CDW suppression and photoinduced gap states in BaBiO3

Solid State Communications, Vol. 81, No. 5, pp. 419-423, 1992. Printed in Great Britain.

0038-1098/92 $5.00 + .00 Pergamon Press plc

CDW SUPPRESSION AND PHOTOINDUCED GAP STATES IN BaBiO3 X. Wei,t A.J. Pal,* L. Chen,§ G. Ruani,* C. Taliani,* Z.V. Vardenyt and R. Zamboni* tDepartment of Physics, University of Utah, Salt Lake City, UT 84112, USA *Instituto di Spettroscopia Molecolare, C.N.R., Via de' Castagnoli, i, 40126 Bologna, Italy §Division of Engineering, Brown University, Providence, RI 02912, USA

(Received 25 July 1991 by J. Tauc) We have studied the electronic and vibrational photoexcitations in high quality polycrystalline BaBiO3 by the photomodulation spectroscopy technique in a broad spectral range (50-20 000 cm ~) and time interval (10-6S to 100S). We have identified two distinct photogenerated gap states with below gap electronic transitions and associated bleaching of i.r.-active vibrations. We interpret these spectral features as evidence of photogenerated polarons and bipolarons and explain their distinct generation processes and recombination kinetics. The photobleaching of i.r.-active vibrations is explained as due to suppression of the 3-D charge-density-waves.

THE CUBIC perovskite alloy Ba~ ,K,.BiO 3 with i.r.-active vibrations which are associated with the x = 0.4 has been discovered to be a superconductor CDW, indicating its suppression. Although the two at relatively high temperature (T, = 29K) [1], thus photoinduced gap states have different recombination approaching the T, value of the prototype cuprate kinetics, relaxation time, temperature dependence and perovskites, i.e. La2 ,.SrxCuO4 [2]. This discovery has associated i.r.-active vibrations and, therefore, do not renewed interest in the semiconducting parent com- share a common origin, they still have identical pound BaBiO3, a three dimensional (3-D) non- response upon bias illumination, therefore, indicating magnetic system with an optical gap of 2 eV [3] and that they are correlated. We interpret the photoinduced a transport activation energy of 0.24eV [4]. This bands as due to intermediate-state polarons which compound has been understood in the context of decay in about 10-3S into the more stable bipolarons, charge density disproportionation where the half- by analogy with the photoexcitations in quasi I-D filled Bi(6s)-O(2p) conduction band [5] is split, due to conducting polymers [1 I]. PM spectroscopy requires two light sources: a Bi site "dimerization" [6] caused by a 3-D perfect nesting of the Fermi surfaces. BaBiO3 may be, there- pump beam for photogeneration of carriers and a fore, viewed as a commensurate Peierls insulator probe beam for measuring the photoinduced changes involving a bond-order charge-density-wave (CDW) in transmission [12]. For the steady state measurewhere the Bi-O bond lengths are alternating [7]. ments in the visible and near i.r. spectral ranges [12] Indeed several theoretical works [7, 8] have predicted the pump was a mechanically chopped (35Hz < polarons and/or bipolarons as the primary electronic f < 3.5kHz) Ar ÷ laser beam with an intensity excitations in BaBiO3, which consequently suppress IL ~< 500 mW cm -2 and the probe beam was derived from an incandescent lamp dispersed by a monothe 3-D CDW and induced states in the gap [8, 9]. In this work we study the photogenerated elec- chromator. The transmission Tand its modulation AT tronic and vibrational excitations in BaBiO3 using the were recorded in the spectral range of 0.1 to 2.2eV. transient photomodulation (PM) technique [10] in a For the PM spectrum in the mid i.r. range we used broad spectral range (50 to 20000cm -j) and time [13] a modified FT-i.r. (Bruker) to allow sample interval (!/~s to 100s). Photoinjection of e-h pairs in illumination by the Ar ÷ laser beam. FT-i.r. spectra in BaBiO3 allows us to investigate the system in extremely the range of 50-3000cm t were recorded collecting low doping conditions (i.e. 10t6-10~Scarrierscm 3). consecutive interferograms with laser on and off We found two distinct electronic gap-states which (chopping frequency f = 0.01 Hz) and signal averborrow their oscillator strengths from the interband aging until the desired signal to noise ratio of - A T [ T transitions above the optical gap. Correlated with was obtained. The /as transient PM measurements the electronic transitions is the photobleaching of were done [10] using a pulsed N d : Y A G (yttrium419

420

CDW GAP STATES IN BaBiO3

aluminium-garnet) laser with a frequency doubler. The pulse duration was about 10ns, the energy per pulse was 50 nJ at 532 nm and the repetition rate was 20 Hz; the illuminated area on the sample was 1/2 cm". The probe beam was produced by an incandescent lamp whose light was spectrally resolved using a set of interference filters in the spectral range 0.2-1.8eV with an approximate resolution of 0.1 eV. The spectra were recorded using two signal averagers in the time range of 300 ns-30ms. PM data is presented in the form of - A T / T which is approximately equal to dA~ (dis the film thickness and A~ is the modulated change in the sample absorption ~) in the transparent spectral regions [14]. BaBiOs was prepared from a mixture of Ba(BO3)2 and Bi203 which were ground together in an AI203 mortar, placed in a covered Pt crucible and heated to !110°C for 2h. The microcrystals were subsequently annealed in an oxygen atmosphere at 1125°C. The crystallites were golden coloured and X-ray study showed that they were of the correct phase. The PM measurements were done on pressed pellets where the polycrystalline samples were ground and mixed with polyethylene, Csl or KBr in an agate mortar and then pressed to obtain optically transparent pellets which corresponded to an equivalent thickness of about 0.5#m. In Fig. 1 we show the steady state PM spectra of BaBiO3 in the visible-i.r, spectral range; Fig. l(a) also shows the absorbance spectrum (~d) of the sample with a peak at 2eV indicating an optical gap in the near i.r. range [3, 15]. The PM spectra of Fig. 1 can be easily decomposed into two photoinduced absorption (PA) bands with peaks at 0.4eV (LE band) and 0.9eV (HE band), respectively and photobleaching (PB) starting at 1.4eV with maximum bleaching at about 2eV. The PB of the interband transitions indicates that the two PA bands were created at the expense of electronic states in the continuum bands, showing their intrinsic nature [1 I, 12]. As shown in Fig. I for the PM spectra taken at various IL and)q, the two PA bands do not share a common origin. The H E PA band increases with IL more than the LE, as seen in Figs. l(a) and l(b) respectively, with It of 100 and 5 0 0 m W c m - 2 ; in fact we found (by adding various neutral density filters in front of the laser beam) that the HE PA strength increases linearly with IL whereas the LE PA strength is proportional to 1°6. Also the HE band decays much slower than the LE band, as shown in Fig. l(c) where the H E intensity decreases with f more than the LE, showing a stronger dependence due to its slower kinetics. The different kinetics of the two PA bands are clearly shown in Fig. 2 where the PM spectra were

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Fig. 1. PM spectra of BaBiO3 at 80 K and at various laser modulation frequency f and intensity It. (a) f = 40Hz, It = 5 0 0 m W c m 2; (b) f = 40Hz, It = 100mWcm 2; (c) f = 500Hz and IL = 5 0 0 m W c m 2 . The LE and HE PA bands are assigned. The sample's optical density spectrum is also shown in (a) in broken lines.

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recorded at 10 6 10 3 and 2 × 10 2s following the pulse excitation. At short time we can only see the LE PA band, with a much stronger intensity than at steady state (Fig. i); at 80K it decays with a characteristic time of 10 _3 S. The H E PA band, on the other hand, does not clearly appear for times up to a b o u t 10-3s. This is consistent with the assumption that the H E band is due to photoexcited species with a delayed generation, probably associated with the decay of the species related to the LE band. Strong evidence that the two gap states associated with the LE and H E PA bands are correlated in spite of their different I L , f a n d decay kinetics shown above, is presented in Fig. 3; where we show two PM spectra at 80 K taken under different bias illumination intensities lb. lh was provided by a separate tungsten lamp, whose power controlled beam was simultaneously focused on the same illuminated spot on the sample used for the PM measurements. We see from Fig. 3 that the LE band decreases whereas the H E band increases with lb. The changes of the two PA bands with lh are summarized in Fig. 3 (inset). This shows a linear decrease of the LE band with Ih whereas the HE band increases linearly with lh; in fact the relative changes of the two PA bands with Ih are the same. This indicates that the H E band increases at the expense of 'he LE band showing correlated kinetics. The i.r. absorption spectrum of BaBiO3 shows four i.r. active vibrations (Fig. 4(a)). This is inconsistent with a simple cubic structure (O~,) with 3 i.r. modes at q = 0, but it is in agreement with a cubic symmetry with two formula units in the primitive cell (Oj~), as suggested in [16]. In this model the 162cm vibration is an acoustic zone-boundary mode made i.r. active due to the Brillouin zone folding associated with the appearance of CDW distortion. The other three i.r. active vibrations seen in Fig. 4(a) at 102, 270

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0 200 400 600 WAVENUMBER (cm -1) Fig. 4. (a) I.r. absorption spectrum of a pressed pellet of BaBiO3 in KBr measured at 5 K. (b) PM spectrum of BaBiO 3 pellet at 5 K measured with IL = 80 mW cm 2. (c) Thermomodulation spectrum of the BaBiO3 pellet obtained by subtracting T~20K)-TcsKI. and 473cm ~ have been assigned [16] to the BiO 3 external mode, stretching mode and bending mode, respectively. The PM of i.r. active phonons shown in Fig. 4(b), consists of PB of the 3 higher energy vibrations. The PB of the 162 cm ~ band is the most pronounced and varies linearly with IL. The thermomodulation spectrum [13] is shown in Fig. 4(c) for comparison in order to identify artifacts due to photoinduced temperature modulation. The band at 162cm ~ shows small thermal induced changes in the opposite direction of that observed in the PM measurements. We conjecture, therefore, that this band is an instrinsic photoinduced feature. A double peak PB structure (Fig. 4(b)), whose intensity is proportional to I °6 (Fig. 2) is observed at about 270cm ~, no thermal modulation structure is detectable for this band. On the other hand, the bleaching of the 473 cm ~and also the "pseudo" PA band at 555 c m - ' coincide almost perfectly with the structures due to heating (Fig. 4(b), (c)) and thus are related to a temperature modulation due to the laser illumination [17]. The similarity between the dependence of It. of the vibrational PB at 162 and at 270cm -~ with that of the

422

CDW G A P STATES IN BaBiO3

LE and HE electronic PA bands indicates that a correlation exists between them. We, therefore, tentatively correlate the LE PA band with the PB of the stretching vibration of BiO3 at 270cm ~ and the HE PA band with the PB of the bending vibration of BiO3 at 162cm ~ Upon hole doping the CDW in BaBiO3 is suppressed [15] due to loss of commensurate Peierls condensation energy due to the dopant induced holes. However, only the oscillator strength of the 162 cm zone-boundary mode is strongly reduced upon d o p i n g and this decrease is directly correlated with the CDW suppression [16]. With this in mind, we conclude that only the HE PA band, which is associated with the PB of the BiO 3 bending vibration at 162cm ~, efficiently suppresses the CDW by analogy with the effect caused by doping [16], and, therefore, it is the more stable electronic excitation. This is consistent with the decay of the LE PA band into the more stable HE band. It is also worthwhile comparing the PA results in the electronic spectral range with the induced changes in the optical conductivity of BaBiO3 upon dilute doping with potassium [18]. If the change of the optical conductivity spectra caused by potassium doping is represented as a difference spectrum such as - A T / T , then a broad peak appears which is centered at about i eV followed by a bleaching corresponding to the maximum of the optical conductivity of the undoped BaBiO3. This is similar to our - A T ] T spectrum obtained by PM at low modulation frequency (Fig. I) again indicating that the H E band is associated with the more stable photoexcitations whereas the LE PA band is due to transient species. By analogy with the electronic excitations in quasi-l-D conducting polymers, where polarons decay into the more stable bipolarons upon doping or photoexcitation [11], we tentatively identify the LE PA band and the associated PB at 270 cm ~in BaBiO3 as due to photogenerated polarons (P) whereas, the HE PA band and its associated PB at 160cm ~are due to the more stable bipolarons (BP). Polarons are the transient species: they decay faster and their recombination increases upon bias illumination, probably due to the BP which are photogenerated by the bias light. As in conducting polymers [1 I], two polarons of opposite charges may recombine to the ground state (P+ + P --* g.s.), whereas two polarons of the same charges may recombine into bipolarons (P+ + P+ --, BP 2+-). This can explain the transient PM seen in Fig. 2 where the generation of the H E PA band is delayed; it probably appears due to polaron (LE band) recombination. Furthermore the polaron enhanced recombination into BP may explain the unique feature shown in Fig. 3, where the H E band

Vol. 81, No. 5

(BP) intensifies with lh at the expense of the LE (P) band. Finally [19] we show that the different IL dependencies of the two PA bands can be readily explained within the polaron-bipolaron recombination. The recombination kinetics equations for the polaron density n under constant generation I~ may be written as: dn dt

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This result is consistent with the steady state 1°.6 dependence for the LE band (polarons). By contrast, the linear It dependence for the HE band (BP) is consistent with a monomolecular recombination kinetics that can be expressed as: dN dt

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Such monomolecular recombination kinetics can be understood to result from low BP mobility compared to that of polarons, which result in a trap-controlled (Shockley-Read type) recombination. In summary, by using the PM spectroscopy technique in BaBiO~ in a broad energy range and time interval, we have identified photogenerated polarons and bipolarons with distinct generation processes, recombination kinetics and relaxation times. These photogenerated defects are genuine electronic excitations of BaBiO3, which are created at the expense of electronic states in the continuum bands and consequently suppress the 3-D CDW.

Acknowledgements - The crystals came from the collection of the late J.P. Remeika of A.T. & T Bell Laboratories. We would like to thank Mrs A.S. Cooper for kindly supplying them. The work at the University of Utah was supported in part by the DOE grant no. DE-FG-02-89, ER-45409 and the work at the lstituto di Spettroscopia Molecolare was supported by the P.F. "SuCryTech" of CNR. We are grateful to Drs M. Marezio and M.J. Rice for helpful discussions. We also thank Dr J.F. Federici for sending us his manuscript [9] prior to publication.

CDW GAP STATES IN BaBiO3

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C. Taliani, R. Zamboni, G. Ruani, F.C. Matacotta & K.I. Pokhodnya, Solid State Commun. 66, 487 (1988). H.T. Grahm, C. Thomsen & J. Tauc, Opt. Commun. 58, 226 (1986). S. Tajima, S. Uchida, A. Masaki, H. Takagi, K. Kitazawa, S. Tanaka & S. Sugai, Phys. Rev. B35, 696 (1987). S. Uchida, S. Tajima, A. Masaki, S. Sugai, K. Kitazawa & S. Tanaka, J. Phys. Soc. Japan 54, 4395 (1985). By comparing the PM spectrum of BaBiO3 in the phonon region with those obtained for the semiconducting phases of the HT, cuprates perovskites [! 3] it has to be pointed out that the former does not show any PA bands. In the cuprate system the local deformation due to the photogenerated charge carriers relaxes the selection rules, making some Raman modes of the doped system to becomes infrared active. In the case of doped BaBiO3 which has a cubic ABO 3 structure there are no Raman active modes, therefore, we do not expect any photoinduced phonon band in BaBiO3. H. Sato, S. Tajima, H. Takagi & S. Uchida, Nature 338, 241 (1989). We also attempted to identify the two PA bands by measuring light induced ESR at 5K. We found a null result in both steady-state and double modulation. This indicates that most of the long-lived photogenerated carriers are spinless, consistent with spinless BP.