Photoinduced Phase Transitions in MX-Chain Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 59

transition and superconductivity at low temperature is observed by substituting
Nd by Ce through the electron-carrier doping [ 55 ]. In these materials, the I-M
transitions and the collapse of the gap are clearly reflected by the optical conduc-
tivity spectra in the near-IR to visible region. Figure 5.3b shows the doping
concentration ( x ) dependence of

s

) spectra in
La 2 x Sr x CuO 4 , which are obtained by the Kramers-Kronig (KK) analyses of the
reflectivity spectra [ 54 ]. The parent compound (La 2 CuO 4 ) has a clear peak at
around 2 eV corresponding to the charge-transfer (CT) gap. With increasing x ,
the spectral weight of the CT-gap transition is transferred into the intragap region.
Such a huge spectral change over a wide energy region is the ubiquitous feature of
the Mott transition in the 2D cuprate and also other strongly correlated electron
systems of 3d transition-metal oxides [ 50 ]. Our purpose is to trigger a similar Mott
transition by a photocarrier doping in the Ni-chain compound.

the optical conductivity (

5.3.1 Ultrafast Photoinduced Transition from Mott Insulator
to Metal in Bromine-Bridged Nickel-Chain Compound

The polarized reflectivity spectrum of the Ni-Br chain compound is presented in
Fig. 5.5a . A sharp peak at around 1.3 eV is due to the CT-gap transition. The
transient reflectivity (TR) spectra observed at the delay time t d after the photoexci-
tation are presented by the dots and lines. The excitation photon energy is 1.55 eV
just above the CT gap. The intensity of the irradiated light was 3.6 mJ/cm 2 . Under
this condition, the average excitation density x ph of the absorbed photon is 0.5 per
Ni site within the absorption depth (460 ˚ ), as estimated by taking account of the
reflection loss (30 %) and the unit cell volume (8.68 10 21 cm 3 ). Immediately

following the photoexcitation ( t d ¼ 0.1 ps), the reflectivity in the mid-IR region

considerably increases, being reminiscent of the Drude-like metallic response,
while the reflectivity around the CT band decreases due to photoinduced bleaching.
The magnitude of the transient reflectivity R 0 at t d ¼ 0.1 ps reaches about 70 % at

the lowest photon energy of the probe light (0.12 eV), where the change of
reflectivity ð D R=R ¼ðR 0 RÞ=RÞ

is as large as 260 % of the original reflectivity

R . The optical conductivity s

spectrum was obtained by performing the KK
analyses of the original reflectivity spectrum and the transient ones, which are
shown in Fig. 5.5b . As seen in the figure, the

0.1 ps monotonically
increases with lowering the probe photon energy to 0.12 eV, suggesting the closing
of the optical gap. Such a noticeable photoinduced feature is observed only for the
probe light polarization ( E ) parallel to the Ni-Br chain ( E // b ) and not for E ⊥ b at all,
indicating the photogeneration of a quasi-1D metallic state.
To clarify the photoinduced change of the electronic state in more detail, the
excitation density x ph dependence of the TR has been investigated. Spectra of the
TR and

s at t d ¼

s at t d ¼ 0.1 ps for various x ph are shown in Fig. 5.6 . For the weak