Photon Factory Activity Report 2004 #22 Part B (2005)
Materials Science

1B, 4C, 16A2/2001S2002

The order of electron degrees of freedom studied by resonant x-ray scattering
Youichi MURAKAMI*1,2, Hironori NAKAO1,Takeshi MATSUMURA1, Kazuma HIROTA3
Yusuke WAKABAYASHI4, Hiroshi SAWA4, Masato KUBOTA4, Hiroyuki OHSUMI5,Takashi KIYAMA6, Takahisa ARIMA7, Yoshinori TOKURA8, Fumitoshi IGA91
Department of Physics, Tohoku University, Sendai 980-8578, Japan
2
Synchrotron Radiation Research Center, JAERI, Sayo, 679-5148, Japan
3
ISSP, University of Tokyo, Kashiwa 277-8581, Japan
4KEK-PF, Tsukuba, Ibaraki 305-0801, Japan
5Japan Synchrotron Radiation Research Institute, Sayo 679-5198, Japan
6Department Phsics, Nagoya University, Nagoya, 464-8602, Japan
7
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Japan
8
Departmet of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan
9
Department of Quantum Matter, ADSM, Hiroshima University, Higasihiroshima 739-8526, Japan
Introduction
The strong electron correlation in solids shows richphenomena such as superconductivity and colossal
magnetoresistance, and so on. It has been recognized thatelectron degrees of freedom (charge, spin, and orbital)
plays very important roles in their electronic andmagnetic properties. In order to observe the ordering
states of the degrees of freedom we have applied theresonant x-ray scattering (RXS) technique to transition
metal oxides and quadrupolar ordering f-electron systems.
In this S2 project (2001 Oct. – 2004 Sep.) we havedeveloped the tecnique to observe the orbital ordering andapplied it to the study of the order-disroder transition in
manganese and titanium oxides and RB2C2 (R: rare earthelements).
Experimental Results
Development of the interference technique
Until now, we have observed only antiferro-orbitalordering. On the other hand it is very difficult to obsere
ferro-orbital ordering because the signals appear at Γpoints (the Brillouin-zone centers) where a very large
Bragg peak intensity is superposed. However, we haverecently succeeded in observing the signal from ferroorbital ordering of a manganite films with superlattice
structure by exploring the interference term between σand π polarization components of the scattered x-ray.
Analyzing energy profiles of the interference intensity,
one can reveal the energy level scheme relating to theresonance process.
Charge and Orbital Ordering of Manganites (eg system)
We have studied charge and orbital states in the single
layered and the bi-layered manganites. The wave numberof the superlattice due to the charge and orbital ordering
is shifted when we increase the hole concentration fromthe half doped state. However, we have found that thewave number shows a constant value in a finite
concentration region. Namely, the charge and orbital
ordering has a lock-in-structure like the devil's staircase.

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This structure and its temperature dependence werediscussed on the basis of the charge and orbital
correlations.
Orbital Ordering in Titanates (t2g system)
Orbitally ordered states in Y1-xCaxTiO3 have beensystematically investigated by RXS. The RXS intensities
at 1s to 3d transition energy (pre-edge) reflecting the 3dorbital ordering dramatically decrease with increasing Ca
concentration toward the ferromagnetic-paramagnetic
phase boundary (xFP=0.15). The intensity remaining
above xFP decreases gradually and almost disappears at themetal-insulator transition (xMI=0.4). Consequently, theorbital ordering is strongly suppressed toward xFP, andnearly vanishes at xMI.
The hole concentration
dependence of Jahn-Teller distortion determined by the xray structural analysis is also consistent with that of theorbitally ordered state. We also investigated orbital
ordered states in RTiO3 (R=Y, Gd, Sm, Nd, and La). It
has been found that orbital state of LaTiO3 is different
from those of RTiO3 (R=Y, Gd, Sm).
Quadrupolar Ordering in RB2C2 (R = Ho, Tb) (f system)
We have observed the orderings of the quadrupolar
moments of the 4f-electron systems. Following thesuccess of the observation of the antiferro quadrupolarordering in DyB2C2, we have extended the study to
HoB2C2 and TbB2C2 in which the competition between
magnetic and quadrupolar interactions is more important
for the physical properties. In HoB2C2, temperaturedependence of the order parameter shows that magneticand quadrupolar orderings of long range appear at thesame temperature of 5 K, indicating the crucial role of thequadrupolar moment for the unusual magnetic structure.
* murakami@iiyo.phys.tohoku.ac.jp