Browsing by Author "Kim, Y."

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    Effects of electron-electron interactions on the electronic Raman scattering of graphite in high magnetic fields
    (American Physical Society, 2014) Ma, Y.; Kim, Y.; Kalugin, N.G.; Lombardo, A.; Ferrari, A.C.; Kono, J.; Imambekov, A.; Smirnov, D.
    We report the observation of strongly temperature (T)-dependent spectral lines in electronic Raman-scattering spectra of graphite in a high magnetic field up to 45 T applied along the c axis. The magnetic field quantizes the in-plane motion, while the out-of-plane motion remains free, effectively reducing the system dimension from 3 to 1. Optically created electron-hole pairs interact with, or shake up, the one-dimensional Fermi sea in the lowest Landau subbands. Based on the Tomonaga-Luttinger liquid theory, we show that interaction effects modify the spectral line shape from (ω−Δ)−1/2 to (ω−Δ)2α−1/2 at T = 0. At finite T, we predict a thermal broadening factor that increases linearly with T. Our model reproduces the observed T-dependent line shape, determining the electron-electron interaction parameter α to be ∼0.05 at 40 T.
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    Measurement of Filling-Factor-Dependent Magnetophonon Resonances in Graphene Using Raman Spectroscopy
    (American Physical Society, 2013-05-29) Kim, Y.; Poumirol, J.M.; Lombardo, A.; Kalugin, N.G.; Georgiou, T.; Kim, Y.J.; Novoselov, K.S.; Ferrari, A.C.; Kono, J.; Kashuba, O.; Fal’ko, V.I.; Smirnov, D.
    We perform polarization-resolved Raman spectroscopy on graphene in magnetic fields up to 45 T. This reveals a filling-factor-dependent, multicomponent anticrossing structure of the Raman G peak, resulting from magnetophonon resonances between magnetoexcitons and E2g phonons. This is explained with a model of Raman scattering taking into account the effects of spatially inhomogeneous carrier densities and strain. Random fluctuations of strain-induced pseudomagnetic fields lead to increased scattering intensity inside the anticrossing gap, consistent with the experiments.
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    MeV x-ray production from a petawatt laser in the regime of a relativistically transparent preplasma, with applications to radiography
    (AIP Publishing, 2024) Strehlow, J.; Yin, L.; Wong, C.-S.; Luedtke, S. V.; Palaniyappan, S.; Stark, D. J.; Huang, C.-K.; Bogale, A.; Cage, B.; Coffman, T. A.; Figueroa Bengoa, A.; Fitzgarrald, R.; Mix, L. T.; Nedbailo, R.; Rusby, D. R.; Schmidt, J. L.; Twardowski, J.; Van Pelt, A.; Day, T. H.; Jones, B. J.; Bruce, S. A.; Helal, A.; Spinks, M. M.; Quevedo, H. J.; Beg, F. N.; Chowdhury, E. A.; Ditmire, T.; Liang, E.; Thomas, A. G. R.; Fernández, J. C.; Gautier, D. C.; Hunter, J.; Kim, Y.; Meaney, K. D.; Albright, B. J.
    Bright sources of mega-electron volt (MeV) x-rays have many unique applications, including nuclear physics, radiation oncology, and imaging high areal density systems. High intensity lasers (> 1018 W cm−2) incident on mm-thick metal targets can deliver MeV x-rays via the bremsstrahlung process, providing sources with ultrashort duration ( ∼ps) and small source size (∼100  μm). Here, we report on a reproducible regime of laser-driven MeV x-ray sources, where the x-ray dose can be further increased by 60% by coating the metal target with micrometers of plastic. High fidelity numerical simulations indicate that the interaction is a result of relativistic transparency in the preplasma. Though relativistic transparency is present in both cases, the greater sound speed and smaller ion inertia of the plastic target allow the laser to more deeply penetrate and couple more efficiently to electrons. Radiography with this system demonstrates a resolving power < 300  μm, important for imaging applications.