Physics Colloquium

Spring 2012

Thursdays 4:00 p.m., Room 104 Physics
Refreshments served at 3:40 p.m.
  Colloquium organizer: Yew San Hor
(Link to main colloquium page)

Green - open date
Yellow - tentative (reserved)
Red - firm commitment


Date Host Speaker Title of the talk Abstract
Jan. 19

Jan. 26 Vojta
Dr. Adrian Del Maestro
University of Vermont
A Luttinger Liquid Core Inside Helium-4 Filled Nanopores As helium-4 is cooled below 2.17 K in undergoes a phase transition to a fundamentally new state of matter known as a superfluid which supports flow without viscosity. This type of dissipationless transport can be observed by forcing helium to travel through a narrow constriction that the normal liquid could not penetrate. Recent advances in nanofabrication techniques allow for the construction of smooth pores with nanometer  radii, that approach the truly one dimensional limit. In one dimension, it is believed that a system of bosons (like helium-4) may have startlingly different behavior than in three dimensions. The one dimensional system is predicted to have a linear hydrodynamic description known as Luttinger liquid theory, where no type of long range order can be sustained. In the limit where the pore radius is small, helium inside the channel would behave as a sort of quasi-supersolid with all correlations decaying as power-laws at zero temperature.  We have performed large scale quantum Monte Carlo simulations of helium-4 inside nanopores of varying radii at low temperatures with realistic helium-helium and helium-pore interactions. The results indicate that helium inside the nanopore forms concentric cylindrical layers surrounding a core that can be fully described via Luttinger liquid theory and provides insights towards the exciting possibility of the experimental detection of a Luttinger liquid.
Feb. 2 Hor
Dr. Deborah Hanuscin
UM - Columbia
Scientific Illiteracy - A Gap in Our Syllabi?

Scientific literacy involves being able to be informed consumers of scientific information and the ability to make sense of socioscientific issues. An important component of this is understanding what science is and how science works. Nonetheless, addressing topics such as the nature of scientific knowledge and how it is that we know what we know in science are often neglected in the curriculum of science courses. In this talk, we'll take a though-provoking look and what our students need to understand in order to be scientifically literate and identify gaps in our syllabi and how these can be addressed.

Feb. 9 Yamilov
Dr. Jie Gao
MAE, Missouri S&T
Chip-scale Photonic Devices for Light-matter Interactions and Quantum Information Processing Chip-scale photonic devices such as microdisks, photonic crystal cavities and slow-light photonic crystal waveguides possess strong light localization and long photon lifetime, which will significantly enhance the light-matter interactions and can be used to implement new functionalities for both classical and quantum information processing, optical computation and optical communication in integrated nanophotonic circuits. In this talk, I will present design and characterization of asymmetric resonate cavity with radiation directionality and air-slot photonic crystal cavity with ultrasmall effective mode volume, which have important applications in chip-scale lasers, chemical sensor and nonlinear optics. Then I will talk about the exciton-photon interactions between emitters (ensemble PbS nanocrystals and single InAs quantum dot) and high Q localized mode in photonic crystal devices. These results provide opportunities and challenges in future research of light-matter interactions, quantum computation and information processing in solid-state integrated photonic circuits. 
Feb. 16 Jentschura
Dr. Christian Schubert
Michoacan University
The First-Quantized Approach to Quantum Field Theory Almost simultaneously with inventing the formalism of modern second-quantized quantum electrodynamics, Feynman in 1951 and 1952 also presented an alternative representation of the QED S-matrix in terms of first-quantized particle path integral. This ''worldline formalism'' has been generalized to some other quantum field theories, but has only in recent years gained some popularity as a calculational alternative to Feynman diagrams, partly due to developments in string theory. In this talk, I will first discuss the rather involved history of this subject, and then present a number of sample calculations, mostly taken from QED.
Feb. 23 Yamilov
Dr. Ildar Gabitov
University of Arizona
Stochastic phenomena in high speed optical fiber communication systems: gross impact of slim chances Optical fiber communications is the chief technology in high-speed data transmission. Modeling of information flow through optical networks leads to numerous challenges both in the complete physical description and the corresponding mathematical study. Evaluation of errors in fiber communication systems is an important example of tone such challenge, one great industrial potential, which require using modern methods of mathematics and statistical physics. Structural disorder in optical fibers and temporal noise introduced by optical amplifiers, integrated to telecommunication systems, represent a source of errors in high speed optical fiber communications. Modern requirements in optical communications require limiting acceptable bit error rate in the range of 10 -9 - 10 -12. Evaluation of statistical characteristics of errors at such low level is a very difficult problem. We address this problem analytically using an instantonic approach. We show the existence of long extended tails of statistical distribution of system parameters characterizing performance and present comparison of obtained results with experimental data. We also demonstrate bifurcation phenomena for special type of optical pulses in optical fiber systems, which can be used to increase bit rate of communication systems.
Mar. 1
Dr. Anton Malko
University of Texas - Dallas
Spectroscopy of colloidal nanocrystals: Multiexcitons, non-blinking giant dots and excitonic energy transfer

Colloidal nanocrystals (NCs) have been at the forefront of the optoelectronics research since their initial discovery by Brus and co-workers. Strong research efforts have been applied to understand and engineer dynamics of multiexciton (MX) states in such NCs, specifically to address carrier multiplication and PL fluorescence intermittency (blinking) issues. Recently1, we developed a new class of “giant” CdSe/CdS multishell nanocrystals of CdSe cores overcoated with multiple layers of inorganic shells (CdS) and observed complete blinking suppression at time scales from milliseconds to minutes for dots with n>12 monolayers of CdS. In single dot PL measurements of such g-NCs we observed MX emission in steady-state PL at low (10 K) temperatures2. By analyzing steady-state and time-resolved PL traces we found that rates of Auger non-radiative recombination for MXs can be much less their radiative rates, allowing for significant fraction of multiexcitons to recombine radiatively.3 We also suggested that Auger rates for charged excitons (trions) depend crucially on carrier localization length and observed large differences for recombination rates of negative and positive trions. These observations allowed us to propose an ingenuous mechanism that explains observed blinking suppression.4 In following experiments, we studied hybrid film structures consisting of a thin layer of NCs “anchored” to a monolayer of J-aggregates (JA) of a cyanine dye. Time-resolved and steady-state PL measurements indicated strong Forster energy transfer from NCs to JA layer. Such energy transfer might be of importance for hybrid photovoltaic and LED technologies.

[1] JACS 130(15), 5026 (2008);
[2] Nano Letters10(7), 2401-2407 (2010);
Phys. Rev. Lett. 106, 187401 (2011);
Nano Letters 11, 5213 (2011)

Mar. 8 Vojta
Dr. Raj Narayanan
Indian Institute of Technology - Madras
Effects of disorder and dissipation on quantum phase transitions Quenched disorder or "frozen-in" impurities can have remarkable effects on a system undergoing a phase transition. These effects are particularly stark near a class of zero-temperature phase transitions called quantum phase transitions.
Near these quantum phase transitions, disorder can lead to a wealth of interesting phenomena like infinite-disorder critical points, quantum Griffiths singularities, and even the smearing of the transition. In this talk I will discuss how the presence of dissipation in some physical systems affects these disorder induced phenomena.
Mar. 15 Hagen
Dr. Sung Ho Salk
Korea Institute of Advanced Study
Role of Antiferromagnetic Spin Fluctuations in High Tc Superconductivity
For the last two decades since the advent of high temperature superconductivity, noticeable progress has been made to understand both normal and superconducting states by paying attention
to the observed phase diagrams, electronic structures, thermal properties, superfluidity and both spin and charge dynamics. Despite the progress there exists no satisfactory theory which consistently reproduces various physical properties of cuprate oxides in association with the phase diagram. High Tc superconductivity is involved with low (two) dimensional systems of strongly correlated electrons, namely the cuprate oxides acting as Mott insulators. To tackle this problem, theoretical efforts have been made using either the Hubbard Hamiltonian or the t-J Hamiltonian. Electrons have three ”faces”: namely, the spin, charge and quantum phase. To meet the three essential faces, earlier we proposed both U(1) and SU(2) gauge theoretic slave-boson theories of the t-J Hamiltonian in which coupling between the spin pairing and charge paring orders are properly taken care of, contrary to other slave-boson theories. Thanks to the fact that the Cooper pair is realized as a composite of the spinon (spin) and holon (charge) pairing orders in our slave-boson treatment, not only the dome-shaped phase diagram but other physical properties are consistently reproduced in agreements with measurements : superfluid weight, spectral function, optical conductivity, magnetic susceptibility and the universal scaling behavior of T.
In this talk we present a study of both the spin and charge dynamics. We explain how the two different dynamics are coupled to cause various exotic physical phenomena such as the dome-shaped bose condensation temperature, the boomerang behavior of superfluid weight and peak-dip-hump structures of both spectral function and optical conductivity. Regarding the charge dynamics we find that the peak-dip-hump structure in optical conductivity is attributed to coupling between the spin and charge degrees of freedom, but not to the spin-charge separation. Further, quantum critical point will found in light of charge dynamics. Regarding the spin dynamics we unveil physics involved with both temperature and doping dependence of magnetic resonance. We find from this study that the onset temperature of magnetic resonance which occurs in spin-exciton channel is the pseudogap (spin gap) temperature T in agreement with measurements of Mook and coworkers. The resonance peak energy Eres is shown to have a linear scaling behavior with the superconducting transition emperature, Tc, in agreements with observations of Keimer and coworker. We demonstrate that the spin pairing correlations are responsible for both the linear scaling of Eres ~ Tc and the universal scaling behavior of T=Tc. Finally an integrated view of physics of the above novel features willbe highlighted in the language of coupling between the spin and charge degrees of freedom and the spin and charge dynamics.
Mar. 22 Madison
Dr. David Schultz
University of North Texas
New Explorations of Atomic Interactions Using Large Scale Simulation

Contemporary computational methods to treat few-body, atomic-scale interactions have opened opportunities to study them at a new level of detail and to uncover unexpected phenomena. Such interactions within gaseous, plasma, and even material environments are fundamental to such diverse phenomena as low temperature plasma processing of semiconductors, collapsing giant molecular clouds forming stars, fluorescent lighting, radiation treatment of disease, and the chemistry of earth’s atmosphere.  I will illustrate progress using examples from recent work treating very simple systems, for which our knowledge has been both subtly refined and significantly changed.  In particular, using a direct computational approach, the origin of unexpected vortices in atomic-scale wavefunctions has been elucidated.  These wavefunctions describe interactions such as atomic collisions and the response of atoms to short electric field pulses, involving as few as one electron, whereas, in contrast, vortices are usually associated with systems containing large numbers of particles.

Apr. 5 Hor Dr. Liang Fu
Topological Insulators and Superconductors The discovery of quantum Hall effects at high magnetic fields in 1980s revealed the existence of topologically ordered phases of matter without spontaneous symmetry breaking. In the last few years, a variety of new topological phases has been theoretically predicted to exist in band insulators with strong spin-orbit coupling and superconductors with unconventional pairing symmetry. In this talk, I will describe the unifying topological structure of these electronic phases, experimental realizations of topological insulators, and ongoing search for topological superconductors.
Apr. 12 Madison Dr. Itzik Ben-Itzhak
Kansas State University
Probing molecular-ion beams with intense few-cycle laser pulses1 – two-color controlled dissociation
We have studied laser-induced fragmentation of molecular-ion beams using coincidence 3D momentum imaging, with direct separation of all the reaction products measured simultaneously. These measurements provide detailed kinetic energy release and angular distributions of the different fragmentation processes. We mainly focus on the fundamental H2+ and H3+ molecules (in 7-50 fs laser pulses having 1012-1016 W/cm2 peak intensity) as models for more complex systems, and at times we do explore more complex molecules such as O2+ and CO2+.
In this talk, we will discuss electron localization on specific nuclei during strong-field dissociation of molecular-ion beams which is controlled by the relative phase between the 790 and 395 nm components of an ultrashort laser pulse.
In addition, clear experimental and theoretical evidence for the intriguing zero-photon dissociation (ZPD) process of H2+ will be presented. The key role of the laser-pulse bandwidth will be discussed. Moreover, we will explore control over the final dissociation product of HD+, either H+ + D or H + D+ – usually referred to as channel asymmetry.
Others contributing to this work: B. Gaire, M. Zohrabi, J. McKenna, U. Ablikim, A.M. Sayler, N.G. Johnson, K.D. Carnes, D. Ursrey, F. Anis, J. Hernandez, J.J. Hua, and B.D. Esry.
1Supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, US Department of Energy
Apr. 19 Hor Dr. Taylor Hughes
University of Illinois - Urbana Champaign
Time-reversal invariant topological insulators and superconductors New states of matter have recently been discovered which are reminiscent of the quantum Hall effect found in two-dimensional electron systems, but which do not require the extreme environment to exist (e.g. high magnetic fields, low temperature). These states are the so-called time-reversal invariant topological insulators which were theoretically predicted and subsequently experimentally confirmed to exist in 2D and 3D. These states are nominally insulating in the bulk but harbor low-energy metallic states on their edges/surfaces which are robust to disorder and other imperfections. I will discuss the basic physics of topological insulators which can be modeled with simple, free-fermion Dirac Hamiltonians. My focus will be on the criteria that led the search for realistic materials, and on the novel response of these insulators to electromagnetic fields. In addition to this discussion, I will mention related topological superconducting states of matter as well as some interesting topological insulator/ferromagnet/superconductor hetero-structures which have recently received attention due to possible quantum computation applications.
Apr. 26 Hor 41st Annual Harold Q. Fuller Prize Colloquium

Anti-spam link