107 research outputs found
Evidences of spin-temperature in Dynamic Nuclear Polarization: an exact computation of the EPR spectrum
In dynamic nuclear polarization (DNP) experiments, the compound is driven
out-of-equilibrium by microwave (MW) irradiation of the radical electron spins.
Their stationary state has been recently probed via electron double resonance
(ELDOR) techniques showing, at low temperature, a broad depolarization of the
electron paramagnetic resonance (EPR) spectrum under microwave irradiation. In
this theoretical manuscript, we develop a numerical method to compute exactly
the EPR spectrum in presence of dipolar interactions. Our results reproduce the
observed broad depolarisation and provide a microscopic justification for
spectral diffusion mechanism. We show the validity of the spin-temperature
approach for typical radical concentration used in dissolution DNP protocols.
In particular once the interactions are properly taken into account, the
spin-temperature is consistent with the non-monotonic behavior of the EPR
spectrum with a wide minimum around the irradiated frequency.Comment: 8 pages, 7 figures. Title and abstract change
Relevance of electron spin dissipative processes to dynamic nuclear polarization via thermal mixing
The available theoretical approaches aiming at describing Dynamic Nuclear
spin Polarization (DNP) in solutions containing molecules of biomedical
interest and paramagnetic centers are not able to model the behaviour observed
upon varying the concentration of trityl radicals or the polarization
enhancement caused by moderate addition of gadolinium complexes. In this
manuscript, we first show experimentally that the nuclear steady state
polarization reached in solutions of pyruvic acid with 15 mM trityl radicals is
substantially independent from the average internuclear distance. This
evidences a leading role of electron (over nuclear) spin relaxation processes
in determining the ultimate performances of DNP. Accordingly, we have devised a
variant of the Thermal Mixing model for inhomogenously broadened electron
resonance lines which includes a relaxation term describing the exchange of
magnetic anisotropy energy of the electron spin system with the lattice. Thanks
to this additional term, the dependence of the nuclear polarization on the
electron concentration can be properly accounted for. Moreover, the model
predicts a strong increase of the final polarization on shortening the electron
spin-lattice relaxation time, providing a possible explanation for the effect
of gadolinium doping.Comment: 13 pages, 12 figure
Tuning the magnetic and structural phase transitions of PrFeAsO via Fe/Ru spin dilution
Neutron diffraction and muon spin relaxation measurements are used to obtain
a detailed phase diagram of Pr(Fe,Ru)AsO. The isoelectronic substitution of Ru
for Fe acts effectively as spin dilution, suppressing both the structural and
magnetic phase transitions. The temperature of the tetragonal-orthorhombic
structural phase transition decreases gradually as a function of x. Slightly
below the transition temperature coherent precessions of the muon spin are
observed corresponding to static magnetism, possibly reflecting a significant
magneto-elastic coupling in the FeAs layers. Short range order in both the Fe
and Pr moments persists for higher levels of x. The static magnetic moments
disappear at a concentration coincident with that expected for percolation of
the J1-J2 square lattice model
Role of the glassy dynamics and thermal mixing in the dynamic nuclear polarization and relaxation mechanisms of pyruvic acid
The temperature dependence of H and C nuclear spin-lattice
relaxation rate has been studied in the 1.6 K - 4.2 K temperature range
in pure pyruvic acid and in pyruvic acid containing trityl radicals at a
concentration of 15 mM. The temperature dependence of is found to
follow a quadratic power law for both nuclei in the two samples. Remarkably the
same temperature dependence is displayed also by the electron spin-lattice
relaxation rate in the sample containing radicals. These results are
explained by considering the effect of the structural dynamics on the
relaxation rates in pyruvic acid. Dynamic nuclear polarization experiments show
that below 4 K the C build up rate scales with , in
analogy to C and consistently with a thermal mixing scenario
where all the electrons are collectively involved in the dynamic nuclear
polarization process and the nuclear spin reservoir is in good thermal contact
with the electron spin system.Comment: 14 pages, 13 figure
Evidence for impurity-induced frustration in La2CuO4
Zero-field muon spin rotation and magnetization measurements were performed
in La2Cu{1-x}MxO4, for 0<x< 0.12, where Cu2+ is replaced either by M=Zn2+ or by
M=Mg2+ spinless impurity. It is shown that while the doping dependence of the
sublattice magnetization (M(x)) is nearly the same for both compounds, the
N\'eel temperature (T_N(x)) decreases unambiguously more rapidly in the
Zn-doped compound. This difference, not taken into account within a simple
dilution model, is associated with the frustration induced by the Zn2+ impurity
onto the Cu2+ antiferromagnetic lattice. In fact, from T_N(x) and M(x) the spin
stiffness is derived and found to be reduced by Zn doping more significantly
than expected within a dilution model. The effect of the structural
modifications induced by doping on the exchange coupling is also discussed.Comment: 4 pages, 4 figure
Effects of Age and Calving Season on Lactation Curves of Milk Production Traits in Italian Water Buffaloes
Test day (TD) records of milk production traits (milk yield, fat, and protein percentages) of 534 Italian buffalo cows were analyzed with a mixed linear model in order to estimate lactation curves pertaining to different ages at calving and different seasons of calving. Milk yield lactation curves of younger animals were lower than those of older animals until 20 wk from parturition. No effect of age at calving could be observed for fat and protein percentages. Season of calving affected milk yield only in the first phase of lactation, with the lowest production levels for summer calvings; no effect could be observed on fat and protein contents. Average correlations among TD measures within lactation were 0.59, 0.31, and 0.36 for milk yield, fat, and protein percentages, respectively. Five standard linear functions of time were able to reconstruct the average lactation curves. Goodness of fit was satisfactory for all models considered, although only the five-parameter model was flexible enough to fit all the three traits considered with excellent results
Cluster charge-density-wave glass in hydrogen-intercalated TiSe
The topotactic intercalation of transition-metal dichalcogenides with atomic
or molecular ions acts as an efficient knob to tune the electronic ground state
of the host compound. A representative material in this sense is
1-TiSe, where the electric-field-controlled intercalations of lithium
or hydrogen trigger superconductivity coexisting with the charge-density wave
phase. Here, we use the nuclear magnetic moments of the intercalants in
hydrogen-intercalated 1-TiSe as local probes for nuclear magnetic
resonance experiments. We argue that fluctuating mesoscopic-sized domains
nucleate already at temperatures higher than the bulk critical temperature to
the charge-density wave phase and display cluster-glass-like dynamics in the
MHz range tracked by the H nuclear moments. Additionally, we observe a
well-defined independent dynamical process at lower temperatures that we
associate with the intrinsic properties of the charge-density wave state. In
particular, we ascribe the low-temperature phenomenology to the collective
phason-like motion of the charge-density wave being hindered by structural
defects and chemical impurities and resulting in a localized oscillating
motion.Comment: 9 pages, 4 figure
Exotic atoms at extremely high magnetic fields: the case of neutron star atmosphere
The presence of exotic states of matter in neutron stars (NSs) is currently
an open issue in physics. The appearance of muons, kaons, hyperons, and other
exotic particles in the inner regions of the NS, favored by energetic
considerations, is considered to be an effective mechanism to soften the
equation of state (EoS). In the so-called two-families scenario, the softening
of the EoS allows for NSs characterized by very small radii, which become
unstable and convert into a quark stars (QSs). In the process of conversion of
a NS into a QS material can be ablated by neutrinos from the surface of the
star. Not only neutron-rich nuclei, but also more exotic material, such as
hypernuclei or deconfined quarks, could be ejected into the atmosphere. In the
NS atmosphere, atoms like H, He, and C should exist, and attempts to model the
NS thermal emission taking into account their presence, with spectra modified
by the extreme magnetic fields, have been done. However, exotic atoms, like
muonic hydrogen or the so-called Sigmium , could
also be present during the conversion process or in its immediate aftermath. At
present, analytical expressions of the wave functions and eigenvalues for these
atoms have been calculated only for H. In this work, we extend the existing
solutions and parametrizations to the exotic atoms and
, making some predictions on possible transitions. Their
detection in the spectra of NS would provide experimental evidence for the
existence of hyperons in the interior of these stars.Comment: 10 pages, 6 figures, proceedings of the "International Conference on
Exotic Atoms and Related Topics - EXA2017", Austrian Academy of Sciences,
Austria, September 11-15, 201
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