Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Mittwoch
fields are calculated using a model that considers the competition between
double exchange (DE), super exchange (SE) and electron-phonon
interactions. We show a comparison to transport and magnetization experiments
in related compounds.
TT 24.43 Mi 14:30 Poster A
Comparison of LSDA+U calculations and photo emission
data of Fe3O4 — •David Schrupp 1 , Ivan Leonov 1 , Alexander
Yaresko 2 , Shigemasa Suga 3 , Vladimir Anisimov 4 , and Ralph
Claessen 1 — 1 Institut für Physik, Universität Augsburg, 86135
Augsburg, Germany — 2 Max Planck Institute for Physics of Complex
Systems, 01187 Dresden, Germany — 3 Department of Material Physics,
Osaka University, Osaka 560-8531, Japan — 4 Institute of Metal Physics,
Ekaterinburg GSP-170, Russia
Magnetite, undergoes a first-order phase transition, reflected by an
increase of electrical resistivity by two orders of magnitude, called the
Verwey transition. Although known for a long time the Verwey transition
is currently a matter of intensive debate questioning fundamental
issues such as the charge ordering of Fe 2+ ions which is widely believed
to be the driving mechanism of the Verwey transition.
The LSDA+U calculations based on new structure refinements in the
low temperature phase of magnetite result in a charge and orbitally ordered
insulator. The self-consistent solution corresponding to charge order
does not satisfy the widely-accepted Anderson condition of minimum
electrostatic repulsion, but agrees with photoemission spectra, taken below
the Verwey transition temperature.
These spectra where obtained by using polished, sputtered, annealed
and post-oxidized single crystals. The correct stoichiometry and longrange
order of the thus prepared surfaces were proven by LEED, STM,
and XPS. The samples were measured with high energy photoemission
(¯hω ≈ 700 eV), which means an enhanced information depth.
TT 24.44 Mi 14:30 Poster A
Dimerization versus Orbital Moment Ordering in the Mott insulator
YVO3 — •Peter Horsch1 , Giniyat Khaliullin1 und Andrzej
M. Oles2 — 1Max-Planck-Institut f. Festkoerperforschung, D-
70569 Stuttgart, Germany. — 2Smoluchowski Institute of Physics, PL-
30059 Krakow, Poland.
We investigate the magnetic and orbital ordering within the spinorbital
model [1] for cubic vanadates using exact diagonalization in combination
with mean-field theory [2]. Increasing Hund’s exchange JH triggers
a crossover from the valence-bond orbital states to C-type antiferromagnetic
(AF) phase, while a finite spin-orbit coupling induces the
staggered component of magnetization which couples to the t2g orbital
moments. Our results provide a qualitative explanation of the observed
spin canting and large reduction of magnetization in the C-AF phase
of YVO3 [3]. At finite temperature an orbital instability in the C-type
antiferromagnetic phase induces modulation of magnetic exchange constants
even in the absence of lattice distortions. This dimerization should
be distinguished from a usual Peierls instability, as it emerges at finite
temperatures (and vanishes at zero temperature) due to an interplay of
quantum effects and thermal fluctuations, which open the way towards
dimerized orbital- and spin-correlations. The calculated spin structure
factor shows a magnon splitting at �q = (0, 0, π)
due to the orbital di-
2
merization, similar to the spinwave dispersions measured by Ulrich et
al.
[1] G. Khaliullin, P. Horsch, A.M. Oles, Phys. Rev. Lett. 86,3879 (2001).
[2] P. Horsch, G. Khaliullin, A.M. Oles, Phys. Rev. Lett. in print (2003).
[3] C. Ulrich et al., Phys. Rev. Lett. in print (2003).
TT 24.45 Mi 14:30 Poster A
Orbital assisted metal insulator transition in VO2 — •L.H.
Tjeng 1 , T. Koethe 1 , M.W. Haverkort 1 , Z. Hu 1 , A. Tanaka 2 ,
S. Streltsov 3 , M. Korotin 3 , V. Anisimov 3 , W. Reichelt 4 , H.H.
Hsieh 5 , H.-J. Lin 5 , and C.T. Chen 5 — 1 II. Physikalisches Institut,
Universität zu Köln — 2 ADSM, Hiroshima University, Japan — 3 IMP,
Ekaterinburg, Russia — 4 Institut für Anorganische Chemie, Dresden —
5 NSRRC, Hsinchu, Taiwan
VO2 is a non-magnetic oxide that undergoes a metal-to-insulator transition
at 340 Kelvin. Above this temperature, VO2 is metallic and has a
rutile (TiO2) structure (R-phase). At low temperatures, it is an insulator
with a monoclinic structure (M1-phase), in which V-V pairs are formed.
The long-standing debate about this compound concerns the nature of
the metal-to-insulator transition. The issue is whether the non-magnetic
insulating state would be regarded as a Peierls-insulator with the char-
acter of a band insulator (one-electron picture), or whether it should be
viewed as a Mott-insulator (many-body picture).
We have used polarization dependent soft-X-ray absorbtion at the V
L2,3 edges to investigate the local electronic structure of VO2, and have
found that the V 3d orbital occupation symmetry changes dramatically
across the metal-insulator transition. From a comparison with LDA and
LDA+U calculations we infer that this phase transition is assisted by
the orbital degrees of freedom, i.e. requiring an explanation beyond the
classical Peierls mechanism.
TT 24.46 Mi 14:30 Poster A
Orbital occupation and momentum in LaTiO3 — •M.W.
Haverkort 1 , Z. Hu 1 , H. Roth 1 , T. Lorenz 1 , C. de Nadai 2 , N.B.
Brookes 2 , A. Tanaka 3 , H.H. Hsieh 4 , H.-J. Lin 4 , C.T. Chen 4 , and
L.H. Tjeng 1 — 1 II. Physikalisches Institut, Universität zu Köln —
2 ESRF, Grenoble, France — 3 ADSM, Hiroshima, Japan — 4 NSRRC,
Hsinchu, Taiwan
LaTiO3 is an antiferromagnetic insulator with a magnetic ordering
temperature of TN ≈ 145 K. Neutron measurements reveal a magnetic
moment of 0.45 ∼ 0.57 µB, significantly less than the 1 µB expected for
a S = 1
2 system (Ti = 3d1 ). One may envision that this discrepancy can
be ascribed to the presence of an anti-parallel aligned orbital momentum
in this quasi cubic material. However, neutron data also found an almost
perfect isotropic spin wave spectrum with an extremely low spin gap,
indicative for the absence of an orbital moment. An orbital liquid model
is then proposed to explain these data. We have carried out detailed temperature
dependent soft-x-ray absorption and circularly-polarized/spinresolved
photoemission experiments. We find that the orbital moment
is practically quenched, in agreement with the neutron spin wave data.
Using the results from LDA and LDA+U calculations, we infer that this
quenching is caused by non-cubic crystal fields associated with the small
but non-negligible non-cubic distortions in LaTiO3. These crystal fields
are much stronger than the spin-orbit interaction, and crucial for the
explanation of the lineshape and the lack of temperature dependence of
the spectra. It seems that these conditions do not favor the formation of
an orbital liquid.
TT 24.47 Mi 14:30 Poster A
Orbital excitations in LaTiO3 and YTiO3 investigated by infrared
and Raman spectroscopy — •A. Gössling 1 , R. Rückamp 1 ,
M. Grüninger 1 , M. Cwik 1 , H Roth 1 , T. Lorenz 1 , A. Freimuth 1 ,
B. Keimer 2 und C. Ulrich 2 — 1 II.Physikalisches Institut, Universität
zu Köln — 2 Max-Planck-Institut FKF, Stuttgart
There is still discussion about the puzzling compound LaTiO3 [1,2,3].
We compare LaTiO3 with orbitally ordered YTiO3 using IR and Raman
spectroscopy. With IR we observe broad peaks at approximately 0.30 eV
in both compounds, with Raman at 0.25 eV. The difference in energy
is due to the simultaneous excitation of a phonon in IR spectroscopy,
breaking the parity selection rule. This clearly reveals the even parity of
the excitation, typical for orbital excitations. We discuss the origin of the
peaks within the framework of different theoretical models. Supported by
the DFG through SFB 608.
[1] B.Keimer et al., Phys.Rev.Lett.85, 3946 (2000).
[2] G.Khaliullin et al., Phys.Rev.Lett.85, 3950 (2000).
[3] M.Cwik et al., Phys.Rev.B 68, 060401 (2003).
TT 24.48 Mi 14:30 Poster A
Search for orbital excitations in YTiO3: calculation of phonon
dispersion relations — •M. Guennou 1 , C. Ulrich 1 , C. Frost 2 ,
S. Miyasaka 3 , Y. Taguchi 3 , Y. Tokura 3 , and B. Keimer 1 —
1 Max-Planck-Institut FKF, Stuttgart — 2 ISIS, Oxford, England —
3 Departement of Applied Physics, University of Tokyo, Japan
The purpose of our study was to search for elementary excitations of
the orbital magnetization, the so-called orbitons, in YTiO3. We have performed
single crystal inelastic neutron scattering experiments up to high
energies (350 meV) at the spectrometer MAPS at the ISIS facility in Oxford.
In order to give an interpretation of the features observed at high
energies, we have calculated the phonon dispersion relations and the first
and higher order density of states. There is a good agreement between
the calculated phonon dispersion relations and the first order phonon
modes measured by neutron and Raman scattering. These results will
be compared with the theoretical predictions for the orbital dispersion
relations given by G. Khaliullin [1].
[1] G. Khaliullin et al., Phys.Rev.Lett. 89, 167201 (2002).