ramans study of y1−xprxba2cu3o7−δ and yb2cu3−xznxo7−δ single crystals

ELSEVIER Physica C 250 (1995)275-281

PHYSICA

Raman study of Yl_xPrxBa2Cu307_8 and YBa2Cu3_xZnxO7_ single crystals

H y e - S o o Sh in a, I n - S a n g Y a n g a, * , W.-C. Lee b

a Department of Physics, Ewha Womans University, Seou1120-750, Korea b Department of Physics, Sookmyung Women's University, Seoul, 140-742, Korea

Received 22 March 1995; revised manuscript received 11 May 1995

Abstract

Single crystals with known T c values of Yl_xPrxBa2CU3OT_ 8 (Y-Prl : 2 : 3) and YBa2Cu3_xZnxO 7_ ~ (Y-Znl : 2 : 3) systems are studied by Raman measurements. The Raman spectra for Y-Prl : 2 : 3 single crystals show that the frequencies of Ba and O z modes increase as the Pr content increases. The results are consistent with the hole-localization scheme proposed for the suppression of superconductivity in the polycrystalline Y - P r l : 2 : 3 systems. On the other hand, in the Y-Znl : 2 : 3 system, all the Raman modes do not change in frequencies. However, the FWHM of the Cu(2) mode increases with the decrease of T c, indicating strong scattering of charge carriers by the substituted Zn ions in the CuO 2 planes. The induced disorder in the CuO 2 planes may be related with suppression of T c in the Y - Z n 1 : 2 : 3 system. Thus, the suppression mechanism in the Y-Znl : 2: : 3 systems seems to be different from that in the Y-Prl : 2 : 3 systems.

1. Introduction

It is well known that the Yl_xPrxBa2Cu307_ ~ ( Y - P r l : 2 : 3) and YBa2Cu3_xZnxO7_ 8 ( Y - Z n 1 : 2 : 3 ) systems show strong suppression of superconductivity by Pr or Zn ions. The Y - P r l : 2 : 3 system is interesting because it is isostructural to the Y1 : 2 : 3 system, yet the superconductivity is strongly suppressed with increasing Pr content [1,2]. The substitution of Cu in Y1 : 2 : 3 by the transition met- als, particularly Zn, results in the suppression of the transition temperature (To) , too [3,4]. The divalent Zn ion is nonmagnetic, and is believed to occupy the Cu(2) site at low concentration [5].

There are various suggestions for the suppression of T c when Pr is substituted in Y1 : 2 : 3, such as the

* Corresponding author.

possibility of a mixed valence state of Pr-ions [6], pair-breaking mechanism of the Pr ions [1,2], Pr 4 f - C u / O hybridization [7], and hole localization at the B a - O planes [8]. There are some explanations for the suppression of T c in Y - Z n l : 2 : 3 , yet its mechanism is also not dearly understood. Several mechanisms have been proposed, including the dis- ordering of the CuO 2 planes [9] and the decrease of the carder concentration at room temperature due to the Zn ions [10].

Yang et al. have measured Raman spectra of the polycrystalline Y - P r l : 2 : 3, E u - P r l : 2 : 3, E r - Prl : 2 : 3 and Y - Z n l : 2 : 3 systems in order to study the suppression mechanism of superconductivity in the 1 : 2 : 3 systems [11]. They found no change in the frequency of Raman modes of the Y - Z n l : 2 : 3 systems in contrast to the increase in the frequency of Ba and O z modes with increase of Pr content in

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276 H.-S. Shin et al. / Physica C 250 (1995) 275-281

the Pr-doped 1 : 2 : 3 systems. The binding energies of the core-levels studied by X-ray photoemission spectroscopy (XPS) of the Y - Z n l : 2 : 3 system did not change at all within the experimental resolution. However, the binding energies of the Ba core-levels shifted to higher values as the Pr content increased in the Pr-doped systems, Y-P r l : 2 : 3, Eu-Pr l : 2 : 3 and E r - P r l : 2 : 3 . The shifts in the binding energies of the Ba core-levels in the Pr-doped 1 : 2 : 3 systems occur regardless of the relative magnitude of the magnetic moment of the Y-site ions. From these various observations, they suggested that holes are localized in local bands, such as Ba 5d, in the Pr-doped 1 : 2 : 3 systems and that a different suppression mechanism is responsible in the Y - Znl : 2 : 3 systems.

However, polycrystalline Y - P r l : 2 : 3 ( Y - Z n l : 2 : 3 ) samples could be mixtures of different single crystals with various Pr (Zn) contents and, thus, with different values of T c. Therefore, it is important that we study the Raman spectra of Y - Prl : 2 : 3 and Y - Z n l : 2 : 3 single crystals to compare with the results of polycrystalline samples. In this paper, for the first time, single crystals with known T~ values of the Y - P r l : 2 : 3 and Y - Z n l : 2 : 3 systems are studied by Raman measurements.

2. Experimental

0.2

-0.2

~ -0.6

~ -1.0

~ -1.4

@ -I .8 E

-2.2

-2.6 0

• • • • • • l l l l l l l l l ~ | I I

Reld-Cooled

o

Y-Pr 1:2:3, Tc = 71K H= 1G, H//c

Zero-Field-Cooied #

P ° l ° q [] P o 'oooP~ I

5O I I

100

1.0 A

E -I.0

-3.0

¢i E -5.0

-7.0

Field-Cooled ~ m •

Y-Zn 1:2:3, Tc = 60K a

H= 1G, H / / c o

o Zero-Field-Cooled

o o D o o o o i11) o

I I I I I !

0 40

I

80

Temperature (K)

Fig. 1. Magnetization curves for typical Y - P r l : 2 : 3 and Y - Zn1:2:3 single crystals. The magnetic field of I G was applied in the direction parallel to the c-axis of the crystals.

ization of the incident wave. The slit widths were set at 500 Ixm and thus the overall resolution of the spectra was about 4 -5 cm-1.

The single crystals of Y - P r l : 2 : 3 and Y - Znl : 2 : 3 were grown by heating CuO-rich fluxes of appropriate powders in zirconia crucibles. The values of T c were determined from magnetization measurements (Fig. 1) using a SQUID magnetometer. The Tc's of the Y - P r l : 2: 3 crystals decreased from 88 to 33 K (and 0 K for P r l : 2 : 3 ) and those of Y - Znl : 2 : 3 crystals from 85 to 56 K [12].

Raman spectra were measured at room temperature using a Jobin Yvon U1000 double monochroma- tor. The 514.5 nm or the 488.0 nm line of an Ar-ion laser was used as the excitation source for the spectra. The laser power used was below 100 p,W to avoid unnecessary heating of the crystals. The diam- eter of the laser spot on the sample surface was about 2 -3 Ixm. The incident light was polarized linearly and the scattered light was analyzed in the parallel or in the perpendicular direction to the polar-

3. Results and discussion

Figs. 2 and 3 show the Raman spectra of the Y - P r l : 2 : 3 and Y - Z n l : 2 : 3 systems in Y(ZZ)}" geometry. The sharp line near 116 cm -1 is an As-plasma line when the 514.5 nm wavelength is used. In order to avoid the plasma line near the Ba mode ('-, 118 cm-1), a 488.0 nm wavelength was used in measuring the spectra of the Ba and Cu modes (Figs. 4 and 5). These figures show Ba Alg peaks clearly for both the Y - P r 1 : 2 : 3 and Y - Znl : 2 : 3 systems.

From the Raman spectra of Y - P r l : 2 : 3 single crystals (Figs. 2 and 4), the Ba A ls mode and the apical oxygen O~ Als mode are observed to shift towards higher frequency as the Pr-ion content increases. Normally, substitution by a larger atom (Pr) for a smaller atom (Y) would result in the decrease

H.-S. Shin et al. / Physica C 250 (1995) 275-281 277

l - : 3

._z, t -

O

E w

100 200 300 400 500 600 700 Raman Shift(era 1 )

Fig. 2. Raman spectra of Y - P r l : 2 : 3 single crystals in Y(ZZ)~" geometry excited by the 514.5 nm line of an Ar-ion laser. The T c value of each crystal is denoted.

0 E

Y-Pr1:2:3 Y(Z Z)~( 488.0nm cu(2)

Pr 1::2:3

I I 120 140

I 160

Raman Shift(cm -1 )

Fig. 4. Raman spectra of Y-Pr l :2 :3 single crystals in Y(ZZ)Y geometry excited by the 488.0 nm line of an At-ion |ascr.

e -

¢.,.

Y-Zn1:2:3 Y(Z Z)~( I 514.5nm Oz I

Cu(2) ~J~ I

i

I I I I I

100 200 300 400 500 600 700

Raman Shift(era 1 )

Fig. 3. Raman spectra of Y - Z n l : 2 : 3 single crystals in Y(ZZ)~" geometry excited by the 514.5 nm line of an Ar-ion laser. The T¢ value of each crystal is denoted.

E

Y-Zn 1:2:3 488.0nm /

! !

120 140

Cu(2) Y(Z Z)C(

!

160 Raman Shift(era -1 )

Fig. 5. Raman spectra of Y - Z n l : 2 : 3 single crystals in Y(ZZ)}" geometry excited by the 488.0 nm line of an Ar-ion laser.

278 H.-S. Shin et aL /Physica C 250 (1995) 275-281

in the Raman frequencies due to expansion of the lattice parameters [11]. Therefore, it is believed that the increase of the Raman frequency of the Ba and the O z modes of the Y - P r l : 2 : 3 system is not due to changes in the bondlengths themselves, but due to changes in the electronic structure induced by the substituting Pr ions.

In contrast, the frequencies of the Raman modes in Y - Z n l : 2 : 3 single crystals (Figs. 3 and 5) do not show any change with increasing,Zn concentration. The ionic radius of Zn 2+ is 0.74 A and is similar to that (0.72 ,~) of Cu 2+. Raman results mean that, when Cu(2) is substituted by Zn in the Y 1 : 2 : 3 structure, the force constant of the Zn-O bonds is almost the same as that of the Cu-O bonds.

The Big mode of the O(2), 0(3) atoms in the CuO 2 planes is shown in Figs. 6 and 7. These spectra are measured from the xy surfaces of the crystals in Z(XX)Z geometry. The intensity of the Oz Alg mode is very weak for both Y - P r l : 2 : 3 and Y - Z n l : 2 : 3 systems, indicating that the xx or yy element of the scattering tensor of the O z A lg mode is much smaller than the zz element throughout the series of crystals [13]. The Big mode of the Y - Z n l : 2 : 3 system is much sharper than that of the Y - P r l : 2 : 3 system, even though the CuO 2 plane

O(2l-O(3) s lg Y-Pr1:2:3 Z(X X)Z 1

300 400 500 Raman Shi f t (cm I )

Fig. 6. The Big mode of 0(2), 0(3) atoms m the CuO 2 planes of Y-Prl : 2:3 single crystals in Z(XX)Z geometry.

I A0(2)-0(3) B 1 g

300 400 500 Raman Shift(cm ~ )

Fig. 7. The Big mode of 0(2), 0(3) atoms in the CuO 2 planes of Y-Zn1:2:3 single crystals in Z(XX)Z geometry.

itself is disrupted in the Y - Z n l : 2 : 3 system. This supports that, in the Y 1 : 2 : 3 structure, the Zn-O bonds are similar to the Cu-O bonds in the force constant.

Yang et al. observed two-mode behavior of the 0 (2 ) -0 (3 ) Big mode from the polycrystalline samples of Y - P r l : 2 : 3 [11]. Bogachev et al. [14] also found that the superconductor-to-nonsuperconductor transition correlates with the observation of the two- mode behavior of the 0 (2 ) -0 (3 ) Big mode in Ln0.sPr0.sBa2Cu307_a, where Ln stands for any lanthanide element. The two-mode behavior that oc- curs with increasing Ln-Pr radius difference A r was explained with Ln-rich and Pr-rich microstructures. We do not observe two-mode behavior in our Y - Prl : 2 :3 single crystals. This is reasonable for single crystals, since there should be only one phase and no microstructures for each single crystal.

The relation between the full-width at the half- maximum (FWHM) of the Cu(2) mode of Y - Znl : 2 : 3 and the values of T c is illustrated in Fig. 8. With decrease in To, the values of the FWHM show a rapid increase. This supports that the Zn ions act as strong scattering centers to the charge carriers in the CuO 2 planes [15]. The increase in the disorder of the

H.-S. Shin et al. /Physica C 250 (1995) 275-281 279

10

t Y-Zn1:2:3 " Cu(2) Alo

E=8 .E

~ -

!

50 60 70 80

Tc (K) Fig. 8. The relation of the FWHM of the Cu(2) Alg values of Tc of the Y - Z n l : 2 : 3 single crystals.

90

mode and the

CuO 2 plane due to the substituted Zn-ions might be related with the suppression of T c in Y - Z n l : 2 : 3 . The direct causality of the disorder to the suppression of Tc is yet to be proved. To mention only a few suggestions, Ata-Allah et al. [16] argue that the substituted Zn disturbs the alignment of Cu 3dx2 y~ and O 2p,, orbitals resulting in the carrier localization in the CuO 2 planes. Kim et al. [17] explain that the Zn (and Ni) ions act as elastic scattering centers that induce strong gaplessness in the superconducting density of states, which they interpreted as a support for d-wave models. Semba et al. propose that T~ suppression can be explained by the increase in scattering if the superconducting gap has nodes [18].

The results of Raman measurements of Y - Prl : 2 : 3 and Y - Z n l : 2 : 3 single crystals are consistent with those of polycrystalline samples [11]. Fig. 9 and Table 1 summarize the Raman frequencies of all the modes as a function of T c of Y - P r l : 2 : 3 and Y - Z n l : 2 : 3 single crystals. The results clearly show that the Raman frequencies of the Ba and O z modes increase as Tc decreases in the Y-Pr l : 2 : 3 crystals. Within the solution limit of the Zn ion in the Y1 : 2 : 3 system, all the Raman modes do not show any change in the frequency.

The change of the Raman frequencies of the Ba

525

515

5O5

E 340

g 300

,~ lS3 E nr"

147

130

OY-Pr1:2:3 [3Y-Zn1:2:3

Oz Alg I

~ a ' " i'ta-~

Cu(2) Alg o . . . . . . . . . . . . _._e__ e. o. p_=.___,_ . . . . a

4

20 40 60 80

Tc (K) Fig. 9. Raman frequencies of several modes vs. T e of Y-Pr l : 2: 3 (filled circles) and Y-Zn1:2 :3 (open squares) single crystals. Lines are guides for the eye.

Table 1 Summary of the Raman frequencies of several modes and the values of T c for the Y-Pr l : 2: 3 and Y-Zn l : 2: 3 single crystals. For the Y-Pr l : 2: 3 system, the signal-to-noise ratio of the Raman spectra was not favorable to discern the position of the 0(2) -0(3) Alg mode near 430 cm -1

(a) Y-Pr l : 2: 3

T c (K) Raman frequency (cm- 1 ) Ba Cu(2) 0(2) -0(3) O z

88 118.1 150.2 335 503 71 119.9 151.7 334 507 51 120.8 150.5 327 509 46 121.7 150.5 324 510 38 123.2 151.0 321 513 33 121.4 149.6 - 511

0 125.9 149.9 300 520

(b) Y-Zn l : 2: 3

T c (K) Raman frequency (era- 1 ) Ba Cu(2) 0(2)-0(3) O~

85 119.6 150.8 339 435 504 73 119.0 152.0 339 438 502 60 118.1 150.8 337 437 504 56 118.4 150.8 338 436 503

280 H.-S. Shin et al. / Physica C 250 (1995) 275-281

and Oz modes towards higher frequencies in Y - Prl : 2 : 3 as Pr content increases reflects the growing strength of the binding between Ba and O z in the Y-Pr system. Yang et al. observed that the binding energies of Ba core levels increase with the increase of Pr content in the Y - P r l : 2 : 3 system. They explained that the increase of the Ba core level is related with hole localization at a localized band such as the Ba 5d band [8]. This localization might be caused by the hybridization of Pr 4f -O 2p [7].

If the hole-localization mechanism is the correct one in the Pr-doped 1 : 2 : 3 system, what would happen if the Ba-site itself is disturbed? There is an independent Raman study by Bogachev et al. on the changes in the Ba and Cu(2) modes as a function of the content of Sr substituting the Ba sites [19]. They observed Raman spectra of NdBa 2- x SrxCu 307 - 8 in the low-frequency range. In this case, there should be a direct effect on the localization of holes at Ba-sites in the localization scheme proposed above. The results showed a strong hardening of the O~ mode upon Sr doping. They understood the large shift of the Ba(Sr) and the Cu(2) Raman modes as due to the change of the force constant between the Ba(Sr) and O z atoms. As Sr ions substitute the Ba sites, the binding of the Ba-O z bonds hardens in- creasingly. These results are consistent with the in- terpretation in the hole-localization mechanism. That is, the increase of the frequency of the Ba and Oz Raman modes is due to transfer of holes from a bonding band such as Ba 6s-O 2p into a localized band such as Ba 5d. Thereby, T¢ is suppressed by the Sr substitution for the Ba sites.

We can understand that, in the Pr-doped 1 : 2 : 3 systems and possibly the NdBa2_xSr~Cu307_ 8 system, electron charges in a localized band such as Ba 5d transfer into a bonding orbital such as Ba 6s-O 2p, and thereby the holes are localized at Ba sites. The superconductivity is strongly suppressed due to the high degree of hole localization at the Ba sites in these systems. In the Y - Z n l : 2 : 3 system, on the other hand, the suppression mechanism seems to be different from that in the Pr-doped 1 : 2 : 3 systems. The increase in the FWHM of the Cu(2) Raman mode with the decrease in T~ of the Y - Z n l : 2 : 3 system suggests that the suppression of superconductivity may be related with the disorderedness in the CuO 2 planes due to the substitution of Zn for Cu.

4. Conclusion

Single crystals with known T c values of the Y - Prl : 2 : 3 and Y - Z n l : 2 : 3 systems have been studied by Raman measurements in order to study the suppression mechanism of superconductivity in the Y1 : 2 : 3 system. The effects of substitution of Cu(2) sites were compared with those of Y-site substitution by Pr in the Y1 : 2 : 3 system. The Raman spectra for Y -P r l : 2 : 3 single crystals show that the frequencies of Ba and O Z modes increase as the Pr content increases. The Raman results mean that the overlap of Ba and apical oxygen bonding orbitals gets stronger as Pr increases. The results are consistent with the hole-localization scheme proposed for the suppression of superconductivity in the polycrystalline Y - P r l : 2 : 3 systems [8].

On the other hand, in the Y - Z n l : 2 : 3 system, all the Raman modes do not change in frequency. How- ever, the FWHM of the Cu(2) mode increases with the decrease of To, indicating strong scattering of charge carriers by the substituted Zn ions in the CuO 2 planes. The induced disorder in the CuO2 planes may be related with suppression of T~ in the Y - Z n l : 2 : 3 system. Thus, the suppression mechanism in the Y - Z n l : 2 : 3 systems seems to be different from that in the Y - P r l : 2 : 3 systems.

Acknowledgement

The authors are grateful for support from the Korea Science & Engineering Foundation under con- tract Nos. 931-0200-032-2 (ISY) and 941-0200-005-2 (WCL).

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ramans study of y1−xprxba2cu3o7−δ and yb2cu3−xznxo7−δ single crystals

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