389 research outputs found
An alternate model for magnetization plateaus in the molecular magnet V_15
Starting from an antiferromagnetic Heisenberg Hamiltonian for the fifteen
spin-1/2 ions in V_15, we construct an effective spin Hamiltonian involving
eight low-lying states (spin-1/2 and spin-3/2) coupled to a phonon bath. We
numerically solve the time-dependent Schrodinger equation of this system, and
obtain the magnetization as a function of temperature in a time-dependent
magnetic field. The magnetization exhibits unusual patterns of hysteresis and
plateaus as the field sweep rate and temperature are varied. The observed
plateaus are not due to quantum tunneling but are a result of thermal
averaging. Our results are in good agreement with recent experimental
observations.Comment: Revtex, 4 pages, 5 eps figure
Combining ability studies in restorer lines of sunflower (Helianthus annuus L.)
The restorer lines are used to get fertile hybrids in hybrid seed production system of sunflower. Improve-ment of R-lines for fertility and oil content of sunflower is required to get desirable hybrids upon crossing. In the pre-sent study, 6 restorer lines were crossed in full diallele and both direct and reciprocal crosses along with parents were evaluated for their combining ability. The analysis of variance revealed higher magnitude of SCA variance than GCA variance for all the characters studied except for days to 50 per cent flowering. Among six parents, GMU -520 and R-GM-41 were best general combiner for plant height, head diameter, test weight and leaf size in the desired direction. However, GMU-520 has advantage of yield per plant and oil content. From 15 direct crosses R-GM-41 x R -GM-49 exhibited significant specific combining ability for viz., days to 50 per cent flowering (-1.08), plant height (13.69), head diameter (4.79), leaf size (0.74), yield per plant (17.14), volume weight (2.54), test weight (1.75) and oil content (0.34) followed by RCR-630 x GMU-520, R-GM-41 x EC-602060, R-GM-49 x GMU-520 and EC-602060 x GMU-520 each exhibiting good specific combining ability for majority of characters. Among reciprocal crosses R-GM -41 x RCR-8 exhibited a high reciprocal effects for head diameter (4.57), yield per plant (15.16), test weight (2.02) and leaf size (0.75)
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
Magnetic Anisotropy in the Molecular Complex V15
We apply degenerate perturbation theory to investigate the effects of
magnetic anisotropy in the magnetic molecule V15. Magnetic anisotropy is
introduced via Dzyaloshinskii-Moriya (DM) interaction in the full Hilbert space
of the system. Our model provides an explanation for the rounding of
transitions in the magnetization as a function of applied field at low
temperature, from which an estimate for the DM interaction is found. We find
that the calculated energy differences of the lowest energy states are
consistent with the available data. Our model also offers a novel explanation
for the hysteretic nature of the time-dependent magnetization data.Comment: Final versio
A pilot study of same-day MRI-only simulation and treatment with MR-guided adaptive palliative radiotherapy (MAP-RT)
We conducted a prospective pilot study evaluating the feasibility of same day MRI-only simulation and treatment with MRI-guided adaptive palliative radiotherapy (MAP-RT) for urgent palliative indications (NCT#03824366). All (16/16) patients were able to complete 99% of their first on-table attempted fractions, and no grades 3-5 toxicities occurred
Pre-pregnancy predictors of hypertension in pregnancy among Aboriginal and Torres Strait Islander women in north Queensland, Australia; a prospective cohort study
BACKGROUND Compared to other Australian women, Indigenous women are frequently at greater risk for hypertensive disorders of pregnancy. We examined pre-pregnancy factors that may predict hypertension in pregnancy in a cohort of Aboriginal and Torres Strait Islander women in north Queensland. METHODS Data on a cohort of 1009 Indigenous women of childbearing age (15–44 years) who participated in a 1998–2000 health screening program in north Queensland were combined with 1998–2008 Queensland hospitalisations data using probabilistic data linkage. Data on the women in the cohort who were hospitalised for birth (n = 220) were further combined with Queensland perinatal data which identified those diagnosed with hypertension in pregnancy. RESULTS Of 220 women who gave birth, 22 had hypertension in the pregnancy after their health check. The mean age of women with and without hypertension was similar (23.7 years and 23.9 years respectively) however Aboriginal women were more affected compared to Torres Strait Islanders. Pre-pregnancy adiposity and elevated blood pressure at the health screening program were predictors of a pregnancy affected by hypertension. After adjusting for age and ethnicity, each 1 cm increase in waist circumference showed a 4% increased risk for hypertension in pregnancy (PR 1.04; 95% CI; 1.02-1.06); each 1 point increase in BMI showed a 9% adjusted increase in risk (1.09; 1.04-1.14). For each 1 mmHg increase in baseline systolic blood pressure there was an age and ethnicity adjusted 6% increase in risk and each 1 mmHg increase in diastolic blood pressure showed a 7% increase in risk (1.06; 1.03-1.09 and 1.07; 1.03-1.11 respectively). Among those free of diabetes at baseline, the presence of the metabolic syndrome (International Diabetes Federation criteria) predicted over a three-fold increase in age-ethnicity-adjusted risk (3.5; 1.50-8.17). CONCLUSIONS Pre-pregnancy adiposity and features of the metabolic syndrome among these young Aboriginal and Torres Strait Islander women track strongly to increased risk of hypertension in pregnancy with associated risks to the health of babies.Sandra K Campbell, John Lynch, Adrian Esterman and Robyn McDermot
Model Exact Low-Lying States and Spin Dynamics in Ferric Wheels; Fe to Fe
Using an efficient numerical scheme that exploits spatial symmetries and
spin-parity, we have obtained the exact low-lying eigenstates of exchange
Hamiltonians for ferric wheels up to Fe. The largest calculation
involves the Fe ring which spans a Hilbert space dimension of about 145
million for M=0 subspace. Our calculated gaps from the singlet ground state
to the excited triplet state agrees well with the experimentally measured
values. Study of the static structure factor shows that the ground state is
spontaneously dimerized for ferric wheels. Spin states of ferric wheels can be
viewed as quantized states of a rigid rotor with the gap between the ground and
the first excited state defining the inverse of moment of inertia. We have
studied the quantum dynamics of Fe as a representative of ferric wheels.
We use the low-lying states of Fe to solve exactly the time-dependent
Schr\"odinger equation and find the magnetization of the molecule in the
presence of an alternating magnetic field at zero temperature. We observe a
nontrivial oscillation of magnetization which is dependent on the amplitude of
the {\it ac} field. We have also studied the torque response of Fe as a
function of magnetic field, which clearly shows spin-state crossover.Comment: Revtex, 24 pages, 8 eps figure
Magnetization of Mn_12 Ac in a slowly varying magnetic field: an ab initio study
Beginning with a Heisenberg spin Hamiltonian for the manganese ions in the
Mn_12 Ac molecule, we find a number of low-energy states of the system. We use
these states to solve the time-dependent Schrodinger equation and find the
magnetization of the molecule in the presence of a slowly varying magnetic
field. We study the effects of the field sweep rate, fourth order anisotropic
spin interactions and a transverse field on the weights of the different states
as well as the magnetization steps which are known to occur in the hysteresis
plots in this system. We find that the fourth order term and a slow field sweep
rate are crucial for obtaining prominent steps in magnetization in the
hysteresis plots.Comment: LaTeX, 11 pages, 12 eps figure
One entropy function to rule them all
We study the entropy of extremal four dimensional black holes and five
dimensional black holes and black rings is a unified framework using Sen's
entropy function and dimensional reduction. The five dimensional black holes
and black rings we consider project down to either static or stationary black
holes in four dimensions. The analysis is done in the context of two derivative
gravity coupled to abelian gauge fields and neutral scalar fields. We apply
this formalism to various examples including minimal supergravity.Comment: 29 pages, 2 figures, revised version for publication, details adde
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