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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.
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Selected Publications
Double scheelite-type oxides of the form A+A′3+(WO4)2 are promising hosts for rare-earth-doped solid-state lasing materials. Using synchrotron X-ray and neutron powder diffraction with total scattering and pair distribution function analysis, the interplay between long-range symmetry and short-range distortions across the AA′(WO4)2 (A+ = Li, Na, K; A′3+ = La, Lu, Bi) series has been resolved. The Na- and La-containing oxides display an average tetragonal I41/a scheelite-type structure with disorder across the two A-site cations. Pronounced local-scale deviations arise from differences in A-site cation size, polarizability, and the presence of 6s2 lone pair activity, with symmetry-lowering required to model the bonding requirements of each (A/A′)O8 polyhedron. These requirements are better captured by the partial cation ordering and displacements allowed by a monoclinic I2 small-box model. When A+ = Li or K, the Lu-containing analogues order across the A site and display long-range symmetry lowering to nonscheelite monoclinic structures, consistent with the less polarizable LuO8 polyhedra. Reverse Monte Carlo modeling reveals an interplay between cation size and the polarizability of A-site cations, explaining and establishing structural design principles for tuning local environments and optimizing emission bandwidths in scheelite-based solid-state laser hosts.
Magnetostructural coupling plays a crucial role in numerous types of quantum materials and functional magnetic materials. Here, we investigate the magnetostructurally active ferromagnet MnSb, whose derivative compounds Mn1+xSb are strong candidates for magnetocaloric applications. We reveal an unusually large spontaneous magnetostriction effect in MnSb, resulting in a relative unit cell expansion of up to 1% and an extended temperature region of negative thermal expansion of the crystallographic axis. This magnetovolume effect is exceptional not only because of its magnitude, but also because the volume increase begins in the paramagnetic phase and exhibits unconventional linear scaling with the short-range ferromagnetic correlations as the temperature is lowered, before switching to quadratic scaling with the long-range-ordered magnetic moment below the transition. We explain this exceptional behavior through unusual trilinear couplings among the lattice strain and domains of distinct magnetic order parameters. Heat capacity measurements also provide new thermodynamic information about MnSb, including determination of the Debye and Einstein temperatures to be 164 and 296 K, respectively. These findings greatly expand our understanding of MnSb and provide insights into the fundamental materials physics of magnetostructural coupling in the presence of short-range magnetic correlations, with potential for broad applicability.
Non-topological solitons, such as Q-balls, may contribute to the cosmological dark matter. The formation and evolution of Q-balls in the early universe requires an understanding of solitons with nonzero angular momentum. We derive (rather than assume) the schematic form of the scalar field configurations that produce rotating Q-balls, which produce their well known quantized angular momentum. This analysis leads to additional insight into the properties of these rotating solitons, including a method for computing their characteristic angular velocity. By considering rotating solitons in two spatial dimensions, we investigate these attributes concretely. We develop analytical approximations for the solitons and their defining quantities. We show that they agree with numerical results and exhibit the general properties of rotating solitons.
We present new spectroscopic observations of the inner circumgalactic medium (CGM) of NGC 891 taken with the Mid-Infrared Imager/Medium Resolution Spectroscopy instrument on board JWST, in four positions: two near the bulge and two at galactocentric radii (r) of ∼1.5, 4.7 kpc. Each pair of pointings has one position along the minor axis (h) at ∼0.5 kpc and one at ∼1 kpc away from the mid-plane. We analysed 1D spectra and 3D cubes using the dust emission model PAHFIT to extract properties of typical mid-IR features. These spectra reveal that the earlier reported mid-IR emission out to 4 kpc is dominated by the emission of polycyclic aromatic hydrocarbons (PAHs), and not hot dust continuum, thus providing direct evidence of the survival of PAHs in the inner CGM of NGC 891. Comparing PAH band ratios with other environments (Orion, M51), it is obvious that the 11.2 μm PAH feature – and not the usual 7.7 μm – dominates in NGC 891, which seems to imply the presence of more neutral, large PAHs in the CGM. Overall, PAH-to-continuum ratios show little variations with scale-height and radius in NGC 891, which suggests little PAH processing. However, we do see a decrease in the PAH feature strengths with the [Ne III]/[Ne II] ratio, which points to elevated dust processing with increased radiation field hardness. We also confirm a tight correlation between H2 and PAH features, which suggests that the two tracers must be co-spatial, and hence implies that PAH emission predominantly arises from cool dense parts of cloudlets entrained in galactic outflows. Finally, we report the clear detection of a previously unidentified PAH feature at 16.72 μm.
Only three binaries have been identified among the Centaur population. Because their perihelia are significantly closer than those of other trans-Neptunian binaries (TNBs), these systems allow a detailed look at tight binaries in the broader trans-Neptunian object population and provide critical insight into the disruption of binaries as they enter the Centaur population. Using recent and archival Hubble Space Telescope observations, along with Keck data, we present a spin–orbit study of Typhon–Echidna. We find that the binary’s mutual orbit is inconsistent with a Keplerian orbit; more detailed non-Keplerian fits show that the mutual orbit is rapidly precessing. We measure Typhon’s dynamical oblateness, J2, at ∼10σ confidence and find that Typhon’s rotation pole is ≳20° misaligned with the binary’s mutual orbit. Assuming Typhon has a triaxial shape, our results, combined with rotational light curves and thermal measurements from the literature, suggest ellipsoidal semiaxes of km, km, and km. We further investigate the observational consequences of the complex spin–orbit dynamics, including light-curve alteration by axial precession of Typhon and substantial changes to the system’s mutual event season. Based on the system’s dynamically excited state, we suggest a recent encounter with a giant planet may have substantially altered the system, potentially consistent with a binary in an early stage of disruption. This hypothesis can be tested with resolved photometric observations of the system. Our investigation highlights how non-Keplerian dynamics enhance our understanding of TNB systems and motivates ongoing observations of TNBs with astrometry, photometry, and stellar occultations.
