Isotope effect and 70 meV feature in YBa2Cu3O7−x single crystals

PHYSICA

Physica C 194 (1992) 411-414 North=Holland

Isotope effect and 70 meV feature in YBa2Cu307_x single crystals A.V. B a z h e n o v a n d K.B. R e z c h i k o v Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka. Moscowdistrict. 142432, Russia

Received 28 January 1992

Substituting ~sOfor ~60 in superconductingYBa2Cu3Ov_xsingle crystals suggests that the 70 meV feature involvesan electron transition and a phonon.

1. Introduction

ing oxygen content in the superconducting crystal.

The ~ 7 0 meV (570 cm -~ ) feature active in the EII C polarization was observed in FIR spectra of

2. Experimental

YBa2Cu307_x ( 123 ) single crystals [ 1-3 ]. Different opinions were expressed about its nature. In refs. [ 1,2 ] we have investigated single crystals with Tc = 60 K and attributed the 70 meV feature to the electron transition between the Cu 3d,..~+y., + O 2p.,_,ybands of the cuprate planes. An analogous line was observed in 123 powder transmission spectra [4] and in Raman spectra [5 ], being interpreted as a vibrational mode due to crystal defects, namely, vacancies of a bridging oxygen 0 ( 4 ) [5]. In ref. [6] the 569 cm -~ line was interpreted as a normal optical mode governed by an antiphase motion of C u ( 1 ) and O( 1 ) + 0 ( 4 ) atoms along the C-axis of the crystals (Cu( 1 ) and O( 1 ) denote atoms of the chain plane, O (4) apical oxygen ). In the case of a phonon nature of this feature, the vibrations of oxygen atoms have to make a dominant contribution to this mode because of its high energy. Therefore, this feature should shift towards the long-wave side with substitution of ~60 with ~80. If the 70 meV feature is an electron transition, then its spectral position will not alter upon isotopic substitution. In order to elucidate the nature of the 70 meV feature we have investigated the reflection spectra of 123 single crystals upon the isotopic oxygen substitution. It appeared to be a combination of an electron transition and a phonon. The phonon intensity in relation to the electron transition increases with increas-

The reflectance spectra were measured at T = 300 K for the Ell C polarization in the spectral range from 50 to 5500 cm-~. A broad spectral range ensured a correct calculation of the dielectric function by the Kramers-Kronig method in the range from 50 to 700 cm-~. 123 single crystals with typical dimensions 2 × 2 × 0 . 3 mm 3, grown by the flux method in an alundum crucible, were studied. The samples were doped with A1 and had the composition YBa2Cu3_.~fl~.l.vO7_x, where y = 0.1. "As-grown" crystals were superconductors with a broad transition near 40 K as a result of inhomogeneous oxygen distribution and oxygen deficiency. The isotopic substitution was conducted as follows. The samples placed in a quartz ampule were annealed in a vacuum of 10 -5 bar at 450°C for 6 h. At this stage chain oxygen was removed from the crystals. Then, the temperature was raised to 600°C and ~802 isotope was supplied to the ampule at a pressure somewhat lower than 1 bar. Finally, the temperature was increased to 840°C to ensure an effective migration of any oxygen atoms in the crystal and the samples were annealed for 13 h. The samples were cooled at a rate of 15°C/h. At 550°C and 400°C the cooling was interrupted for 12 and 48 h, respectively, so that the bridging ( 0 ( 4 ) ) and chain (O( 1 ) ) oxygens be ordered. The prepared samples became

0921-4534/92/$05.00 © 1992 ElsevierSciencePublishers B.V. All rights reserved.

412

A. F,, Bazhenov, K.B. Rezchikov / 70 me Vfeature in YBa.,Cu~OT_x

superconducting at Tc=93 K, the transition width AT¢ being smaller than 0.3 K (measured by a shielding method). Single crystals with J60 and T¢=91 K were prepared under the same annealing conditions as isotopic substitution. Besides these, we have investigated 123 single crystals with To=80 K (ATe=3 K) prepared with the help of low-temperature heat treatment in oxygen atmosphere at 400°C for two weeks. 3. Experimental results and discussion

Figure 1 shows the reflectance spectrum R and imaginary part of the dielectric function e2 of superconducting 123 single crystals with T¢=80 K. The 570 c m - t feature consists of a comparatively narrow line at 568 c m - t ( T = 300 K) and a shoulder starting at 510 cm-~. Single crystals with T¢ < 80 K have a feature similar to that shown in fig. 1. No such feature is observed in the nonsuperconducting phase. It appears in the spectrum when the distance between the CuO2 plane and oxygen 0 ( 4 ) decreases in a stepwise fashion with increasing oxygen content in the crystal [7,8]. The intensity of the shoulder in the spectrum of e2 equals approximately 5 relative to the

background. It should be noted that the 568 c m line intensity, in relation to the shoulder, increases appreciably with growing Tc and, accordingly, oxygen content in the crystal. This is clearly seen when comparing the spectra of fig. 1 (To=80 K) with the spectra of single crystals with T~=90-93 K (figs. 2 and 3). In the latter case, the shoulder transforms to the band at 510-625 cm -t. The spectrum of an 'SO-annealed sample with T~= 93 K for Ell C polarization exhibits a long-wave shift of all optical phonons with energy above 120 c m - t (figs. 2 and 3). Table 1 shows the observed transverse ( T O ) optical phonon frequencies at T = 300 K and their relative shifts after isotopic substitution. Also given are the relative shifts for complete isotopic substitution in 123 ceramics [9]. In 1.0

I

I

I

I

I

I

I

I

I

I

0.8

0.6

O.,tl

I

0

200

I

400

600

/~,cm"I

Fig. 2. Reflectance spectra of YBa2Cu3OT_x single crystals with Tc=93 K containing ~60 (solid line) and tsO (dash). EIIC, T=300 K.

1O0 0.80 060

~2

0.40

8O

I

I

I

rY

x•ftl

020 0 80 40

\ /v

40

0 0 200

400

600

/~,cm "1

Fig. h Reflectivity R and imaginary part of the dielectric function ~2 as a function of frequency for the YBa2Cu307_ x single crystals ( Tc = 80 K) in the Ell C polarization. The solid line shows the feature under study. T= 300 K.

I 200

I

400

I

i

600

7}, cra "~

Fig. 3. Imaginary part of the dielectric function as a function of frequency for YBa2Cu3Ov_xsingle crystals (Tc=93 K). e2 was obtained by the Kramers-Kronig transformation of the spectra of fig. 2. Designations are analogous to fig. 2. Dot-dash line is a guide to the eye. EUC, T=300 K.

413

A. F. Bazhenov, K.B. Rezchikov / 70 reeFfeature in YBa2CusOz_x

Table 1 Frequencies of IR transverse optical (TO) phonons for EIIC polarization and RS-activeA8 mode (R) in YBa2Cu3OT_x single crystals with oxygen ~80 and ~60. 8v/v is the relative change of the phonon frequency at the isotopic oxygen substitution. Also given are 5u/v measured in ceramic with complete oxygen isotope substitution [9] v (cm -t )

0(4) - ? 0(2,3) 0(2,3) Y Ba,O(4) Cu(2) O(4)(R)

~vlu (%)

YBa2Cu3t607

YBa2Cu318Ov_x16Ox

Exp.

Exp. [9]

572 313 291 197 154 119 500

559 303 285 194 151 119 482

2.3+0.2 3.2+0.3 2.1 _+0.4 1.5_+0.3 1.6_+0.3 0.0_+0.3 3.6_+0.2

4.4+0.2 5.0_+0.3 5.3_+0.4 1.6+0.5 1.6_+0.7 5.5_+0.4

the spectral range of main interest, the relatively narrow line Uxo= 572 cm-~, halfwidth A u = 16 cm-~, is displaced to the long-wave side, becoming UTo= 559 c m - t as a result of the isotope effect. However, the 510-625 cm -~ band did not change its spectral position. So, this experiment shows incontrovertibly that the 70 meV feature is a superposition of an electron transition ( 510-625 c m - 1band ) and an optical phonon (Vxo= 572 cm-~ ) whose energy is markedly contributed to by oxygen atom vibrations. Certain doubts may arise in the interpretation of the high-energy part of the 510-625 cm-~ band as an electron transition in the crystal with Tc=93 K. The fact is that the spectrum of the crystal with To=80 K involves a weak line 608 cm -t. This line shifts to 640 c m - ~and its intensity grows with decreasing oxygen content in the crystal. In this case, the phonon nature of this line determined by the stretching vibration of oxygen 0 ( 4 ) is indubitable [1 ]. If one assumes that the percentage of the t 6 0 ( 4 ) substitution with t80(4) in our case is small, then the highenergy part of the 510-625 cm-~ band could be explained in terms of phonon presentations by a small ~80(4) content in the investigated crystals and its nonuniform distribution in the sample. To estimate the degree of oxygen substitution we have used the following simple considerations. The eigenvectors of IR-active phonons generally involve all atoms in the unit cell s o a s to fulfill the requirement of center-of-mass invariance. Optical phonons active in Raman scattering (RS) are more simple. The RS-active 500 cm-~ stretching mode is gov-

erned predominantly by vibrations of apical 0 ( 4 ) atoms [9]. If ~60 and ~so atoms are uniformly distributed in a random fashion in oxygen sites of the crystalline lattice, then the oxygen effective mass 1/ m * = x / 1 8 + ( 1 - x ) / 1 6 , where x is the quota of ~so and ( 1 - x ) that of t60. In this case, a change of the phonon frequency 5u/v at the isotopic substitution complies with x = 9 8 u / u ( 2 - ~ u / u ) = 18~u/v for vibrational modes with a dominant contribution from oxygen atoms to their eigenfrequencies (u is the phonon frequency in a crystal with t60). We have investigated the change in the stretching vibration of 0 ( 4 ) of Ag symmetry using a RS method. In RS spectra this line is very intensive, in contrast to that from IR-spectra. It turned out that as a result of the isotopic substitution this RS-active mode shifted from 500 cm - t (Tc=93 K), usual for a crystal with ~60, to 482 cm -~. The width of this line did not change as a result of isotopic substitution and no additional lines appeared in the RS spectra. So, RS study suggests that an oxygen isotope is distributed uniformly in the crystal. The isotopic shift of this line was found to be fir~u=3.6+_0.2% so that 65+_3% of oxygen O (4) is substituted with ~so. In RS-spectra of 123 ceramic samples with complete oxygen substitution by ~so, the frequency of this mode is 471 c m - t , 5 u/v = 5.5 _+0.4% [9 ]. Simple division of the experimentally observed isotopic shift, 3.6%, by the relative shift for complete isotopic substitution, 5.5%, gives the same percentage for the 0 ( 4 ) substitution. An analogous procedure gives the 65% substitution of the CuO2 plane oxygens, 0 ( 2 ) and 0 ( 3 ) . The

414

A. V. Bazhenov, K.B. Rezchikov / 70 meV feature in YBa2Cu~O7_~

above analysis shows that, if the high-energy threshold o f the 510-625 c m - ~band is governed by the IRactive stretching vibration o f O (4), it should shift to the long-wave side by an easily measurable value of 20 c m - 1. So, the results presented in figs. 2 and 3 indicate unambiguously the electron nature of the whole 510-625 cm -~ band. The nature o f the 572 cm-~ phonon is unclear. In ref. [6] it was regarded as a normal mode governed by the antiphase motion o f C u ( 1 ) and O( 1 ) + 0 ( 4 ) atoms along the C-axis o f the crystal. In this case one has to suggest the interaction o f this mode with the 608 c m - ~mode for the explanation o f their intensity alteration, that is, the transfer o f the oscillator strength of the phonon v > 600 c m - t to the 572 c m mode with increasing oxygen content in the crystal. The phonon nature o f the 6 0 8 - 6 4 0 cm-~ line (Ell C) determined by the stretching vibration of oxygen 0 ( 4 ) is indubitable for low oxygen content [ 1 ]. In our opinion, the 572 c m - line is due to the same stretching vibration of the apical oxygen O (4) at high oxygen content. There may exist a two-well potential for the oxygen 0 ( 4 ) . A change in the occupation o f potential wells under increasing oxygen content may be accounted for by the transfer of the line intensity v > 6 0 0 cm -~ to the 572 cm -~ mode. Another, perhaps more realistic, possibility is an interpretation o f the 572 and v > 6 0 0 cm -~ lines as 0 ( 4 ) stretching modes of the two-coordinated Cu ( 1 ) (Cu(1)+20(4)) and four-coordinated C u ( 1 ) (Cu( 1 ) + 2 0 ( 4 ) + 2 0 ( 1 ) ) configurations characteristic o f the insulating and metallic phases o f the 123 compound, respectively. So, the intensities o f these lines are determined by the concentrations o f these two configuarations as a function o f the oxygen content.

4. Conclusion Substituting ~80 for 160 oxygen in YBa2Cu3OT_x

single crystals with To= 93 K has been employed to show that the 70 meV feature active in the Ell C polarization is a combination o f the low-energy electron transition and a phonon. This combination may be interesting as a mediator for the pairing o f free carriers in high-To superconductors. Besides, it is possible to answer the question, currently being discussed, about one-electron approximation for the description o f the spectrum o f electronic states in high-T¢ superconductors. The observed electronic transition is evidence for the applicability o f these calculations, in particular, the theory [ 10] for the description o f the low-energy electron transitions in YBa2CuaO7_x. In accord with ref. [10], the origin o f the 510-625 cm -~ electron transition is the existence of two nearly degenerate quasi-two-dimensional bands, Cu 3d.,.2+ ,2 + O 2px,y, related to the existence o f two CuO2 planes per unit cell.

References [I]A.V. Bazhenov and V.B. Timofeev, Physica C 162-164 (1989) 1247. [2] A.V. Bazhenov and V.B. Timofeev, Spct 3 (1990) 1174. [3] A.V. Bazhenov, in: USSR-FRG Bilateral Seminar (TaIlin, 1989), Investigation Of High Temperature Superconductors, ed. L.A. Chernozatonsky (Moscow, 1990) p. 149. [4] G. Ruani, C. Talliani, R. Zamboni, D. Citlone and F.K. Matacotta, Physica C 153-155 (1988) 645. [ 5 ] K.F. McCarty, J.C. Hamilton, R.N. Shelton and D.S. Ginley, Phys. Rev. B 38 (1988) 2914. [6] L. Genzel, A. Winlin, M. Bauer, M. Cardona, E. Schonherr and A. Simon, Phys. Rev. B 40 (1989) 2170. [7]A.V. Bazhenov, A.V. Gorbunov and K.B. Rezchikov, Physica C 176 (1991) 35. [8]R.G. Cava, A.W. Hewat, E.A. Hewat, B. Batlogg, M. Marerio, K.M. Kabe, J.J. Krajewski, W.F. Peck and L.W. Rupp, Physica C 165 (1990) 419. [9] C. Thomsen, Hi. Mattausch, M. Bauer, M. Bauhofer, R. Liu, L. Genzel and M. Cardona, Solid State Commun. 67 (1988) 1069. [10] E.G. Maksimov, S.N. Rasheev, S. Ju. Savrasov and Ju.A. Uspenskii, Phys. Rev. Lett. 63 (1989) 1880.