23 research outputs found
Quantum Nucleation in a Ferromagnetic Film Placed in a Magnetic Field at an Arbitrary Angle
We study the quantum nucleation in a thin ferromagnetic film placed in a
magnetic field at an arbitrary angle. The dependence of the quantum nucleation
and the temperature of the crossover from thermal to quantum regime on the
direction and the strength of the applied field are presented. It is found that
the maximal value of the rate and that of the crossover temperature are
obtained at a some angle with the magnetic field, not in the direction of the
applied field opposite to the initial easy axis.Comment: 15 pages, RevTex, 3 PostScript figures. To appear in Phys. Rev.
Oscillatory Tunnel Splittings in Spin Systems: A Discrete Wentzel-Kramers-Brillouin Approach
Certain spin Hamiltonians that give rise to tunnel splittings that are viewed
in terms of interfering instanton trajectories, are restudied using a discrete
WKB method, that is more elementary, and also yields wavefunctions and
preexponential factors for the splittings. A novel turning point inside the
classically forbidden region is analysed, and a general formula is obtained for
the splittings. The result is appled to the \Fe8 system. A previous result for
the oscillation of the ground state splitting with external magnetic field is
extended to higher levels.Comment: RevTex, one ps figur
Quenched Spin Tunneling and Diabolical Points in Magnetic Molecules: II. Asymmetric Configurations
The perfect quenching of spin tunneling first predicted for a model with
biaxial symmetry, and recently observed in the magnetic molecule Fe_8, is
further studied using the discrete phase integral (or
Wentzel-Kramers-Brillouin) method. The analysis of the previous paper is
extended to the case where the magnetic field has both hard and easy
components, so that the Hamiltonian has no obvious symmetry. Herring's formula
is now inapplicable, so the problem is solved by finding the wavefunction and
using connection formulas at every turning point. A general formula for the
energy surface in the vicinity of the diabolo is obtained in this way. This
formula gives the tunneling apmplitude between two wells unrelated by symmetry
in terms of a small number of action integrals, and appears to be generally
valid, even for problems where the recursion contains more than five terms.
Explicit results are obtained for the diabolical points in the model for Fe_8.
These results exactly parallel the experimental observations. It is found that
the leading semiclassical results for the diabolical points appear to be exact,
and the points themselves lie on a perfect centered rectangular lattice in the
magnetic field space. A variety of evidence in favor of this perfect lattice
hypothesis is presented.Comment: Revtex; 4 ps figures; follow up to cond-mat/000311
Magnetic Field Dependence of Macroscopic Quantum Tunneling and Coherence of Ferromagnetic Particle
We calculate the quantum tunneling rate of a ferromagnetic particle of diameter in a magnetic field of arbitrary angle. We consider the
magnetocrystalline anisotropy with the biaxial symmetry and that with the
tetragonal symmetry. Using the spin-coherent-state path integral, we obtain
approximate analytic formulas of the tunneling rates in the small -limit for the magnetic field normal to the easy axis (), for the field opposite to the initial easy axis (),
and for the field at an angle between these two orientations (). In addition, we obtain numerically the tunneling rates for
the biaxial symmetry in the full range of the angle of the magnetic
field (), for the values of \epsilon =0.01 and
0.001.Comment: 25 pages of text (RevTex) and 4 figures (PostScript files), to be
published in Phys. Rev.
Spin Tunneling in Magnetic Molecules: Quasisingular Perturbations and Discontinuous SU(2) Instantons
Spin coherent state path integrals with discontinuous semiclassical paths are
investigated with special reference to a realistic model for the magnetic
degrees of freedom in the Fe8 molecular solid. It is shown that such paths are
essential to a proper understanding of the phenomenon of quenched spin
tunneling in these molecules. In the Fe8 problem, such paths are shown to arise
as soon as a fourth order anisotropy term in the energy is turned on, making
this term a singular perturbation from the semiclassical point of view. The
instanton approximation is shown to quantitatively explain the magnetic field
dependence of the tunnel splitting, as well as agree with general rules for the
number of quenching points allowed for a given value of spin. An accurate
approximate formula for the spacing between quenching points is derived
Surface superconductivity and in UPt
Surface superconductivity is studied within Ginzburg-Landau theory for two
classes of models for the order parameter of UPt. The first class assumes
two independent one-dimensional order parameters ( models), while the
second assumes a single two-dimensional order parameter ( models).
is calculated for all cases where the surface normal and magnetic field lie
along high symmetry directions. Assuming specular reflection, it is found that
except when , the ratio is either
unity or equals its `s-wave' value 1.695, although the precise vs.
curve predicted by the and models differs for various geometries. The
results are compared with recent experiments, and predictions are made for
future experiments.Comment: 26 pages, with 6 figure
Spin tunnelling in mesoscopic systems
We study spin tunnelling in molecular magnets as an instance of a mesoscopic
phenomenon, with special emphasis on the molecule Fe8. We show that the tunnel
splitting between various pairs of Zeeman levels in this molecule oscillates as
a function of applied magnetic field, vanishing completely at special points in
the space of magnetic fields, known as diabolical points. This phenomena is
explained in terms of two approaches, one based on spin-coherent-state path
integrals, and the other on a generalization of the phase integral (or WKB)
method to difference equations. Explicit formulas for the diabolical points are
obtained for a model Hamiltonian.Comment: 13 pages, 5 figures, uses Pramana style files; conference proceedings
articl
The Reliability of Parafoveal Cone Density Measurements
Background Adaptive optics scanning light ophthalmoscopy (AOSLO) enables direct visualisation of the cone mosaic, with metrics such as cone density and cell spacing used to assess the integrity or health of the mosaic. Here we examined the interobserver and inter-instrument reliability of cone density measurements.
Methods For the interobserver reliability study, 30 subjects with no vision-limiting pathology were imaged. Three image sequences were acquired at a single parafoveal location and aligned to ensure that the three images were from the same retinal location. Ten observers used a semiautomated algorithm to identify the cones in each image, and this was repeated three times for each image. To assess inter-instrument reliability, 20 subjects were imaged at eight parafoveal locations on one AOSLO, followed by the same set of locations on the second AOSLO. A single observer manually aligned the pairs of images and used the semiautomated algorithm to identify the cones in each image.
Results Based on a factorial study design model and a variance components model, the interobserver study\u27s largest contribution to variability was the subject (95.72%) while the observer\u27s contribution was only 1.03%. For the inter-instrument study, an average cone density intraclass correlation coefficient (ICC) of between 0.931 and 0.975 was calculated.
Conclusions With the AOSLOs used here, reliable cone density measurements can be obtained between observers and between instruments. Additional work is needed to determine how these results vary with differences in image quality
Case Reports1. A Late Presentation of Loeys-Dietz Syndrome: Beware of TGFβ Receptor Mutations in Benign Joint Hypermobility
Background: Thoracic aortic aneurysms (TAA) and dissections are not uncommon causes of sudden death in young adults. Loeys-Dietz syndrome (LDS) is a rare, recently described, autosomal dominant, connective tissue disease characterized by aggressive arterial aneurysms, resulting from mutations in the transforming growth factor beta (TGFβ) receptor genes TGFBR1 and TGFBR2. Mean age at death is 26.1 years, most often due to aortic dissection. We report an unusually late presentation of LDS, diagnosed following elective surgery in a female with a long history of joint hypermobility. Methods: A 51-year-old Caucasian lady complained of chest pain and headache following a dural leak from spinal anaesthesia for an elective ankle arthroscopy. CT scan and echocardiography demonstrated a dilated aortic root and significant aortic regurgitation. MRA demonstrated aortic tortuosity, an infrarenal aortic aneurysm and aneurysms in the left renal and right internal mammary arteries. She underwent aortic root repair and aortic valve replacement. She had a background of long-standing joint pains secondary to hypermobility, easy bruising, unusual fracture susceptibility and mild bronchiectasis. She had one healthy child age 32, after which she suffered a uterine prolapse. Examination revealed mild Marfanoid features. Uvula, skin and ophthalmological examination was normal. Results: Fibrillin-1 testing for Marfan syndrome (MFS) was negative. Detection of a c.1270G > C (p.Gly424Arg) TGFBR2 mutation confirmed the diagnosis of LDS. Losartan was started for vascular protection. Conclusions: LDS is a severe inherited vasculopathy that usually presents in childhood. It is characterized by aortic root dilatation and ascending aneurysms. There is a higher risk of aortic dissection compared with MFS. Clinical features overlap with MFS and Ehlers Danlos syndrome Type IV, but differentiating dysmorphogenic features include ocular hypertelorism, bifid uvula and cleft palate. Echocardiography and MRA or CT scanning from head to pelvis is recommended to establish the extent of vascular involvement. Management involves early surgical intervention, including early valve-sparing aortic root replacement, genetic counselling and close monitoring in pregnancy. Despite being caused by loss of function mutations in either TGFβ receptor, paradoxical activation of TGFβ signalling is seen, suggesting that TGFβ antagonism may confer disease modifying effects similar to those observed in MFS. TGFβ antagonism can be achieved with angiotensin antagonists, such as Losartan, which is able to delay aortic aneurysm development in preclinical models and in patients with MFS. Our case emphasizes the importance of timely recognition of vasculopathy syndromes in patients with hypermobility and the need for early surgical intervention. It also highlights their heterogeneity and the potential for late presentation. Disclosures: The authors have declared no conflicts of interes