Beiträge auf der Frühjahrstagung 2005:

2001-2002-2003-2004-2005-2006-2007-2008-2009-2010 - 2011 - 2012 - 2013 - 2014

Abstracts der Deutsche Physikalische Gesellschaft e. V. (DPG)

TT 21.8 Vortrag   Symposium Frustrierte Spinsysteme

Raman spectroscopy on frustrated spin systems

Peter Lemmens, Inst. for Physics of Condensed Matter, TU Braunschweig, D-38106 Braunschweig, Germany and MPI for Solid State Research, D-70569 Stuttgart, Germany

Many transition metal oxides realize competing interactions based on spin, charge or orbital degrees of freedom (1). Raman scattering has contributed to the understanding of this interplay and the effect of frustration to a great extend due to its unprecedented sensitivity, resolution
and additional symmetry information that may be gained considering selection rules. It will be discussed how ground state degeneracy and / or suppressed long range magnetic ordering due to novel spin topologies effect the excitation spectrum of a quantum spin system. Respective
model systems are the tetrahedral cluster compound Cu2Te2O5Br2 and the frustrated dimer system SrCu2(BO3)2. These compounds demonstrate a cross over from low energy singlet bound states to longitudinal magnons. In addition, multiband effects and polarons will be addressed
in the hydrated superconductor NaxCoO2 yH2O.

This work was supported by DFG SPP1073 and INTAS 01-278.
[1] Spin - Orbit - Topology, a triptych, P. Lemmens and P. Millet, in "Quantum Magnetism", Ed. U. Schollwöck, J. Richter, B.J.J. Farell, R.F. Bishop, Springer, Heidelberg (2004).


TT 16.76 Poster    

Co3+ spin state transition detected by phonon Raman scattering in GdBaCo2O5.5

Yu.G. Pashkevich1, V.P. Gnezdilov2, K.-Y. Choi3, P. Lemmens4;5, B. Keimer5, C. Ambrosch-Draxl6, S.N. Barilo7, S.-V. Shiryaev7, and G.-L. Bychkov7

1 A.A. Galkin Donetsk Phystech NASU, 83114 Donetsk, Ukraine
2 B.I. Verkin Inst. for Low Temp. Physics NASU, 61164 Kharkov, Ukraine
3 Inst. for Materials Research, Tohoku University, Sendai 980-8577, Japan
4 Inst. for Physics of Condensed Matter, TU Braunschweig, D-38106 Braunschweig, Germany
5 MPI-FKF, D-70569 Stuttgart, Germany
6 Inst. für Theoretische Physik, Universität Graz, A-8010 Graz, Austria
7 Inst. for Physics of Solids & Semiconductors, Academy of Sciences, 220072 Minsk, Belarus

The change of the spin state of Co3+ as function of temperature or pressure is an intriguing feature of cobalt-based perovskite compounds. In the layered cobaltites RBaCo2O5O5:5 this problem is rather complicated due to the octahedral and opened square pyramidal coordinations of Co with oxygen. Raman spectroscopy can provide information about subtle changes in spin state through detecting changes of the phonon spectra, which are sensitive to the O-Co-O bonding length and angles. Raman spectra of single crystal GdBaCo2O5O5:5 were measured in the temperature range 5 - 400 K and remarkable changes of frequencies and intensities were observed. Frequencies and eigenvectors of Raman active phonon modes have been de¯ned using frozen phonon ab initio band structure calculations and structural data at 300 K in Pmmm setting.


TT 16.75 Poster C    

Low temperature mixed spin state of Co3+ in LaCoO3 evidenced from local lattice distortions

V. Gnezdilov1, P. Lemmens2,3, Yu.G. Pashkevich4, K.-Y. Choi5, S. Shiryaev6, G. Bychkov6, and S. Barilo6
1 B.I. Verkin Inst. for Low Temp. Physics NASU, 61164 Kharkov, Ukraine
2 Inst. for Physics of Condensed Matter, TU Braunschweig, D-38106 Braunschweig, Germany
3 MPI-FKF, D-70569 Stuttgart, Germany
4 A.A. Galkin Donetsk Phystech NASU, 83114 Donetsk, Ukraine
5 Inst. for Materials Research, Tohoku University, Sendai 980-8577, Japan
6 Inst. of Physics of Solids & Semiconductors,
Academy of Sciences, 220072 Minsk, Belarus

Single- and multi-phonon excitations of the single crystalline LaCoO3 were studied using Raman spectroscopy in the temperature region of 5 K -300 K. First-order Raman spectra show a larger number of phonon modes than allowed for the rhombohedral (D6 3d) structure. Additional phonon
modes are interpreted in terms of activated modes due to local lattice distortions arising from the Jahn-Teller (JT) activity of the intermediate- spin (IS) state of Co3+ ions. The temperature dependence of the breathing - and stretching-type phonon modes on cooling suggests the presence of Co3+ ions in the intermediate spin state, even at lowest temperatures. The anomalous temperature dependence of the second-order phonon excitations spectra is in accordance with the Franck-Condon mechanism that is characteristic for a JT orbital order.


TT 16.52 Poster C    

Magnetic structure of DyBaCo2O5.5

•H. Luetkens1, H.-H. Klauss2, A. Bosse2, D. Cheptiakov3, S. Streule3, E. Pomjakushina3,
A. Podlesnyak3, K. Conder3, and P. Lemmens4

1 Laboratory for Muon-Spin Spectroscopy, Paul Scherrer Institut CH-5232 Villigen PSI, Switzerland
2 IMNF, TU Braunschweig, D-38106 Braunschweig, Germany
3 Laboratory for Neutron Scattering ETHZ & PSI, CH-5233 Villigen PSI, Switzerland 
4 IHO, TU Braunschweig, D-38106 Braunschweig, Germany

The magnetic perovskite REBaCo2O5.5 system shows a rich magnetic and electronic phase diagram, with a MI transition at higher temperatures and magnetic ordering just below room temperature. Here, we report on neutron diffraction and muon spin rotation (MSR) measurements
on a powder of DyBaCo2O5:5. From the neutron diffraction measurements an AFM (G-type) phase below 285 K is indicated by a doubling of the magnetic unit cell along both the crystallographic a and c axis. Further cooling below 260 K leads to a second magnetic phase which can be equally well described by two different spin-state ordered AFM structures. Microscopically, three magnetic transitions at 285 K, 260 K and 162 K have been observed by MSR. The high as well as the low temperature phase both show well defined muon spin precession characteristic for long range magnetic order. In the high temperature regime it was possible to independently measure the magnetic order parameter as well as the magnetic volume fraction. Additionally, at low temperatures a slowing down of the fluctuating Dy moment is observed by a characteristic peak in the longitudinal relaxation rate.


MA 5.6 Vortrag    
Angular dependent absorption spectroscopy of a de-twinned DyBaCo2O5.5 single crystal

Myrsini Lafkioti1, Stefan Gold1, Eberhard Goering1, Gisela Schütz1, Joachim
Deisenhofer2, and Peter Lemmens3
1 Max-Planck-Institut für Metallforschung, 70569 Stuttgart
2 Lst. für Eperimentalphysik V, Uni-Augsburg, 86159 Augsburg
3 Inst. für Halbleiterphysik und Optik, TU Braunschweig, 38106 Braunschweig

The Perovskite DyBaCo2O5.5 exhibits a very rich phase diagram including two different paramagnetic, a ferromagnetic, and an antiferromagnetic phase as a function of temperature[1]. In addition to these unusual properties, the system exhibits a very pronounced and strong magnetocrystalline anisotropy, with the easy axis along the crystallographic a-axis[2]. We will show detailed angular and temperature dependent X-ray magnetic circular dichroism and nonmagnetic linear polarized absorption spectroscopy results at the Co L2,3 and O K edges, performed
on a de-twinned single crystal. A clear and quantitative correspondence between the microscopic and macroscopic magnetic behavior has been found. O K edge spectra reveal angular dependent variations, which give insight to the orbital character of the unoccupied states. Our results
provide detailed information about the microscopic physics behind the observed unusual and also technical very promising properties. [1] A. Maignan et al., Journal of Solid State Chemistry 142, 247 (1999). [2] A. A. Taskin et al., Phys.Rev.Lett. 90, 227201-1 (2003).