Beiträge auf der Frühjahrstagung 2004
Excitation spectrum of the cobaltite NaxCoO2 yH2O as function of Na content and hydration
•P. Lemmens1,2, K.-Y.
Choi2, V. Gnezdilov3, N.N. Kovaleva1, H.
Sakurai4, K. Takada4, T. Sasaki4, E.
Takayama-Muromachi4, F.C. Chou5, C.T. Lin1 und
We report on a Raman scattering study on the superconducting cobaltite NaxCoO2 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 a1g level from the other t2g states of Co4+ and superconductivity.
Work supported by DFG SPP1073, NATO PST.CLG.9777766, INTAS 01-278, CREST of JST (Japanese Science and Technology Corporation) and MRSEC Program of NSF under award number DMR 02-13282.
X-ray magnetic and natural circular dichroism at the vicinity of the Co low-spin to high-spin transition in REBaCo2O5.5 (RE= Dy; Gd) system
•S. Gold1, E. Goering1, P.
Lemmens2, A. Podlesnyak3, G. Bychkov4, J.
Deisenhofer5, A. Loidl5 und G. Schütz1
The magnetic perovskite REBaCo2O5.5 (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 L2,3 XMCD. We interpret this double peak by spectroscopic distinguishable octahedral and pyramidal Co sites. The Co L2,3 XMCD has been separated from the overlapping Ba M4,5 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
|Giant phonon anomalies in
the pseudo-gap phase of TiOCl
•P. Lemmens1,2, K.-Y. Choi2,
G. Caimi3, L. Degiorgi3,4, A. Seidel5 und
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 2Dspin approx. 430 K.
Work supported by the DFG SPP1073, MRSEC Program of NSF under award number DMR 02-13282, NATO PST.CLG.9777766, INTAS 01-278, and the Swiss National Foundation for the Scientific Research.
|Orbital-spin coupling in La1-xSr1+xMnO4
observed by Raman spectroscopy
•K.-Y. Choi1, D. Heydhausen1, T. Sahaoui1,
P. Reutler2, B. Büchner2, P. Lemmens1,3
und G. Güntherodt1
We present an inelastic light scattering study of the layered manganites La1-xSr1+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 TN = 125 K for x = 0 as well as of an orbital ordering with a dominant character of the d3z2-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 TN 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 TN. 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 dx2-y2 with d3z2-r2 orbitals as well as of the motion of holes.
Work supported by DFG SPP1073
anomalies in (La1-yPry)1-xSrxMnO3
•K.-Y. Choi1, P. Lemmens1,2, G.
Güntherodt1, Yu. Pashkevich3, V. Gnezdilov4,
P. Reutler5, B. Büchner5 und A. Revcolevschi6
We present an inelastic light scattering study of single crystalline (La1-yPry)1-xSrxMnO3 ( 0 < x < 0.14, y = 0 and x = 1/8, 0 < y < 0.5) and LaMnO3+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.
Work supported by DFG SPP1073