14 research outputs found
Avoided quasiparticle decay and enhanced excitation continuum in the spin-1/2 near-Heisenberg triangular antiferromagnet Ba3CoSb2O9
We explore the magnetic excitations of the spin-1/2 triangular
antiferromagnet Ba3CoSb2O9 in its 120 degree ordered phase using single-crystal
high-resolution inelastic neutron scattering. Sharp magnons with no decay are
observed throughout reciprocal space, with a strongly renormalized dispersion
and multiple soft modes compared to linear spin wave theory. We propose an
empirical parametrization that can quantitatively capture the complete
dispersions in the three-dimensional Brillouin zone and explicitly show that
the dispersion renormalizations have the direct consequence that one to two
magnon decays are avoided throughout reciprocal space, whereas such decays
would be allowed for the unrenormalized dispersions. At higher energies, we
observe a very strong continuum of excitations with highly-structured intensity
modulations extending up at least 4x the maximum one-magnon energy. The
one-magnon intensities decrease much faster upon increasing energy than
predicted by linear spin wave theory and the higher-energy continuum contains
much more intensity than can be accounted for by a two-magnon cross-section,
suggesting a significant transfer of spectral weight from the high-energy
magnons into the higher-energy continuum states. We attribute the strong
dispersion renormalizations and substantial transfer of spectral weight to
continuum states to the effect of quantum fluctuations and interactions beyond
the spin wave approximation, and make connections to theoretical approaches
that might capture such effects. Finally, through measurements in a strong
applied magnetic field, we find evidence for magnetic domains with opposite
senses for the spin rotation in the 120 degree ordered ground state, as
expected in the absence of Dzyaloshinskii-Moriya interactions, when the sense
of spin rotation is selected via spontaneous symmetry breaking.Comment: 20 pages, 13 figure
Tuning the confinement potential between spinons in the Ising chain CoNb2O6 using longitudinal fields and quantitative determination of the microscopic Hamiltonian
The Ising chain realizes the fundamental paradigm of spin fractionalization,
where locally flipping a spin creates two domain walls (spinons) that can
separate apart at no energy cost. In a quasi-one-dimensional system, the
mean-field effects of the weak three-dimensional couplings confine the spinons
into a Zeeman ladder of two-spinon bound states. Here, we experimentally tune
the confinement potential between spinons in the quasi-one-dimensional Ising
ferromagnet CoNb2O6 by means of an applied magnetic field with a large
component along the Ising direction. Using high-resolution single crystal
inelastic neutron scattering, we directly observe how the spectrum evolves from
the limit of very weak confinement at low field (with many closely-spaced bound
states with energies scaling as the field strength to the power 2/3) to very
strong confinement at high field (where it consists of a magnon and a
dispersive two-magnon bound state, with a linear field dependence). At
intermediate fields, we explore how the higher-order bound states disappear
from the spectrum as they move to higher energies and overlap with the
two-particle continuum. By performing a global fit to the observed spectrum in
zero field and high field applied along two orthogonal directions, combined
with a quantitative parameterization of the interchain couplings, we propose a
refined single chain and interchain Hamiltonian that quantitatively reproduces
all observed dispersions and their field dependence.Comment: 20 pages, 13 figure
Tuning the confinement potential between spinons in the Ising chain CoNb2O6 using longitudinal fields and quantitative determination of the microscopic Hamiltonian
The Ising chain realizes the fundamental paradigm of spin fractionalization, where locally flipping a spin creates two domain walls (spinons) that can separate apart at no energy cost. In a quasi-one-dimensional system, the mean-field effects of the weak three-dimensional couplings confine the spinons into a Zeeman ladder of two-spinon bound states. Here, we experimentally tune the confinement potential between spinons in the quasi-one-dimensional Ising ferromagnet CoNb2O6 by means of an applied magnetic field with a large component along the Ising direction. Using high-resolution single crystal inelastic neutron scattering, we directly observe how the spectrum evolves from the limit of very weak confinement at low field (with many closely-spaced bound states with energies scaling as the field strength to the power 2/3) to very strong confinement at high field (where it consists of a magnon and a dispersive two-magnon bound state, with a linear field dependence). At intermediate fields, we explore how the higher-order bound states disappear from the spectrum as they move to higher energies and overlap with the two-particle continuum. By performing a global fit to the observed spectrum in zero field and high field applied along two orthogonal directions, combined with a quantitative parameterization of the interchain couplings, we propose a refined single chain and interchain Hamiltonian that quantitatively reproduces all observed dispersions and their field dependence
The problem of constitutional legitimation: what the debate on electoral quotas tells us about the legitimacy of decision-making rules in constitutional choice
Proponents of electoral quotas have a ‘dependent interpretation’ of democracy, i.e. they have formed an opinion on which decision-making rules are fair on the basis of their prior approval of the outcomes these rules are likely to generate. The article argues that this position causes an irresolvable problem for constitutional processes that seek to legitimately enact institutional change. While constitutional revision governed by formal equality allows the introduction of electoral quotas, this avenue is normatively untenable for proponents of affirmative action if they are consistent with their claim that formal equality reproduces biases and power asymmetries at all levels of decision-making. Their critique raises a fundamental challenge to the constitutional revision rule itself as equally unfair. Without consensus on the decision-making process by which new post-constitutional rules can be legitimately enacted, procedural fairness becomes an issue impossible to resolve at the stage of constitutional choice. This problem of legitimation affects all instances of constitutional choice in which there are opposing views not only about the desired outcome of the process but also about the decision-making rules that govern constitutional choice
Tuning the confinement potential between spinons in the Ising chain CoNb2O6 using longitudinal fields and quantitative determination of the microscopic Hamiltonian: data archive
The deposited package contains experimental single crystal inelastic neutron scattering data probing the spin dynamics in the quantum Ising chain magnet CoNb2O6 in longitudinal magnetic field together with corresponding theoretical calculations of the spin dynamics. A matlab script to read and plot all data in ASCII files is also supplied
Transport of root-respired CO₂ via the transpiration stream affects aboveground carbon assimilation and CO₂ efflux in trees
Upward transport of CO2 via the transpiration stream from belowground to aboveground tissues occurs in tree stems. Despite potentially important implications for our understanding of plant physiology, the fate of internally transported CO2 derived from autotrophic respiratory processes remains unclear.
We infused a (CO2)-C-13-labeled aqueous solution into the base of 7-yr-old field-grown eastern cottonwood (Populus deltoides) trees to investigate the effect of xylem-transported CO2 derived from the root system on aboveground carbon assimilation and CO2 efflux.
The C-13 label was transported internally and detected throughout the tree. Up to 17% of the infused label was assimilated, while the remainder diffused to the atmosphere via stem and branch efflux. The largest amount of assimilated C-13 was found in branch woody tissues, while only a small quantity was assimilated in the foliage. Petioles were more highly enriched in C-13 than other leaf tissues.
Our results confirm a recycling pathway for respired CO2 and indicate that internal transport of CO2 from the root system may confound the interpretation of efflux-based estimates of woody tissue respiration and patterns of carbohydrate allocation