Professor Nadia Fomin

University of Tennessee

Title: Life and Death of the Free Neutron

Abstract: Modern neutron sources provide extraordinary opportunities to study a wide variety of physics topics, including the physical system of the neutron itself. One of the processes under the microscope, neutron beta decay, is an archetype for all semi-leptonic charged-current weak processes. Precise measurements of the correlation parameters in neutron beta decay as well as the neutron lifetime itself are required for tests of the Standard Model and for searches of new physics. The state of the field will be presented and a program of current and future experiments and potential impacts explored.

Dr. Turgut Yilmaz,
Department of Physics,
University of Connecticut

Topological insulators and their interaction with magnetic impurities and superconductors


Topological properties of materials have recently attracted much attention in the condensed matter physics community due, in part to their possible applications in quantum electronic devices. In particular, topological insulators (TIs) that possess gapless surface states that are topologically protected and can lead to dissipationless quantum electronics. TI's surface state exhibit relativistic momentum dispersion with the Dirac point (DP) located at the time reversal invariant momenta point while the bulk bands are inverted and gapped [1]. Currently, two majority experimental efforts have concentrated on inducing the magnetic and superconducting properties in TI, which would lead to the experimental realizations of quantum anomalous Hall effect (QAHE) and Majorana fermions, respectively [2-3].

Magnetism can be induced in TIs by incorporating impurities into the bulk or on the surface of TIs. If there is a net out-of-plane magnetic moment, an energy gap opens at the DP due to broken time reversal symmetry, which would give rise to QAHE. On the other hand, the superconductivity in can be induced in TI through the proximity effect by growing TIs on the surface of a superconductor. Both, the gapped DP and the superconductivity can be probed by angle-resolve photoemission spectroscopy (ARPES). In this talk, I will present our photoemission studies on the search of broken time reversal symmetry in magnetic TIs and proximity induced superconductivity in TI/superconductor heterostructures. Also, the experimental setup to grow and study TIs at the Physics Department of the University of Connecticut will be presented.

[1] L. Fu, C. L. Kane, and E. J. Mele, Phys. Rev. Lett. 98, 106803 (2007).

[2] X.-L. Qi and S.-C. Zhang, Rev. Mod. Phys. 83, 1057 (2011).

[3] T. Yilmaz, et al., Appl. Surf. Sci. 407, 371 (2017).

Prof. Serge M. Nakhmanson, Department of Materials Science & Engineering, and Institute of Materials Science, University of Connecticut


Mesoscale simulations of ferroic functionalities with finite elements: MOOSE, Ferret and other animals


Ferret is an open-source highly scalable real-space finite-element-method (FEM) based code for simulating transitional behavior of materials systems with coupled physical properties at mesoscale. This code is built on MOOSE, Multiphysics Object Oriented Simulation Environment, and is being developed by a team of collaborators at the University of Connecticut and Argonne National Laboratory. MOOSE allows efficient solving of coupled-physics problems, has a proven record of scaling of execution on up to 10K nodes, and is easily extendable to include new physics and couplings. Real-space FEM approach allows treatment of materials systems possessing complicated geometry and morphology, and evaluation of property dependencies on the system shape, size, microstructure and applied boundary conditions. In this presentation we provide an overview of computational approach utilized by the code, as well as its technical features and the associated software within its tool chain. We also highlight a variety of examples of the code applications, some of which are being pursued in collaboration with a number of different experimental groups. These applications include (a) evaluation of size- and microstructure-dependent elastic and electronic properties of semiconducting core-shell nanostructures; (b) investigation of transitional behavior and topological phases in ferroelectric films, islands, particles and particle-based ferroelectric/dielectic materials; and (c) understanding the workings of photoelectric and electro-optic effects in piezoelectric and ferroelectric materials and nanostructures.

SN acknowledges the efforts of his students John Mangeri, Krishna Chaitanya Pitike and Lukasz Kuna, and senior collaborators S. Pamir Alpay (UConn), and Olle G. Heinonen (ANL), all of whom made major contributions to the projects discussed in this presentation. Funding support from the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program is acknowledged. The SCGSR program is administered by the Oak Ridge Institute for Science and Education (ORISE) for the US Department of Energy. ORISE is managed by ORAU under contract number DE-SC0014664. US National Science Foundation (award number DMR 1309114) is also acknowledged for partial funding of some activities related to the development of the Ferret code.

Dr. Chiara Mingarelli,
Center for Computational Astrophysics, Flatiron Institute


Pulsar Timing Arrays: The Next Window to Open on the Gravitational-Wave Universe


Galaxy mergers are a standard aspect of galaxy formation and evolution, and most (likely all) large galaxies contain supermassive black holes. As part of the merging process, the supermassive black holes should in-spiral together and eventually merge, generating a background of gravitational radiation in the nanohertz to microhertz regime. An array of precisely timed pulsars spread across the sky can form a galactic-scale gravitational wave detector in the nanohertz band. I describe the current efforts to develop and extend the pulsar timing array concept, together with recent limits which have emerged from international efforts to constrain astrophysical phenomena at the heart of supermassive black hole mergers.

Adam Ginsburg, National Radio Astronomy Observatory, Socorro, New Mexico

High mass star and cluster formation

High mass stars are responsible for most of the photons, heavy elements, and energy in galaxies, thereby driving their evolution. They tend to form in dense environments and clusters, which means we can only observe their formation at relatively large distances. I will discuss observational programs, primarily with ALMA, that show how stars form differently in denser parts of galaxies. At higher densities, more stars form in clusters, more stars form in the vicinity of massive stars, and the stellar initial mass function (IMF) shifts to favor higher masses. ALMA data at high resolution show how the feedback from massive stars shapes their environments. The upcoming ALMA-IMF large program will extend this work, showing how the stellar IMF is shaped throughout our Galaxy.

Dr. Aaron LaForge, Department of Physics, University of Connecticut


Excitation dynamics in helium nanodroplets induced by XUV radiation


As opposed to purely molecular systems where electron dynamics proceed only through intramolecular processes, weakly-bound complexes like helium nanodroplets offer an environment where local excitations can interact with neighboring molecules leading to new intermolecular relaxation mechanisms. Here, we present a systematic, time-resolved study of the excitation dynamics when helium nanodroplets are resonantly excited by XUV free electron laser radiation. At low pulse energy, a single atom is excited leading to the formation of a bubble state which relaxes to the droplet surface on a femtosecond timescale. At high pulse energy, a network of excited atoms is formed within a nanodroplets which then collectively autoionizes with high efficiency. Using the pump-probe technique, we were able to time resolve this process.

John Donoghue, UMass Amherst


A possible QFT pathway for UV complete quantum gravity


This is a study of a renormalizable quantum field theory for gravity. It makes use of a gravitational sector which is weakly coupled at all scales, and a helper Yang Mills gauge theory which becomes strong at the Planck scale. While it has an unstable ghost in the propagator, and other unusual properties, unitarity is satisfied at the one loop level as determined by explicit calculation. Further
explorations will be discussed.

22nd Annual Katzenstein Distinguished Lecture

Exploration of the Universe with Gravitational Waves

Professor Rainer Weiss,
Massachusetts Institute of Technology,
2017 Nobel Prize Winner,
Member of the Laser Interferometer Gravitational-Wave Observatory (LIGO) Scientific Collaboration

The observations of gravitational waves from the merger of binary black holes and from a binary neutron star coalescence followed by a set of astronomical measurements is an example of investigating the universe by "multi-messenger" astronomy. Gravitational waves will allow us to observe phenomena we already know in new ways as well as to test General Relativity in the limit of strong gravitational interactions -- the dynamics of massive bodies traveling at relativistic speeds in a highly curved space-time. Since the gravitational waves are due to accelerating masses while electromagnetic waves are caused by accelerating charges, it is reasonable to expect new classes of sources to be detected by gravitational waves as well. The lecture will start with some basic concepts of gravitational waves. Briefly describe the instruments and the methods for data analysis that enable the measurement of gravitational wave strains of 10-21 and then present the results of recent runs. The lecture will end with a vision for the future of gravitational wave astrophysics and astronomy.


Refreshments will be served at 3:00 p.m. outside of GW-38

Dr. Yuya Tanizaki, RIKEN-BNL


Constraints on possible dynamics of QCD by symmetry and anomaly

Low-energy dynamics of Quantum Chromodynamics (QCD) is an important subject for nuclear and hadron physics, but it is a strongly coupled system and difficult to solve. In this situation, symmetry and also anomaly give us an important guide to make a solid conclusion on possible
behaviors of QCD. We will give a brief review on recent development of anomaly matching condition, starting from a simple quantum mechanical example. After that, we explain the newly found anomaly of QCD in chiral limit, and discuss the conclusion of anomaly matching.

Prof. Roger Falcone, University of California, Berkeley

(tentative, to be confirmed)

Title and abstract: forthcoming

Dr. Charlotte A. Mason,
Harvard-Smithsonian Center for Astrophysics

What Can Galaxies Tell Us About Reionization

The reionization of intergalactic hydrogen in the universe's first billion years was likely driven by the first stars and galaxies. The timeline of reionization is currently uncertain but if it is accurately measured it can unveil the properties of 'first light' sources. Lyman alpha (Lyα) emission from galaxies can potentially probe the intergalactic medium (IGM) at high redshifts but requires modeling physics from pc to Gpc scales. I will introduce a new forward-modeling framework which combines cosmological IGM simulations with interstellar medium models to enable Bayesian inference of the IGM neutral hydrogen fraction from observations of Lyα from galaxies. I will present our new measurements of the neutral fraction at z~7 and z~8 and place them in the context of the reionization history. I will describe ongoing efforts with the HST survey GLASS to build larger spectroscopic samples of galaxies at z>6 to further measure the reionization timeline, and future prospects with JWST.

Prof. J. Brad Marston,
Physics Department, Brown University

El Nino as a Topological Insulator: A Surprising Connection Between
Climate and Quantum Physics

Symmetries and topology play central roles in our understanding of physics. Topology, for instance, explains the precise quantization of the
Hall effect and the protection of surface states in topological insulators
against scattering from disorder or bumps. However discrete symmetries and
topology have so far played little role in thinking about the fluid dynamics of oceans and atmospheres. In this talk I show that, as a consequence of the rotation of the Earth that breaks time reversal symmetry, equatorially trapped Kelvin and Yanai waves emerge as
topologically protected edge modes. Thus the oceans and atmosphere of Earth naturally share basic physics with topological insulators. As
equatorially trapped Kelvin waves in the Pacific ocean are an important component of El Nino Southern Oscillation and other climate processes,
these new results demonstrate that topology plays a surprising role in Earth's climate system.

Dr. Jeff Powell,
Department of Physics,
University of Connecticut

Photoelectron emission from metal and dielectric nanoparticles in intense laser fields

Nanoparticles provide a unique platform to study the mechanisms of light interactions with complex systems, in particular, the motion of electrons driven by strong fields and excitation of plasmonic resonances. Here I report on the systematic analysis of the photoelectron emission from single, isolated, gas-phase, spherical nanoparticles as a function of particle size, composition and laser intensity. Gold, silica (SiO2), and silica core/gold shell nanoparticles ranging in size from 5nm to 750nm were irradiated by 800nm, 25fs laser pulses at 0.2-20 TW/cm2 peak intensities. A high energy velocity-map imaging spectrometer was used to determine the cutoff photoelectron energy for each sample and intensity. Nanoparticle size and composition influence the electron cutoff energy, which provides insight into photoelectron trajectories and rescattering dynamics, both of which are affected by near-field enhancement and plasmonic excitation. In particular, I observe that photoelectrons emitted from gold nanoparticles are much more energetic than those from silica, which, in turn, are much faster than those from isolated atoms or molecules exposed to the same light pulses. This work reveals the complex interplay between the external optical field and the induced near-field of the nanoparticle.

Prof. Dawn Meredith, Department of Physics, University of New Hampshire

Instructor and student resources in the physics course for life science students

Over a decade ago, professionals in life science called for the education for the next generation of practitioners to be more interdisciplinary and quantitative. Physicists listened, and in the last decade we have learned a lot about the students and the potential of this course. I will share some insights about what makes this course different from the course for engineers, best practices and new curricula, and the soon-to-be-live Living Physics Portal that provides community and resources for instructors. In the second part of the talk, I'll discuss research that uncovers some productive cognitive resources that life science students bring to the physics classroom on which new understandings can be built.​

Coffee and tea will be served prior to the talk, at 3:00 p.m., In Room GW-103

Prof. Steve Finkelstein, Department of Astronomy,
University of Texas, Austin

Understanding the Reionization of the Universe Now and with JWST


The universe just after the Big Bang was a dark place, while the remnant radiation from the Big Bang cooled, and gas began to pile into dark matter halos. After a few hundred million years, the as-yet unseen first galaxies began to form, and the ionizing radiation produced by their massive stars ionized the intergalactic medium, in a process known as reionization. The search for these first galaxies, and an understanding of the process of reionization, has been a major goal of extragalactic astronomy for decades. While it is commonly assumed that massive stars within star-forming galaxies provide the needed ionizing photons to reionize the intergalactic medium, the commonly assumed ionizing photon escape fractions in the range of 10-50% have thus far exceeded what has been observed (except in a few extreme cases). I will discuss the results of a new semi-empirical model, combining simulated galaxy escape fractions with observed rest-UV galaxy luminosity functions. We find that we can successfully match all observational constraints on reionization by 1) forming stars to physically-motivated halo mass limits; 2) allowing galaxies to be more efficient ionizing photon producers at higher redshift, and 3) allowing a (not strong) contribution from AGNs. This predicts that reionization is a smoother process than previous models, with a predicted volume-averaged ionized fraction of 20% at z=10. JWST will soon dramatically improve our view of the z~10 universe and validate whether significant star-formation is occurring in that epoch, and I will conclude by discussing a recently-approved JWST Early Release Science program which will begin to probe this epoch.

Shany Danieli, Yale University

Hunting low surface brightness galaxies with the Dragonfly Telephoto Array


Dwarf galaxies are a significant component of the galaxy population. Their number density, among other properties, provides strong constraints on theories of galaxy formation, stellar feedback processes, and the distribution and nature of dark matter. However, their low central surface brightnesses makes them difficult to detect and study beyond the Local Group, leading to a biased view of the full galaxy population. To address this problem a new robotic telescope was developed and built, called the Dragonfly Telephoto Array, that is optimized to explore the universe at surface brightness levels below 30 mag/arcsec^2. In this talk I'll describe the technology behind the 1m f/0.4 refractor and present some early results in the dwarf galaxy regime. I will then present our strategy for finding these low surface brightness galaxies as part of the on-going Dragonfly Wide Field Survey that will cover 500 sq. deg. upon survey completion. We will use these data to empirically constrain the Luminosity Function of field galaxies down to the low mass end which is essential for understanding the nature of low-mass dark matter halos.

Shany Danieli is a Physics PhD student at Yale University, working with Prof. Pieter van Dokkum. She specializes in the formation and evolution of dwarf galaxies and Ultra Diffuse Galaxies, both in galaxy groups and in the field (isolated galaxies), using various observational tools such as the Dragonfly Telephoto Array and the Hubble Space Telescope.

Prof. Gerald Dunne, Department of Physics, University of Connecticut

Resurgence and Phase Transitions

Resurgence is a new approach to quantum field theory that combines perturbative and non-perturbative effects in a unified framework. I'll give an overview of the key ideas and discuss applications to phase transitions in physical problems. The main motivation is to find a new way to probe the QCD phase diagram. In this approach, a phase transition is interpreted as a change of dominant saddle in a path integral or partition function. Examples include the Ising model, chiral symmetry breaking in the Gross-Neveu model, the Thirring and Hubbard models, and the large N phase transition in 2d lattice gauge theory.

Dr. Peter D Drummond FAA,
Centre for Quantum and Optical Science, Swinburne University of Technology, Melbourne, Australia

Testing quantum mechanics with modern technology

Wendell Furry [1] and Sydney Coleman [5] were two of Harvard's most original quantum theorists. What can modern quantum technology contribute to their work? Experimentally tested simulations of opto-mechanics[4] and atom interferometers [3], with thermal noise and losses, will be used to analyse proposals for testing Furry's nonlocal decoherence, and Coleman's QFT tunnelling to the true vacuum. Both these ideas are fundamental to our understanding of the quantum universe. Nonlocal decoherence was proposed in Furry's response to Einstein's EPR paper. Today, this idea is becoming testable for massive objects at low temperatures. Entanglement decoherence and massive Schrodinger cats are testable [4] through an optomechanical memory, simulated using the positive-P representation, thus combining technology at Yale and JILA. Coleman's proposal of a false vacuum models fluctuations in the early universe, leading to large-scale structure in the microwave background. Coleman's ideas will be applied to construct a proposal for a laboratory model of the quantum fluctuations in the 'Big Bang', using a coupled BEC experiment in ultracold 41K, simulated with a Wigner phase-space representation [6].

[1] W. H. Furry, Phys. Rev. 49, 393 (1936).

[2] A. Einstein, B. Podolsky, and N. Rosen, Phys. Rev. 47, 777 (1935).

[3] M. Egorov et. al, Phys. Rev. A 84, 021605 (2011).

[4] S. Kiesewetter, R. Y. Teh, P. Drummond and M. Reid, Phys. Rev. Lett. 119, 023601 (2017).

[5] S. Coleman, Phys. Rev. D 15, 2929 (1977).

[6] O. Fialko et. al., J. Phys. B 50 024003 (2017).


Coffee and tea will be served prior to the talk, at 3:00 p.m., In Room GW-103

Jennifer Bergner, Harvard-Smithsonian Center for Astrophysics, Harvard University

Chemical complexity during star and planet formation: new observational and laboratory insights

Planets form by accreting material from a protoplanetary disk, meaning that the inventory of molecules in the disk sets what subsequent chemistry can occur on a newly formed planet. Constraining the organic chemistry in disks and their precursors is therefore key to origins of life studies. I will discuss recent progress in detecting and characterizing simple (HCN and C2H) and complex (CH3CN and HC3N) organic molecules in protoplanetary disks using ALMA, as well as implications for the chemistry at play. As we are sensitivity-limited in the level of molecular complexity that can be detected in disks, observations of organic molecules around low-mass protostars are providing important windows into the initial chemical conditions of disk and planet formation. To complement these observational
approaches, we use laboratory experiments to understand how reactions proceed under conditions of low temperature and low pressure. I will outline our recent work exploring new pathways to chemical complexity in laboratory analogs of
astrophysical ices.

About the speaker: Jennifer Bergner is a graduate student in (Astro)Chemistry at Harvard working with Prof. Karin Oberg at the Harvard-Smithsonian Center for Astrophysics. She is a National Science Foundation graduate research fellow and
specializes in laboratory astrochemistry, focused on the physics and chemistry of interstellar ice analogs.

Dr. Shimshon Kallush,
Physics and Optical Engineering Department, ORT Braude College, Karmiel, and
The Fritz Haber Center for Molecular Dynamics, The Hebrew University, Jerusalem, Israel

Molecular alignment in hot and cold regimes, and its relationship to quantum coherence

The phenomena of field-free instantaneous quantum revivals of molecular alignment and orientation driven by optical and/or terahertz fields will be introduced. A brief survey over a few applications of the phenomena will be presented within this context:

1. Ultracold photoassociation will be shown to intrinsically contain molecular alignment. Its characteristics and the feasibility to detect directional preferences for the photoassociated species will be discussed.

2. Rotational echoes of alignment under multiple pulses will be shown to lead to unique spatial-temporal structures of the refractive index. Several applications of this feature will be suggested.

3. The quantum picture of alignment raises questions that relate to the ability to embed quantum coherences into thermal quantum systems. The problem of quantification of coherences is defined and some results that predict the ability to induce field-driven coherences will be presented

Dr. Ming Li, Department of Physics, University of Connecticut

High Baryon Densities Achievable in the Fragmentation Regions of High Energy Heavy-Ion Collisions

In high energy heavy-ion collisions, the two colliding nuclei pass through each other leaving behind an almost baryon-free central region. Most of the baryon charges are carried away by the receding nuclear remnants. During the collisions, a large amount of kinetic energy is deposited in the central region where a new form of matter called quark-gluon plasma is formed. At the same time, the colliding nuclei are highly compressed, resulting in very high baryon densities in the nuclear remnants. In this talk, I will explore the high baryon density achievable in the nuclear remnants and study the hydrodynamic evolution of the high baryon density matter.

This seminar was originally scheduled for September 10.

Professor Alexandra Pope, Department of Astronomy,
University of Massachusetts Amherst


Rise of dust: Completing our Census of Cosmic Star Formation


Observations at (sub)millimeter wavelengths (~0.25-1.4 mm) provide a unique view of galaxy formation, revealing hidden star formation in the early Universe (a few Gyr after the Big Bang) that cannot be seen at optical wavelengths due to extreme dust obscuration. Previous surveys from single-dish telescopes were sensitive to only the most extreme galaxies forming stars >100 times the rate of the Milky Way, while the more typical galaxies that dominate the total energy budget at these wavelengths remain elusive. I will discuss recent observations of dusty galaxies with the next generation of (sub)millimeter wavelength telescopes -- including the Large Millimeter Telescope (LMT) and the Atacama Large Millimeter Array (ALMA) -- to push studies of dust-obscured activity down to the more typical galaxies that dominate the cosmic star formation history. Finally, I will introduce TolTEC, a revolutionary new camera for the LMT, and the public legacy surveys that will be completed from 2019-2021. These surveys will make a complete census of dust-obscured activity in the Universe.

Coffee and tea will be served prior to the talk, at 3:00 p.m., In Room GW-103 (aka PB-103)

The Physics Graduate Student Association (PGSA) invites all members of the Physics Department to our annual ice cream social event, to introduce our new graduate students and undergraduate majors and welcome everyone back who has recently joined us or returned for the new academic year.

THIS SEMINAR HAS BEEN RESCHEDULED FOR MONDAY, SEPTEMBER 17, 2018

Dr. Ming Li, Department of Physics, University of Connecticut

High Baryon Densities Achievable in the Fragmentation Regions of High Energy Heavy-Ion Collisions

Title: Contributions of Dr. Nora Berrah to the Growth of the Physics Department

Abstract: Dr. Berrah was the Department Head of Physics for the past five years. During that time, she had a major impact on all areas of the department, including important hires in all the major research areas of the department (Particles, Astrophysics, and Nuclear Physics; Atomic, Molecular, and Optical Physics; Condensed Matter Physics), including a new one (Astronomy); encouraging new teaching styles, mentoring early-career faculty, further improving the welcoming climate for undergraduate and graduate students; and raising the profile of the department in the university. Of special interest are four major initiatives she started in Astronomy, Studio Physics, AMO Physics, and Condensed Matter Physics, all of which required considerable effort on her part. In this talk, we will hear about the various hires as well as the four initiatives.

Dr. Marein Rahn, Condensed Matter and Magnet Science Group, Los Alamos National Laboratory

RIXS study of the Anderson Lattice compound
CePd₃


Intermediate valence (IV) materials represent one of the longest standing issues in quantum materials physics [1]. In CePd₃, the correlation of localized and itinerant electrons results in an ambivalent state that strongly fluctuates between the Ce⁴⁺ (4f⁰) and Ce³⁺ (4f¹) configurations. The most striking manifestation of this Kondo hybridization of 4f electrons with a sea of transition metal states occurs at low temperatures, where the valence fluctuations may develop coherency. The few available neutron spectroscopic studies of such coherent "Anderson Lattices" have
revealed that the phenomenon is associated with unusual quasiparticle excitations across the hybridization gap [2]. The low temperature dynamic magnetic susceptibility thus acquires a characteristic energy-/momentum-space texture. The recent advances in soft x-ray resonant inelastic scattering (RIXS) are among the most anticipated developments at 3rd generation synchrotrons. With an energy resolution on the order of E/dE=30000, soft RIXS is now able to address the small energy scale of the IV phenomenon [3]. I present the first use of Ce M-edge RIXS to reveal the evolution of a low temperature coherently hybridized state in CePd₃.

[1] J. M. Lawrence et al., Valence Fluctuation Phenomena, Rep. Prog. Phys 44, 1 (1981)

[2] E. Goremychkin et al., Coherent band excitations in CePd₃, Science 359, 186 (2018)

[3] N. B. Brookes et al., The beamline ID32 at the ESRF, Nuc. Inst. Meth. Phys. Res. Sec. A. 903, 175 (2018)

Fryderyk Lyzwa, Department of Physics, University of Fribourg, Switzerland


Spectroscopic ellipsometry study of LAO/STO (001) heterostructures

The heterostructure LaAlO₃/SrTiO₃ (001) has been intensively studied, after the discovery of a 2-dimensional electron gas forming at its interface [1]. This was surprising since both materials are band-insulators with a relatively high bandgap (>3eV). Moreover, applying a backgate voltage exhibits several phenomena, such as the ability to tune from an insulating into a conducting state [2]. However, the origin of the electric-field effect on this material system is still not understood. In this study, we performed spectroscopic ellipsometry in the mid-infrared region and use the Berreman-mode to analyze the behavior of the conducting layer while applying a backgate voltage.

[1] A. Ohtomo & H. Y. Hwang, Nature 427, 423 (2004)

[2] A. D. Caviglia et. al., Nature 456, 4 (2008)

Connor A. Occhialini, Department of Physics, University of Connecticut


Negative thermal expansion near two structural quantum phase transitions


Recent experimental work has revealed that the unusually strong, isotropic structural negative thermal expansion in cubic perovskite ionic insulator ScF3 occurs in excited states above a ground state tuned very near a structural quantum phase transition, posing a question of fundamental interest as to whether this special circumstance is related to the anomalous behavior. To test this hypothesis, we report an elastic and inelastic x-ray scattering study of a second system Hg2I2 also tuned near a structural quantum phase transition while retaining stoichiometric composition and high crystallinity. We find similar behavior and significant negative thermal expansion below 100 K for dimensions along the body-centered-tetragonal c axis, bolstering the connection between negative thermal expansion and zero-temperature structural transitions. We identify the common traits between these systems and propose a set of materials design principles that can guide discovery of new materials exhibiting negative thermal expansion.


References:

1. Connor A. Occhialini, Sahan U. Handunkanda, Gian G. Guzman-Verri, Jason N. Hancock, "Negative thermal expansion near two structural quantum phase transitions", Frontiers in Physical Chemistry and Chemical Physics (2018)

2. Connor A. Occhialini, Sahan U. Handunkanda, Ayman Said, Sudhir Trivedi, Gian G. Guzman-Verri, Jason N. Hancock, "Negative thermal expansion near two structural quantum phase transitions", Phys. Rev. Materials 1, 070603R (2017)

3. Connor A. Occhilaini, Sahan U. Handunkanda, Erin B. Curry, Jason N. Hancock, "Classical, quantum, and thermodynamics of a model exhibiting structural negative thermal expansion", Phys. Rev. B, 95, 094106 (2017)

4. Sahan U. Handunkanda, Connor A. Occhialini, Ayman H. Said, and Jason N. Hancock, "Two-dimensional nanoscale correlations in the strong negative thermal expansion material ScF3", Phys. Rev. B, 94, 214102 (2016)

Dr. Peter Walter, SLAC National Accelerator Laboratory


The TMO Instrument: Opportunities and Plans for Time-resolved Atomic, Molecular and Optical Science at LCLS-II


The unique capabilities of LCLS, the world's first hard X-ray FEL, have had significant impact on
advancing our understanding across a broad range of science, from fundamental atomic and molecular
physics, to condensed matter, to catalysis and to structural biology.

A major upgrade of the LCLS facility, the LCLS-II project, is now underway. LCLS-II is being developed
as a high-repetition rate X-ray laser with two simultaneously operating, independently tunable
FELs. It features a 4 GeV continuous wave superconducting linac that is capable of producing
uniformly spaced (or programmable) ultrafast X-ray laser pulses at a repetition rate up to 1 MHz spanning the energy range from 0.25 to 5 keV.


The Time resolved AMO (TMO) instrument, one of the four new LCLS-II instruments, will support AMO science, strong-field and nonlinear science, and a new dynamic reaction microscope. TMO will support
many experimental techniques not currently available at LCLS and will locate two experimental endstations. A new reaction microscope endstation will house a COLTRIMS type spectrometer to accommodate extreme vacuum, sub-micron focus spot size, and target purity requirements as dictated by coincidence experiments. The accumulation of data will be performed on an event-by-event basis using the the 1MHz full repetition rate of LCLS-II. A second TMO
endstation will be optimized for performing high energy, high resolution, time- but also angular-resolved
photoelectron spectroscopic measurements. By leveraging its existing suite of spectrometers (i.e. high
resolution iTOF, eTOF, LAMP double-sided VMIs, Kaesdorf spectrometers) a high resolution, high throughput, hemispherical electron analyzer will also be available. The photon fluence at TMO should reach up to 10^22 photons/cm^2, thus allowing to be tailored to specific experimental needs.

This talk will present some of the important science opportunities, new capabilities and instrumentation being planned for NEH 1.1 (TMO) at LCSL-II.

Dale L Smith,
Department of Physics,
University of Connecticut

Velocity Map Imaging of the Single Ionization of
Molecular Iodine

Single ionization in molecules is a critical first step in many higher order process such as high harmonic generation. However, single ionization remains poorly understood even in simple diatomic systems. We have found that molecular iodine, I2, has a complicated ionization processes which does not originate from valance orbitals, as would be expected, but from more tightly bound inner orbitals comprised of 5s electrons. I will discuss two experiments which use ultrafast lasers to investigate the ionization of diatomic I2. Ultrafast laser pulses have a duration in the tens of femtoseconds (fs). Since the vibrational period of I2 is ≈180 fs, we are able to ionize the molecule before it dissociates. Velocity map imaging allows us to observe the kinetic energy release (KER) of charged fragments from dissociation after single ionization. In the first experiment, we have performed a wavelength study from 800 to 400 nm. We have found in the I + I+ dissociation channel (which we denote as the (1,0)) the KER of the fragments was mostly inconsistent with ionization to the X-, A-, or B-states of I2+ which implies ionization through deeper orbitals. A pump-probe Fourier technique also showed that modulation in the cation products only occurs below 0.2 eV, which is consistent with dissociation through the B-state and ionization of high-lying molecular orbitals. This rules out a two-step process through the X- and A-states for much of the observed KER. Employing intensity-, polarization-, and wavelength-dependent experiments we were able to eliminate bond softening, electron rescattering, and photon mediation through the X- or A-states. In the second experiment, we performed a detailed wavelength analysis of I2 around its one-photon B-state resonance at 530 nm where we have identified a strong enhancement in the (1,0) channel. The resonance enhancement is found in both ionization from outer orbitals through the B-state as well as ionization from inner orbitals. Interestingly, the branching ratio for inner orbital ionization reaches over 98% at 519 nm and the peaks in the branching ratios of the two channels occur at slightly different wavelengths. For double ionization into an excited state of I2+ we find that the branching ratio closely follows the branching ratio of the single ionization of deeply bound inner orbitals. This implies that excitation of molecules comes about through inner orbital ionization. The finding of these experiments are inconsistent with current tunneling ionization theory which postulates that ionization arises through the least bound, outer orbital electrons.

Udaya Raj Dahal,
Department of Physics,
University of Connecticut

Polyethylene Oxide Hydration in Solutions, Nanoconfinement and Nanostructures


Amphiphilic water-soluble polymers are actively used in designing novel nanomaterials and have been the subject of extensive experimental and simulation studies. Polyethylene oxide (PEO) is a water soluble, biocompatible, non-toxic synthetic polymer capable of preventing protein adsorption which is widely used in industry and biomedicine for protein crystallization, control of particle aggregation and drug delivery. Most of the applications of PEO and PEO-based nanoparticles are utilized in an aqueous environment as PEO is highly soluble in water due to its ability to form hydrogen bonds with water and therefore understanding the role of water on the conformation and dynamics of PEO and PEO-based nanomaterials is essential in improving and designing new nanomaterials for a variety of applications.

Using all-atom molecular dynamics simulations, we studied PEO in bulk solutions, under nanoconfinement in carbon nanotubes and in nanostructures (PEO brushes grafted to a planar surface and nanoparticles). In the bulk solution, we find that PEO forms a globule like structure in hexane, coil-like structure in water or benzene and an extended rod-like or helical structure is isobutyric acid. The conformation and mobility of PEO in water and in isobutyric acid is dictated by the hydrogen bonding with PEO. As part of our confinement studies, we found that PEO is spontaneously encapsulated from aqueous solution into carbon nanotubes and forms rod-like, helical and wrapped chain conformation depending on the size of the carbon nanotube. The stable helix inside the carbon-nanotube is a consequence of the stable water arrangement around PEO.

As part of our studies of PEO brushes we investigated PEO grafted to gold nanoparticles and planar gold surfaces at varying grafting densities. We found that PEO hydration in the brush depends on grafting density and for gold nanoparticles also varies with the radial distance and nanoparticle radius of curvature. Our simulation results agree well with the classical scaling theories and we were able to explain the curvature dependent hydration based on bulk-solution PEO behavior.

Di Shu, Department of Physics,
University of Connecticut


Threshold Resonance Effects and the Phase-Amplitude Approach


A fundamental aspect of scattering is the appearance of resonances, which are rather ubiquitous. Although their effect is often lost due to averaging at room temperatures or higher, they can become dominant features at low or ultralow temperatures, where only a few partial waves contribute to the scattering process. Our ability to form and manipulate ultracold molecules provides the seed to study in a precise and controlled fashion the role of single partial waves, and state-to-state processes in chemical systems. We will explore resonances occurring due to the existence of a quasi-bound state in the entrance channel of a scattering system and explain the energy scaling due to these near threshold resonances (NTR) based on the properties of the Jost functions. We will also investigate the threshold resonance effects in the Efimov systems which have been studied in a variety of context, such as three-body Coulomb systems and nuclear three-body systems. Numerical schemes based on Milne's phase-amplitude approach are invented to analyze NTR effects. We will present a simple and practical approach to solve the long-standing problem of finding the non-oscillatory solution to Milne's equation. Our numerical approach also gives an integral representation of scattering phase shift and allows us to compute ultra-narrow shape resonances.