3,315 research outputs found
Spin-lattice order in frustrated ZnCr2O4
Using synchrotron X-rays and neutron diffraction we disentangle spin-lattice
order in highly frustrated ZnCrO where magnetic chromium ions occupy
the vertices of regular tetrahedra. Upon cooling below 12.5 K the quandary of
anti-aligning spins surrounding the triangular faces of tetrahedra is resolved
by establishing weak interactions on each triangle through an intricate lattice
distortion. The resulting spin order is however, not simply a N\'{e}el state on
strong bonds. A complex co-planar spin structure indicates that antisymmetric
and/or further neighbor exchange interactions also play a role as ZnCrO
resolves conflicting magnetic interactions
Structural Anomalies at the Magnetic and Ferroelectric Transitions in (R=Tb, Dy, Ho)
Strong anomalies of the thermal expansion coefficients at the magnetic and
ferroelectric transitions have been detected in multiferroic . Their
correlation with anomalies of the specific heat and the dielectric constant is
discussed. The results provide evidence for the magnetic origin of the
ferroelectricity mediated by strong spin-lattice coupling in the compounds.
Neutron scattering data for indicate a spin reorientation at the
two low-temperature phase transitions
Legacy Vehicle Fuel System Testing with Intermediate Ethanol Blends
The effects of E10 and E17 on legacy fuel system components from three common mid-1990s vintage vehicle models (Ford, GM, and Toyota) were studied. The fuel systems comprised a fuel sending unit with pump, a fuel rail and integrated pressure regulator, and the fuel injectors. The fuel system components were characterized and then installed and tested in sample aging test rigs to simulate the exposure and operation of the fuel system components in an operating vehicle. The fuel injectors were cycled with varying pulse widths during pump operation. Operational performance, such as fuel flow and pressure, was monitored during the aging tests. Both of the Toyota fuel pumps demonstrated some degradation in performance during testing. Six injectors were tested in each aging rig. The Ford and GM injectors showed little change over the aging tests. Overall, based on the results of both the fuel pump testing and the fuel injector testing, no major failures were observed that could be attributed to E17 exposure. The unknown fuel component histories add a large uncertainty to the aging tests. Acquiring fuel system components from operational legacy vehicles would reduce the uncertainty
First-order nature of the ferromagnetic phase transition in (La-Ca)MnO_3 near optimal doping
Neutron scattering has been used to study the nature of the ferromagnetic
transition in single crystals of La_0.7Ca_0.3MnO_3 and La_0.8Ca_0.2MnO_3, and
polycrystalline samples of La_0.67Ca_0.33MnO_3 and La_5/8Ca_3/8MnO_3 where the
naturally occurring O-16 can be replaced with the O-18 isotope. Small angle
neutron scattering on the x=0.3 single crystal reveals a discontinuous change
in the scattering at the Curie temperature for wave vectors below ~0.065 A^-1.
Strong relaxation effects are observed for this domain scattering, for the
magnetic order parameter, and for the quasielastic scattering, demonstrating
that the transition is not continuous in nature. There is a large oxygen
isotope effect observed for the T_C in the polycrystalline samples. For the
optimally doped x=3/8 sample we observed T_C(O-16)=266.5 K and T_C(O-18)=261.5
K at 90% O-18 substitution. The temperature dependence of the spin-wave
stiffness is found to be identical for the two samples despite changes in T_C.
Hence, T_C is not solely determined by the magnetic subsystem, but instead the
ferromagnetic phase is truncated by the formation of polarons which cause an
abrupt transition to the paramagnetic, insulating state. Application of
uniaxial stress in the x=0.3 single crystal sharply enhances the polaron
scattering at room temperature. Measurements of the phonon density-of-states
show only modest differences above and below T_C and between the two different
isotopic samples.Comment: 13 pages, 16 figures, submitted to Phys. Rev.
Division of labour and the evolution of multicellularity
Understanding the emergence and evolution of multicellularity and cellular
differentiation is a core problem in biology. We develop a quantitative model
that shows that a multicellular form emerges from genetically identical
unicellular ancestors when the compartmentalization of poorly compatible
physiological processes into component cells of an aggregate produces a fitness
advantage. This division of labour between the cells in the aggregate occurs
spontaneously at the regulatory level due to mechanisms present in unicellular
ancestors and does not require any genetic pre-disposition for a particular
role in the aggregate or any orchestrated cooperative behaviour of aggregate
cells. Mathematically, aggregation implies an increase in the dimensionality of
phenotype space that generates a fitness landscape with new fitness maxima, and
in which the unicellular states of optimized metabolism become fitness saddle
points. Evolution of multicellularity is modeled as evolution of a hereditary
parameter, the propensity of cells to stick together, which determines the
fraction of time a cell spends in the aggregate form. Stickiness can increase
evolutionarily due to the fitness advantage generated by the division of labour
between cells in an aggregate.Comment: 28 pages, 2 figure
Spectroscopy with random and displaced random ensembles
Due to the time reversal invariance of the angular momentum operator J^2, the
average energies and variances at fixed J for random two-body Hamiltonians
exhibit odd-even-J staggering, that may be especially strong for J=0. It is
shown that upon ensemble averaging over random runs, this behaviour is
reflected in the yrast states. Displaced (attractive) random ensembles lead to
rotational spectra with strongly enhanced BE2 transitions for a certain class
of model spaces. It is explained how to generalize these results to other forms
of collectivity.Comment: 4 pages, 4 figure
Recommended from our members
Structural Phase Transition in AuZn Alloys
AuxZn1-x alloys undergo a shape memory martensitic transformation whose temperature and nature (continuous or discontinuous) is strongly composition dependent. Neutron diffraction experiments were performed on single crystals of x=50 and 52 to explore the structural changes occurring at the transition temperature. A transverse modulation with wavevector q0=(1/3,1/3,0) develops below the transition temperature, with no observable change in lattice parameter. However, the Bragg peak width shows a broadening suggesting an unresolved rhombohedral distortion similar to what has been observed in NiTi-Fe alloys
Observation of a continuous phase transition in a shape-memory alloy
Elastic neutron-scattering, inelastic x-ray scattering, specific-heat, and
pressure-dependent electrical transport measurements have been made on single
crystals of AuZn and Au_{0.52}Zn_{0.48} above and below their martensitic
transition temperatures (T_M=64K and 45K, respectively). In each composition,
elastic neutron scattering detects new commensurate Bragg peaks (modulation)
appearing at Q = (1.33,0.67,0) at temperatures corresponding to each sample's
T_M. Although the new Bragg peaks appear in a discontinuous manner in the
Au_{0.52}Zn_{0.48} sample, they appear in a continuous manner in AuZn.
Surprising us, the temperature dependence of the AuZn Bragg peak intensity and
the specific-heat jump near the transition temperature are in favorable accord
with a mean-field approximation. A Landau-theory-based fit to the pressure
dependence of the transition temperature suggests the presence of a critical
endpoint in the AuZn phase diagram located at T_M*=2.7K and p*=3.1GPa, with a
quantum saturation temperature \theta_s=48.3 +/- 3.7K.Comment: 6 figure
- …