192 research outputs found
A numerical method to calculate the muon relaxation function in the presence of diffusion
We present an accurate and efficient method to calculate the effect of random
fluctuations of the local field at the muon, for instance in the case muon
diffusion, within the framework of the strong collision approximation. The
method is based on a reformulation of the Markovian process over a discretized
time base, leading to a summation equation for the muon polarization function
which is solved by discrete Fourier transform. The latter is formally
analogous, though not identical, to the integral equation of the original
continuous-time model, solved by Laplace transform. With real-case parameter
values, the solution of the discrete-time strong collision model is found to
approximate the continuous-time solution with excellent accuracy even with a
coarse-grained time sampling. Its calculation by the fast Fourier transform
algorithm is very efficient and suitable for real time fitting of experimental
data even on a slow computer.Comment: 7 pages, 3 figures. Submitted to Journal of Physics: Condensed Matte
Muon contact hyperfine field in metals: A DFT calculation
In positive muon spin rotation and relaxation spectroscopy it is becoming
nowadays customary to take advantage of Density Functional Theory (DFT) based
computational methods to aid the experimental data analysis. DFT aided muon
site determination is especially useful for measurements performed in magnetic
materials, where large contact hyperfine interactions may arise. Here we
present a systematic analysis of the accuracy of the ab initio estimation of
muon's hyperfine contact field on elemental transition metals, performing state
of the art spin-polarized plane wave DFT and using the projector augmented
pseudopotential approach, which allows to include the core state effects due to
the spin ordering. We further validate this method in not-so-simple,
non-centrosymmetric metallic compounds, presently of topical interest for their
spiral magnetic structure giving rise to skyrmion phases, such as MnSi and
MnGe. The calculated hyperfine fields agree with experimental values in all
cases, provided the spontaneous spin magnetization of the metal is well
reproduced within the approach. To overcome the known limits of the
conventional mean field approximation of DFT on itinerant magnets, we adopt the
so-called reduced Stoner theory [L. Ortenzi et al.,Phys. Rev. B 86, 064437
(2012)]. We establish the accuracy of the estimated muon contact field in
metallic compounds with DFT and our results show improved agreement with
experiments compared to those of earlier publications.Comment: 8 pages, 4 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
Quantum effects in muon spin spectroscopy within the stochastic self-consistent harmonic approximation
In muon spin rotation experiments the positive implanted muon vibrates with
large zero point amplitude by virtue of its light mass. Quantum mechanical
calculations of the host material usually treat the muon as a point impurity,
ignoring this large vibrational amplitude. As a first order correction, the
muon zero point motion is usually described within the harmonic approximation,
despite the large anharmonicity of the crystal potential. Here we apply the
stochastic self-consistent harmonic approximation, a quantum variational method
devised to include strong anharmonic effects in total energy and vibrational
frequency calculations, in order to overcome these limitations and provide an
accurate ab initio description of the quantum nature of the muon. We applied
this full quantum treatment to the calculation of the muon contact hyperfine
field in textbook-case metallic systems, such as Fe, Ni, Co including MnSi and
MnGe, significantly improving agreement with experiments. Our results show that
muon vibrational frequencies are strongly renormalized by anharmonicity.
Finally, in contrast to the harmonic approximation, we show that including
quantum anharmonic fluctuations, the muon stabilizes at the octahedral site in
bcc Fe.Comment: 10 page
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
Robotic total gastrectomy with intracorporeal robot-sewn anastomosis. A novel approach adopting the double-loop reconstruction method
Gastric cancer constitutes a major health problem. Robotic
surgery has been progressively developed in this field. Although the
feasibility of robotic procedures has been demonstrated, there are
unresolved aspects being debated, including the reproducibility of
intracorporeal in place of extracorporeal anastomosis.
Difficulties of traditional laparoscopy have been described and there
are well-known advantages of robotic systems, but few articles in
literature describe a full robotic execution of the reconstructive phase
while others do not give a thorough explanation how this phase was run.
A new reconstructive approach, not yet described in literature, was
recently adopted at our Center.
Robotic total gastrectomy with D2 lymphadenectomy and a socalled
‘‘double-loop’’ reconstruction method with intracorporeal robotsewn
anastomosis (Parisi’s technique) was performed in all reported
cases.
Preoperative, intraoperative, and postoperative data were collected
and a technical note was documented.
All tumors were located at the upper third of the stomach, and no
conversions or intraoperative complications occurred. Histopathological
analysis showed R0 resection obtained in all specimens. Hospital
stay was regular in all patients and discharge was recommended starting
from the 4th postoperative day. No major postoperative complications
or reoperations occurred.
Reconstruction of the digestive tract after total gastrectomy is one of
the main areas of surgical research in the treatment of gastric cancer and
in the field of minimally invasive surgery.
The double-loop method is a valid simplification of the traditional
technique of construction of the Roux-limb that could increase the
feasibility and safety in performing a full hand-sewn intracorporeal
reconstruction and it appears to fit the characteristics of the robotic
system thus obtaining excellent postoperative clinical outcome
Proof-of-concept Quantum Simulator based on Molecular Spin Qudits
The use of -level qudits instead of two-level qubits can largely increase
the power of quantum logic for many applications, ranging from quantum
simulations to quantum error correction. Molecular Nanomagnets are ideal spin
systems to realize these large-dimensional qudits. Indeed, their Hamiltonian
can be engineered to an unparalleled extent and can yield a spectrum with many
low-energy states. In particular, in the last decade intense theoretical,
experimental and synthesis efforts have been devoted to develop quantum
simulators based on Molecular Nanomagnets. However, this remarkable potential
is practically unexpressed, because no quantum simulation has ever been
experimentally demonstrated with these systems. Here we show the first
prototype quantum simulator based on an ensemble of molecular qudits and a
radiofrequency broadband spectrometer. To demonstrate the operativity of the
device, we have simulated quantum tunneling of the magnetization and the
transverse-field Ising model, representative of two different classes of
problems. These results represent an important step towards the actual use of
molecular spin qudits in quantum technologies
Ab initio modeling and experimental investigation of FeP by DFT and spin spectroscopies
FeP alloys have been identified as promising candidates for magnetic
refrigeration at room-temperature and for custom magnetostatic applications.
The intent of this study is to accurately characterize the magnetic ground
state of the parent compound, FeP, with two spectroscopic techniques,
SR and NMR, in order to provide solid bases for further experimental
analysis of FeP-type transition metal based alloys. We perform zero applied
field measurements using both techniques below the ferromagnetic transition
. The experimental results are reproduced and interpreted
using first principles simulations validating this approach for quantitative
estimates in alloys of interest for technological applications.Comment: 10 pages, 2 figure
Addressing the magnetic properties of sub-monolayers of single-molecule magnets by X-ray magnetic circular dichroism
We report on a comparative study of electronic and magnetic properties of Mn6 single-molecule magnets (SMMs) grafted on gold surface. Two derivatives with spin-ground states S ¼ 4 andhave been functionalized with 3-tp-CO2 (3-thiophene carboxylate, tpc) ligands and characterized thick films (TFs) as well as sub-monolayers (sMLs) by synchrotron based techniques. X-ray absorption spectroscopy at the Mn L2,3 edges shows the modification of the spectral lineshape in the sMLs respect to the TFs suggesting that the local symmetry at the Mn sites changes once the molecules deposited on gold surface. In spite of this, the expected MnIII oxidation state is preserved. X-ray magnetic circular dichroism (XMCD) spectra show that the total magnetic moment is only spin part because of the quenched orbital moment. Moreover, variable temperature and variable XMCD spectra reveal an effective decrease of the Mn spin moment for both derivatives
Magnetic properties of spin diluted iron pnictides from muSR and NMR in LaFe1-xRuxAsO
The effect of isoelectronic substitutions on the microscopic properties of
LaFe1-xRuxAsO, for 0< x< 0.8, has been investigated by means of muSR and 139La
NMR. It was found that Ru substitution causes a progressive reduction of the
N\`eel temperature (T_N) and of the magnetic order parameter without leading to
the onset of superconductivity. The temperature dependence of 139La nuclear
spin-lattice relaxation rate 1/T_1 can be suitably described within a two-band
model. One band giving rise to the spin density wave ground-state, while the
other one is characterized by weakly correlated electrons. Fe for Ru
substitution yields to a progressive decrease of the density of states at the
Fermi level close to the one derived from band structure calculations. The
reduction of T_N with doping follows the predictions of the J_1-J_2 model on a
square lattice, which appears to be an effective framework to describe the
magnetic properties of the spin density wave ground-state.Comment: 6 pages, 8 figure
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