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Optical studies of BaBiO3: A 3D charge density wave (CDW) insulator
Synthetic Metals, 41--43 (1991) 3977-3980
3977
OPTICAL STUDIES OF BaBiO3: A 3D CHARGE DENSITY WAVE (CDW) INSULATOR
G. RUANI, .N.J. PAL, C. TALIANI and R. ZAMBONI
Istituto di Spettroscopia Molecolare, C.N.R., via de'Castagnoli L 40126 BOLOGNA, Italy. X. WEI, L. CHEN* and Z.V. VARDENY
Department of Physics, Universityof Utah, Salt Lake City, Utah 84112, USA.
ABSTRACT The photomodulation spectra of high quality polycrystalline BaBiO3 have been studied in the spectral range of 100 to 20000 cm d. Clear evidence of photogeneration of self-localized charged carriers has been observed. This includes photobleaching of IR-active phonons at 270 and 162 cm"1, the latter associated with the CDW indicating its suppression, and the correlated creation of two PA electronic bands with peaks at 0.4 and 0.9 eV, respectively. The possibility of polaron and bipolaron photogeneration is discussed. INTRODUCTION The BaBiO3 potassium doped system is, up to now, the only material which shows superconductivity [1] up to 30 K that is: i) perfectly three dimensional, ii) without Cu-O layers and iii) has no magnetic ions. The existence of such a material, if considered a HTcS, opens some doubts on the role played by Cu and low dimensionality in the high Tc superconductivity mechanism. The undoped BaBiO3 compound presents an oxygen breathing-mode distortion which gives rise to a commensurate CDW, a typical 3D Peierls distortion, that determines the opening of a semiconducting gap around the Fermi energy. The commensurability of the CDW is reduced upon doping and completely disappears when the material becomes metallic at x = 0.35 in BaPbl-xBixO3 and at x -- 0.2 in BaBil-xKxO3. It has been shown that the photogenerated charged quasiparticles in the semiconducting phases of other HTc superconducting systems (La2CuO4 [2], YBa2Cu306 +x [3] and TI2Ba2Cal xGdxCu208 [4]) evolve in local electronic states within the semiconducting gap, which are associated with lattice distortions; such mid-gap states are very similar to those induced in these systems by doping, suggesting that the charges induced by doping and those photoinduced behave similarly. We note that a fast photoinduced electronic response has been recently observed in BaBiO3 [5]. In this work, we carried out photoinduced absorption (PA) studies on BaBiO3 in order to find common characteristics which might exist in all of the HTc superconducting families.
*
Dept. Phys., Brown Univ., Providence, Rhode Island 02912, USA
0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
3978
PHOTOINDUCED ABSORPTION IN THE PHONON SPECTRAL RANGE. Optical measurements were carried out on pressed pellets. Polycrystalline samples were ground and mixed with polyethylene CsI or KBr in an agate mortar in a controlled atmosphere and then pressed for obtaining optically transparent pellets in the far IR, mid IR, and near IR - visible spectral range, respectively. Considering an homogeneous distribution of the material in the transparent supporting media, the 0.25 mg of BaBiO3 used for a 13 mm diameter pellet corresponds to a sample with an equivalent thickness of approximately 0.5. 0.m The IR absorbance spectrum of this sample shows four bands in the phonon spectral region. As reported by Uchida et al. [6], this is consistent with a cubic symmetry but with two formula units in the primitive cell (Oh5) obtained when taking into account only the breathing distortion of the structure. They suggested that the vibration at 162 cm 1 is a zoneboundary mode which becomes IR active due to the folding of the Brillouin zone by the CDW distortion. This explanation was based on the disappearance of this band upon doping. The other three bands at 473, 270 and 102 cm "1 were assigned to the BiO3 bending, stretching and external mode respectively. Experimental details of the photoinduced absorption technique were reported previously in Ref 3a. An Ar + laser was used for photoexcitation at 2.41 eV at different cw power density (20 - 350 mW/cm2). All measurements were carried out at 5 K. 1
-1
-2 ~-'-3
a)
oe--
f
I
I
,
,
,
-
X
4 i--
i 0 -2 -4
~
i
i
200 400 600 Wavenumber [cm-1] Fig. 1. a) Photoindueed absorption s l~ectrum of BaBiO3 at 5 K for excitation laser at 2.41 eV and power density of 80 mW cm". b) Differential absorption spectrum of BaBiO3 between 5 and 20 K.
3979
The photoinduced absorption spectrum (see Fig la) shows photobleaching (PB) of the three bands at higher energy. The PB of the 162 cm "1 band is the most pronounced. The decrease in intensity of the 162 cm "l band is linear with the laser intensity showing saturation effects at higher laser power. As shown in Fig. lb this PB cannot be ascribed to a thermal effect: the temperature increase shows some detectable changes of the absorption intensity only with an increase of about 15K but in the opposite direction to the one observed in the PA measurements. A double peak structure PB, of which intensity is proportional to IL0"6 is observed in the 270 cm "1 band range; no thermal modulation structure is detectable in this spectral range. Contrary to what is observed in the case of the other two bands, the bleaching of the 473 cm "1 and also the "pseudo" PA band at 555 cm -1 coincide almost perfectly with the structures due to heating (Fig. lb) and thus are related to a thermomodulation effect. P H O T O I N D U C E D ABSORPTION IN T H E ELECTRONIC SPECTRAL R A N G E We have also used the standard photomodulation (PM) technique [7] in the visible/nearIR spectral range to study the photogeneration of self-localized carriers with states in the gap. The experiments were done using an Ar + laser beam with intensity of 500 mW cm "2 (corresponding to a fluence of 1023 pbotons/cm2sec) which was modulated between 40 and 3500 Hz; here we report our PM measurements at 80K. 4
'
I HE
3 -
o
LE /
/
a) _
/
\
.~..,.f
,
--
-
"
0
t-.-
I
/\--
21
'
x_
..-'
A
r"''~ I
]
LE ~%.: .//
0.5 /
'¢
ME ~\
b)
,.-
0
"-
-0.5
,
0
I
I
. ' .....
,
I
1.0 2.0 Photon energv [eV ]
Fig. 2. The photomodulation spectrum of BaBiO3 at 80K using a) 40 Hz b) 5~0 Hz modulation for the excitation laser at 2.41eV and power density of 500 mW cm". The two PA bands at 0.4 eV and.0.9 eV are assigned.
3980
Fig. 2a shows the PM spectrum at f=40 Hz. The spectrum contains two PA bands with peaks at 0.4 eV (LE) and at 0.9 eV (HE) respectively and photobleaching starting at 1.4 eV with maximum at 2 eV. This shows that the two gap states borrow the IR oscillator strength from the interband transitions, indicating their intrinsic nature. The two PA bands do not share a common origin as evidenced as follows. The LE PA band increases with IL with a power of 0.6, whereas the HE PA band increases linearly with IL Also the HE band is much slower than the LE band, as shown in Fig. 2b. At 500 Hz modulation the LE intensity decays by a factor of 5, whereas the HE band is hardly influenced by the higher f. This indicates a shorter lifetime for the LE state. We may tentatively correlate the PA electronic bands with the PB of the IR phonons based on the different IL dependencies. The LE PA band is thus correlated with the PB at 270 cm"1. The HE PA band is related to the PB of the 160 cm "1 phonon. Since only the latter has been shown to be directly correlated with the CDW in BaBiO3, we conjecture that the LE PA band is an intermediate electronic state which may eventually decay into the state associated with the HE band. Based on their IL dependence and the resultant recombination kinetics and other evidences [8], we tentatively identify the LE PA band as due to photogenerated polarons, whereas the HE band may be due to photogenerated bipolarons. This conclusion, however, may be seriously considered only after experiments aimed to elucidate the spin states of the photogenerated carriers are completed. ACKNOWLEDGEMENTS The work at the University of Utah was supported in part by DOE grant no. DE-FG-0289, ER-45409 and the work at the Istituto di Spettroscopia Molecolare was financed by the P.F. "SuCryTec" of CNR. We wish to thank Messrs A.Martiniello, P.Mei and G.Tasini for valuable technical support REFERENCES 1. R.J. Cava, B. Batlogg, J.J. Krajewski, R. Farrow, LW. Rupp Jr., A.E. White, W.F. Peck and T. Kometani, Nature 332 (1988) 814. 2. a) Y.H. Kim, A.J. Heeger, L Acedo, G. Stucky and F. Wudl, Phys, Rev. B 36 (1987) 7252. b) J.M. Ginder, M.G. Roe, Y. Song, R.P. McCall, J.R. Gaines, E. Ehrenfreund and A. Epstein, Phys. Rev. B 37 (1988) 7506. c) X.Wei, L Chen, Z.V. Vardeny, C. Taliani, R. Zamboni, A.J. Pal and G. Ruani, Physica C 162-164 (1989) 1109. 3. a) C. Taliani, R. Zamboni, G. Ruani, F.C. Matacotta and K.I.Pokhodnya, Solid State Commun. 66 (1988) 487. b) Y.H. Kim, C.M. Foster, AJ. Heeger, S. Cox and G. Stuck'y, Phys Rev B 38 (1988) 6478. c) C. Taliani, R. Zamboni, G. Ruani, A.J. Pal, F.C. Matacotta, Z. Vardeny and X. Wei in "Electronic Structure of high ~ Superconductors", eds by A. Bianconi and A. Marcelli, Pergamon Press, Oxford (UK) (1989) 95. 4. C.M. Foster, A.J. Heeger, G. Stuck'y, N.Herron, Solid State Commun. 71 (1989) 945. 5. J.F. Federici, B.I. Green, E.H. Hartford and E.S. Hellman, Phy~, Rev. B. in press. 6. S. Uchida, S. Tajima, A. Masaki, S. Sugai, K. Kitazawa and S. Tanaka, ~1.Phys. Soc. Japan 54 (1985) 4395. 7. Z.V. Vardeny, T.X. Zhou, H.A. Stoddart and J.Taue Solid State Commun, ~i~ (1988) 1049. 8. X. Wei, L. Chen, Z.V. Vardeny, G. Ruani, A.J. Pal, C. Taliani and R. Zamboni, to be published.
3977
OPTICAL STUDIES OF BaBiO3: A 3D CHARGE DENSITY WAVE (CDW) INSULATOR
G. RUANI, .N.J. PAL, C. TALIANI and R. ZAMBONI
Istituto di Spettroscopia Molecolare, C.N.R., via de'Castagnoli L 40126 BOLOGNA, Italy. X. WEI, L. CHEN* and Z.V. VARDENY
Department of Physics, Universityof Utah, Salt Lake City, Utah 84112, USA.
ABSTRACT The photomodulation spectra of high quality polycrystalline BaBiO3 have been studied in the spectral range of 100 to 20000 cm d. Clear evidence of photogeneration of self-localized charged carriers has been observed. This includes photobleaching of IR-active phonons at 270 and 162 cm"1, the latter associated with the CDW indicating its suppression, and the correlated creation of two PA electronic bands with peaks at 0.4 and 0.9 eV, respectively. The possibility of polaron and bipolaron photogeneration is discussed. INTRODUCTION The BaBiO3 potassium doped system is, up to now, the only material which shows superconductivity [1] up to 30 K that is: i) perfectly three dimensional, ii) without Cu-O layers and iii) has no magnetic ions. The existence of such a material, if considered a HTcS, opens some doubts on the role played by Cu and low dimensionality in the high Tc superconductivity mechanism. The undoped BaBiO3 compound presents an oxygen breathing-mode distortion which gives rise to a commensurate CDW, a typical 3D Peierls distortion, that determines the opening of a semiconducting gap around the Fermi energy. The commensurability of the CDW is reduced upon doping and completely disappears when the material becomes metallic at x = 0.35 in BaPbl-xBixO3 and at x -- 0.2 in BaBil-xKxO3. It has been shown that the photogenerated charged quasiparticles in the semiconducting phases of other HTc superconducting systems (La2CuO4 [2], YBa2Cu306 +x [3] and TI2Ba2Cal xGdxCu208 [4]) evolve in local electronic states within the semiconducting gap, which are associated with lattice distortions; such mid-gap states are very similar to those induced in these systems by doping, suggesting that the charges induced by doping and those photoinduced behave similarly. We note that a fast photoinduced electronic response has been recently observed in BaBiO3 [5]. In this work, we carried out photoinduced absorption (PA) studies on BaBiO3 in order to find common characteristics which might exist in all of the HTc superconducting families.
*
Dept. Phys., Brown Univ., Providence, Rhode Island 02912, USA
0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
3978
PHOTOINDUCED ABSORPTION IN THE PHONON SPECTRAL RANGE. Optical measurements were carried out on pressed pellets. Polycrystalline samples were ground and mixed with polyethylene CsI or KBr in an agate mortar in a controlled atmosphere and then pressed for obtaining optically transparent pellets in the far IR, mid IR, and near IR - visible spectral range, respectively. Considering an homogeneous distribution of the material in the transparent supporting media, the 0.25 mg of BaBiO3 used for a 13 mm diameter pellet corresponds to a sample with an equivalent thickness of approximately 0.5. 0.m The IR absorbance spectrum of this sample shows four bands in the phonon spectral region. As reported by Uchida et al. [6], this is consistent with a cubic symmetry but with two formula units in the primitive cell (Oh5) obtained when taking into account only the breathing distortion of the structure. They suggested that the vibration at 162 cm 1 is a zoneboundary mode which becomes IR active due to the folding of the Brillouin zone by the CDW distortion. This explanation was based on the disappearance of this band upon doping. The other three bands at 473, 270 and 102 cm "1 were assigned to the BiO3 bending, stretching and external mode respectively. Experimental details of the photoinduced absorption technique were reported previously in Ref 3a. An Ar + laser was used for photoexcitation at 2.41 eV at different cw power density (20 - 350 mW/cm2). All measurements were carried out at 5 K. 1
-1
-2 ~-'-3
a)
oe--
f
I
I
,
,
,
-
X
4 i--
i 0 -2 -4
~
i
i
200 400 600 Wavenumber [cm-1] Fig. 1. a) Photoindueed absorption s l~ectrum of BaBiO3 at 5 K for excitation laser at 2.41 eV and power density of 80 mW cm". b) Differential absorption spectrum of BaBiO3 between 5 and 20 K.
3979
The photoinduced absorption spectrum (see Fig la) shows photobleaching (PB) of the three bands at higher energy. The PB of the 162 cm "1 band is the most pronounced. The decrease in intensity of the 162 cm "l band is linear with the laser intensity showing saturation effects at higher laser power. As shown in Fig. lb this PB cannot be ascribed to a thermal effect: the temperature increase shows some detectable changes of the absorption intensity only with an increase of about 15K but in the opposite direction to the one observed in the PA measurements. A double peak structure PB, of which intensity is proportional to IL0"6 is observed in the 270 cm "1 band range; no thermal modulation structure is detectable in this spectral range. Contrary to what is observed in the case of the other two bands, the bleaching of the 473 cm "1 and also the "pseudo" PA band at 555 cm -1 coincide almost perfectly with the structures due to heating (Fig. lb) and thus are related to a thermomodulation effect. P H O T O I N D U C E D ABSORPTION IN T H E ELECTRONIC SPECTRAL R A N G E We have also used the standard photomodulation (PM) technique [7] in the visible/nearIR spectral range to study the photogeneration of self-localized carriers with states in the gap. The experiments were done using an Ar + laser beam with intensity of 500 mW cm "2 (corresponding to a fluence of 1023 pbotons/cm2sec) which was modulated between 40 and 3500 Hz; here we report our PM measurements at 80K. 4
'
I HE
3 -
o
LE /
/
a) _
/
\
.~..,.f
,
--
-
"
0
t-.-
I
/\--
21
'
x_
..-'
A
r"''~ I
]
LE ~%.: .//
0.5 /
'¢
ME ~\
b)
,.-
0
"-
-0.5
,
0
I
I
. ' .....
,
I
1.0 2.0 Photon energv [eV ]
Fig. 2. The photomodulation spectrum of BaBiO3 at 80K using a) 40 Hz b) 5~0 Hz modulation for the excitation laser at 2.41eV and power density of 500 mW cm". The two PA bands at 0.4 eV and.0.9 eV are assigned.
3980
Fig. 2a shows the PM spectrum at f=40 Hz. The spectrum contains two PA bands with peaks at 0.4 eV (LE) and at 0.9 eV (HE) respectively and photobleaching starting at 1.4 eV with maximum at 2 eV. This shows that the two gap states borrow the IR oscillator strength from the interband transitions, indicating their intrinsic nature. The two PA bands do not share a common origin as evidenced as follows. The LE PA band increases with IL with a power of 0.6, whereas the HE PA band increases linearly with IL Also the HE band is much slower than the LE band, as shown in Fig. 2b. At 500 Hz modulation the LE intensity decays by a factor of 5, whereas the HE band is hardly influenced by the higher f. This indicates a shorter lifetime for the LE state. We may tentatively correlate the PA electronic bands with the PB of the IR phonons based on the different IL dependencies. The LE PA band is thus correlated with the PB at 270 cm"1. The HE PA band is related to the PB of the 160 cm "1 phonon. Since only the latter has been shown to be directly correlated with the CDW in BaBiO3, we conjecture that the LE PA band is an intermediate electronic state which may eventually decay into the state associated with the HE band. Based on their IL dependence and the resultant recombination kinetics and other evidences [8], we tentatively identify the LE PA band as due to photogenerated polarons, whereas the HE band may be due to photogenerated bipolarons. This conclusion, however, may be seriously considered only after experiments aimed to elucidate the spin states of the photogenerated carriers are completed. ACKNOWLEDGEMENTS The work at the University of Utah was supported in part by DOE grant no. DE-FG-0289, ER-45409 and the work at the Istituto di Spettroscopia Molecolare was financed by the P.F. "SuCryTec" of CNR. We wish to thank Messrs A.Martiniello, P.Mei and G.Tasini for valuable technical support REFERENCES 1. R.J. Cava, B. Batlogg, J.J. Krajewski, R. Farrow, LW. Rupp Jr., A.E. White, W.F. Peck and T. Kometani, Nature 332 (1988) 814. 2. a) Y.H. Kim, A.J. Heeger, L Acedo, G. Stucky and F. Wudl, Phys, Rev. B 36 (1987) 7252. b) J.M. Ginder, M.G. Roe, Y. Song, R.P. McCall, J.R. Gaines, E. Ehrenfreund and A. Epstein, Phys. Rev. B 37 (1988) 7506. c) X.Wei, L Chen, Z.V. Vardeny, C. Taliani, R. Zamboni, A.J. Pal and G. Ruani, Physica C 162-164 (1989) 1109. 3. a) C. Taliani, R. Zamboni, G. Ruani, F.C. Matacotta and K.I.Pokhodnya, Solid State Commun. 66 (1988) 487. b) Y.H. Kim, C.M. Foster, AJ. Heeger, S. Cox and G. Stuck'y, Phys Rev B 38 (1988) 6478. c) C. Taliani, R. Zamboni, G. Ruani, A.J. Pal, F.C. Matacotta, Z. Vardeny and X. Wei in "Electronic Structure of high ~ Superconductors", eds by A. Bianconi and A. Marcelli, Pergamon Press, Oxford (UK) (1989) 95. 4. C.M. Foster, A.J. Heeger, G. Stuck'y, N.Herron, Solid State Commun. 71 (1989) 945. 5. J.F. Federici, B.I. Green, E.H. Hartford and E.S. Hellman, Phy~, Rev. B. in press. 6. S. Uchida, S. Tajima, A. Masaki, S. Sugai, K. Kitazawa and S. Tanaka, ~1.Phys. Soc. Japan 54 (1985) 4395. 7. Z.V. Vardeny, T.X. Zhou, H.A. Stoddart and J.Taue Solid State Commun, ~i~ (1988) 1049. 8. X. Wei, L. Chen, Z.V. Vardeny, G. Ruani, A.J. Pal, C. Taliani and R. Zamboni, to be published.