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Mid-gap states in BaBiO3: A photo-induced reflectance study
PIIIU
Physica C 185-189 (1991) 995-996 North-Holland
MID-GAP STATES IN BaBiO3: A PHOTO4NDUCED REFLECTANCE STUDY W.Markowitseh*, V.Schlosser*, W.Lang*, K,Remsehnig**, P.Rogl** *Ins6tut fiir Feslk0rperphysik der Universitti~.Wicn und Ludwig Boltzmann Insti~t fiir Festk~rperphysik, Kopemikusgasse 15, A-1060 Wien, Austria + **Institut ftir Physikalische Chemie, Universit~t Wien, Austria Photo-induced reflectance measurements of .~ ,W,Os are reported. Utilizing a double modulation technique, several reflectivity peaks around the midgap energy (~ 0.9 eV) are observed when exciting the system at a photon energy of 1.95 eV. This is evidence for a lattice relaxation after removing electrons from the valence band. As a consequence energy levels appear within the gap. The results confirm that the gap in BaBiO3 originates from a Peierls distortion (charge density wave).
1. INTRODUCTION Since the discovery of high temperature supercon-
the samples were single phase with monoclinic struc
ductivity in the Bal_xKxBiO3 system 1, the "parent" insulating compound BaBiO 3 has attracted interest
+ 0.002 ~, c = 8.668 + 0.002 ,/~ and a tilt angle fl =
again. The most interesting feature in this material is the band gap around 2 eV 2, which has been inter-
ture. Lattice constants a = 6.183 5:0.001 A, b = 6.137 90.19 + 0 . 4 9 w e r e found. For the optical experiments the samples were polished on a soft cloth. One half of the polished
preted as a Peierls gap although BaBiO 3 is not a quasi-l-d system. In order to test this interpretation
surface was coated with a thin film of AI by evapo-
we have carried out photo-induced optical measure-
face structure of the sample. The coated part of the
ments. It is well known from quasi-l-d conductors like
sample served as a reference for the reflection
polyacetylene3 that exciting carriers across the band
measurements. The results show that this method is
gap results in the creation of "midgap states". In
a good correction for the surface roughness.
BaBiO 3 the same should be the case if the gap arises from a charge density wave (CDW) formation. Usually photo-induced experiments are carried out as transmission measurements. This requires thin films of the material. Our samples were bulk samples, so we
ration. The film was thin enough to maintain the sur-
The retqection q,ectra of the samples (without laser excitation) were measured with a Hitachi spectrophotometer in the visible to near infrared and "Mth a Bruker Fourier spectrometer in the middle infrared. The setup for the photo-induced (PI) experiments
measured the reflectance. Since the effect is expected
consisted of a He-Ne-laser, a double grid monoch~mma-
to be smaller in this way, we developed a double modu-
tor, a Ge detector and two Lock in amplifiers. The
lation technique (see section 2).
laser beam with a photon energy of 1.95 eV was inten-
2. SAMPLE PREPARATION AND EXPERIMENTAL
chopper. In the light path of the double grid m o n a
sity modulated with 13 cycles/see by a mechanicM TECHNIQUES Bulk samples of BaBiO 3 were prepared by the usual
chromator one mirror was icp~a,.~., ~,7 a plezo d~ven. mirror, allowing to periodically tilt the mirror at
solid state reaction technique. A stoichiometric mix-
frequencies up to 300 cycles,lsec. The assembly was
ture of Bi20 3 and Ba(NO3) 2 was sintered at 820°C for
adjusted to produce a wavelength modulation of -1 nm.
10 days in air. Additionaly an oxygen annealing was
The detector was connected with a Lock In amplifier
carried out at 450°C. X-ray diffraction showed that
locked to the wavelength modulation signal which
+ This work was supported by the Fonds zur Frrderung der wissen~haftlichen For~hung, grant # P8t80, P8t69 0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights resets,cA.
996
W. Markowitsch et at / Mid-gap states in BaBiO~
means the derivative of the reflectance. The output of this Lock In amplifier was picked up by a second one that was locked to the light chopper frequency. Thus the resulting signal is zero when there is no change
T ,, 291K
+i t sample: BanjOs
in the reflectance due to the laser exitation. Peaks in the spectra are detected by their derivative spectra which is much more sensitive to changes than the original reflection spectra. 3. RESULTS Figure 1 shows the reflection spectrum of a BaBiO 3 sample which resembles the results from single crystals2. The inset of Figure 1 shows the absorption
°_,f'"V 0.7
O.B
0.9
t.0
t.i
1.2
PHOTON ENERGY [ eV ]
coefficient, Ix(late), estimated from a Kramers-Kronig analysis. The gap absorption starts at about 1.7 eV and has a maximum value of - 1.5.105 em "1 at 2.1 eV.
FIGURE 2 Photo-~nduced reflectance spectrum of a BaBiO3 sample. Thin line: as measured; thick line: smoothed
.8
.7
~-,f,2
,=.
.6
4. DISCUSSION The energy gap, estimated from tx(hto) (Figure 1), is - 1.7 eV. The energy levels at 0.82 eV and 0.88 eV
,-=
~.5
1=¢
(Figure 2) give strong evidence for a midgap absorp-
i-
~.4 .Oi
t.O eV
.l
~.3 ¸
tion like in polyacetylene3. We offer a qualitative interpretation. When electrons are removed from the valence band by laser excitation the CDw is locally
.2
destroyed. This process results in an electron connec-
.!
ted with a lattice deformation Cpolaron"). The defect
0 .01
PHOTON ENERGY [ eV ]
introduces energy levels within the gap which are observed in the PI reflection measurements. The presence of 2 peaks may indicate a sample inhomogenity
FIGURE I
too small for X-ray detection.
.t
i.O
Reflection spectrum of a BaBiO 3 sample. Inset: absorption coefficient, Ix(la~), from a Kramers-Kronig analysis Figure 2 shows the P! spectrum of the same sample,
ACKNOWLEDGEMENTS We wish to thank T.Pichler and H.Kuzmany for exneip. perimental ......
The signal means the change of the derivative of the reflectance. Reflectance peaks induced by laser excitation appear as zero-crossings in Figure 2. Despite the noise, due to the small effect, the features at 0.82 eV and 0.88 eV were proved as reproducible.
REFERENCES 1. R.J.Cava et al., Nature 332, 814(1988) 2. T.J.Tajima et al., Phys.Rev.B32, 6302(1985) 3. G.B.Blanchet et al., Phys.Rev.Lett.50, 1938(1983)
Physica C 185-189 (1991) 995-996 North-Holland
MID-GAP STATES IN BaBiO3: A PHOTO4NDUCED REFLECTANCE STUDY W.Markowitseh*, V.Schlosser*, W.Lang*, K,Remsehnig**, P.Rogl** *Ins6tut fiir Feslk0rperphysik der Universitti~.Wicn und Ludwig Boltzmann Insti~t fiir Festk~rperphysik, Kopemikusgasse 15, A-1060 Wien, Austria + **Institut ftir Physikalische Chemie, Universit~t Wien, Austria Photo-induced reflectance measurements of .~ ,W,Os are reported. Utilizing a double modulation technique, several reflectivity peaks around the midgap energy (~ 0.9 eV) are observed when exciting the system at a photon energy of 1.95 eV. This is evidence for a lattice relaxation after removing electrons from the valence band. As a consequence energy levels appear within the gap. The results confirm that the gap in BaBiO3 originates from a Peierls distortion (charge density wave).
1. INTRODUCTION Since the discovery of high temperature supercon-
the samples were single phase with monoclinic struc
ductivity in the Bal_xKxBiO3 system 1, the "parent" insulating compound BaBiO 3 has attracted interest
+ 0.002 ~, c = 8.668 + 0.002 ,/~ and a tilt angle fl =
again. The most interesting feature in this material is the band gap around 2 eV 2, which has been inter-
ture. Lattice constants a = 6.183 5:0.001 A, b = 6.137 90.19 + 0 . 4 9 w e r e found. For the optical experiments the samples were polished on a soft cloth. One half of the polished
preted as a Peierls gap although BaBiO 3 is not a quasi-l-d system. In order to test this interpretation
surface was coated with a thin film of AI by evapo-
we have carried out photo-induced optical measure-
face structure of the sample. The coated part of the
ments. It is well known from quasi-l-d conductors like
sample served as a reference for the reflection
polyacetylene3 that exciting carriers across the band
measurements. The results show that this method is
gap results in the creation of "midgap states". In
a good correction for the surface roughness.
BaBiO 3 the same should be the case if the gap arises from a charge density wave (CDW) formation. Usually photo-induced experiments are carried out as transmission measurements. This requires thin films of the material. Our samples were bulk samples, so we
ration. The film was thin enough to maintain the sur-
The retqection q,ectra of the samples (without laser excitation) were measured with a Hitachi spectrophotometer in the visible to near infrared and "Mth a Bruker Fourier spectrometer in the middle infrared. The setup for the photo-induced (PI) experiments
measured the reflectance. Since the effect is expected
consisted of a He-Ne-laser, a double grid monoch~mma-
to be smaller in this way, we developed a double modu-
tor, a Ge detector and two Lock in amplifiers. The
lation technique (see section 2).
laser beam with a photon energy of 1.95 eV was inten-
2. SAMPLE PREPARATION AND EXPERIMENTAL
chopper. In the light path of the double grid m o n a
sity modulated with 13 cycles/see by a mechanicM TECHNIQUES Bulk samples of BaBiO 3 were prepared by the usual
chromator one mirror was icp~a,.~., ~,7 a plezo d~ven. mirror, allowing to periodically tilt the mirror at
solid state reaction technique. A stoichiometric mix-
frequencies up to 300 cycles,lsec. The assembly was
ture of Bi20 3 and Ba(NO3) 2 was sintered at 820°C for
adjusted to produce a wavelength modulation of -1 nm.
10 days in air. Additionaly an oxygen annealing was
The detector was connected with a Lock In amplifier
carried out at 450°C. X-ray diffraction showed that
locked to the wavelength modulation signal which
+ This work was supported by the Fonds zur Frrderung der wissen~haftlichen For~hung, grant # P8t80, P8t69 0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights resets,cA.
996
W. Markowitsch et at / Mid-gap states in BaBiO~
means the derivative of the reflectance. The output of this Lock In amplifier was picked up by a second one that was locked to the light chopper frequency. Thus the resulting signal is zero when there is no change
T ,, 291K
+i t sample: BanjOs
in the reflectance due to the laser exitation. Peaks in the spectra are detected by their derivative spectra which is much more sensitive to changes than the original reflection spectra. 3. RESULTS Figure 1 shows the reflection spectrum of a BaBiO 3 sample which resembles the results from single crystals2. The inset of Figure 1 shows the absorption
°_,f'"V 0.7
O.B
0.9
t.0
t.i
1.2
PHOTON ENERGY [ eV ]
coefficient, Ix(late), estimated from a Kramers-Kronig analysis. The gap absorption starts at about 1.7 eV and has a maximum value of - 1.5.105 em "1 at 2.1 eV.
FIGURE 2 Photo-~nduced reflectance spectrum of a BaBiO3 sample. Thin line: as measured; thick line: smoothed
.8
.7
~-,f,2
,=.
.6
4. DISCUSSION The energy gap, estimated from tx(hto) (Figure 1), is - 1.7 eV. The energy levels at 0.82 eV and 0.88 eV
,-=
~.5
1=¢
(Figure 2) give strong evidence for a midgap absorp-
i-
~.4 .Oi
t.O eV
.l
~.3 ¸
tion like in polyacetylene3. We offer a qualitative interpretation. When electrons are removed from the valence band by laser excitation the CDw is locally
.2
destroyed. This process results in an electron connec-
.!
ted with a lattice deformation Cpolaron"). The defect
0 .01
PHOTON ENERGY [ eV ]
introduces energy levels within the gap which are observed in the PI reflection measurements. The presence of 2 peaks may indicate a sample inhomogenity
FIGURE I
too small for X-ray detection.
.t
i.O
Reflection spectrum of a BaBiO 3 sample. Inset: absorption coefficient, Ix(la~), from a Kramers-Kronig analysis Figure 2 shows the P! spectrum of the same sample,
ACKNOWLEDGEMENTS We wish to thank T.Pichler and H.Kuzmany for exneip. perimental ......
The signal means the change of the derivative of the reflectance. Reflectance peaks induced by laser excitation appear as zero-crossings in Figure 2. Despite the noise, due to the small effect, the features at 0.82 eV and 0.88 eV were proved as reproducible.
REFERENCES 1. R.J.Cava et al., Nature 332, 814(1988) 2. T.J.Tajima et al., Phys.Rev.B32, 6302(1985) 3. G.B.Blanchet et al., Phys.Rev.Lett.50, 1938(1983)