T. Shiroka

s-wave pairing in the optimally doped LaO_{0.5}F_{0.5}BiS_{2} superconductor

We report on the magnetic and superconducting properties of LaO0.5F0.5BiS2 by means of zero- (ZF) and transverse-field (TF) muon-spin spectroscopy measurements (uSR). Contrary to previous results on iron-based superconductors, measurements in zero field demonstrate the absence of magnetically ordered phases. TF-uSR data give access to the superfluid density, which shows a marked 2D 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.

Correlated trends of coexisting magnetism and superconductivity in optimally electron-doped oxypnictides.

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 SmFe(1-x)Ru(x)AsO(0.85)F(0.15) for 0.1≤x≲0.5. The two reduced order parameters coexist within nanometer-size domains in the FeAs layers and eventually disappear around a common critical threshold x(c)~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 x(c).

Unusual polymerization in the Li4C60 fulleride

Li4C60, one of the best representatives of lithium intercalated fullerides, features a novel type of 2D polymerization. Extensive investigations, including laboratory x-ray and synchrotron radiation diffraction, 13C NMR, MAS and Raman spectroscopy, show a monoclinic I2/m structure, characterized by chains of [2+2]-cycloaddicted fullerenes, sideways connected by single C-C bonds. This leads to the formation of polymeric layers, whose insulating nature, deduced from the NMR and Raman spectra, denotes the complete localization of the electrons involved in the covalent bonds.

Magnetic-superconducting phase boundary of SmFeAsO1-xFx studied via muon spin rotation: Unified behavior in a pnictide family

We present uSR investigations on SmFeAsO_1-xF_x showing coexistence of magnetic order and superconductivity only in a very narrow F-doping range. The sharp crossover between the two types of order is similar to that observed in LaFeAsO_1-xF_x, suggesting a common behavior for the 1111 pnictides. The analysis of the muon asymmetry demonstrates that the coexistence must be nanoscopic, i.e. the two phases must be finely interspersed over a typical length-scale of few nm. In this regime both the magnetic and the superconducting transition temperatures collapse to very low values. Our data suggest a competition between the two order parameters

A multiple-quantum nuclear magnetic resonance study of interstitial Li clusters in LixC60

We characterize the local Li structure in the low-temperature phase of LixC60 with 6⩽x⩽15 using single and multiple-quantum 7Li nuclear magnetic resonance spectroscopy. We find clear evidence for Li cluster intercalated structures in all compositions. The inferred cluster sizes are significantly smaller than those suggested by the stoichiometry and could be explained by a more disordered LixC60 low-temperature phase than expected from the current models.

Long- to short-range magnetic order in fluorine-doped CeFeAsO

as a function of theuorine content xwas investigated primarily via zero- eld muon-spin spectroscopy. The long-rangemagnetic order observed in the undoped compound gradually turns into a short-range order atx= 0:04, seemingly accompanied/induced by a drastic reduction of the magnetic moment of theiron ions. Superconductivity appears upon a further increase in doping (x>0:04) when, unlike inthe cuprates, the Fe magnetic moments become even weaker. The resulting phase diagram evidencesthe presence of a crossover region, where the superconducting and the magnetic order parameterscoexist on a nanoscopic range.

Distribution of NMR Relaxations in a Random Heisenberg Chain

NMR measurements of the (29)Si spin-lattice relaxation time T(1) were used to probe the spin-1/2 random Heisenberg chain compound BaCu(2)(Si(1-x)Ge(x))(2)O(7). Remarkable differences between the pure (x=0) and the fully random (x=0.5) cases are observed, indicating that randomness generates a distribution of local magnetic relaxations. This distribution, which is reflected in a stretched exponential NMR relaxation, exhibits a progressive broadening with decreasing temperature, caused by a growing inequivalence of magnetic sites. Compelling independent evidence for the influence of randomness is also obtained from magnetization data and Monte Carlo calculations. These results suggest the formation of random-singlet states in this class of materials, as previously predicted by theory.

Nanoscopic coexistence of magnetic and superconducting states within the FeAs layers of CeFeAsO 1 − x F x

We report on the coexistence of magnetic and superconducting states in

Recovering metallicity in A 4 C 60 : The case of monomeric Li 4 C 60

{\text{CeFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}

Field-Induced Quantum Soliton Lattice in a Frustrated Two-Leg Spin-1/2 Ladder

for