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Physics Colloquium, Fall 2016
Thursdays 4:00 p.m., Room 104 Physics
Refreshments served at 3:40 p.m.
Title: Imaging light-induced dynamics of small quantum systems: From infrared to hard X-ray domain
Abstract: Ultrashort light pulses provide experimental tools capable of tracing in real time the motion of atomic nuclei or sometimes even electrons. This opens up a variety of new possibilities to study dynamics of different physical, chemical or biological processes in time domain, revealing the structure of transient states and reaction intermediates, which are often not accessible by energy (frequency) domain spectroscopies. The extension of femtosecond (or even sub-femtosecond) light sources from optical to XUV and X-ray wavelengths (nowadays down to ~ 1 Angstrom) allowed probing these dynamics with atomic spatial resolution and enabled studying a particular site in an extended system by element-specific inner shell excitations. For many light-induced reactions in relatively small systems it became possible to obtain a very intuitive picture of what happens after the initial photoexcitation by acquiring a sequence of timed snapshots of the nuclei positions and (in certain cases) evolving electronic structure. In this talk I will present an overview of our ongoing effort to image a broad range of ultrafast photo-induced processes in molecules employing the so-called "momentum microscopy" (i.e., three-dimensional mapping of momentum vectors of charged reaction products) to acquire the snapshots of molecular structure. This activity combines experiments using optical lasers and their high-order harmonics at the J.R. Macdonald Laboratory at KSU with measurements performed at accelerator-based free-electron laser facilities such as LCLS in Stanford and FLASH in Hamburg. A basic underlying idea is to exploit the availability of short-pulsed light sources in a very broad span of wavelength and parameters to ensure optimal probing conditions, and to obtain the most comprehensive (often complimentary) information for each particular reaction. The examples to be considered include imaging of light-induced wave packets in simple molecules, structural rearrangement reactions like isomerization or proton migration, and charge transfer dynamics after inner-shell photoabsorption.
Host: Daniel Fischer
Title: Epitaxial superconductor-semiconductor two-dimensional systems: platforms for quantum circuits
Abstract: A key challenge in fabrication of hybrid semiconductor-superconductor devices is forming highly transparent contacts between the active electrons in the semiconductor and the superconducting metal. It has been shown that a near perfect interface and a highly transparent contact can be achieved using epitaxial growth of aluminum on InAs nanowires . Recently, this method was extended to two-dimensional systems and epitaxial aluminum on top of our near-surface InAs 2DESs were grown . Quantum devices fabricated by selective etching of Al exhibited unprecedented quality for hybrid superconductor-semiconductor weak links. This has sparked a great deal of interest in low power classical and quantum computation as gate-controlled properties of the semiconductor allows scaling of such devices.
In this work, we present recent progress in optimization of the growth of Al on InAs near surface quantum wells. We show the growth of InGaAs layers on top of InAs facilitates lower strain energy at the interface with Al and results in a flat and smooth growth allowing ultra thin superconducting Al films. We further extend this work to growth of higher Tc superconductors (such as Nb) on these thin film Al and compare the results to direct growth of these superconductors.
InAs is a particularly interesting material as it has strong spin orbit coupling that can lead to engineering novel states such as topological superconductivity. However, this method can be extended to topological insulators (in-situ growth of superconductor) and silicon for superconducting qubits. We discuss the challenges and our preliminary results in these areas. These exciting developments might lead to a number of useful applications ranging from novel low-power classical to quantum computing.
 P. Krogstrup et al. Nature Materials (2015)
 J. Shabani et al. Phys Rev B (2016)
Host: Cihan Kurter
Title: Search for variation of fundamental constants
Abstract: In many theories beyond the Standard Model of elementary particles and general relativity dimensionless fundamental constants become dynamic fields. Such theories include string theories, discrete quantum gravity and loop quantum gravity, various dark energy models, and others. The search for the variation of the fundamental constants is also a test of the local position invariance hypothesis and thus of the equivalence principle. Since any detection of the variation of fundamental constants would be an unambiguous signal of new physics, this has been a subject of active investigation in several fields of physics. From the standpoint of metrology and other precision experiments, testing for the variation of fundamental constants is necessary to ensure that the experiments are reproducible at the level of their uncertainties. This became particularly important due to exceptional improvement of atomic precision metrology in recent years. I will give an overview of the atomic and molecular searches for the variation of fundamental constants, focusing on recent key results and future proposals.
Host: Daniel Fischer
Title: Exploring quantum measurement with spontaneous emission
Abstract: Spontaneous emission of light by atoms is one of the most basic light-matter interactions and is responsible for the majority of the visible light that we see. The process of spontaneous emission can also be viewed in the context of quantum measurement, the light-matter interaction entangles the atom with the electromagnetic field and subsequent measurements of the field convey information about the state of the atom. For example, if the emitted light is detected in the form of energy quanta, the detection of an individual photon results in an instantaneous jump of the atom to a lower energy state. However, if the emission is instead measured with a detector that is not sensitive to quanta, but rather the amplitude of the field, the atom’s state will undergo different dynamics. In this talk I will describe experiments where we use superconducting circuits to explore this canonical problem in quantum measurement elucidating the process of wave function collapse.
Host: Cihan Kurter
Title: Muon g-2 and the quest for new physics
Abstract: One of the most imperative questions in particle physics today is whether or not new physics will emerge at the few TeV scale. Observational hints for new physics have arisen from several sectors with exciting theoretical implications that can potentially be explained by supersymmetry, dark matter, or other exotic models. One of the most persistent hints comes from the Brookhaven muon g-2 experiment, where an ultra-precise measurement of the muon anomalous magnetic moment differs by >3 sigma from the theoretical expectation. The anomalous magnetic moment of the muon provides a unique window into the TeV scale, and a new effort is underway at Fermilab to improve the experimental precision. A review of the physics, the principles behind the experiment,and the incredible journey to bring the experiment to the point it is at today will be discussed. From a personal perspective, the speaker will also relate how opportunities as a student at Rolla led to an exciting career in particle physics.
Host: Dan Waddill
Host: Daniel Fischer
Title: Can Waves Be Chaotic?
Abstract: There is great interest in the wave and quantum properties of systems that show chaos in the classical (short wavelength) limit. These ‘wave chaotic’ systems appear in many contexts: nuclear physics, acoustics, two-dimensional quantum dots, and electromagnetic enclosures, for example. It has been hypothesized that Random Matrix Theory (RMT) predicts universal fluctuating properties of quantum/wave systems that show chaos in the classical/ray limit. From a practical standpoint there is a need to understand electromagnetic interference on electronics located inside metallic enclosures. When the wavelength of the impinging radiation is much smaller than the typical length scale of the enclosure, the distribution of energy within such cavities is highly sensitive to small changes in the frequency, the structure of the cavity, as well as the nature of the channels which couple EM energy into the cavity. In this context we developed a stochastic model, the “Random Coupling Model” (RCM), which can accurately predict the probability density functions of voltages and electromagnetic field quantities on objects within such cavities, given a minimum of information about the cavity and the nature of its internal details. The RCM is formulated in terms of electrical impedance, essentially equivalent to Wigner’s reaction matrix in quantum mechanics, rather than the more commonly studied scattering matrix. I will discuss how the RCM predictions have been tested in a series of experiments using normal metal and superconducting quasi-two-dimensional and three-dimensional electromagnetic billiards. The model has been extended in many new directions and I will summarize a few of the surprising results.
This work is supported by ONR and ONR/DURIP, and an AFOSR Center of Excellence Grant. For more information see: http://anlage.umd.edu/AnlageQChaos.htm.
Host: Daniel Fischer
Title: Atomic Physics in X-ray Absorption Astrophysics
Abstract: An outstanding challenge in modeling the chemical evolution of the universe is the determination of elemental abundances in terms of their various phases: atomic, molecular, and/or solid-state. The answer to this problem can be found through an analysis of X-ray telescope observations using a combination of independent disciplines, commonly grouped into the subfield of "Laboratory Astrophysics". These disciplines include astrophysical spectral modeling of the interstellar medium (ISM), experimental atomic physics measurements, and theoretical atomic physics calculations.
This talk will first give a brief review of x-ray spectroscopy and how it is used to study the chemical evolution of the universe. The underlying atomic physics processes will also be described, with an emphasis on the resonance process that are strongly imprinted on the observed x-ray spectra. I will conclude by discussing the extension from simple atomic photoabsorption to absorption in multi-centered (molecular or solid-state) systems. Absorption cross sections in composite-matter systems are needed for modeling the fractional phase distributions of heavy elements in cosmic clouds.
Host: Daniel Fischer
Title: Electron Impact Ionization of Atoms, Molecules and Clusters
Abstract: Ionizing collisions of electrons with atoms or molecules on one hand are very fundamental few-body reactions and on the other hand they are ubiquitous in our environment and in applications like in natural and technical plasmas and in radiation therapy. By measuring the momentum vectors of all charged particles involved we get detailed insight into the ionization dynamics. After an introduction some of our recent studies on atoms and small molecules are presented. Then new ionization and fragmentation reactions are discussed which are observed if target atoms or molecules are embedded in an environment as it is the case in small clusters or in the condensed phase. We have identified interatomic energy and charge transfer processes in argon dimers with the coincident detection of three electrons and two ions. Furthermore, we discuss ionization of the biologically relevant ring molecule tetrahydrofuran (C4H8O, THF) which is the simplest analog of deoxyribose in the DNA backbone. In pure and water-mixed clusters of THF we observed strongly modified fragmentation channels compared to ionization of monomers. In addition we identified Intermolecular Coulombic Decay induced by energy transfer from an ionized neighboring water molecule to the THF molecule. Such reactions are suspected to enhance radiation damages which are induced by ionizing radiation in biological cells.
Host: Daniel Fischer
Title: Strange phenomena in flatland: Physics Nobel Prize 2016