Publications
Phenotypic models of evolution and development: Geometry as destiny
Quantitative models of development that consider all relevant genes typically are difficult to fit to embryonic data alone and have many redundant parameters. Computational evolution supplies models of phenotype with relatively few variables and parameters that allows the patterning dynamics to be reduced to a geometrical picture for how the state of a cell moves. The clock and wavefront model, that defines the phenotype of somitogenesis, can be represented as a sequence of two discrete dynamical transitions (bifurcations).
X-ray imaging detectors
Fluctuation broadening in carbon nanotube resonators
We simulated the behavior of suspended carbon nanotube resonators over a broad range of temperatures to explore the physics of semiflexible polymers in underdamped environments. We find that thermal fluctuations induce strong coupling between resonance modes. This effect leads to spectral fluctuations that readily account for the experimentally observed quality factors Q ∼ 100 at 300 K. Using a mean-field approach to describe fluctuations, we analytically calculate Q and frequency shifts in tensioned and buckled carbon nanotubes and find excellent agreement with simulations.
Edge states for topological insulators in two dimensions and their Luttinger-like liquids
Topological insulators in three spatial dimensions are known to possess a precise bulk-boundary correspondence, in that there is a one-to-one correspondence between the five classes characterized by bulk topological invariants and Dirac Hamiltonians on the boundary with symmetry protected zero modes. This holographic characterization of topological insulators is studied in two dimensions. Dirac Hamiltonians on the one-dimensional edge are classified according to the discrete symmetries of time reversal, particle hole, and chirality, extending a previous classification in two dimensions.
Strong enhancement of light-matter interaction in graphene coupled to a photonic crystal nanocavity
We demonstrate a large enhancement in the interaction of light with graphene through coupling with localized modes in a photonic crystal nanocavity. Spectroscopic studies show that a single atomic layer of graphene reduces the cavity reflection by more than a factor of one hundred, while also sharply reducing the cavity quality factor. The strong interaction allows for cavity-enhanced Raman spectroscopy on subwavelength regions of a graphene sample. A coupled-mode theory model matches experimental observations and indicates significantly increased light absorption in the graphene layer.
Structural and kinetic effects on changes in the CO2 binding pocket of human carbonic anhydrase II
This work examines the effect of perturbing the position of bound CO 2 in the active site of human carbonic anhydrase II (HCA II) on catalysis. Variants of HCA II in which Val143 was replaced with hydrophobic residues Ile, Leu, and Ala were examined. The efficiency of catalysis in the hydration of CO2 for these variants was characterized by 18O exchange mass spectrometry, and their structures were determined by X-ray crystallography at 1.7-1.5 Å resolution.
A tunable low-energy photon source for high-resolution angle-resolved photoemission spectroscopy
We describe a tunable low-energy photon source consisting of a laser-driven xenon plasma lamp coupled to a Czerny-Turner monochromator. The combined tunability, brightness, and narrow spectral bandwidth make this light source useful in laboratory-based high-resolution photoemission spectroscopy experiments. The source supplies photons with energies up to ∼7 eV, delivering under typical conditions >1012 ph/s within a 10 meV spectral bandwidth, which is comparable to helium plasma lamps and many synchrotron beamlines.
Effect of cation sublattice ordering on structure and raman scattering of znGeN2
The semiconductor ZnGeN2 was grown by a vapor-liquid-solid mechanism. Ordering of the Zn-Ge sublattice with growth temperature and Zn partial pressure was investigated by powder X-ray diffraction and was found to be sensitive to the growth temperature and insensitive, over the range explored, to the Zn and NH3 partial pressures. The degree of disorder on the cation sublattice was observed to correlate with the suppression of predicted Raman peaks and the emergence of phonon density-of-states features. © 2013 Materials Research Society.
Electronic liquid crystal physics of underdoped cuprates
Recent observations of broken symmetries have partly demystified the pseudogap phase. Here we review evidence for long-range intra-unit-cell (IUC) nematic order and its unexpectedly strong coupling to the phase of the fluctuating stripes in the pseudogap states of underdoped Bi 2Sr 2CaCu 2O 8+δ.
Generalized rotational susceptibility studies of solid 4He
Using a novel SQUID-based torsional oscillator (TO) technique to achieve increased sensitivity and dynamic range, we studied TO's containing solid 4He. Below ∼250 mK, the TO resonance frequency f increases and its dissipation D passes through a maximum as first reported by Kim and Chan. To achieve unbiased analysis of such 4He rotational dynamics, we implemented a new approach based upon the generalized rotational susceptibility χ 4He -1(ω, T ).