Research area 1 Superconductivity and superconductors

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Reports from Research areas Sergey V. Borisenko, Alexander A. Kordyuk, Sibylle Legner, Konstantin Nenkov, Mark S. Golden, Jörg Fink Fig.: Fermi surface maps of Pb-Bi2212 compounds with different hole doping levels: underdoped (UD), as grown (AG) and overdoped (OD). Critical temperatures are also indicated. Funded by: BMBF, DFG Holger Bitterlich, Günter Behr, Stephan-Ludwig Drechsler, Jörg Fink, Günter Fuchs, Gerald Graw, Wolfgang Löser, Konstantin Nenkov, Ludwig Schultz Fig. Upper critical fields H c2 ll and H c2 ll vs. temperature T of a Tb 0.2 Y 0.8 Ni 2 B 2 C single crystal for different orientations parallel to the a-axis and parallel to the c-axis, respectively. Inset: Magnetization vs. external field plots for two different orientations M(H ll ) and M(H ll ) at T = 4.5 K. Funded by: DFG Doping dependence of the Fermi surface in Bi-based superconducting cuprates We have continued our systematic studies of the Fermi surface (FS) of Pb 0.44 Bi 1.56 Sr 2 CaCu 2 O 8+d by studying the evolution of the FS topology as a function of the hole doping level range, covering from underdoped (Tc=76 K) to overdoped (Tc=69K) samples. Utilizing the high energy and momentum resolution offered by our angle-scanning photoemission set-up, we recorded maps of the momentum distribution of the Fermi level photemission intensity, which give direct image of the FS (see Fig.). A visual inspection of the data already reveals that the size of the rounded barrels (representing the hole-like FS) increases on going from the underdoped to the overdoped regime. A more quantitative analysis is currently underway to determine the charge carrier concentration directly from the ARPES results. A further result is that the FS topology tends towards stronger nesting as the doping increases – i.e. the form changes from more circular to a squarer shape (with rounded corners). We stress however, that for all doping levels concerned, the FS remains hole-like, indicating that even for the strongest overdoped crystals the flat bands located near to the [p,0] point are still below the chemical potential. As regards the lifetime of the states involved, an analysis of the width of the E F momentum distribution curves crossing the FS shows that the lifetime varies along the FS following sin 2 (2f), where f is the angle with respect to the nodal direction measured with [p,p] as the origin. This would fit into a hot-spot / cold-spot scenario describing the k-dependence of the interactions in the system. Remarkably, however, the f dependence of the lifetime is independent of doping, which would not be expected from this picture. Finally, we note that relative intensity of the shadow FS to the main FS is found to be maximal at optimal doping, decreasing on going either to the under- or overdoped side, which suggests that the origin of this feature is important for our understanding the mechanism of the high temperature superconductivity. Cooperation: Institut de Physique Applique, EPFL, Lausanne, Switzerland, DP/IGA, EPFL, Lausanne, Switzerland, Univ. Birmingham, MPI für Festkörperforschung Stuttgart Crystal growth and intrinsic properties of Rare earth-Transition metal intermetallic compounds Bulk single crystals of Y 1-x Tb x Ni 2 B 2 C borocarbides have been grown by the floating zone method with inductive and optical heating, respectively, in order to study the interplay of paramagnetic Tb-ions with superconducting properties. Substitution of Y- by Tb-ions not only reduces the upper critical field H c2 , but an anisotropy of H c2 along the c-axis and in the plane perpendicular to the c-axis was induced. Moreover, the magnitude of the anisotropy can be tuned by changing the fraction x of Tb-ions. The maximum anisotropy was reached at x = 0.2 (Fig. 1). The anisotropy H c2 ll[001] > H c2 ll[100] is induced by the in-plane magnetic moments of Tb-ions, which is emphasized by the magnetization measurements. The behaviour is explained by a simplified theoretical model, which is primarily based on the interaction of conduction electrons with magnetic moments of the rare earth ions. For YNi 1-x Cu x BC compounds the effect of alloy composition on phase relations and superconducting properties was revealed. The peritectic solidification mode could be proven. A maximum Cu solubility of x Å 0.42 of YNi 1-x Cu x BC compounds was inferred. The superconducting transition temperature increases with both the Cu and B content up to a maximum T C = 9.1 K. For the Tb 2 PdSi 3 intermetallic compound the interaction between magnetic moments of Tb-ions with conduction electrons leads to a considerable anisotropy of the negative giant magnetoresistance (GMR) at low temperatures. An unusual anisotropy of the magnetocaloric effect was found for the first time for bulk Tb 2 PdSi 3 single crystals prepared. Cooperation: INPG-ENSPG Grenoble, Univ. Frankfurt, Tata Institute of Fundamental Research India, TU Clausthal, TU Dresden, Univ. of Minsk 54
Reports from Research areas Karl-Hartmut Müller, Günter Fuchs, Axel Handstein, Stefan-Ludwig Drechsler, Helge Rosner, Sergej Shulga, Jens Freudenberger, Konstantin Nenkov, Kerstin Häse, Stuart Wimbush, Bernhard Holzapfel, Holger Bitterlich, Wolfgang Löser, Günter Behr, Ludwig Schultz Fig.: Several quantities for Y x Lu 1-x Ni 2 B 2 C compounds determined by resisitivity (left panel) and specific heat measurements (right panel) vs. Y concentration: Superconducting transition temperature T c , the upper critical field parameters a and H c2 * (according to H c2 (T)=H c2 *[1-T/T c ] 1+a ), the residual resisitivity ratio RRR, the parameter ( of the relation g(H)/g N µ [H/H c2 (0)] 1-b and the Sommerfeld parameter g N . Funded by: DFG (SFB 463) Magnetism and superconductivity of borocarbides The effect of substitutional disorder-induced local lattice distortions on the superconducting properties of nonmagnetic Y x Lu 1-x Ni 2 B 2 C compounds was studied by resistivity and specific heat measurements. Substitutional disorder was found to reduce several relevant quantities as the superconducting transition temperature T c , the upper critical field H c2 (0) at T=0, a characteristic positive curvature of H c2 (T) observed for these compounds, the Sommerfeld parameter g N in the normal state as well as the curvature of g (H) with g as the electronic specific heat coefficient in the mixed state. These quantities have their highest values for the pure compounds and show a minimum for the highest degree of substitutional disorder at x ~ 0.5 without reaching the case of dirty limit where the curvatures of g(H) and of H c2 (T) at T c would disappear (i.e. b=0 and a=0). Starting from a two-band model description of H c2 (T) within the clean limit, a correlation between H c2 (0) and the unusual g(H) dependence was established and related to the impurity scattering rate. Also epitaxial YNi 2 B 2 C and HoNi 2 B 2 C thin films were prepared for the first time by UHV laser deposition. Epitaxial c-axis oriented films were produced at high deposition temperatures, whereas a-axis oriented YNi 2 B 2 C films could be prepared by a low deposition temperature. The availibilty of epitaxial thin films of nonmagnetic borocarbides enables phase-sensitive tunneling experiments which could help to clarify the symmetry of the superconducting order parameter in borocarbides. Furthermore, the effect of paramagnetic Tb-ions on H c2 (T) of Tb x Y 1-x Ni 2 B 2 C bulk single crystals was studied. With increasing Tb content x both T c and H c2 are reduced. Different from YNi 2 B 2 C a reversed anisotropy H c2 || (001) > H c2 || (100) was observed resulting from the in-plane magnetic moments of uncoupled Tb-ions. The magnitude of this anisotropy was found to rise up to a Tb concentration of x = 0.2. The reduction of the anisotropy for higher Tb concentrations can be explained by magnetic ordering of Tb-ions. Cooperation: TU Dresden, MPG cPfS, ILL Grenoble Stefan-Ludwig Drechsler, Helge Rosner, Ingo Opahle, Sergey Shulga, Helmut Eschrig Fig.1 Fermi surface and Fermi velocity (in atomic units) distribution of a band composed mainly by Ni 3d xy and Ni 3d z2 states for the Y(NiB) 2 C superconductor. Fig.2 Fermi surface and Fermi velocity (in atomic units) distribution of a band composed mainly by Ni 3d xy and Ni 3d z2 states for the La(NiB) 2 C nonsuperconductor. Electronic structure and superconductivity of transition metal borocarbides The role of the electronic structure in the mechanism of superconductivity and selected thermodynamic properties has been investigated in the frame of modern density functional theory as well as multi-band Eliashberg theory. The bandstructure, density of states, the distribution of Fermi velocities v F , the orbital character, and nesting properties of rather complex Fermi surfaces of both superconducting as well as nonsuperconducting members of the R(TB) 2 C family with R=Y, Lu, Sc, Th, La, Ho and T=Ni, Co, Pt, Pd, Re, Ru have been studied in detail. The great complexity of the first glance bandstructure can be resolved into relatively separate subsystems providing a well-defined basis for a systematic study of interesting many-body problems in near future. R(TB) 2 C-Superconductors are characterized by a broad distribution of Fermi velocities containing both very slow and very fast electrons as well as the presence of nested regions which play a decisive also in the phonon softening as well as in special incommensurate magnetic structures for R(TB)2C compounds with magnetic rare earth ions R. These features are absent in the nonsuperconducting members of the R(TB) 2 C family. Candidates for relatively isolated bands have been figured out which might alter conventional properties such as the shape of the upper critical field as well as the field dependence of the electronic specific heat in the mixed state. 55
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