Beiträge auf der Frühjahrstagung 2003
- Montag, den 24.3.2001: Vortrag MA4.1, 10.15; Poster TT6.26
- Dienstag, den 25.3.2001: Poster TT11.16, TT11.20, TT11.30, TT11.31, MA14.150
|Kondo/Schwer-Fermionen/Quasi-Niedrigdimensionale Systeme (gemeinsam mit TT)|
|MA 4.1||Vortrag||Mo 10:15||HSZ/405|
Raman scattering study of S=1/2 2D HAF K2V3O8
•K.-Y. Choi1, G. Güntherodt1, B.C. Sales2, S.E. Nagler2 und P. Lemmens1,3
12. Physikalisches Institut, RWTH Aachen
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 TN) with J=12.8 K (TN= 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
|Postersitzung I (Amorphe- u. Tunnelsysteme, Niedrigdimensionale Systeme, Supraleitung: Elektronenstruktur, Phononen, Tunneln, Ordnungsparameter)|
|TT 6.26||Poster||Mo 14:30||P2c, P2d|
Magnetic order in the tetrahedral quantum spin system Cu2Te2O5(Br1-xClx)2
•H.-H. Klauss1, P. Lemmens1, A. Bosse1, D. Baabe1, D. Mienert1,
1Institut für Metallphysik und Nukleare Festkörperphysik, TU Braunschweig
We present studies of the magnetic correlations in the weakly coupled spin tetrahedra system Cu2Te2O5(Br1-xClx)2. Due to quantum fluctuations these systems show strongly reduced magnetic transition temperatures TNCl = 18.2 K and TNBr = 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.
Sitzung TT 11:
|Postersitzung II (Metall-Isolator-Übergänge, Phasenübergänge in Quantensystemen, Theorie: Systeme korrelierter Elektronen)|
|TT 11.16||Poster||Di 14:30||P2c, P2d|
Experimental quest for orbitons in LaMnO3
•R. Rückamp1, M. Grüninger1, C. Hilgers1, A. Freimuth1, P. Reutler2,3,
1II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937
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.  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 . 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.
 E. Saitoh et al., Nature 410, 180 (2001).  M. Grüninger et al., Nature 418, 39 (2002).
|TT 11.20||Poster||Di 14:30||P2c, P2d|
Spectroscopic investigation of the Spin-Peierls-like instability in TiOCl
•P. Lemmens1,2, K.-Y. Choi2, T. Sahaoui2, A. Seidel3 und F.C. Chou3
1MPI for Solid State Research, D-70569 Stuttgart
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 t2g 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 TC=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
|TT 11.30||Poster||Di 14:30||P2c, P2d|
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
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.
|TT 11.31||Poster||Di 14:30||P2c, P2d|
Observation of a longitudinal magnon in the tetrahedral quantum spin system Cu2Te2O5Br2
•Claudius Gros1, P. Lemmens2,3, M. Vojta4, Roser Valenti1, K. -Y. Choi2,
1Fakultät 7, Theoretische Physik, Universität des Saarlandes, 22. Physikalisches Institut, RWTH Aachen,
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 TN. This energy is determined experimentally. The ordering temperature TN has an inverse-log dependence on the coupling parameters near quantum criticality.
Sitzung MA 14:
|MA 14.150||Poster||Di 15:15||Zelt|
Micro-Raman spectroscopy and magnetic characterization of the s=1/2 Dimer system CuTe2O5
•A.-M. Carsteanu1, J. Schoenes1, P. Lemmens2,3, R.K. Kremer3 und M. Johnsson4
1Institut für Halbleiterphysik und Optik, TU Braunschweig, 38106
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.