Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Mittwoch
TT 24.49 Mi 14:30 Poster A
Giant phonon anomalies in the pseudo-gap phase of TiOCl —
•P. Lemmens 1,2 , K.-Y. Choi 2 , G. Caimi 3 , L. Degiorgi 3,4 , A. Seidel
5 , and F.C. Chou 5 — 1 MPI for Solid State Research, D-70569
Stuttgart — 2 2. Physikalisches Institut, D-52056 RWTH Aachen —
3 Laboratorium für Festkörperphysik, ETH Zürich, CH-8093 Zürich —
4 Paul Scherrer Institute, CH-5232 Villigen — 5 Center 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 2∆spin ≈ 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.
TT 24.50 Mi 14:30 Poster A
Orbital-spin coupling in La1−xSr1+xMnO4 observed by Raman
spectroscopy — •K.-Y. Choi 1 , D. Heydhausen 1 , T. Sahaoui 1 , P.
Reutler 2 , B. Büchner 2 , P. Lemmens 1,3 , and G. Güntherodt 1 —
1 2. Physikalisches Institut, RWTH Aachen — 2 Institute for Solid State
Research, IFW Dresden — 3 MPI for Solid State Research, Stuttgart
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 d 3z 2 −r 2 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 d x 2 −y 2 with d 3z 2 −r 2 orbitals as well as of the
motion of holes. Work supported by DFG SPP1073
TT 24.51 Mi 14:30 Poster A
Orbital-induced phonon anomalies in (La1−yPry)1−xSrxMnO3
and LaMnO3+δ — •K.-Y. Choi 1 , P. Lemmens 1,2 , G. Güntherodt 1 ,
Yu. Pashkevich 3 , V. Gnezdilov 4 , P. Reutler 5 , B. Büchner 5 ,
and A. Revcolevschi 6 — 1 2. Physikalisches Institut, RWTH Aachen
— 2 MPI for Solid State Research, Stuttgart — 3 A. A. Galkin Donetsk
Phystech NASU, Ukraine — 4 B. I. Verkin Inst. for Low Temp. Physics
NASU, Ukraine — 5 Institute for Solid State Research, IFW Dresden —
6 Laboratoire de Physico-Chimie, Université Paris-Sud, France
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+δ (0.071 ≤ δ ≤ 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 ≤ δ ≤ 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
δ = 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
TT 24.52 Mi 14:30 Poster A
Electron Spin Resonance in charge and orbitally ordered manganites
— •J. Deisenhofer 1 , H.-A. Krug von Nidda 1 , A. Loidl 1 ,
T. Nakajima 2 , and Y. Ueda 2 — 1 EP V, Center for Electronic Correlations
and Magnetism, Institute for Physics, Augsburg University, D-
86135 — 2 Material Design and Characterization Laboratory,Institute for
Solid State Physics, University of Tokyo, 5-1-5
Both the paramagnetic and the magnetically ordered regime have been
investigated for La1−xSrxMnO3 (0 ≤ x ≤ 0.2) single crystals by ESR: For
x=0.05 the analysis of the temperature dependence and the anisotropy
of ESR linewidth and g-value in the orbitally ordered phase allows to
determine unambiguously the orbital mixing parameter above TN, indicating
that also in pure LaMnO3 orbital ordering is stabilized to a
large extent by superexchange processes in the ferromagnetic bonds. For
0.075 ≤ x < 0.15 we observe anisotropic quasi-ferromagnetic resonance
signals persisting far into the paramagnetic regime, indicating the existence
of a Griffiths-like phase above long-range magnetic order on crossing
from the canted antiferromagnetic state at x=0.075 to a purely ferromagnetic
ground state at x=0.175. Moreover, we report on correlations
between the ESR linewidth and resistivity at the charge-ordering transition
in the layered manganites RBaMn2O6 (R=Y,Dy).
TT 24.53 Mi 14:30 Poster A
Magnetic and orbital correlations in single layered manganates
— •D. Senff 1 , O. Friedt 1 , M. Ben Omar 1 , Y. Sidis 2 , P. Reutler
3,4 , A. Revcolevschi 4 , and M. Braden 1 — 1 II. Physikalisches
Institut, Universität zu Köln, Zülpicher Str. 77, D-50937 Köln —
2 Laboratoire Léon Brillouin,CE-Saclay, F-91191 Gif-sur-Yvette —
3 IfW Dresden, Helmholtzstr. 20, D-01069 Dresden — 4 Laboratoire de
Physico-Chimie de l’Etat Solide, Université Paris Sud, F-91405 Orsay
Cedex
We have studied the magnetic and charge/orbital order in
La1−xSr1+xMnO4 by elastic and inelastic neutron scattering. The
antiferromagnetic transition and the spinwave dispersion of the parent
compound LaSrMnO4 show a typical 2D behavior. There is a large
anisotropy gap of 9meV, but we find additional localized magnetic
scattering below this spin-wave gap whose intensities increase upon
minor Sr-content enhancement. For higher Sr-concentrations, x>0.4,
the system exhibits at low temperatures charge-orbital ordering, best
established for the concentration of x=0.5. In La0.4Sr1.6MnO4 (x=0.6)
we find incommensurate ordering most likely induced by the additional
number of Mn 4+ -ions. The resulting orbital and magnetic structure may
be interpreted in a stripe picture similar to the nickelates.
supported by DFG through SFB 608
TT 24.54 Mi 14:30 Poster A
Orbital ordering in single-crystal and thin-film La7/8Sr1/8MnO3
— •Y. Su 1 , D. Wermeille 2 , O.H. Seeck 1 , A. Fattah 1 , J. Persson
1 , P. Foucart 1 , K. Istomin 1 , D. Hupfeld 1 , Th. Brueckel 1 ,
Zhi-Hong Wang 3 , G. Cristiani 3 , H.-U. Habermeier 3 , F. Wu 4 und
R.L. Johnson 4 — 1 Institut fuer Festkoerperforschung, Forschungszentrum
Juelich GmbH, D-52425 Juelich, Germany — 2 MuCAT, Advanced
Photon Source, Argonne National Laboratory, 9700 S. Cass Ave., Argonne,
IL 60439, USA — 3 Max-Planck-Institut fuer Festkoerperforschung,
Heisenbergstrasse 1, D-70569 Stuttgart, Germany — 4 II Institut fuer Experimentalphysik,
Universitaet Hamburg, D-22761 Hamburg, Germany
The orbital ordering (OO) is believed to be a key ingredient for the
metal-to-insulator transition in the ferromagnetic regime of lightly doped
manganites. By utilising the resonant X-ray scattering (RXS) technique,
a novel OO was identified in the ferromagnetic insulating (FI) regime
of single-crystal La7/8Sr1/8MnO3. Meanwhile, Templeton scattering due
to anisotropic structural effects was systematically measured at a set of
forbidden Bragg reflections in the full temperature range, enabling us to
quantitatively determine the atomic scattering tensor in both orbital ordered
and disordered states. The complex structural phase transition and
lattice modulations associated with this novel OO were also comprehensively
examined by high-energy X-ray diffraction. A definite model on the
OO in the FI regime will be proposed based on these new X-ray scattering
results. In addition, latest results on the RXS investigations of orbital ordering
and related structural modulation in thin-film La7/8Sr1/8MnO3
will also be presented