Beiträge auf der Frühjahrstagung 2004
in Regensburg:

Excitation spectrum of the cobaltite Na_xCoO₂ yH₂O as function of Na content and hydration

•P. Lemmens^1,2, K.-Y. Choi², V. Gnezdilov³, N.N. Kovaleva¹, H. Sakurai⁴, K. Takada⁴, T. Sasaki⁴, E. Takayama-Muromachi⁴, F.C. Chou⁵, C.T. Lin¹ und B. Keimer^1
4Supercond. Mat. Center and Adv. Mat. Lab., NIMS, Tsukuba, Ibaraki 305-0044
⁵Center for Material Science and Engineering, MIT, Cambridge, MA 02139
¹MPI for Solid State Research, D-70569 Stuttgart
²2. Phys. Inst., RWTH Aachen, D-52056 Aachen
³B.I. Verkin Inst. for Low Temp. Phys., NASU, 61164 Kharkov

We report on a Raman scattering study on the superconducting cobaltite Na_xCoO₂ yH₂O as function of Na content and hydration (x approx. 1/3, 3/4 and y approx. 0, 2/3, 4/3). The observed phonon scattering is analyzed in terms of lattice strain due to the structural misfit and disorder. Hydration, due to the intercalation of H₂O layers, releases a part of this strain. We will show experimental evidence for a pseudo gap formation in the electronic scattering continuum. Furthermore, our Raman data suggest a connection between disorder on the partly occupied Na sites, the split off of the a_1g level from the other t_2g states of Co⁴⁺ and superconductivity.

•S. Gold¹, E. Goering¹, P. Lemmens², A. Podlesnyak³, G. Bychkov⁴, J. Deisenhofer⁵, A. Loidl⁵ und G. Schütz^1
1MPI für Metallforschung, Stuttgart
²MPI für Festkörperforschung, Stuttgart
³PSI, ETHZ&PSI, Villingen
⁴Inst. of Solid State & Semicond. Phys., Nat. Acad. Sci. Minsk
⁵EP5, Universität Augsburg

The magnetic perovskite REBaCo₂O_5.5 (RE= Dy; Gd) system shows a rich magnetic and electronic phase diagram, with a MI transition at higher temperatures and ferromagnetic ordering just below room temperature. With a further reduction of temperature the ferromagnetic magnetization is strongly reduced. This has been discussed in terms of a Co spin state transition and/or antiferromagnetism. A double peak like structure could be observed in the Co L_2,3 XMCD. We interpret this double peak by spectroscopic distinguishable octahedral and pyramidal Co sites. The Co L_2,3 XMCD has been separated from the overlapping Ba M_4,5 spectra by a recently developed XMCD analysis tool called moment analysis. This analysis exhibits an unusual large orbital/spin-moment ratio. In addition, we could observe an extreme large X-ray natural circular dichroism (XNCD) signal, which is about two orders of magnitude larger compared to other published results and about 20% of the corresponding white line intensity. This suggests a large non centrosymmetric charge distribution at the Co site. Nevertheless, the orgin of this extreme large XNCD effect is not clear up to now.

Work supported by DFG SPP1073

•P. Lemmens^1,2, K.-Y. Choi², G. Caimi³, L. Degiorgi^3,4, A. Seidel⁵ und F.C. Chou^5
1MPI for Solid State Research, D-70569 Stuttgart
²2. Physikalisches Institut, D-52056 RWTH Aachen
³Laboratorium für Festkörperphysik, ETH Zürich, CH-8093 Zürich
⁴Paul Scherrer Institute, CH-5232 Villigen
⁵Center for Material Science and Engineering, MIT, Cambridge, MA 02139

We report infrared and Raman spectroscopy results of the spin-1/2 quantum magnet TiOCl. Giant anomalies are found in the temperature dependence of the phonon spectrum, which hint to unusual coupling of the electronic degrees of freedom to the lattice. These anomalies develop over a broad temperature interval, suggesting the presence of an extended fluctuation regime. This defines a pseudo-gap phase, characterized by a local spin-gap. Below 100 K a dimensionality cross-over leads to a dimerized ground state with a global spin-gap of about 2D_spin approx.  430 K.

•K.-Y. Choi¹, D. Heydhausen¹, T. Sahaoui¹, P. Reutler², B. Büchner², P. Lemmens^1,3 und G. Güntherodt^1
12. Physikalisches Institut, RWTH Aachen
²Institute for Solid State Research, IFW Dresden
³MPI for Solid State Research, Stuttgart

We present an inelastic light scattering study of the layered manganites La_1-xSr_1+xMnO₄ (x = 0 and 1/8). The samples have the K₂NiF₄-type tetragonal structure (symmetry group I4/mmm ) and show a C-type antiferromagnetic ordering at T_N = 125 K for x = 0 as well as of an orbital ordering with a dominant character of the d_3z²_-r² orbital. The undoped sample shows a pronounced two-magnon spectrum (2MS) in the ab plane. The 2MS undergoes a strong damping already above 1.5 T_N and evolves into quasielastic scattering consisting of two components. The latter implies the rapid suppression of short-range antiferromagnetic correlations while there appear other magnetic correlations. In addition, a symmetry-forbidden phonon mode is observed around 700 cm^-1 in the high-temperature region for in-plane polarizations. Surprisingly, new modes appear upon cooling below T_N. This is due to zone-folded modes induced by the formation of a superstructure. This provides evidence for induced orbital ordering through spin-orbital coupling in the plane. As to x = 1/8 the 2MS is totally suppressed while activated phonon modes develop which are not Raman-active for x = 0. This feature is discussed in terms of the mixture of d_x²_-y² with d_3z²_-r² orbitals as well as of the motion of holes.

•K.-Y. Choi¹, P. Lemmens^1,2, G. Güntherodt¹, Yu. Pashkevich³, V. Gnezdilov⁴, P. Reutler⁵, B. Büchner⁵ und A. Revcolevschi^6
2MPI for Solid State Research, Stuttgart
³A. A. Galkin Donetsk Phystech NASU, Ukraine
⁴B. I. Verkin Inst. for Low Temp. Physics NASU, Ukraine
⁵Institute for Solid State Research, IFW Dresden
⁶Laboratoire de Physico-Chimie, Université Paris-Sud, France
¹2. Physikalisches Institut, RWTH Aachen

We present an inelastic light scattering study of single crystalline (La_1-yPr_y)_1-xSr_xMnO₃ ( 0 < x < 0.14,  y = 0 and x = 1/8,  0 < y < 0.5) and LaMnO_3+d (0.071 < d < 0.125). The studied samples range from canted antiferromagnetic insulating (CAF) to ferromagnetic insulating (FMI) phase. A giant softening up to 20 -30 cm^-1 of the Mn-O breathing mode around 610 cm^-1 is observed only for the FMI samples (0.11 < x < 0.14 and 0.085 £ d £ 0.125) upon cooling below the Curie temperature. Moreover, Pr-doping leads to the gradual suppression of the softening. This is attributed to a coupling of the breathing mode to orbital polarons, giving evidence for their presence in the FMI phase. No obvious saturation of the softening signals the instability of an orbital polaron state. In addition, the CAF samples (0 < x < 0.1 and d = 0.071) exhibit pronounced multi-phonon features at 1000-1300 cm^-1. Unexpectedly, they undergo a strong softening of the peak energy and a damping as a function of temperature and doping. This odd behavior is interpreted in terms of the change of orbital forms.