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We present an open-source software for the simulation of observables in magnetic resonance experiments, including nuclear magnetic/quadrupole resonance NMR/NQR and electron spin resonance (ESR), developed to assist experimental research in the design of new strategies for the investigation of fundamental quantum properties of materials, as inspired by magnetic resonance protocols that emerged in the context of quantum information science (QIS). The package introduced here enables the simulation of both standard NMR spectroscopic observables and the time-evolution of an interacting single-spin system subject to complex pulse sequences, i.e. quantum gates. The main purpose of this software is to facilitate in the development of much needed novel NMR-based probes of emergent quantum orders, which can be elusive to standard experimental probes. The software is based on a quantum mechanical description of nuclear spin dynamics in NMR/NQR experiments and has been widely tested on available theoretical and experimental results. Moreover, the structure of the software allows for basic experiments to easily be generalized to more sophisticated ones, as it includes all the libraries required for the numerical simulation of generic spin systems. In order to make the program easily accessible to a large user base, we developed a user-friendly graphical interface, Jupyter notebooks, and fully-detailed documentation. Lastly, we portray several examples of the execution of the code that illustrate the potential of a novel NMR paradigm, inspired by QIS, for efficient investigation of emergent phases in strongly correlated materials., Comment: 51 pages

We present an extended X-ray absorption fine structure study at the Mo K-edge of A2(Mo1yMny)2O7 (A Gd, Ho; y 0.05 and 0.10) pyrochlores, as a function of temperature and composition, coupled to diffractometric and magnetic characterizations. Extending the study reported in our previous paper on the pristine A2Mo2O7 compounds to these Mo/Mn partially substituted samples, where we hypothesize a competition between double-exchange and superexchange couplings, we aim to check which structure parameters are related to the nature of the nearest-neighbor magnetic interactions, looking for the presence and evolution of order and structure anomalies. Two Ho samples Ho2(Mo1yMny)2O7 (y 0.05 and 0.10) keep the spin-glass nature of the parent compositions with a strong distortion of the MoO(1)6 octahedron and of the MoHo and MoMo second shell. On the other hand, two Gd2(Mo1yMny)2O7 samples, ferromagnetic at low temperature in the undoped case, show the appearance on a local scale of a spin-glass transition mirrored by a quite high structure disorder and by a distortion coherent with a lattice frustration after Mn-doping. Therefore, the Gd and Ho Mn-doped samples display a similar frustrated behavior, differently from the corresponding undoped ones. We gratefully acknowledge the Spanish CRG at the ESRF for providing beamtime under experiment HC-3343. MS thanks Prof. Monica Dapiaggi for useful discussions. GL thanks Prof. Fabio M. Canepa for illuminating discussions. F.R.-M. gratefully acknowledge the support of the AEI (Spanish Government) Project (No. MAT2017-86450-C4-1-R). F.R.-M. is indebted to MINECO for the Ramon y Cajal contract (Ref: RyC-2015-18626), which is co-financed by the European Social Fund.

We have used muon spin rotation and relaxation (

In the heavy-fermion system $Yb_2Pd_2In_{1-x}Sn_x$, the interplay of crystal-field splitting, Kondo effect, and Ruderman-Kittel-Kasuya-Yosida interactions leads to complex chemical-, pressure-, and magnetic-field phase diagrams, still to be explored in full detail. By using a series of techniques, we show that even modest changes of parameters other than temperature are sufficient to induce multiple quantum-critical transitions in this highly susceptible heavy-fermion family. In particular, we show that, above $\sim 10$ kbar, hydrostatic pressure not only induces an antiferromagnetic phase at low temperature, but it likely leads to a reorientation of the Yb magnetic moments and/or the competition among different antiferromagnetic configurations.

Measurements of proton nuclear magnetic resonance ($^{1}\mathrm{H}\phantom{\rule{0.16em}{0ex}}\mathrm{NMR}$) spectra and relaxation and of muon spin relaxation (${\ensuremath{\mu}}^{+}\mathrm{SR}$) have been performed as a function of temperature and external magnetic field on two isostructural lanthanide complexes, Er(trensal) and Dy(trensal) [where ${\mathrm{H}}_{3}\mathrm{trensal}=2,{2}^{\ensuremath{'}},{2}^{\ensuremath{'}\ensuremath{'}}\ensuremath{-}\mathrm{tris}\ensuremath{-}(\mathrm{salicylideneimino})\mathrm{triethylamine}$], featuring crystallographically imposed trigonal symmetry. Both the nuclear $1/{T}_{1}$ and muon \ensuremath{\lambda} longitudinal relaxation rates (LRRs) exhibit a peak for temperatures $T$ 30 K, associated to the slowing down of the spin dynamics, and the width of the NMR absorption spectra starts to increase significantly at $T$ \ensuremath{\sim} 50 K, a temperature sizably higher than the one of the LRR peaks. The LRR peaks have a field and temperature dependence different from those previously reported for all molecular nanomagnets. They do not follow the Bloembergen-Purcell-Pound scaling of the amplitude and position in temperature and field and thus cannot be explained in terms of a single dominating correlation time ${\ensuremath{\tau}}_{\mathrm{c}}$ determined by the spin slowing down at low temperature. Further, for $T50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ the spectral width does not follow the temperature behavior of the magnetic susceptibility \ensuremath{\chi}. We suggest, using simple qualitative considerations, that the observed behavior is due to a combination of two different relaxation processes characterized by the correlation times ${\ensuremath{\tau}}_{\mathrm{LT}}$ and ${\ensuremath{\tau}}_{\mathrm{HT}}$, dominating for $T$ 30 K and $T50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, respectively. Finally, the observed flattening of LRR for $T$ 5 K is suggested to have a quantum origin.

We present UNDI, an open-source program to analyze the time dependent spin polarization of an isolated muon interacting with the surrounding nuclear magnetic dipoles in the context of standard muon spin rotation and relaxation spectroscopy experiments. The code can perform both exact and approximated estimates of the muon polarization function in presence of external fields and electric field gradients on the nuclei surrounding the muon. We show that this tool, combined to ab initio estimations of the electric field gradient at the nuclei interacting with the muon, can become a valuable complement to supercell based identifications of muon sites in crystals when large nuclear magnetic moments are present in the sample. In addition, it allows to properly investigate physical properties influenced by the presence of a non-negligible electric field gradient such as avoided level crossing resonance, nature of the ground state, disentanglement of electronic and nuclear magnetic moments or charge ordered states. The efficiency and effectiveness of this method is shown along the lines of three realistic examples. Program summary Program Title: UNDI CPC Library link to program files: http://dx.doi.org/10.17632/grp8njy6gz.1 Developer’s repository link: https://github.com/bonfus/undi Code Ocean capsule: https://codeocean.com/capsule/8365303 Licensing provisions: GPLv3 Programming language: Python Nature of problem: To simplify and potentially automate the analysis of the nuclear contribution to muon polarization functions in crystalline materials. Solution method: A python library that provides a set of tools to efficiently solve the spin Hamiltonian describing the interaction between the muon and its neighboring nuclei. The code provides the time development of the spin polarization of the muon subject to external fields and accounts for quadrupolar interactions at the nuclear sites. The solution is sought at quantum level accuracy and Hilbert spaces with dimension up to one million can be handled, thus providing accurate results for all standard experimental conditions.

We investigate the specific influence of structural disorder on the suppression of antiferromagnetic order and on the emergence of cuprate superconductivity. We single out pure disorder, by focusing on a series of Y$_{z}$Eu$_{1-z}$Ba$_2$Cu$_3$O$_{6+y}$ samples at fixed oxygen content $y=0.35$, in the range $0\le z\le 1$. The gradual Y/Eu isovalent substitution smoothly drives the system through the Mott-insulator to superconductor transition from a full antiferromagnet with N\'eel transition $T_N=320$ K at $z=0$ to a bulk superconductor with superconducting critical temperature $T_c=18$ K at $z=1$, YBa$_2$Cu$_3$O$_{6.35}$. The electronic properties are finely tuned by gradual lattice deformations induced by the different cationic radii of the two lanthanides, inducing a continuous change of the basal Cu(1)-O chain length, as well as a controlled amount of disorder in the active Cu(2)O$_2$ bilayers. We check that internal charge transfer from the basal to the active plane is entirely responsible for the doping of the latter and we show that superconductivity emerges with orthorhombicity. By comparing transition temperatures with those of the isoelectronic clean system we deterime the influence of pure structural disorder connected with the Y/Eu alloy., Comment: 10 pages 11 figures, submitted to Journal of Physics: Condensed Matter, Special Issue in memory of Prof. Sandro Massida

Li6C60 has been chosen as the most representative system to study the hydrogenation mechanism in alkali-cluster intercalated fullerides. We present here a muon spin relaxation (mu SR) experiment that hints the chance to achieve a higher storage capacity on fullerene with respect to the values suggested in literature. Moreover, a linear relationship between the muonium adduct radical hyperfine frequency and the level of C-60 hydrogenation was found and it can be exploited to probe the C-60 hydrogenation level, giving more credit to this technique in the field of hydrogen storage materials. (C) 2017 Elsevier Ltd. All rights reserved.

Lithium and hydrogen dynamics in Li12C60 and Li12C60Hy are investigated by means of 7Li and 1H solid state Nuclear Magnetic Resonance (NMR) in the temperature range 80–550 K. Differential scanning calorimetry characterization on the hydrogen sorption and X-rays structural analysis are also reported. In the pure phase, the 7Li results show a thermally activated dynamics that can be associated to Li motions within the crystal interstices. Upon hydrogenation, Li ionic motion is considerably hindered by the presence of hydrofullerene molecules. The 1H measurements show that C–H bonds are stable on the local scale up to 400–450 K. The NMR results at higher temperatures are compatible with a H diffusion mechanism which anticipates the H desorption process.

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 nearest-neighbour sites of arsenic. It is found that the low-frequency (Ru-free) NQR peak remains almost unaffected upon ruthenium substitution, providing an experimental confirmation that ruthenium does not introduce delocalized carriers in the iron plane.

In this paper we report a detailed $\mu^{+}$SR and {}$^{19}$F-NMR study of the La$_{0.7}$Y$_{0.3}$FeAsO$_{1-x}$F$_{x}$ class of materials. Here, the diamagnetic La$_{1-y}$Y$_{y}$ substitution increases chemical pressure and, accordingly, sizeably enhances the optimal superconducting transition temperature. We investigate the magnetic-superconducting phase transition by keeping the Y content constant ($y = 0.3$) and by varying the F content in the range $0.025 \leq x \leq 0.15$. Our results show how magnetism and superconductivity coexist for $x = 0.065$. Such coexistence is due to segregation of the two phases in macroscopic regions, resembling what was observed in LaFeAsO$_{1-x}$F$_{x}$ materials under applied hydrostatic pressure. This scenario is qualitatively different from the nanoscopic coexistence of the two order parameters observed when La is fully substituted by magnetic rare-earth ions like Sm or Ce.

Muon spin rotation ($\ensuremath{\mu}\mathrm{SR}$) and $^{19}\mathrm{F}$ nuclear magnetic resonance (NMR) measurements were performed to investigate the effect of Mn for Fe substitutions in ${\mathrm{La}}_{1\ensuremath{-}y}{\mathrm{Y}}_{y}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ superconductors. While for $y=0$ a very low critical concentration of Mn ($x=0.2%$) is needed to quench superconductivity, as $y$ increases the negative chemical pressure introduced by Y for La substitution stabilizes superconductivity and for $y=20%$ it is suppressed at Mn contents an order of magnitude larger. A magnetic phase arises once superconductivity is suppressed both for $y=0$ and for $y=20%$. Low-energy spin fluctuations give rise to a peak in $^{19}\mathrm{F}$ NMR $1/{T}_{1}$ with an onset well above the superconducting transition temperature and whose magnitude increases with $x$. Also the static magnetic correlations probed by $^{19}\mathrm{F}$ NMR linewidth measurements show a marked increase with Mn content. The disruption of superconductivity and the onset of the magnetic ground state are discussed in the light of the proximity of ${\mathrm{LaFeAsO}}_{0.89}{\mathrm{F}}_{0.11}$ to a quantum critical point.

We investigated the effect of hole doping and quenched disorder on the phase diagram of Y 1 - x Ca x Ba 2 Cu 3 O 6 + y , by exploiting the double doping mechanism of Y–Ca substitution and O intercalation in the undoped-to-heavily underdoped regime. We show that the insulator to metal transition, governed by the mobile hole concentration, reflects only the charges transferred by chain oxygen (the y fraction in the chemical formula). The transition is preceded by the suppression of antiferromagnetic order, which is replaced by a cluster spin glass ground state. We discuss the effect of doping and disorder on both magnetic states and on the appearance of superconductivity.

$^{19}\mathrm{F}$ NMR measurements in optimally electron-doped ${\mathrm{La}}_{1\text{\ensuremath{-}}y}{\mathrm{Y}}_{y}{\mathrm{Fe}}_{1\text{\ensuremath{-}}x}{\mathrm{Mn}}_{x}{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ superconductors are presented. The effect of Mn doping on the superconducting phase is studied for two series of compounds $(y=0$ and $y=0.2)$ where the chemical pressure is varied by substituting La with Y. In the $y=0.2$ series superconductivity is suppressed for Mn contents an order of magnitude larger than for $y=0$. For both series a peak in the $^{19}\mathrm{F}$ NMR nuclear spin-lattice relaxation rate $1/{T}_{1}$ emerges upon Mn doping and becomes significantly enhanced on approaching the quantum phase transition between the superconducting and magnetic phases. $^{19}\mathrm{F}$ NMR linewidth measurements show that for similar Mn contents magnetic correlations are more pronounced in the $y=0$ series, at variance with what one would expect for $\stackrel{P\vec}{Q}=(\ensuremath{\pi}/a,0)$ spin correlations. These observations suggest that Mn doping tends to reduce fluctuations at $\stackrel{P\vec}{Q}=(\ensuremath{\pi}/a,0)$ and to enhance other low-frequency modes. The effect of this transfer of spectral weight on the superconducting pairing is discussed along with the charge localization induced by Mn.

Angle-resolved photoemission spectroscopy (ARPES) is one of most powerful techniques to unravel the electronic properties of layered materials and, in recent decades, it has lead to significant progress in the understanding of the band structures of cuprates, pnictides, and other materials of current interest. On the other hand, its application to Mott-Hubbard insulating materials where a Fermi surface is absent has been more limited. Here we show that in these latter materials, where electron spins are localized, ARPES may provide significant information on the spin correlations which can be complementary to the one derived from neutron-scattering experiments. Sr2Cu1-xZnxO2Cl2, a prototype of a diluted spin S=1/2 antiferromagnet (AF) on a square lattice, was chosen as a test case and a direct correspondence between the amplitude of the spectral weight beyond the AF zone boundary derived from ARPES and the spin-correlation length ξ estimated from Cl35 NMR was established. It was found that even for correlation lengths of a few lattice constants, a significant spectral weight in the backbended band is present which depends markedly on ξ. Moreover, the temperature dependence of that spectral weight is found to scale with the x-dependent spin stiffness. These findings prove that the ARPES technique is very sensitive to short-range correlations and its relevance in the understanding of the electronic correlations in cuprates is discussed.

The superconducting properties of ${\mathrm{LaFeAsO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}$ under conditions of optimal electron doping are investigated upon the application of external pressure up to $\ensuremath{\sim}23\text{ }\text{ }\mathrm{kbar}$. Measurements of muon-spin spectroscopy and dc magnetometry evidence a clear mutual independence between the critical temperature ${T}_{c}$ and the low-temperature saturation value for the ratio ${n}_{s}/{m}^{*}$ (superfluid density over effective band mass of Cooper pairs). Remarkably, a dramatic increase of $\ensuremath{\sim}30%$ is reported for ${n}_{s}/{m}^{*}$ at the maximum pressure value while ${T}_{c}$ is substantially unaffected in the whole accessed experimental window. We argue and demonstrate that the explanation for the observed results must take the effect of nonmagnetic impurities on multiband superconductivity into account. In particular, the unique possibility to modify the ratio between intraband and interband scattering rates by acting on structural parameters while keeping the amount of chemical disorder constant is a striking result of our proposed model.

We present an extended x-ray absorption fine structure study of the pyrochlore ${\mathrm{Y}}_{2}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ (8--298 K). We find evidence, on a local scale, of a significant magnetoelastic coupling at the N\'eel temperature ${T}_{\mathrm{N}}\ensuremath{\sim}77\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ pointed out by a huge Debye-Waller ${\ensuremath{\sigma}}^{2}$ factor deviation from a correlated temperature dependent Debye-like local order behavior plus a temperature independent static contribution. Moreover, we notice the occurrence of a potential local order-disorder structural phase transition at ${T}^{*}=150$ K. This anomalous behavior is consistent with the pyrochlore's predisposition towards structural disorder and with a strong spin-phonon correlation. Remarkably the low-temperature order competes with the tendency of magnetic frustration to induce a less symmetric local structure.

Coplanar microwave resonators made of 330 nm-thick superconducting YBa2Cu3O7 have been realized and characterized in a wide temperature (T, 2-100 K) and magnetic field (B, 0-7 T) range. The quality factor (Q(L)) exceeds 10(4) below 55 K and it slightly decreases for increasing fields, remaining 90% of Q(L) (B = 0) for B = 7 T and T = 2K. These features allow the coherent coupling of resonant photons with a spin ensemble at finite temperature and magnetic field. To demonstrate this, collective strong coupling was achieved by using di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium organic radical placed at the magnetic antinode of the fundamental mode: the in-plane magnetic field is used to tune the spin frequency gap splitting across the single-mode cavity resonance at 7.75 GHz, where clear anticrossings are observed with a splitting as large as similar to 82 MHz at T = 2 K. The spin-cavity collective coupling rate is shown to scale as the square root of the number of active spins in the ensemble. (c) 2015 AIP Publishing LLC.

For electro-oxidation of [Pt(Me 2 pipdt) 2 ][Pt(mnt) 2 ] (Me 2 pipdt N , N ′-dimethyl-piperazine-2,3-dithione; mnt = maleonitrile-2,3-dithiolate) ( 1a ) the novel salt [Pt(Me 2 pipdt) 2 ][Pt(mnt) 2 ] 2 ( 2a ) has been prepared and characterised. 2a exhibits segregated chains of anionic and cationic complexes. The anions generate a non-regular stack with the separation between the planes of 3.558(5) A [3.60(1) A is the Pt⋯C C (mid point) separation] and 3.643(5) A [3.980(3) A is the separation between Pt and S-atoms]. The cations are perpendicular to the anions and interposed between those stacks in a ribbon fashion. Magnetic measurements show the presence of a magnetic transition between the behaviour of an antiferromagnetic alternating-exchange chain and the behaviour of a set of antiferromagnetic dimers.

We report μ SR experiments on two lightly doped superconducting YBa 2 Cu 3 O 6 + x samples ( 0.39 ⩽ x ⩽ 0.41 ) , performed with the initial muon spin polarization rotated by 47 ∘ from the applied field direction. At low temperature, the muon asymmetry of both samples exhibits a dominant component with very large depolarization rates, due to the presence of frozen magnetic clusters. A two-component model, accounting for muon sites close to the clusters and far from them, respectively, is presented. The model provides a prediction for the time evolution of both the longitudinal and transverse projections of the muon polarization, and yields a good simultaneous fit to the data of each sample at all the applied fields. The best-fit parameters demonstrate that the static internal fields from the magnetic clusters are disordered, in agreement with the absence of long-range magnetic Bragg peaks in neutron diffraction.

A preparation method to simultaneously synthesize and consolidate bulk MgB2 superconductors from Mg and B commercial elemental powders by means of the spark plasma sintering technique is reported. The influence of process parameters on sintering process dynamics as well as product characteristics, determined by transport and magnetic measurements, is investigated. The superconducting properties of the obtained samples, and particularly the critical current density, are comparable or better than those corresponding to other MgB2 preparation techniques. Thus, the superconductive properties of the bulk MgB2 materials synthesized in this work are suitable for selected applications, such as magnetic levitation, magnetic screening, and fault current limiters. It should be finally noted that the proposed method represents a particularly rapid preparation route as compared to other techniques. © 2006 American Institute of Chemical Engineers AIChE J, 2006

Here we present a reviewed phase diagram of the high-T c superconducting YBa2Cu3O6+ x compound, finely mapped in the strongly underdoped region (0

We present the results of an extensive μSR reinvestigation of the doping dependence of the AF order in polycrystalline YBa 2 Cu 3 O 6+ x samples (0 x x >0.2, we detected a spin freezing correlated to the charge freezing of the holes in the CuO 2 planes. The data may be fitted to a model of size-dependent reduction of the staggered magnetization implied in a stripe scenario. The hole concentration dependence of these freezing phenomena is mapped onto the phase diagram of YBa 2 Cu 3 O 6+ x .

The charge and spin freezing phenomena in high T c YBa 2 Cu 3 O 6+x superconductor is investigated in the low doping region (00.2, the observed spin freezing is correlated to the charge freezing of the holes in the Cu(2)O2 planes. The data may be fitted to a model of size-dependent reduction of the staggered magnetization implied in a stripe scenario. The hole concentration dependence of these freezing phenomena is mapped onto the phase diagram of YBa 2 Cu 3 O 6+x.

We report $^{63}$Cu nuclear magnetic resonance and muon spin rotation measurements on the S=1/2 antiferromagnetic Heisenberg spin chain compound Sr$_{1.9}$Ca$_{0.1}$CuO$_3$. An exponentially decreasing spin-lattice relaxation rate 1/T$_1$ indicates the opening of a spin gap. This behavior is very similar to what has been observed for the cognate zigzag spin chain compound Sr$_{0.9}$Ca$_{0.1}$CuO$_2$, and confirms that the occurrence of a spin gap upon Ca doping is independent of the interchain exchange coupling $J'$. Our results therefore generally prove the appearance of a spin gap in an antiferromagnetic Heisenberg spin chain induced by a local bond disorder of the intrachain exchange coupling $J$. A low temperature upturn of 1/T$_1$ evidences growing magnetic correlations. However, zero field muon spin rotation measurements down to 1.5 K confirm the absence of magnetic order in this compound which is most likely suppressed by the opening of the spin gap.

Neutron diffraction and muon spin relaxation measurements are used to obtain a detailed phase diagram of ${\mathrm{PrFe}}_{1\ensuremath{-}x}{\mathrm{Ru}}_{x}\mathrm{AsO}$. The isoelectronic substitution of Ru for Fe acts effectively as spin dilution, suppressing both the structural and magnetic phase transitions. The temperature ${T}_{S}$ of the tetragonal-orthorhombic structural phase transition decreases gradually as a function of $x$. Slightly below ${T}_{S}$, 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 $J{}_{1}\text{\ensuremath{-}}J{}_{2}$ square-lattice model.

We report on a ìSR and 55 Mn NMR investigation of the magnetic order parameter as a function of temperature in the optimally doped La(5/8)(CaySr(1.y))(3/8)MnO3 and in the underdoped La1.xSrxMnO3 and La1.xCaxMnO3 metallic manganite families. The study is aimed at unravelling the effect of lattice distortions, implicitly controlled by the Ca-Sr isoelectronic substitution, from that of hole doping x on the Curie temperature TC and the order of the magnetic transition. At optimal doping, the transitions are second order at all y values, including the y = 1 (La(5/8)Ca(3/8)MnO3) end member. By contrast, they are first order in the underdoped samples, which show a finite (truncated) order parameter at the Curie point, including La(0.75)Sr(0.25)MnO3 whose TC is much higher than that of La(5/8)Ca(3/8)MnO3. The order parameter curves, on the other hand, exhibit a very minor dependence on x, if truncation is excepted. This suggests that the effective exchange interaction between Mn ions is essentially governed by local distortions, in agreement with the original double-exchange model, while truncation is primarily, if not entirely, an effect of under- or over-doping. A phase diagram, separating in the x.y plane polaron-driven first order transitions from regular second order transitions governed by critical fluctuations, is proposed for the La(1.x)(CaySr(1.y))xMnO3 system.

Bulk defective graphene produced by thermal exfoliation of graphite oxide was treated under H-2 and investigated with X-ray photoemission spectroscopy, neutron spectroscopy, and solid state nuclear magnetic resonance. Graphene defects appear effective in dissociating H2 molecule and in promoting H covalent absorption on the carbon backbone. Measured generalized phonon density of states shows the presence of localized peaks ascribed to C H bending modes already in pristine graphene, whose intensities enhance when samples are treated under H2 at 1273 K. However, H-1 NMR evidences a thermally activated dynamics with a correlation time of a few microseconds assigned to a part of H atoms bound onto the graphene plane. These findings point toward a diffusive dynamics of the hydrogen chemically e, bound to graphene sheets, already active at room temperature.

Anisotropic Cu-O strain has been measured and a relation between the superconducting critical temperature and the strain is studied in the YBa 2 Cu 3 O 6+ k (Y123). oxygen ordered YBa 2 Cu 3 O 6+ k phases. We show that the critical temperature scales both with the strain along the a axis (ε a ,) and along the b axis (ε b ). While undoped, the system is close to the critical strain, ε c ~ 0.043. The formation of chains with doping gives rise to a decrease of the ε b to take the system away from the critical point approaching the dynamical quantum stripe fluctuations (ε b c ) and the ε a increases to approach the phase of superconducting stripes (ε a >ε c ).

The authors investigate the tetragonal(T)-orthorhombic(O) transient zone of YBa 2 Cu 3 O 6+x , in a comparative way on pair samples one of which is produced by deintercalation [D] and the other by intercalation [I] of intrinsic oxygen in the basal Cu(1)O x plane and having the same oxygen content (k) and thermal history. Resistive and electron diffraction measurements, performed on [D] k - [I] k pairs within the composition range 0.28 ≤ k ≤ 0.32, are reported. Direct evidence is provided that the T-OII transient is a structural and electronic two phase region characterized by the (T+OII) [D] 0.30 non-superconducting (non-SC) and (OII+OIII*) [I] 0.30 superconducting (SC) phases. The OII superstructure represents the minority component of [D] 0.30 phase and the majority one of [I] 0.30 and OIII* is a new orthorhombic anti-OIII superstructure. The nature of the non-SC and SC behaviours has been comparatively investigated by NQR analysis at the T-OII borderline. The evaluated different values of the average chain lengths emphasize the role of the Cu-O chain length on the hole doping efficiency in YBCO at constant oxygen content.

The oxygen-order dependent emergence of superconductivity in YBa2Cu3O6+x is studied, for the first time in a comparative way, on pair samples having the same oxygen content and thermal history, but different Cu(1)Ox chain arrangements deriving from their intercalated and deintercalated nature. Structural and electronic non-equivalence of pairs samples is detected in the critical region and found to be related, on microscopic scale, to a different average chain length, which, on being experimentally determined by nuclear quadrupole resonance (NQR), sheds new light on the concept of critical chain length for hole doping efficiency.

A comparative investigation, performed on Oxygen-Chain-equalised(k) Deintercalated [ OCD ] k and Intercalated [ OCD ] k pair-samples ( k -pairs) of YBa 2 Cu 3 O 6+x by resistive T c and electron diffraction (ED) at steps Δk=0.02-0.03, revealed that equivalent or non-equivalent microscopic oxygen-chain arrangements are produced, at the same oxygen content k, by intercalation and deintercalation of oxygen in the Cu(1)Ox chains. These arrangements manifest in Tc -singularities or Tc -splittings of the k -pairs, whose occurrence can be straightforwardly explained by taking into account the non equivalence of the structural phase diagrams of YBCO, pointed out by electron diffraction, when produced by intercalation of oxygen in the tetragonal T structure or by deintercalation from the orthorhombic OI structure. Tc -singularities are produced in the k ranges for which a superimposition of the phase diagrams occurs, whereas Tc -splitting are pointed out in correspondence of splitted structural regions. The maximum Tc -splitting is observed in proximity of the semiconductor-metal transition with the appearing of superconductivity related to an anti-OIII (OIII*) superstructure in intercalated samples at k =0.30, where the corresponding deintercalated samples are tetragonal and semiconductor. This result confirms that the vanishing of the antiferromagnetic ordering, the emergence of hole doped superconductivity and the symmetry breaking at the T-O transition are nearly coincident phenomena in YBCO. However the related critical oxygen stoichiometry is strictly dependent on the ordering parameter in the Cu(1) plane, i.e. on the formation of extended chains. The OIII* structure, for the first time observed in a systematic way at the T-O transition in intercalated samples, seems to exhibit in analogy with OII and OIII a characteristic Tc .

The authors report on 63 Cu NMR relaxation measurements in YBa 2 Cu 3 O 6+x samples having similar T c's but a different structural ordering of the Cu(1)O chains. It is found that the gap in the spin excitations estimated from 63 Cu (2) nuclear spin-lattice relaxation rate 1/T1, varies from 110 K up to 190 K depending on the sample. This observation suggests that there is not a one to one relationship between the hole concentration and the amplitude of the spin-gap in the normal state of YBa 2 Cu 3 O 6+x. The sizeable variation in the gap, related to the different ordering of the Cu(1)O chains, is tentatively explained in the framework of a phase separation scenario where Cu(1)O chains act as pinning wells for the stripes.

We report on the magnetic and superconducting properties of LaO${}_{0.5}$F${}_{0.5}$BiS${}_{2}$ by means of zero- and transverse-field (ZF/TF) muon-spin spectroscopy measurements ($\ensuremath{\mu}$SR). Contrary to previous results on iron-based superconductors, measurements in zero field demonstrate the absence of magnetically ordered phases. TF-$\ensuremath{\mu}$SR data give access to the superfluid density, which shows a marked two-dimensional character with a dominant $s$-wave temperature behavior. The field dependence of the magnetic penetration depth confirms this finding and further suggests the presence of an anisotropic superconducting gap.

We have studied the electronic structure of unoccupied states measured by O K-edge and Cu L-edge x-ray absorption spectroscopy (XAS), combined with crystal structure studied by high resolution powder x-ray diffraction (HRPXRD), of charge-compensated layered superconducting CaxLa(1-x)Ba(1.75-x)La(0.25+x)Cu3Oy (0, Comment: 14 pages, 11 figures, accepted for publication in Journal of Physics and Chemistry of Solids

The appearance of static magnetism, nanoscopically coexisting with superconductivity, is shown to be a general feature of optimally electron-doped $L$Fe${}_{1\ensuremath{-}x}$Ru${}_{x}$AsO${}_{1\ensuremath{-}y}$F${}_{y}$ superconductors ($L$ represents a 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 temperatures (${T}_{c}$) 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 $x\ensuremath{\simeq}0.6$.

Muon spin spectroscopy is one of the most powerful tools to investigate the microscopic properties of superconductors. In this manuscript, an overview on some of the main achievements obtained by this technique in the iron-based superconductors (IBS) are presented. It is shown how the muons allow to probe the whole phase diagram of IBS, from the magnetic to the superconducting phase, and their sensitivity to unravel the modifications of the magnetic and the superconducting order parameters, as the phase diagram is spanned either by charge doping, by an external pressure or by introducing magnetic and non-magnetic impurities. Moreover, it is highlighted that the muons are unique probes for the study of the nanoscopic coexistence between magnetism and superconductivity taking place at the crossover between the two ground-states., 28 pages, 18 figures

We investigate the phonon-induced relaxation dynamics in the Fe-7 magnetic molecule, which is made of two Fe3+ triangles bridged together by a central Fe3+ ion. The competition between different antiferromagnetic exchange interactions leads to a low-spin ground state multiplet with a complex pattern of low-lying excited levels. We theoretically investigate the decay of the time correlation function of molecular observables, such as the cluster magnetization, due to the spin-phonon interaction. We find that more than one time contributes to the decay of the molecular magnetization. The relaxation dynamics is probed by measurements of the nuclear spin-lattice relaxation rate 1/T-1. The interpretation of these measurements allows the determination of the magnetoelastic coupling strength and to set the scale factor of the relaxation dynamics time scales. In our theoretical interpretation of 1/T-1 data we also take into account the wipeout effect at low temperatures.

Ac susceptibility and static magnetization measurements were performed in the nearly optimally doped LaFeAsO${}_{0.9}$F${}_{0.1}$ and CeFeAsO${}_{0.92}$F${}_{0.08}$ superconductors, complementing earlier results on SmFeAsO${}_{0.8}$F${}_{0.2}$ [Phys. Rev. B 83, 174514 (2011)]. The magnetic field-temperature phase diagram of the mixed superconducting state is drawn for the three materials, displaying a sizable reduction of the liquid phase upon increasing ${T}_{c}$ in the range of applied fields ($H\ensuremath{\leqslant}5$ T). This result indicates that SmFeAsO${}_{0.8}$F${}_{0.2}$ is the most interesting compound among the investigated ones in view of possible applications. The field-dependence of the intragrain depinning energy ${U}_{0}$ exhibits a common trend for all the samples with a typical crossover field value (2500 Oe $\ensuremath{\lesssim}{H}_{\mathrm{cr}}\ensuremath{\lesssim}5000$ Oe) separating regions where single and collective depinning processes are at work.

The effect of isoelectronic substitutions on the microscopic properties of LaFe${}_{1\ensuremath{-}x}$Ru${}_{x}$AsO, for $0\ensuremath{\le}x\ensuremath{\le}0.8$, has been investigated by means of $\ensuremath{\mu}$SR and ${}^{139}$La 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 ${}^{139}$La nuclear spin-lattice relaxation rate $1/{T}_{1}$ can be suitably described within a two-band model. One band giving rise to the metallic antiferromagnetic ground state, while the other one is characterized by weakly correlated electrons. Ru for Fe substitution yields 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 increasing Ru content 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 magnetic ground state.

The effect of isoelectronic substitutions on the microscopic properties of LaFe1−xRuxAsO, for 0< x< 0.8, has been investigated by means of μSR and 139La NMR. It was found that Ru substitution causes a progressive reduction of the Néel temperature (TN) and of the magnetic order parameter without leading to the onset of superconductivity. The temperature dependence of 139La nuclear spin-lattice relaxation rate 1/T1 can be suitably described within a two-band model. One band giving rise to the metallic antiferromagnetic ground state, while the other one is characterized by weakly correlated electrons. Ru for Fe substitution yields a progressive decrease of the density of states at the Fermi level close to the one derived from band structure calculations. The reduction of TN with increasing Ru content follows the predictions of the J1-J2 model on a square lattice, which appears to be an effective framework to describe the magnetic properties of the magnetic ground state.