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Background: Coherent {\pi}0 photoproduction on heavy nuclei has been suggested as a reliable tool to infer neutron skin thicknesses. To this aim, various experiments have been performed, especially on 208Pb. Purpose: We analyze the sensitivity of that reaction process to the nucleonic density, and especially to the neutron skin thickness, for 12C, 40Ca and 208Pb, for which reliable data exist, and on 116,124Sn, for which measurements have been performed in Mainz. We study also the role played by the {\pi}0-nucleus final-state interaction. Method: A model of the reaction is developed at the impulse approximation considering either plane waves or distorted waves to describe the {\pi}0-nucleus scattering in the outgoing channel. Results: Our calculations are in good agreement with existing data, especially for 208Pb. The sensitivity of the theoretical cross sections to the choice of the nucleonic density is small, and below the experimental resolution. Conclusions: Coherent {\pi}0 photoproduction is mostly an isoscalar observable that bares no practical sensitivity to the neutron skin thickness. To infer that structure observable it should be coupled to other reaction measurements, such as electron scattering, or by comparing experiments performed on isotopes of the same chemical element., Comment: Accepted for publication in Phys. Rev. C. One new Ref.[21]

The scalar dipole polarizabilities, $\alpha_{E1}$ and $\beta_{M1}$, are fundamental properties related to the internal dynamics of the nucleon. The currently accepted values of the proton polarizabilities were determined by fitting to unpolarized proton Compton scattering cross section data. The measurement of the beam asymmetry $\Sigma_{3}$ in a certain kinematical range provides an alternative approach to the extraction of the scalar polarizabilities. At the Mainz Microtron (MAMI) the beam asymmetry was measured for Compton scattering below pion photoproduction threshold for the first time. The results are compared with model calculations and the influence of the experimental data on the extraction of the scalar polarizabilities is determined., Comment: 6 pages, 5 figures

The possibility of multi-band conductivity and multi-gap superconductivity is explored in oriented V3Si thin films by means of reflectance and transmittance measurements at terahertz frequencies. The temperature dependence of the transmittance spectra in the normal state gives evidence of two bands contributing to the film conductivity. This outcome is consistent with electronic structure calculations performed within density functional theory. On this basis, we performed a detailed data analysis and found that all optical data can be consistently accounted for within a two-band framework, with the presence of two optical gaps in the superconducting state corresponding to 2D=kTc values close to 1.8 and 3.8., Comment: 6 pages with 4 figures

In this paper the magnetic behaviour of undoped and 15% F doped SmFeAsO (Sm-1111) and LaFeAsO (La-1111) samples is presented and discussed. Magnetization measurements are not a simple tool to use for the characterisation of the new family of Fe-based superconductors because magnetic impurities can be easily formed during the preparation procedure and may affect the magnetic signal. In spite of this problem bulk magnetization measurements, properly treated, may give very useful information. In the undoped samples we gathered the main aspects of the physical behavior of the 1111 phase, i.e. the onset of the Spin Density Wave (SDW), the antiferromagnetic ordering at the Sm sublattice and the susceptibility increase with increasing temperature above the SDW temperature, and, in addition, we were able to estimate the Pauli contribution to susceptibility and therein the Wilson ratio both for LaFeAsO and SmFeAsO compounds, and the amplitude of the jump at the SDW temperature. In the doped samples, while the presence of magnetic signals due to impurities is dominating in the normal state, the superconducting behavior may be clearly observed and studied. In particular, in the Sm-1111 superconducting sample the coexistence-competition between superconductivity and antiferromagnetic ordering of the Sm ions was clearly observed., Comment: in press on Journal of Magnetism and Magnetic Materials

We measure magnetotransport of F doped SmFeAsO samples up to 28T and we extract the upper critical fields, using different criteria. In order to circumvent the problem of criterion-dependence Hc2 values, we suggest a thermodynamic estimation of the upper critical field slope dHc2/dT based on the analysis of conductivity fluctuations in the critical regime. A high field slope as large as -12T/K is thus extracted for the optimally doped sample. We find evidence of a two-dimensional lowest Landau level (LLL) scaling for applied fields larger than mu_0H_LLL=8T. Finally, we estimate the coherence length values and we observe that they progressively increase with decreasing Tc. In all cases, the coherence length values along the c axis are smaller than the interplanar distance, confirming the two-dimensional nature of superconductivity in this compound., Comment: 14 pages, 4 figures, 3 tables Submitted to Phys.Rev.B

We report measurements of resistivity, magnetoresistivity, Hall effect, Seebeck coefficient, infrared reflectivity of undoped SmFeAsO and lightly doped SmFeAs(O0.93F0.07) oxypnictides. All the properties measured on SmFeAsO are characterized by clear signatures of the magnetic instability. A self-consistent picture emerges in which below the magnetic transition carrier condensation occurs due to the opening of spin density wave (SDW) gap. This is accompanied by the mobility increase of not gapped carriers due to the suppression of electron-electron scattering. SmFeAs(O0.93F0.07) exhibits an increase of the metallic character on cooling consistent with electron doping, even though at room temperature values of all the properties nearly overlaps with those of SmFeAsO. However, with temperature decrease all anomalies related to the SDW instability are missed and the superconducting transition occurs. This suggests that doping breaks abruptly the symmetries of the Fermi surface inhibiting the SDW formation in favor of the superconducting transition, with no substantial changes in the density of states or in the effective mass., Comment: Submitted to SUST

In this paper the magnetic behaviour of SmFeAsO and SmFeAs(O0.85F0.15) samples is presented and discussed. Molar susceptibility of SmFeAsO exhibits a local peak at T around 140K due to the establishment of a long range antiferromagnetic ordering of the Fe moments in Fe-As layers. This feature has already been observed with different techniques, and frequently ascribed to the onset of a Spin Density Wave (SDW). At TN around 6K another peak, which we attribute to the establishment of antiferromagnetic ordering of Sm ion sublattice, is observed. Furthermore, a temperature independent signal (Pauli paramagnetism, Landau and core diamagnetism...) is also present in the magnetic behaviour of this sample. In SmFeAs(O0.85F0.15) the antiferromagnetic ordering in Fe-As plane is suppressed and superconductivity occurs at T = 52 K, whereas the antiferromagnetic ordering of Sm ions at low temperature persists, leading to the coexistence and competition between superconducting and magnetic orderings. Above the transition temperature, after the subtraction of the Sm ion sublattice paramagnetic contribution and of the temperature independent contribution to the experimental susceptibility data, a Curie-Weiss behaviour for Fe is observed, with a magnetic moment of 1.4 Bohr magneton., Comment: submitted to Physical Review B

SmFeAsO and the isostructural superconducting SmFeAs(O0.93F0.07) samples were prepared. Characterization by means of Rietveld refinement of X-ray powder diffraction data, scanning electron microscope observation, transmission electron microscope analysis, resistivity and magnetization measurements were carried out. Sintering treatment strongly improves the grain connectivity, but, on the other hand, induces a competition between the thermodynamic stability of the oxy-pnictide and Sm2O3, hence worsening the purity of the sample. In the pristine sample both magnetization and resistivity measurements clearly indicate that two different sources of magnetism are present: the former related to Fe ordering at 140 K and the latter due to the Sm ions that orders antiferromagnetically at low temperature. The feature at 140 K disappears in the F-substituted sample and, at low temperatures a superconducting transition appears. The magnetoresistivity curves of the F-substituted sample probably indicates very high critical field values., Comment: Submitted to: Superconductor Science and Technology

The crystal and magnetic structure of (La0.70Ca0.30)(CryMn1-y)O3 for y = 0.70, 0.50 and 0.15 has been investigated using neutron powder diffraction. The three samples crystallize in the Pnma space group at both 290 K and 5 K and exhibit different magnetic structures at low temperature. In (La0.70Ca0.30)(Cr0.70Mn0.30)O3, antiferromagnetic order with a propagation vector k = 0 sets in. The magnetic structure is Gx, i.e. of the G-type with spins parallel to the a-axis. On the basis of our Rietveld refinement and the available magnetisation data, we speculate that only Cr3+ spins order, whereas Mn4+ act as a random magnetic impurity. In (La0.70Ca0.30)(Cr0.50Mn0.50)O3 the spin order is still of type Gx, although the net magnetic moment is smaller. No evidence for magnetic order of the Mn ions is observed. Finally, in (La0.70Ca0.30)(Cr0.15Mn0.85)O3 a ferromagnetic ordering of the Mn spins takes place, whereas the Cr3+ ions act as random magnetic impurities with randomly oriented spins., Comment: 7 pages, 5 figures, 2 tables

Oxygen vacancies are introduced in the RuO2 and possibly CuO2 planes of RuSr2GdCu2O8 by annealing in vacuum above 600 K. The diffusive jumps of the O vacancies are accompanied by a reorientation of the local distortion, and are probed by measuring the elastic energy loss and modulus versus temperature at 1-10 kHz. An intense acoustic absorption peak develops near 670 K at 1 kHz and finally stabilizes after heating up to 920 K in vacuum. The analysis of the peak shows a barrier for the O diffusion of ~1.4 eV, and a slowing down of Curie-Weiss type, with Tc = 400-470 K, due to the interaction among the O vacancies. A secondary peak is attributed to O vacancies trapped at defects in the RuO2 planes, or to vacancies in the CuO2 planes. No sign of structural phase transformation is found up to 920 K., Comment: 5 pages, 2 figures, submitted to Phys. Rev. B

A systematic study on the synthesis of the Ru-1212 compound by preparing a series of samples that were annealed at increasing temperatures and then quenched has been performed. It results that the optimal temperature for the annealing lies around 1060-1065 C; a further temperature increase worsens the phase formation. Structural order is very important and the subsequent grinding and annealing improves it. Even if from the structural point of view the samples appear substantially similar, the physical characterization highlight great differences both in the electrical and magnetic properties related to intrinsic properties of the phase as well as to the connection between the grains as inferred from the resistive and the Curie Weiss behaviour at high temperature as well as in the visibility of ZFC anf FC magnetic signals., Comment: 17 pages, 12 figures. Proc. Int. Workshop " Ruthenate and rutheno-cuprate materials: theory and experiments", Vietri, October 2001. To be published on LNP Series, Springer Verlag, Berlin, C. Noce, A. Vecchione, M. Cuoco, A. Romano Eds, 2002

The evolution of an anelastic relaxation process occurring around 80 K in La{2-x}Sr{x}CuO{4} at a measuring frequency of ~1 kHz has been followed from x = 0.0075 to the overdoped region, x = 0.2, where it disappears. The dependence of the peak intensity on doping is consistent with a polaronic mechanism, identified with the disordered charge stripes overcoming pinning centers. A marked decrease of the peak amplitude occurs at x > 0.045, the same doping range where a change of the stripe order from parallel to diagonal with respect to the Cu-O bonds has been observed by neutron diffraction. Both the energy barrier and peak amplitude also exhibit a rise near x = 1/8., Comment: 5 pages, 4 figures

The anelastic spectra of La$_{2-x}$Sr$_{x}$CuO$_{4}$ have been measured at liquid He temperatures slightly below and above the concentration $% x_{c}\simeq 0.02$ which is considered to separate the spin-glass phase from the cluster spin-glass (CSG) phase. For $x\le x_{c}$ all the elastic energy loss functions show a step below the temperature $T_{g}(x=0.02) $ of freezing into the CSG state, similarly to what found in samples well within the CSG phase, but with a smaller amplitude. The excess dissipation in the CSG state is attributed to the motion of the domain walls between the clusters of antiferromagnetically correlated spin. These results are in agreement with the recent proposal, based on inelastic neutron scattering, of an electronic phase separation between regions with $x\sim 0$ and $x\sim 0.02$, at least for $x>0.015$, Comment: 5 pages, 3 figures, submitted to Phys. Rev. B

The complex dynamic Young's modulus of ceramic Nd2-xCexCuO4 with x = 0, 0.05 and 0.20 has been measured from 1.5 to 100 K at frequencies of 1-10 kHz. In the undoped sample the modulus starts decreasing below ~20 K, instead of approaching a constant value as in a normal solid. The modulus minimum has been interpreted in terms of paraelastic contribution from the relaxation of the Nd^3+ 4f electrons between the levels of the ground state doublet, which is split by the interaction with the antiferromagnetically ordered Cu sublattice. The value of the splitting is found to be 0.34 meV, in excellent agreement with inelastic neutron scattering, infrared and specific heat experiments. With doping, the anomaly shifts to lower temperature and decreases in amplitude, consistently with a reduction of the local field from the Cu sublattice., Comment: 5 pages, 2 figures, submitted to Eur. Phys. J. B

We report on structural, electrical and magnetic properties of RuSr2GdCu2O8 samples made by systematic synthesis work so that they differ from one onether in number of annealings and sintering temperature. Our aim is to highlight the different behaviours as a consequence of the thermal treatments. In particular, we deal with the onset of superconductivity in relation to the homogenisation process, and its influence on resistivity, structural and magnetic ordering. Finally, the problems related to the magnetic measurements due to the superimposition of different magnetic states are examined., Comment: 10 pages, 5 figures, 1 table. to be published in Physica C

We made point-contact measurements with Au tips on La_{2-x}Sr_{x}CuO_{4} samples with 0.08 < x < 0.20 to investigate the relationship between superconducting gap and pseudogap. We obtained junctions whose conductance curves presented typical Andreev reflection features at all temperatures from 4.2 K up to T_c^A close to the bulk T_c. Their fit with the BTK-Tanaka-Kashiwaya model gives good results if a (s+d)-wave gap symmetry is used. The doping dependence of the low temperature dominant isotropic gap component Delta_{s} follows very well the T_{c} vs. x curve. These results support the separation between the superconducting (Andreev) gap and the pseudogap measured by angle-resolved photoemission spectroscopy (ARPES) and tunneling., Comment: 4 pages, 5 eps figures, 1 table. SNS 2001 Conference

The anelastic spectra (elastic energy loss and dynamic modulus versus temperature) of La1-xCaxMnO3 have been measured for 0.33< x < 0.5. A peak in the imaginary part of the elastic susceptibility is found slightly below the Curie temperature, whose temperature and frequency dependences are typical of the dielectric and magnetic susceptibilities of relaxor ferroelectrics, spin glasses and other inhomogeneous frustrated systems. The fluctuations can be associated with the magnetic clusters which are observed by several techniques or with charge-ordered insulating domains in the low temperature inhomogeneous phase. The temperature dependent distribution function of the fluctuation energy barriers has been extracted by the analysis of the experimental curves., Comment: 4 pages, 3 figures

We present new Au/La_{2-x}Sr_{x}CuO_{4} (LSCO) point-contact conductance measures as a function of voltage and temperature in samples with 0.08 <= x <= 0.2. Andreev reflection features disappear at about the bulk Tc, giving no evidence of gap for T > Tc. The fit of the normalized conductance at any T < Tc supports a (s + d)-wave symmetry of the gap, whose dominant low-T s component follows the Tc(x) curve in contrast with recent angle-resolved photoemission spectroscopy and quasiparticle tunneling data. These results prove the separation between pseudogap and phase-coherence superconducting gap in LSCO at x <= 0.2., Comment: 4 pages, 4 eps figures, 1 table (RevTeX). Labels added to Fig. 1; Fig. 3 resized; references added; short discussion about ballistic contact regime added

We compare the low frequency (~1 kHz) anelastic spectra of La_{2-x}Sr_xCuO_4 and La_{2-x}Ba_xCuO_4 at x = 0.03 and 0.06 in the temperature region where the freezing into the cluster spin-glass (CSG) phase occurs and is accompanied by an increase of the acoustic absorption. The dependence of the amplitude of the anelastic relaxation on doping is explained in terms of movement of the domain walls (DW) in the CSG phase between the Sr (Ba) pinning points. The LBCO sample at x = 0.06 transforms into the LTT structure below 40 K and the amplitude of the anelastic anomaly is 7 times smaller than expected, indicating pinning of the DW which run parallel to the LTT modulation. Such DW can be identified with the stripes of high hole density, and the present measurements show that they are mobile between the Sr (Ba) pinning points down to few kelvin, but become static in the presence of LTT modulation also far from the condition x = 1/8 for commensurability between stripe and lattice periodicities., Comment: 4 figures

An increase of the acoustic absorption is found in La_{2-x}Sr_{x}CuO_{4} (x = 0.019, 0.03 and 0.06) close to the temperatures at which freezing of the spin fluctuations in antiferromagnetic-correlated clusters is expected to occur. The acoustic absorption is attributed to changes of the sizes of the quasi-frozen clusters induced by the vibration stress through magnetoelastic coupling., Comment: LaTeX, 2 PostScript figures, submitted to Phys. Rev. B

The anelastic spectrum of La{2-x}Sr{x}CuO{4} (x = 0, 0.008, 0.019, 0.032) has been measured down to 1.5 K, in order to see the effect of doping on the intrinsic lattice fluctuations already found in stoichiometric La{2}CuO{4}, and identified with tunneling driven tilt modes of the O octahedra. Slight doping with Sr causes a drastic increase of the transition rates and relaxation strength of the tunneling systems. The influence of doping on the relaxation rate is interpreted in terms of direct coupling between between the tilts of the octahedra and the hole excitations. However, the observed fast dependence of the rate on temperature cannot be explained in terms of the ususal models of coupling between a tunneling system and the conduction electrons., Comment: LaTeX, 5 figures in a single PostScript file, submitted to Phys. Rev. B