In the organic matter dynamic and fluid phases cannot survive down to temperatures of a few kelvins, temperature at which only low-inertial-mass groups, such as methyls, may exhibit fast rotation. Here we fabricate 3D-architectures of spontaneous molecular rotors by engineering in metal organic frameworks full-fledged barrierless rotors with exceptionally-low activation energy of 6.2 small cal mol-1. The trigonal bipyramidal symmetry of the rotator in the struts was frustrated by its arrangement in the cubic crystal cell, generating high multiplicity of 12 shallow minima per turn, with a benchmark 10 10 Hertz frequency persistent even below 2K. The nearly degenerated energy landscape allows for continuous unidirectional hyper-fast rotation, which lasts for hundreds of turns, by 'overflying' several minima. Such an impressive dynamic performance in solid organic matter is only comparable to that of methyl rotation, opening new fields of application whenever hyper-fast dynamics at extremely-low temperatures and minimization of thermal-noise are needed.

Ac susceptibility and static magnetization measurements were performed in the optimally doped SmFeAsO0.8F0.2 superconductor. The field -temperature phase diagram of the superconducting state was drawn and, in particular, the features of the flux line lattice derived. The dependence of the intra-grain depinning energy on the magnetic field intensity was derived in the thermallyactivated flux creep framework, enlightening a typical 1/H dependence in the high-field regime. The intra-grain critical current density was extrapolated in the zero temperature and zero magnetic field limit, showing a remarkably high value Jc0(0) ∼ 2 · 10 7 A/cm 2 , which demonstrates that this material is rather interesting for the potential future technological applications.

We report on the recovery of the short-range static magnetic order and on the concomitant degradation of the superconducting state in optimally F-doped SmFe1−xRuxAsO0.85F0.15 for 0.1 ≤ x 0.5. The two reduced order parameters coexist within nanometer-size domains in the FeAs layers and finally disappear around a common critical threshold xc ∼ 0.6. Superconductivity and magnetism are shown to be closely related to two distinct well-defined local electronic environments of the FeAs layers. The two transition temperatures, controlled by the isoelectronic and diamagnetic Ru substitution, scale with the volume fraction of the corresponding environments. This fact indicates that superconductivity is assisted by magnetic fluctuations, which are frozen whenever a short-range static order appears, and totally vanish above the magnetic dilution threshold xc.The appearance of high-T c superconductivity (SC) close to the disruption of static magnetic (M) order is a general feature of the Fe-based superconductors either as a function of doping or external pressure. In the REFeAsO family (RE1111) it is found that SC and M strongly compete and hardly coexist simultaneously [1, 2], apart for RE=Sm and Ce [3,4] within a small doping range where both order parameters are depressed. Coexistence implies short range magnetic order, that is detected only by local probes such as muon-spin rotation (µSR) [4] or nuclear quadrupole resonance (NQR) [5], since it eludes long coherence probes such as powder diffraction [6]. The competition between the superconducting and magnetic ground-states must be reconciled with the prevailing models of pairing mediated by spin fluctuations [7]. These models are seemingly in contradiction with the evidence that the two mutually excluding orders coexist only when phase separation occurs.Here we show, by means of µSR and 75 As NQR, that magnetism is surprisingly still at play in optimally F-doped SmFe 1−x Ru x AsO 0.85 F 0.15 . The isoelectronic Fe:Ru substitution is found to deteriorate the superconducting state in optimally electron-doped SmFeAsO 0.85 -F 0.15 samples and simultaneously to recover static magnetism within the FeAs layers, for 0.1 ≤ x 0.5. This is accompanied by a local electronic rearrangement within the FeAs layers. When Ru doping approaches the critical threshold x c = 0.6, corresponding to percolation of a magnetic square lattice with nearest neighbor (n.n.) and next-nearest neighbor hopping, both magnetism and SC vanish.The investigated polycrystalline SmFe 1−x Ru x AsO 0.85 -F 0.15 samples are the same of Ref. 8. From 19 F nuclear magnetic resonance the relative fluorine content was found to be constant within ∆ 0.01 in the whole set of samples investigated. To investigate the bulk character of the superconducting state we carried out transverse field (TF)-µSR measurements, where a sample is fieldcooled (FC) in a magnetic field larger than the lower superconducting critical field H c1 , applied perpendicular to the initial muon-spin orientation (H ⊥ S µ ). A flux-line lattice (FLL) i...

The evolution of the antiferromagnetic order parameter in CeFeAsO1−xFx as a function of the fluorine content x was investigated primarily via zero-field muon-spin spectroscopy. The long-range magnetic order observed in the undoped compound gradually turns into a short-range order at x=0.04, seemingly accompanied or induced by a drastic reduction of the magnetic moment of the iron ions. Superconductivity appears upon a further increase in doping (x>0.04) when, unlike in the cuprates, the Fe magnetic moments become even weaker. The resulting phase diagram evidences the presence of a crossover region, where the superconducting and the magnetic order parameters coexist on a nanoscopic range

We report on the coexistence of magnetic and superconducting states in CeFeAsO 1−x F x for x = 0.06͑2͒, characterized by transition temperatures T m = 30 K and T c = 18 K, respectively. Zero-field and transverse-field muon-spin-relaxation measurements show that below 10 K the two phases coexist within a nanoscopic scale over a large volume fraction. This result clarifies the nature of the magnetic-to-superconducting transition in the CeFeAsO 1−x F x phase diagram, by ruling out the presence of a quantum critical point which was suggested by earlier studies.The recent discovery of high-T c superconductivity ͑SC͒ close to the disruption of magnetic ͑M͒ order in Fe-based compounds has stimulated the scientific community to further consider the role of magnetic excitations in the pairing mechanism. In order to address this point it is necessary to understand how the ground state evolves from the M to the SC phase within each family of Fe-based superconductors. In the REFeAsO 1−x F x family ͑hereafter RE1111, with RE =La or a rare earth͒ early experiments have suggested that the M-SC crossover is RE dependent. For instance, a smooth reduction in the magnetic and superconducting ordering temperatures, T m and T c , respectively, was found for RE = Ce, 1 suggesting the presence of a quantum critical point. 2 For RE = Sm a partial coexistence of the M and SC states was found 3 while a first-order transition seems to occur for RE = La. 4 Successive studies 5-7 have shown that the doping region where T m and T c are both nonzero is virtually pointlike in Sm1111, demonstrating that the cases of RE = La and Sm can be reconciled under a unique behavior. 5 Recently, nanoscale electronic inhomogeneities have been shown to be present in both RE = La and Sm in a wide range above the crossover region. 8 Actually also the case of RE = Ce is susceptible to further investigation concerning the presence of electronic inhomogeneities in the superconducting dome or even the possible microscopic coexistence of magnetic ordering and superconductivity in the FeAs layers, 2 which might have eluded previous neutron-diffraction studies. 1 In fact, contrary to diffraction techniques, which cannot detect short-range magnetic order, muons act as local magnetic probes, hence making muon spectroscopy ͑SR͒ an ideal tool for this sort of investigations. For this reason SR has long been employed to study the M-SC coexistence in cuprates 9-13 as well as in other superconducting compounds, such as the ruthenocuprates, 14 or the heavy-fermion superconductors. [15][16][17] Here we report on zero-field ͑ZF͒-and transverse-field ͑TF͒-SR measurements on a sample of CeFeAsO 1−x F x which unambiguously show the coexistence of superconductivity and short-range magnetic order on a nanoscopic length scale. While in contradiction with previous experimental findings on the same compound, 1 this result closely resembles the behavior of Sm1111 at the M-SC crossover. 3,5-7

We report a detailed investigation of RECoPO (RE = La, Pr) and LaCoAsO materials performed by means of muon spin spectroscopy. Zero-field measurements show that the electrons localized on the Pr 3+ 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 La 3+and Pr 3+ , 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 sizeable modification in the band structure of the material upon increasing pressure. This scenario is qualitatively supported by ab-initio density-functional theory calculations.