6,043 research outputs found
Spin-lattice order in frustrated ZnCr2O4
Using synchrotron X-rays and neutron diffraction we disentangle spin-lattice
order in highly frustrated ZnCrO where magnetic chromium ions occupy
the vertices of regular tetrahedra. Upon cooling below 12.5 K the quandary of
anti-aligning spins surrounding the triangular faces of tetrahedra is resolved
by establishing weak interactions on each triangle through an intricate lattice
distortion. The resulting spin order is however, not simply a N\'{e}el state on
strong bonds. A complex co-planar spin structure indicates that antisymmetric
and/or further neighbor exchange interactions also play a role as ZnCrO
resolves conflicting magnetic interactions
High-magnetic field phase diagram and failure of magnetic Gr\"uneisen scaling in LiFePO
We report the magnetic phase diagram of single-crystalline LiFePO in
magnetic fields up to 58~T and present a detailed study of magneto-elastic
coupling by means of high-resolution capacitance dilatometry. Large anomalies
at \tn\ in the thermal expansion coefficient imply pronounced
magneto-elastic coupling. Quantitative analysis yields the magnetic Gr\"uneisen
parameter ~mol/J. The positive
hydrostatic pressure dependence ~K/GPa is dominated
by uniaxial effects along the -axis. Failure of Gr\"uneisen scaling below
~K, i.e., below the peak temperature in the magneto-electric
coupling coefficient [\onlinecite{toft2015anomalous}], implies several
competing degrees of freedom and indicates relevance of recently observed
hybrid excitations~[\onlinecite{yiu2017hybrid}]. A broad and strongly
magnetic-field-dependent anomaly in in this temperature regime
highlight the relevance of structure changes. Upon application of magnetic
fields -axis, a pronounced jump in the magnetisation implies
spin-reorientation at ~T as well as a precursing phase at 29~T
and ~K. In a two-sublattice mean-field model, the saturation field
~T enables the determination of the effective
antiferromagnetic exchange interaction ~meV as well as
the anisotropies ~meV and ~meV
Anisotropic strains and magnetoresistance of La_{0.7}Ca_{0.3}MnO_{3}
Thin films of perovskite manganite La_{0.7}Ca_{0.3}MnO_{3} were grown
epitaxially on SrTiO_3(100), MgO(100) and LaAlO_3(100) substrates by the pulsed
laser deposition method. Microscopic structures of these thin film samples as
well as a bulk sample were fully determined by x-ray diffraction measurements.
The unit cells of the three films have different shapes, i.e., contracted
tetragonal, cubic, and elongated tetragonal for SrTiO_3, MgO, and LaAlO_3
cases, respectively, while the unit cell of the bulk is cubic. It is found that
the samples with cubic unit cell show smaller peak magnetoresistance than the
noncubic ones do. The present result demonstrates that the magnetoresistance of
La_{0.7}Ca_{0.3}MnO_{3} can be controlled by lattice distortion via externally
imposed strains.Comment: Revtex, 10 pages, 2 figure
Refined Simulations of the Reaction Front for Diffusion-Limited Two-Species Annihilation in One Dimension
Extensive simulations are performed of the diffusion-limited reaction
AB in one dimension, with initially separated reagents. The reaction
rate profile, and the probability distributions of the separation and midpoint
of the nearest-neighbour pair of A and B particles, are all shown to exhibit
dynamic scaling, independently of the presence of fluctuations in the initial
state and of an exclusion principle in the model. The data is consistent with
all lengthscales behaving as as . Evidence of
multiscaling, found by other authors, is discussed in the light of these
findings.Comment: Resubmitted as TeX rather than Postscript file. RevTeX version 3.0,
10 pages with 16 Encapsulated Postscript figures (need epsf). University of
Geneva preprint UGVA/DPT 1994/10-85
The definability criterions for convex projective polyhedral reflection groups
Following Vinberg, we find the criterions for a subgroup generated by
reflections \Gamma \subset \SL^{\pm}(n+1,\mathbb{R}) and its finite-index
subgroups to be definable over where is an integrally
closed Noetherian ring in the field . We apply the criterions for
groups generated by reflections that act cocompactly on irreducible properly
convex open subdomains of the -dimensional projective sphere. This gives a
method for constructing injective group homomorphisms from such Coxeter groups
to \SL^{\pm}(n+1,\mathbb{Z}). Finally we provide some examples of
\SL^{\pm}(n+1,\mathbb{Z})-representations of such Coxeter groups. In
particular, we consider simplicial reflection groups that are isomorphic to
hyperbolic simplicial groups and classify all the conjugacy classes of the
reflection subgroups in \SL^{\pm}(n+1,\mathbb{R}) that are definable over
. These were known by Goldman, Benoist, and so on previously.Comment: 31 pages, 8 figure
Spontaneous Formation of Gold Nanoparticles on Graphene by Galvanic Reaction through Graphene
We demonstrate an effective and facile method for the deposition of gold nanoparticles (AuNPs) on graphene by using spontaneous galvanic reaction. Despite the interest and importance of the hybrid structure of noble metal-deposited graphene has been considerably increased for its fundamental knowledge in chemical and physical sciences and for its various applications, the progress of this subject is very slow mainly because of the lack of synthetic methods for such structures, especially that are not free from chemical contamination and usage of complex and expensive equipment. Therefore, we developed a new method allowing chemically pure AuNPs/graphene hybrid structures employing galvanic reaction. The spontaneous galvanic reaction was derived from reductant/graphene/oxidant sandwich structures, such as Au ions/graphene/Ge wafer and Au ions/graphene/copper foil, by placing Au ion solution droplets on graphene transferred on a germanium wafer or as made graphene on Cu foil, respectively. According to scanning electron microscopy and atomic force microscopy results, it was confirmed that AuNPs were successfully formed on the graphene surface. This result implies two important points. One is that the formation of pure AuNPs on graphene is possible without using other chemicals frequently required for conventional NP preparation. The other one is that it was experimentally demonstrated that there are electronic communications between the oxidant and reductant that are separated by graphene, through which electrons can pass freely.11Ysciescopu
Orbital Correlations in Doped Manganites
We review our recent x-ray scattering studies of charge and orbital order in
doped manganites, with specific emphasis on the role of orbital correlations in
Pr_1-xCa_xMnO_3. For x=0.25, we find an orbital structure indistinguishable
from the undoped structure with long range orbital order at low temperatures.
For dopings 0.3<x<0.5, we find scattering consistent with a charge and
orbitally ordered CE-type structure. While in each case the charge order peaks
are resolution limited, the orbital order exhibits only short range
correlations. We report the doping dependence of the correlation length and
discuss the connection between the orbital correlations and the finite magnetic
correlation length observed on the Mn^3+ sublattice with neutron scattering
techniques. The physical origin of these domains, which appear to be isotropic,
remains unclear. We find that weak orbital correlations persist well above the
phase transitions, with a correlation length of 1-2 lattice constants at high
temperatures. Significantly, we observe similar correlations at high
temperatures in La_0.7Ca_0.3MnO_3, which does not have an orbitally ordered
ground state, and we conclude that such correlations are robust to variations
in the relative strength of the electron-phonon coupling.Comment: 22 pagegs, 7 figure
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