Synthesis and physical properties of LiBC intermetallics

Polycrystalline samples of LiBC compounds, which were predicted as possible candidate for high-Tc superconductivity, have been synthesised by a flux method and investigated by means of electrical resistivity and magnetic susceptibility. Scanning electron microscopy and X-ray diffraction patterns showed a plate-like morphology and a single-phase nature of LiBC samples for starting composition of Li1.25BC (flux composition). The lattice constants a, c display a systematic variation with x and has maximum volume of the hexagonal unit cell at x = 1.25. Electrical resistivity measurements revealed an extrinsic semi-conducting behaviour of the single-phase LiBC with an activation energy of 18 meV and a maximum specific resistivity of 2.5 Wcm at 300 K. In contrast to the theoretical prediction of high Tc, no superconducting features were detected down to 2 K both, by measurements of electrical resistivity and magnetic susceptibility.Comment: 17 pages, 6 figure

Distinct itinerant spin-density waves and local-moment antiferromagnetism in an intermetallic ErPd2 Si2 single crystal

Identifying the nature of magnetism, itinerant or localized, remains a major challenge in condensed-matter science. Purely localized moments appear only in magnetic insulators, whereas itinerant moments more or less co-exist with localized moments in metallic compounds such as the doped-cuprate or the iron-based superconductors, hampering a thorough understanding of the role of magnetism in phenomena like superconductivity or magnetoresistance. Here we distinguish two antiferromagnetic modulations with respective propagation wave vectors at Q± = (H ± 0.557(1), 0, L ± 0.150(1)) and QC = (H ± 0.564(1), 0, L), where (H, L) are allowed Miller indices, in an ErPd2Si2 single crystal by neutron scattering and establish their respective temperature- and field-dependent phase diagrams. The modulations can co-exist but also compete depending on temperature or applied field strength. They couple differently with the underlying lattice albeit with associated moments in a common direction. The Q± modulation may be attributed to localized 4f moments while the QC correlates well with itinerant conduction bands, supported by our transport studies. Hence, ErPd2Si2 represents a new model compound that displays clearly-separated itinerant and localized moments, substantiating early theoretical predictions and providing a unique platform allowing the study of itinerant electron behavior in a localized antiferromagnetic matrix

High-energy magnetic excitations from heavy quasiparticles in CeCu2_2Si2_2

Magnetic fluctuations is the leading candidate for pairing in cuprate, iron-based and heavy fermion superconductors. This view is challenged by the recent discovery of nodeless superconductivity in CeCu$_2$Si$_2$, and calls for a detailed understanding of the corresponding magnetic fluctuations. Here, we mapped out the magnetic excitations in \ys{superconducting (S-type)} CeCu$_2$Si$_2$ using inelastic neutron scattering, finding a strongly asymmetric dispersion for $E\lesssim1.5$~meV, which at higher energies evolve into broad columnar magnetic excitations that extend to $E\gtrsim 5$ meV. While low-energy magnetic excitations exhibit marked three-dimensional characteristics, the high-energy magnetic excitations in CeCu$_2$Si$_2$ are almost two-dimensional, reminiscent of paramagnons found in cuprate and iron-based superconductors. By comparing our experimental findings with calculations in the random-phase approximation,we find that the magnetic excitations in CeCu$_2$Si$_2$ arise from quasiparticles associated with its heavy electron band, which are also responsible for superconductivity. Our results provide a basis for understanding magnetism and superconductivity in CeCu$_2$Si$_2$, and demonstrate the utility of neutron scattering in probing band renormalization in heavy fermion metals

Metastable phase formation in undercooled Fe-Cr-Ni alloy melts

The solidification behaviour of undercooled Fe69Cr31-xNix melts, which represents the basis for the technically important stainless steels, was investigated in a wide composition range (7 at.% < x < 22 at.%) with respect to the competitive formation of ferrite (bcc) and austenite (fcc). The electromagnetic levitation technique was used for undercooling of bulk samples. The primary solidifying phase was identified by the analysis of time-resolved recalescence profiles that were detected by a fast responding photo diode at a rate of 1 MHz. Under equilibrium conditions the solidification mode changes from ferrite to austenite if the atomic fraction ratio of Ni/Cr = 0.5 is exceeded. It is shown that crystallization of bcc phase is preferred even at compositions where bcc is metastable. With rising undercooling a transition from primary fcc to primary bcc solidification occurs at a critical undercooling. The experimental results are compared to the predictions of the classical nucleation theory and an improved theory taking into account a finite thickness of the interface between the nucleus and the undercooled melt. The financial support from the Deutsche Forschungsgemeinschaft under contract no. He 1601/3 is gratefully acknowledged

Searching Dynamic Communities with Personal Indexes

Often the challenge of finding relevant information is reduced to find the &apos;right&apos; people who will answer our question. In this paper we present innovative algorithms called INGA (Interest-based Node Grouping Algorithms) which integrate personal routing indices into semantic query processing to boost performance