Publications (22)59.2 Total impact

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ABSTRACT: We revise the simple and yet powerful approach of the Double Adiabatic Approximation (DAA) to study positive muon embedding sites in condensed matter. This method introduces a tractable and reasonably accurate description of the quantum nature of the muon in Density Functional Theory (DFT) simulations aiming at identifying muon sites. Moreover, this approach overcomes the limitations of the harmonic approximation in terms of both computational efforts and accuracy. Notably, it also improves treatment of muon diffusion. We use the DAA and Nudged Elastic Band calculations to determine the muon sites and the activation energies between equivalent sites in MnSi and Copper. In metallic Cu, the proposed phonon assisted mechanism for the low temperature muon diffusion in the lattice is confirmed.  [Show abstract] [Hide abstract]
ABSTRACT: Among the widely studied superconducting ironpnictide compounds belonging to the Ln1111 family (with Ln a lanthanide), a systematic investigation of the crossover region between the superconducting and the antiferromagnetic phase for the Ln = Nd case has been missing. We fill this gap by focusing on the intermediate doping regime of NdFeAsO(1x)F(x) by means of dcmagnetometry and muonspin spectroscopy measurements. The longrange order we detect at low fluorine doping is replaced by shortrange magnetic interactions at x = 0.08, where also superconductivity appears. In this case, longitudinalfield muonspin spectroscopy experiments show clear evidence of slow magnetic fluctuations that disappear at low temperatures. This fluctuating component is ascribed to the glassylike character of the magnetically ordered phase of NdFeAsO at intermediate fluorine doping.  [Show abstract] [Hide abstract]
ABSTRACT: 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 tetragonalorthorhombic 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 magnetoelastic 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 J1J2 square lattice model.  [Show abstract] [Hide abstract]
ABSTRACT: By making a systematic study of the hydrogendoped LaFeAsO system by means of dc resistivity, dc magnetometry, and muonspin spectroscopy, we addressed the question of universality of the phase diagram of rareearth1111 pnictides. In many respects, the behaviour of LaFeAsO1−xHx resembles that of its widely studied Fdoped counterpart, with H− realizing a similar (or better) electron doping in the LaO planes. In an x = 0.01 sample we found a longrange spindensity wave (SDW) order with TN = 119 K, while at x = 0.05 the SDW establishes only at 38 K and, below Tc = 10 K, it coexists at a nanoscopic scale with bulk superconductivity. Unlike the abrupt magneticsuperconducting transition found in the La1111 compound, the presence of a crossover region makes the Hdoped system qualitatively similar to other Sm1111, Ce1111, and Nd1111 families.  [Show abstract] [Hide abstract]
ABSTRACT: Transversefield muonspin rotation ($\mu$SR) experiments were performed on a single crystal sample of the noncentrosymmetric system MnSi. The observed angular dependence of the muon precession frequencies matches perfectly the one of the Mndipolar fields acting on the muons stopping at a 4a position of the crystallographic structure. The data provide a precise determination of the magnetic dipolar tensor. In addition, we have calculated the shape of the field distribution expected below the magnetic transition temperature $T_C$ at the 4a muonsite when no external magnetic field is applied. We show that this field distribution is consistent with the one reported by zerofield $\mu$SR studies. Finally, we present ab initio calculations based on the densityfunctional theory which confirm the position of the muon stopping site inferred from transversefield $\mu$SR. In view of the presented evidence we conclude that the $\mu$SR response of MnSi can be perfectly and fully understood without invoking a hypothetical magnetic polaron state. 
Article: 75As NQR signature of the isoelectronic nature of ruthenium for iron substitution in LaFe1−xRuxAsO
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ABSTRACT: We have investigated the effects of the isovalent ruthenium substitution in LaFeRuAsO, by extensive 75As NQR (nuclear quadrupole resonance) measurements, supported by DFT (density functional theory) calculations, in order to characterize both the lattice and electronic structure details. The evidence for five different local configurations around the arsenic site emerges upon increasing ruthenium for iron substitution. DFT calculations confirm the attribution of the measured electric field gradients (EFGs) to ruthenium atom occupancies (0, 1, 2, 3, and 4) on the nearestneighbour sites of arsenic. It is found that the lowfrequency (Rufree) NQR peak remains almost unaffected upon ruthenium substitution, providing an experimental confirmation that ruthenium does not introduce delocalized carriers in the iron plane.  [Show abstract] [Hide abstract]
ABSTRACT: A superconductingtomagnetic transition is reported for LaFe$_{1x}$Mn$_x$AsO$_{0.89}$F$_{0.11}$ where a per thousand amount of Mn impurities is dispersed. By employing local spectroscopic techniques like muon spin rotation (muSR) and nuclear quadrupole resonance (NQR) on compounds with Mn contents ranging from x=0.025% to x=0.75%, we find that the electronic properties are extremely sensitive to the Mn impurities. In fact, a small amount of Mn as low as 0.2% suppresses superconductivity completely. Static magnetism, involving the FeAs planes, is observed to arise for x > 0.1% and becomes further enhanced upon increasing Mn substitution. Also a progressive increase of low energy spin fluctuations, leading to an enhancement of the NQR spinlattice relaxation rate 1/T1, is observed upon Mn substitution. The analysis of 1/T1 for the sample closest to the the crossover between superconductivity and magnetism (x = 0.2%) points towards the presence of an antiferromagnetic quantum critical point around that doping level.  [Show abstract] [Hide abstract]
ABSTRACT: One of the most fundamental limitations of a muonspin relaxation experiment can be the lack of knowledge of the implantation site of the muon and the uncertainty about the muon's perturbation of its host. Here we review some of the work done on the `muon site problem' in the solid state and highlight some recent applications of electronic structure calculations that have successfully characterized the quantum states of muons in a number of insulating compounds containing fluorine, in a number of pnictide superconductors, and in ZnO.  [Show abstract] [Hide abstract]
ABSTRACT: We report on the magnetic and superconducting properties of LaO_{0.5}F_{0.5}BiS_{2} by means of zero and transversefield (ZF/TF) muonspin spectroscopy measurements (μSR). Contrary to previous results on ironbased superconductors, measurements in zero field demonstrate the absence of magnetically ordered phases. TFμSR data give access to the superfluid density, which shows a marked twodimensional character with a dominant swave temperature behavior. The field dependence of the magnetic penetration depth confirms this finding and further suggests the presence of an anisotropic superconducting gap.  [Show abstract] [Hide abstract]
ABSTRACT: We provide direct experimental evidence for the identical effect of the inplane Fe$_{1x}$Co$_{x}$ and of the outofplane O$_{1x}$F$_{x}$ chemical dilutions on the itinerant spindensitywave (SDW) magnetic phase in CeFeAsO. Remarkably, the suppression of SDW is not sensitive at all to the different kinds of disorder introduced in the two cases. Still, it is clearly shown that the sizeable inplane disorder induced by the Fe$_{1x}$Co$_{x}$ substitution is highly effective in suppressing $T_{\textrm{c}}$. Differently from what is observed in CeFeAsO$_{1x}$F$_{x}$, the ordered magnetic phase of the Ce sublattice is preserved throughout the whole phase diagram in CeFe$_{1x}$Co$_{x}$AsO ($x \leq 0.2$). An intriguing effect is encountered, whereby the magnetic coupling among Ce$^{3+}$ ions is enhanced by the superconducting phase.  [Show abstract] [Hide abstract]
ABSTRACT: The appearance of static magnetism, nanoscopically coexisting with superconductivity, is shown to be a general feature of optimally electrondoped LnFe(1x)Ru(x)AsO(1y)F(y) superconductor (Ln  lanthanide ion) upon isovalent substitution of Fe by Ru. The magnetic ordering temperature T_N and the magnitude of the internal field display a domelike dependence on x, peaked around x=1/4, with higher T_N values for those materials characterized by a larger z cell coordinate of As. Remarkably, the latter are also those with the highest superconducting transition temperature (T_c) for x=0. The reduction of T_c(x) is found to be significant in the x region of the phase diagram where the static magnetism develops. Upon increasing the Ru content superconductivity eventually disappears, but only at x=0.6.  [Show abstract] [Hide abstract]
ABSTRACT: The appearance of static magnetism, nanoscopically coexisting with superconductivity, is shown to be a general feature of optimally electrondoped LnFe(1x)Ru(x)AsO(1y)F(y) superconductor (Ln  lanthanide ion) upon isovalent substitution of Fe by Ru. The magnetic ordering temperature T_N and the magnitude of the internal field display a domelike dependence on x, peaked around x=1/4, with higher T_N values for those materials characterized by a larger z cell coordinate of As. Remarkably, the latter are also those with the highest superconducting transition temperature (T_c) for x=0. The reduction of T_c(x) is found to be significant in the x region of the phase diagram where the static magnetism develops. Upon increasing the Ru content superconductivity eventually disappears, but only at x=0.6. 
Article: Charge doping versus disorder in CeFeAsO: do the in and outofplane dilutions play the same role?
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ABSTRACT: We provide direct experimental evidence for the identical effect of the inplane Fe$_{1x}$Co$_{x}$ and of the outofplane O$_{1x}$F$_{x}$ chemical dilutions on the itinerant spindensitywave (SDW) magnetic phase in CeFeAsO. Remarkably, the suppression of SDW is not sensitive at all to the different kinds of disorder introduced in the two cases. Still, it is clearly shown that the sizeable inplane disorder induced by the Fe$_{1x}$Co$_{x}$ substitution is highly effective in suppressing $T_{\textrm{c}}$. Differently from what is observed in CeFeAsO$_{1x}$F$_{x}$, the ordered magnetic phase of the Ce sublattice is preserved throughout the whole phase diagram in CeFe$_{1x}$Co$_{x}$AsO ($x \leq 0.2$). An intriguing effect is encountered, whereby the magnetic coupling among Ce$^{3+}$ ions is enhanced by the superconducting phase.  [Show abstract] [Hide abstract]
ABSTRACT: The evolution of magnetic order in Fe1+ySexTe1x crystals as a function of Se content was investigated by means of ac/dc magnetometry and muonspin spectroscopy. Experimental results and selfconsistent density functional theory calculations both indicate that muons are implanted in vacant ironexcess sites, where they probe a local field mainly of dipolar origin, resulting from an antiferromagnetic (AFM) bicollinear arrangement of iron spins. This longrange AFM phase becomes progressively disordered with increasing Se content. At the same time all the tested samples manifest a marked glassy character that vanishes for high Se contents. The presence of local electronic/compositional inhomogeneities most likely favours the growth of clusters whose magnetic moment 'freezes' at low temperature. This glassy magnetic phase justifies both the coherent muon precession seen at short times in the asymmetry data, as well as the glassy behaviour evidenced by both dc and ac magnetometry.  [Show abstract] [Hide abstract]
ABSTRACT: We report on an ab initio strategy based on density functional theory to identify the muon sites. Two issues must be carefully addressed: muon delocalization about candidate interstitial sites and local structural relaxation of the atomic positions due to mu(+)sample interaction. Here, we verify our strategy's validity focusing on two wide band gap materials, LiF and YF3, where both mu(+) delocalization and crystal lattice relaxation play an important role in determining the mu(+) stopping site positions. DOI: 10.1103/PhysRevB.87.115148 
Article: Common Effect of Chemical and External Pressures on the Magnetic Properties of RCoPO (R=La,Pr)
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ABSTRACT: We report a detailed investigation of RCoPO (R = La, Pr) and LaCoAsO materials performed by means of muon spin spectroscopy. Zerofield measurements show that the electrons localized on the Pr3+ ions do not play any role in the static magnetic properties of the compounds. Magnetism at the local level is indeed fully dominated by the weakly itinerant ferromagnetism from the Co sublattice only. The increase of the chemical pressure triggered by the different ionic radii of La3+ and Pr3+, on the other hand, plays a crucial role in enhancing the value of the magnetic critical temperature and can be mimicked by the application of external hydrostatic pressure up to 24 kbar. A sharp discontinuity in the local magnetic field at the muon site in LaCoPO at around 5 kbar suggests a sizable modification in the band structure of the material upon increasing pressure. This scenario is qualitatively supported by ab initio densityfunctionaltheory calculations. 
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ABSTRACT: We investigate the effect of external pressure on magnetic order in undoped LnFeAsO (Ln = La, Ce, Pr, La) by using muonspin relaxation measurements and abinitio calculations. Both magnetic transition temperature $T_m$ and Fe magnetic moment decrease with external pressure. The effect is observed to be lanthanide dependent with the strongest response for Ln = La and the weakest for Ln = Sm. The trend is qualitatively in agreement with our DFT calculations. The same calculations allow us to assign a value of 0.68(2) $\mu_B$ to the Fe moment, obtained from an accurate determination of the muon sites. Our data further show that the magnetic lanthanide order transitions do not follow the simple trend of Fe, possibly as a consequence of the different $f$electron overlap. 
Article: Magnetic properties of spindiluted iron pnictides from μ SR and NMR in LaFe 1 − x Ru x AsO
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ABSTRACT: The effect of isoelectronic substitutions on the microscopic properties of LaFe1xRuxAsO, 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 spinlattice relaxation rate 1/T_1 can be suitably described within a twoband model. One band giving rise to the spin density wave groundstate, 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_1J_2 model on a square lattice, which appears to be an effective framework to describe the magnetic properties of the spin density wave groundstate.
Publication Stats
173  Citations  
59.20  Total Impact Points  
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Institutions

20132015

Università degli studi di Parma
 Department of Physics and Earth Sciences
Parma, EmiliaRomagna, Italy 
Paul Scherrer Institut
 Laboratory for Muon Spin Spectroscopy (LMU)
Aargau, Switzerland


20122013

Azienda Unità Sanitaria Locale Parma
Parma, EmiliaRomagna, Italy 
University of Pavia
 Department of Physics
Ticinum, Lombardy, Italy
