Beiträge auf der Frühjahrstagung 2003
in Dresden:

Raman scattering study of S=1/2 2D HAF K₂V₃O₈

•K.-Y. Choi¹, G. Güntherodt¹, B.C. Sales², S.E. Nagler² und P. Lemmens^1,3

¹2. Physikalisches Institut, RWTH Aachen
²Oak Ridge National Laboratory, Tennesse 37831, USA
³MPI for Solid State Research, Stuttgart

We present an inelastic light scattering study of the two-dimensional antiferromagnetic Heisenberg spin system (2D HAF) K₂V₃O₈. This S=1/2 square lattice antiferromagnet is formed by a vanadium layer which consists of corner-sharing magnetic (S=1/2) VO₅ square pyramids and nonmagnetic (S=0) VO₄ tetrahedra with K ions separating the layers. This compound may be used as a model system to understand the effect of a low symmetry exchange path and depletion of the magnetic sites on the excitation spectrum of a 2D HAF. In Raman scattering we observe a broad double peak maximum in the (a'b') geometry that is identified as two-magnon scattering. Furthermore similar symmetry-forbidden signals in (aa) and (a'b') symmetries arise. These scattering intensities persist up to 5J (15 T_N) with J=12.8 K (T_N= 4 K) determined from previous neutron scattering and susceptibility experiments. We will discuss these effects in the context of interlayer coupling and a possible electronic soft mode arising due to a local distortion of the VO₅ square pyramids. Work supported by DFG SPP1073

•H.-H. Klauss¹, P. Lemmens¹, A. Bosse¹, D. Baabe¹, D. Mienert¹,
H. Luetkens¹, F.J. Litterst¹, K.-Y. Choi², R. Scheuermann³ und M. Johnsson⁴

¹Institut für Metallphysik und Nukleare Festkörperphysik, TU Braunschweig
²II. Physikalisches Institut, RWTH Aaachen
³Paul Scherrer Institut, Villigen, Switzerland
⁴Department of Inorganic Chemistry, Stockholm University, Sweden

We present studies of the magnetic correlations in the weakly coupled spin tetrahedra system Cu₂Te₂O₅(Br_1-xCl_x)₂. Due to quantum fluctuations these systems show strongly reduced magnetic transition temperatures T_N^Cl = 18.2 K and T_N^Br = 11.4 K in comparison with a magnetic exchange of J » 40 K. Susceptibility, specific heat and magnetic raman scattering experiments reveal that the Bromide system (x = 0) is close to a quantum phase transition (P. Lemmens et al. PRL 87 227201 (2001)). We performed muon spin relaxation (mSR) experiments on six samples with 0 £ x £ 1. All samples show two spontaneous muon spin precession frequencies below their respective ordering temperatures which decrease monotonously with decreasing x. An enhanced longitudinal spin relaxation is attributed to the dominantly low dimensional character of the system.

•R. Rückamp¹, M. Grüninger¹, C. Hilgers¹, A. Freimuth¹, P. Reutler^2,3,
K.-Y. Choi², P. Lemmens², B. Büchner², G. Güntherodt² und A. Revcolevschi³

¹II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln
²II. Physikalisches Institut, RWTH-Aachen
³Lab. de Chimie des Solides, Université Paris-Sud, France

Orbital physics currently attracts much interest. In an orbitally ordered state one expects new collective elementary excitations, namely orbital waves or orbitons, which involve transitions between different orbital states. Recently, Saitoh et al. [1] reported three peaks in the Raman data of orbitally ordered LaMnO₃ and interpreted this as the first observation of orbital waves as elementary excitations in a solid. We determined the optical conductivity s(w) of LaMnO₃. The peaks observed in s(w) are very similar to the Raman features. However, direct orbital excitations are not infrared active due to parity. Therefore the observation of similar peak frequencies in the Raman spectrum and in s(w) strongly challenges the interpretation of these peaks as orbitons [2]. We identify these features as multi-phonons and present Raman data which show two-phonon, three-phonon and four-phonon scattering. We discuss the search for orbital excitations in s(w) at higher energies and in related materials.

[1] E. Saitoh et al., Nature 410, 180 (2001). [2] M. Grüninger et al., Nature 418, 39 (2002).

Spectroscopic investigation of the Spin-Peierls-like instability in TiOCl

•P. Lemmens^1,2, K.-Y. Choi², T. Sahaoui², A. Seidel³ und F.C. Chou³

¹MPI for Solid State Research, D-70569 Stuttgart
²2. Physikalisches Institut, D-52056 RWTH Aachen
³Center for Material Science and Engineering, MIT, Cambridge, MA 02139, USA

We present a Raman light scattering study of the layered titanium oxyhalide TiOCl. In this system LDA+U calculations propose the formation of S=1/2 Heisenberg chains by a direct overlap of ordered t_2g orbitals of Ti³⁺ ions [1,2]. Experimentally, a broad maximum is observed in the high temperature magnetic susceptibility consistent with an afm coupling of J = 660 K and a sudden decrease for temperatures below T_C=100 K. Using Raman light scattering we studied excitations of the phonon system as well as high energy scattering attributed to electronic/magnetic degrees of freedom. The observed phonon anomalies point to strong dimer fluctuations. The scattering selection rules of the signals in the high-energy sector are consistent with the theoretical model. However, the line-shape of this scattering does not follow the Loudon-Fleury scenario of two-magnon scattering. We propose a novel interplay of orbital and spin excitations as the origin of this scattering. Work supported by DFG SPP1073

[ 1] A. Seidel, C.A. Marianetti, F.C. Chou, G. Ceder, P.A. Lee, cond-mat/0206374 (2002).
[ 2] A. Seidel and P.A. Lee, cond-mat/0212199 (2002).

Electronic structure of the coupled-tetrahedra quantum spin systems Cu₂Te₂O₅Br₂ and Cu₂Te₂O₅Cl₂

•Roser Valenti¹, Tanusri Saha-Dasgupta², Claudius Gros¹, Helge Rosner³ und Peter Lemmens⁴

¹Fakultät 7, Theoretische Physik, Universität des Saarlandes
²S.N. Bose National Centre for Basic Sciences, JD Block, Sector 3,Salt Lake City, Kolkata 700098, India
³MPI for Chemical Physics of Solids, Dresden
⁴MPI for Solid State Research, Stuttgart

We present a comparative ab initio study of the electronic properties of the isostructural coupled-tetrahedra compounds Cu₂Te₂O₅Br₂ and Cu₂Te₂O₅Cl₂. A detailed investigation of the copper-copper interaction pathes reveals that the halogen-ions play an important role in the inter-tetrahedral couplings via X₄-rings (X=Br, Cl). We find that, contrary to initial indications, both systems show a similar electronic behavior with long range exchange pathes mediated by the X₄-rings.

Observation of a longitudinal magnon in the tetrahedral quantum spin system Cu₂Te₂O₅Br₂

•Claudius Gros¹, P. Lemmens^2,3, M. Vojta⁴, Roser Valenti¹, K. -Y. Choi²,
H. Kageyama⁵, Z. Hiroi⁵, N.V. Mushnikov⁵, T. Goto⁵, M. Johnsson⁶ und P. Millet⁷

¹Fakultät 7, Theoretische Physik, Universität des Saarlandes, ²2. Physikalisches Institut, RWTH Aachen,
³MPI for Solid State Research, Stuttgart, ⁴ITKM, Universität Karlsruhe, ⁵ISSP, Kashiwa, Japan,
⁶Dept. of Inorg. Chem., Stockholm, Sweden, ⁷CEMES/CNRS, Toulouse, France

We present a comprehensive study of the coupled tetrahedra-compound Cu₂Te₂O₅Br₂ by theory and experiments in external magnetic fields. We report the observation of a longitudinal magnon in Raman scattering in the ordered state close to quantum criticality. We show that the excited tetrahedral-singlet sets the energy scale for the magnetic ordering temperature T_N. This energy is determined experimentally. The ordering temperature T_N has an inverse-log dependence on the coupling parameters near quantum criticality.

Micro-Raman spectroscopy and magnetic characterization of the s=1/2 Dimer system CuTe₂O₅

•A.-M. Carsteanu¹, J. Schoenes¹, P. Lemmens^2,3, R.K. Kremer³ und M. Johnsson⁴

¹Institut für Halbleiterphysik und Optik, TU Braunschweig, 38106 Braunschweig, Germany
²IMNF, TU Braunschweig, 38106 Braunschweig, Germany
³MPI-FKF Stuttgart
⁴Dept. Inorg. Chemistry, Stockholm Univ., 10691 Stockholm, Sweden

The quantum antiferromagnet CuTe₂O₅ represents a clean s=1/2 dimer system of Cu²⁺, 3d⁹ moments with negligible interdimer interaction or frustration. The monoclinic crystal structure contains well separated Cu₂O₁₀ dioctahedra. The magnetic susceptibility can very well be fitted to the Bleaney expression using an antiferromagnetic exchange constant of J=90K. We have investigated the Raman response of the system as function of temperature and compare these results with a detailed factor group analysis. Further magnetic and Raman investigations involved Zn- and Ni-substitutions on the Cu site of this system. We acknowledge support by DFG SPP1073.