813 research outputs found
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.
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
Single-ion and exchange anisotropy effects and multiferroic behavior in high-symmetry tetramer single molecule magnets
We study single-ion and exchange anisotropy effects in equal-spin
tetramer single molecule magnets exhibiting , , ,
, , or ionic point group symmetry. We first write the
group-invariant quadratic single-ion and symmetric anisotropic exchange
Hamiltonians in the appropriate local coordinates. We then rewrite these local
Hamiltonians in the molecular or laboratory representation, along with the
Dzyaloshinskii-Moriay (DM) and isotropic Heisenberg, biquadratic, and
three-center quartic Hamiltonians. Using our exact, compact forms for the
single-ion spin matrix elements, we evaluate the eigenstate energies
analytically to first order in the microscopic anisotropy interactions,
corresponding to the strong exchange limit, and provide tables of simple
formulas for the energies of the lowest four eigenstate manifolds of
ferromagnetic (FM) and anitiferromagnetic (AFM) tetramers with arbitrary .
For AFM tetramers, we illustrate the first-order level-crossing inductions for
, and obtain a preliminary estimate of the microscopic
parameters in a Ni from a fit to magnetization data.
Accurate analytic expressions for the thermodynamics, electron paramagnetic
resonance absorption and inelastic neutron scattering cross-section are given,
allowing for a determination of three of the microscopic anisotropy
interactions from the second excited state manifold of FM tetramers. We also
predict that tetramers with symmetries and should exhibit both
DM interactions and multiferroic states, and illustrate our predictions for
.Comment: 30 pages, 14 figures, submitted to Phys. Rev.
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.
RELATION BETWEEN BIRTH WEIGHT OF LAMBS AND MILK PRODUCTION IN EWES
Čini se da je proizvodnja mlijeka u ovaca povezana s težinom janjadi pri janjenju. To se vjerojatno može pripisati laktogenom hormonu placente čije izlučivanje ovisi o težini placente. Obavljena su 4 pokusna ispitivanja, 2 na ovcama Sarda (S) i 2 na Merino(M) i Awassi x Merino (A x M) ovcama, u razdoblju izmedu 5. i 8. tjedna post partum. Sakupljeni su podaci o tjelesnoj težini ovaca, težini janjadi pri janjenju i proizvodnji mlijeka. Analize linearne regresile između grama janjadi kod janjenja na kg tjelesne težine ovce te grama mlijeka proizvedenog na kg tjelesne težine ovce pokazuju da su ove dvije varijable u pozitivnoj uzajamnoj vezi i da uklanjanje mlijeka u slučaju janjaca blizanaca povećava davanje mlijeka.Milk production in sheep seem to be related to lamb weight at birth. This is probably due to the lactogen placental hormone whose secretion depends on the weight of the placenta. 4 experimental trials were carried out, 2 on Sarda ewes (S) and 2 on Merino (M) and Awasi x Merino (A x M) ewes, during the period between 5-th and 8-th week post partum. Data of body weight of ewes, weight at birth weight of lambs and milk production were collecteds. Linear regresion analysis between grams of lamb at birth per kg of ewe\u27s body weight and grams of milk produced per kg of ewe\u27s body weight show that the two variables are positively correlated and that milk removal by twin lambs increased milk yield
Experimental study of CO2 sequestration by ECBM recovery: the case of Sulcis coal.
An ECBM (Enhanced Coal Bed Methane) feasibility study started for the Sulcis Coal Province (SW Sardinia, Italy) in December 2004: geochemical, structural-geology, stratigraphic and reservoir engineering considerations are discussed. The first newly gathered experimental data are discussed, including: fluid geochemistry (major and minor elements, dissolved gases, C and He isotopic ratios) of the reservoir, coal composition and experimental data on CO2/CH4 adsorption on coal. A MapInfo GIS structure was built up including stratigraphical, geo-structural, hydrogeochemical, coal-compositional and environmental impact information as well as the CO2 sources location and typology. Even if these data could be preliminary with respect to the coal characteritics effectively located at the future injection depth, they highlighted both the challenging positive and negative aspects of the Sulcis Coal Province versus the exploitation of the ECBM technique. The most important objective of this phase I of the project is the selection of the best Sulcis ECBM test-pilot site, which will be followed (Phase II) by the choice of a scaled up site and possibly by a future network (Phase III). These phases are foreseen to be accompanied by the selection of progressively added CO2 industrial sources, to be used within the project economic spreadsheet model, actually in evolution.
CO2 geological storage and CH4 production potentials in Sulcis have been grossly evaluated as a whole, in the frame of the Sardinia region CO2 sources, including the coal-fired power plants, both existent and foreseen (hundreds of millions of tonns of CO2 are possible to be stored underground in the next decades). The reservoir estimates, both for the CO2 injection and for the CH4 production are clearly involving to start the test-site phase exploitation, in the frame of an auspicabile international operative project
Thermal anomalies and fluid geochemistry framework in occurrence of the 2000-2001 Nizza Monferrate seismic sequence (northern Italy): Episodic changes in the fault zone heat flow or chemical mixing phenomena?
International audienceThe paper discusses the correlation between the heating of shallow groundwater over a 10 × 20 km wide area close to the town of Nizza Monferrato (Piemonte Region, Northern Italy) and the concomitant local seismic sequences during the period August 2000 ? July 2001. The first seismic sequence started on 21 August 2000 with a Ml = 5.2 earthquake. Within few hours, the local authorities received calls alerting that the groundwater temperature rose from 10 to 30°C in many shallow wells. Our geochemical experimental data and the geological-seismotectonic framework do not allow the hypothesis of simple fluid mixing between the thermal reservoir of Acqui Terme and the Nizza-Monferrato shallow groundwater to explain the observed thermal anomalies. On the other hand, we invoke more complex processes such as frictional heating, mechano-chemistry, fault-valve mechanism, adiabatic decompression and hydrogeologically driven heat flow i.e., thermal effects due to variations of basin-scale permeability field. All these processes are able to transmit heat to the surface and to generate a transient incremental heat flow better than the mass transfer occurring typically when fluids from different reservoirs mix
Exchange-coupling constants, spin density map, and Q dependence of the inelastic neutron scattering intensity in single-molecule magnets
The Q dependence of the inelastic neutron scattering (INS) intensity of
transitions within the ground-state spin multiplet of single-molecule magnets
(SMMs) is considered. For these transitions, the Q dependence is related to the
spin density map in the ground state, which in turn is governed by the
Heisenberg exchange interactions in the cluster. This provides the possibility
to infer the exchange-coupling constants from the Q dependence of the INS
transitions within the spin ground state. The potential of this strategy is
explored for the M = +-10 -> +- 9 transition within the S = 10 multiplet of the
molecule Mn12 as an example. The Q dependence is calculated for powder as well
as single-crystal Mn12 samples for various exchange-coupling situations
discussed in the literature. The results are compared to literature data on a
powder sample of Mn12 and to measurements on an oriented array of about 500
single-crystals of Mn12. The calculated Q dependence exhibits significant
variation with the exchange-coupling constants, in particular for a
single-crystal sample, but the experimental findings did not permit an
unambiguous determination. However, although challenging, suitable experiments
are within the reach of today's instruments.Comment: 11 pages, 6 figures, REVTEX4, to appear in PR
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