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Raman study of the high-Tc superconductors ABa2Cu3O7-x (A=Y, Ho, Y0.1Gd0.9 and Y0.9Sm0.1)

This article has been downloaded from IOPscience. Please scroll down to see the full text article. 1988 J. Phys. C: Solid State Phys. 21 L41 (http://iopscience.iop.org/0022-3719/21/3/001) View the table of contents for this issue, or go to the journal homepage for more

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J . Phys. C: Solid State Phys. 21 (1988) L41-L45. Printed in the UK

LETTER TO THE EDITOR

Raman study of the high-T, superconductors ABa,Cu,O,-. (A = Y, Ho, Y,.,Gd,., and Yo.9Smo.l) P Choudhury?, A K SuklaS, H S Mavi$, K P JainS, S C AbbiS, P Mandalt, A Poddart, A N Das? and B Ghosht t Saha Institute of Nuclear Physics, 92 Acharya Prafulla Chandra Road, Calcutta 700 009, India f Laser Technology Research Programme, Indian Institute of Technology, New Delhi 110 016, India

Received 3 November 1987

Abstract. The Raman spectra of polycrystalline A-Ba-Cu-0 (A = Y, Ho, YolGdo9 and Yo ,) are studied from 200 to 800 cm-' at room temperature. To distinguish the Cu-0 modes of the A-Ba-Cu-0 systems, the Raman spectrum of the Hc-Ba-Ni-0 system is also recorded. The assignment of the observed bands has been made by comparing spectra of these samples and on the basis of the information available in the literature. The mode at 338 cm-I, which shows anomalous behaviour at the superconducting transition temperature, is assigned to the Cu-0 mode of the ABa2Cu307-,compound. The other bands observed have been assigned to different phases present in these samples.

Recent interest in the high-T, superconductors has mainly been concentrated on the YBa-Cu-O system and its analogues. The phase which is responsible for the superconducting transition at about 90 K (T,) in Y-Ba-Cu-0 systems is Y B a 2 C ~ 3 0 7 -( xx = 0.1). The structure of this phase is orthorhombic (Cava et a1 1987, Le Page et a1 1987, Hazen et a1 1987, Tarascon et a1 1987) and consists of three perovskite layers such that the Y atoms are separated by C u 0 2 , BaO, CuO, BaO and CuOz planes. The oxygen vacancies in the middle CuO layers are ordered so that Cu-0 chains are formed. Vacuum annealing of the system removes oxygen from the centre CuO layers of the system and the material thus obtained is tetragonal in structure and a non-superconductor (Tarascon etal 1987, Izumi etal 1987). The substitution of Y in the Y B a 2 C ~ 3 0 7system -x with other trivalent rare-earth ions does not affect T, (Hor et a1 1987, Tarascon et a1 1987, Poddar et a1 1987,1988). On the other hand, the substitution of Cu with other transition-metal elements lowers T, appreciably (Xiao et a1 1987, Maeno et a1 1987a, b, Mandal et a1 1987). This indicates that Cu-0 chains play a vital role in the high-temperature superconductivity of these systems, but the fundamental mechanism which leads to the superconductivity has not yet been understood. Raman spectroscopy is an important tool to investigate the role of phonon participation in the superconductivity of these high-T, systems. Raman spectra of the Y-Ba-Cu-0 system have been studied by many workers (Batlogg er a1 1987, Hemley and Mao 1987, Udagawa et a1 1987, Liu et a1 1987, Macfarlane et a1 1987, Rosen er a1 1987, Stavola et a1 1987, Iqbal et a1 1987). However, there is not much resemblance in the spectra recorded by them. This may be due to the 0022-3719/88/030041 + 05 $02.50 @ 1988 IOP Publishing Ltd c3

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letter to the Editor

fact that YBa2Cu307-, is difficult to obtain in pure single-phase form; generally, other non-superconducting phases such as Y2BaCu05,BaCu02etc are also present with the superconducting phase. The presence of these phases in varying proportions would give different spectra. In order to identify the vibrational modes of YBa,Cu,O,-, systems, we have studied Raman spectra of A B a 2 C ~ 3 0 7 -(A x = Y, Ho, Yo.lGdo.9 and Yo,9Smo.l) systems. We have also studied the Raman spectra of HoBa2Ni307-, systems to distinguish the Cu-0 modes. Assignment of the bands has been made comparing our spectra of different samples and information available in the literature. The samples used in this study had the nominal stoichiometry corresponding to ~ O were ~ - ~ . AB~,CU~O (A~=- Y, ~ Ho, Yo.lG4.9 and Yo,9Smo,l)and H O B ~ ~ N ~They prepared by the solid state reactions from the mixture of appropriate amounts of A 2 0 3 , B a C 0 3 and CuO or NiO. For A = Yo.lGdo,9and Yo.9Smo.lsystems, we followed the preparation procedure as described in our earlier communication (Poddar eta1 1987a, b) ; the only difference was that this time, after sintering, the furnace was cooled slowly ~-~, and H O B ~ * N ~ systems ~ O ~ - the ~ to 400 "C in 6 h. For Y B ~ , C U ~ OHoBa2Cu307-, mixture was heated in flowing oxygen at 900 "C for 24 h and 940 "C for 15 h with intermediate grinding. The product was then powdered, pressed into pellets at 5 kbar pressure and sintered at 940 "C in oxygen for 17 h and then the furnace was cooled slowly to 400 "C over 6 h. Superconductivity in all the Cu systems was confirmed by resistivity and magnetic moment measurements prior to Raman experiments. Raman scattering experiments were performed from pellets of these samples in a standard back-scattering geometry using a Jobin-Yvon Ramanor HG-2S double monochromator and photon counting system. The excitation power was 150-250mW at 5145 8, from a coherent Innova-90 argon-ion laser. The laser beam was focused to a point on the sample with a maximum power density of 125 W cm-2. The spectral slit width was held constant to 500pm throughout the experiment. Raman data were recorded at room temperature from 200 to 800cm-' with a scanning speed of 1 cm-' min-' and the integration time 10 s of the photon counting mode, Raman spectra of YBa2Cu307-,, Yo.1G&,9Ba2C~307-, and Yo,9Smo,lBa2Cu307-, systems from 200 to 800 cm-' at room temperature are shown in figure 1.Figure 2 shows the same for HoBa2Cu307-, and HoBa2Ni307-, systems. The peak positions and the assignment of the bands (in the possible cases) are given in table 1. We have mentioned earlier that the samples of ABa2Cu307-, used in this study are expected to have phases like A2BaCu05,BaCu02 etc. Moreover, because of large Raman cross section and relatively large transparency of the impurity phases at an excitation wavelength of 5145 8, in comparison with the ABa2Cu307-, phase (Rosen et a1 1987) the Raman lines of these phases occur with reasonable intensity. From table 1it is seen that the bands at 338,370 and 572cm-' appear in the Cu systems but not in the Ni system. Hemley and Mao (1987) observed a band at 338 cm-I in the superconducting composition YBa2Cu307-,. Macfarlane et a1 (1987) detected anomalous behaviour of this band near the T, for this composition. This suggests that the 338 cm-' band corresponds to the Cu-0 mode of the superconducting phase. Udagawa er a1 (1987) observed a 370cm-' band in the composition Y2BaCu05and its intensity is very weak in comparison with the intensity of the 391 cm-' band of the same phase. However, we observed a weak band at 391 cm-' whereas the intensity of the 370 cm-' mode is reasonable. Hence, the 370 cm-' band corresponds to a Cu-0 vibration but at present it is not possible to assign the phase due to which it is appearing. The 572 cm-' band, which is seen here for the first time in the A-Ba-Cu-0 system, isvery weak and we are not sure whether this band does not appear in the Ni system because of its weakness or because it is a band corresponding to a

Letter to the Editor

300

L43

400

500

600

700

Wavenumber shift ( cm-l ) Figure 1. The Raman spectra of YBa2Cu307_,, Yo,Gd,,Ba2Cu307-, and Yo ,Ba2Cu3O7-, compounds. Excitation: 5145 A, 15G250 mW. Scanning speed 1 cm-’ min-’. The background has been subtracted.

I

I

I

I

I

300

400

500

600

700

Wavenumber shift

( cm-’ )

Figure 2. The Raman spectra of HoBa2Cu307_, and HoBa2Ni7_,compounds. Specifications are as given in figure 1.

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Letter to the Editor Table 1. Raman frequencies (cm-I) and assignments of the Raman lines of YBa2Cu307_, r C), (sample A), Yo lGd,,9Ba2C~317-r(sample B), Y O 9 S 91 B a 2 C ~ 3 0 7 - (sample HoBa2Cu307_,(sample D) and HoBa2Ni307.., (sample E) at room temperature. A stands for Y, Ho, Gd or Sm. A

B

C

D

E

Assignments

222 318 338

220

221

220 315

338

336

220 318 338

372 396 413 432 442 452 462 480 503 557 572 598 611 634 672 726 739 724

370

370 392 412 432 441 449 462 478 501 556 571 596 610 633 674 724 739 771

A,BaCuO, phase A2BaCu05phase Cu-0 vibration; ABa,Cu307_,phase Ni-0 vibration Cu-0 vibration

348

410 441 450 460 503 557 571 609 672 724 739 772

370 410

410

440 450 460

440 450 458 478 502 556

502 556 572 610

597 610

672 725 739 770

670 723 738 772

A,BaCuO, phase A,BaCuOS phase Ba-0 vibration; ABazCu3O7-,phase A,BaCuO, phase ABa2Cu307_,phase A2BaCu05phase BaCu02phase

Cu-0 vibration. Thirteen bands, at frequencies 220,410,441,450,460,502,556, 597, 610, 672, 724, 739 and 772cm-', appear in the Raman spectra of all the samples we studied. Of these, the bands at 220,441,556 and 610 cm-' have been observed by one group or the other in YBazCu05phase (Udagawa et af 1987, Rosen et a1 1987, Hemley and Mao 1987). The 502 cm-' band is a strong one and has been detected by several groups of workers (Batlogg et a1 1987, Liu et af 1987, Macfarlane et a1 1987, Iqbal et af 1987). Udagawa eta1 (1987) and Rosen et a1 (1987) did not observe this line in Y2BaCu05 or BaCu02 compounds. Liu et af (1987) and Iqbal et af (1987) assigned this band to the Cu-0 stretching modes. We, however, observed this band in the Ni sample also. Moreover, BaO has a band near this frequency. This indicates that the 502 cm-' band x The 597 cm-' band arises from corresponds to the Ba-0 mode of A B a 2 C ~ 3 0 7 -phase. the superconducting phase and this has also been observed by Hemley and Mao (1987). The 410, 450, 460, 672, 724, 739 and 772 cm-' bands have not been observed by any other workers. We do not assign them to any particular phase. Besides these bands, we have also observed some weak lines. They did not appear in all the analogous samples, probably due to rough scattering surfaces. The 318 and 478 cm-' bands correspond to the A2BaCu05phase (Udagawa et a1 1987, Rosen et a1 1987, Hemley and Mao 1987). The 348 cm-' band appears only in the compound and it is assigned to the Ni-0 mode. The 432 cm-' band observed in two systems (YBa2Cu307 and Y,,,Smo,1Ba,Cu,07 -J may arise from the superconducting phase as was observed by several workers (Iqbal et af 1987, Liu et a1 1987, Stavola et af 1987, Batlogg et a1 1987). We observed a very weak line at 634 cm-' in Y B a 2 C ~ 3 0 7 - x

Letter to the Editor

L45

and Yo,$mo,lBa2Cu307--* systems which coincides with the strongest line of BaCuOz (Stavola et a1 1987). This indicates that the amount of BaCuOz phase in our samples is very small. In conclusion, we have assigned the bands observed in A-Ba-Cu-0 systems to different phases present in our samples. Macfarlane et a1 (1987) observed an anomalous behaviour in the temperature-dependent frequency shift of the 338 cm-' mode in the vicinity of the transition temperature of the YBa2Cu307-xcompound. Our Raman study has confirmed that this band is arising from a Cu-0 vibration of this compound. This suggests that Cu-0 vibration is coupled to the superconducting transition (order parameter) of the system and supports the idea that Cu-0 modes play an important role in the superconducting transition of ABa2Cu307 systems. We thank Professor C K Majumdar and Mr S Ghosh for their kind interest in this work and Mr S N Datta and Mr A Pal for technical help.

References BatloggB,CavaRJ, JayaramanA,vanDoverRB, KourouklisGA, SunshineS,MurphyDW , R u p p L W , Chen H S, White A, Short K T, Mujsce A M and Rietman E A 1987 Phys. Rev. Lett. 58 2333 Cava R J , Batlogg B, van Dover R B, Murphy D W, Sunshine S , Siegrist T, Remeika J P, Rietman E A, Zahurak S and Espinosa G P 1987 Phys. Rev. Lett. 58 1676 Hazen R M, Finger L W, Angel R J, Prewit C T, Ross N L, Mao H K and Hadidiacos C G 1987 Phys. Rev.B 35 7238 Hemley R J and Mao H K 1987 Phys. Rev. Lett. 58 2340 Hor P H, Meng R L, Wang Y Q , Gao L, Huang Z J , Bechtold J, Forster K and Chu C W 1987 Phys. Rev. Lett. 58 1891 Iqbal Z , Steinhauser S W, Bose A, Cipollini N and Eckhardt H 1987 Phys. Rev. B 36 2283 IzumiF, AsanoH, IshigakiT, Takayama-Muromachi E , UchidaY, WatanabeN andNishikawaT 1987Japan. J. Appl. Phys. 26 L649 Le Page Y , McKinnon W R, Tarascon J M, Greene L H, Hull G Wand Hwang D M 1987 Phys. Rev. B 35 7245 Liu R , Merlin R , Cardona M, Mattausch H , Bauhofer W, Simon A, Garcia-Alvarado F, Moran E, Vallet M, Gonzalez-Calbet J M and Alario M A 1987 Solid State Commun. 63 839 Macfarlane R M, Rosen H and Seki H 1987 Solid Stare Commun. 63 831 Maeno Y, Nojima T, Aoki Y, Kato M, Hoshino K, Minami A and Fujita T 1987a Japan. J . Appl. Phys. 26 L774 Maeno Y ,Timita T, Kyogoku M, Awaji S , Aoki Y , Hoshino K, Minami A and Fujita T 1987b Nature 328 512 Mandal P, Poddar A , Choudhury P, Das A N and Ghosh B 1987 J . Phys. C: Solid State Phys. 20 L553 Poddar A, Mandal P, Choudhury P, Das A N and Ghosh B 1987 J . Phys. C: Solid State Phys. 20 669 -1988 J . Phys. C: Solid State Phys. submitted Rosen H, Engler E M, Strand T C, Lee V Y and Bethune D 1987 Phys. Rev. B 36 726 Stavola M, Krol D M, Weber W, Sunshine SA, Jayaraman A, Kourousklis G A, Cava R J and Rietman E A 1987 Phys. Rev. €3 36 850 Tarascon J M, Mckinnon W R, Greene L H, Hull G Wand Vogel E M 1987 Phys. Rev. B 36 226 Udagawa M, Ogita N, Fukumoto A, Utsunomiya Y and Ohbayashi K 1987Japan. J . Appl. Phys. 26 L858 Xiao G , Streitz F H , Garvin A, Du Y W and Chein C L 1987 Phys. Reu. B 35 8782