Frontier 2

Correlated Electrons:
Science to Technology
Yoshinori TOKURA
Department of Applied Physics, University of Tokyo,Tokyo113-8656,Japan,
Joint Research Center for Atom Technology (JRCAT) and Correlated Electron Research Center (CERC)
Tsukuba,Ibaraki305-0046,Japan

Abstract
Light irradiation creates a magnet in a nonmangetic medium, and an electric or magnetic field turns a material from an insulator to ametal: such unconventional control of the state of matter is possible by exploiting strongly correlated electrons. Correlated electrons in solids,
bear multiple degrees of freedom not only in charge sector but also in the spin and orbital sectors. Strong mutual interaction in the respective
sectors, as well as cooperation/competition between the different sectors, can give rise to astonishing electronic properties/functions, such as
high-temperature superconductivity and colossal magnetoresistance. Electronic and magnetic phases can be controlled by utilizing cooperative response of correlated electrons to external electric/magnetic fields, stress, and photo-excitation. The critical-state phase control is a key
concept for correlated-electron technology in the future.

eg orbitals
z
x

y

x2-y2

3z2-r2t2g orbitals

zx

yz

xy

Fig. 1.
Five d orbitals. In the cubic crystal field, the five-fold degeneracy is lifted
to two eg orbitals ( x2-y2, 3z2-r2) and three t2g orbitals ( xy, yz, zx).

1. Introduction
An electron in a solid, when it is bound toor nearly localized on a specific atom, has three
attributes; charge (-e), spin (S=1/2), and orbital(see the case of the d orbital in Fig.1). An orbital, which represents the electron’s probability-density distribution, may be viewed as theshape of an electron in a solid. Conventional
electronics utilizes the charge degree of freedom and its coupling with external electric field

12

JSAP International No.2 (July 2000)

and light. Recent extensive R&D on spintronics,
on the other hand, is directed toward the useof both the spin and the charge degrees of freedom of electrons. The correlated-electron technology (CET) outlined in this article will utilize
all these attributes of electrons including theorbital.
In correlated electron systems, where theelectron-electron Coulomb repulsion interaction is strong and electrons are almost local-

ized or barely mobile, a naive single-particle approximation (considering a moving electron as
an independent particle in the effective medium) does not hold. In this context, the concept of CET is just opposite that of single-electron manipulation: its emphasis is laid on thecontrol of the electronic phase which interacting many electrons can form. The charge, spin,
orbital degrees of freedom - and their coupled
dynamics - can produce complex phases andphenomena such as liquid-like, crystal-like, liquid-crystal-like states of electrons, and electronic phase separation or pattern formation,
as typical of complex system. The electronic/
magnetic phase of a material containing correlated electrons can be controlled in unconventional ways (Fig. 2) and in an ultrafast way(at teraherz frequencies), so CET may provide
a seed for a new class of electronics.
The correlation of electrons in a solid beyond the single-particle approximation typicallyshows up in cooperation or competition between magnetism and electrical conductance,
and this has been a long-standing important(but difficult) problem in the field of condensed
matter physics. Since the discovery of hightemperature superconducting (high-Tc) copperoxides, a more general interest in the Mott transition, which is the metal-insulator transition
in a correlated electron system,1) has revived
peripheral fields of science as well. The high-Tc
copper oxides are composed of CuO2 sheets
separated from each other by atomic barrier
units called “block layers”. The CuO2 sheet isoriginally insulating because of the large cor-

2. Colossal magnetoresistance(CMR)

electric field,photon

chargecolossalmagnetoresistancefield induced insulator-metaltransitionpressure induced
insulator-metal transitionpressuremagnetic fieldorbitalspin

field induced
structural transition

magnetic control

electrical control

photonic control

conductivity

magnetization

magnetization
conductivity

magnetic field

electric fieldlight intensity

Fig.2.
Spin-charge orbital coupling for correlated d electron systems (upper) and unconventional phase control (lower).

relation effect, while possessing one conduction electron (or hole) per Cu site. This is a typical example of the Mott insulator. The high-Tc
superconductivity with unconventional (perhaps d-wave) symmetry emerges upon the filling-controlled Mott transition, namely, as soon
as the charge of the ionic block layer, or equivalently the change in the band filling (numberof conduction electrons), causes the Mott transition. However, the strong antiferromagnetic
correlation, originated in the Mott-insulating
CuO2 sheets, clearly subsists in the metallic state
and this has been believed to be most relevantto the mechanism of high-Tc superconductivity.
In the course of the renaissance of the correlated-electron science during this decade, a
lot of new aspects of the Mott transition phenomena have been unraveled from a modernized viewpoint.1) One of the fruits of this renaissance was the rediscovery of the so-calledcolossal magnetoresistance (CMR) phenomena,2) which is a gigantic decrease of resistancethat is induced by application of a magnetic
field and that is typically observed in perovskite
manganese oxides. In the following of this article, we will demonstrate some examples ofdrastic phase changes in CMR manganites
which arise from a close interplay amongcharge, spin, orbital, and lattice degrees of freedom.
The CET is exploiting a broad range of materials and electronic functionality in addition
to the high-Tc-related and CMR-related ones.
The recent advance of the fabrication technologies used to prepare thin films and superlattices
has begun to produce correlated-electron junctions and atomically controlled layered structures that can be viewed as tailored correlatedelectron materials. Another newly foundcharateristic of correlated electrons is the photonic functionality. It has also become known
recently3) that the third-order nonlinear opticalsusceptibility, which corresponds to the properties of two-photon absorption and the optical Kerr effect, is gigantic for some one-dimensional Mott insulators. Indeed, the third-ordernonlinear optical susceptibility of these Mott
insulators is comparable to or greater than thatof so-far known nonlinear optical materials.
These new trends in the CET research will also
be dealt with in the following parts of this article.

The CMR phenomena arise from thestrong coupling, known as the double-exchange interaction,4,5) between a conduction
electron (eg orbital) and local spin (t2g orbital),
which are both d electrons in origin but areapparently decoupled (i.e., t2g electrons are localized as local spins) because of the strong
correlation effect. As shown in Fig. 3(a), thespin of the conduction electron is alwaysaligned parallel to the direction of the local spinbecause of the strong Hund’s-rule coupling.
The conduction electron cannot hop to theneighbor site when the local spins on the adjacent sites are antiparallel (antiferromagnetic).
By contrast, the electron can conduct when the
spin-parallel (ferromagnetic) configuration is
induced thermally or by application of an external magnetic field. This atomic switchingmechanism is the simplest explanation of the
CMR effect. As illustrated in Fig. 3(c), when amagnetic field is applied to a perovskite-type(Nd,Sm)0.5Sr0.5MnO3 in the antiferromagnetic(but no long-range spin-ordered) high-resistive
state above the Curie temperature (Tc), the resistivity is reduced to 10-3 of its zero-field value
and the magnetism changes from antiferromagnetic to ferromagnetic.6)
A similar switching of the resistive state
coupled with the change of spin configuration
can be observed for a layered crystal of manganese oxide with bilayer-units of Mn-O sheets(Fig. 4).7,8) In this oxide, the spin configuration
between the ferromagnetic bilayer units is antiferromagnetic. The fully spin-polarized electron can therefore move freely within the layered unit but cannot hop along the interplane(c-axis) direction. But the confinement of carriers can be relaxed by the application of amagnetic field that can align all the spins inthe same direction (i.e., that can cause themetamagnetic transition), resulting in the gigantic magentoresistnace effect (the right-handpanel of Fig. 4). The magnetic field strength
needed for this interlayer spin alignment is lessthan a few hundred milliteslas, much smaller
than that needed for the aforementioned
atomic switching. This is viewed as the tunneling magentoresistance (TMR) effect that canusually be realized in artificial tunneling structures. Although the gigantic TMR effect canbe observed only at temperatures below 100
K, the extremely large TMR value (4000%) observed under optimized condition (under pressures) may give a prospect of utilizing a half-

JSAP International No.2 (July 2000)

13

(a)

(b)

(c)

orbital liquid-crystal

conduction
electron

115K

1.0

125K

(Nd,Sm) 0.5Sr 0.5MnO3

135K

localized spin

240K

H

H

orbital liquid(H)/ (0)

0.8

220K
0.6
200K
0.4
180K

0.2
conduction
electron

145K

160K

0.0

localized spin

0

1

2

3

4

5

magnetic field[T]FM(Ferromagnetic Metal)c (H)/Hc (0.8T) %Fig.3.
(a) Spin-charge coupling for the colossal magnetoresistnace(CMR) manganite. (b) Liquid-crystal ( antiferromagnetic) stateand liquid (ferromagnetic metallic) state of the conduction electrons. (c) CMR observed in a (Nd,Sm)0.5Sr0.5MnO3 crystal.

4000

4.2 K

3000

FM

I

FM

La, Sr Mn O

Interplane magnetoresistanceI(Nonmagnetic Insulator)

2000

1000

layered magnite(La,Sr) 3Mn2O7
7kbar

Fe/Al2 O3/Fe
tunnel junction
10

0kbar

0
-0.5

0
Magnetic Field [T]0.5

Fig.4.
Left: Layered perovskite structure viewed as a tunneling magnetoresistance system. Right: Interplane tunnelingmagnetoresistance for (La,Sr)3Sr2Mn2O7 crystal.

metallic (i.e., fully spin-polarized) transitionmetal oxide as the component of the TMR devices.2)

3. Electron orbital as a control
parameter
An origin of the CMR effect is thus thestrong spin-charge coupling inherent in the cor-

14

JSAP International No.2 (July 2000)

related electrons. However, the physics of CMR
is more complex and profound. As noted inSec. 1, an important hidden attribute of correlated electrons is the orbital degree of freedom.9,10) The orbital ordering and correlation
as well as their strong coupling to the spin andlattice structure is essentially important for understanding of the CMR effect. The d electron

shows the five-fold degenerate states, whose
respective orbitals represent spatially anisotropic electron-cloud (see Fig. 1). The crystal field
lifts partially the degeneracy, but the electron
orbital often behaves as a pseudo spin. It haslong been well known that the orbital degreeof freedom plays an important role in anisotropic magnetic interaction in transition-metaloxides11,12) as well as in the spontaneous lattice
deformation arising from the so-called JahnTeller interaction.13,14) Recent studies of the CMR
oxides have further been revealing that the dynamical or quantum fluctuation of orbital isstrongly coupled with charge dynamics in thebarely metallic state.1,2,9)
The manganese oxide (manganite) possesses the conduction electron which is accommodated in the e g orbital state and tied by
Hund’s rule to the local t2g spin of S=3/2. Therefore, the dominating interactions are not only
the ferromagnetic double-exchange interaction, and the antiferromagnetic superexchangeinteraction between t2g local spins, but also theexchange interaction in the orbital sector of theeg conduction electron and the orbital-ordering induced ferromagentic superexchange interaction. The eg orbital, which directs the surrounding oxygen sites (see Fig. 1), strongly
couples with the deformation of the MnO6 octahedron (Jahn-Teller interaction). For example, the 3z2-r2 state tends to elongate theoctahedron along the z direction, while the x2y2 state tends to stretch the octahedron later-

Frontier 2

Correlated Electron:
Scinece to Technology
the substrate. It is also based on the local density functional calculation on the tetragonal
manganites.16) The interplay of the doping andthe strain causes the slanted phase boundaries
for the F (orbital-disordered), A (x2-y2 ordered),and C (3z2-r2 ordered) states. As a general trend,
the decrease of hole doping in this region enlarges the F state, while the increase (decrease)in the c/a ratio stabilizes the C (A) state as expected. In fact, thin films of La 1-xSr xMnO3(x=0.5) epitaxially grown on three different
perovskite substrates show the respective
ground states and the similar transport properties to those shown in the case of Nd1-xSrxMnO3(Fig. 5(a)). Thus, near the phase boundaries, aminute perturbation, stress or carrier doping,can induce the metal-insulator or ferromagnetic-antiferromagnetic transition or both.
The orbital plays an essential role in the
CMR effect itself.2,9,21) In the highly resistive

state near above the Tc (Fig. 3(c)), the orbitalsof the conduction electrons are regularly arranged and directed along the same direction
over some short distance. Such a dynamicalordering of charge and orbital can be viewed
as “electron liquid-crystal” that is intermediate in nature between the electron crystal(charge/orbital-ordered insulator) and electron
liquid (metal). The electron liquid-crystal staterealized just above the Curie temperature (Tc)
is antiferromagnetic in nature but is easily converted by an external magnetic field to a ferromagnetic electron-liquid state ( Fig. 3(b)).

4. Unconventional phase
control
In correlated electron systems with multiple degrees of freedom, unconventional
phase-control like that depicted in Fig. 2 may
become possible. The spin-charge-orbital cou-

C

F

A

3z 2-r 2

x 2-y 2& 3 z2-r 2

x 2-y 2

Nd1-xSr xMnO3
single crystal
(a)

cm)

A

-2

10

1.08

C

0.55
(0.97)

-3

10

1.04

-4

0.45
F (0.99)

0

1.02
1.00

0H = 0 T

10

(b)
LAO

1.06

x=
0.67
(1.04)

c/a

-1

10

Resistivity (ally. Conversely speaking, the uniaxial strain
with respect to the MnO6 octahedron can serve
as a pseudo magnetic field on the orbital (thepseudo spin). The orbital ordering gives rise toanisotropy of the electron transfer interaction,
which anisotropy either enhances or suppresses
the double-exchange and superexchange interactions, thereby giving rise to complex spinorbital coupled states.
One typical example is the case of LaMnO3
or NdMnO3, in which the in-plane (ab plane)
alternate ordering of 3x2-r2/3y2-r2 states realizes the in-plane ferromagnetic, while antiferromagnetic along the c axis, state. In the holedoped manganites, in which the double-exchange interaction emerges with doping-dependent strength, various orbital-ordered anddisordered states show up accompanying therespective spin-ordering features,15,16) as illustrated in the top panel of Fig. 5. Let us here
take the case of Nd1-xSrxMnO3. With appreciable
doping on the parent compound NdMnO3, theorbital order melts into a quantum-disorderedstate and the compound shows the ferromagnetic-metallic (F) state when 0.3<x<0.5. Withfurther doping (0.5<x<0.7), the kinetic energyof carriers decreases and the compound shows
the two-dimensional metallic state with layeredtype antiferromagnetic (A) state.17,18) This A
state is realized because of the ferromagnetic(homogeneous) order of x2-y2 orbital. The overlap between the x2-y2 orbitals on the adjacentplane is zero in the cubic perovskite structure,and hence the electron transfer is almost prohibited along the c axis. This produces the antiferromagnetic coupling between the doubleexchange ferromagnetic metallic sheets (abplanes), producing the two-dimensional halfmetal sate in spite of the nearly cubic crystal
structure.18) Doping above x=0.7 further modifies the magnetic structure to the chain-type(C),19) as shown in Fig. 5, which is described by
the order of 3z2-r2 orbital.
Instead of changing the carrier kinetic energy by doping, one can use the lattice strain
as a biasing field on the orbital state via the
Jahn-Teller interaction.20) Figure 5(b) shows aschematic spin-orbital phase diagram for themoderately doped (0.3<x<0.7) manganite on
the plane of the doping level vs. the uniaxial
strain measured as the ratio of lattice parameter c/a (or almost equivalently the ratio of theapical to equatorial Mn-O bond length). The
phase diagram is based on experimental results20) obtained using the epitaxial single-crystalline films of La1-xSrxMnO3 with coherent lattice strain due to the lattice mismatching with

3T
5T
7T

100 200 300 400
Temperature (K)

0.98
0.96

F
LSAT

C

NSMO
bulk

LSMO bulk

STO

A

0.2 0.3 0.4 0.5 0.6 0.7
xFig. 5.
Spin-orbital phase diagram for a perovskite manganese oxide. The top panel shows the orbital and spin order in the holedoped manganese oxides. (a) Temperature dependence of the resistivity of Nd1-xSrxMnO3 under various magnetic fields
for the respective magnetic phases(F,A, and C). The number in parenthesis represents the uniaxial lattice strain(c/a) ratio,
indicating the coupling of the magnetism to the orbital order as shown in the top panel. (b) The schematic phase diagramin the plane of lattice strain c/a and doping level x. The data labeled LAO, LSATand STO represent the results for thecoherently strained epitaxial thin films of La1-xSrxMnO3 grown on the perovskite single-crystal substrates of LaAlO3,(La,Sr)(Al,Ta)O3, and SrTiO3, respectively. LSMO-bulk and NSMO-bulk stand for the results for the bulk single crystals of
La1-xSrxMnO3and Nd1-xSrxMnO3.

JSAP International No.2 (July 2000)

15

pling produces complex and intriguing electronic phases. The energy-scale of the dominating interactions in the correlated electron
system is of the order of eV, while the competition of the phases (e.g. metallic vs. insulating, or ferromagnetic vs. antiferromagnetic) is
usually so subtle, as described on a low-energy
scale. This means that a minute perturbation
as the input can cause a gigantic response accompanying the fast electronic-phase switching as the output.
The charge- and/or orbital-ordered state(electron-crystal state) is widely seen in a classof perovskite manganites and also in other transition-metal oxides.1) This is true even for hightemperature superconducting copper oxides,
which are known to show the charge-spin
stripe order.22,23) The charge ordering in the
manganites accompanies the concomitant orbital ordering. The most prototypical case,called the CE type,24-26) is realized at doping level(x) of 0.5 and is shown as the projection onto
the ab plane in Fig. 6(a). In the pseudo cubic
perovskite, the ab planes are coupled
antiferromagnetically while keeping the samecharge and orbital pattern. In the single-layered perovskite La1-xSr1+xMnO4 (x=0.5), having

(a)
Mn3+
Mn4+
a
b

(b) T T co(c) T T co
a
b
out
in

100 m
Fig.6.
(a) The orbital(3x2-r2/3y 2-r2) and charge order of the CE type projected on the MnO2 sheet (ab plane). Lower panel:
Polarization microscope image with cross polarizations parallel to a and b axes for a La0.5Sr1.5MnO4 (x=0.5) crystal. The left
bright image is for the charge/orbital ordered state at 77k . The orbital disordered state at 298K shows the isotropic
optical response and hence gives the dark image.

the so-called K2NiF4 structure like that of thefirst high-temperature supercondcutor discovered, La2CuO4, the same charge-orbital pattern

Pr1 xCaxMnO3, x=0.4
1Pr1-xCaxMnO3 (x=0.4)
6

Reflectivity

10

E//b
4

0.5

3T
0T

2

0

1

1

cm )

10

0
3

10

10

75K

2

30K
10

30 K, E//b

140K

4

0

4
8
Temperature (K)12Optical Conductivity (10Resistivity ( cm)

10

7T
6T
5T
4T

1

0.1

1
Photon Energy (eV)
7T
6T
5T
4T

0.5
3T
0T
0
0

30 K, E//b

0.5
1
Photon Energy (eV)1.5

Fig.7.
(a) Melting (insulator-metal transition) of the charge/orbital-ordered state (electron-crystal state) for Pr1-xCaxMnO3 (x=0.3)
by application of magnetic field at various tempeartues. Spectra of (a) optical reflectivity and (b) corresponding opticalconductivity for a Pr1-xCaxMnO3 (x=0.3) crystal at 30K at various magnetic fields (in the increasing run) in the course of theinsulator-metal transition.

16

JSAP International No.2 (July 2000)

displayed in Fig. 6 was confirmed by the resonant X-ray scattering method.26) In this compound, the charge and orbital ordering occurs
concomitantly at TCO=220 K, and then the CEtype spin ordering transition sets in at TN=150
K.27) At higher temperatures above TCO the average structure of the crystal is tetragonal, andhence the optical property is isotropic on thelateral plane. Upon the orbital ordering, thecrystal is deformed to an orthorhombic one,though this deformation can hardly be detectedby conventional diffraction measurements.
However, the anisotropic ordering of the orbital causes a fairly large in-plane anisotropy
for the optical electronic transitions, and hencein the cross-Nichol configuration of light polarization we can visualize the orbital-ordereddomain.28) The polarization microscope images
for the La1-xSr1+xMnO4 (x=0.5) are shown in Figs.
6 (b) and 6(c). The orbital-disordered state
above TCO is optically isotropic in the plane, giving the extinction of cross-polarized reflection
light (Fig. 6(c)), while we observe the globallybright image for the orbital ordered state below TCO where the orthorhombic domain anddomain walls (dark stripes) are clearly visible(Fig. 6(b)). (A periodic structure of the domains
arises perhaps form the slight residual strain
introduced during the crystal growth.)
The CE-type charge/orbital ordered statein the mangantie is generally amenable to application of magnetic field. Figure 7(a) illustrates a magnetic field induced insulator-metal

Frontier 2

Correlated Electron:
Scinece to Technology
transition for a crystal of perovskite
Pr 0.6Ca0.4MnO 3 that undergoes the CE-typecharge-orbital ordering at TCO =240 K. At low
temperatures below TCO, the application ofmagnetic field of several teslas induces themelting of charge/orbital order and changes thephase from an antiferromagnetic insulator to aferromagnetic metal. This phase change is accompanied by a gigantic decrease of resistivity.29) With decreasing temperature, this firstorder phase transition shows an increasing fieldhysteresis, indicating the occurrence of a supercooling or of a superheating effect in thefield-induced melting or solidifying process. At
sufficiently low temperatures, the field-induced
insulator-metal transition becomes irreversible
and is accompanied by a change in resistivity
from above 10+10Ωcm to 10-4Ωcm.
The insulator-metal transition in the manganite is usually accompanied by a large change
in electronic structure that shows up as achange of the optical spectrum in the infrared
to visible region.30) The magnetic field dependence of reflectivity and (transformed) opticalconductivity spectra for a Pr0.6Ca0.4MnO3 crys-tal at 30 K is shown in Figs. 7(b) and 7(c). The
spectra show a conspicuous but continuouschange with increasing magnetic field strength
up to 6 T as if they prepared for the insulatormetal transition occurring around 7T. In particular, the optical gap (see Fig. 7(c)) seems tocontinuously decrease with increases in fieldstrength. In accord with the insulator-metaltransition at this temperature (30 K) shown in
Fig. 7(a), the gap is closed at 7 T, and the metallic high-reflectivity band shows up in Fig. 7(b).
The change of reflectivity spectra extends up
to 3 eV, i.e. visible region. This is not a simple
magneto-optical effect but should be called
magnetochromism.
The charge/orbital ordered state or electron crystal state can also be modified by application of an electric field.31) Figure 8 illustrates
the I-V characteristics (representing the insulator-metal transition) at 20 K of a circuit withan manganite crystal, Pr1-xCaxMnO3 (x=0.3) subjected to various magnetic fields. The I-V characteristic at 4 T represents the reference curveof the ferromagnetic-metallic state, since at 4
T the charge/orbital ordered state is completely

Pr 1-xCax MnO3 x=0.3
V

I
Rload
(1M‰)

R
A

Applied voltage-2

10Current (A)

10-3
10-4

(a)

T=20K
4T

(b)

T=20K

10-5

T=20K

4T

10-6
10-7

(c)

4T

0T

2T

3T

10-8
-9

10

10

melted at this temperature. The electric-field
induced change of the resistance at 0 T indicates the local insulator-metal transition due
to the dielectric breakdown of the charge/orbital-ordered state. With increasing magnetic
field, the breakdown threshold voltage is decreased and hysteresis is enhanced. Since thecurrent path (metallic patch) is expected to beferromagnetic-metallic, this phenomenon may
represent the functionality of electromagnet
that can be built in the material.
A similar transformation from the chargeordered insulator to the ferromagnetic metallic state can be caused by photo-excitation323,33)
or X-ray irradiation.34) Figure 9 shows an example of the photo-induced dielectric breakdown for the same crystal, Pr 1-x Ca x MnO 3(x=0.3).The sample resistance across the gold
electrodes with a gap of 200 µm was more than
1 GΩ at 30 K. A single nanosecond laser pulse
with a photon energy well-above the chargegap (0.5 eV, see Fig. 7(c)) exteremely decreases
the resistance to a few tens of ohm or generates a large persistent photoconductivity. Under a microscope, the local insulator-metal transition between the electrode gap (200 µm) is
quite visible as the current path (the brightened
image) in the upper panel of Fig. 9. Such a
local light-induced insulator-metal transition isdue to the local melting of the charge/orbital
ordered state, which gives rise to a large changeof the reflectivity even in the visible region asdemonstrated in Fig. 7 for the case of magnetic-field induced melting. The metallic pathcan be sustained by passing the current. According to a pump-probe experiment35) using
100-fs light pulses, the photo-induced meltingof the charge/orbital ordered state or the local
insulator-metal transition is completed within
1 ps, and is accompanied by a large change in
optical reflectivity and perhaps also by the onset of the ferromagnetism. Such an ultrafast
phase-control would be a great advantage in
use of correlated-electron materials.

100
1000 10100
1000 10
100
1000
Applied voltage (V)Applied voltage (V)Applied voltage (V)Fig.8.
I-V characteristics of the circuit with a Pr1-xCaxMnO3 (x=0.3) crystal in series ( upper panel) at 20K under magnetic fields of(a) 0, (b) 2,and (c) 3T. The crystal at this temperature undergoes the magnetic field induced insulator-metal transition at
4T, and hence the curve for 4T is indicated as a reference for the bulk ferromagnetic metallic crystal.

5. Tailored materials withcorrelated electrons
The availability of high-quality single crystals of transition-metal oxides, typically prepared by the floating-zone method, has been
expediting recent researches on the physics ofcorrelated electron systems. In addition, a recent advance in thin-film technology has provided a new tool to explore correlated-electron
systems. With use of the molecular beam epitaxy (MBE) or laser ablation (laser MBE) method,
thin films can be grown in a layer-by-layer mode

JSAP International No.2 (July 2000)

17

200 mLaser Pulse(5 ns)
8

Pr0.7Ca0.3MnO3

(

)

10

Resistancefor most of perovskite transition-metal oxides.
Figure 10 shows an atomic force microscope(AFM) image of a 100-nm-thick La0.6Sr0.4MnO3(LSMO) film that was fabricated on a single crystal substrate of SrTiO3 (STO) with the same
perovskite structure by using a laser MBE
method.36) Stripes represent the 4-nm-highatomic steps corresponding to the perovskite
unit cell, and the terrace region between steps
shows the atomically flat region. The nearly
identical atomic pattern for the surfaces of thesubstrate and the thin film ensures the atomically defined layer-by-layer epitaxy. A straightforward application of this technique is to thefabrication of perovskite superlattices composed of a few-u.c-thick perovskite-layers withdifferent compositions.
A good example of the tailored materials
based on the perovskite superlattice is themiltilayer system that can mimic the naturally
layered magnatites with a gigantic tunnelingmagnetoresistance as shown in Fig.4. The
superlattice may realize even a higher magnetictransition temperature than the bilayered material. For this purpose, the superlattice film
can be fabricated as alternate m-units of ferromagnetic La0.7Sr0.3MnO3 (LSMO) layers and nunits of insulating SrTiO3 (STO) layers (m=1-10,

10 6
10 4
100
1
-100

0

100
200
Time ( S)n=0-5).37) The left-hand panel of Fig. 11 shows
the cross-section transmission electron-microscope image (courtesy by Dr. T. Chikyo) of the

2.0 nM

1.0 nM

0.0 nM

Fig.10.
Atomic force microscope (AFM) images of 100-nm-thick La0.6Sr0.4MnO3 (LSMO) film (upper panel) grown epitaxially bylaser ablation method on a si ngle-crystal substrate (lower panel) of SrTiO3 (STO).

18

JSAP International No.2 (July 2000)

400

Fig.9.
The photo-induced insulator-metal transition for a Pr1-xCaxMnO3 (x=0.3) crystal at 30K. The resistance measured with theelectrond gap of 200µm shows a gigantic decrease form above 1GΩ to a few tens of ohms upon the photo-excitation witha single shot of nanosecond laser (2.4 eV) pulse. Upper panel shows the image (a bright region) of the photo-induced
metallic current path between the electrodes (gold). The metallic path is sustained by flowing the current.

AFM image of 100nm LSMO film on STO(001)

SrTiO3 (001) substrate300

m=n=5 superlattice fabricated in a layer-bylayer mode by monitoring RHEED oscillation.
The temperature variation of the in-plane resistivity in magnetic fields is illustrated in theright-hand panel of Fig. 11 for (m,n)=(5,1),(5,2), and (5,5), i.e. with varying the thicknessof the insulating barrier-layer. The Tc for the(m,n)=(5,1) superlattice appears to be increased
up to above 200 K, while the colossal
magetoresistance (CMR) behavior is seen as in
the bilayer compounds La2-2xSr1+2xMn2O7. However, the thick (n=5) intervening STO layer leads
to the carrier localization for the thin LSMO
(m=5) layer, as seen in the resistivity curve forthe (5,5) superlattice. This suggests the importance of the interlayer hopping of the carriers(or electronic three-dimensionality) in producing the metallic behavior free from the localization effect.
The detailed study of this kind of
superlattice films is now revealing unexpectedly rich aspects of the electronic features of
the interfaces, such as charge-transfer throughthe interface and resultant modification of spin
structure at the interfaces, and through-barrier
mixing of electronic states, and relevant localization effect.37) At the early stage in the study
on the CMR manganites, a trilayer junction
made of La1-xSrxMnO3 layers intervened by an
insulating SrTiO 3 layer was fabricated as amodel of tunneling magnetoresistance (TMR)
device. Reflecting the half-metallic nature ofFrontier 2

Correlated Electron:
Scinece to Technology[LSMO/STO]= [5/5]

1

Resistivity( cm)

10

0T
1T
3T
5T
7T

[5/2]

100

10-1

[5/1]
-2

10

0

100

200

300

Temperature(K)Fig.11.
Magnetoresistance properties of a supperlattice composed 20 repeated sequences of [LSMO]m[STO]n (LAMO=La0.7Sr0.3MnO3, STO=SrTiO3; m=5,n=1,2,5). This superlattice can be viewed as
tailored layered ferromagnetic compound. The left-hand panel is a cross-section image taken with a transmission electron microscope for (m,n)=(5,5) supperlattice ( courtesy ofDr.T.Chikyo) .

0.30

4
0T
1T
3T
5T
7T

LSMO
single layer
30.20
superlattice
0.15

2

0.10
1

Resistivity (10 -3 cm)

0.25

Resistivity ( cm)the manganites, the TMR value is very large at
low temperatures but unfortunately becomes
exteremely small above about 150 K2,38). Thisis perhaps due to the spin structure modification arising from such an interface effect, whichshould be further pursued and will be hopefully much improved by compositional tuningof the interface region.
Let me now show the magnetotransport
properties of another magnetoelectronic
superlattice, those of a typical spin-superlatticecomposed of ferromagnetic-metallic layers(La0.6Sr0.4MnO3; abbreviated here LSMO) andantiferromagnetic insulating layers(La0.6Sr0.4FeO3; LSFO).39) In Fig. 12 the properties of the the single-layer (LSMO) film are compared with those of the superlattice[LSMO]10[LSFO]3 (suffixes indicate the numbersof repeated unit cells (u.c.) of the respective
layers.) LSFO shows the G-type (alternate spinup and spin-down) antiferromagnetic state. The
superlattice whose properties are shown in Fig.
12 was epitaxially grown on a (100) substrateof perovskite SrTiO 3 with coherent strain.
Therefore the interface of originally ferromagnetic LSMO and antiferromagnetic LSFO layers
shows a spin frustration irrespective of the signof the magnetic coupling at the interface. The
much enhanced resistivity of the superlatticeat zero field is due not only to the confinement
effect of the charge carriers in the thin (10u.c.) LSMO layer but also to the frustration-in-

0.05

0.00
0

100

200

300

0
400

Temperature(K)

AF
CAF
FM

LSFO
LSMO
LSFO
H=0

H›0

Fig.12.
Magnetoresistance properties of the superlatticecontaining 20 repeated sequences of [LSMO]10[LSFO]3(LAMO=La 0.6 Sr 0.4 MnO 3 and LSFO=La 0.6 Sr 0.4 FeO 3 ) incomparison with those of LSMO (thick) films . The bottom
panel depicts the mecanism of the colossal
magnetoresistance of the supperlattice:The canted
afntiferromagnetic (CAF) region near the LSFO/LSMO
interface is converted by an external magnetic field to aferromagnetic (FM) one, increasing the effective thicknessof the FM region as a wave-guide for the spin-polarized
carriers.

duced modification of the spin structure at theinterface (perhaps from ferromagnetic to spincanted antiferromagnetic). This was confirmedby examining the LSFO-layer thickness dependence of the resistivity and magnetic response
for the superlattice while keeping the thicknessof LSMO constant (10 u.c.): with increasing
LSFO layer thickness, the resistivity increases andthe net magnetization of the constituent LSMO
layers decreases. As shown in Fig.12, an extremely large magnetoresistance effect wasobserved for this (10,3) superlattice down to
the lowest temperature, whereas the LSMO
single-layer film showed the conventional MR
only around Tc. This difference between thetwo kinds of films can be accounted for by thefield-induced modification of the canted spin
structure near the interface (see the lower panelof Fig. 12). The original spin canting near theatomic interface reduces the thickness of the
conducting region of the LSMO layer, but theeffective thickness of the conducting region isincreased when an external field forces thecanted spins into ferromagnetic alignment.
The perovskite superlattices can be composed of a variety of combinations of layerswith various compositions, and not only the
charge gap and spin structure but also the orbital state of the constituent layers play important roles in determining the properties of thesupperlattices. The charge transfer across theinterface, which transfer is affected by the

JSAP International No.2 (July 2000)

19

0.4

Room Temperature

pump@1400nm

0.2

probe@1200nm

T/ T

0.0
Sr2CuO3

-0.2
-0.4

Sr
Cu

-0.6

O

-0.8
E

-1.0
-1

0

1

2

3

4

t [ps]

Fig.13.
Photo-switching (modulation of optical transmittance) experiment at room temperature using a Sr2CuO3 crystal (as astrong two-photon absorber) ,which is a prototypical Mott insulator with C u-O chain structure. Pump and probe pulses
were 100fs in duration and a train of the two pulses separated by 2ps causes the distict optical switching.

chemical potential as well as by the orbital-spinstate, is particularly important, because it affects the spin structure at the interface andhence the transport property. It is not so easy
at the moment but quite a rewarding task toconjecture and design electronic and magnetic
properties of superlattices composed of thetransition-metal oxide perovskites. This emerging research field assures an abundant gold
mine for new correlated-electron materials.

6. Photonic functionality ofcorrelated electron systems
So far little known but potentially usefulcharacteristic of the correlated electron systems
is their role in nonlinear optical (NLO) materials
and photo-switching device materials. In the
Mott insulator, a charge gap is formed as a result of the large on-site Coulomb energy,
termed U. Occasionally, and in particular forthe transition-metal oxides such as nickelates
and cuprates, the actual charge gap is formed
not between the U-split bands (lower and upper Hubbard bands) of d electrons, but between

20

JSAP International No.2 (July 2000)

the oxygen (or halogen) filled p-band and upper Hubbard (unoccupied) d-band and istermed a charge-transfer gap.40) The charge
gap transition with the p-d character bears alarge transition dipole but occurs well above
1eV, and hence there is still a large transparent
window in the near-infrared region, which is
the wavelength region used in optical-fiber
communication systems.
Up to now, materials with large third-order NLO susceptibility have been sought in thefamily of one-dimensional semiconductors likeorganic conjugate polymers and polysilanes (linear Si-polymers).41) This is partly because the
NLO susceptibility is favored by the linear extension of the excited state wave functions aswell as the large oscillator strengths of the essential-state transitions, such as those between
the ground state and the first excited state andbetween the first and higher excited states. In
this context, there is an important class of onedimensional systems with correlated electrons
among transition-metal oxide and halide materials. In these systems there is strong p-d elec-tron hybridization that enhances the transition
dipole moments between the above-mentioned
essential states, while the electron correlation
effect keeps the appreciable charge gap magnitude. In fact, a recent systematic study3) of
one-dimensional charge-transfer insulatorssuch as halogen-bridged Ni complexes and
Sr2CuO3 has shown that there is a large transition dipole between the first and second excited states which are almost energetically degenerate. This situation can produce a gigantic third-order optical nonlinear susceptibility
ranging from 10-8 - 10-4 esu in the resonanceenergy region.
Figure 13 demonstrates the fast photoswitching, at room temperature, of near-infrared lights by using a typical one-dimensionalcharge-transfer insulator, Sr2CuO3 (see the inset for its structure).42) This compound is a onedimensional analog of a high-temperature superconductor parent compound La2CuO4: it
contains Cu-O chains instead of CuO2 sheets.
When the polarization of two incident lightbeams is parallel to the Cu-O chain and thesum of the two photon energies is 2-3 eV,strong two-photon absorption (one of the thirdorder NLO properties) occurs. Figure 13 shows
the transmittance change of this compound at
1200nm due to the two-photon absorptionthat is induced with two pump pulses with 2psapart in a train. The use of the two-photon
absorption that occurs in the conventional semiconductors is not suitable for all-optical switching devices because the accumulation of twophoton generated electron-hole pairs becomesa big obstacle to the fast recovery of the groundsate. In Fig. 13, however, the tail componentof the transmittance change seems smallenough. This is perhaps because the opticallysilent high-energy spin excitations provide aneffective recovery-channel to the ground-sate.
This would be a great advantage of the correlated electron system in their use in NLO devices. Thus, the use of the correlated-electronmaterials is a quite promising approach to explore new NLO materials and ultrafast (THz)
optical switching devices.

7. Correlated-electron
technology
We have seen several examples of intriguing electronic phases produced by strongly correlated electrons. This article has focused on
the manganites, but another promising familyof correlated-electron are, needless to say, thehigh-temperature supercondcuting copper oxides (cuprates) with perovskite-related struc-Frontier 2

Correlated Electron:
Scinece to Technology
tures similar to those of the mangantites. Both
the CMR manganites and the high-temperature superconducting cuprates will be major
element systems in correlated-electron technology (CET). There are, however, also many othercompounds we might use for CET. Those compounds, including some organic molecularcompounds, have various instabilities of charge/
spin/orbital ordering and are thus on the vergeof the electronic phase transformation. In manyof those compounds, exotic spin and/or orbital
textures, e.g., a chiral spin state and an incommensurate charge/spin/orbital stripe, show up,
which would provide another CET functionality. The use of those correlated-electron materials would be based on the concept of thephase-control of complex electronic states.
Because of an inherently large energy-scale ofthe electronic parameters in correlated electron
systems, external stimuli can be expected tocause a gigantic and ultrafast response in electrical, magnetic, optical, and dielectric properties. In conclusion, let me summarize the future subjects of CET research:[1] critical-state phase control of correlated
electrons[2] tailoring new materials phases bysuperlattice technology[3] fabrication of junctions and interfaces ofcorrelated-electron system[4] searching for photonic materials withstrong electron correlation[5] devising new principles for CET
Among them, new principles for CET
would be highly desired, that can coherently
describe complex quantal phases, utilize thosequantal phase dynamics, and substantiate theirapplication in quite a different way from prevailing single-electron technology.

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The author would like to thank N.
Nagaosa, M. Kawasaki, K. Miyano, M. KuwataGonokami, H. Okamoto,T. Ogasawara, K.
Terakura, Y. Tomioka, T. Kimura, and M. Izumi
for their help in writing this article and for enlightening discussions. The works presented
here were done at JRCAT under the support of
the NEDO and in the COE research project at
Department of Applied Physics (Butsuko), University of Tokyo under the support of the Ministry of Education.

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