1,212 research outputs found
Spin and exchange coupling for Ti embedded in a surface dipolar network
We have studied the spin and exchange coupling of Ti atoms on a
CuN/Cu(100) surface using density functional theory. We find that
individual Ti have a spin of 1.0 (i.e., 2 Bohr Magneton) on the CuN/Cu(100)
surface instead of spin-1/2 as found by Scanning Tunneling Microscope. We
suggest an explanation for this difference, a two-stage Kondo effect, which can
be verified by experiments. By calculating the exchange coupling for Ti dimers
on the CuN/Cu(100) surface, we find that the exchange coupling across a
`void' of 3.6\AA\ is antiferromagnetic, whereas indirect (superexchange)
coupling through a N atom is ferromagnetic. We confirm the existence of
superexchange interactions by varying the Ti-N angle in a model trimer
calculation. For a square lattice of Ti on CuN/Cu(100), we find a novel
spin striped phase
Magnetic ground state and 2D behavior in pseudo-Kagome layered system Cu3Bi(SeO3)2O2Br
Anisotropic magnetic properties of a layered kagome-like system
Cu3Bi(SeO3)2O2Br have been studied by bulk magnetization and magnetic
susceptibility measurements as well as powder and single-crystal neutron
diffraction. At T_N = 27.4 K the system develops an alternating
antiferromagnetic order of (ab) layers, which individually exhibit canted
ferrimagnetic moment arrangement, resulting from the competing ferro- and
antiferro-magnetic intralayer exchange interactions. A magnetic field B_C ~ 0.8
T applied along the c axis (perpendicular to the layers) triggers a
metamagnetic transition, when every second layer flips, i.e., resulting in a
ferrimagnetic structure. Significantly higher fields are required to rotate the
ferromagnetic component towards the b axis (~7 T) or towards the a axis (~15
T). The estimates of the exchange coupling constants and features indicative of
an XY character of this quasi-2D system are presented.Comment: 7 pages, 6 figures, final versio
Many-spin effects in inelastic neutron scattering and electron paramagnetic resonance of molecular nanomagnets
Many molecular magnetic clusters, such as single-molecule magnets, are
characterized by spin ground states with defined total spin S exhibiting
zero-field-splittings. In this work, the spectroscopic intensities of the
transitions within the ground-state multiplet are analyzed. In particular, the
effects of a mixing with higher-lying spin multiplets, which is produced by
anisotropic interactions and is neglected in the standard single-spin
description, are investigated systematically for the two experimental
techniques of inelastic neutron scattering (INS) and electron paramagnetic
resonance (EPR), with emphasis on the former technique. The spectroscopic
transition intensities are calculated analytically by constructing
corresponding effective spin operators perturbationally up to second order and
consequently using irreducible tensor operator techniques. Three main effects
of spin mixing are observed. Firstly, a pronounced dependence of the INS
intensities on the momentum transfer Q, with a typical oscillatory behavior,
emerges in first order, signaling the many-spin nature of the wave functions in
exchange-coupled clusters. Secondly, as compared to the results of a
first-order calculation, the intensities of the transitions within the spin
multiplet are affected differently by spin mixing. This allows one, thirdly, to
differentiate the higher-order contributions to the cluster magnetic anisotropy
which come from the single-ion ligand-field terms and spin mixing,
respectively. The analytical results are illustrated by means of the three
examples of an antiferromagnetic heteronuclear dimer, the Mn-[3 x 3] grid
molecule, and the single-molecule magnet Mn12.Comment: 18 pages, 3 figures, REVTEX4, to appear in PR
Spin anisotropy effects in dimer single molecule magnets
We present a model of equal spin dimer single molecule magnets. The
spins within each dimer interact via the Heisenberg and the most general set of
four quadratic anisotropic spin interactions with respective strengths and
, and with the magnetic induction . We solve the model
exactly for , and for antiferromagnetic Heisenberg couplings
(), present curves at low for these cases. Low-
curves for and electron paramagnetic susceptibility
for are also provided. For weak anisotropy
interactions, we employ a perturbative treatment, and show that the Hartree and
extended Hartree approximations lead to reliable analytic results at low
and large for these quantities and for the inelastic neutron scattering
cross-section . Our results are discussed with
regard to existing experiments on Fe dimer
single molecule magnets, and suggest that one of them contains a substantial
amount of single-ion anisotropy, without any sizeable global spin anisotropy.
We urge further experiments of the above types on single crystals of Fe and
on some [Mn] dimers, in order to elucidate the precise values
of the various microscopic interactions.Comment: 30 pages, 25 figures, submitted to Phys. Rev.
Direct Observation of Quantum Coherence in Single-Molecule Magnets
Direct evidence of quantum coherence in a single-molecule magnet in frozen
solution is reported with coherence times as long as T2 = 630 ns. We can
strongly increase the coherence time by modifying the matrix in which the
single-molecule magnets are embedded. The electron spins are coupled to the
proton nuclear spins of both the molecule itself and interestingly, also to
those of the solvent. The clear observation of Rabi oscillations indicates that
we can manipulate the spin coherently, an essential prerequisite for performing
quantum computations.Comment: 5 Pages, 4 Figures, final version published in PR
Mandible reconstruction with iliac crest graft and implant assisted prosthesis after resection of mandibular myxoma
La reconstrucción de los maxilares luego de las resecciones por tumores odontogénicos agresivos sigue siendo un tema de debate entre los cirujanos bucomaxilofaciales, en cuanto a la técnica a emplear y los momentos oportunos para implementarla. En nuestra experiencia, ante la presencia de tumores odontogénicos agresivos, la resección con margen de seguridad es mandataria, y de ser posible la reconstrucción inmediata con injerto de cresta ilíaca para una posterior rehabilitación oral implanto-asistida. En el presente trabajo reportamos un casos clínico de una paciente con un mixoma mandibular resecado y reconstruido en forma inmediata con injerto de cresta ilíaca y su posterior rehabilitación protética implanto asistida.Mandible reconstruction after resection due to aggressive odontogenic tumors continues to be a subject of discussion among buco-maxillofacial surgeons, especially regarding the technique used and the appropriate time to implement it. In our experience, in the presence of aggressive odontogenic tumors, resection with safety margins is compulsory, if possible, accompanied by the immediate reconstruction with iliac crest graft for subsequent implantassisted oral rehab. A case report of a patient with resected mandibular myxoma immediately reconstructed with iliac crest graft and implant-assisted prosthetic rehab is described in this paper.Facultad de Odontologí
Spin electric effects in molecular antiferromagnets
Molecular nanomagnets show clear signatures of coherent behavior and have a
wide variety of effective low-energy spin Hamiltonians suitable for encoding
qubits and implementing spin-based quantum information processing. At the
nanoscale, the preferred mechanism for control of quantum systems is through
application of electric fields, which are strong, can be locally applied, and
rapidly switched. In this work, we provide the theoretical tools for the search
for single molecule magnets suitable for electric control. By group-theoretical
symmetry analysis we find that the spin-electric coupling in triangular
molecules is governed by the modification of the exchange interaction, and is
possible even in the absence of spin-orbit coupling. In pentagonal molecules
the spin-electric coupling can exist only in the presence of spin-orbit
interaction. This kind of coupling is allowed for both and
spins at the magnetic centers. Within the Hubbard model, we find a relation
between the spin-electric coupling and the properties of the chemical bonds in
a molecule, suggesting that the best candidates for strong spin-electric
coupling are molecules with nearly degenerate bond orbitals. We also
investigate the possible experimental signatures of spin-electric coupling in
nuclear magnetic resonance and electron spin resonance spectroscopy, as well as
in the thermodynamic measurements of magnetization, electric polarization, and
specific heat of the molecules.Comment: 31 pages, 24 figure
Pressure Dependence of the Magnetic Anisotropy in the "Single-Molecule Magnet" [Mn4O3Br(OAc)3(dbm)3]
The anisotropy splitting in the ground state of the single-molecule magnet
[Mn4O3Br(OAc)3(dbm)3] is studied by inelastic neutron scattering as a function
of hydrostatic pressure. This allows a tuning of the anisotropy and thus the
energy barrier for slow magnetisation relaxation at low temperatures. The value
of the negative axial anisotropy parameter changes from
-0.0627(1) meV at ambient to -0.0603(3) meV at 12 kbar pressure, and in the
same pressure range the height of the energy barrier between up and down spins
is reduced from 1.260(5) meV to 1.213(9) meV. Since the bond is
significantly softer and thus more compressible than the bonds,
pressure induces a tilt of the single ion Mn anisotropy axes, resulting
in the net reduction of the axial cluster anisotropy.Comment: 4 pages, 3 figure
Bounding and approximating parabolas for the spectrum of Heisenberg spin systems
We prove that for a wide class of quantum spin systems with isotropic
Heisenberg coupling the energy eigenvalues which belong to a total spin quantum
number S have upper and lower bounds depending at most quadratically on S. The
only assumption adopted is that the mean coupling strength of any spin w.r.t.
its neighbours is constant for all N spins. The coefficients of the bounding
parabolas are given in terms of special eigenvalues of the N times N coupling
matrix which are usually easily evaluated. In addition we show that the
bounding parabolas, if properly shifted, provide very good approximations of
the true boundaries of the spectrum. We present numerical examples of
frustrated rings, a cube, and an icosahedron.Comment: 8 pages, 3 figures. Submitted to Europhysics Letter
Spin-Triplet Excitons in the Gapped Antiferromagnet BaCuSiO: Electron Paramagnetic Resonance Studies
BaCuSiO, a quantum antiferromagnet with a double-layer
structure of Cu ions in a distorted planar-rectangular coordination and
with a dimerized spin singlet ground state, is studied by means of the electron
paramagnetic resonance technique. It is argued that multiple absorptions
observed at low temperatures are intimately related to a thermally-activated
spin-triplet exciton superstructure. Analysis of the angular dependence of
exciton modes in BaCuSiO allows us to accurately estimate anisotropy
parameters. In addition, the temperature dependence of EPR intensity and
linewidth is discussed.Comment: Submitted to Phys. Rev.
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