A. Richter

Invariant-mass spectroscopy of 10Li and 11Li

Break-up of secondary Li-11 ion beams (280 MeV/nucleon) on C and Pb targets into Li-9 and neutrons is studied experimentally. Cross sections and neutron multiplicity distributions are obtained, characterizing different reaction mechanisms. Invariant-mass spectroscopy for Li-11 and Li-10 is performed. The E1 strength distribution, deduced from electromagnetic excitation of Li-11 up to an excitation energy of 4 MeV comprises similar to 8% of the Thomas-Reiche-Kuhn energy-weighted sumrule strength. Two low-lying resonance-like structures are observed for Li-10 at decay energies of 0.21(5) and 0.62(10) MeV, the former one carrying 26(10)% of the strength and likely to be associated with an s-wave neutron decay. A strong di-neutron correlation in Li-11 can be discarded. Calculations in a quasi-particle RPA approach are compared with the experimental results for Li-10 and Li-11

Nanoindentation of diamond, graphite and fullerene films

Abstract The recently developed method of nanoindentation is applied to various forms of carbon materials with different mechanical properties, namely diamond, graphite and fullerite films. A diamond indenter was used and its actual shape determined by scanning force microscopy with a calibration grid. Nanoindentation performed on different surfaces of synthetic diamond turned out to be completely elastic with no plastic contributions. From the slope of the force–depth curve the Youngs modulus as well as the hardness were obtained reflecting a very large hardness of 95 GPa and 117 GPa for the {100} and {111} crystal surfaces, respectively. Investigation of a layered material such as highly oriented pyrolytic graphite again showed elastic deformation for small indentation depths but as the load increased, the induced stress became sufficient to break the layers after which again an elastic deformation occurred. The Young’s modulus was calculated to be 10.5 GPa for indentation in a direction perpendicular to the layers. Plastic deformation of a thin fullerite film during the indentation process takes place in the softer material of a molecular crystalline solid formed by C 60 molecules. The hardness values of 0.24 GPa and 0.21 GPa for these films grown by layer epitaxy and island growth on mica and glass, respectively, vary with the morphology of the C 60 films. In addition to the experimental work, molecular dynamics simulations of the indentation process have been performed to see how the tip–crystal interaction turns into an elastic deformation of atomic layers, the creation of defects and nanocracks. The simulations are performed for both graphite and diamond but, because of computing power limitations, for indentation depths an order of magnitude smaller than the experiment and over indentation times several orders of magnitude smaller. The simulations capture the main experimental features of the nanoindentation process showing the elastic deformation that takes place in both materials. However, if the speed of indentation is increased, the simulations indicate that permanent displacements of atoms are possible and permanent deformation of the material takes place.

Discovery of a Superhard Iron Tetraboride Superconductor

Single crystals of novel orthorhombic (space group Pnnm) iron tetraboride FeB4 were synthesized at pressures above 8 GPa and high temperatures. Magnetic susceptibility and heat capacity measurements demonstrate bulk superconductivity below 2.9 K. The putative isotope effect on the superconducting critical temperature and the analysis of specific heat data indicate that the superconductivity in FeB4 is likely phonon mediated, which is rare for Fe-based superconductors. The discovered iron tetraboride is highly incompressible and has the nanoindentation hardness of 62(5) GPa; thus, it opens a new class of highly desirable materials combining advanced mechanical properties and superconductivity.

Nanoscale mechanical properties of polymers irradiated by UV

Abstract The application of depth sensing nanoindentation to determine mechanical properties of three different polymers is described in this work using three different techniques to calibrate the measurement system. The nano-hardness and the elastic indentation modulus of polyvinyl chloride, polyethylene oxide and polyacrylic acid were inferred from nanomechanical tests, and the influence of ultraviolet irradiation on the mechanical properties of measured polymers is studied. A multicycling test—a sequence of several loading and unloading procedures—allowed the measurement of changes in the sample viscoelasticity. The nano-hardness of the polymers is shown to increase with radiation dose while the viscoelasticity decreases.

Longitudinal momentum distributions of 16,18C fragments after one-neutron removal from 17,19C

The fragment separator FRS at GSI was used as an energy-loss spectrometer to measure the longitudinal momentum distributions of C-16,C-18 fragments after one-neutron removal reactions in C-17,C-19 impinging on a carbon target at about 910 MeV/u. The distributions in the projectile frames are characterized by a FWHM of 141 +/- 6 MeV/c for C-16 and 69 +/- 3 MeV/c for C-18. Th, results are compared with experimental data obtained at lower energies and discussed within existing theoretical models

Aggregated diamond nanorods, the densest and least compressible form of carbon

We report the synthesis of aggregated diamond nanorods (ADNRs) from fullerene C60 at 20(1) GPa and 2200  °C using a multianvil apparatus. Individual diamond nanoroads are of 5–20 nm in diameter and longer than 1μm. The x-ray and measured density of ADNRs is ∼0.2%–0.4% higher than that of usual diamond. The extremely high isothermal bulk modulus KT=491(3)GPa [compare to KT=442(4)GPa of diamond] was obtained by in situ x-ray diffraction study. Thus, ADNRs is the densest among all carbon materials and it has the lowest so far experimentally determined compressibility.

Atomistic modelling of nanoindentation in iron and silver

Experimental and theoretical investigations of nanoindentation into fcc silver and bcc iron were performed, including an investigation of the effect near the grain boundaries. Experimentally, micrograph images of the surfaces and force-depth curves were obtained which were used to determine the hardness and Youngs modulus of the materials. Molecular dynamics simulations, on smaller systems than those investigated experimentally, exhibit the main experimental attributes, showing the plastic deformation of the substrates with piling-up of the work material along the indenter sides. The simulations also show how defects in the substrates form and these are contrasted with the various materials under investigation.

Atomistic modelling of ploughing friction in silver, iron and silicon

Molecular dynamics (MD) simulations of atomic-scale stick–slip have been performed for a diamond tip in contact with the (100) surface of fcc Ag, bcc Fe, Si and H-terminated Si, at a temperature of 300 K. Simulations were carried out at different support displacements between 5 and 15 A. The simulations illustrate the important mechanisms that take place during stick–slip. In particular, for the case of the metals they show a direct link between tip slip events and the emission of dislocations from the point of contact of the tip with the substrate. This occurs both during indentation and scratching. For the case of silicon, no slip events were observed and no subsurface dislocations were generated underneath the scratch groove. At the deeper support displacement of 15 A the silicon atoms undergo some local phase transformations and the atom coordination number varies between 5 and 8, with the majority being five-fold or six-fold coordinated. Both the dynamic and the static friction coefficients were found to be higher for Si compared to the corresponding values for H-terminated Si. Comparisons were made between the MD simulations and experimental measurements for indentation on the (100) surface of Si and Al. A good qualitative agreement was observed between the experimental and theoretical results. However, in both the cases of Si and metals the MD simulations give a contact pressure under load that is depth dependent and values that are higher than experimental nanohardness values.

Fine structure of the E1 response in 140Ce below the particle threshold

Abstract The E1 response of the semi-magic nucleus 140Ce below the particle threshold was measured in a (γ,γ′) experiment utilizing the new Euroball Cluster detector at the S-DALINAC. While the energy averaged data are in good agreement with tagged photon results, here they are resolved for the first time into 54 individual transitions. A quasiparticle-phonon model calculation including up to three-phonon configurations compares well to the extracted strength distribution. The interference between one- and two-phonon contributions is essential for a quantitative reproduction.

Invariant mass spectrum and alpha-n correlation function studied in the fragmentation of He-6 on a carbon target

Momentum distributions and invariant mass spectra from the breakup of He-6 ions with an energy of 240 MeV/u interacting with a carbon target have been studied. The data were used to extract information about the reaction mechanism which is influenced by the structure of He-6. It is found that the dominant reaction mechanism is a two-step process: knock out of one neutron followed by the decay of the He-5 resonance. The shape of the (alpha+n) two-body invariant mass spectrum is interpreted as mainly reflecting the 5He ground state which is a J(pi) = 3/2(-) resonance. However, no evidence for correlations between cu particles and neutrons is observed in the momentum widths of the distributions. It is demonstrated that a combined analysis of the two-body invariant mass spectrum and an appropriate correlation function may be used to determine the properties of the intermediate resonance