Project description:Percolation, describing critical behaviors of phase transition in a geometrical context, prompts wide investigations in natural and social networks as a fundamental model. The introduction of quantum coherence and superposition brings percolation into quantum regime with more fascinating phenomena and unique features, which, however, has not been experimentally explored yet. Here we successfully map these large-scale porous structures into a photonic chip using femtosecond laser direct writing techniques and present an experimental demonstration of quantum transport in hexagonal percolation lattices, probed by coherent light. A quantum percolation threshold of 80% is observed in the prototyped laser-written lattices with up to 1,600 waveguides, which is significantly larger than the classical counterpart of 63%. We also investigate the spatial confinement by localization parameters and exhibit the transition from ballistic to diffusive propagation with the decrease of the occupation probability. Direct observation of quantum percolation may deepen the understanding of the relation among materials, quantum transport, geometric quenching, disorder and localization, and inspire applications for quantum technologies.
Project description:Near-zero-index (NZI) media have been theoretically identified as media where electromagnetic radiations behave like ideal electromagnetic fluids. Within NZI media, the electromagnetic power flow obeys equations similar to those of motion for the velocity field in an ideal fluid, so that optical turbulence is intrinsically inhibited. Here, we experimentally observe the electromagnetic power flow distribution of such an ideal electromagnetic fluid propagating within a cutoff waveguide by a semi-analytical reconstruction technique. This technique provides direct proof of the inhibition of electromagnetic vorticity at the NZI frequency, even in the presence of complex obstacles and topological changes in the waveguide. Phase uniformity and spatially-static field distributions, essential characteristics of NZI materials, are also observed. Measurement of the same structure outside the NZI frequency range reveals existence of vortices in the power flow, as expected for conventional optical systems. Therefore, our results provide an important step forward in the development of ideal electromagnetic fluids, and introduce a tool to explore the subwavelength behavior of NZI media including fully vectorial and phase information.
Project description:Natural light harvesting as well as optoelectronic and photovoltaic devices depend on efficient transport of energy following photoexcitation. Using common spectroscopic methods, however, it is challenging to discriminate one-exciton dynamics from multi-exciton interactions that arise when more than one excitation is present in the system. Here we introduce a coherent two-dimensional spectroscopic method that provides a signal only in case that the presence of one exciton influences the behavior of another one. Exemplarily, we monitor exciton diffusion by annihilation in a perylene bisimide-based J-aggregate. We determine quantitatively the exciton diffusion constant from exciton-exciton-interaction 2D spectra and reconstruct the annihilation-free dynamics for large pump powers. The latter enables for ultrafast spectroscopy at much higher intensities than conventionally possible and thus improves signal-to-noise ratios for multichromophore systems; the former recovers spatio-temporal dynamics for a broad range of phenomena in which exciton interactions are present.
Project description:Nanofluidic devices have channel dimensions which come to within one order of magnitude of the Debye length of common aqueous solutions. Conventionally, external driving is used to create concentration polarization of ions and biomolecules in nanofluidic devices. Here we show that long-range ionic strength gradients intrinsic to all nanofluidic devices, even at equilibrium, also drive a drift of macromolecules. To demonstrate the effect, we confine long DNA to straight nanochannels of constant, rectangular cross-section (100 × 100 nm2) which are connected to large microfluidic reservoirs. The motion of DNA is observed in absence of any driving. We find that at low ionic strengths, molecules in nanochannels migrate toward the nano-micro interface, while they are undergoing purely diffusive motion in high salt. Using numerical models, we demonstrate that the motion is consistent with the ionic strength gradient at the micro-nano interface even at equilibrium, and that the dominant cause of the drift is diffusophoresis.
Project description:The oxygen isotope compositions of carbonate and phosphatic fossils hold the key to understanding Earth-system evolution during the last 500 million years. Unfortunately, the validity and interpretation of this record remain unsettled. Our comprehensive compilation of Phanerozoic δ18O data for carbonate and phosphate fossils and microfossils (totaling 22,332 and 4615 analyses, respectively) shows rapid shifts best explained by temperature change. In calculating paleotemperatures, we apply a constant hydrosphere δ18O, correct seawater δ18O for ice volume and paleolatitude, and correct belemnite δ18O values for 18O enrichment. Similar paleotemperature trends for carbonates and phosphates confirm retention of original isotopic signatures. Average low-latitude (30° S-30° N) paleotemperatures for shallow environments decline from 42.0 ± 3.1 °C in the Early-to-Middle Ordovician to 35.6 ± 2.4 °C for the Late Ordovician through the Devonian, then fluctuate around 25.1 ± 3.5 °C from the Mississippian to today. The Early Triassic and Middle Cretaceous stand out as hothouse intervals. Correlations between atmospheric CO2 forcing and paleotemperature support CO2's role as a climate driver in the Paleozoic.
Project description:The characterization of self-assembling molecules presents significant experimental challenges, especially when associated with phase separation or precipitation. Transparent window infrared (IR) spectroscopy leverages site-specific probes that absorb in the "transparent window" region of the biomolecular IR spectrum. Carbon-deuterium (C-D) bonds are especially compelling transparent window probes since they are non-perturbative, can be readily introduced site selectively into peptides and proteins, and their stretch frequencies are sensitive to changes in the local molecular environment. Importantly, IR spectroscopy can be applied to a wide range of molecular samples regardless of solubility or physical state, making it an ideal technique for addressing the solubility challenges presented by self-assembling molecules. Here, we present the first continuous observation of transparent window probes following stopped-flow initiation. To demonstrate utility in a self-assembling system, we selected the MAX1 peptide hydrogel, a biocompatible material that has significant promise for use in drug delivery and medical applications. C-D labeled valine was synthetically introduced into five distinct positions of the twenty-residue MAX1 β-hairpin peptide. Consistent with current structural models, steady-state IR absorption frequencies and linewidths of C-D bonds at all labeled positions indicate that these side chains occupy a hydrophobic region of the hydrogel and that the motion of side chains located in the middle of the hairpin is more restricted than those located on the hairpin ends. Following a rapid change in ionic strength to initiate self-assembly, the peptide absorption spectra were monitored as function of time, allowing determination of site-specific time constants. We find that within the experimental resolution, MAX1 self-assembly occurs as a cooperative process. These studies suggest that stopped-flow transparent window FTIR can be extended to other time-resolved applications, such as protein folding and enzyme kinetics.
Project description:The interplay between magnetism and superconductivity can lead to unconventional proximity and Josephson effects. A related phenomenon that has recently attracted considerable attention is the superconducting diode effect, in which a nonreciprocal critical current emerges. Although superconducting diodes based on superconductor/ferromagnet (S/F) bilayers were demonstrated more than a decade ago, the precise underlying mechanism remains unclear. While not formally linked to this effect, the Fulde-Ferrell-Larkin-Ovchinikov (FFLO) state is a plausible mechanism due to the twofold rotational symmetry breaking caused by the finite center-of-mass-momentum of the Cooper pairs. Here, we directly observe asymmetric vortex dynamics that uncover the mechanism behind the superconducting vortex diode effect in Nb/EuS (S/F) bilayers. Based on our nanoscale SQUID-on-tip (SOT) microscope and supported by in-situ transport measurements, we propose a theoretical model that captures our key results. The key conclusion of our model is that screening currents induced by the stray fields from the F layer are responsible for the measured nonreciprocal critical current. Thus, we determine the origin of the vortex diode effect, which builds a foundation for new device concepts.
Project description:The defining feature of bacterial phase variation is a stochastic 'all-or-nothing' switching in gene expression. However, direct observations of these rare switching events have so far been lacking, obscuring possible correlations between switching events themselves, and between switching and other cellular events, such as division and DNA replication. We monitored the phase variation of type 1 fimbriae in individual Escherichia coli in real time and simultaneously tracked the chromosome replication process. We observed distinctive patterns of fim (fimbriae) expression in multiple genealogically related lineages. These patterns could be explained by a model that combines a single switching event with chromosomal fim replication, as well as the epigenetic inheritance of expressed fim protein and RNA, and their dilution by growth. Analysis of the moment of switching at sub-cell-cycle resolution revealed a correlation between fim switching and cell age, which challenges the traditional idea of phase variation as a random Poissonian phenomenon.
Project description:Folded proteins can exist in multiple conformational substates. Each substate reflects a local minimum on the free-energy landscape with a distinct structure. By using ultrafast 2D-IR vibrational echo chemical-exchange spectroscopy, conformational switching between two well defined substates of a myoglobin mutant is observed on the approximately 50-ps time scale. The conformational dynamics are directly measured through the growth of cross peaks in the 2D-IR spectra of CO bound to the heme active site. The conformational switching involves motion of the distal histidine/E helix that changes the location of the imidazole side group of the histidine. The exchange between substates changes the frequency of the CO, which is detected by the time dependence of the 2D-IR vibrational echo spectrum. These results demonstrate that interconversion between protein conformational substates can occur on very fast time scales. The implications for larger structural changes that occur on much longer time scales are discussed.
Project description:Fundamental concepts of the crystal formation suggest that the growth and decomposition are determined by simultaneous embedding and removal of the atoms. Apparently, by changing the crystal formation conditions one can switch the regimes from the growth to decomposition. To the best of our knowledge, so far this has been only postulated, but never observed at the atomic level. By means of in situ environmental transmission electron microscopy we monitored and examined the atomic layer transformation at the conditions of the crystal growth and its decomposition using CuO nanowires selected as a model object. The atomic layer growth/decomposition was studied by varying an O2 partial pressure. Three distinct regimes of the atomic layer evolution were experimentally observed: growth, transition and decomposition. The transition regime, at which atomic layer growth/decomposition switch takes place, is characterised by random nucleation of the atomic layers on the growing {111} surface. The decomposition starts on the side of the nanowire by removing the atomic layers without altering the overall crystal structure, which besides the fundamental importance offers new possibilities for the nanowire manipulation. Understanding of the crystal growth kinetics and nucleation at the atomic level is essential for the precise control of 1D crystal formation.