News and Events
The properties of a material are often thought to be determined primarily by the elements from which it is made. But what happens when the geometry of a crystal lattice itself becomes a source of new quantum phenomena? In this talk, I will explore this question through the physics of kagome metals—materials whose atoms form a network of corner-sharing triangles. The kagome lattice provides a remarkable example of how geometry can reshape the electronic landscape of a metal. Quantum interference between electronic pathways produces nearly flat electronic bands, while the same lattice can host Dirac-like band crossings and van Hove singularities, where the density of electronic states becomes strongly enhanced. These features can dramatically amplify the effects of interactions between electrons, creating an unusually rich environment for collective quantum phenomena. Kagome materials have consequently emerged as a platform for studying charge-density waves, magnetism, unconventional superconductivity, nematicity, anomalous Hall responses, and other forms of intertwined and potentially topological order. I will discuss how these ideas emerge from the electronic structure and how they manifest experimentally in real materials. In particular, I will highlight recent work from my group using the experimental technique of angle-resolved photoemission spectroscopy (ARPES) to directly visualize the electronic states of several kagome metals and to follow their evolution across the various phase transitions. These experiments reveal an intimate connection between the underlying band structure and the collective behavior of the electrons, while also highlighting the ways in which real materials can depart from simple theoretical pictures.
| Temp: | 82 °F | N2 Boiling: | 75.9 K |
| Humidity: | 21% | H2O Boiling: | 368.4 K |
| Pressure: | 85 kPa | Sunrise: | 6:57 AM |
| Wind: | 2 m/s | Sunset: | 7:55 PM |
| Precip: | 0 mm | Sunlight: | 0 W/m² |
Selected Publications
We describe efforts to develop broadband mirror coatings with high performance that will extend from the far-ultraviolet (FUV) to infrared wavelengths. Our team at the Goddard Space Flight Center has developed a reactive physical vapor deposition (rPVD) process that combines a fluorination with a XeF2 gas (which grants a thin AlF3 layer) in between the Al and the metal-fluoride protection layer (either LiF or MgF2) that are done with the conventional PVD process. This recently developed rPVD process produces protected Al mirrors coatings with an improved average FUV reflectance between 10% and 15% higher (when compared with conventionally prepared samples). We have termed these coatings as XeLiF when the dielectric overcoat is LiF or XeMgF2 when the dielectric overcoat is MgF2. The XeLiF-coated Al mirrors meet current goals for advanced broadband mirrors in the FUV (R>70% at 103 nm and R>80 above 110 nm), whereas the XeMgF2 provides R>80% above 115 nm. The IR/Vis/UV reflectance for either XeLiF or XeMgF2 mirrors is similar to the theoretical reflectance of bare aluminum at wavelengths >200 nm. In addition, long-term lifetime testings of XeLiF mirrors indicate the rPVD process produces more environmentally stable coatings, where a XeLiF sample showed a degradation in the average FUV reflectance of around 1% to 2% when stored in a relative humidity of 40% over a period of 3.5 years. These results are a remarkable improvement when compared with conventionally prepared Al+LiF samples that would degrade their FUV reflectance in a matter of weeks or months when exposed to those kinds of relative humidity levels. Surface topographies on several XeLiF samples with varying Al and LiF thicknesses have been measured with an atomic force microscope (AFM). The root mean square (RMS) roughness (σ) values derived from these AFM results have ranged between 0.6 and 0.9 nm. For comparison, samples without the Xe process start off by having an RMS roughness that is 30% larger than samples treated with the XeF2 gas. We have also determined that these roughness values are showing a slight increase, ranging between 0.9 and 1.0 nm, when samples are exposed to room temperature and relative humidity as high as 50% over one week. Both of these key performance parameters (environmental stability in reflectance and smoothness of ≤1 nm) are key considerations for using the XeLiF coating in the primary and secondary mirrors of the Habitable Worlds Observatory (HWO). We also show evidence that the rPVD coating process is compatible with deposition on Si-based gratings. It is known that XeF2 vapor is a strong Si etchant, thus the demonstration that the native SiO2 layer on Si test samples is sufficient to protect the groove profile of E-beam-ruled Si gratings from degradation is an important and significant finding.
We present ground-based multiband light curves of the AGN Mrk 509, NGC 4151, and NGC 4593 obtained contemporaneously with Swift monitoring. We measure cross-correlation lags relative to Swift UVW2 (1928 Å) and test the standard prediction for disc reprocessing, which assumes a geometrically thin optically thick accretion disc where continuum interband delays follow the relation . For Mrk 509 the 273-d Swift campaign gives well-defined lags that increase with wavelength as , steeper than the thin-disc prediction, and the optical lags are a factor of longer than expected for a simple disc-reprocessing model. This ‘disc-size discrepancy’ as well as excess lags in the u and r bands (which include the Balmer continuum and H , respectively) suggest a mix of short lags from the disc and longer lags from nebular continuum originating in the broad-line region. The shorter Swift campaigns, 69 d on NGC 4151 and 22 d on NGC 4593, yield less well-defined shorter lags d. The NGC 4593 lags are consistent with but with uncertainties too large for a strong test. For NGC 4151 the Swift lags match , with a small U-band excess, but the ground-based lags in the r, i, and z bands are significantly shorter than the B and g lags, and also shorter than expected from the thin-disc prediction. The interpretation of this unusual lag spectrum is unclear. Overall these results indicate significant diversity in the relation across the optical/UV/NIR, which differs from the more homogeneous behaviour seen in the Swift bands.
Computational enzyme design remains a powerful yet imperfect tool for optimizing biocatalysts, especially when targeting non-natural substrates. Using design tools we investigated Pseudomonas aeruginosa LipA, a lipase with a flexible lid domain crucial for substrate binding and turnover, aiming to enhance its hydrolysis of the industrially relevant substrate Roche ester. We generated an initial set of single-point mutations based on structural proximity to the active site and evaluated their effects using a computational pipeline integrating molecular dynamics (MD) simulations, density functional theory (DFT) calculations, and ensemble-based energy scoring. While we identified several active variants, attempts to rank them by activity using structural features, such as hydrogen bond formation or residue flexibility, failed. Deep learning models, applied post hoc for structural analysis via AlphaFold3, produced nearly identical active site geometries across variants, irrespective of activity. Reaction pathway analysis revealed energy barriers varying by 5–15 kcal/mol depending on substrate conformation, with the nucleophile addition step consistently rate-limiting. However, these small energetic shifts, likely critical for incremental activity changes, were indistinguishable by current computational or deep learning methods. Our results highlight the limitations of existing approaches in resolving subtle functional differences and underscore the need for improved benchmarks, reactive force fields, and more sensitive ranking metrics. Advancing these areas will be essential for designing enzymes with gradual, evolution-like activity improvements and for bridging the gap between structural prediction and catalytic function.
We provide experimental evidence for the absence of a magnetic moment in bulk RuO2, a candidate altermagnetic material, by using a combination of Mössbauer spectroscopy, nuclear forward scattering, inelastic X-ray and neutron scattering, and density functional theory calculations. Using complementary Mössbauer and nuclear forward scattering, we determine the Ru magnetic hyperfine splitting to be negligible. Inelastic X-ray and neutron scattering-derived lattice dynamics of RuO2 are compared to density functional theory calculations of varying flavors. Comparisons among theory with experiments indicate that electronic correlations, rather than magnetic order, are key in describing the lattice dynamics.
The glockenspiel is a bright, resonant percussion instrument with a series of simple bars mounted next to each other in a frame. Its acoustic radiation remains underexplored, particularly in its full instrument configuration. This study investigates the acoustic radiation and vibrational behavior of a glockenspiel bar in different mounting conditions. Directivity measurements and the scanning laser Doppler vibrometer were used to compare a single bar in free-free, baffled, and full-instrument configurations. The results show that the mounting significantly alters radiation patterns of the bar, particularly at higher modes. Torsional modes exhibited greater deviation from free-free predictions than bending modes, especially in the full-instrument case. The findings highlight the importance of considering frame and structural interactions in modeling glockenspiel vibration and radiation.