. For triboelectrification we do not have an ab-initio theory of charge transfer. And for ferroelectrics we do not have an ab-initio theory of the limits of spontaneous polarization which can be designed.

March 14
No Colloquium
Spring Break 

March 21

The Significance of Dimensionality in Cellular Dynamics and Function in beta Insulin cells

Norbert Scherer
Department of Chemistry
University of Chicago

View Abstract

Cells and the cellular context are generally 3-dimensional in vivo, yet typical measurements (i.e. imaging) are done in 2-D. While sequentially measured stacks of 2-D (e.g. confocal) slices are fine for static or fixed cells, such measurements are problematic and can even lead to erroneous results for dynamical systems. In this talk I will discuss recent work from my group on the topic of intracellular transport and the "subordinated" random walk that we have discovered for insulin granules in beta cells. I will discuss the structural and dynamical features that give rise to this transport. Then, by way of new clonal cell lines that we have created, I will present new results showing the importance of the function of these cells in 3-D organizations (i.e. in pseudo-islets) as opposed to 2-D cell culture. In particular, stimulated insulin secretion and also differential RNA expression of key genes and pathways is strongly affected by dimensionality. If time permits, I will briefly discuss new 3-D imaging methods that we are developing to enhance our ability to study dynamical phenomena in 3D in cells.

March 28

Cold Atom Quantum Emulation

David Weld
Assistant Professor
Dept of Physics
UC Santa Barbara

View Abstract

Ultracold atoms in optical lattices represent a new frontier for the investigation of outstanding problems in many-body quantum mechanics. The ability to exert full spatiotemporal control over the evolution of quantum degenerate gases is enabling a new generation of experiments at the boundary between condensed matter and atomic physics. In these experiments, the precision and control of atomic physics is brought to bear on important topics in condensed matter, from ultrafast electron dynamics to quasicrystals. I will discuss two experimental platforms based around ultracold lithium and strontium, which enable the creation and study of new, highly tunable, and strongly correlated phases of matter.

April 4

From Black Holes to Graphene

Andrew Lucas
Harvard University

View Abstract

Perhaps surprisingly, the dynamics of a strongly interacting finite temperature quantum system can quickly become describable by classical hydrodynamics. I will propose that fluid-like behavior among interacting electrons may be responsible for the emergence of physics beyond Landau's Fermi liquid theory in (at least some) strongly interacting metallic phases. I will discuss how recent developments linking black holes with finite temperature strongly interacting metals (the AdS/CFT correspondence) have led to a universal hydrodynamic theory for the electrical and thermal conductivities of these clean metals. This theory predicts and explains electrical and thermal transport measurements in the recent experimental realization of a strongly interacting fluid of electrons in charge neutral graphene.

April 11

Resolution of the black hole information paradox

Samir Mathur
Dept of Physics
Ohio State University

View Abstract

Some 40 years ago Hawking found a remarkable contradiction: if we accept the standard behavior of gravity in regions of low curvature, then the evolution of black holes will violate quantum mechanics. Resolving this paradox would require a basic change in our understanding of spacetime and/or quantum theory. This paradox has found an interesting resolution through string theory. While quantum gravity is normally expected to be important only at distances of order planck length, the situation changes when a large number N of particles are involved, as for instance in the situation where we make a large black hole. Then the length scale of quantum gravity effects grows with N, altering the black hole structure to a "fuzzball"; this effect resolves the paradox.

April 18

Combining Mechanistic Modeling with Big Data to Better Understand Vascular Systems and Allometric Scaling

Van Savage
Associate Professor
Dept of Ecology and Evolutionary Biology and Dept of Biomathematics

View Abstract

I will present work that links mechanistic, mathematical models with big-data approaches to address fundamental questions in physiology. Advances in imaging together with the compilation and analysis of large databases enable mechanistic models to be thoroughly grounded in and tested with empirical biological data. Specifically I will describe new software for automatically measuring vessel dimensions and geometry from three-dimensional angiographic (e.g., CT and MRI) images. This software leads to much faster collection of larger amounts of data than was previously possible. By analyzing these vascular data, I show how we test leading models for the structure and dynamics of vasculature and how we identify new patterns for the asymmetry and self similarity of vascular branching. Based on these results, I describe approaches to developing new models for the structure and dynamics of vascular networks and to exploring whether constraints in real systems exist at the level of the whole network or of individual branching nodes. To conclude, I will discuss how these recent findings impact allometric scaling as well as models of tumor angiogenesis and growth.