Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Dienstag
tisch geordneten Tieftemperaturphase. Darüber hinaus ergeben sich Belege
für eine temperaturabhängige Umbesetzung der Orbitale. Eine geringe
Dotierung (x=1/8) mit Löchern, d.h. das Einschalten von konkurrierenden
ferromagnetischen Doppelaustauschwechselwirkungen, verringert die
Stabilität der antiferromagnetischen Tieftemperaturphase. Dabei zeigen
Messungen der Magnetisierung das Auftreten zusätzlicher xy-artiger magnetischer
Momente in der geordneten Phase, die als stark anisotrope
magnetische Polaronen verstanden werden können.
TT 14.7 Di 16:20 H18
Bond centered vs. site-centered charge ordering and ferroelectricity
in oxides. — •Dmitry Efremov 1 , Jeroen van den Brink 2 ,
and Daniel Khomskii 3 — 1 Technische Universität Dresden, Institut
für Theoretische Physik, 01062 Dresden — 2 Institut-Lorentz for Theoretical
Physics, Universiteit Leiden, 2300 RA Leiden, The Netherlands
— 3 Universität zu Köln, 50937 Köln
Many transition metal oxides show charge ordering, which up to now
was always treated as alternation of valency of transition metal ions. We
show that besides this site-centered charge ordering there may exist another
type of charge and orbital ordering, namely bond-centered ordering.
We demonstrate that such type of ordering should exist in manganites
close to half-filling. Moreover we show that these two types of ordering
may coexist making the system ferroelectric.
16:35 Pause
Hauptvortrag TT 14.8 Di 16:50 H18
Resonant Soft Energy X-ray Diffraction of Charge, Spin and
Orbital Ordering — •Peter Hatton — Dept. of Physics, University
of Durham, UK
Soft x-ray resonant diffraction is a new technique pioneered by our
group. We have published examples of the huge resonant enhancements
of charge and magnetic scattering that can be obtained at the L-edges
of 3d transition metal oxides as well as the first direct observation of
orbital ordering. In this talk we will review the technique of soft x-ray
diffraction as well as giving recent examples of results In particular we
will report the first resonant soft x-ray scattering observations of orbital
ordering from a bulk single crystal. We have studied the low temperature
phase of La1.5Sr0.5MnO4 that displays charge, spin and orbital ordering.
Previous claims of orbital ordering in such materials have relied on observations
at the K edge. These claims have attracted a barrage of controversy
from theoretical studies. Instead we have employed resonant soft
x-ray scattering at the manganese LIII and LII edges which provides
a direct measurement of the orbital ordering. Energy scans at constant
wavevector have been compared to theoretical predictions and show that
at all temperatures there are two separate contributions to the observed
scattering, direct Goodenough orbital ordering and strong cooperative
Jahn-Teller distortions of the Mn 3+ ions.
Fachvortrag TT 14.9 Di 17:20 H18
The interplay of orbital and lattice degrees of freedom in titanates
— •M. Grüninger, R. Rückamp, A. Gößling, M. Cwik,
H. Roth, J. Baier, M. Kriener, T. Lorenz, M. Braden, and A.
Freimuth — II. Physikalisches Institut, Universität zu Köln, Germany
Recently, several interesting phenomena have been proposed in which
orbitals play a key role, e.g. orbital order, dispersing orbital waves or
orbital liquids. However, the driving force in most theoretical treatments
is based on electronic interactions, the possible role of the lattice is often
neglected.
One interesting phenomenon which still has to be confirmed experimentally
is the existence of orbital liquids. The pseudo-“cubic” d 1 antiferromagnet
LaTiO3 seemed a promising candidate. We discuss the structure,
the thermal expansion and the optical conductivity of LaTiO3 and of
orbitally ordered YTiO3. A consistent description of the data of both
compounds is obtained within a Jahn-Teller scenario. Orbital excitations
are observed in the optical conductivity spectra of both LaTiO3 and
YTiO3 at about 0.25eV, which is difficult to reconcile with an orbital
liquid ground state of LaTiO3. On contrary, the data suggest that the
coupling to the lattice is of dominant importance not only for eg systems,
but also for t2g electrons. This point of view is corroborated by the optical
conductivity data of TiOCl. There, the splitting of the t2g levels is
larger than 0.5eV.
TT 14.10 Di 17:40 H18
Spin-orbital physics in Raman and optical spectra of Mott
insulators — •Giniyat Khaliullin — a:Max-Planck-Institut FKF,
Heisenbergstr.1, 70569 Stuttgart
We review our recent work which focuses on Raman [1] and optical
[2] spectroscopy in Mott insulators. First, we discuss different Raman
scattering channels involving orbital excitations in titanates. Next, we
consider the spectral weights of optical transitions in titanates and cubic
vanadates. We show that the temperature and polarization dependences
of the Raman and optical spectra are directly determined by the underlying
spin and orbital correlations, and discuss the role played by orbital
quantum fluctuations.
[1] G. Khaliullin, (unpublished).
[2] G. Khaliullin, P. Horsch, and A.M. Ole´s, (unpublished).
TT 14.11 Di 17:55 H18
Direct observation of charge order in La1.8Sr0.2NiO4 —
•C. Schüßler-Langeheine 1 , J. Schlappa 1 , Z. Hu 1 , M. W.
Haverkort 1 , M. Benomar 1 , O. Friedt 1 , E. Schierle 2 , H. Ott 2 ,
E. Weschke 2 , G. Kaindl 2 , G. A. Sawatsky 3 , A. Tanaka 4 , M.
Braden 1 , and L. H. Tjeng 1 — 1 II. Physikalisches Institut, Universität
zu Köln — 2 Institut für Experimentalphysik, Freie Universität Berlin —
3 Department of Physics and Astronomy, University of British Columbia,
Vancouver, Canada — 4 ADSM, Hiroshima University, Japan
We report on the direct observation of charge order in Sr-doped
La2NiO4 using resonant soft x-ray scattering at the Ni L2,3 and La
M4,5 resonances. Unlike conventional scattering techniques, which are
essentially probing lattice modulations, resonant soft x-ray scattering is
very sensitive to the oxidation state and hence particularly suited to
study charge-order phenomena. The charge-order superstructure peak
from La1.8Sr0.2NiO4 shows a resonant enhancement predominantly at Ni
L2,3 edges, in contrast to the x-ray absorption signal, which is dominated
by the La absorption. The details of this energy dependence agree
well with charge order in the Ni system, providing a direct experimental
observation of this ordered phase. Furthermore, the superstructure peak
observed with resonant soft x-ray scattering is much narrower in momentum
space than known from conventional scattering techniques, because
the comparably short penetration depth of soft x-rays together with the
availability of well focused x-ray beams makes this technique sensitive to
slight local inhomogeneities of the doping level, which cannot be probed
by other techniques.
TT 14.12 Di 18:10 H18
Extended moment formation and magnetic ordering in the trigonal
chain compound Ca3Co2O6 — •Volker Eyert 1 , Christian
Laschinger 1 , Thilo Kopp 1 , and Raymond Frésard 2 — 1 Institut für
Physik, Universität Augsburg, Germany — 2 Laboratoire Crismat, UMR
CNRS-ENSICAEN (ISMRA) 6508, Caen, France
The results of electronic structure calculations for the one-dimensional
magnetic chain compound Ca3Co2O6 are presented. The calculations are
based on density functional theory and the local density approximation
and used the augmented spherical wave (ASW) method. Our results allow
for deeper understanding of recent experimental findings. In particular,
alternation of Co 3d low- and high-spin states along the characteristic
chains is related to differences in the oxygen coordination at the inequivalent
cobalt sites. Strong hybridization of the d states with the O 2p
states lays ground for polarization of the latter and the formation of extended
localized magnetic moments centered at the high-spin sites. In
contrast, strong metal-metal overlap along the chains gives rise to intrachain
ferromagnetic exchange coupling of the extended moments via the
d 3z 2 −r 2 orbitals of the low-spin cobalt atoms.
TT 14.13 Di 18:25 H18
Excitation spectrum of the cobaltite NaxCoO2 yH2O as function
of Na content and hydration — •P. Lemmens 1,2 , K.-Y. Choi 2 ,
V. Gnezdilov 3 , N.N. Kovaleva 1 , H. Sakurai 4 , K. Takada 4 , T.
Sasaki 4 , E. Takayama-Muromachi 4 , F.C. Chou 5 , C.T. Lin 1 , and
B. Keimer 1 — 1 MPI for Solid State Research, D-70569 Stuttgart — 2 2.
Phys. Inst., RWTH Aachen, D-52056 Aachen — 3 B.I. Verkin Inst. for Low
Temp. Phys., NASU, 61164 Kharkov — 4 Superconducting Mat. Center
and Advanced Mat. Lab., NIMS, Tsukuba, Ibaraki 305-0044 — 5 Center
for Material Science and Engineering, MIT, Cambridge, MA 02139
We report on a Raman scattering study on the superconducting
cobaltite NaxCoO2 yH2O as function of Na content and hydration