1,783 research outputs found
Theory of spin-polarized scanning tunneling microscopy applied to local spins
We provide a theory for scanning tunneling microscopy and spectroscopy using
a spin-polarized tip. It it shown that the tunneling conductance can be
partitioned into three separate contributions, a background conductance which
is independent of the local spin, a dynamical conductance which is proportional
to the local spin moment, and a conductance which is proportional to the noise
spectrum of the local spin interactions. The presented theory is applicable to
setups with magnetic tip and substrate in non-collinear arrangement, as well as
for non-magnetic situations. The partitioning of the tunneling current suggests
a possibility to extract the total spin moment of the local spin from the
dynamical conductance. The dynamical conductance suggests a possibility to
generate very high frequency spin-dependent ac currents and/or voltages. We
also propose a measurement of the dynamical conductance that can be used to
determine the character of the effective exchange interaction between
individual spins in clusters. The third contribution to the tunneling current
is associated with the spin-spin correlations induced by the exchange
interaction between the local spin moment and the tunneling electrons. We
demonstrate how this term can be used in the analysis of spin excitations
recorded in conductance measurements. Finally, we propose to use spin-polarized
scanning tunneling microscopy for detailed studies of the spin excitation
spectrum.Comment: 12 pages, 4 figure, updated to match the published version, to appear
in the Phys. Rev.
The electronic and magnetic properties of anion doped (C, N, S) GaFeO3; an ab initio DFT study
AbstractIn this study we present ab initio DFT calculations performed on stoichiometric and anion doped GaFeO3 substituting O by a C, N and S atom, respectively. Stoichiometric GaFeO3 has an antiferromagnetic (AFM) ground state. The Fe atoms of the sublattices Fe1 and Fe2 couple antiferromagnetically via the O atoms through the superexchange mechanism. Replacing the superexchange mediating O atom with p-elements of a different valence electron configuration changes the underlying magnetic exchange mechanism and influence the ground state properties. This may be used for tuning properties interesting for technical applications. Four different doping configurations were examined revealing a cell site dependent influence on the magnetic properties. Carbon, for example, changes the AFM coupling present in the Fe1–O–Fe2 configuration into a ferrimagnetic exchange for the Fe1–C–Fe2 bond. Depending on the respective cell site C substitution introduces a ferrimagnetic or AFM ground state. Nitrogen alters the ground state magnetic moment as well and sulfur introduces large structural distortions affecting the band gap and the overall AFM coupling inside the doped GaFeO3 simulation cell. We give a detailed discussion on the respective magnetic exchange mechanisms and electronic properties with regard to applications as photocatalysis and use the predictive power of ab initio DFT simulations that may trigger future experiments in the very promising field of tunable multifunctional devices
Magnetic properties of 3d-impurities substituted in GaAs
We have calculated the magnetic properties of substituted 3d-impurities
(Cr-Ni) in a GaAs host by means of first principles electronic structure
calculations. We provide a novel model explaining the ferromagnetic long rang
order of III-V dilute magnetic semiconductors. The origin of the ferromagnetism
is shown to be due to delocalized spin-uncompensated As dangling bond
electrons. Besides the quantitative prediction of the magnetic moments, our
model provides an understanding of the halfmetallicity, and the raise of the
critical temperature with the impurity concentration
Spontaneous separation of two-component Fermi gases in a double-well trap
The two-component Fermi gas in a double-well trap is studied using the
density functional theory and the density profile of each component is
calculated within the Thomas-Fermi approximation. We show that the two
components are spatially separate in the two wells once the repulsive
interaction exceeds the Stoner point, signaling the occurrence of the
ferromagnetic transition. Therefore, the double-well trap helps to explore
itinerant ferromagnetism in atomic Fermi gases, since the spontaneous
separation can be examined by measuring component populations in one well.Comment: 6 pages, 6 figures, to appear in ep
Stable longitudinal associations of family income with children's hippocampal volume and memory persist after controlling for polygenic scores of educational attainment
Despite common notion that the correlation of socioeconomic status with child cognitive performance may be driven by both environmentally- and genetically-mediated transactional pathways, there is a lack of longitudinal and genetically informed research that examines these postulated associations. The present study addresses whether family income predicts associative memory growth and hippocampal development in middle childhood and tests whether these associations persist when controlling for DNA-based polygenic scores of educational attainment. Participants were 142 6-to-7-year-old children, of which 127 returned when they were 8-to-9 years old. Longitudinal analyses indicated that the association of family income with children's memory performance and hippocampal volume remained stable over this age range and did not predict change. On average, children from economically disadvantaged background showed lower memory performance and had a smaller hippocampal volume. There was no evidence to suggest that differences in memory performance were mediated by differences in hippocampal volume. Further exploratory results suggested that the relationship of income with hippocampal volume and memory in middle childhood is not primarily driven by genetic variance captured by polygenic scores of educational attainment, despite the fact that polygenic scores significantly predicted family income
Continuous isotopic composition measurements of tropospheric CO<sub>2</sub> at Jungfraujoch (3580 m a.s.l.), Switzerland: real-time observation of regional pollution events
A quantum cascade laser based absorption spectrometer (QCLAS) is applied for the first time to perform in situ, continuous and high precision isotope ratio measurements of CO<sub>2</sub> in the free troposphere. Time series of the three main CO<sub>2</sub> isotopologue mixing ratios (<sup>12</sup>C<sup>16</sup>CO<sub>2</sub>, <sup>13</sup>C<sup>16</sup>CO<sub>2</sub> and <sup>12</sup>C<sup>18</sup>O<sup>16</sup>O) have simultaneously been measured at one second time resolution over two years (from August 2008 to present) at the High Altitude Research Station Jungfraujoch (3580 m a.s.l., Switzerland). This work focuses on periods in February 2009 only, when sudden and pronounced enhancements in the tropospheric CO<sub>2</sub> were observed. These short-term changes were closely correlated with variations in CO mixing ratios measured at the same site, indicating combustion related emissions as potential source. The analytical precision of 0.046‰ (at 50 s integration time) for both δ<sup>13</sup>C and δ<sup>18</sup>O and the high temporal resolution allowed the application of the Keeling plot method for source signature identification. The spatial origin of these CO<sub>2</sub> emission sources was then determined by backward Lagrangian particle dispersion simulations
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