Correlation of electronic structure and ordered charge and orbital patterns for single-layered
manganites in a wide hole-doping range „0 x 1…
Y. S. Lee,
1,2
T. Arima,
1,3
S. Onoda,
1
Y. Okimoto,
4
Y. Tokunaga,
1
R. Mathieu,
1
X. Z. Yu,
1
J. P. He,
1
Y. Kaneko,
1
Y. Matsui,
5
N. Nagaosa,
4,6
and Y. Tokura
1,4,6
1
Spin Superstructure Project and Multiferroics Project, ERATO, Japan Science and Technology Agency, Tsukuba 305-8562, Japan
2
Department of Physics, Soongsil University, Seoul 156-743, Korea
3
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
4
Correlated Electron Research Center (CERC), National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba
Central 4, Tsukuba 305-8562, Japan
5
Advanced Materials Laboratory, National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan
6
Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan
Received 29 January 2007; published 6 April 2007
We investigated the doping-dependent evolution of the charge-orbital ordering CO-OO phase using optical
spectroscopy of a series of single-layer manganites, La
1-x
Sr
1+x
MnO
4
x =0 and 1 and Pr
1-x
Ca
1+x
MnO
4
0.3
x 0.7. We found that the electronic response of the CO-OO phase changes asymmetrically with respect to
the addition or removal of e
g
electrons from x =1/2. Especially, low-energy optical spectra depend strongly on
the modulation vector of striped phase in a higher doping region x 1/2. This finding highlights the role of
the nanoscale structure of e
g
ordering in the electronic property of the striped phase in the layered manganite.
DOI: 10.1103/PhysRevB.75.144407 PACS numbers: 75.47.Gk, 75.30.Et, 78.40.Ha
I. INTRODUCTION
The physics of doped Mott insulators is at the focus of
current research on strongly correlated electronic system be-
cause of a variety of intriguing ground states that originate
from strong correlations among the doped holes.
1
One of the
interesting examples is the one-dimensional 1D charge-spin
superstructure, referred to as stripe. The charge-spin striped
phase has been observed with a strong doping concentration
dependence in some two-dimensional 2D Cu and Ni ox-
ides. On the other hand, in a situation where the orbital de-
gree of freedom is active, the striped pattern is even more
peculiar, which is the case of charge-orbital stripe observed
for some Mn oxides in close relation to the magnetic
correlation.
2–5
For example, at half doping x =1/2, the zig-
zag chain-type e
g
orbital arrangement along the direction
perpendicular to the charge stripe is formed below the
charge-orbital ordering CO-OO transition temperature
T
CO-OO
and drives the CE-type magnetic order below Néel
temperature T
N
T
CO-OO
: the ferromagnetic FM spin or-
dering along the zigzag chain and the antiferromagnetic
AFM coupling between neighboring chains. These peculiar
properties of the charge-orbital stripe imply that the manga-
nite may provide an important laboratory to investigate the
role of the orbital degree of freedom in determining the elec-
tronic property of the striped phase.
In pseudocubic perovskite 113 manganites, the compe-
tition between CO-OO and FM correlations leads to a com-
plex nature of phase diagram with a metal-insulator
transition.
5
Moreover, various ordering types are observed
with doping. For example, the ground states of
Nd
1-x
Sr
x
MnO
3
are FM for x = 0.4, CO-OO and CE-type
AFM for x = 0.5, A type for x = 0.55, and C type for
x = 0.65.
6,7
In a single-layer 214 structure, on the other
hand, the low-temperature FM correlation is almost
suppressed
8
and insulating phases are dominant in an entire
doping range. Indeed, the CO-OO phase of the
Pr
1-x
Ca
1+x
MnO
4
PCMO series is robust in a relatively wide
doping range of 0.35 x 0.75.
9,10
In this situation, it is fea-
sible to access the generic doping-dependent evolution of the
CO-OO state without any disturbance of FM correlation. No-
tably, the phase diagram for the CO-OO phase in the 214
structure shows an asymmetric behavior around x =1/2: the
CO-OO correlation is weakened for x 1/2, whereas the
modulation period of the charge-orbital stripe changes sys-
tematically with doping for x 1/2.
8,9,11,12
Most likely this
asymmetric doping dependence is attributed to a peculiar e
g
orbital correlation. To elucidate this, optical spectroscopy on
the electronic structure of the insulating CO-OO state will be
useful. In this paper, we report on the in-plane optical study
of the Pr
1-x
Ca
1+x
MnO
4
series where the CO-OO state is ro-
bust in a relatively wide doping range of 0.35 x 0.75.
9,10
In an effort to reveal the electronic structure in the layered
manganites including the end member compounds, it is
found that the low-energy absorption formed by introducing
holes depends strongly on the CO-OO pattern as well as x.
The significant change in the spectral shape with the stripe
periodicity is compared with a theoretical calculation.
II. EXPERIMENT
Pr
1-x
Ca
1+x
MnO
4
0.3 x 0.7, LaSrMnO
4
x =0, and
Sr
2
MnO
4
x =1 single crystals were grown by the floating-
zone method.
11
To characterize the CO-OO phase of the
PCMO series, we performed measurements of dc resistivity,
magnetization, and electron diffraction pattern measure-
ments. T-dependent reflectivity spectra R at nearly nor-
mal incidence were measured for the Mn-O plane ab plane
at photon energies from 0.06 to 5 eV with T variation from
10 to 400 K and connected smoothly to room-temperature
R above 5 – 40 eV, which was measured with synchrotron
PHYSICAL REVIEW B 75, 144407 2007
1098-0121/2007/7514/1444076 ©2007 The American Physical Society 144407-1