160 research outputs found
Breeding Bird Populations in Iowa, 1968-1980
A comparison of Breeding Bird Survey (BBS) data in Iowa between 1968-1970 and 1978-1980 suggests that 19 species have increased in numbers and that 18 have declined over that period. Most species showing increases are associated with agricultural and grassland habitats. Most species showing decreases are associated with agricultural and edge-oldfield habitats. A variety of factors is probably responsible for these declines. BBS results are fairly consistent with the National Audubon Society\u27s Blue List - birds thought to be declining - but show little correlation with that group\u27s list of species of special concern. The BBS detected at least 68% of the birds known to breed in Iowa, although not all of those were detected each year. About the same number of species were reported from each of five regions in the years considered, although the same species were not seen in all areas. Still, 73 species reported in all five regions dominate the state\u27s avifauna. Some biases related to the species\u27 detectability and observer competence are evident in the data. Overall, BBS data do seem to be meeting their goal of providing quantitative information useful in detecting long-term changes in bird populations
Imaging stress and magnetism at high pressures using a nanoscale quantum sensor
Pressure alters the physical, chemical and electronic properties of matter.
The development of the diamond anvil cell (DAC) enables tabletop experiments to
investigate a diverse landscape of high-pressure phenomena ranging from the
properties of planetary interiors to transitions between quantum mechanical
phases. In this work, we introduce and utilize a novel nanoscale sensing
platform, which integrates nitrogen-vacancy (NV) color centers directly into
the culet (tip) of diamond anvils. We demonstrate the versatility of this
platform by performing diffraction-limited imaging (~600 nm) of both stress
fields and magnetism, up to pressures ~30 GPa and for temperatures ranging from
25-340 K. For the former, we quantify all six (normal and shear) stress
components with accuracy GPa, offering unique new capabilities for
characterizing the strength and effective viscosity of solids and fluids under
pressure. For the latter, we demonstrate vector magnetic field imaging with
dipole accuracy emu, enabling us to measure the pressure-driven
phase transition in iron as well as the complex
pressure-temperature phase diagram of gadolinium. In addition to DC vector
magnetometry, we highlight a complementary NV-sensing modality using T1 noise
spectroscopy; crucially, this demonstrates our ability to characterize phase
transitions even in the absence of static magnetic signatures. By integrating
an atomic-scale sensor directly into DACs, our platform enables the in situ
imaging of elastic, electric and magnetic phenomena at high pressures.Comment: 18 + 50 pages, 4 + 19 figure
Direct measurement of discrete valley and orbital quantum numbers in bilayer graphene
The high magnetic field electronic structure of bilayer graphene is enhanced by the spin, valley isospin, and an accidental orbital degeneracy, leading to a complex phase diagram of broken symmetry states. Here, we present a technique for measuring the layer-resolved charge density, from which we directly determine the valley and orbital polarization within the zero energy Landau level. Layer polarization evolves in discrete steps across 32 electric field-tuned phase transitions between states of different valley, spin, and orbital order, including previously unobserved orbitally polarized states stabilized by skew interlayer hopping. We fit our data to a model that captures both single-particle and interaction-induced anisotropies, providing a complete picture of this correlated electron system. The resulting roadmap to symmetry breaking paves the way for deterministic engineering of fractional quantum Hall states, while our layer-resolved technique is readily extendable to other two-dimensional materials where layer polarization maps to the valley or spin quantum numbers.United States. Department of Energy. Office of Basic Energy Sciences (Contract FG02-08ER46514)Gordon and Betty Moore Foundation (Grant GBMF2931
Spin-orbit driven band inversion in bilayer graphene by van der Waals proximity effect
Spin orbit coupling (SOC) is the key to realizing time-reversal invariant
topological phases of matter. Famously, SOC was predicted by Kane and Mele to
stabilize a quantum spin Hall insulator; however, the weak intrinsic SOC in
monolayer graphene has precluded experimental observation. Here, we exploit a
layer-selective proximity effect---achieved via van der Waals contact to a
semiconducting transition metal dichalcogenide--to engineer Kane-Mele SOC in
ultra-clean \textit{bilayer} graphene. Using high-resolution capacitance
measurements to probe the bulk electronic compressibility, we find that SOC
leads to the formation of a distinct incompressible, gapped phase at charge
neutrality. The experimental data agrees quantitatively with a simple
theoretical model in which the new phase results from SOC-driven band
inversion. In contrast to Kane-Mele SOC in monolayer graphene, the inverted
phase is not expected to be a time reversal invariant topological insulator,
despite being separated from conventional band insulators by electric field
tuned phase transitions where crystal symmetry mandates that the bulk gap must
close. Electrical transport measurements, conspicuously, reveal that the
inverted phase has a conductivity , which is suppressed by
exceptionally small in-plane magnetic fields. The high conductivity and
anomalous magnetoresistance are consistent with theoretical models that predict
helical edge states within the inversted phase, that are protected from
backscattering by an emergent spin symmetry that remains robust even for large
Rashba SOC. Our results pave the way for proximity engineering of strong
topological insulators as well as correlated quantum phases in the strong
spin-orbit regime in graphene heterostructures.Comment: 7 pages of main text + 13 pages supplementary material and figures.
More information available at http://www.afylab.com/publications
- …