[{"type":"speech","title":"Structural properties of Ca\u2082\u208b\u2093Sr\u2093RuO\u2084","issued":{"date-parts":[["2005"]]},"author":[{"family":"Schumann","given":"O."},{"family":"Steffens","given":"P."},{"family":"M\u00fcller","given":"R."},{"family":"Andre","given":"G."},{"family":"Radaelli","given":"P. G."},{"family":"Adelmann","given":"P."},{"family":"Nakatsuji","given":"S."},{"family":"Maeno","given":"Y."},{"family":"Braden","given":"M."}],"note":"Physik seit Albert Einstein : 69.Jahrestagung der DPG, Berlin, 4.-9.M\u00e4rz 2005 Verhandlungen der Deutschen Physikalischen Gesellschaft, R.6, B.40(2005) TT 16.73","abstract":"TT 16.67 Sa 11:00 Poster TU C\nItinerant electron metamagnetism and weak ferromagnetism\n\u00a8\nin LaCo9 Si4 and YCo9 Si4 \u2014 \u2022H. Michor1 , S. Ozcan1 , M.\nKhmelevskyi2 , P. Mohn2 , P. Rogl3 , and H. Rosner4 \u2014 1 Institut\nf\u00a8r Festk\u00a8rperphysik, T.U. Wien, A-1040 Wien, Austria \u2014 2 Center\nfor Computational Materials Science, T.U. Wien, Austria \u2014 3 Institut\nf\u00a8r Physikalische Chemie, Universit\u00a8t Wien, Austria \u2014 4 Max-Planck\nInstitute for Chemical Physics of Solids, D-01187 Dresden, Germany\nLaCo9 Si4 is a strongly exchange enhanced Pauli paramagnet with an\ninstability towards weak ferromagnetism, i.e. exhibits itinerant electron\nmetamagnetism at about 3.5 T for H||c and 6 T for H\u22a5c, which is the lowest value ever found for rare earth intermetallic compounds [1]. Despite of\nthe smaller unit cell volume isostructural and isoelectronic YCo9 Si4 exhibits a weak itinerant ferromagnetic ground state (TC 25 K) already in\nzero-\ufb01eld. The ground state properties of La- and YCo9 Si4 are discussed\non basis of magnetisation, speci\ufb01c heat, and resistivity measurements and\nvia ab-initio electronic structure calculations. The band structure calculations result in a ferromagnetic groundstate for both compounds with\nmoments substantialy larger than the experimentally observed moments.\nThe origin of these discrepancies is brie\ufb02y discussed. [1] H. Michor et\nal., Phys.Rev. B 69 (2004) 081404(R).\nTT 16.68 Sa 11:00 Poster TU C\nMagnetic Ordering in Trigonal Chain Compounds \u2014 \u2022Christian\n\u00b4\nKopp1 , Raymond Fresard2 , and Ulrich Eckern1 \u2014 1 University of\nAugsburg \u2014 2 Institut des Sciences de la Mati`re et du Rayonnement,\nWe investigate the microscopic origin of the ferromagnetic and antiferromagnetic spin exchange couplings in the quasi-one-dimensional cobalt\ncompounds Ca3 ABO6 with A = Fe, Co and B = Co, Rh. From electronic\nstructure calculations we \ufb01nd A 3d low spin and high spin states alternating along the characteristic chains. In addition strong d-p hybridisation\nleads to the formation of extended localized magnetic moments centered\nat the high spin sites. Antiferromagnetic coupling along the chains is induced by a strong metal-metal overlap via the d3z2 \u2212r2 orbitals of the low\nspin sites. It competes with ferromagnetic exchange, which originates in\na cyclic exchange through the ligand atoms.\nTT 16.69 Sa 11:00 Poster TU C\nDimerization pattern or two-dimensional spin systems with spin\nphonon coupling in the adiabatic limit \u2014 \u2022Carsten Aits1 , Ute\nL\u00a8 w1 , and Andreas Kl\u00a8mper2 \u2014 1 Universit\u00a8t zu K\u00a8ln, Institut f\u00a8r\nLittle is known about the ground state phase diagram of twodimensional spin systems with spin phonon coupling. In the adiabatic\nlimit, however, they correspond to spin models with inhomogeneous\ncouplings. In contrast to the one-dimensional case, where the dimerization pattern is unique, it is not clear how the two-dimensional lattice\nresponds to a non-vanishing spin phonon coupling. As far as the S=1\/2\nHeisenberg model is concerned, di\ufb00erent choices of inhomogenous\npatterns of couplings lead to rather di\ufb00erent ground state properties\nwith magnetic energy gain that may or may not compete with the\nenergy loss of the phonon system.\nWe apply a loop algorithm in continuous Trotter time to clarify which\ndistortion pattern is energetically favored. Our approach is twofold. First,\nwe extrapolate the ground state energies and magnetizations for various\npatterns of alternating couplings and analyze whether a transition to a\ngapped state appears. Second, we consider an expansion of the free energy of the distorted models at the point of vanishing distortion. In the\nadiabatic limit, this corresponds to an analysis of spin layers coupled\nto three dimensional phonons at \ufb01nite temperatures. We determine the\ncoe\ufb03cients of the leading order from the (Euclidean) dynamical dimer\ncorrelation functions, which are directly accessible within our method.\nTT 16.70 Sa 11:00 Poster TU C\nperovskites \u2014 \u2022Alexei Sherman1 and Michael Schreiber2 \u2014\n1\nInstitute of Physics, University of Tartu, Estonia \u2014 2 Institut f\u00a8r\nThe incommensurate magnetic response observed in normal-state\ncuprate perovskites is interpreted based on the memory function formalism and the t-J model of Cu-O planes. In agreement with experiment\nSamstag\nthe calculated dispersion of maxima in the susceptibility has the shape\nof two parabolas with upward and downward branches which converge at\nthe antiferromagnetic wave vector. The maxima are located at the momenta ( 1 , 1 \u00b1 \u03b4), ( 1 \u00b1 \u03b4, 1 ) and at ( 1 \u00b1 \u03b4, 1 \u00b1 \u03b4), ( 1 \u00b1 \u03b4, 1 \u03b4) in the lower\n2\n2\n2\n2\n2\n2\n2 2\nand upper parabolas, respectively. The upper parabola re\ufb02ects the dispersion of magnetic excitations of the localized Cu spins, while the lower\nparabola arises due to a dip in the spin-excitation damping at the antiferromagnetic wave vector. For moderate doping this dip stems from the\nweakness of the interaction between the spin excitations and holes near\nthe hot spots. The frequency dependence of the susceptibility is shown\nto depend strongly on the hole bandwidth and damping and varies from\nthe shape observed in YBa2 Cu3 O7\u2212y to that inherent in La2\u2212x Srx CuO4 .\nTT 16.71 Sa 11:00 Poster TU C\nStructural properties of RETiO3 and Y1\u2212x Cax TiO3 \u2014 \u2022A.\n\u00b4\nWe have studied the crystal structure of the RETiO3 by di\ufb00erent\ndi\ufb00raction techniques as function of temperature. In all compounds we\n\ufb01nd signi\ufb01cant distortions of the TiO6 -octahedra which lead to a lifting of the t2g -orbital degeneracy. A comparison with literature data on\nisostructural compounds with a 3d0 or a 4d0 -con\ufb01guration shows that\nsuch distortions not necessarily are caused by orbital physics. In the\nRETiO3 -series, however, the temperature dependence clearly points to\na direct coupling. The octahedron distortions depend more sensitively\non temperature than the tilt and rotation angles; and, in particular, we\n\ufb01nd strong anomalies at the N\u00b4el-temperatures in all antiferromagnetic\nRETiO3 compounds.\nNeutron di\ufb00raction on a sample of Y0.62 Ca0.38 TiO3 yields strong evidence for charge ordering, which may be the key element to understand,\nwhy Ca-doped YTiO3 stays non-metallic till rather high doping.\nTT 16.72 Sa 11:00 Poster TU C\nOrbital excitations in transition-metal compounds \u2014 \u2022R.\nR\u00a8ckamp1 , A. G\u00a8 ssling1 , M. Gr\u00a8ninger1 , H. Roth1 , A.\nFreimuth1 , L. Jongen2 , A. M\u00a8 ller2 , G. Meyer2 , T.T.M.\nPalstra3 , A. Nugroho3 , and S.-W. Cheong4 \u2014 1 II. Physikalisches\nUniversit\u00a8t zu K\u00a8ln \u2014 3 Materials Science Centre, University of\nGroningen \u2014 4 Department of Physics & Astronomy, Rutgers University,\nNew Jersey\nIn recent years orbital physics has attracted much interest since novel\nphenomena - such as an orbital liquid state or new elementary excitations in an orbitally ordered state - have been predicted. In order to\nobserve these phenomena, one has to look for a system in which the\norbital (electron-electron) coupling dominates over the coupling to the\nlattice (Jahn-Teller e\ufb00ect). We have studied orbital excitations in the\noptical conductivity spectra of several transition-metal compounds such\nas RTiO3 , RVO3 , TiOX or Y2 BaNiO5 by measuring both transmittance\nand re\ufb02ectance of single crystals. The energies of the orbital (d-d) transitions are compared with the results of a point-charge model including\nthe hybridisation with the ligand ions. We \ufb01nd good agreement between\nexperiment and theory, which suggests that the coup","kit-publication-id":"230060616"}]