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 $d$-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 Fe2_2P by DFT and spin spectroscopies

Fe$_2$P 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, Fe$_2$P, with two spectroscopic techniques, $\mu$SR and NMR, in order to provide solid bases for further experimental analysis of Fe$_2$P-type transition metal based alloys. We perform zero applied field measurements using both techniques below the ferromagnetic transition $T_C=220~\mathrm K$. 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

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