25th European Crystallographic Meeting, ECM 25, İstanbul, 2009 Acta Cryst. (2009). A65, s 343 Page s 343 FA5-MS07-P01 Dissolution of Boron in Diamond under High Pressures: Experimental Evidences. Natalia Dubrovinskaiaa, Richard Wirthb, Jochen Wosnitzac, Thomas Papageorgiouc, Hans F. Braund, Nobuyoshi Miyajimae, Leonid Dubrovinskye. aMineralphysik, Institut für Geowissenschaftent, Universität Heidelberg, Heidelberg, Germany. bGeoForschungsZentrum Potsdam, Experimental Geochemistry and Mineral Physics, Potsdam, Germany. cHochfeld-Magnetlabor Dresden (HLD), Forschungszentrum Dresden-Rossendorf, Dresden, Germany. dPhysikalisches Institut, Universität Bayreuth, Bayreuth, Germany. eBayerisches Geoinstitut, Universität Bayreuth, Bayreuth,Germany. E-mail: Natalia.Dubrovinskaia@min.uni-heidelberg.de

The measured thermodynamic phase diagram of the quasi-two-dimensional magnet [Cu(HF{sub 2})(pyz){sub 2}]BF{sub 4}(pyz=pyrazine=N{sub 2}C{sub 4}H{sub 4}) exhibits an unusual nonmonotonic dependence of the Neel temperature T{sub N} as a function of magnetic field H. The nonmonotonic behavior of T{sub N}(H) results from two competing effects induced by the field: while H suppresses the amplitude of the order parameter by polarizing the spins along a given direction, it also reduces the phase fluctuations by changing the order parameter space from the sphere S{sup 2} to the circle S{sup 1}. The latter effect dominates at low fields only if the system is close enough to its lower critical dimension (d{sub c} = 2), i.e., when fluctuations become important. Our theoretical results reproduce the measured phase diagram and demonstrate that this unusual effect is realized in [Cu(HF{sub 2})(pyz){sub 2}]BF{sub 4}.

Structural, magnetic and transport properties of undoped and Mn-doped quasi one-dimensional ZnO nano-wires formed as dendrite crystals of different width have been studied. All Mn-doped nanowires (Mn content 16% and 27 %) exhibit a ferromagnetic behavior, which is significantly temperature dependent. A small magnetic signal detected up to the room temperature is supposed to be due to small amounts of magnetic impurities. Analysis of the temperature dependencies of magnetization shows that the observed magnetic properties can be caused by the host ferromagnetic matrix with Curie temperature TC  40 K.

P. Sengupta,1,2 C. D. Batista,1 R. D. McDonald,2 S. Cox,2 J. Singleton,2 L. Huang,3 T. P. Papageorgiou,3 O. Ignatchik,3 T. Herrmannsdörfer,3 J. L. Manson,4 J. A. Schlueter,5 K. A. Funk,5 and J. Wosnitza3 1Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA 2MPA-NHMFL, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA 3Hochfeld-Magnetlabor Dresden (HLD), Forschungszentrum Dresden-Rossendorf, P.O. Box 510119, D-01314 Dresden, Germany 4Department of Chemistry and Biochemistry, Eastern Washington University, Cheney, Washington 99004, USA 5Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA Received 26 September 2008; published 23 February 2009

Poster Sessions controlled by the O atom modulation in the CuO2.We have further investigated the temperature dependence of the atomic modulations in Sr14Cu24O41, particularly in the CuO2 chain in which the spin-gap behavior accompanied by the formation of the spin-dimerized state is realized at low temperature.By single-crystal x-ray-diffraction method, we have confirmed that superspace group of the modulated structure remains unchanged from room temperature to 150K.The hole distribution has been considered on the basis of the changes of lattice constants, the atomic modulation and the interatomic distances between Cu in the Cu2O3 and O atom in the CuO2.It is indicated that the small amount of holes doped in the Cu2O3 have been backtransferred to the CuO2 and that almost all of the holes are localized in the CuO2 at low temperature.Moreover, the possible hole-ordered structure with the Zhang-Rice singlet in the CuO2 are mainly due to the O atom modulation in the CuO2 and the ZR-singlet site with rectangular CuO4 unit is possible in the CuO2, which is analogous to the local CuO4 coordination in the CuO2 plane of high-Tc cuprates.

We report the carbon-isotope effect for boron-doped diamond (BDD). Resistive as well as specific-heat measurements reveal a 0.2 K shift in the superconducting transition temperature T-c between BDDs containing C-13 and C-12. This is more than two times larger than could be expected from the mass difference by the use of the simple BCS formula in case the phenomenon is related to electron-phonon mediated superconductivity in BDD.

TT 32.1 Thu 14:00 Poster B Matrix product state approach for a two-lead, multi-level Anderson impurity model — •Andreas Holzner1,2, Andreas Weichselbaum2, and Jan von Delft2 — 1Institute for Theoretical Physics C, RWTH Aachen, D-52056 Aachen, Germany — 2Physics Department, Arnold Sommerfeld Center for Theoretical Physics, and Center for NanoScience, Ludwig-Maximilians-Universität München, D-80333 München, Germany

Results of magnetization and high-field ESR studies of the new spin-1 Haldane-chain material [Ni(C2H8N2)2NO2](BF4) (NENB) are reported. A definite signature of the Haldane state in NENB was obtained. From the analysis of the frequency-field dependence of magnetic excitations in NENB, the spin-Hamiltonian parameters were calculated, yielding Delta = 17.4 K, g_parallel = 2.14, D = 7.5 K, and |E| = 0.7 K for the Haldane gap, g factor and the crystal-field anisotropy constants, respectively. The presence of fractional S = 1/2 chain-end states, revealed by ESR and magnetization measurements, is found to be responsible for spin-glass freezing effects. In addition, extra states in the excitation spectrum of NENB have been observed in the vicinity of the Haldane gap, which origin is discussed.

The discovery of superconductivity in polycrystalline boron-doped diamond (BDD) synthesized under high pressure and high temperatures [Ekimov, et al. (2004) Nature 428:542–545] has raised a number of questions on the origin of the superconducting state. It was suggested that the heavy boron doping of diamond eventually leads to superconductivity. To justify such statements more detailed information on the microstructure of the composite materials and on the exact boron content in the diamond grains is needed. For that we used high-resolution transmission electron microscopy and electron energy loss spectroscopy. For the studied superconducting BDD samples synthesized at high pressures and high temperatures the diamond grain sizes are ≈1–2 μm with a boron content between 0.2 ( 2 ) and 0.5 ( 1 ) at %. The grains are separated by 10- to 20-nm-thick layers and triangular-shaped pockets of predominantly (at least 95 at %) amorphous boron. These results render superconductivity caused by the heavy boron doping in diamond highly unlikely.

Magnetization measurements in pulsed magnetic fields up to 47 T were utilized to estimate the average Ru moment in polycrystalline RuSr2GdCu2O8. A NbSr2GdCu2O8 sample was used as reference. The extracted Ru moment of about 1.8 mu(B) at 4.2 K and 47 T is indicative of a mixed-valence state of Ru involving Ru5+ and Ru4+ ions with magnetic moments of 2 mu(B)/Ru5+ and 0.9 mu(B)/Ru4+. The estimated ratio of Ru5+:Ru4+approximate to 87%:13% corresponds to a hole concentration p approximate to 0.065 in the CuO2 planes suggesting an underdoped nature of the superconducting state. We propose that the magnetic structure of the Ru moments corresponds to an antiferromagnetic phase, involving only Ru5+ ions, interrupted by ferromagnetic stripes, where charge transfer between Ru4+ and Ru5+ ions takes place.

We have investigated the magnetic properties of transition-metal clusters with a single grain size of about 1 nm. These metallic nanoclusters have been deposed on a biological substrate. This substrate is a purified self-assembling paracrystalline surface layer (S-layer) of the Bacillus sphaericus strain JG-A12, which exhibits square symmetry and is composed of identical protein monomers. First data of the magnetic susceptibility, taken in a SQUID magnetometer at 0<B<7T and 1.8 K<T<400 K, reveal unusual magnetic properties. The Stoner enhancement factor of the d conduction-electron susceptibility in the Pd and Pt nanoclusters is dramatically reduced compared to the one of the corresponding bulk transition metals. The weakened magnetism of the 5d electrons is considered to play a crucial role for the occurrence of superconductivity in microgranular Pt by adjusting the balance between electron-phonon interactions and competing magnetic interactions. (C) 2006 Elsevier B. V. All rights reserved.

The superconducting properties of the magnetic (TM≈130K) superconductors RuSr2RECu2O8 (RE=Eu,Gd) (Tc,Eu≈27.5K, Tc,Gd≈46K) were investigated using resistance and AC-susceptibility measurements. Tc,Eu showed a magnetic-field dependence up to 14T which can be described by the empirical relation Tc(B)=Tc(0)[1-B/Bc(0)]1/2, contrary to previous reports pointing out the possibility of phase separation in the ruthenocuprates. A plausible explanation of this discrepancy is based on the significance of the Ru5+/Ru4+ ratio in the ruthenocuprates affecting the competition between antiferromagnetic superexchange and ferromagnetic double exchange in these compounds.

We present an investigation of magnetic ordering in the two-dimensional S=1/2 quantum magnet Cu(Pz)(2)(ClO4)(2) using specific heat and zero-field muon-spin relaxation (mu+SR). The magnetic contribution to the specific heat is consistent with an exchange strength of 17.7(3) K. We find unambiguous evidence for a transition to a state of three-dimensional long-range order below a critical temperature T-N=4.21(1) K using mu+SR even though there is no feature in the specific heat at that temperature. The absence of a specific heat anomaly at T-N is consistent with recent theoretical predictions. The ratio of T-N/J=0.24 corresponds to a ratio of intralayer to interlayer exchange constants of parallel to J(')/J parallel to=6.8x10(-4), indicative of excellent two-dimensional isolation. The scaled magnetic specific heat of [Cu(Pz)(2)(HF2)]BF4, a compound with an analogous structure, is very similar to that of Cu(Pz)(2)(ClO4)(2) although both differ slightly from the predicted value for an ideal 2D S=1/2 Heisenberg antiferromagnet.

Crystal structure, thermodynamic properties and electron spin resonance spectra of Cu(NH3)2Ag2(CN)4 have been studied. The structure of the studied compound consists of interpenetrating wave-shaped two-dimensional (2d) arrays, in which CuN4N2 octahedra axially elongated due to the Jahn–Teller effect are linked by diamagnetic [Ag(CN)2]− anions forming a square lattice. Although the susceptibility and specific heat data confirm the existence of short-range order at about 2 K, quantitative analysis of the experimental data reveals that the magnetic behavior differs from that of the crystal structure expected for 2d Heisenberg magnet on the square lattice. Structural features responsible for the observed difference are discussed. In addition, it is suggested, that additional degrees of freedom contribute to the thermodynamic equilibrium properties in the millikelvin temperature range.

In this study we report the synthesis and characterization of four samples prepared with a nominal RuSr2GdCu2O8 starting composition following a two-step procedure, involving the Sr2GdRuO6 compound as precursor, in a controlled atmosphere. A chemical-vapour-transport process in an open system is used to control the Ru content in these samples during the annealing cycle. We observe that high Ru-oxides mass transport results in a change of the phases in equilibrium at the RuSr2GdCu2O8 composition and a multiphase product is obtained. Rietveld refinement analysis and SEM-EDX studies are carried out in order to estimate the Ru content in the as-prepared RuGd1212.

Magnetization measurements of the magnetic (T-M approximate to 138 K) superconductor (T-S approximate to 42 K) RuSr2GdCu2O8 in pulsed magnetic fields up to 47 T reveal at 48 K and the highest available magnetic field a Ru contribution to the measured magnetic moment of about 2.2 mu(B)/formula unit. This value is indicative of a mixed-valence state of the Ru ions involving Ru5+(S = 3/2) and Ru4+(S = 1) ions. We propose that the mixed-valence state combined with charge transport in the RuO2 planes results in a competition between ferromagnetic double exchange involving Ru5+ and Ru4+ ions and antiferromagnetic superexchange involving Ru5+ ions. This causes magnetic-phase separation in ferromagnetic (non-superconducting) and anti ferromagnetic (superconducting) domains. We conclude that the magnetic and superconducting properties of the ruthenocuprates critically depend on the Ru5+/Ru4+ ratio which can be affected by the preparation conditions. (C) 2007 Elsevier B.V. All rights reserved.