R. De Renzi

Evidence of orbital reconstruction at interfaces in ultrathin La0.67Sr0.33MnO3 films.

Linear dichroism (LD) in x-ray absorption, diffraction, transport, and magnetization measurements on thin La(0.7)Sr(0.3)MnO(3) films grown on different substrates, allow identification of a peculiar interface effect, related just to the presence of the interface. We report the LD signature of preferential 3d-e(g)(3z(2)-r(2)) occupation at the interface, suppressing the double exchange mechanism. This surface orbital reconstruction is opposite to that favored by residual strain and is independent of dipolar fields, the chemical nature of the substrate and the presence of capping layers.

Preparation and characterization of LaMnO3 thin films grown by pulsed laser deposition

We have grown LaMnO3 thin films on (001) LaAlO3 substrates by pulsed laser deposition. X-ray diffraction confirms that the films are only slightly relaxed and are oriented “square on square” relative to the substrate. The measured Raman spectra closely resemble that observed in bulk LaMnO3, which indicates no relevant distortions of the MnO6 octahedra induced by the epitaxial strain. Therefore, no detectable changes in the lattice dynamics occurred in our LaMnO3 strained films relative to the bulk case. Mn55 nuclear magnetic resonance identifies the presence of localized Mn4+ states. Superconducting quantum interference device magnetization measures TN=131(3)K and a saturation moment μ=1.09μB∕Mn, revealing a small concentration of Mn4+ and placing our films within the antiferromagnetic insulating phase.

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).

Observation of superconductivity in TDAE-C60

Abstract SQUID magnetometry performed on the fullerene based charge transfer salt TDAE-C 60 shows the presence of a superconducting phase below T c = 17.4 K, coexisting with the already known magnetic phase. A large fraction Meissner effect ( f ≈ 0.3) appears only when the compound is slowly cooled across the temperatures corresponding to (partial) freezing of the molecular rotations. The ferromagnetic response prevails over the Meissner effect for H > 6 Oe. A strong diamagnetic contribution to the RF susceptibility, appearing below T c , confirms these findings. This phenomenon sheds new light on the complementary nature of super-conductivity and magnetism in low dimensional Mott-Hubbard systems.

Ultraslow Polaron Dynamics in Low-Doped Manganites fromL139aNMR-NQR and Muon Spin Rotation

We report a (139)La NMR investigation of low-doped insulating manganite samples (LaMnO(3+delta) and La(1-y)Ca(y)MnO(3+delta)) as a function of temperature. A volume fraction with fast nuclear relaxations was revealed by the inhomogeneous loss of the NMR signal over a broad temperature interval. Comparison with muon spin rotation data demonstrates that the wipeout of the (139)La signal is mainly due to slowly fluctuating electric field gradients. This provides strong evidence for the slow diffusion of lattice excitations, identified with Jahn-Teller small polarons.

Structural and magnetic properties of In1−xMnxSb: Effect of Mn complexes and MnSb nanoprecipitates

Structural and magnetic properties of the group III-V diluted magnetic semiconductor In1−xMnxSb with x = 0.005–0.06, including the nuclear magnetic resonance (NMR) investigations, are reported. Polycrystalline In1−xMnxSb samples were prepared by direct alloying of indium antimonide, manganese and antimony, followed by a fast cooling of the melt with a rate of 10–12 K/s. According to the X-ray diffraction data, part of Mn is substituted for In, forming the In1−xMnxSb matrix. Atomic force microscopy and scanning tunneling microscopy investigations provide evidence for the presence of microcrystalline MnSb inclusions (precipitates), having a size of ∼100–600 nm, and the fine structure of nanosize grains with a Gaussian distribution around the diameter of ∼24 nm. According to the NMR spectra, the majority of Mn enters the MnSb inclusions. In addition to the single Mn ions, which contribute to the magnetization M (T) only in the low-temperature limit of T < 10–20 K, and MnSb nanoprecipitates responsible for th...

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

Vortex dynamics and irreversibility line in optimally doped SmFeAsO0.8F0.2from ac susceptibility and magnetization measurements

Ac susceptibility and static magnetization measurements were performed in the optimally doped SmFeAsO

Magnetic states of lightly hole-doped cuprates in the clean limit as seen via zero-field muon spin spectroscopy

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