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Schoop L.M.,Graduate School Material Science in Mainz | Schoop L.M.,Johannes Gutenberg University Mainz | Medvedev S.A.,Max Planck Institute for Chemistry | Ksenofontov V.,Johannes Gutenberg University Mainz | And 10 more authors.
Physical Review B - Condensed Matter and Materials Physics

Copper doping of FeSe destroys its superconductivity at ambient pressure, even at low doping levels. Here we report the pressure-dependent transport and structural properties of Fe1.01-xCuxSe with 3% and 4% Cu doping and find that the superconductivity is restored. Metallic resistivity behavior, absent in Cu-doped FeSe, is also restored. At the low pressure of 1.5 GPa, superconductivity is seen at 6 K for 4% Cu doping, somewhat lower than the 8 K Tc of undoped FeSe. Tc reaches its maximum of 31.3 K at 7.8 GPa, lower than the maximum superconducting temperature in the undoped material under pressure (Tc max of 37 K) but still very high. X-ray diffraction shows that applied pressure decreases the lattice parameter in the basal plane, counteracting the structural effect of Cu doping, providing a possible explanation for the restoration of the superconductivity. © 2011 American Physical Society. Source

Trefz B.,Graduate School Material Science in Mainz | Trefz B.,Johannes Gutenberg University Mainz | Virnau P.,Johannes Gutenberg University Mainz
Journal of Physics Condensed Matter

Large scale molecular dynamics simulations on graphic processing units (GPUs) are employed to study the scaling behavior of ring polymers with various topological constraints in melts. Typical sizes of rings containing 31, 51 knots and catenanes made up of two unknotted rings scale like N1/3 in the limit of large ring sizes N. This is consistent with the crumpled globule model and similar findings for unknotted rings. For small ring lengths knots occupy a significant fraction of the ring. The scaling of typical ring sizes for small N thus depends on the particular knot type and the exponent is generally larger than 0.4. © 2015 IOP Publishing Ltd. Source

Schoop L.M.,Princeton University | Allred J.M.,Princeton University | Ni N.,Princeton University | Hirai D.,Princeton University | And 5 more authors.
Journal of Solid State Chemistry

The structure and elementary physical properties of a new intermetallic compound, β-HfCuGe, are reported. β-HfCuGe has a tetragonal structure (space group I4/mmm) with lattice constants of a=3.7634(11) Å and c=13.499(4) Å. The structure, which consists of double layers of Hf stacked with edge-sharing CuGe4 squares, is not typical for intermetallic compounds and appears to be a new structure type. The compound is a weak paramagnet and a normal metal down to 0.4 K. © 2012 Elsevier Inc. All rights reserved. Source

Schoop L.,Graduate School Material Science in Mainz | Schoop L.,Princeton University | Muchler L.,Max Planck Institute for Chemical Physics of Solids | Schmitt J.,Johannes Gutenberg University Mainz | And 8 more authors.
Physical Review B - Condensed Matter and Materials Physics

The ternary superconductor NaAlSi, isostructural with LiFeAs, the "111" iron pnictide superconductor, is investigated under pressure. The structure remains stable up to 15 GPa. Resistivity and susceptibility measurements show an increase of Tc up to 2 GPa, followed by a decrease until superconductivity disappears at 4.8 GPa. Band structure calculations show that pressure should have a negligible effect on the electronic structure and the Fermi surface and thus the disappearance of superconductivity under pressure must have a different origin. We compare the electronic structure of NaAlSi under pressure with that of nonsuperconducting isostructural NaAlGe. © 2012 American Physical Society. Source

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