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Professor Olivier Delaire
Department of Mechanical Engineering and Materials Science
Duke University
George P. Williams, Jr. Lecture Hall, (Olin 101)
Wednesday, Oct. 30, 2019, at 3:00 PM

There will be a reception in the Olin Lounge at approximately 4 PM following the colloquium. All interested persons are cordially invited to attend.

ABSTRACT

A detailed view of atomic motions in materials is needed to refine microscopic theories of transport and thermodynamics, and to design next-generation energy materials. In particular, the behavior of atomic vibrations (phonons) is key to rationalize numerous functional properties, ranging from ferroelectrics for sonar, to superionics for safer solid batteries, to thermoelectrics for waste-heat harvesting, or metal-insulator transitions for ultrafast transistors. Near phase transitions associated with phonon instabilities, one needs to properly account for the effect of strong anharmonicity, which disrupts the quasiharmonic phonon gas model through large phonon-phonon coupling terms. Large phonon amplitudes can also amplify the electron-phonon interaction and lead to renormalization of a material’s electronic structure. These interactions, while often neglected in textbooks and traditional studies, could open the door to further tuning of materials properties for improved functionality.

This presentation will highlight results from our investigations of atomic dynamics in several classes of materials impacted by lattice instabilities, such as ferroelectrics and multiferroics (EuTiO3, YMnO3) [1], thermoelectrics (PbTe, SnSe) [2-4], superionic conductors [5], and VO2 across its metal insulator transition [6,7]. Our group takes advantage of advances in modern neutron and x-ray instrumentation, which have revolutionized our ability to probe atomic dynamics. By mapping phonon spectral functions throughout reciprocal space, phonon anharmonicity and couplings to other degrees of freedom can now be revealed in great detail. Such mode-resolved investigations bring direct insights into phonon scattering mechanisms, including anharmonicity, electron-phonon coupling, spin-phonon coupling, or scattering by defects and nanostructures. Increasingly, first-principles simulations of atomic dynamics enable the quantitative rationalization of these effects, for example with ab-initio molecular dynamics simulations or anharmonic renormalization techniques at finite-temperature, and our group systematically integrates such modeling with our scattering experiments. The presentation will conclude with some possible scientific opportunities.

[1] D. Bansal, J. L. Niedziela, R. Sinclair, V. O. Garlea, D. L. Abernathy, S. Chi, Y. Ren, H. Zhou, and O. Delaire, “Momentum-resolved observations of the phonon instability driving geometric improper ferroelectricity in yttrium manganite”, Nature Communications 9, 15 (2018).
[2] O. Delaire, J. Ma, K. Marty, A. F. May, M. A. McGuire, M.-H. Du, D. J. Singh, A. Podlesnyak, G. Ehlers, M. Lumsden, B. C. Sales, “Giant Anharmonic Phonon Scattering in PbTe”, Nature Materials 10, 614 (2011).
[3] C.W. Li, O. Hellman, J. Ma, A.F. May, H.B. Cao, X. Chen, A.D. Christianson, G. Ehlers, D.J. Singh, B.C. Sales, and O. Delaire, “Phonon self-energy and origin of anomalous neutron scattering spectra in SnTe and PbTe thermoelectrics”, Physical Review Letters 112, 175501 (2014).
[4] C.W. Li,* J. Hong,* A.F. May, D. Bansal, S. Chi, T. Hong, G. Ehlers and O. Delaire, “Orbitally-driven giant phonon anharmonicity in SnSe”, Nature Physics 11, 1063 (2015).
[5] J. L. Niedziela, D. Bansal, A. F. May, J. Ding, T. Lanigan-Atkins, G. Ehlers, D. L. Abernathy, A. Said & O. Delaire, “Selective Breakdown of Phonon Quasiparticles across Superionic Transition in CuCrSe2”, Nature Physics, 15, 73–78 (2019)
[6] J. D. Budai*, J. Hong*, M. E. Manley, E. D. Specht, C. W. Li, J. Z. Tischler, D. L. Abernathy, A. H. Said, B. M. Leu, L. A. Boatner, R. J. McQueeney, and O. Delaire, “Metallization of vanadium dioxide driven by large phonon entropy”, Nature 515, 535–539 (2014).
[7] S. Lee, et al., “Anomalously low electronic thermal conductivity in metallic vanadium dioxide” Science, 355 (6323): 371 (2017).

Funding from US DOE, Office of Basic Energy Sciences, Materials Science and Engineering Division.
Link to Delaire Research Group Web Page

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Professor Troy Stich
Department of Chemistry
Wake Forest University
George P. Williams, Jr. Lecture Hall, (Olin 101)
Wednesday, Oct. 9, 2019, at 3:00 PM

There will be a reception in the Olin Lounge at approximately 4 PM following the colloquium. All interested persons are cordially invited to attend.

ABSTRACT

The radical SAM (S-adenosyl-L-methionine) superfamily of enzymes catalyzes a dizzying array of chemistries triggered by reductive cleavage of SAM to yield the primary carbon radical 5′-deoxyadenosyl (5’dAdo●). 5’dAdo● can pluck off H-atoms with bond dissociation enthalpies <105 kcal/mol from substrate molecules to initiate carbon skeleton rearrangements. We hypothesize that amino acids within the active site, a triose phosphate isomerase (TIM) barrel, are key in conducting these rearrangements down the evolutionary-designed path. Our research effort employs a combined biochemical, spectroscopic, and computational approach to determine atomic level details of these mechanisms. We further use substrate analogs that can slow or halt the chemistry at key points, allowing us to verify mechanistic hypotheses. Today, I will present a few examples that illustrate our progress toward unveiling the factors that control these exotic reactions.

Link to Professor Stich’s web page

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WFU Physics Faculty and Students
George P. Williams, Jr. Lecture Hall, (Olin 101)
Wednesday, Sept. 18, 2019, at 3:00 PM

There will be a reception in the Olin Lounge at approximately 4 PM following the colloquium. All interested persons are cordially invited to attend.

PROGRAM

This colloquium will highlight physics research at Wake Forest University. During the colloquium, Physics Department faculty members and/or their students will present the essence of their research programs in the Physics Department. This forum for sharing ideas will hopefully inspire collaborations between students and faculty and between research groups.

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The Society for Physics Students (SPS) is hosting a cookout outside of Polo Hall on Monday Sept. 2, 2019 starting at 5:30 PM.   All Physics students (undergraduate majors, minors, and potentials, graduate students, etc.), faculty, and staff are welcome to join. Please sign up here.

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Professor Jing Li
Department of Chemistry and Chemical Biology
Rutgers University
Piscataway, NJ USA
George P. Williams, Jr. Lecture Hall, (Olin 101)
Wednesday,Sept. 4, 2019, at 3:00 PM

There will be a reception in the Olin Lounge at approximately 4 PM following the colloquium. All interested persons are cordially invited to attend.

ABSTRACT

Metal-organic frameworks (MOFs) are a unique class of highly porous crystalline solids composed of periodically ordered and covalently bonded metal building units and organic ligands. In the past two decades MOFs have become one of the most intensively and extensively explored material families due to their enormous potential for a wide range of applications. MOFs are particularly promising as a new type of adsorbents for gas storage, capture and separation. They have demonstrated numerous advantages over conventional/traditional sorbent materials, not only due to their exceptionally high surface area, but also because of their nearly unlimited structural tunability and remarkable surface functionalizability.

Adsorptive separation of industrially relevant hydrocarbons is of paramount importance as it may substantially reduce the energy consumption required for the current distillation-based technology. However, finding an ideal adsorbent has been challenging as it requires precise control of the porosity (e.g. pore size, pore aperture, pore shape) and sorbent-sorbate interaction in order to meet the stringent performance requirement.

Guided by topological design strategy, we have recently succeeded in designing several MOFs with optimum pore structure.1-3 Built on zirconium and calcium metals and tetratopic carboxylate linkers they exhibit excellent stability towards heat, moisture and hush chemical environment. They show highly efficient separation of selected hydrocarbon mixtures, including alkane isomers and propane/propylene, with a performance surpassing benchmark adsorbents.

References:

  1. Wang, H.; Dong, X.L.; Lin, J.Z.; Teat, S.J.; Jensen, S.; Cure, J.; Alexandrov, E.V.; Xia, Q.B.; Wang, Q.N.; Olson, D.H.; Proserpio, D.M.; Chabal, Y.J.; Thonhauser, T.; Sun, J.L.; Han, Y.; Li, J. “Topologically Guided Tuning of Zr-MOF Pore Structures for Highly Selective Separation of C6 Alkane Isomers”, Nat. Commun., 2018, 9:1745.
  2. Wang, H.; Dong, X.L.; Velasco, E.; Olson, D.H.; Han, Y.; Li, J. “One-of-A-Kind: The First Example of Adsorptive Separation of Three Alkane Isomers by A Microporous Metal-Organic Framework via Temperature- and Adsorbate-Dependent Molecular Sieving”, Ene & Env Sci, 2018, 11, 1226-1231.
  3. Wang, H., Dong, X.L.; Colombo, V.; Wang, Q.N.; Liu, Y.Y.; Liu, W.; Wang, X.L.; Huang, X.Y.; Proserpio, D.M.; Sironi, S.; Han, Y.; Li, J. “Tailor-Made Microporous Metal-Organic Frameworks for the Full Separation of Propane from Propylene through Selective Size Exclusion”, Adv. Mater., 2018, 30, 201805088.

Link to Professor Jing Li’s web page

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George P. Williams, Jr. Lecture Hall, (Olin 101)
Wednesday, August 28, 2019, at 3 PM

There will be a reception in the Olin Lounge at approximately 4 PM following the colloquium. All interested persons are cordially invited to attend.

PROGRAM

The purpose of this first seminar is to help new, returning, and prospective students (including both undergraduate and graduate students), faculty, and staff to become acquainted with each other and with the Physics Department. We will meet in the George P. Williams, Jr. Lecture Hall (Olin 101) at 3:00 PM for presentations by some undergraduate students highlighting their summer research experiences, followed by general welcoming statements and departmental announcements.

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WFU senior physics students will present highlights of their honors theses
George P. Williams, Jr. Lecture Hall, (Olin 101)
Wednesday, April 24, 2019, at 4:00 PM

There will be a reception with refreshments at 3:30 PM in the lounge. All interested persons are cordially invited to attend.

PROGRAM

  • Jacquelyn Sharpe— Mentor: Prof. Guthold — “Mechanical Properties of Electrospun 50:50 Fibrinogen:PCL Nanofibers”
  • Sean Yan— Mentor: Prof. Carroll — “All Inorganic Lead Halide Perovskite Core Shell Structure Nano-Inclusion Based Thin Film Light-Emitting Device Optimization
  • Cole Teander— Mentor: Prof. Thonhauser — “Using DFT to Predict the Elastic Moduli of Fourth Generation Metal-Organic Framework Materials”

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Peiyun Li
Public Presentation in ZSR Library Auditorium
Monday, April 15, 2019 at 11 AM

There will be a reception with refreshments following the defense in Olin Lounge. All interested persons are cordially invited to attend the public talk and the reception.

ABSTRACT

Recent discoveries of rare-earth and alkaline-earth halides with scintillation activators and co-dopants showing excellent properties for spectroscopic gamma radiation detection attract a surge in research activity on their scintillation mechanisms. There is still much to learn about excited states in these materials. Understanding behaviors of the free carries and excitons in the first picoseconds are crucial for determining the speed and nonlinearity of response. Questions remain on whether and when the free electrons are trapped on holes, dopants or defects. The nature of interaction and recombination between the photon-excited species are also important. In this thesis, the crucial early evolution of excited populations is studied with picosecond spectroscopy of optical absorption induced by interband excitation.

We identified the self-trapped exciton (STE) absorption bands in LaBr3:Ce and CeBr3 samples along with a comparative study on the effects of Ce concentration on the STE absorption decay rate. The dominant scintillation mechanism of both LaBr3:Ce and CeBr3 is attributed to dipole-dipole energy transfer from the STE to Ce3+ dopant ions on the basis of the transient absorption bands. We identified the charge-transfer excitation of excited Ce3+* ions for the first time. The population rise time of the Ce3+* excited states in CeBr3 (~540 fs) is observed to be faster than in LaBr3:Ce, and reasons are described. We conclude that our picosecond absorption spectroscopy provides a unique method to assist in the improvement of timing resolution by isolating the rise time of population in the emitting state from the rise time of detected scintillation light, aiming for ultrafast time-of-flight detection.

We also studied the effects of interband excitation on undoped BaBrCl and on BaBrCl doped with Eu and/or Au, as measured by picosecond transient absorption spectroscopy. Aside from the identification of STE absorption bands in BaBrCl samples, we concluded that subsequent dipole-dipole energy transfer from STE to Eu is the dominant energy transfer mechanism. Au co-dopant in BaBrCl:Eu has been found to improve the scintillation light yield, and these transient absorption studies support that the mechanism involves suppression of the concentration of pre-existing halide vacancies.

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Dr. K. Maria Mills,
Laboratory of Molecular Biophysics, NIH
George P. Williams, Jr. Lecture Hall, (Olin 101)
Wednesday, December 5, 2018, at 4:00 PM

There will be a reception with refreshments at 3:30 PM in the lounge. All interested persons are cordially invited to attend.

ABSTRACT

Type IA topoisomerases cleave single-stranded DNA and relieve topological strain by passing an intact DNA strand through the broken strand. Although it is assumed that these enzymes accomplish this strand passage via a protein-mediated DNA gate, opening of this gate has never been observed. We developed a single-molecule magnetic tweezers assay to directly measure gate opening of the E. coli type IA topoisomerases, topo I and topo III. Following cleavage of single-stranded DNA, the protein gate opens by as much as 6.6 nm and can close against forces in excess of 16 pN. The force-dependent kinetics reveal that the gate dynamics of these enzymes are remarkably fast. Key differences in the cleavage, ligation, and gate dynamics of these two enzymes provide insights into their different cellular functions. The single-molecule results are broadly consistent with conformational changes associated with gate-opening obtained from molecular dynamics simulations. These results allow us to develop a complete mechanistic model of type IA topoisomerase-ssDNA interactions.