PHYSICAL REVIEW B

VOLUME 62, NUMBER 21

1 DECEMBER 2000-I

Spectroscopic study of photoinduced charge-gap collapse in the correlated insulators
Pr1Àx Cax MnO3
1

2T. Tonogai,1 T. Satoh,1 K. Miyano,1 Y. Tomioka,2 and Y. Tokura1,2Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan
Joint Research Center for Atom Technology (JRCAT), Tsukuba 305-0046, Japan
共Received 6 June 2000兲

We observed an instantaneous reflectance change triggered by pulse laser irradiation. The laser heating
cannot account for the action spectra, which are indicative of the charge-gap collapse. It is found in
Pr1⫺x Cax MnO3 for a wide range of Ca doping, x⫽0, 0.1, 0.2, 0.31, 0.4, and 0.5, which suggests that thephotoinduced charge gap collapse is not characteristic of charge-ordered insulators. We propose that thephotoinduced charge gap collapse can be a general behavior in strongly correlated insulators.

The phase control in strongly correlated transition metaloxides has been an attractive subject for many years. The
perovskite-type manganites R 1⫺x A x MnO3 , where R is atrivalent rare-earth ion and A is a divalent alkaline-earth ion,
have been extensively investigated since the rediscovery ofcolossal magnetoresistance 共CMR兲.1 The mother material,
RMnO3 , is a typical Mott insulator, while the system tends tobe metallic with hole doping as A 2⫹ substitutes for R 3⫹ . The
metallic state is accompanied by ferromagnetic spin ordering
through a double-exchange mechanism, which is a dominant
driving force of CMR phenomena.2,3 However,Pr1⫺x Cax MnO3 , which has a smaller one-electron bandwidthW, shows no ferromagnetic metallic phase and undergoes acharge-ordering phase transition.4,5 In the charge-ordered
共CO兲 insulating state, Mn3⫹ and Mn4⫹ sites are alternatelyaligned accompanied by spin and orbital ordering because ofstrong intersite Coulomb interactions and cooperative JahnTeller lattice distortion. In previous works, a metallic state
has been obtained by magnetic fields,6 pressure,7 electricfields,8 x-ray irradiation,9,10 and electron beam irradiation.11
These external fields act on the charge, spin, and/or latticewhich are strongly coupled. In a certain temperature region,
the system is metastable between CO insulating and metallic
phases, resulting in an irreversible field-induced insulatormetal transition.
We have recently reported that the visible-IR pulse laser
irradiation also induces an insulator-conductor transition in
the CO phase of Pr1⫺x Cax MnO3 .12,13 By illuminating thesample with a pulse laser in the presence of a static electricfield, a stable current path is formed across electrodes, which
can be clearly visualized through a change of reflectivity.14The current path formation is supposed to consist of twostages.15 First, metallic clusters are induced by laser emission, which can be detected as instantaneous photocurrent
and reflectance change. When the applied electric fields
across the electrodes are small, the photocurrent and reflectance changes decay to zero in a few ␮ s. In the second stage,
when a large electric field is applied across the electrodes,
the metallic clusters are stabilized by electron injection fromthe electrodes, to form a static current path. In contrast to themagnetic field and x-ray-induced cases, the photoinduced
conducting state is unstable when the electric field is removed. This may signal a fundamentally different nature of
the photoinduced phase change, which warrants further investigation.
0163-1829/2000/62共21兲/13903共4兲/$15.00

PRB 62In order to study the role of photoexcitation, we will concentrate on the first stage without electric field in this paper.
We measured the action spectra of the reflectance change in
Pr1⫺x Cax MnO3 for a wide range of Ca doping (x⫽0, 0.1,0.2, 0.31, 0.4, and 0.5兲 by a pump and probe experiment. As
shown in Fig. 1, this doping level covers entirely different
phases: antiferromagnetic insulators for all temperature
ranges (x⫽0 and 0.1兲, paramagnetic and ferromagnetic insulators (x⫽0.2), and paramagnetic and CO insulators (x⫽0.31, 0.4, and 0.5兲. Single-crystal samples ofPr1⫺x Cax MnO3 were synthesized by the floating-zone technique, which has been reported in detail elsewhere.6 The
polarization dependences are not discussed in this paper because all the samples are multidomain crystals. An opticalparametric oscillator 共OPO兲, pumped with a third-harmonicof a Nd:YAG laser with 7 ns light pulse, was used for thepumping laser pulse of a wide energy range 共0.5–1.5 eV兲.
The reflectance change of a cw Nd:YAG laser light (␭⫽1.06 ␮ m) was monitored with an electrically cooled InGaAs photodiode. In order to avoid thermal heating, theprobe laser beam was mechanically chopped to a width of
1.8 ms at a frequency of 10 Hz, synchronized with the pump-FIG. 1. The electronic phase diagram of Pr1⫺x Cax MnO3 . Abbreviations represent paramagnetic insulating 共PI兲, spin-canted insulating 共CI兲, ferromagnetic insulating 共FI兲, and charge-ordered insulating 共COI兲 states, respectively. T N , T C , T CO , and T CA stand for
antiferromagnetic, ferromagnetic, charge-ordering, and spin-canting
transition temperatures, respectively.
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FIG. 2. Pumping energy dependencies of reflectance change,detected at ប ␻ ⫽1.16 eV (␭⫽1.06 ␮ m). The irradiated photon
number was kept constant to be 1.6⫻1017 photons per cm2 . The
inset shows the time dependence of reflectance in x⫽0.31 sample,
pumped by a 1.4 eV laser pulse of 7 ns.

ing pulse laser. All the photoexcitation measurements wereperformed at 30 K in a He-flow-type cryostat. The opticalconductivity spectrum of each sample was obtained via a
Kramers-Kronig transform of respective reflectivity data,measured with a combination of a Fourier transform spectrometer and a grating monochromator from 0.05 eV to 6 eV
at 10 K.
The growth of conducting clusters and the recovery to theinsulating state are observed as a change of reflectance. The
time dependences of the reflectance at 1.06 ␮ m 共1.16 eV兲were measured in the range from ⫺5 ␮ s to ⫹40 ␮ s of laserpulse. As shown in the inset of Fig. 2, a sharp decrease of thereflectance and a gradual recovery, with a time of as long as
about 10 ␮ s, are observed in all compounds. In the following, we refer to the peak change ⌬R, defined as (R⫺R 0 )/R 0 , where R 0 and R is the reflectance before and at
the instance of laser irradiation, respectively. 兩 ⌬R 兩 increases
almost linearly to the pump laser intensity. In our presentexperiments, the incident photon number was kept constantat 1.6⫻1017 photons per cm2 at each pulse with a spot size ofabout 500 ␮ m. We plot 兩 ⌬R 兩 in Fig. 2, detected at 1.06 ␮ m,
as a function of pumping photon energy E pump . A monotonical decrease of 兩 ⌬R 兩 toward low energy is observed in allsamples. However, we can see a clear x dependence in theintensity. The spectra for x⫽0.2, 0.31, 0.4, and 0.5 are almost identical. It is striking that the x⫽0.2 sample 共a ferromagnetic insulator兲 behaves similarly to the x⫽0.31–0.5
compounds, because, in previous works, external fieldinduced insulator-metal transitions were considered to becharacteristic phenomena of CO insulators. The value of兩 ⌬R 兩 is about 0.1 at 1.4 eV pumping and decreases to zero atabout 0.5 eV, indicating an energy gap ⌬ ref⬇0.5 eV. The
lightly and undoped samples x⫽0 and 0.1 show smaller兩 ⌬R 兩 , about 1/3 in x⫽0.1 and 1/5 in x⫽0 at E pump
⫽1.4 eV, compared to highly doped compounds. The reflectance change decreases toward the low-energy region, which
disappears at 0.9 eV and 1.0 eV in x⫽0.1 and x⫽0, respectively.

PRB 62

FIG. 3. Optical conductivity spectra of Pr1⫺x Cax MnO3 (x⫽0,0.1, 0.2, 0.31, 0.4, and 0.5兲 at 10 K. The peak structures at about 1
eV are assigned to the excitation energy of an electron from O2⫺ toMn4⫹ as shown in the left inset, while the broad peaks above 2 eV
are ascribed to the CT excitation, O2⫺ to Mn3⫹ , as shown in theright inset.

In order to investigate the origin of the x dependences of
the reflectance change, the electronic structure of the compounds was studied by optical reflectivity measurements fora wide energy region in each sample. The obtained opticalconductivity spectra at 10 K, ␴ ( ␻ ), are shown in Fig. 3. All
spectra show insulating behaviors, growing up from ␴ ( ␻ )⫽0 at ប ␻ ⫽0. The spiky structures around 0.06 eV are due
to optical phonon modes. One can see broad peaks at 1 eV in
the x⫽0.31, 0.4, and 0.5 samples. As reported previously intemperature- and magnetic-field-dependent optical spectroscopy measurements,16 the broad peak structure at about 1 eVin the x⫽0.4 system is assigned to the charge-ordering gap,
corresponding to the excitation energy of an electron fromthe O2⫺ to Mn4⫹ site. It is natural that the observed peaks inthe x⫽0.31 and 0.5 spectra are also ascribed to the sameorigin. On the other hand, in the x⫽0 spectrum, a monotonical increase of ␴ ( ␻ ) is observed up to 2 eV. Arima et al.
have reported an optical spectrum of LaMnO3 ,17 in which apeak of ␴ ( ␻ ) is observed around 2 eV, which is assigned toa charge transfer 共CT兲 gap. The PrMnO3 spectrum in ourexperiments is similar to their results, indicating that theweak peak structure at about 2 eV can be assigned to the CT
excitation gap, electron transfer from O2⫺ to Mn3⫹ . Therefore, a gradual spectral weight transfer from 2 eV to 1 eV
while increasing x corresponds to an increase of the transfer
probability of O2⫺ →Mn4⫹ , because Ca doping on the Pr site
introduces holes on the Mn sites.
The x dependence of the gap ⌬ ref on the reflectancechange measurements can be naturally understood in termsof the spectral weight shift in ␴ ( ␻ ): ⌬ ref⬇0.5 eV for theO2⫺ →Mn4⫹ transfer for x⭓0.2 and ⌬ ref⬇1 eV for O2⫺→Mn3⫹ transfer for x⭐0.1. The x dependence of 兩 ⌬R 兩 represents the stability of the insulating state. The x⫽0 and 0.1
compounds show small 兩 ⌬R 兩 , while large reflectancechanges are observed in x⭓0.2. All experimental results indicate that the instability of the insulating phase increases
with the hole doping, which saturates at x⫽0.2.
In any photoexcitation experiment, one cannot avoid thePRB 62

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FIG. 5. Absorption coefficient 共a兲, pumping energy dependenceof the calculated surface temperature change 共b兲, and action spectrum of the reflectance change detected at ប ␻ ⫽1.16 eV 共c兲, allnormalized at 1.4 eV.

FIG. 4. Pumping power dependence of reflectance change 兩 ⌬R 兩
at 1.16 eV in the x⫽0.4 sample at 30 K 共a兲 and temperature dependence of reflectance 共b兲, detected at ប ␻ ⫽1.16 eV (␭⫽1.06 ␮ m).
共c兲 represents pumping power dependence of the expected surface
temperature T deduce deduced from 共a兲 and 共b兲.

bolometric effect completely. The ns pulses used here are
more prone to the bolometric effect compared to sub-pspulses. In order to estimate the temperature swing during thepulse irradiation, we show the reflectance change at 1.06 ␮ min the x⫽0.4 sample, 兩 ⌬R 兩 , as functions of the pumping
laser power and temperature, in Figs. 4共a兲 and 4共b兲, respectively. 兩 ⌬R 兩 grows up to 0.1 with increasing pumping power,while the temperature dependence of 兩 ⌬R 兩 is dull and tends
to saturate around room temperature. If we assume that thetemperature increase by laser irradiation causes the reflectance change, the expected surface temperature vs the irradiated laser power would look like Fig. 4共c兲. The upward concave shape of the temperature T vs the laser power isinconsistent with a local heating picture. It is reported thatthe specific heat of manganites increases monotonically withtemperature.18 Therefore, the surface temperature T should
be an upward convex function of the impute power.
Further evidence excluding the simple heating picture is
the action spectra of the reflectivity change. We estimatedthe local temperature rise by laser irradiation assuming thatthe absorbed light energy is instantaneously distributed to all
degrees of freedom within the absorbing volume. Using a setof measured parameters 共heat capacity18 and optical absorption coefficient兲, we calculated the surface temperaturechange, from which the expected reflectance change wasread off from Fig. 4共b兲. Figures 5共a兲, 5共b兲, and 5共c兲 show theabsorption coefficient, pumping energy dependence of calculated surface temperature change, and action spectrum of reflectance change for the x⫽0.31 sample, respectively. Allvalues are arbitrarily normalized at 1.4 eV. The overall trend
in the calculated temperature change is not consistent withthe experimentally observed action spectrum. In the lowenergy region, absorbed photons which are supposed tocause local heating do not contribute to the reflectancechange, which implies that action spectra of reflectancechange cannot be explained by a simple heating picture. Justas in ordinary solids, we expect that the tail of the absorption
gap consists of localized states below the ‘‘band gap 共CT
excitation兲.’’ As such, photoexcitation in this range of spectrum does not contribute to a drastic alteration of the electronic state. Figure 5 thus signifies that photoexcitation into
the proper electronic states is essential for the reflectancechange. The large change signals the collapse of the energy
gap, be it the charge-ordered gap or the Mott gap. It is to beadded, however, that a considerable surface temperatureswing is possible as well. In order to obtain a better estimateof the bolometric effect, measurements employing thin films
are under way.
In summary, we observed a sudden decrease of the reflectance at 1.16 eV synchronized with pulse laser irradiation in
Pr1⫺x Cax MnO3 (x⫽0, 0.1, 0.2, 0.31, 0.4, and 0.5兲. The optical conductivity spectra show a gradual crossover of thespectral weight from 2 eV to 1 eV with hole doping, which isconsistent with x dependences of the photoexcitation gap inreflectance change measurements. The action spectra in thereflectance change cannot be explained by a laser heatingeffect. We conclude that laser irradiation excites localized
electrons to destroy the charge gap, which may be a generalbehavior not only of CO insulators but also of strongly correlated insulators in general.

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