Aug. 29
Departmental Safety Colloquium
Betsy Howe, UNL
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Friday, Sept. 13
Non-equilibrium Structural Dynamics and Phase Transitions in Solids
Ralph Ernstorfer, Fritz Haber Institute, Berlin
Intense ultrashort laser pulses allow for the preparation of transient states of matter exhibiting strong non-equilibrium between electrons and lattice. The optical and structural properties as well as the temporal evolution of such states provide insight into the mutual dependence of electronic and atomic structure. We investigate optical and structural properties of non-equilibrium states with femtosecond optical spectroscopy and femtosecond electron diffraction. Atomic-level views of melting have been obtained under strongly-driven conditions for metal, semimetal and semiconductor films. The effect of intense excitation on the inter-atomic potential in a solid and its implication on the nature of a subsequent phase transition are strongly material dependent. In the case of semiconductors and semimetals, strong electronic excitation gives rise to an electronically-driven, non-thermal disordering mechanism [1,2]. In contrast, gold excited into the regime of warm dense matter exhibits disordering of the lattice slower compared to the energy transfer from the electronic to the vibrational degrees of freedom indicative of electronic bond hardening [3].
In addition to the effect of carrier generation, we investigate the possible effect of carrier relaxation dynamics on the evolution of excited-state potential energy surfaces. We study the dynamics of the coherent A1g optical phonon in TiO2 after above bandgap excitation using ultrashort ultraviolet pulses. A phase shift of the phonon oscillation compared to a purely instantaneous displacive excitation indicates a significant contribution to the displacive force driving the lattice vibration due to the cooling of the excited hot electron-hole plasma [4]. Finally, I briefly discuss photo-induced ultrafast dynamics in the optical response of the phase change material Ge2Sb2Te5 (GST) which is widely-used as optical data-storage medium.
[1] M. Harb et al., Phys. Rev. Lett. 100, 155504 (2008).
[2] G. Sciaini et al., Nature 458, 56 (2009).
[3] R. Ernstorfer et al., Science 323, 1033 (2009).
[4] E.M. Bothschafter et al., Phys. Rev. Lett. 110, 067402 (2013).
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Sept. 26
Anion Photodetachment Imaging: Electron-Neutral Molecule Interactions from Anion Precursors
Richard Mabbs, Washington University, St. Louis, MO
Molecular anions afford interesting opportunities to investigate interactions of free electrons with neutral molecules. Using a photodetachment approach the anion serves as an in situ electron source, excitation allowing us to access the electron-neutral molecule continuum. This will be illustrated using the AgF− anion as an example. In a way, the AgF−→ AgF + e− process represents a half scattering event. Careful observation of the energy evolution of the photoelectron angular distribution (PAD) as a function of incident photon energy using velocity mapped imaging (VELMI). VELMI allows simultaneous measurement of the photoelectron spectrum (PES) and photoelectron angular distribution (PAD). The results clearly indicate the presence of strong interactions between the departing electron and the residual AgF molecule. In particular, the PADs give a strikingly sensitive indication of indirect processes (elastic scattering/autodetachment ) which lead to the same (energetically indistinguishable) outcome as direct detachment.
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Oct. 3
The First Measurement of Spatially-Localized Viscous Heating
John Goree, University of Iowa
Viscous heating is ubiquitous. It is the way that energy is lost as air flows over a car, water flows past a swimmer, and blood flows through a body's vessels. Viscous heating happens most intensely where there is a large gradient or shear in the fluid's velocity. While it is easy enough to heat a fluid by causing a shear flow, it is surprisingly difficult to detect a temperature increase that is localized where the shear is greatest. It is so difficult, in fact, that it seems nobody had ever reported such an experiment, according to our literature search. The reason is that most fluid substances have such a high thermal conductivity that heat is carried away too rapidly to leave a hot spot. By using an extreme substance, we have now observed such a localized heating. The substance is a strongly-coupled dusty plasma. Dusty plasmas are common in interstellar nebulae; they are a mixture of small solid particles, electrons, ions and gas. They can also be made in the laboratory by introducing polymer microspheres into a low-pressure gas that is partially ionized. The polymer microspheres are electrically levitated. Under laboratory conditions, the collection of microspheres behaves like molecules in a liquid, but with an extraordinarily low density. The radiation pressure force from a laser is used to drive a flow of the polymer microspheres, which are imaged using a high-speed video camera.
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Oct. 10
The Science and Engineering of Functional Complex Oxide Thin Films
Lane Martin, University of Illinois at Urbana-Champaign
Complex oxide materials possess a range of interesting properties and phenomena that make them candidates for next-generation devices and applications. But before these materials can be integrated into state-of-the-art devices, it is important to understand how to control and engineer the response of these often complex materials. In this talk we will discuss the science and engineering of thin-film versions of these materials. We will explore the role of epitaxial thin-film growth and the use of lattice mismatch strain to engineer domain structures and properties in ferroelectric systems. The use of epitaxial strain enables the production of model versions of these complicated materials and the subsequent deterministic study of field-dependent response. In particular we will investigate how epitaxial constraints can enhance electric field and temperature susceptibilities (i.e., dielectric, pyroelectric, and electrocaloric effects) in ferroelectrics. The presentation will highlight a comprehensive approach to the understanding of field-dependent response of materials. This includes aspects of design of new high-performance materials using phenomenological models, application of epitaxial thin-film strain to produce controlled domain structures and exotic new phases, identification of domain wall contributions to response, the development of novel measurement techniques, and the fabrication and testing of rudimentary devices based on these materials. In particular, we will explore the synthesis and properties of highly engineered ferroic thin films (i.e., single layer, bi-layer, and compositionally graded PbZr1-xTixO3 and others) that have been optimized for enhanced dielectric/pyroelectric/electrocaloric responses. The discussion will range from the development of a fundamental understanding of the physics that lies at the heart of these effects, to an illustration of routes to manipulate and control these effects, to the demonstration of solid-state devices based on these materials.
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Oct. 17
Ultrafast Imaging of Molecules
Martin Centurion, UNL
We have recently demonstrated 3D imaging of a symmetric top molecule by using a femtosecond laser to align the molecules, and a femtosecond electron pulse to capture the diffraction pattern while the molecules are aligned*. The 3D structure of the molecule was retrieved by combining the information from multiple diffraction patterns corresponding to different projections of the molecule. This method is ideal to study structural dynamics in molecules on very fast time scales, as it simultaneously provides sub-Angstrom and femtosecond resolution. We are currently working to extend this method to more complex molecules and to image the effect of strong laser fields on molecules. In addition, a new electron gun is being constructed with the goal of achieving atomic imaging with 100 fs resolution.
*Focus: First Frame from 3D Molecular Movie
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Friday, Nov. 1
Elements of Theoretical Strong-Field and Attosecond Physics
Lars Bojer Madsen, Department of Physics and Astronomy, Aarhus University, Denmark
I first describe how recent technological advances have enabled ultrashort, ultraintense laser pulses with frequencies that permit unprecedented investigations of time-resolved electron and nuclear motion in molecular systems. Femto- and attosecond pulses thus open new avenues for time-domain studies and promise major breakthroughs in our understanding of many- electron quantum dynamics in atoms, molecules and solids on their natural time- and length-scales (see, e.g., the review [1]).
Among the different strong-field processes, tunneling ionization plays a prominent role as the initial key process triggering subsequent strong-field dynamics. I will discuss tunneling ionization of molecules and I will address the question of how an exact treatment of the nuclear motion affects the electron tunneling dynamics. I will show that the Born-Oppenheimer approximation breaks down at sufficiently weak fields, since retardation caused by the finiteness of the electron's velocity prevents the electron to adjust to an instantaneous internuclear configuration at large electron-nuclei distance [2].
In the last part of the talk, I will elucidate the fundamental question about how the energy deposited by an intense laser pulse is shared between electrons and nuclei in molecules. How does the energy sharing depend on the number of absorbed photons? How does the width of the different photon absorption channels change with increasing photon absorption? Why do we see a difference in the structure of the above-threshold absorption spectra between linear and circularly polarized light? Following recent theoretical [3] and experimental [4] works, the joint energy spectrum (JES) for electrons and nuclei is identified as the appropriate observable.
- F. Krausz and M. Ivanov, Attosecond physics, Rev. Mod. Phys. 81, 163 (2009).
- O. I. Tolstikhin and L. B. Madsen, Retardation effects and the Born-Oppenheimer approximations: Theory of tunneling ionization of molecules revisited, Phys. Rev. Lett. Accepted September 2013.
- C. B. Madsen et al., Multiphoton Above Threshold Effects in Strong-Field Fragmentation, Phys. Rev. Lett. 109, 163003 (2012); R. E. F. Silva et al., Correlated Electron and Nuclear Dynamics in Strong Field Photoionization of H2+, Phys. Rev. Lett. 110, 113001 (2013).
- J. Wu et al. Electron-Nuclear Energy Sharing in Above-Threshold Multiphoton Dissociative Ionization of H2, Phys. Rev. Lett. 111, 023002 (2013).
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Nov. 7
Global Climate Change and Atmospheric Ozone Depletion: Understanding and Perspective from a Physicist
Qing-Bin Lu, University of Waterloo, ON
In the world major environmental and climate problems, it might be seen as a mystery that despite increasing CO2 levels, observed global surface temperature has strikingly stopped rising or even showed a declining trend since about a decade ago. Another not well-known mystery is that no clear trend in recovery of the Antarctic ozone hole has been detected, while the Montreal Protocol has led to the decline in atmospheric level of chlorofluorocarbons (CFCs, the major ozone depleting molecules) since the turn of this century. This talk will discuss the possible solutions to these two mysteries. It will be focused on the cosmic-ray-driven electron-induced-reaction (CRE) theory of halogenated molecules for the formation of the polar ozone hole [1, 2] and the greenhouse theory of halogenated molecules for recent global warming [2]. Recent in-depth analyses of comprehensive measured datasets and theoretical calculations have convincingly shown that both the CRE mechanism and the CFC-warming mechanism not only provide new fundamental understandings of the ozone hole and global climate change but have superior predictive capabilities, compared with the conventional models [3].
- QB Lu & TE Madey, J. Chem. Phys. 111, 2861 (1999); Phys. Rev. Lett. 82, 4122 (1999). QB Lu & L Sanche, Phys. Rev. Lett. 87, 078501 (2001); Phys. Rev. B63, 153403 (2001). QB Lu, Phys. Rev. Lett. 102, 118501 (2009).
- QB Lu, Physics Reports 487, 141 (2010); QB Lu, J. Cosmology 8, 1846 (2010).
- QB Lu, Int. J. Mod. Phys. B 27, 1350073 (2013).
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Monday, Nov. 11
Study on Magnetoelectric Effect in Thin Film Cr2O3 Sesquioxide and Electrical Switching of HEX and Residual Magnetization
Masashi Sahashi, Tohoku University
Magneto-Electric (ME) effect has been paid much attention from the perspective of voltage controlled magnetization switching. Cr2O3 oxide is a typical sesquioxide that shows ME effect and its antiferromagnetic Neel temperature is TN =307 K, which is highest in ME materials. A robust isothermal electric control of exchange-bias field at RT is actually reported for bulk single crystal Cr2O3 (0001) substrate/Pd 0.5 nm/(Co 0.6 nm/ Pd 1.0 nm)3 exchange-biased system after initial ME annealing ( E=1 kV/cm and H=778 Oe), where isothermal-field exposure is under E=26 kV/cm and H= 1.54 kOe (|EH| product ~ 40 kV/cm･ kOe), respectively [1]. But it is of much note that ME effect has been not yet confirmed in thin film form, which is the key for the whole evolution to device application, because of its large leakage current while ME effect like behavior is reported to be observed up to 200K in an ultrathin Cr2O3/Fe2O3 nanooxide layer by investigating the training effect (partial surface spin reversal by the ME effect) [2]. Considering the application of ME effect to storage/memory technology for voltage-controlling magnetization switching, there are many concerns including the above, which should be resolved. The first is to realize and design an effectually high exchange-bias filed between Cr2O3 and FM thin film layers in the higher temperature range than RT, which means high blocking temperature (TB), where the properly low coercive force of FM is also required. The second is to invest FM layer with a perpendicular anisotropy, which is thought to be caused by both of the hybridization of FM 3d and O 2p orbitals (interface anisotropy; KS) and the exchange anisotropy (KEX) at the interface between FM and Cr2O3 besides the bulk anisotropy (KV) of FM layer. The third is to confirm ME effect in the thin film Cr2O3 after getting Cr2O3 thin film which shows good electrical properties. In this study, magnetoelectric effect of the thin film Cr2O3 sesquioxide with good leakage-current property (~10-5A/cm2) and the exchange-bias property were investigated. We successfully confirmed ME effect of the Cr2O3 thin film, and observed clear electrical switching of HEX and the residual magnetization in M-H curve for the perpendicular magnetized FM layer with low coercivity. The electrical properties in the perpendicular direction of our thin film were as follows; The leakage current density at E = 20 [kV/cm] is as small as 3 × 10 -6 [A/cm2]. The parasitic resistance, the film resistance and the capacitance were 16.6 [Ω], 80.2 [kΩ] and 17.2 [nF], respectively. Dielectric constant εr calculated from these results was 13.8, which is almost same as that reported (εr = 11.9). In addition, we also successfully observed the effect of Fe2O3 buffer layer on Hex and TB. Exchange biased system with Cr2O3 thin film and thin Fe2O3 buffer (5 nm) shows high JK up to 0.44 (erg/cm2) and higher TB more than 200K. In presentation, the control of both of Neel temperature for Cr2O3 and Morin temperature for Fe2O3 is also discussed from a perspective of future device application.
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Nov. 21
From Atoms to Solids: Mapping Structure with Electrons
Carlos Trallero, Kansas State University
We can study the structure of atoms, molecules and nano-sized samples using laser driven electrons. In molecules and atoms this can be done through the process of high harmonic generation, in vacuum where the molecule’s own electrons are used to reveal their structure. In solids, this is not so trivial and I will present preliminary results on the interaction of strong laser pulses with nanowires. Our results show that a new, strong-field approach of the interaction between electrons and the electric field is necessary. Finally I will present some of the new optics developments that are taking place at the J.R. Macdonald Laboratory
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Dec. 5
The Standard Model and the Higgs Boson at the LHC
Joao Guimaraes da Costa, Harvard
Scientists at CERN have been exploring the high energy frontier with the Large Hadron Collider since March 2010. The substantial dataset accumulated thus far, albeit at lower energy than initially foreseen, already yielded a Nobel prize award. The new boson, discovered in 2012 by the ATLAS and CMS collaborations, has been proven to behave very much like the long-sought-after Higgs Boson, and hence it completes the discovery of the Standard Model of Particle Physics. The LHC will resume operations in 2015 with increased center of mass energy, opening the possibility for yet new major breakthroughs. Precise measurements of the Standard Model phenomena at these unprecedented energies are a key element of any such discoveries, and allows us to constrain physics beyond the Standard Model. This talk will review several measurements at the LHC, with an emphasis on the most recent Higgs results, and discusses their interplay with the ever continuing search for new answers.
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Jan. 16
Escape Trajectories from Traditional Condensed Matter
Jerry Seidler, University of Washington
We are entering a golden age of facility capability and user access for the preparation of extreme states of matter. While the campaign for inertial confinement fusion at the National Ignition Facility is both a major driver and a central highlight of this era, the entire achievable phase space of temperature, pressure, and magnetic field is rich with scientific opportunity in correlated electron physics, laboratory astrophysics, and fusion energy science. Here, I focus on the transition regimes from ‘traditional’ condensed matter to dense correlated plasma and ‘warm dense matter’ states. First, I will survey the physical phenomena already observed, already predicted, or that can be reasonably expected at temperatures up to 100 eV (1 million K) and pressures up to tens of megabars where surprising commonalities exist between contemporary issues in correlated plasma physics and those in condensed matter physics. Second, I will address the large uncertainties, often uncontrolled and under-appreciated, in basic material properties under such extreme conditions. In this context, there is again strong overlap between the major problems in the determination of state variables in dense plasma physics and the canonical issues in condensed matter physics, such as the need to orthogonalize the valence and core electron wavefunctions. This raises the question of whether traditional, low-density plasma physics can be perturbatively expanded into any truly dense regime, or if instead modern, nonperturbative, condensed phase approaches need to be extended to consider the elevated temperatures and densities that occur in the warm dense matter regime. [1]
[1] Mattern, Seidler, Kas, arXiv:1308.2990 [physics.plasm-ph]
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FRIDAY
Jan. 31
249 JH
The Smallest Free Particles in Saturn’s Rings
Rebecca Harbison
The Cassini spacecraft, which entered orbit around Saturn in 2004, has provided a wealth of observations at resolutions and geometries unavailable from Earth, thereby expanding our knowledge of the Saturnian system. In particular, observations of Saturn’s rings not only give us an understanding both of this remarkable system, but a close look at the nearest debris disk to the Earth, offering insight into the behavior of these disks in an astrophysical context.
Cassini’s orbit about Saturn allows for solar occultations of the rings, a geometry impossible to achieve from Earth. During these occultations, the icy particles making up the rings diffract infrared light at angles detectable by the Visible-Infrared Mapping Spectrometer (VIMS) instrument onboard, thus giving information about the size distribution of ring particles. Using VIMS, I have measured the minimum free ring particle size of the A and C Ring. While the C Ring result agrees with other studies, my observations of the A Ring show particles smaller than one millimeter, conflicting with previous results. However, I also discovered that including the A Ring’s self gravity wakes – temporary 50-100 meter-sized aggregates of ring particles – makes a noticeable difference in any attempt to model the ring’s particle-size distribution, and previous studies did not account for this non-homogenous ring structure.
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Feb. 27
Electron Diffraction – From Time-Averaged to Time-Resolved Experiments
Derek Wann, University of York
For more than 80 years electron diffraction has been used as a tool for determining the static structures of molecular species. Such work is still important, especially in the gas phase where the analogous technique of X-ray diffraction has limited use. I will talk about some work performed in my group that has used molecular dynamics simulations to better understand vibrations of molecules in the gas phase in an attempt to yield electron diffraction structures for molecules with very low-amplitude modes of vibration.
In recent years electron diffraction has found uses in time-resolved experiments where ultrafast lasers are used both to produce electrons from a photocathode, and also to pump structural changes. I will discuss the work that has been going on in my group in the UK to develop our own system capable of studying structural dynamics. I will focus on the apparatus design, simulations of resolution, and our initial candidates for study using the set-up, presenting some of the first diffraction images recorded.
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March 14
Why Isn't God Ambidextrous?
Timothy Gay, University of Nebraska-Lincoln
Until 1957, scientists thought that the fundamental laws of Nature must be the same whether they were applied to our Universe or the Universe that is a mirror reflection of our own. The implications of the discovery that this is not true – essentially that Nature is "handed" – will be discussed. Some interesting applications of handedness, or "chirality" in agriculture, biology, chemistry, and physics will be presented. I will also talk about some new physics experiments on chirality that may shed light on how life began on this planet.
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March 20
Anyonics: Designing exotic circuitry with non-Abelian anyons
Kirill Shtengel, University of California, Riverside
Non-Abelian anyons are widely sought for the exotic fundamental physics they harbor as well as for their possible applications for quantum information processing. Currently, there are numerous blueprints for stabilizing the simplest type of non-Abelian anyon, a Majorana zero energy mode bound to a vortex or a domain wall. One such candidate system, a so-called "Majorana wire" can be made by judiciously interfacing readily available materials; the experimental evidence for the viability of this approach is presently emerging. Following this idea, we introduce a device fabricated from conventional fractional quantum Hall states, s-wave superconductors and insulators with strong spin-orbit coupling. Similarly to a Majorana wire, the ends of our "quantum wire" would bind "parafermions", exotic non-Abelian anyons which can be viewed as fractionalised Majorana zero modes. I will discuss their properties and describe how such parafermions can be used to construct new and potentially useful circuit elements which include current and voltage mirrors, transistors for fractional charge currents and "flux capacitors".
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April 10
Novel States and Functions of Magnetic and Polar Solids at the Nanoscale
Wolfgang Kleemann, University of Duisburg-Essen
Magnetic and electric dipolar interactions may give rise to novel states in aggregates of nanoparticles. First, different supermagnetic states are observed in dense magnetic single domained nanoparticle systems [1]. Superspin glasses have even been proposed to serve as model systems to test microscopic spin glass theories [2]. Second, in so-called ‘relaxor ferroelectrics’ such as PbMg1/3Nb2/3O3 or BaTi1-xSnxO3 polar nanoregions first emerge via intrinsic random electric fields and afterwards organize a cluster glass ground state [3]. Arguments against a one-step spin-glass theory [4] will be discussed.
Stress-strain coupling in magnetoelectrically nanocomposed thick films of BaTiO3/CoFe2O4¬¬ (BTO/CFO) developing self-organized nanopillars (NPs) of CFO [5] may be used for novel RAM devices. Magnetostrictive deformation of the NPs by in-plane x and y directed magnetic fields stresses the BTO environment uniaxially [6] and piezoelectrically induces binary in-plane polarization patterns to be read magnetoresistively.
[1] W. Kleemann et al., Phys. Rev. B63 (2001) 134423; S. Bedanta et al., in: Handbook Magn. Mater., ed. K.H.J. Buschow (Elsevier, 2014).
[2] R. Mathieu et al., Eur. Phys. Lett. 102 (2013) 67002.
[3] W. Kleemann, in: Mesoscopic phenomena in multifunctional materials, eds. A. Saxena, A. Planes (Springer, 2014).
[4] D. Sherrington, Phys. Rev. Lett. 111 (2013) 227601.
[5] H. Zheng et al., Science 303 (2004) 661.
[6] C. Antoniak et al., Nat. Comm. 4 (2013) 2015.
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TUESDAY
April 15
The Hidden Ocean of Europa: Exploring a Potentially Habitable World
Robert Pappalardo, JPL, California Institute of Technology
This is a joint colloquium between the Physics & Astronomy Department and the School of Biological Sciences. Dr. Pappalardo’s visit is made possible by the American Astronomical Society’s Harlow Shapley Visiting Lectureship Program.
Galileo spacecraft data suggest that a global ocean exists beneath the frozen ice surface of Jupiter’s moon Europa. Magnetometry data indicates an induced magnetic field at Europa, implying that a salt-water ocean exists today. A paucity of large craters argues for a surface on average only ~40–90 Myr, and two multi-ring structures suggest impacts punched through an ice shell ~20 km thick. Europa’s ocean and surface are inherently linked through tidal deformation of the floating ice shell, and tidal flexing and nonsynchronous rotation may generate stresses that fracture and deform the surface to create ridges and bands. Dark spots, domes, and chaos terrain are probably related to tidally driven ice convection, along with partial melting within the ice shell. Europa’s geological activity and probable direct contact between its ocean and rocky mantle may permit the chemical ingredients for life to be present within the ocean. Fascinating geology and geophysics, combined with high astrobiological potential, make Europa a top priority for future spacecraft exploration. The Europa Clipper is a mission concept currently being studied by NASA, which would make multiple flybys of Europa from Jupiter orbit, to investigate its potential habitability.
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April 17
Hidden Momentum
David Griffiths, Reed College
Electromagnetic fields carry energy, momentum, and even angular momentum. The momentum density is ε0(E×B), and it accounts (among other things) for the pressure of light. But even static fields can harbor momentum, and this would appear to contradict a general theorem: if the center of energy of a closed system is at rest, then its total momentum must be zero. Evidently in such cases there lurks some other momentum, not electromagnetic in nature, which cancels the field momentum. But finding this "hidden momentum" can be surprisingly subtle. I'll discuss a particularly nice example.
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May 1
Graphene Plus
Jun Zhu, Penn State University
Graphene is a fascinating 2D material from the viewpoint of both fundamental physics studies and applications. Its all-surface nature and intimate interactions with its environment present challenges as well as opportunities to engineer new properties. I will attempt to give a short update on the progress of the field in the last several years and discuss a few works in my lab in controlling existing and imparting new properties and functionalities in graphene and few-layer graphene systems. Functionalization of graphene using chemisorbed fluorine adatoms can profoundly modify its charge transport, leading to a band insulator. It also enhances the intrinsically weak spin-orbit coupling in pristine sheet and results in additional spin relaxation mechanisms that are tunable electrically. These new properties are potentially useful in spintronic applications. Bilayer and Trilayer graphene possess band gaps that are tunable via an electric field from zero to a few hundred meV, the exploitation of which could lead to optoelectronic devices.