PHYSICAL REVIEW B

VOLUME 60, NUMBER 11

15 SEPTEMBER 1999-I

Action spectra of the two-stage photoinduced insulator-metal transition in Pr1ⴚx Cax MnO3
M. Fiebig*
Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japanand Japan Science and Technology Corporation (JST), Tokyo 171-0031, Japan

K. Miyano and T. Satoh
Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan

Y. Tomioka
Joint Research Center for Atomic Technology (JRCAT), Tsukuba 305-0046, Japan

Y. Tokura
Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japanand Joint Research Center for Atomic Technology (JRCAT), Tsukuba 305-0046, Japan
共Received 6 November 1998; revised manuscript received 1 April 1999兲
The action spectra for the photoinduced insulator-metal transition in Pr1⫺x Cax MnO3 were investigated in the
0.45⫺3.05 eV range. The photon-density threshold for the transition exhibits a gaplike behavior at ⬃0.4 eVwith anisotropic features, and it is insensitive to the degree of carrier doping x. A two-stage mechanism is
proposed for the transition. In the first stage, photoexcited charge carriers melt down the charge-ordered stateby screening effects, which is followed by a local formation of metastable metallic domains. In the second
stage, the barrier between the metallic domains is overcome by an applied electric field resulting in a continuous conducting path that remains stable as long as the current is maintained. 关S0163-1829共99兲03935-1兴

I. INTRODUCTION

Perovskite manganites with the formula R 1⫺x A x MnO3 (Rand A being rare- and alkaline-earth ions, respectively兲 are amodel system for extensive testing of the concept of ‘‘phase
control.’’ Charge, spin, orbital, and lattice act as independent
yet strongly coupled physical degrees of freedom that can be
influenced by a variation of the compositional degrees of
freedom R, A, and x. The interplay is subtle and leads tohighly complex phase diagrams and a number of unusual
magnetic and electronic properties of these compounds.1–3
The most spectacular of these may be a change of resistivityof up to more than ten orders of magnitude in the course of
an insulator-metal transition 共IMT兲 upon the application of amagnetic field, the so-called colossal magnetoresistance
共CMR兲.4
In the CMR compound Pr1⫺x Cax MnO3 , the small ionicradii of the cations at the A site lead to a small transfer
integral between Mn ions. Charge localization is therefore
strong, and the compound shows insulating behavior over thewhole composition range. For 0.3⭐x⭐0.5, the material ischarge ordered at low temperature, exhibiting a CE-type
structure 关Fig. 1共b兲, inset兴, which is incommensurate for x⫽0.5.5,6 The charge-ordered state is energetically nearly degenerate with the charge delocalized metallic state driven by
the double-exchange mechanism. Therefore, a first-orderphase transition from the insulating ‘‘charge lattice’’ into a
conducting ‘‘charge liquid’’ can be induced when a criticalmagnetic field is exceeded.7 As the Mn4⫹ sites are replacedby Mn3⫹ ions, the charge transfer along the c axis is progressively promoted. The material thus undergoes the IMT at
0163-1829/99/60共11兲/7944共6兲/$15.00

PRB 60a systematically lower magnetic field as the composition x
varies from 0.5 to 0.3.8
The application of external forces other than a magnetic
field can also lead to a breakdown of charge ordering andtherefore an IMT.9–12 The photoinduced IMT 共Ref. 12兲 isoutstanding in the sense that only a well-localized conducting path is generated whereas the bulk of the sample remains
insulating. The phase transition is therefore distinctly
localized.13 Not much is known about light induced phasetransitions14 and, in particular, about the microscopic mechanism and its relation to the well-studied magnetic field induced IMT in our case. Here we report the action spectra of
the photoinduced IMT, from which we propose that the transition proceeds in two distinct steps: creation of metallic domains by light followed by their fusion by the applied electric field.
II. EXPERIMENTAL

Five different bulk samples of Pr1⫺x Cax MnO3 with x⫽0.30, 0.35, 0.40, 0.45, and 0.50 and lateral dimensions of
1–4 mm were investigated. The x⫽0.40 sample was an
orthorhombic single-domain single crystal whose a, b, and caxes had been determined by x-ray diffraction. In the othersamples, the almost cubic symmetry of the crystal lattice led
to a multi-domain structure of the principal axes. After applying the same polishing process to all the samples,4,12
some of them exhibited mirrorlike surfaces whereas others
appeared blurred due to an infrequent pattern of microscopic
defects. A pair of gold electrodes separated by a straight 25
␮ m gap was vacuum evaporated onto the surface of thesamples and connected to a dc regulating power supply pro7944©1999 The American Physical Society

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ACTION SPECTRA OF THE TWO-STAGE . . .

7945

Once the conducting path has been formed, it remains
stable without further illumination as long as a current isflowing between the electrodes, but the sample returns to theinitial insulating state immediately after the current has beenswitched off. The path is always formed inside the illuminated region.13 Consequently, the phase transition is mosteasily induced if the laser spot is well focused and fully
covers the space between the electrodes at an arbitrary position along the gap. Since the light-intensity threshold for thetransition is highly sensitive even to tiny misalignments,
great care had to be taken to achieve and maintain an optimal
adjustment. For this purpose the image of the sample wasmonitored with a CCD camera equipped with a telephotolens. This enabled the adjustment of the laser beam andhelped to detect and avoid damages of the sample surface or
the electrodes. Still, frequent fine alignment was requiredsince wavelength tuning of the OPO led to defocusing and abeam walk off. The criterion for the best adjustment was
minimization of the pulse energy required for the photoinduced IMT. Gradually occurring effects, due to the unstableperformance of the OPO laser beam or to microscopic damages of the electrodes after recurrent induction of the phasetransition, were taken into account by repeating the measurement several times during one experimental run. Asymmetries originating in an elliptical laser-beam profile were compensated by verifying the threshold for the IMT for selected
photon energies with the sample rotated by 90°.
FIG. 1. 共a兲 Spectral and polarization dependence of the photon
density ␴ required to trigger the IMT in a Pr0.6Ca0.4MnO3 single
crystal at 30 K for a voltage of 50 V applied to the sample electrodes separated by a 25 ␮ m gap. Circles and squares denote atransition induced by applying the voltage 共U兲 along the b or c axisof the crystal, respectively. Open and closed symbols denote a polarization of the incident light along the b or c axis of the crystal,respectively. The inset shows the spectral dependence of the anisotropy ratio ␴ (b)/ ␴ (c). 共b兲 Optical conductivity spectrum of thesample at 10 K for electric fields of the incident light (U opt) polarized along the b and c axis, respectively. The inset shows the collinear CE-type spin-, orbital-, and charge-ordering structure. Lobes
denote the manganese d 3x 2 ⫺r 2 and d 3y 2 ⫺r 2 orbitals.

tected by a 500-⍀ resistance. On the x⫽0.4 single-domain
sample, two pairs of electrodes with gaps perpendicular toeach other were evaporated so that the electric field could beapplied along the b and c axes simultaneously with independent power supplies. The samples were mounted in acontinuous-flow helium cryostat and kept at a temperature of
30 K unless otherwise stated. As light source, an opticalparametric oscillator 共OPO兲 pumped by the third harmonicof a Nd:YAG laser was used. The IMT was induced by illuminating the gap between the electrodes with 5 ns lightpulses from the OPO at a photon energy of 0.45⫺3.05 eV,
while applying a fixed voltage of up to 50 V to the electrodes. The transition was noted as a sudden drop of theresistance between the electrodes after a single light pulse.12
The polarization and intensity of the light pulses were controlled with three revolving Glan-Thompson prisms. The
pulse energy was monitored with a flat-response Joule meter,
and the beam diameter was measured with a traveling knifeedge. The absorbed photon density ␴ was calculated using
results of these measurements and was corrected for the reflectivity.

III. RESULTS AND DISCUSSION

Figure 1共a兲 shows the action spectrum, that is, the spectraland polarization dependence of the minimum photon density
␴ required to trigger the IMT in the Pr0.6Ca0.4MnO3 single
crystal with 50 V applied. Since ␴ does not vary much at
photon energies E⬎1.0 eV, only the low-energy range isshown in Fig. 1共a兲. There is distinct evidence for an energy
gap at E⬃0.4 eV. The threshold density remains independent of the polarization of the incident light, but below E⫽1.0 eV an anisotropy appears with respect to the directionof the applied electric field, the anisotropy ratio ␴ (b)/ ␴ (c)being shown in the inset in Fig. 1共a兲. For comparison, Fig.
1共b兲 shows the optical conductivity spectrum for light polarized along the b and c axis, respectively.15 Contrary to theaction spectrum, the optical conductivity starts to rise atlower energy with a clear anisotropy with respect to the polarization of the incident light.
Figure 2 shows the action spectra of all samples with 50
V applied to the electrodes. The x⫽0.40 spectrum of Fig. 2
was derived by averaging over the four spectra of Fig. 1共a兲.
Since all other samples were not single domain, the directional dependence of their spectra was not studied. For E⬎2.0 eV, ␴ saturates at a value quite insensitive to bothphoton energy and carrier doping. By averaging over the fivedifferent curves, the saturated photon density is found to be␴ sat⫽(24.6⫾2.9)⫻1015 cm⫺2 in the 2.0–3.0 eV energy
range 共inset in Fig. 2兲. For E⬍2.0 eV, ␴ increases withdecreasing photon energy. A steep increase of ␴ at the lowest photon energies indicates the presence of an energy gap
at E⬃0.4 eV. At 0.45 eV, ␴ is about 50 times larger than␴ sat with a variation of ⫾50% for the different samples. The
variation, however, does not systematically depend on the
degree of carrier doping x. It is rather due to the aforementioned differences in sample quality or long-term variationsof the laser performance. Nonetheless, the anisotropy

7946

FIEBIG, MIYANO, SATOH, TOMIOKA, AND TOKURAPRB 60

FIG. 2. Spectral dependence of the photon
density ␴ required to trigger the IMT in
Pr1⫺x Cax MnO3 at 30 K for a voltage of 50 V
applied to the sample electrodes separated by a25 ␮ m gap. The inset shows the high-energy
range of the spectrum averaged over all fivesamples and a linear fit to the data 共line兲.

关 ␴ (b)/ ␴ (c)⬃1.5 at E⫽0.45 eV] for the x⫽0.4 sample
关Fig.1共a兲兴 is reliable, since the data were gained from a single
sample and in a single run.
The insensitivity to the degree of carrier doping of thespectra shown in Fig. 2 is the most remarkable feature. As
shown in Ref. 7 and mentioned earlier, it becomes easier toinduce the IMT in a magnetic field upon a decrease from x⫽0.5 to x⫽0.3. The absence of this tendency in our experiments shows that the mechanism underlying the photoinduced IMT acts on the charge-ordered state in a completely
different way.
The magnetic-field induced IMT described in Sec. I is anequilibrium process. The insulating state with low transfer
integral at zero field and the metallic state with higher transfer integral at a nonzero field are about degenerate, and the
Zeeman energy simply biases the fluctuations around thesystem equilibrium to find a path from the insulating to themetallic ground state. In this sense, the transition is governedby the weakest path through phase space between the two
ground states.
The optical transition, on the other hand, is a nonequilibrium process and may be divided into steps as follows: 共1兲
From the localized state, charge carriers are excited acrossthe Mn3⫹ →Mn4⫹ charge-transfer gap by a light pulse. 共2兲
Due to screening by the delocalized electrons, the chargeordered state melts, and 共3兲 the formation of a multidomain
metallic phase occurs instead. 共4兲 The decay of this metastable state is interrupted by the electric field by fusing thedomains into a continuous conducting path between the electrodes. Once the path is established, the dc current provides
the screening and maintains the metallic phase for an infinite
time.
共1兲 The relevant optical excitation process in this model
can be seen by comparing Figs. 1共a兲 and 1共b兲. The lowenergy tail in the optical conductivity for the c-polarized
light is due to the extended one-dimensional electronic statealong the c axis mediated by the partial filling of the e gorbitals at Mn4⫹ sites as discussed before. The optical excitation within this extended state, however, does not disrupt
the charge-ordering pattern and hence cannot lead to the metallic state, which must be three-dimensional.15 Clearly,

the photoexcitation has to overcome the highest barrier inall directions against charge delocalization for the photoinduced IMT. This should be the charge-transfer excitationfrom Mn3⫹ to Mn4⫹ in the CE-type charge- and orbitalordered state. The insensitivity of the gap energy and of␴ to the composition x is reasonable since the CE-type ordering, which is overcome by the photoinduced screening,
persists throughout the entire composition range 0.3⭐x⭐0.5.6
共2兲 Phase transition due to screening by photoinduced
charge carriers are known in ferroelectric materials where
screening modifies the frequency of the soft polarization vibration mode and therefore the Curie temperature.16 As to
the manganite compounds, screening attenuates the longrange Coulomb interaction, which, besides Jahn-Teller andother local lattice distortions, is responsible for the charge
localization.17,18 The Hamiltonian for the double-exchangeinteraction is modified by the screening to a much lesser
extent.19 When the screening is strong enough, the charge
localization therefore becomes inferior to the double exchange, and the IMT occurs.
共3兲 The photoinduced metallic phase is created locally andtherefore strongly competes with the surrounding antiferromagnetic phase.20 Due to this ‘‘pressure,’’ the metallic state
is not stable, even though it is surprisingly long lived with alifetime of about 1 ␮ s.21,22 If the sample has fully undergone
the IMT in a magnetic field, the lifetime is greatly extended23
due to the absence of the background pressure so that themetallic state is regarded as stable. Note that the describedphotoexcitation is a quantum process. Therefore, its quantum
efficiency saturates at high photon energies 共Fig. 2, inset兲. On
the other hand, the power efficiency goes down as the photon
energy goes up, disproving that laser heating is of any
significance.12,13,25
共4兲 It is known that the photoinduced phase isferromagnetic.24 In the absence of the external magnetic
field, however, the ferromagnetic domains are randomly oriented and cannot easily merge. The role of the external electric field in the photoinduced IMT is to fuse the domains andto form a continuous conducting path between the electrodes,
which prevents the charge carriers from relocalization as

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ACTION SPECTRA OF THE TWO-STAGE . . .

7947

FIG. 3. Voltage dependence of the photon
density ␴ required to trigger the IMT in
Pr0.6Ca0.4MnO3 at 30 K. The threshold values
␴ (U) were normalized to ␴ (50 V) and averaged
over six different photon energies (E⫽0.5, 0.6,
0.7, 0.9, 1.2, and 1.6 eV兲. The electric field wasapplied along 共a兲 the b axis or 共b兲 the c axis of thesample, respectively.

long as the current is maintained through it. The fusing process requires essentially the electrical breakdown in the insulating parts between the domains. Creation of metastable
metallic domains by light followed by their fusion into astable metallic path by the applied voltage constitute our
two-stage photoinduced IMT model.
The above scenario is buttressed by the observation thatthe resistance of the conducting path 共see below兲 is independent of the degree of carrier doping whereas the ohmic resistance of the samples in the insulating state changes by
three orders of magnitude while going from x⫽0.5 to x
⫽0.3.25 The threshold ␴ thus reflects the resistive propertiesof the metallic state, which corroborates that ␴ is determinedby the electric field applied to the sample in the state after
the laser irradiation when the metallic state is alreadypresent. The two-stage model is also supported by Fig. 3, in
which the threshold photon density is plotted as a function of
the applied voltage. The threshold values at six different
photon energies (E⫽0.5, 0.6, 0.7, 0.9, 1.2, and 1.6 eV兲 forthe electric field applied along the b or c axis of the x⫽0.4
sample, respectively, were normalized to the threshold at U
⫽50 V and averaged over all six energies to arrive at
具 ␴ norm(U) 典 ⫽ 具 ␴ (U)/ ␴ (50 V)典. The small 5% variance of

the averaged value and the insensitivity to the direction of
the applied electric field confirm that ␴ norm(U) is independent of both E and of the orientation of the crystal. According to the two-stage model, the voltage required to trigger thegeneration of the conducting path should only depend on thedensity of metallic domains, irrespective of the photon energy of the excitation. Even though the required photon density depends on E 共because the density of metallic domains
does兲, the normalized threshold value ␴ norm(U) should not,and indeed it does not.
Let us further note two distinct features in Fig. 3. The firstis the linear relationship between the reciprocal normalized
threshold and the applied voltage, and the second is the 5 V
offset at the abscissa. Both cannot be understood in terms of
simple percolation26 or Ostwald ripening phenomena.27 In
either case, more photons create more metallic domains,which at some point should fill up the space between theelectrodes resulting in the metallic path at zero applied voltage. One would expect an abrupt drop of applied voltage as
1/␴ norm approaches zero, and a nonzero intercept on the ordinate rather than on the abscissa. The behavior shown in
Fig. 3 can be understood if we consider the internal degree ofFIG. 4. Voltage drop Ũ across the conducting
path as a function of the current flowing throughit along the b or c axis of the x⫽0.4 sample,
respectively. Temperature is 30 K. The inset
shows the ratio Ũ(b)/Ũ(c).

7948

FIEBIG, MIYANO, SATOH, TOMIOKA, AND TOKURAPRB 60

FIG. 5. Temperature dependence of the photon density ␴ required to trigger the IMT in a
Pr0.6Ca0.4MnO3 single crystal with light at 1.2 eV
for a voltage of 30 V applied to the sample electrodes separated by a 25 ␮ m gap. The dashed line
marks the temperature above which the material
becomes instable against the electric-field induced IMT.

freedom of the metallic domains, the magnetization. Because
the total magnetization should be conserved during the photoexcitation, the domains that appear out of the antiferromagnetic background must be accompanied by domains with opposite spin, the boundary being the antiferromagnetic
insulating wall. This is not to be confused with a domain
wall in an ordinary ferromagnetic metal. Even at the limit of
infinite light intensity, the boundary between the domains
is left. The applied field is needed to overcome this boundary
whereby the 5 V offset must be exceeded in order tomaintain the continuous metallic path in accordance with
Fig. 4.
With the above view, the anisotropy in Fig. 1共a兲 can beeasily understood in terms of the aforementioned ‘‘pressure.’’ One signature of such an effect is the current-voltage(I-Ũ) relationship after the IMT, which is shown in Fig. 4.
The resistance Ũ/I represents the ‘‘stress’’ needed to maintain the metallic path against the surrounding antiferromagnetic state. With the width of the conducting path corresponding to the path current, the I-Ũ curve shows strong
negative differential resistance dŨ/dI.25,28 According to Fig.
4, the resistance for a given current along the b axis is 50%
higher than for the c axis. This reveals a corresponding anisotropy of the background pressure which in turn explains
the anisotropy of ␴ with respect to the direction of the electric field in Fig. 1共a兲.
The final evidence that the photoinduced IMT occurs far
away from the thermal equilibrium and that it is not affectedby laser heating is presented in Fig. 5, in which ␴ is plotted
as a function of the temperature. The incident photon energy
and the applied voltage were fixed at 1.2 eV and 30 V, respectively. The insensitivity of the photoinduced IMT to thetemperature below 53 K is obvious. Above 53 K, the material becomes unstable against the electric-field induced IMT
共Ref. 9兲 so that the phase transition already occurs in theabsence of laser irradiation. If the underlying mechanisms
for the two types of IMT’s were related to each other, theireffects would add up, and ␴ should smoothly decrease towards zero at 53 K. Lack of such a behavior indicates thatthe role of the applied field in the respective IMT’s is completely different.
IV. CONCLUSION

In summary, the application of optical spectroscopy revealed that the photoinduced IMT in Pr1⫺x Cax MnO3 can be
regarded as a process with two largely independent stages.
At the first stage, charge carriers are photoinduced in acharge-transfer excitation. The screening effect weakens thestrong correlation between the electrons leading to the reorganization of the local electronic structure from that of thecharge-ordered insulator to that of the metallic phase. At thesecond stage, the insulating barriers between the isolated metallic domains are overcome by an applied electric field, anda metallic path is formed between the electrodes. Unlike themagnetic-field induced IMT, which works via the amplification of double exchange, the photoinduced IMT works via
the attenuation of charge ordering. It is therefore insensitive
to the degree of carrier doping.
According to the proposed model, the presence of themetallic state is due to photoexcitation whereas the external
electric field enables its persistency. Hence, evidence for the
metallic state should be found in a pump-and-probe experiment in which the spectral change of reflectivity accompanying the phase transition is probed in the absence of anelectric field. An investigation of the temporal evolution of
the photoinduced IMT would therefore offer a different approach to the problem of ‘‘phase control.’’
ACKNOWLEDGMENTS

This work was supported by the Grant-in-Aid for COE
research. The work at JRCAT was supported by NEDO.
The work of M.F. was supported by the CREST project ofJST.

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ACTION SPECTRA OF THE TWO-STAGE . . .

*Author to whom correspondence should be addressed. Electronic
address: fiebig@ap.t.u-tokyo.ac.jp
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