85 research outputs found
Quantum tunneling in a three dimensional network of exchange coupled single-molecule magnets
A Mn4 single-molecule magnet (SMM) is used to show that quantum tunneling of
magnetization (QTM) is not suppressed by moderate three dimensional exchange
coupling between molecules. Instead, it leads to an exchange bias of the
quantum resonances which allows precise measurements of the effective exchange
coupling that is mainly due to weak intermolecular hydrogen bounds. The
magnetization versus applied field was recorded on single crystals of [Mn4]2
using an array of micro-SQUIDs. The step fine structure was studied via minor
hysteresis loops.Comment: 4 pages, 4 figure
DFT calculation of the intermolecular exchange interaction in the magnetic Mn dimer
The dimeric form of the single-molecule magnet
[MnOCl(OCEt)(py)] recently revealed interesting
phenomena: no quantum tunneling at zero field and tunneling before magnetic
field reversal. This is attributed to substantial antiferromagnetic exchange
interaction between different monomers. The intermolecular exchange
interaction, electronic structure and magnetic properties of this molecular
magnet are calculated using density-functional theory within
generalized-gradient approximation. Calculations are in good agreement with
experiment.Comment: 4 page
Thermal conductivity of sand and its effect on the temperature of Loggerhead Sea Turtle (Caretta Caretta) nests
The conductivity of sand at a depth of 30–50 cm was measured at 15 sites on the beach at Captiva Island in south-west Florida which is used by nesting loggerhead turtles (Caretta caretta). The mean daily temperature of the sand was correlated with conductivity at the same depth measured the same day (r=0·611). When day to day variation was removed the correlation between nest temperature and conductivity increased to 0·694. The sand was highly variable in its grain structure. The dominant variability (80·6%) was redescribed by the first two principal components of a Principal Components Analysis (PCA). These two components were influenced mostly by percentages of large (> 1 mm) and small (< 500 μm) grains respectively. Conductivity was strongly correlated with the grain structure of the sand. The first three principal components describing sand grain structure, explained 84·1% of the variation in conductivity. Moisture content of the sand (always < 5%) was not an important factor. Sites dominated by larger grains generally had poorer conductivity and were cooler. Comparisons of eight nests to seven adjacent random sites revealed no strong evidence for directional selection in nest placement relative to sand conductivity. The variance in conductivities recorded at nests was also not significantly different from the variance at random sites
Properties of low-lying states in some high-nuclearity Mn, Fe and V clusters: Exact studies of Heisenberg models
Using an efficient numerical scheme that exploits spatial symmetries and spin
parity, we have obtained the exact low-lying eigenstates of exchange
Hamiltonians for the high nuclearity spin clusters, Mn_{12}, Fe_8 and V_{15}.
The largest calculation involves the Mn_{12} cluster which spans a Fock space
of a hundred million. Our results show that the earlier estimates of the
exchange constants need to be revised for the Mn_{12} cluster to explain the
level ordering of low-lying eigenstates. In the case of the Fe_8 cluster,
correct level ordering can be obtained which is consistent with the exchange
constants for the already known clusters with butterfly structure. In the
V_{15} cluster, we obtain an effective Hamiltonian that reproduces exactly, the
eight low-lying eigenvalues of the full Hamiltonian.Comment: Revtex, 12 pages, 16 eps figures; this is the final published versio
Observation of a Distribution of Internal Transverse Magnetic Fields in a Mn12-Based Single Molecule Magnet
A distribution of internal transverse magnetic fields has been observed in
single molecule magnet (SMM) Mn12-BrAc in the pure magnetic quantum tunneling
(MQT) regime. Magnetic relaxation experiments at 0.4 K are used to produce a
hole in the distribution of transverse fields whose angle and depth depend on
the orientation and amplitude of an applied transverse ``digging field.'' The
presence of such transverse magnetic fields can explain the main features of
resonant MQT in this material, including the tunneling rates, the form of the
relaxation and the absence of tunneling selection rules. We propose a model in
which the transverse fields originate from a distribution of tilts of the
molecular magnetic easy axes.Comment: 4 page
Spin-parity dependent tunneling of magnetization in single-molecule magnets
Single-molecule magnets facilitate the study of quantum tunneling of
magnetization at the mesoscopic level. The spin-parity effect is among the
fundamental predictions that have yet to be clearly observed. It is predicted
that quantum tunneling is suppressed at zero transverse field if the total spin
of the magnetic system is half-integer (Kramers degeneracy) but is allowed in
integer spin systems. The Landau-Zener method is used to measure the tunnel
splitting as a function of transverse field. Spin-parity dependent tunneling is
established by comparing the transverse field dependence of the tunnel
splitting of integer and half-integer spin systems.Comment: 4 pages, 6 figure
Do Movement Patterns Differ Between Laboratory and Field Suction Feeding Behaviors in a Mexican Cichlid?
Recommended from our members
Overview of the Historic and Current Vegetation Near the 100th Meridian in North Central United States
The Northern Great Plains contains a diverse group of vegetative communities, primarily dominated by grassland communities. Precipitation declines along an east-west gradient, ranging from 27.4 inches at Detroit Lakes, Minnesota to 12.4 inches at Miles City, Montana, and productivity follows a similar decline. Precipitation falls primarily during the growing season, which combined with the lower mean annual temperature results in productive, high-quality, cool-season dominated grasslands. Although the region is primarily dominated by areas of tallgrass, midgrass, and shortgrass prairie, there are outcrops of limber (Pinus flexilis) and ponderosa pine (P. ponderosa) along the Little Missouri River and stands of quaking aspen (Populus tremuloides) in the Turtle Mountains. Besides climate and soils, fire, drought, and grazing have also contributed to the rich diversity of communities in the region. Recent invasions of perennial cool-season grasses are threatening historic plant communities; whether these invasions can be reversed and altered environmental services restored are the primary questions facing grassland managers.The Rangelands archives are made available by the Society for Range Management and the University of Arizona Libraries. Contact [email protected] for further information
- …