76 research outputs found
Role of dipolar and exchange interactions in the positions and widths of EPR transitions for the single-molecule magnets Fe8 and Mn12
We examine quantitatively the temperature dependence of the linewidths and
line shifts in electron paramagnetic resonance experiments on single crystals
of the single-molecule magnets Fe and Mn, at fixed frequency, with
an applied magnetic field along the easy axis. We include inter-molecular
spin-spin interactions (dipolar and exchange) and distributions in both the
uniaxial anisotropy parameter and the Land\'{e} -factor. The temperature
dependence of the linewidths and the line shifts are mainly caused by the
spin-spin interactions. For Fe and Mn, the temperature dependence of
the calculated line shifts and linewidths agrees well with the trends of the
experimental data. The linewidths for Fe reveal a stronger temperature
dependence than those for Mn, because for Mn a much wider
distribution in overshadows the temperature dependence of the spin-spin
interactions. For Fe, the line-shift analysis suggests two competing
interactions: a weak ferromagnetic exchange coupling between neighboring
molecules and a longer-ranged dipolar interaction. This result could have
implications for ordering in Fe at low temperatures.Comment: published versio
Spin-Wave Description of Nuclear Spin-Lattice Relaxation in Mn_{12}O_{12} Acetate
In response to recent nuclear-magnetic-resonance (NMR) measurements on the
molecular cluster Mn_{12}O_{12} acetate, we study the nuclear spin-lattice
relaxation rate 1/T_1 developing a modified spin-wave theory. Our microscopic
new approach, which is distinct from previous macroscopic treatments of the
cluster as a rigid spin of S=10, not only excellently interprets the observed
temperature and applied-field dependences of 1/T_1 for ^{55}Mn nuclei but also
strongly supports the ^{13}C NMR evidence for spin delocalization over the
entire molecule.Comment: to be published in Phys. Rev. Lett., 4 pages, 4 figures embedde
Semiconductive and Photoconductive Properties of the Single Molecule Magnets Mn-Acetate and FeBr
Resistivity measurements are reported for single crystals of
Mn-Acetate and FeBr. Both materials exhibit a
semiconductor-like, thermally activated behavior over the 200-300 K range. The
activation energy, , obtained for Mn-Acetate was 0.37 0.05
eV, which is to be contrasted with the value of 0.55 eV deduced from the
earlier reported absorption edge measurements and the range of 0.3-1 eV from
intramolecular density of states calculations, assuming = , the
optical band gap. For FeBr, was measured as 0.73 0.1 eV,
and is discussed in light of the available approximate band structure
calculations. Some plausible pathways are indicated based on the crystal
structures of both lattices. For Mn-Acetate, we also measured
photoconductivity in the visible range; the conductivity increased by a factor
of about eight on increasing the photon energy from 632.8 nm (red) to 488 nm
(blue). X-ray irradiation increased the resistivity, but was insensitive
to exposure.Comment: 7 pages, 8 figure
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Vocal Hygiene Habits and Vocal Handicap Among Conservatory Students of Classical Singing
Objectives: This study sought to assess classical singing students' compliance with vocal hygiene practices identified in the literature and to explore the relationship between self-reported vocal hygiene practice and self-reported singing voice handicap in this population. The primary hypothesis was that increased attention to commonly recommended vocal hygiene practices would correlate with reduced singing voice handicap.
Study Design: This is a cross-sectional, survey-based study.
Methods: An anonymous survey assessing demographics, attention to 11 common vocal hygiene recommendations in both performance and nonperformance periods, and the Singing Voice Handicap Index 10 (SVHI-10) was distributed to classical singing teachers to be administered to their students at two major schools of music.
Results: Of the 215 surveys distributed, 108 were returned (50.2%), of which 4 were incomplete and discarded from analysis. Conservatory students of classical singing reported a moderate degree of vocal handicap (mean SVHI-10, 12; range, 0â29). Singers reported considering all 11 vocal hygiene factors more frequently when preparing for performances than when not preparing for performances. Of these, significant correlations with increased handicap were identified for consideration of stress reduction in nonperformance ( P = 0.01) and performance periods ( P = 0.02) and with decreased handicap for consideration of singing voice use in performance periods alone ( P = 0.02).
Conclusions: Conservatory students of classical singing report more assiduous attention to vocal hygiene practices when preparing for performances and report moderate degrees of vocal handicap overall. These students may have elevated risk for dysphonia and voice disorders which is not effectively addressed through common vocal hygiene recommendations alone.
Keywords: vocal hygiene, vocal handicap, voice handicap, singer, dysphoni
Nuclear spin-lattice relaxation in ferrimagnetic clusters and chains: A contrast between zero and one dimensions
Motivated by ferrimagnetic oligonuclear and chain compounds synthesized by
Caneschi et al., both of which consist of alternating manganese(II) ions and
nitronyl-nitroxide radicals, we calculate the nuclear spin-lattice relaxation
rate 1/T_1 employing a recently developed modified spin-wave theory. 1/T_1 as a
function of temperature drastically varies with the location of probe nuclei in
both clusters and chains, though the relaxation time scale is much larger in
zero dimension than in one dimension. 1/T_1 as a function of an applied field
in long chains forms a striking contrast to that in finite clusters, diverging
with decreasing field like inverse square root at low temperatures and
logarithmically at high temperatures.Comment: to be published in Phys. Rev. B 68 August 01 (2003
High frequency resonant experiments in Fe molecular clusters
Precise resonant experiments on Fe magnetic clusters have been
conducted down to 1.2 K at various tranverse magnetic fields, using a
cylindrical resonator cavity with 40 different frequencies between 37 GHz and
110 GHz. All the observed resonances for both single crystal and oriented
powder, have been fitted by the eigenstates of the hamiltonian . We have identified the
resonances corresponding to the coherent quantum oscillations for different
orientations of spin S = 10.Comment: to appear in Phys.Rev. B (August 2000
Magnetic Field Effects on the Far-Infrared Absorption in Mn_12-acetate
We report the far-infrared spectra of the molecular nanomagnet Mn_12-acetate
(Mn_12) as a function of temperature (5-300 K) and magnetic field (0-17 T). The
large number of observed vibrational modes is related to the low symmetry of
the molecule, and they are grouped together in clusters. Analysis of the mode
character based on molecular dynamics simulations and model compound studies
shows that all vibrations are complex; motion from a majority of atoms in the
molecule contribute to most modes. Three features involving intramolecular
vibrations of the Mn_12 molecule centered at 284, 306 and 409 cm-1 show changes
with applied magnetic field. The structure near 284 cm displays the
largest deviation with field and is mainly intensity related. A comparison
between the temperature dependent absorption difference spectra, the gradual
low-temperature cluster framework distortion as assessed by neutron diffraction
data, and field dependent absorption difference spectra suggests that this mode
may involve Mn motion in the crown.Comment: 5 pages, 4 figures, PRB accepte
Detailed single crystal EPR lineshape measurements for the single molecule magnets Fe8Br and Mn12-ac
It is shown that our multi-high-frequency (40-200 GHz) resonant cavity
technique yields distortion-free high field EPR spectra for single crystal
samples of the uniaxial and biaxial spin S = 10 single molecule magnets (SMMs)
[Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O and [Fe8O2(OH)12(tacn)6]Br8.9H2O. The
observed lineshapes exhibit a pronounced dependence on temperature, magnetic
field, and the spin quantum numbers (Ms values) associated with the levels
involved in the transitions. Measurements at many frequencies allow us to
separate various contributions to the EPR linewidths, including significant
D-strain, g-strain and broadening due to the random dipolar fields of
neighboring molecules. We also identify asymmetry in some of the EPR lineshapes
for Fe8, and a previously unobserved fine structure to some of the EPR lines
for both the Fe8 and Mn12 systems. These findings prove relevant to the
mechanism of quantum tunneling of magnetization in these SMMs.Comment: Phys. Rev. B, accepted with minor revision
Equal Sensitivity of the Two Strands of øX174 Replicative DNA to Breakage by Ionizing Radiation
The supertwisted, double-stranded, replicative intermediate of øX174 DNA (RFI) has been used to determine whether one of the two strands of the double helix is uniquely sensitive to induction of single-strand breaks by ionizing radiation. This could result from a particularly sensitive base sequence or a transfer of energy to a specific location of the DNA molecule. The results indicate that both strands of the double helix are equally broken, even though their base compositions are significantly different. If there are âhot spotsâ in the strands, then they are present in equal amounts in each strand
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