18,722 research outputs found
Heisenberg exchange in magnetic monoxides
The superexchange intertacion in transition-metal oxides, proposed initially
by Anderson in 1950, is treated using contemporary tight-binding theory and
existing parameters. We find also a direct exchange for nearest-neighbor metal
ions, larger by a factor of order five than the superexchange. This direct
exchange arises from Vddm coupling, rather than overlap of atomic charge
densities, a small overlap exchange contribution which we also estimate. For
FeO and CoO there is also an important negative contribution, related to Stoner
ferromagnetism, from the partially filled minority-spin band which broadens
when ionic spins are aligned. The corresponding J1 and J2 parameters are
calculated for MnO, FeO, CoO, and NiO. They give good accounts of the Neel and
the Curie-Weiss temperatures, show appropriate trends, and give a reasonable
account of their volume dependences. For MnO the predicted value for the
magnetic susceptibility at the Neel temperature and the crystal distortion
arising from the antiferromagnetic transition were reasonably well given.
Application to CuO2 planes in the cuprates gives J=1220oK, compared to an
experimental 1500oK, and for LiCrO2 gives J1=4 50oK compared to an experimental
230oK.Comment: 21 pages, 1 figure, submitted to Phys. Rev. B 1/19/07. Realized
J=4V^2/U applies generally, as opposed to J=2V^2/U from one-electron theory
(1/28 revision
Electronic structure and magnetic properties of pyroxenes (Li,Na)TM(Si,Ge)2O6: novel low-dimensional magnets with 90 bonds
The results of the LSDA+U calculations for pyroxenes with diverse magnetic
properties (Li,Na)TM(Si,Ge)O, where TM is the transition metal ion
(Ti,V,Cr,Mn,Fe), are presented. We show that the anisotropic orbital ordering
results in the spin-gap formation in NaTiSiO. The detailed analysis of
different contributions to the intrachain exchange interactions for pyroxenes
is performed both analytically using perturbation theory and basing on the
results of the band structure calculations. The antiferromagnetic
exchange is found to decrease gradually in going from Ti to Fe.
It turns out to be nearly compensated by ferromagnetic interaction between
half-filled and empty orbitals in Cr-based pyroxenes. The
fine-tuning of the interaction parameters by the crystal structure results in
the ferromagnetism for NaCrGeO. Further increase of the total number of
electrons and occupation of sub-shell makes the contribution
and total exchange interaction antiferromagnetic for Mn- and Fe-based
pyroxenes. Strong oxygen polarization was found in Fe-based pyroxenes. It is
shown that this effect leads to a considerable reduction of antiferromagnetic
intrachain exchange. The obtained results may serve as a basis for the analysis
of diverse magnetic properties of pyroxenes, including those with recently
discovered multiferroic behavior.Comment: 11 pages, 10 figure
Growth of Epitaxial Oxide Thin Films on Graphene
The transfer process of graphene onto the surface of oxide substrates is well known. However, for many devices, we require high quality oxide thin films on the surface of graphene. This step is not understood. It is not clear why the oxide should adopt the epitaxy of the underlying oxide layer when it is deposited on graphene where there is no lattice match. To date there has been no explanation or suggestion of mechanisms which clarify this step. Here we show a mechanism, supported by first principles simulation and structural characterisation results, for the growth of oxide thin films on graphene. We describe the growth of epitaxial SrTiO3 (STO) thin films on a graphene and show that local defects in the graphene layer (e.g. grain boundaries) act as bridgepillar spots that enable the epitaxial growth of STO thin films on the surface of the graphene layer. This study, and in particular the suggestion of a mechanism for epitaxial growth of oxides on graphene, offers new directions to exploit the development of oxide/graphene multilayer structures and devices
Thermal structure and exhumation history of the Lesser Himalaya in central Nepal
The Lesser Himalaya (LH) consists of metasedimentary rocks that have been scrapped off from the underthrusting Indian crust and accreted to the mountain range over the last ~20 Myr. It now forms a significant fraction of the Himalayan collisional orogen. We document the kinematics and thermal metamorphism associated with the deformation and exhumation of the LH, combining thermometric and thermochronological methods with structural geology. Peak metamorphic temperatures estimated from Raman spectroscopy of carbonaceous material decrease gradually from 520°–550°C below the Main Central Thrust zone down to less than 330°C. These temperatures describe structurally a 20°–50°C/km inverted apparent gradient. The Ar muscovite ages from LH samples and from the overlying crystalline thrust sheets all indicate the same regular trend; i.e., an increase from about 3–4 Ma near the front of the high range to about 20 Ma near the leading edge of the thrust sheets, about 80 km to the south. This suggests that the LH has been exhumed jointly with the overlying nappes as a result of overthrusting by about 5 mm/yr. For a convergence rate of about 20 mm/yr, this implies underthrusting of the Indian basement below the Himalaya by about 15 mm/yr. The structure, metamorphic grade and exhumation history of the LH supports the view that, since the mid-Miocene, the Himalayan orogen has essentially grown by underplating, rather than by frontal accretion. This process has resulted from duplexing at a depth close to the brittle-ductile transition zone, by southward migration of a midcrustal ramp along the Main Himalayan Thrust fault, and is estimated to have resulted in a net flux of up to 150 m^2/yr of LH rocks into the Himalayan orogenic wedge. The steep inverse thermal gradient across the LH is interpreted to have resulted from a combination of underplating and post metamorphic shearing of the underplated units
Local Structure and It's Effect on The Ferromagnetic Properties of LaSrCoO thin films}
We have used high-resolution Extended X-ray Absorption Fine-Structure and
diffraction techniques to measure the local structure of strained
LaSrCoO films under compression and tension. The lattice
mismatch strain in these compounds affects both the bond lengths and the bond
angles, though the larger effect on the bandwidth is due to the bond length
changes. The popular double exchange model for ferromagnetism in these
compounds provides a correct qualitative description of the changes in Curie
temperature , but quantitatively underestimates the changes. A microscopic
model for ferromagnetism that provides a much stronger dependence on the
structural distortions is needed.Comment: 4 pages, 4 figure
Quantum Criticality in an Organic Magnet
Exchange interactions between sites in piperazinium
hexachlorodicuprate produce a frustrated bilayer magnet with a singlet ground
state. We have determined the field-temperature phase diagram by high field
magnetization and neutron scattering experiments. There are two quantum
critical points: T separates a quantum paramagnet phase from a
three dimensional, antiferromagnetically-ordered state while T
marks the onset of a fully polarized state. The ordered phase, which we
describe as a magnon Bose-Einstein condensate (BEC), is embedded in a quantum
critical regime with short range correlations. A low temperature anomaly in the
BEC phase boundary indicates that additional low energy features of the
material become important near .Comment: 4 pages, 4 figures, submitted to Phys. Rev. Lett. Replaced original
text with additional conten
Anomalous Suppression of Valley Splittings in Lead Salt Nanocrystals without Inversion Center
Atomistic sp3d5s* tight-binding theory of PbSe and PbS nanocrystals is
developed. It is demonstrated, that the valley splittings of confined electrons
and holes strongly and peculiarly depend on the geometry of a nanocrystal. When
the nanocrystal lacks a microscopic center of inversion and has T_d symmetry,
the splitting is strongly suppressed as compared to the more symmetric
nanocrystals with O_h symmetry, having an inversion center.Comment: 5 pages, 4 figures, 1 tabl
Matching Conditions in Atomistic-Continuum Modeling of Materials
A new class of matching condition between the atomistic and continuum regions
is presented for the multi-scale modeling of crystals. They ensure the accurate
passage of large scale information between the atomistic and continuum regions
and at the same time minimize the reflection of phonons at the interface. These
matching conditions can be made adaptive if we choose appropriate weight
functions. Applications to dislocation dynamics and friction between
two-dimensional atomically flat crystal surfaces are described.Comment: 6 pages, 4 figure
Contact of Single Asperities with Varying Adhesion: Comparing Continuum Mechanics to Atomistic Simulations
Atomistic simulations are used to test the equations of continuum contact
mechanics in nanometer scale contacts. Nominally spherical tips, made by
bending crystals or cutting crystalline or amorphous solids, are pressed into a
flat, elastic substrate. The normal displacement, contact radius, stress
distribution, friction and lateral stiffness are examined as a function of load
and adhesion. The atomic scale roughness present on any tip made of discrete
atoms is shown to have profound effects on the results. Contact areas, local
stresses, and the work of adhesion change by factors of two to four, and the
friction and lateral stiffness vary by orders of magnitude. The microscopic
factors responsible for these changes are discussed. The results are also used
to test methods for analyzing experimental data with continuum theory to
determine information, such as contact area, that can not be measured directly
in nanometer scale contacts. Even when the data appear to be fit by continuum
theory, extracted quantities can differ substantially from their true values
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