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Yifan Dong
Please join us for a colloquium titled “Unraveling Ultrafast Charge Transfer Dynamics in Organic Solar Cells” at 12:00 PM in C215 ESC.
Thumbnail of NGC 7714: Starburst after Galaxy Collision
Is this galaxy jumping through a giant ring of stars? Probably not. Although the precise dynamics behind the featured image is yet unclear, what is clear is that the pictured galaxy, NGC 7714, has been stretched and distorted by a recent collision with a neighboring galaxy. This smaller neighbor, NGC 7715, situated off to the left of the frame, is thought to have charged right through NGC 7714. Observations indicate that the golden ring pictured is composed of millions of older Sun-like stars that are likely co-moving with the interior bluer stars. In contrast, the bright center of NGC 7714 appears to be undergoing a burst of new star formation. The featured image was captured by the Hubble Space Telescope. NGC 7714 is located about 130 million light years away toward the constellation of the Two Fish (Pisces). The interactions between these galaxies likely started about 150 million years ago and should continue for several hundred million years more, after which a single central galaxy may result.
Mount Timpanogos with sky above
Check current conditions and historical weather data at the ESC.
Image for Reveling in Uncertainty
Despite the inherent time constraints of engaging undergraduate and graduate students in research, Scott Bergeson enjoys teaching this “seek and find” principle to his students, a principle that has become his philosophy for life.
Image for Steve Summers' Insights for Students
Alumni Steve Summers answers interview questions for current students
Image for Wesley Morgan Doubles AP Physics Enrollment
Y Magazine recognizes finalist for the 2023 National Science Foundation’s Presidential Award of Excellence in Mathematics and Science Teaching

Selected Publications

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BYU Authors: Kameron R. Hansen, Blake Romrell, C. Emma McClure, Michele Eggleston, and John S. Colton, published in J. Phys. Chem. C

Alloying mixed ratios of elements into the halide perovskite (HP) structure has proven to be an effective method of tuning these materials’ structural and electronic properties for photovoltaic and other optoelectronic applications. However, the standard spectroscopies used to characterize HP alloys such as absorption and photoluminescence are limited in their ability to detect disorder and phase segregation within the structure. Here, we characterize these properties in 2D HP alloys to a greater degree by using electroabsorption spectroscopy to study thin films with mixed-metal (Pb–Sn) and mixed-halide (Br–Cl and Br–I) compositions. The large spectral separation of band-edge states in 2D HPs allow us to detect a coexistence of elemental-rich domains within the Pb–Sn and Br–I alloys. Meanwhile, we find that the Br–Cl alloys exhibit sharper spectral features and a more uniform electroabsorption response indicative of an ordered structure, albeit still not as ordered as their pure Br and Cl counterparts. The band gap energies of the Br–Cl series (PEA2PbBrxCl4 – x) can be continuously tuned between 3.425 and 4.13 eV via the Br:Cl ratio, while the exciton binding energies can be tuned from 349 to 487 meV.

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BYU Authors: Grant W. Hart and Kent L. Gee, published in Proc. Meet. Acoust.

Historically the radiation efficiency of a rocket plume has been assumed to be 0.5%, but recent measurements have thrown this into some doubt. Determining the sound power level of a rocket is the first step in characterizing its radiation efficiency. Because the sound radiation from a launch vehicle is anisotropic, well-calibrated ground measurement stations are used, along with trajectory data, to obtain sound power. Historically, the effect of ground reflections appears largely to have been neglected in the literature, despite the potential to inflate overall power levels (OAPWL) and therefore radiation efficiency. This study investigates the likely effect of a finite-impedance ground on spectra, overall sound pressure levels (OASPL), and power levels. A single-parameter ground reflection model is used to obtain an estimate for change in OASPL for a model spectrum based on measured space vehicle launches. When the correction is applied to the OAPWL it produces in a nearly 3dB level reduction, therefore reducing the radiation efficiency by a factor of two. This indicates the probability that the radiation efficiency assumed for rockets in the past is too high.

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BYU Authors: Joseph E. Avila, Samuel D. Bellows, Timothy W. Leishman, and Kent L. Gee, published in Proc. Meet. Acoust.

The directivity function of a played musical instrument describes the angular dependence of its acoustic radiation and diffraction about the instrument, musician, and musician’s chair. In this study, high angular resolution directivity data were acquired in an anechoic chamber of a muted trumpet being played by a seated musician. The chair height and horizontal displacement ensured that the geometric center of the instrument’s radiating region fell at the circular center of a computer-controlled semi-circular array of 36 microphones positioned at 5-degree polar-angle increments. Azimuthal rotations progressed in 5-degree increments, such that the measurements involved 2,521 unique positions over a sphere. Additional measurements at a position within the rotating reference frame facilitated post-processing. The musician played chromatic scales at each rotation position, and this process was repeated for straight, cup, and wow wow mutes in order to draw comparisons in the directivity patterns of each mute to the unmuted trumpet. Radiation behind the musician increased as a result of the mute, and mute-dependent changes to the directivity patterns primarily occurred above 1 kHz.

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BYU Authors: Michael B. Muhlestein and Kent L. Gee, published in J. Acoust. Soc. Am.

The nonlinear evolution of high-amplitude broadband noise is important to the psychoacoustic perception, usually annoyance, of high-speed jet noise. One method to characterize the nonlinear evolution of such noise is to consider a characteristic nonlinear waveform distortion length for the signal. A common length scale for this analysis is the shock formation distance of an initially sinusoidal signal. However, application of this length scale to broadband noise, even with the amplitude and source frequency replaced with characteristic values, may lead to underestimates of the overall nonlinear waveform distortion of the noise as indicated by the skewness of the time derivative of the acoustic pressure (or derivative skewness). This paper provides an alternative length scale derived directly from the evolution of the derivative skewness of Gaussian noise that may be more appropriate when analyzing the nonlinear evolution of broadband noise signals. This Gaussian-based length scale is shown to be a useful metric for its relative consistency and its physical interpretation. Various analytical predictions of the evolution of the derivative skewness for an ensemble of numerical simulations of noise propagation are used to highlight various aspects of this new length scale definition.

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BYU Authors: Benjamin D. Boizelle and Michael D. Joner, published in Astrophys. J.

The AGN STORM 2 Collaboration targeted the Seyfert 1 galaxy Mrk 817 for a year-long multiwavelength, coordinated reverberation mapping campaign including Hubble Space Telescope, Swift, XMM-Newton, NICER, and ground-based observatories. Early observations with NICER and XMM revealed an X-ray state 10 times fainter than historical observations, consistent with the presence of a new dust-free, ionized obscurer. The following analysis of NICER spectra attributes variability in the observed X-ray flux to changes in both the column density of the obscurer by at least one order of magnitude (N H ranges from  to ) and the intrinsic continuum brightness (the unobscured flux ranges from 10−11.8 to 10−10.5 erg s−1  cm−2). While the X-ray flux generally remains in a faint state, there is one large flare during which Mrk 817 returns to its historical mean flux. The obscuring gas is still present at lower column density during the flare, but it also becomes highly ionized, increasing its transparency. Correlation between the column density of the X-ray obscurer and the strength of UV broad absorption lines suggests that the X-ray and UV continua are both affected by the same obscuration, consistent with a clumpy disk wind launched from the inner broad-line region.

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BYU Authors: Michael C. Mortenson, Tracianne B. Neilsen, and Mark K. Transtrum, published in J. Theor. Comput. Acoust.

Sensitivity analysis is a powerful tool for analyzing multi-parameter models. For example, the Fisher information matrix (FIM) and the Cramer-Rao bound (CRB) involve derivatives of a forward model with respect to parameters. However, these derivatives are difficult to estimate in ocean acoustic models. This work presents a frequency-agnostic methodology for accurately estimating numerical derivatives using physics-based parameter preconditioning and Richardson extrapolation. The methodology is validated on a case study of transmission loss in the 50-400Hz band from a range-independent normal mode model for parameters of the sediment. Results demonstrate the utility of this methodology for obtaining Cramer-Rao bound (CRB) related to both model sensitivities and parameter uncertainties, which reveal parameter correlation in the model. This methodology is a general tool that can inform model selection and experimental design for inverse problems in different applications.