32 results on '"Davis, Jeffrey P."'
Search Results
2. Imaging topological polar structures in marginally twisted 2D semiconductors
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Vu, Thi-Hai-Yen, Bennett, Daniel, Pallewella, Gayani Nadeera, Uddin, Md Hemayet, Xing, Kaijian, Zhao, Weiyao, Lee, Seng Huat, Mao, Zhiqiang, Muir, Jack B., Jia, Linnan, Davis, Jeffrey A., Watanabe, Kenji, Taniguchi, Takashi, Adam, Shaffique, Sharma, Pankaj, Fuhrer, Michael S., and Edmonds, Mark T.
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Condensed Matter - Mesoscale and Nanoscale Physics ,Condensed Matter - Materials Science - Abstract
Moire superlattices formed in van der Waals heterostructures due to twisting, lattice mismatch and strain present an opportunity for creating novel metamaterials with unique properties not present in the individual layers themselves. Ferroelectricity for example, arises due to broken inversion symmetry in twisted and strained bilayers of 2D semiconductors with stacking domains of alternating out-of-plane polarization. However, understanding the individual contributions of twist and strain to the formation of topological polar nanostructures remains to be established and has proven to be experimentally challenging. Inversion symmetry breaking has been predicted to give rise to an in-plane component of polarization along the domain walls, leading to the formation of a network of topologically non-trivial merons (half-skyrmions) that are Bloch-type for twisted and Neel-type for strained systems. Here we utilise angle-resolved, high-resolution vector piezoresponse force microscopy (PFM) to spatially resolve polarization components and topological polar nanostructures in marginally twisted bilayer WSe2, and provide experimental proof for the existence of topologically non-trivial meron/antimeron structures. We observe both Bloch-type and Neel-type merons, allowing us to differentiate between moire superlattices formed due to twist or heterogeneous strain. This first demonstration of non-trivial real-space topology in a twisted van der Waals heterostructure opens pathways for exploring the connection between twist and topology in engineered nano-devices.
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- 2024
3. Intrinsic defect engineering of CVD grown monolayer MoS$_2$ for tuneable functional nanodevices
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Abidi, Irfan H., Giridhar, Sindhu Priya, Tollerud, Jonathan O., Limb, Jake, Mazumder, Aishani, Mayes, Edwin LH, Murdoch, Billy J., Xu, Chenglong, Bhoriya, Ankit, Ranjan, Abhishek, Ahmed, Taimur, Li, Yongxiang, Davis, Jeffrey A., Bentley, Cameron L., Russo, Salvy P., Della Gaspera, Enrico, and Walia, Sumeet
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Physics - Applied Physics ,Condensed Matter - Materials Science - Abstract
Defects in atomically thin materials can drive new functionalities and expand applications to multifunctional systems that are monolithically integrated. An ability to control formation of defects during the synthesis process is an important capability to create practical deployment opportunities. Molybdenum disulfide (MoS$_2$), a two-dimensional (2D) semiconducting material harbors intrinsic defects that can be harnessed to achieve tuneable electronic, optoelectronic, and electrochemical devices. However, achieving precise control over defect formation within monolayer MoS$_2$, while maintaining the structural integrity of the crystals remains a notable challenge. Here, we present a one-step, in-situ defect engineering approach for monolayer MoS$_2$ using a pressure dependent chemical vapour deposition (CVD) process. Monolayer MoS$_2$ grown in low-pressure CVD conditions (LP-MoS$_2$) produces sulfur vacancy (Vs) induced defect rich crystals primarily attributed to the kinetics of the growth conditions. Conversely, atmospheric pressure CVD grown MoS$_2$ (AP-MoS$_2$) passivates these Vs defects with oxygen. This disparity in defect profiles profoundly impacts crucial functional properties and device performance. AP-MoS$_2$ shows a drastically enhanced photoluminescence, which is significantly quenched in LP-MoS$_2$ attributed to in-gap electron donor states induced by the Vs defects. However, the n-doping induced by the Vs defects in LP-MoS$_2$ generates enhanced photoresponsivity and detectivity in our fabricated photodetectors compared to the AP-MoS$_2$ based devices. Defect-rich LP-MoS$_2$ outperforms AP-MoS$_2$ as channel layers of field-effect transistors (FETs), as well as electrocatalytic material for hydrogen evolution reaction (HER). This work presents a single-step CVD approach for in-situ defect engineering in monolayer MoS$_2$ and presents a pathway to control defects in other monolayer material systems., Comment: 29 pages, 5 figures
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- 2023
4. Floquet engineering in the presence of optically excited carriers
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Conway, Mitchell A., Tollerud, Jonathan O., Vu, Thi-Hai-Yen, Watanabe, Kenji, Taniguchi, Takashi, Fuhrer, Michael S., Edmonds, Mark T., and Davis, Jeffrey A.
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Condensed Matter - Mesoscale and Nanoscale Physics ,Condensed Matter - Other Condensed Matter ,Physics - Optics - Abstract
Floquet engineering provides an optical means to manipulate electronic bandstructures, however, carriers excited by the pump field can lead to an effective heating, and can obscure measurement of the band changes. A recent demonstration of the effects of Floquet engineering on a coherent ensemble of excitons in monolayer WS$_2$ proved particularly sensitive to non-adiabatic effects, while still being able to accurately resolve bandstructure changes. Here, we drive an AC-Stark effect in monolayer WS$_2$ using pulses with constant fluence but varying pulse duration (from 25-235~fs). With shorter pump pulses, the corresponding increase in peak intensity introduces additional carriers via two-photon absorption, leading to additional decoherence and peak broadening (which makes it difficult to resolve the AC-Stark shift). We use multidimensional coherent spectroscopy to create a coherent ensemble of excitons in monolayer WS$_2$ and measure the evolution of the coherence throughout the duration of the Floquet pump pulse. Changes to the amplitude of the macroscopic coherence quantifies the additional broadening. At the same time, the evolution of the average phase allows the instantaneous changes to the bandstructure to be quantified, and is not impacted by the additional broadening. This approach to measuring the evolution of Floquet-Bloch states demonstrates a means to quantify effective heating and non-adiabaticity caused by excited carriers, while at the same time resolving the coherent evolution of the bandstructure.
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- 2023
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5. Effects of Floquet Engineering on the Coherent Exciton Dynamics in Monolayer WS$_2$
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Conway, Mitchell A., Earl, Stuart K., Muir, Jack B., Vu, Thi-Hai-Yen, Tollerud, Jonathan O., Watanabe, Kenji, Taniguchi, Takashi, Fuhrer, Michael S., Edmonds, Mark T., and Davis, Jeffrey A.
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Condensed Matter - Mesoscale and Nanoscale Physics ,Physics - Optics - Abstract
Coherent optical manipulation of electronic bandstructures via Floquet Engineering is a promising means to control quantum systems on an ultrafast timescale. However, the ultrafast switching on/off of the driving field comes with questions regarding the limits of validity of the Floquet formalism, which is defined for an infinite periodic drive, and to what extent the transient changes can be driven adibatically. Experimentally addressing these questions has been difficult, in large part due to the absence of an established technique to measure coherent dynamics through the duration of the pulse. Here, using multidimensional coherent spectroscopy we explicitly excite, control, and probe a coherent superposition of excitons in the $K$ and $K^\prime$ valleys in monolayer WS$_2$. With a circularly polarized, red-detuned, pump pulse, the degeneracy of the $K$ and $K^\prime$ excitons can be lifted and the phase of the coherence rotated. We demonstrate phase rotations during the 100 fs driving pulse that exceed $\pi$, and show that this can be described by a combination of the AC-Stark shift of excitons in one valley and Bloch-Siegert shift of excitons in the opposite valley. Despite showing a smooth evolution of the phase that directly follows the intensity envelope of the pump pulse, the process is not perfectly adiabatic. By measuring the magnitude of the macroscopic coherence as it evolves before, during, and after the pump pulse we show that there is additional decoherence caused by power broadening in the presence of the pump. This non-adiabaticity may be a problem for many applications, such as manipulating q-bits in quantum information processing, however these measurements also suggest ways such effects can be minimised or eliminated.
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- 2023
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6. Exciton-polaron interactions in monolayer WS$_2$
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Muir, Jack B., Levinsen, Jesper, Earl, Stuart K., Conway, Mitchell A., Cole, Jared H., Wurdack, Matthias, Mishra, Rishabh, Ing, David J., Estrecho, Eliezer, Lu, Yuerui, Efimkin, Dmitry K., Tollerud, Jonathan O., Ostrovskaya, Elena A., Parish, Meera M., and Davis, Jeffrey A.
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Condensed Matter - Mesoscale and Nanoscale Physics ,Condensed Matter - Other Condensed Matter ,Physics - Optics - Abstract
Interactions between quasiparticles are of fundamental importance and ultimately determine the macroscopic properties of quantum matter. A famous example is the phenomenon of superconductivity, which arises from attractive electron-electron interactions that are mediated by phonons or even other more exotic fluctuations in the material. Here we introduce mobile exciton impurities into a two-dimensional electron gas and investigate the interactions between the resulting Fermi polaron quasiparticles. We employ multi-dimensional coherent spectroscopy on monolayer WS$_2$, which provides an ideal platform for determining the nature of polaron-polaron interactions due to the underlying trion fine structure and the valley specific optical selection rules. At low electron doping densities, we find that the dominant interactions are between polaron states that are dressed by the same Fermi sea. In the absence of bound polaron pairs (bipolarons), we show using a minimal microscopic model that these interactions originate from a phase-space filling effect, where excitons compete for the same electrons. We furthermore reveal the existence of a bipolaron bound state with remarkably large binding energy, involving excitons in different valleys cooperatively bound to the same electron. Our work lays the foundation for probing and understanding strong electron correlation effects in two-dimensional layered structures such as moir\'e superlattices.
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- 2022
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7. Direct Measurement of Biexcitons in Monolayer WS2
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Conway, Mitchell, Muir, Jack, Earl, Stuart, Wurdack, Matthias, Mishra, Rishabh, Tollerud, Jonathan, and Davis, Jeffrey
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Condensed Matter - Mesoscale and Nanoscale Physics ,Condensed Matter - Materials Science ,Physics - Optics - Abstract
The optical properties of atomically thin transition metal dichalcogenides (TMDCs) are dominated by Coulomb bound quasi-particles, such as excitons, trions, and biexcitons. Due to the number and density of possible states, attributing different spectral peaks to the specific origin can be difficult. In particular, there has been much conjecture around the presence, binding energy and/or nature of biexcitons in these materials. In this work, we remove any ambiguity in identifying and separating the optically excited biexciton in monolayer WS2 using two-quantum multidimensional coherent spectroscopy (2Q-MDCS), a technique that directly and selectively probes doubly-excited states, such as biexcitons. The energy difference between the unbound two-exciton state and the biexciton is the fundamental definition of biexciton binding energy and is measured to be 26 \pm 2 meV. Furthermore, resolving the biexciton peaks in 2Q-MDCS allows us to identify that the biexciton observed here is composed of two bright excitons in opposite valleys.
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- 2021
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8. Short-Term Learners' Motivation Modeling in Web Based Education System
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Shabbir, Shahzad, Ayub, Muhammad Adnan, Khan, Farman Ali, and Davis, Jeffrey
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Recent research regarding personalized web based educational systems demonstrate learners' motivation to be an essential component of the learning model. This is due to the fact that low motivation results in either students' less engagement or complete drop out from the learning activities. A learner motivation model is considered to be a set of perceptions and beliefs that the system has developed about a learner. This includes both short-term and long-term motivation of learners. Short-term motivation encompasses specific, challenging and attainable goals that develop in the limited timespan. On the other hand, long-term motivation indicates a sort of continuing commitment that is required to complete assigned task. Since, short-term motivational problems span for a limited period of time such as a session, therefore, needs to be addressed in real-time to keep the learner engaged in the learning process. This paper proposes the framework of a domain independent short-term learner motivation model based on Keller ARCS motivation theory and Social Cognitive Theory. The proposed motivation identification framework consists of two modules. The primary module deals with real time identification of motivation, and the secondary module maintains the profile of learners associated with the short-term motivation. [For the full proceedings, see ED621620.]
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- 2020
9. Constraining anomalous Higgs boson couplings to virtual photons
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Davis, Jeffrey, Gritsan, Andrei V., Guerra, Lucas S. Mandacaru, Kyriacou, Savvas, Roskes, Jeffrey, and Schulze, Markus
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High Energy Physics - Phenomenology ,High Energy Physics - Experiment - Abstract
We present a study of Higgs boson production in vector boson fusion and in association with a vector boson and its decay to two vector bosons, with a focus on the treatment of virtual loops and virtual photons. Our analysis is performed with the JHU generator framework. Comparisons are made to several other frameworks, and the results are expressed in terms of an effective field theory. New features of this study include a proposal on how to handle singularities involving Higgs boson decays to light fermions via photons, calculation of the partial Higgs boson width in the presence of anomalous couplings to photons, a comparison of the next-to-leading-order electroweak corrections to effects from effective couplings, and phenomenological observations regarding the special role of intermediate photons in analysis of LHC data in the effective field theory framework. Some of these features are illustrated with projections for experimental measurements with the full LHC and HL-LHC datasets., Comment: 32 pages, 14 figures
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- 2021
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10. Lower-Luminosity Obscured AGN Host Galaxies are Not Predominantly in Major-Merging Systems at Cosmic Noon
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Lambrides, Erini, Chiaberge, Marco, Heckman, Timothy, Kirkpatrick, Allison, Meyer, Eileen T., Petric, Andreea, Hall, Kirsten, Long, Arianna, Watts, Duncan J., Gilli, Roberto, Simons, Raymond, Tchernyshyov, Kirill, Rodriguez-Gomez, Vicente, Vito, Fabio, De La Vega, Alexander, Davis, Jeffrey R., Kocevski, Dale D, and Norman, Colin
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Astrophysics - Astrophysics of Galaxies - Abstract
For over 60 years, the scientific community has studied actively growing central super-massive black holes (active galactic nuclei -- AGN) but fundamental questions on their genesis remain unanswered. Numerical simulations and theoretical arguments show that black hole growth occurs during short-lived periods ($\sim$ 10$^{7}$ -10$^{8}$ yr) of powerful accretion. Major mergers are commonly invoked as the most likely dissipative process to trigger the rapid fueling of AGN. If the AGN-merger paradigm is true, we expect galaxy mergers to coincide with black hole accretion during a heavily obscured AGN phase (N$_H$ $ > 10^{23}$ cm$^{-2}$). Starting from one of the largest samples of obscured AGN at 0.5 $<$ $z$ $<$ 3.1, we select 40 non-starbursting lower-luminosity obscured AGN. We then construct a one-to-one matched redshift- and near-IR magnitude-matched non-starbursting inactive galaxy control sample. Combining deep color \textit{Hubble Space Telescope} imaging and a novel method of human classification, we test the merger-AGN paradigm prediction that heavily obscured AGN are strongly associated with galaxies undergoing a major merger. On the total sample of 80 galaxies, we estimate each individual classifier's accuracy at identifying merging galaxies/post-merging systems and isolated galaxies. We calculate the probability of each galaxy being in either a major merger or isolated system, given the accuracy of the human classifiers and the individual classifications of each galaxy. We do not find statistically significant evidence that obscured AGN at cosmic noon are predominately found in systems with evidence of significant merging/post-merging features., Comment: 19 pages, 11 figures, accepted in ApJ
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- 2021
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11. Lunar Volatiles and Solar System Science
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Prem, Parvathy, Kereszturi, รkos, Deutsch, Ariel N., Hibbitts, Charles A., Schmidt, Carl A., Grava, Cesare, Honniball, Casey I., Hardgrove, Craig J., Pieters, Carlรฉ M., Goldstein, David B., Barker, Donald C., Needham, Debra H., Hurley, Dana M., Mazarico, Erwan, Dominguez, Gerardo, Patterson, G. Wesley, Kramer, Georgiana Y., Brisset, Julie, Gillis-Davis, Jeffrey J., Mitchell, Julie L., Szalay, Jamey R., Halekas, Jasper S., Keane, James T., Head, James W., Mandt, Kathleen E., Robinson, Katharine L., Luchsinger, Kristen M., Magaรฑa, Lizeth O., Siegler, Matthew A., Landis, Margaret E., Poston, Michael J., Petro, Noah E., Lucey, Paul G., Killen, Rosemary M., Li, Shuai, Narendranath, Shyama, Shukla, Shashwat, Barrett, Thomas J., Stubbs, Timothy J., Orlando, Thomas M., and Farrell, William M.
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Astrophysics - Instrumentation and Methods for Astrophysics ,Astrophysics - Earth and Planetary Astrophysics - Abstract
Understanding the origin and evolution of the lunar volatile system is not only compelling lunar science, but also fundamental Solar System science. This white paper (submitted to the US National Academies' Decadal Survey in Planetary Science and Astrobiology 2023-2032) summarizes recent advances in our understanding of lunar volatiles, identifies outstanding questions for the next decade, and discusses key steps required to address these questions.
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- 2020
12. Multidimensional Analysis of Excitonic Spectra of Monolayers of Tungsten Disulphide: Towards Computer Aided Identification of Structural and Environmental Perturbations of 2D Materials
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Kolesnichenko, Pavel V., Zhang, Qianhui, Zheng, Changxi, Fuhrer, Michael S., and Davis, Jeffrey A.
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Condensed Matter - Mesoscale and Nanoscale Physics ,Condensed Matter - Materials Science ,Physics - Computational Physics - Abstract
Despite 2D materials holding great promise for a broad range of applications, the proliferation of devices and their fulfillment of real-life demands are still far from being realized. Experimentally obtainable samples commonly experience a wide range of perturbations (ripples and wrinkles, point and line defects, grain boundaries, strain field, doping, water intercalation, oxidation, edge reconstructions) significantly deviating the properties from idealistic models. These perturbations, in general, can be entangled or occur in groups with each group forming a complex perturbation making the interpretations of observable physical properties and the disentanglement of simultaneously acting effects a highly non-trivial task even for an experienced researcher. Here we generalise statistical correlation analysis of excitonic spectra of monolayer WS2, acquired by hyperspectral absorption and photoluminescence imaging, to a multidimensional case, and examine multidimensional correlations via unsupervised machine learning algorithms. Using principle component analysis we are able to identify 4 dominant components that are correlated with tensile strain, disorder induced by adsorption or intercalation of environmental molecules, multi-layer regions and charge doping, respectively. This approach has the potential to determine the local environment of WS2 monolayers or other 2D materials from simple optical measurements, and paves the way towards advanced, machine-aided, characterisation of monolayer matter., Comment: 12 pages, 4 figures, supplementary materials
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- 2020
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13. Disentangling the effects of doping, strain and defects in monolayer WS2 by optical spectroscopy
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Kolesnichenko, Pavel V., Zhang, Qianhui, Zheng, Changxi, Fuhrer, Michael S., and Davis, Jeffrey A.
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Condensed Matter - Materials Science ,Physics - Chemical Physics ,Physics - Optics - Abstract
Monolayers of transition metal dichalcogenides (TMdC) are promising candidates for realization of a new generation of optoelectronic devices. The optical properties of these two-dimensional materials, however, vary from flake to flake, or even across individual flakes, and change over time, all of which makes control of the optoelectronic properties challenging. There are many different perturbations that can alter the optical properties, including charge doping, defects, strain, oxidation, and water intercalation. Identifying which perturbations are present is usually not straightforward and requires multiple measurements using multiple experimental modalities, which presents barriers when attempting to optimise preparation of these materials. Here, we apply highresolution photoluminescence and differential reflectance hyperspectral imaging in situ to CVD-grown WS2 monolayers. By combining these two optical measurements and using a statistical correlation analysis we are able to disentangle three contributions modulating optoelectronic properties of these materials: electron doping, strain and defects. In separating these contributions, we also observe that the B-exciton energy is less sensitive to variations in doping density than A-excitons., Comment: 27 pages, 5 figures
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- 2019
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14. Background-free time-resolved coherent Raman spectroscopy (CSRS and CARS): heterodyne detection of low-energy vibrations and recognition of excited-state contributions
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Kolesnichenko, Pavel, Tollerud, Jonathan O., and Davis, Jeffrey A.
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Physics - Optics ,Physics - Atomic Physics - Abstract
Coherent Raman scattering (CRS) spectroscopy techniques have been widely developed and optimized for different applications in biomedicine and fundamental science. The most utilized CRS technique has been coherent anti-Stokes Raman scattering (CARS), and more recently, stimulated Raman scattering (SRS). Coherent Stokes Raman scattering (CSRS) has been largely ignored mainly because it is often strongly affected by fluorescence, particularly for resonance enhanced measurements. However, in the cases of resonant excitation the information contained in the CSRS signal can be different and complementary to that of CARS. Here we combine the approaches of pulse shaping, interferometric heterodyne detection, 8-step phase cycling and Fourier-transform of time-domain measurements, developed in CARS and 2D electronic spectroscopy communities, to measure resonant CSRS and CARS spectra using a Titanium:sapphire oscillator. The signal is essentially background-free (both fluorescent and non-resonant background signals are suppressed) with high spectral resolution and high sensitivity, and can access low-energy modes down to ~30 cm-1. We demonstrate the ability to easily select between CSRS and CARS schemes and show an example in which acquisition of both CSRS and CARS spectra allows vibrational modes on the excited electronic state to be distinguished from those on the ground electronic state., Comment: Main text: 14 pages, 4 figures. Supplementary: 14 pages, 12 figures
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- 2018
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15. Persistent coherence of quantum superpositions in an optimally doped cuprate revealed by 2D spectroscopy
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Novelli, Fabio, Tollerud, Jonathan O., Prabhakaran, Dharmalingam, and Davis, Jeffrey A.
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Condensed Matter - Superconductivity ,Physics - Optics - Abstract
Understanding of the precise mechanisms of high-temperature superconductivity is elusive. In particular, in order to solve the puzzle of the pairing mechanism, it is important to understand the detailed nature of the excitations at energies around the superconducting gap. While measurements of the dynamics of excited electronic populations have been able to give some insight, they have largely neglected the intricate dynamics of quantum coherence. Here, we apply multidimensional coherent spectroscopy for the first time to a prototypical cuprate and report unprecedented coherent dynamics persisting for ~500 fs, originating directly from the quantum superposition of optically excited states separated by 20 - 60 meV. These results reveal the correlation between high and low energy excitations, and indicate that the interplay between many-body states on different energy scales conserves phase coherence. In revealing these dynamics we demonstrate that multidimensional coherent spectroscopy can address electronic correlations and interrogate many-body quantum systems in unprecedented ways.
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- 2017
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16. Time-domain THz spectroscopy reveals coupled protein-hydration dielectric response in solutions of native and fibrils of human lyso-zyme
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Novelli, Fabio, Pour, Saeideh Ostovar, Tollerud, Jonathan, Roozbeh, Ashkan, Appadoo, Dominique R. T., Blanch, Ewan W., and Davis, Jeffrey A.
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Physics - Biological Physics - Abstract
Here we reveal details of the interaction between human lysozyme proteins, both native and fibrils, and their water environment by intense terahertz time domain spectroscopy. With the aid of a rigorous dielectric model, we determine the amplitude and phase of the oscillating dipole induced by the THz field in the volume containing the protein and its hydration water. At low concentrations, the amplitude of this induced dipolar response decreases with increasing concentration. Beyond a certain threshold, marking the onset of the interactions between the extended hydration shells, the amplitude remains fixed but the phase of the induced dipolar response, which is initially in phase with the applied THz field, begins to change. The changes observed in the THz response reveal protein-protein interactions me-diated by extended hydration layers, which may control fibril formation and may have an important role in chemical recognition phenomena.
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- 2017
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17. Transient THz conductivity of silicon with optical pumping above and below the second indirect transition
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Novelli, Fabio and Davis, Jeffrey A
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Physics - Optics ,Condensed Matter - Other Condensed Matter ,Condensed Matter - Strongly Correlated Electrons - Abstract
Here we perform a series of time-resolved experiments where a 100 fs pump pulse is tuned between 528 nm and 555 nm, across the second indirect gap of intrinsic silicon at ~540 nm which involves electrons in a higher-lying conduction band with minimum at the L point. The photo-injected carriers, after inter- and intra-band relaxations are complete, are subsequently probed with high-field single-cycle terahertz radiation. When the energy of the pump pulses exceeds the second indirect gap, the probed terahertz absorption decreases by a factor 2.7$\pm$0.2. We suggest that this dramatic change could be due to the different phonon populations obtained when the carriers undergo the L to X inter-band scattering, instead of just cooling within the X-valley.
- Published
- 2016
18. Separating Pathways in Double-Quantum Optical Spectroscopy Reveals Excitonic Interactions
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Tollerud, Jonathan and Davis, Jeffrey
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Physics - Optics - Abstract
Techniques for coherent multidimensional optical spectroscopy have been developed and utilised to understand many different processes, including energy transfer in photosynthesis and many-body effects in semiconductor nanostructures. Double-quantum 2D spectroscopy is one variation that has been particularly useful for understanding many-body effects. In condensed matter systems, however, there are often many competing signal pathways, which can make it difficult to isolate different contributions and retrieve quantitative information. Here, a means of separating overlapping pathways while maintaining the fidelity of the relevant peak/s is demonstrated. This selective approach is used to isolate the double-quantum signal from a mixed two exciton state in a semiconductor quantum well. The removal of overlapping peaks allows analysis of the relevant peak-shape and thus details of interactions with the environment and other carriers to be revealed. An alternative pulse ordering identifies a double-quantum state associated only with GaAs defects, the signature of which has previously been confused with other interaction induced effects. The experimental approach described here provides access to otherwise hidden details of excitonic interactions and demonstrates that the manner in which the double-quantum coherence is generated can be important and provide an additional control to help understand the many-body physics in complex systems.
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- 2016
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19. Revealing and Characterizing Dark Excitons Through Coherent Multidimensional Spectroscopy
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Tollerud, Jonathan O., Cundiff, Steven T., and Davis, Jeffrey A.
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Physics - Optics ,Condensed Matter - Mesoscale and Nanoscale Physics ,Condensed Matter - Materials Science ,Condensed Matter - Other Condensed Matter - Abstract
Dark excitons are of fundamental importance in a broad range of contexts, but are difficult to study using conventional optical spectroscopy due to their weak interaction with light. We show how coherent multidimensional spectroscopy can reveal and characterize dark states. Using this approach, we identify different types of dark excitons in InGaAs/GaAs quantum wells and determine details regarding lifetimes, homogeneous and inhomogeneous linewidths, broadening mechanisms and coupling strengths. The observations of coherent coupling between bright and dark excitons hint at a role for a multi-step process by which excitons in the barrier can relax into the quantum wells.
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- 2016
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20. Two-dimensional double-quantum spectroscopy: peak shapes as a sensitive probe of carrier interactions in quantum wells
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Tollerud, Jonathan O. and Davis, Jeffrey A.
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Physics - Optics ,Condensed Matter - Mesoscale and Nanoscale Physics - Abstract
We identify carrier scattering at densities below which it has previously been observed in semiconductor quantum wells. These effects are evident in the peakshapes of 2D double-quantum spectra, which change as a function of excitation density. At high excitation densities ($\geq 10^{9}$ carriers/,cm$^{-2}$) we observe untilted peaks similar to those reported in previous experiments. At low excitation densities (<$10^{8}$ carriers cm$^{-2}$) we observe narrower, tilted peaks. Using a simple simulation, we show that tilted peak-shapes are expected in double-quantum spectra when inhomogeneous broadening is much larger than homogeneous broadening, and that fast pure-decoherence of the double-quantum coherence can obscure this peak tilt. These results show that carrier interactions are important at lower densities than previously expected, and that the `natural' double-quantum peakshapes are hidden by carrier interactions at the excitation densities typically used. Furthermore, these results demonstrate that analysis of 2D peak-shapes in double-quantum spectroscopy provides an incisive tool for identifying interactions at low excitation density.
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- 2016
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21. The Effects of Brownian Motion On Particle Interactions with Patchy Surfaces Bearing Nanoscale Features
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Bendersky, Marina, Santore, Maria M., and Davis, Jeffrey M.
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Condensed Matter - Soft Condensed Matter - Abstract
The effects of Brownian motion on particle interactions with heterogeneous collectors are evaluated by adding stochastic Brownian displacements to the particle trajectories and comparing those trajectories to those where Brownian motion is not included. We define adhesion thresholds as the average patch density on the collector required to adhere colloidal particles and P\'{e}clet numbers that quantify the relative importance of colloidal, shear and Brownian effects. We show that Brownian motion has a negligible influence on particle trajectories over collectors patterned with nano-scale heterogeneity, the non-uniform distribution of which creates locally attractive and repulsive areas within the collector. High energy barriers in strong locally repulsive areas cannot be overcome by Brownian motion, such that particle deposition on patchy collectors is controlled by spatially varying DLVO interactions and not by Brownian motion. The overall adhesive behavior of the system remains unaffected by the introduction of Brownian motion effects in the simulations, and therefore, for particle sizes that are usually used in experiments of particle trajectories over nanoscale heterogeneous collectors, it is reasonable to neglect Brownian motion effects entirely., Comment: 8 pages, 6 figures
- Published
- 2015
22. Counting prime juggling patterns
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Banaian, Esther, Butler, Steve, Cox, Christopher, Davis, Jeffrey, Landgraf, Jacob, and Ponce, Scarlitte
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Mathematics - Combinatorics - Abstract
Juggling patterns can be described by a closed walk in a (directed) state graph, where each vertex (or state) is a landing pattern for the balls and directed edges connect states that can occur consecutively. The number of such patterns of length $n$ is well known, but a long-standing problem is to count the number of prime juggling patterns (those juggling patterns corresponding to cycles in the state graph). For the case of $b=2$ balls we give an expression for the number of prime juggling patterns of length $n$ by establishing a connection with partitions of $n$ into distinct parts. From this we show the number of two-ball prime juggling patterns of length $n$ is $(\gamma-o(1))2^n$ where $\gamma=1.32963879259...$. For larger $b$ we show there are at least $b^{n-1}$ prime cycles of length $n$.
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- 2015
23. A generalization of Eulerian numbers via rook placements
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Banaian, Esther, Butler, Steve, Cox, Christopher, Davis, Jeffrey, Landgraf, Jacob, and Ponce, Scarlitte
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Mathematics - Combinatorics ,05A15, 05A10 - Abstract
We consider a generalization of Eulerian numbers which count the number of placements of $cn$ "rooks" on an $n\times n$ board where there are exactly $c$ rooks in each row and each column, and exactly $k$ rooks below the main diagonal. The standard Eulerian numbers correspond to the case $c=1$. We show that for any $c$ the resulting numbers are symmetric and give generating functions of these numbers for small values of $k$., Comment: 15 pages
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- 2015
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24. A computational study of chemically heterogeneous particles: patchy vs. uniform particles in shear flow
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Bendersky, Marina, Santore, Maria M., and Davis, Jeffrey M.
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Condensed Matter - Soft Condensed Matter - Abstract
The adhesion of flowing particles and biological cells over fixed collecting surfaces is vitally important in diverse situations and potentially controlled by small-scale surface heterogeneity on the particle. Differences in the behavior of patchy particles (flowing over uniform collectors) relative to the reverse case of uniform particles (flowing over patchy collectors) are quantified. Because a particle rotates more slowly than it translates in the shear field near a collecting surface, the effective interaction time of a patch on a particle is larger than that of a patch on the collector, suggesting distinct particle capture tendencies in each case. This paper presents a new computational approach to simulate the near-surface motion (rotation and translation) of particles having nanoscale surface heterogeneities flowing over uniform collectors. Small amounts of ~10 nm cationic patches randomly distributed on a net-negative particle surface produced spatially varying DLVO interactions that were computed via the Grid Surface Integration (GSI) technique and then combined with hydrodynamic forces in a mobility tensor formulation. Statistical analysis of simulated trajectories revealed fewer extrema in the fluctuating particle-collector separation of heterogeneous particles, compared with the reverse system geometry of uniform particles flowing past a heterogeneous fixed surface. Additionally, the patchy particles were captured to a lesser extent on uniform surfaces compared with the case of uniform particles flowing above patchy collectors. Such behavior was dependent on ionic strength, with the greatest differences obtained near a Debye length of $\kappa^{-1} = 4$ nm for the $2a = 500$ nm simulated particles.
- Published
- 2015
25. Vibronic resonances facilitate excited state coherence in light harvesting proteins at room temperature
- Author
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Novelli, Fabio, Nazir, Ahsan, Richards, Gethin H., Roozbeh, Ashkan, Wilk, Krystyna E., Curmi, Paul M. G., and Davis, Jeffrey A.
- Subjects
Physics - Biological Physics ,Physics - Chemical Physics ,Physics - Optics ,Quantum Physics - Abstract
Until recently it was believed that photosynthesis, a fundamental process for life on earth, could be fully understood with semi-classical models. However, puzzling quantum phenomena have been observed in several photosynthetic pigment-protein complexes, prompting questions regarding the nature and role of these effects. Recent attention has focused on discrete vibrational modes that are resonant or quasi-resonant with excitonic energy splittings and strongly coupled to these excitonic states. Here we unambiguously identify excited state coherent superpositions in photosynthetic light-harvesting complexes using a new experimental approach. Decoherence on the timescale of the excited state lifetime allows low energy (56 cm-1) oscillations on the signal intensity to be observed. In conjunction with an appropriate model, these oscillations provide clear and direct experimental evidence that the persistent coherences observed require strong vibronic mixing among excited states.
- Published
- 2015
- Full Text
- View/download PDF
26. Successful Pressing Sequences for a Bicolored Graph and Binary Matrices
- Author
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Cooper, Joshua and Davis, Jeffrey
- Subjects
Mathematics - Combinatorics ,05C50, 15B33, 92D15 - Abstract
We apply matrix theory over $\mathbb{F}_2$ to understand the nature of so-called "successful pressing sequences" of black-and-white vertex-colored graphs. These sequences arise in computational phylogenetics, where, by a celebrated result of Hannenhalli and Pevzner, the space of sortings-by-reversal of a signed permutation can be described by pressing sequences. In particular, we offer several alternative linear-algebraic and graph-theoretic characterizations of successful pressing sequences, describe the relation between such sequences, and provide bounds on the number of them. We also offer several open problems that arose as a result of the present work.
- Published
- 2015
27. Bifurcation in a thin liquid film flowing over a locally heated surface
- Author
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Katkar, Harshwardhan H. and Davis, Jeffrey M.
- Subjects
Physics - Fluid Dynamics - Abstract
We investigate the non-linear dynamics of a two-dimensional film flowing down a finite heater, for a non-volatile and a volatile liquid. An oscillatory instability is predicted beyond a critical value of Marangoni number using linear stability theory. Continuation along the Marangoni number using non-linear evolution equation is used to trace bifurcation diagram associated with the oscillatory instability. Hysteresis, a characteristic attribute of a sub-critical Hopf bifurcation, is observed in a critical parametric region. The bifurcation is universally observed for both, a non-volatile film and a volatile film., Comment: Revised manuscript published at http://dx.doi.org/10.1017/jfm.2013.245
- Published
- 2014
- Full Text
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28. Disentangling electronic and vibrational coherence in the Phycocyanin-645 light-harvesting complex
- Author
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Richards, Gethin H., Wilk, Krystyna E., Curmi, Paul M. G., and Davis, Jeffrey A.
- Subjects
Physics - Chemical Physics ,Physics - Biological Physics ,Physics - Optics - Abstract
Energy transfer between chromophores in photosynthesis proceeds with near unity quantum efficiency. Understanding the precise mechanisms of these processes is made difficult by the complexity of the electronic structure and interactions with different vibrational modes. Two-dimensional spectroscopy has helped resolve some of the ambiguities and identified quantum effects that may be important for highly efficient energy transfer. Many questions remain, however, including whether the coherences observed are electronic and/or vibrational in nature and what role they play. We utilise a two-colour four-wave mixing experiment with control of the wavelength and polarization to selectively excite specific coherence pathways. For the light-harvesting complex PC645, from cryptophyte algae, we reveal and identify specific contributions from both electronic and vibrational coherences and determine an excited state structure based on two strongly-coupled electronic states and two vibrational modes. Separation of the coherence pathways also uncovers the complex evolution of these coherences and the states involved.
- Published
- 2013
- Full Text
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29. Isolating quantum coherence with pathway-selective coherent multi-dimensional spectroscopy
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Tollerud, Jonathan O, Hall, Christopher R, and Davis, Jeffrey A
- Subjects
Physics - Optics ,Condensed Matter - Mesoscale and Nanoscale Physics ,Physics - Chemical Physics - Abstract
Coherent coupling between spatially separated systems has long been explored as a necessary requirement for quantum information and cryptography. Recent discoveries suggest such phenomena appear in a much wider range of processes, including light-harvesting in photosynthesis. These discoveries have been facilitated by developments in coherent multi-dimensional spectroscopy (CMDS) that allow interactions between different electronic states to be identified in crowded spectra. For complex systems, however, spectral broadening and multiple overlapping peaks limit the ability to separate, identify and properly analyse all contributions. Here we demonstrate how pathway-selective CMDS can overcome these limitations to reveal, isolate and allow detailed analysis of weak coherent coupling between spatially separated excitons localised to different semiconductor quantum wells. Selective excitation of the coherence pathways, by spectrally shaping the laser pulses, provides access to previously hidden details and enables quantitative analysis that can facilitate precise and detailed understanding of interactions in this and other complex systems.
- Published
- 2013
- Full Text
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30. Assessment of the Accounting and Joint Accounting/Computer Information Systems Programs.
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La Guardia Community Coll., Long Island City, NY., Appiah, John, Cernigliaro, James, Davis, Jeffrey, Gordon, Millicent, Richards, Yves, Santamaria, Fernando, Siegel, Annette, Lytle, Namy, and Wharton, Patrick
- Abstract
This document presents City University of New York LaGuardia Community College's Department of Accounting and Managerial Studies assessment of its accounting and joint accounting/computer information systems programs report, and includes the following items: (1) description of the mission and goals of the Department of Accounting and Managerial Studies; (2) descriptive analysis of the curriculum and delivery of instruction; (3) assessment of the curriculum and delivery of instruction, which is further subdivided into the following categories: national standards, program objectives, course evaluation, and joint accounting/computer information systems option; (4) computer laboratory services report; (5) interviews of employers; (6) surveys of students enrolled in accounting courses; (7) articulation report involving a review of the course in the accounting curriculum and their transferability; (8) grade distribution report for accounting courses; (9) special focus area that looks at department sponsored student activities, including a variety of accounting clubs; (10) summary of the department's human resources, facilities and equipment, fiscal resources, and college-wide services; (11) summary of recommendations and future goals; and (12) appendix of survey instruments (intermediate accounting survey, accounting graduate applicants survey, and content area survey) used in the study and verbatim responses to question on improvement of facilities. (VWC)
- Published
- 2000
31. Perceptions of Disability-Services Administrators about Assistive Technology at Postsecondary Institutions
- Author
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Davis, Jeffrey S.
- Abstract
The purpose of this study was to investigate the manifestation of assistive technology at postsecondary institutions from the perspectives of disability-services administrators, through their personal experiences. Exploration of assistive-technology-related experiences presents the potential to improve understanding of the operation of this vital student service that had not previously been investigated fully. The population for the purposive sampling was from postsecondary institutions in the southeastern United States. Through SurveyMonkey(TM), the online phenomenological study included collecting participants' responses to an open-ended questionnaire. Responses were analyzed using interpretative phenomenological analysis and resulted in the discovery of emerging themes and subsequently of superordinate themes that emerged from the voice of participants: access through the use of assistive technology, awareness of assistive technology, and leveling the playing field. Superordinate themes emerged from the collective voice of participants, researcher notations, and emerging themes interpreted from responses to the questionnaire. The knowledge gained about superordinate themes of assistive technology at postsecondary institutions from the perspective of disability-services administrators contributes to the existing body of knowledge in the area of assistive technology and postsecondary education and yields operational and educational benefits for administrators and the students they serve. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]
- Published
- 2012
32. Wanted: C4I Warriors. The Requirement for Marine Corps C2 Systems Planners
- Author
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Davis, Jeffrey P., primary, Gran, David G., primary, and Hall, Timothy J., primary
- Published
- 1993
- Full Text
- View/download PDF
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