542 research outputs found
Optical response of ferromagnetic YTiO_3 studied by spectral ellipsometry
We have studied the temperature dependence of spectroscopic ellipsometry
spectra of an electrically insulating, nearly stoichiometric YTiO_3 single
crystal with ferromagnetic Curie temperature T_C = 30 K. The optical response
exhibits a weak but noticeable anisotropy. Using a classical dispersion
analysis, we identify three low-energy optical bands at 2.0, 2.9, and 3.7 eV.
Although the optical conductivity spectra are only weakly temperature dependent
below 300 K, we are able to distinguish high- and low-temperature regimes with
a distinct crossover point around 100 K. The low-temperature regime in the
optical response coincides with the temperature range in which significant
deviations from Curie-Weiss mean field behavior are observed in the
magnetization. Using an analysis based on a simple superexchange model, the
spectral weight rearrangement can be attributed to intersite d_i^1d_j^1
\longrightarrow d_i^2d_j^0 optical transitions. In particular, Kramers-Kronig
consistent changes in optical spectra around 2.9 eV can be associated with the
high-spin-state (^3T_1) optical transition. This indicates that other
mechanisms, such as weakly dipole-allowed p-d transitions and/or
exciton-polaron excitations, can contribute significantly to the optical band
at 2 eV. The recorded optical spectral weight gain of 2.9 eV optical band is
significantly suppressed and anisotropic, which we associate with complex
spin-orbit-lattice phenomena near ferromagnetic ordering temperature in YTiO_3
Nonequilibrium Generalised Langevin Equation for the calculation of heat transport properties in model 1D atomic chains coupled to two 3D thermal baths
We use a Generalised Langevin Equation (GLE) scheme to study the thermal
transport of low dimensional systems. In this approach, the central classical
region is connected to two realistic thermal baths kept at two different
temperatures [H. Ness et al., Phys. Rev. B {\bf 93}, 174303 (2016)]. We
consider model Al systems, i.e. one-dimensional atomic chains connected to
three-dimensional baths. The thermal transport properties are studied as a
function of the chain length and the temperature difference
between the baths. We calculate the transport properties both in the linear
response regime and in the non-linear regime. Two different laws are obtained
for the linear conductance versus the length of the chains. For large
temperatures ( K) and temperature differences ( K), the chains, with atoms, present a diffusive transport regime
with the presence of a temperature gradient across the system. For lower
temperatures( K) and temperature differences ( K), a regime similar to the ballistic regime is observed. Such a
ballistic-like regime is also obtained for shorter chains (). Our
detailed analysis suggests that the behaviour at higher temperatures and
temperature differences is mainly due to anharmonic effects within the long
chains.Comment: Accepted for publication in J. Chem. Phy
Singlet-triplet decoherence due to nuclear spins in a double quantum dot
We have evaluated hyperfine-induced electron spin dynamics for two electrons
confined to a double quantum dot. Our quantum solution accounts for decay of a
singlet-triplet correlator even in the presence of a fully static nuclear spin
system, with no ensemble averaging over initial conditions. In contrast to an
earlier semiclassical calculation, which neglects the exchange interaction, we
find that the singlet-triplet correlator shows a long-time saturation value
that differs from 1/2, even in the presence of a strong magnetic field.
Furthermore, we find that the form of the long-time decay undergoes a
transition from a rapid Gaussian to a slow power law () when
the exchange interaction becomes nonzero and the singlet-triplet correlator
acquires a phase shift given by a universal (parameter independent) value of
at long times. The oscillation frequency and time-dependent phase
shift of the singlet-triplet correlator can be used to perform a precision
measurement of the exchange interaction and Overhauser field fluctuations in an
experimentally accessible system. We also address the effect of orbital
dephasing on singlet-triplet decoherence, and find that there is an optimal
operating point where orbital dephasing becomes negligible.Comment: 12 pages, 4 figure
Could humans recognize odor by phonon assisted tunneling?
Our sense of smell relies on sensitive, selective atomic-scale processes that
are initiated when a scent molecule meets specific receptors in the nose.
However, the physical mechanisms of detection are not clear. While odorant
shape and size are important, experiment indicates these are insufficient. One
novel proposal suggests inelastic electron tunneling from a donor to an
acceptor mediated by the odorant actuates a receptor, and provides critical
discrimination. We test the physical viability of this mechanism using a simple
but general model. Using values of key parameters in line with those for other
biomolecular systems, we find the proposed mechanism is consistent both with
the underlying physics and with observed features of smell, provided the
receptor has certain general properties. This mechanism suggests a distinct
paradigm for selective molecular interactions at receptors (the swipe card
model): recognition and actuation involve size and shape, but also exploit
other processes.Comment: 10 pages, 1 figur
Exchange between deep donors in semiconductors: a quantum defect approach
Exchange interactions among defects in semiconductors are commonly treated
within effective-mass theory using a scaled hydrogenic wave-function. However
such a wave-function is only applicable to shallow impurities; here we present
a simple but robust generalization to treat deep donors, in which we treat the
long-range part of the wavefunction using the well established quantum defect
theory, and include a model central-cell correction to fix the bound-state
eigenvalue at the experimentally observed value. This allows us to compute the
effect of binding energy on exchange interactions as a function of donor
distance; this is a significant quantity given recent proposals to carry out
quantum information processing using deep donors. As expected, exchange
interactions are suppressed (or increased), compared to the hydrogenic case, by
the greater localization (or delocalization) of the wavefunctions of deep
donors (or `super-shallow' donors with binding energy less then the hydrogenic
value). The calculated results are compared with a simple scaling of the
Heitler-London hydrogenic exchange; the scaled hydrogenic results give the
correct order of magnitude but fail to reproduce quantitatively our
calculations. We calculate the donor exchange in silicon including inter-valley
interference terms for donor pairs along the direction, and also show
the influence of the donor type on the distribution of nearest-neighbour
exchange constants at different concentrations. Our methods can be used to
compute the exchange interactions between two donor electrons with arbitrary
binding energy.Comment: 11 pages, 10 figures, RevTeX
Quantum Electrical Dipole in Triangular Systems: a Model for Spontaneous Polarity in Metal Clusters
Triangular symmetric molecules with mirror symmetry perpendicular to the
3-fold axis are forbidden to have a fixed electrical dipole moment. However, if
the ground state is orbitally degenerate and lacks inversion symmetry, then a
``quantum'' dipole moment does exist. The system of 3 electrons in D_3h
symmetry is our example. This system is realized in triatomic molecules like
Na_3. Unlike the fixed dipole of a molecule like water, the quantum moment does
not point in a fixed direction, but lies in the plane of the molecule and takes
quantized values +/- mu_0 along any direction of measurement in the plane. An
electric field F in the plane leads to a linear Stark splitting +/- mu_0 F}. We
introduce a toy model to study the effect of Jahn-Teller distortions on the
quantum dipole moment. We find that the quantum dipole property survives when
the dynamic Jahn-Teller effect is included, if the distortion of the molecule
is small. Linear Stark splittings are suppressed in low fields by molecular
rotation, just as the linear Stark shift of water is suppressed, but will be
revealed in moderately large applied fields and low temperatures. Coulomb
correlations also give a partial suppression.Comment: 10 pages with 7 figures included; thoroughly revised with a new
coauthor; final minor change
Ferromagnetism in substituted zinc oxide
Room-temperature ferromagnetism is observed in (110) oriented ZnO films
containing 5 at % of Sc, Ti, V, Fe, Co or Ni, but not Cr, Mn or Cu ions. There
are large moments, 1.9 and 0.5 muB/atom for Co- and Ti-substituted oxides,
respectively. Sc-substituted ZnO shows also a moment of 0.3 muB/Sc.
Magnetization is very anisotropic, with variations of up to a factor three
depending on the orientation of the applied field relative to the R-cut
sapphire substrates. Results are interpreted in terms of a spin-split donor
impurity band model, which can account for ferromagnetism in insulating or
conducting high-k oxides with concentrations of magnetic ions that lie far
below the percolation threshold. The variation of the ferromagnetism with
oxygen pressure used during film growth is evidence of a link between
ferromagnetism and defect concentration.Comment: 15 pages, 4 figure
Theory of Umklapp-assisted recombination of bound excitons in Si:P
We present the calculations for the oscillator strength of the recombination
of excitons bound to phosphorous donors in silicon. We show that the direct
recombination of the bound exciton cannot account for the experimentally
measured oscillator strength of the no-phonon line. Instead, the recombination
process is assisted by an umklapp process of the donor electron state. We make
use of the empirical pseudopotential method to evaluate the Umklapp-assisted
recombination matrix element in second-order perturbation theory. Our result is
in excellent agreement with the experiment. We also present two methods to
improve the optical resolution of the optical detection of the spin state of a
single nucleus in silicon.Comment: 9 pages, 6 EPS figures, Revtex
Critical boron-doping levels for generation of dislocations in synthetic diamond
Defects induced by boron doping in diamond layers were studied by transmission electron microscopy. The existence of a critical boron doping level above which defects are generated is reported. This level is found to be dependent on the CH4
/H2 molar ratios and on growth directions. The critical boron concentration lied in the 6.5–17.0 X 10 20 at/cm3 range in the direction and at 3.2 X 1021 at/cm
3 for the one. Strain related effects induced by the doping are shown not to
be responsible. From the location of dislocations and their Burger vectors, a model is proposed, together with their generation mechanism.6 page
Outdoor Worker Sun Protection Project : a mixed methods intervention to improve sun safety among outdoor workers
Queensland, Australia has one of the highest rates of skin cancer in the world. Outdoor workers are regularly exposed to high doses of ultraviolet radiation, and are at increased risk to develop non-melanoma and melanoma skin cancers. In 2010, a health promotion intervention to improve sun protection among outdoor workers in Queensland commenced. The intervention employed a mixed methods approach and a participatory action research framework. Fourteen workplaces were recruited from building and construction, rural and farming, local government, and public sector organisations. Management and workers were engaged in cycles of assessment, reflection and discussion, planning, implementation and reassessing, over a 14-month intervention period. Overall, at least one workplace representative from each workplace (range 1-3) and in depth focus groups were held with a subset of workers (range 3-16) to assess sun safe behaviours pre and post intervention. Workers’ attitudes, beliefs, knowledge and willingness to engage in sun protection differed depending on workplace characteristics and support. A familiar theme among workers spoke of sun safety as being “common sense” and the “workers individual responsibility”. Often there was a discrepancy in the perceptions of the workers, compared to the view of workplace representatives and the workplaces position or policy on sun safety. In larger workplaces, especially Government Departments, workers were more aware and followed sun safe practices compared to smaller workplaces where sun safety was not a high priority. These results indicate that a workplace culture which places high values on safety and polices more broadly may also have a positive effect on sun safety among outdoor workers as well. In addition, the specific characteristics of the workplace and the outdoor work tasks influence workers willingness to engage in sun safety measures
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