RAPID COMMUNICATIONSPHYSICAL REVIEW B 80, 121103共R兲 共2009兲

Optical selection of a multiple phase order in the charge density wave condensate o-TaS3
using a spectrally resolved nonequilibrium measurement
Y. Toda, R. Onozaki, M. Tsubota, K. Inagaki, and S. Tanda
Department of Applied Physics, Hokkaido University, Kita 13 Nishi 8 Kita-ku, Sapporo 060-8628, Japan
共Received 6 June 2009; revised manuscript received 9 August 2009; published 28 September 2009兲
We investigate the spectrally resolved transient reflectivity changes ⌬R共T , ␶ , ␭兲 in the charge density wave
共CDW兲 conductor o-TaS3. A distinct near-infrared resonance in this compound emphasizes the characteristic⌬R共␭兲 resonances, and allows a selection of coexisting CDW phases with different nonequilibrium carrier
dynamics. Furthermore, the spectrally resolved ⌬R共␶兲 characterizes the collective oscillations associated with
the individual states. We believe that this demonstration paves the way for the optical selection of the multiphase order that plays an important role in various macroscopic quantum systems.
DOI: 10.1103/PhysRevB.80.121103

PACS number共s兲: 71.30.⫹h, 71.45.Lr, 74.25.Gz, 78.47.J⫺

I. INTRODUCTION

II. EXPERIMENTAL

The coexistence of several ordered electronic states has
played an important role in various macroscopic quantum
systems such as high-Tc superconductors and charge density
wave 共CDW兲 conductors. Of the various spectroscopic approaches, optical time-resolved spectroscopy is a uniquemethod that probes phase ordering via nonequilibrium carrier
relaxation.1–3 For multiple ground systems, a selective evaluation of the individual states can be realized by determining
the difference between the associated relaxation components
with high time resolution.4–6 Selective enhancement of thesignals from the individual states has also been demonstrated
by changing the resonant conditions for the higher excitedstates.6,7 However, the complexity of the electronic states inthose materials requires more comprehensive approach
which emphasizes the selectivity.
In this Rapid Communication, we demonstrate an opticalselection of the multiphase order in o-TaS3 using a spectrally
resolved pump-probe measurement. Owing to the welldefined excited resonance in this compound, pronounced reflectivity changes ⌬R are observed in both the spectral andtemporal domains across the characteristic temperatures, andcan be identified as responses originating from the two typesof CDW phases. This identification is also supported by thecoherent oscillations whose spectral and temperature dependences highlight the phase transition in each state.
o-TaS3 is one of the most widely studied quasi-onedimensional 共1D兲 compounds that undergoes phase transitions associated with the formation of three-dimensionally
共3D兲 ordered CDWs.8–20 Below the phase-transition temperature 共TCDW兲, a Peierls gap opens at the Fermi level, resulting in a change of the conductivity from metallic to semiconducting. At lower temperatures than TCDW / 2, several
remarkable changes occur; a deviation from the activation
law in linear conductivity, and the appearances of the second
threshold, subgap structure, and new resonance, respectively,
in nonlinear conductivity,9 tunneling spectroscopy,17 and dielectric spectroscopy.11–13,18 In the similar temperature region, a splitting of satellite peak was also observed in a synchrotron x-ray measurement,19 suggesting a coexistence ofdifferent CDW phases. The dielectric measurement provides
evidence of two glass transitions at T ⬃ 40 and ⬃10 K.11–15

High quality samples with a length of a few mm 共along
the conducting axis兲 and several tens of ␮m wide weregrown with a chemical vapor transport technique. Note that
we measured several samples and obtained similar results asreported below. This compound has two different types ofcrystal structure, namely an orthorhombic structure 共o-TaS3兲
and a monoclinic structure 共m-TaS3兲; the former exhibits
only one 3D CDW phase transition at TCDW = 220 K while
the latter exhibits two independent phase transitions at
TCDW = 240 and 160 K. The resistivity measurements of thesamples show that the metallic conductivity at room temperature becomes semiconducting below T = 220 K, indicating an o-TaS3 crystal. This assignment is also supported by
the near-infrared optical reflection spectra 关Fig. 1共a兲兴, inwhich a distinct peak around ␭ = 900 nm is in good agreement with a previous report on o-TaS3.10
For the measurement, we employed a two-color pump andprobe approach, in which the coaxial configuration between
the pump and probe beams allows the excitation of crystals
with small sizes.6 Ultrafast photoexcitation was realized us-

1098-0121/2009/80共12兲/121103共4兲

210K
240K

∆R (arb. units)

Reflectivity:R (arb. units)

280K

210K
170K
100K

170K

100K
10K

10K

0.7

X2

0.8

0.9

1.0

Wavelength (µm)

0

5

10

15

Delay time:τ (ps)FIG. 1. 共Color online兲 共a兲 Optical reflectivity R共␭兲 around aninterband resonance of o-TaS3 at various temperatures. The incident
laser polarization was set parallel to the chain axis 共conducting
axis兲. 共b兲 A typical series of transient reflectivity changes, ⌬R共␶兲, atvarious temperatures. The ⌬R obtained at ␭ pr = 880 nm showing adecrease in R from its equilibrium value is selected to make it easy
to compare results obtained below and above the CDW transition
temperature 共TCDW = 220 K兲. The pump and probe polarizations
were parallel to the conducting axis.

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TODA et al.

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We first outline the transient components, using⌬R共T , ␶ , 880 nm兲 obtained at below, near, and above TCDW
关Fig. 1共b兲兴. In terms of the relaxation time, the transient⌬R共␶兲 is divided into three components: an instantaneous
response 共⬇0.2 ps兲 ⌬Rint that is dominant in the metallic
phase 共T ⬎ TCDW兲, a subsequent exponential decay
共ⱖ0.5 ps兲 ⌬Rgap and a long-lasting offset ⌬Rbg. This combination has been widely observed in various quasi-1D and-two-dimensional CDW compounds,2,6 and ⌬Rint, ⌬Rgap, and⌬Rbg can be associated with an intraband excitation/
relaxation through a continuum, carrier recombination acrossthe CDW gap, and relaxation via localized states within thegap, respectively.1,2 The opening/closing of the gap is evidenced by the distinct changes in ⌬Rgap and ⌬Rbg with varying T across TCDW. Below TCDW, ⌬Rgap exhibits a superposition of coherent oscillations, which can be attributed tocollective excitations including lattice vibrations, and will be
discussed in the latter part of the Rapid Communication.
The left-hand side of Fig. 2 shows 3D plots of ⌬R共T , ␶ , ␭兲at T values ranging from T = 10 K共ⰆTCDW兲 to 240 K
共⬎TCDW兲. Although the amplitude of ⌬R varies significantlywith ␭ and T, the transient ⌬R共␶兲 consists roughly of thethree components mentioned above, i.e., ⌬Rint, ⌬Rgap, and⌬Rbg.
We now focus our attention on the cross-sectional spectraof ⌬R共␶ = 0 , 1 , 20 ps, ␭兲 shown on the right-hand side of Fig.
2, in which one can see the ⌬R resonances that allow aselection of coexisting CDW phase. In each spectrum, thedifference between the gray 关⌬R共0 ps, ␭兲兴 and red
关⌬R共1 ps, ␭兲兴 curves connects the ⌬Rint, and the difference 

III. RESULTS AND DISCUSSIONS

T 


∆R

ing a mode-locked 100 fs Ti-sapphire laser oscillator operating at a repetition rate of about 76 MHz, which synchronously pumped an optical parametric oscillator 共OPO兲. The
probe beam was extracted from the fundamental
共␭ pr = 700– 900 nm兲 with a spectral width of 5 ⬃ 10 nm,
and the pump beam was delivered by the OPO
共␭ pm = 1060– 1260 nm兲. To avoid contributions from thehigher excited states, a probe beam energy is selected thatexceeds that of the pump beam. The cross correlation between the pump and probe pulses in a nonlinear optical crystal exhibited a temporal resolution of 200 fs. The pump andprobe pulse polarizations were parallel to the conducting axis
because ⌬R is dominant in this direction. The total fluence
was kept lower than 50 ␮J / cm2, and the pump/probe ratio
was 2:1. To measure the transient response, the pump pulse
was mechanically delayed with respect to the probe pulses
using a translation stage. The reflectivity change ⌬R in theprobe was detected with a lock-in amplifier. A spectrally resolved investigation was achieved by scanning the wavelength of the probe by tuning the birefringence filter of theoscillator with ␦␭ = 5 nm. We checked that ⌬R was almost
independent of the pump wavelength by comparing it withthe ⌬R signal that we obtained using the second harmonic of
the pump pulse. In this way, we obtained ⌬R as a function oftemperature, delay time and probe wavelength: ⌬R共T , ␶ , ␭兲,
where the ␭ is given by ␭ pr.



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

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 τ(ps)

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

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λ 
FIG. 2. 共Color online兲 共a兲–共e兲 3D view of transient reflectivity
changes, ⌬R共␶ , ␭兲, at various temperatures. 共f兲–共j兲 Transient spectralchanges ⌬R共␭兲 at ␶ = 0 共gray, solid circle兲, 1 共red, solid circle兲, and20 ps 共blue, open circle兲. The arrow indicates a negative peak
whose shape and T dependence roughly match the changes of stationary reflectivity spectra.

between the red 共solid circles兲 and blue 关open circles,⌬R共20 ps, ␭兲兴 connects ⌬Rgap components. The blue 共open
circles兲 curve is almost identical to ⌬Rbg. Above TCDW, only
⌬Rint exhibits distinct values and forms a single spectral
peak 共solid arrow兲 with a negative sign around ␭ = 880 nm,
which is located on the higher energy side of the R peak 关seeFig. 1共a兲兴. When T is decreased below TCDW, this peak becomes clear in both ⌬Rint and ⌬Rgap. In Figs. 2共b兲 and 2共g兲,the enhancement of ⌬Rint over the whole spectral region isobserved, reflecting the icCDW phase transition at
T ⬇ TCDW. When T is decreased further, the peak shifts to ashorter ␭ and forms several oscillatory resonances. At thelowest T 共T = 10 K兲, ⌬R forms more than two oscillations
within the observed spectral range.
The ⌬R共T兲 spectrum at ␶ ⬇ 0 can be associated with thechange induced in the stationary R共T兲 spectra as the sample
temperature is raised from Tb to Ta, where R共Ta兲 and R共Tb兲

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OPTICAL SELECTION OF A MULTIPLE PHASE ORDER…

 

2.5

      

 

 

     

 

 

860nm
840nm
830nm
820nm
800nm
760nm

FIG. 3. 共Color online兲 Temperature dependences of ⌬R 关␶ = 1
共red, solid circle兲 and 20 ps 共blue, open circle兲兴 at various ␭ pr.

720nm

0

5

10

15

20

τ (ps)

25

0

30

1.5

0

1

2

Frequency (THz)

3

4

1.0

∆R (τ=0) (arb. units)

FT amp. (arb. units)

0.0
1.0

0.5

0.0
700

750

800

λpr (nm)

850

0.5

0.0
0

900

20

40

60

80 100 120 140 160 180

Temperature (K)

0.6
0.5

T=10K

∆R (arb. units)reflect the optical transitions of hot and cold carriers,
respectively.4 In Fig. 1共a兲, the R peak slightly shifts toshorter ␭ with decreasing T. The ⌬R共T , ␶ ⬇ 0兲 ⬅ R共Ta兲
− R共Tb兲 thus produces a negative peak on the higher energy
共shorter ␭兲 side of the R peak, which agrees with the
T-dependence of the ⌬R peak in Fig. 2. The blueshift of R
peak provides another positive ⌬R peak on the lower energyside, which is not visible in Fig. 2 because of the wavelength
limit of the probe laser. The shorter ␭ shift is consistent withthe optical transition from the gap state, where the resonanceenergy should be shifted by the amount of gap energy.22 On
the basis of this idea, additional oscillatory ⌬R共␭兲 resonances, which start to appear around 150 K and become
significant below 100 K, suggest that the probe transition
splits into several transitions with decreasing T. Note that thecorresponding spectral changes should be included in thestationary R 共Ref. 21兲 but could not be resolved clearly due
to the low sensitivity compared to the pump-probe 共⌬R兲measurement. Furthermore, the intense broad R peak may
obscure such additional tiny changes. In other words, thetransient ⌬R with high sensitivity allows to emphasize thespectral changes associated with the phase ordering.
As mentioned in Sec. I, below TCDW / 2, the appearance ofa new ordered phase has been suggested in various spectroscopic measurements.11–19 For example, interlayer tunnelingspectroscopy clearly indicates the existence of subgap states
共amplitude soliton states兲 inside the CDW gap for NbSe3 and
TaS3.17 Therefore the oscillatory resonances are associatedwith the optical transitions from the states formed by thecommensurability. As is clearly seen in Figs. 2共e兲 and 2共j兲,
the resonances superimpose on the negative peak, indicating
the coexistence of two types of ground states. A similar result
has been reported by Inagaki et al., who used an angleresolved synchrotron x-ray measurement to demonstrate thatthe ic- and c-CDWs coexist in the cCDW phase.19
When lowering the temperature below 100 K, the ⌬R
spectrum itself does not show any remarkable changes except for the development of spectral oscillations whereas theangle-resolved x-ray measurement shows disappearance ofthe double peak structure below T = 50 K.19 Moreover, in
this temperature range, the dielectric response shows a glasstransition.11,12 In order to clarify the precise T dependence,
the T dependences of ⌬R共␶ = 1 , 20 ps兲 obtained at typicalthree ␭ pr are shown in Fig. 3. The spectral changes at
T = TCDW and TCDW / 2 are confirmed in ⌬R共1 ps, 880 nm兲.
Below 100 K, ⌬R共20 ps兲 ⬇ ⌬Rbg starts to appear, and its
magnitude increases with decreasing temperature at
␭ pr = 820 and 755 nm. The upturn of ⌬R共20 ps, 880 nm兲
with decreasing temperature results from the formation of
the positive peak in the ⌬R spectrum. Because ⌬Rbg reflects

FT amp. (arb. units)

 

λpr
880nm

FT amp. (arb. units)      λ 

20K
30K
40K
50K
60K
70K
80K

0

5

10

τ (ps)

15

20

Frequency (THz)λ 

∆Rosc (arb. units)∆Rτλ τ τ   

0.4
0.3
0.2

ν
Γ

0.1
0.0
0

10 20 30 40 50 60 70 80 90 100

Temperature (K)FIG. 4. 共Color online兲 共a兲 Typical transient oscillation signals
共⌬Rosc兲 obtained at various ␭s at 10 K, where ⌬Rosc is evaluated bysubtracting the exponential contributions from ⌬R. 共b兲 A Fouriertransform 共FT兲 spectrum obtained at ␭ = 830 nm at T = 10 K. Notethat some of the high-frequency modes can also be identified in
optical Raman spectra 共Ref. 20兲. 共c兲 Plots of magnitudes of thecoherent oscillation modes at T = 10 K as a function of ␭, where themagnitudes of individual modes are evaluated by the amplitudes of
the FT spectral peaks. The symbols 共colors兲 are defined in 共b兲. For
comparison, DR spectrum is also plotted on top. 共d兲 T dependencesof the typical three oscillation amplitudes. The lowest frequency
mode disappears around T = 100 K due to a damped-overdamped
transition of the oscillation. 共e兲 ⌬R共␶ , 760 nm兲 in the lowtemperature range. A large oscillation corresponds to the lowest
frequency mode. 共f兲 T dependences of the frequency 共␯兲 and damping constant 共⌫兲 of the lowest mode.

the relaxation via localized states within the gap, the increaseof ⌬Rbg with decreasing the temperature is connected with afreezing of the dielectric response 共glass transition at
T ⬇ 40 K兲.11,12 This property also suggests the complete formation of the commensurate phase, which is consistent withthe disappearance of the double peak structure observed in
the angle-resolved x-ray measurement.19
We now examine the coherent oscillations 共⌬Rosc兲 whose
pronounced variations with both T and ␭ support the distinct
selection of two types of CDW order. We plot the typical⌬Rosc obtained at 10 K in Fig. 4共a兲, where one can clearly
see a spectral variation; the high-frequency oscillation isdominant at ␭ ⬇ 880 nm while ⌬Rosc共␭ ⬍ 800 nm兲 exhibits
only one slow oscillation. The ⌬Rosc共␭ ⬇ 830 nm兲 corresponds to a crossover between the two regimes and its FTamplitude spectrum results in a number of sharp peaks
关Fig. 4共b兲兴. This connection is more clearly defined in Fig.

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TODA et al.

4共c兲, where we plot the FT amplitudes of six typical modes
labeled in Fig. 4共b兲 as a function of ␭. For a comprehensive
comparison, we also plot the ⌬Rint spectrum in the figure.
The high-frequency modes produce a peak positioned aroundthe ⌬R resonance of icCDW while the low mode at 0.55
THz is significant in the shorter ␭. The coexistence of thesetwo different types of oscillation supports the coexistence of
two types of optical transition. The correlations between the⌬Rosc and ⌬R resonances are also evidenced by the T dependence of the oscillation mode amplitudes as summarized inFig. 4共d兲, where the amplitude of the lowest mode 共redcircles兲 becomes small with increasing T and disappears
around T = 100 K while the other high modes are still visible
above T = 100 K and become small as T increases further
关see Fig. 1共b兲 as a typical example兴. At T ⬃ 200 K共⬇TCDW兲,
the high modes also vanish completely. The T dependences
and the spectral correlation suggest that the coherent collective oscillations associated with the individual CDW groundstates can be selectively excited.
Finally we consider the T dependences of the frequency
and damping constant of the low-frequency mode 关Fig. 4共e兲兴
as summarized in Fig. 4共f兲. The frequency of the mode decreases from 0.55 to 0.4 THz as T increases from 10 to 80 K.
The damping constant increases greatly along with the softening, and the oscillation finally becomes overdamped
around 80 K. Similar T-dependent coherent oscillation wasobserved in quasi-1D K0.3MoO3, and was attributed to the
CDW amplitude mode 共AM兲 from the good correspondence

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with Raman and neutron data.2 Therefore, we associate thelow-frequency oscillation with an AM mode of o-TaS3. The
T dependence of the FT amplitude is in good agreement withthat of ⌬Rbg described above, and thus associated with theglass transition of o-TaS3.
IV. CONCLUSION

In conclusion, we have studied the ultrafast carrier dynamics in a quasi-1D compound o-TaS3 using a spectrally
resolved pump-probe measurement. A distinct near-infrared
resonance in this compound emphasizes the characteristic⌬R associated with the CDW phase transition, and allows
selective detection of multiple ground states. The
T-dependent spectral changes of transient ⌬R and coherent
oscillations indicate the coexistence of two types of CDW
ordering at low temperature and clearly disentangle the individual phase transitions. We believe that our demonstration
paves the way for the optical selection of the multiple phaseorder that plays an important role in various macroscopic
quantum systems.
ACKNOWLEDGMENTS

This work was supported by the 21st century COE program “Topological Science and Technology,” MEXT, Japan.
Y.T. acknowledges the Sumitomo foundation and the foundation for opto-science and technology.

13 R. C. Rai and J. W. Brill, Phys. Rev. B

70, 235126 共2004兲.

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21
Indeed, the authors in Ref. 10 found the appearance of the side
peaks in R resonance as decreasing T. This is also supported bya remarkable shrinkage of the peak width at 10 K in Fig. 1共a兲.
22
If we choose the R resonance 共␭ ⬃ 905 nm兲 as the initial position for optical transition above TCDW, the shift of the peak 共␭
⬃ 870 nm at 170 K兲 is evaluated to be ⬃55 meV 共445 cm−1兲.
Although our estimation includes ambiguity, this value is consistent with ⌬ in the previous reports 共Refs. 16 and 17兲.
16

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