Departmental Safety Colloquium
Betsy Howe, UNL
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 .
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 . 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.
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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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.
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.
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.
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 ).
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 .
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  and experimental  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).
Global Climate Change and Atmospheric Ozone Depletion: Understanding and Perspective from a Physicist
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 . 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 .
- 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. B27, 1350073 (2013).
Study on Magnetoelectric Effect in Thin Film Cr2O3 Sesquioxide and Electrical Switching of HEX and Residual Magnetization
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 . 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) . 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.
From Atoms to Solids: Mapping Structure with Electrons
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
The Standard Model and the Higgs Boson at the LHC
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.