Quantum Matter Workshop

While sharply-quantized topological features are conventionally associated with gapped phases of matter, there are a growing number of examples of gapless systems with topologically protected edge states. A particularly striking set of examples are "intrinsically gapless" symmetry-protected topological states (igSPTs), which host topological surface states that could not arise in a gapped system with the same symmetries. Examples include familiar non-interacting Weyl semimetals with Fermi arc surface states, as well as more exotic examples like deconfined quantum critical points with topological edge states. In this talk, I will discuss recent progress in formally understanding the bulk-boundary correspondence of strongly-interacting igSPTs using tools from group cohomology. In these examples, the gapless-ness of the bulk and presence of topological surface states can be understood in a unified way due to the presence of an emergent anomaly. Our formalism allows construction of lattice-models with such emergent anomalies whose topological properties can be deduced exactly.

"Magnetic materials with 4d or 5d transition metals have drawn much attention for their unique magnetic properties arising from J_eff=1/2 magnetic states. Among them, a honeycomb lattice material with unusual bond-dependent interactions called Kitaev interactions is of particular interest due to the potential for realizing the Kitaev quantum spin liquid state. Although much progress has been made in understanding magnetic and spin-orbit excitations in Kitaev materials, such as Na2IrO3 and alpha-RuCl3, using resonant inelastic X-ray scattering (RIXS), there are still many unanswered questions regarding the nature of electronic excitations in these materials. Of particular interest is the sharp peak observed around 0.4 eV in the RIXS spectrum of Na2IrO3, the exact nature of which remains controversial. In this context, it is interesting to note that a similar lower energy “excitonic” peak was observed in our recent RIXS investigation of alpha-RuCl3. Given that the electronic parameters in alpha-RuCl3 are probably very different from those in Na2IrO3 (alpha-RuCl3 has a large bandgap of ~1eV, well above any SO excitation energy scale), the observed similarity is surprising. The RIXS spectra from these two compounds as well as other Kitaev materials will be compared and the origin of common spectral features will be discussed. "

Just seven years after their first detection, gravitational waves (GWs) have revealed the first glimpses of a previously hidden dark Universe. Using the GW signature of distant compact-object collisions, we have discovered a new population of stellar remnants and unlocked new tests of general relativity, cosmology, and ultra-dense matter. Materials with low mechanical loss (and strong constraints on other properties, e.g. reflectivity) are integral to the design and success of the GW detectors making these groundbreaking measurements. I'll summarize recent results from LIGO-Virgo and their wide-reaching implications, and discuss quantum materials advances required to enable future ground-based gravitational wave detectors, including Cosmic Explorer, to sense black hole collisions all the way back to the dawn of cosmic time.