Polar phonon modes in YBa2Cu3O6.4 by infrared reflectivity spectroscopy

~

Solid State Communications, Vol. 69, No. 4, pp.359-362, 1989. Prlnted in Great Britain.

0038-1098/89 $3.00 + .00 Pergamon Press plc

POLAR PHONON MODES IN YBa2 Cu3 06.4 BY INFRARED REFLECTIVITY SPECTROSCOPY Patrick Echegut, Francois Gervais, Krystof Dembinsky, Mouique Gervais and Philippe Odier Centre de Recherches sur la Physique des Hautes Temp6ratures, Centre National de Recherche Scientifique, 45071 Orl6ans, France (Received 8 August 1988 by M. Balkanski) Infrared reflectivity spectra of a highly textured - largely single crystalline - sample of YBa2Cu30 6 4 are reported and analyzed with the factorized form of the dielectric function. Results are compared with predictions "of lattice dynamical models mainly based on Raman data. The anisotropy of vibrational properties is found less marked than in La2CuO4 which was shown to be highly bidimensional. This is related to the existence of short Cu-O bond lengths within the chains along the c axis direction, in addition to those within the basal a-b planes, common to all high-Tc superconductor copper oxide compounds, so that effective charges do not show much anisotropy on an average.

1. INTRODUCTION

anisotropy in polarized infrared light. As-grown samples have a composition such that the plasma frequency is low and phonon modes, therefore, may be measured in good conditions. Results are reported in this paper.

Infrared reflectivity spectroscopy complemented by computer analysis of infrared data is a powerful method to obtain information on the new high-Tc oxide superconductors. Information on at least four types of excitations may be extracted from the refleetivlty spectra. This is the reason why this method has been extensively used during the 18 last months in this field. Two of these excitations, the superconducting gap below T c (see Refs. 1-4) and the plasmon, the bath of free particles from which Cooper pairs may be formed, are directly relevant in the superconducting mechanism. The two other excitations possibly play a part in the superconductivity phenomenon, viz. phonons and narrow electronic gaps seen around 0.5 eV, often assigned to excitons. Even if the weakness of the isotope effect relativizes the role played by plionons in Y-Ba-Cu-O, they cannot be ignored in a realistic description of tlke system. Measurements performed on single crystals of La2NiO4," a parent compound p f La2CuO4, and also in oriented sample of this latter compound," have shown unambiguously that "exciton' and plasmon are confined into the basal h-b plane. Parallel to the c axis, the reflectivity spectrum ressembles that of an insulator. The bidimensionality of electronic properties observed by infrared reflectivity at the microscopic scale is confirmed by the pronounced anisotropy of macroscopic data such as electrical conductivity, magnetic susceptibility, critical current, and so on. A due to understand the bidimensional behavior lies in the comparison of Cu-O bond lengths within a given configuration, for example within the basal ~-b plane and perpendicular to it. Effective iouie charges of oxygen"~ deduced from the Coulombic field which splits polar modes into transverse optical (TO) and longitudinal optical (LO) components, were found to be twice smaller within the a-b plane of La2NiO4, and La2CuO 4 as well, with respect to their values parallel to the c axis. Keeping in mind the correlation bond strength - bond length experimentally .gbserved in any ferroelectric compound investigated in the past,- it appears quite clear that the value of the effective charge within the a-b plane lowered by a factor as large as 2 with respect to the perpendicular direction, is related to the bond overlap which yields "metallic bonding" within the plane and much more ionic locaiised character in the direction parallel to the c axis. It is now well established that the electronic conc~entration at the Fermi level decreases with increasing y between 0 and 1 in YBa2Cu3OT_y, and the compound can no longer undergo a transition to a superconducting state when the carrier concentration becomes too small. Infrared reflectivity spectra of Y-Ba-Cu-O ceramics samples quenched from temperature ranging from 950° C to 500* C have been reported by Sugai.° These results as well as those_obtained on single crystal samples polarized within the a-b plane9 show that phonon modes are nearly completely screened by the plasmon for those samples with y near zero which may become superconductor below T c. Since authors have studied preferentially the superconducting compositions in which phonon modes are masked, rather than compounds with y > 0.5 which are semiconduc.~s but not superconductors, recent lattice dynamical calculationsIU essentially based upon Raman data evidence the lack of reliable polar phonon mode measurements in polarized light. We succeeded in growing large crystals of YBa2Cu30 6 4 which are not yet properly single crystals but however display a good

2. CRYSTAL GROWTH Crystal samples were grown by the method of pseudo-flux reported previously.11"13 It is now commonly accepted that a ternary eutectic lies within the triangle BaCuO 2 -YBa2Cu307 - C u O although its accurate location is presently unknown. Compositions nearby this eutectic may be used as a flux for melting the compound and obtaining the 123 composition as a single crystal at the crystallization. We have obtained relatively large single crystals in the following way. 600 g of a mixture of Y203, BaO 2 and CuO in the ratios 6:33:61 (mole %) was melted at 1150°C for 10 hours and cooled down to 900"C at a rate of 100°C per hour, and then reheated at 1000"C. This cycle was repeated twice. The liquid was finally cooled at 4"C per hour. A large exothermic effect was observed at 800°C, a temperature below which the cooing rate was increased to 300"C per hour. A large volume (6 cm-') of the f'mal product was checked by microprobe analysis to be Y-Ba-Cu-O (123) made of highlg textured material. Single crystal volumes as large as 10x2x2 mm~ were detected by neutron scattering whith however a large mosaicity. Scanning electron microscope studies showed that the samples are made of single crystal sheets of Y-Ba-Cu-O (123) interrupted in some places by BaO-CuO eutectic inclusions, supposed to be the origin of the mosaicity. The structure has been shown by X-ray analysis to be tetragonal with y = 0.6 + 0.1. Annealing of the sample for 3 days at 350°C in oxygen yields the orthorhombic variety which displays a strong Meissner effect in liquid nitrogen, caraeteristie of the superconducting state. Since the Fresnel formula in polarized light involves cosine squared, the sample quality was sufficient to measure the anisotropy of infrared spectra but the process has to be improved for ulterior measurements of phonon dispersion curves by neutron scattering. 3. RESULTS AND DISCUSSION Infrared reflection spectra for the fight polarized parallel and perpendicular to the a-b plane are shown in Fig. 1 and 2, respectively. The spectrum was found isotropic when the a or b axes of a crystal sample cut perpendicular to the c axis was rotated with respect to the direction of polarization, consistent with the tetragonal symmetry of the sample. Best fits of the factorized form of the dielectric function

=

rr "%°---j

j'I.jTO2 _ ¢~2 + i,lfjTOe.~

combined with the Fresnel formula near normal incidence 359

(1)

360

POLAR PHONON MODES IN YBa2Cu306. 4

Y Ba2 Cu306.4

80

/

>I---

> 60 I.-(..3 LIJ ,,_1

Vol. 69, No..4

/~0-

Lt.. Lt.I

20 o

I

,

I

200

400

600 (cm -1)

FREQUENCY

Fig. 1 - Infrared reflection spectmrn for the electric fieM polarized parallel to the tetragonal c axis. Dots are experimental data and the full curve is the best fit of Eq. (1) to experimental spectrum and parameters given in Table 1.

Y

o

80

Ba2Cu306./,

o

>,.

gi;

~6 > (.3

w 40 ..._1

LL W nr

20

I

I

I

200

400 FREQUENCY

600 (cm j )

Fig. 2 - Same as legend to Fig. 1 for the electric field of infrared radiation polarized within the a-b plane.

R = I (~-

1)/(,[~ + 1) [ 2

(2)

to reflectivity spectra are shown in Figs. I and 2. No Drude term has been added, even in the a-b plane. However, the slight deviation of the calculated curve from the experimental data in the reflectivity minima at high frequencies indicates that charge carriers are present in the a-b plane, but at low concentration. Attemps to improve the

fit by adding a Drude term showed that no accurate plasma frequency can be deduced from this spectrum. After these measurements, the samples have been annealed under oxygen as

explained in section 2. Then the sample becomes a superconductor and a plasmon nearly completely screens the phonon in the a-b plane. Oscillator parameters which yield the fit to rcflectivky shown in Figs. 1 and 2 are given in Table 1. The imaginary and imaginary

Vol. 69, No. 4

POLAR PHONON MODES IN YBa2Cu306. 4

inverse dielectric function corresponding to the profiles of TO and LO modes, respectively, are plotted in Figs. 3 and 4. Comparison of spectra with group theory predictions (see e.g. Ref. 10) shows that some weak modes are residue of the other polarization due to the imperfect ~ingle crystal character of our sample. Such 'forbidden" modes are denoted by arrows in Figs. 1 to 4. Finally, if we ignore the lowest-frequency mode for the electric field of the infrared radiation polarized parallel to the e axis (denoted E II e in figures 1.4) whose assigmment is dubious, we are.left with 7 modes for each polarization which agree with group theory predictions for the tetragonal phase. However, the ns~i~ment of the seventh mode within the: basal plane is dubious. It may be either the small band at 450 em"~, which may

361

also be a residue of the otJaer polarization, or a smaller one related to the dip near 650 cm"J'. Residues of the other polarization or dubious modes are ignored in Table 1. The hlgh-frequeney dleleetrie constant is found equal to 4.2 for both polarizations. Present experimental data are eomp$ffed to most recent lattice dynamical calculations by Liu et alI0 and Chaplet.~I~t The caleulatious were based on Raman data and infrared spectra obtained on ceramics sample in tmpolarlzed fight. Although there are large discrepancies with the experiment, Liu et al data appear generally closer to the experimental results than those of Chaplet. The TO-LO splittings are experimentally found larger than in the calculations of Refs. 10, which means that effective ionic charges

100I

YBa2Cu306.t . re" t.-Z Z S) --It.

(.3

0.5

N 5o W 123 >[3"

!

"

~

#

-..,.

~

..._•i

,. A,i.,.,-. ......-

Z L'3

:E

I

I

#

~°°°.°°°e o'°

:S_~

,

400

200

600

FREQUENCY

(cm -1)

Fig. 3 - TO (imaginary dielectric response) and t O (imaginary inverse dielectric response) phonon structures for Au-O~pe modes. The arrow indicates a ~orbidden" mode, residue of the other polarization due to imperfect single crystallinity of the scunple.

100

YB,2cu3o .-

o •...,

Elc

LL

,.

i "

U, 50

:-

,,

!.

tO

". ..........

:

-'...

-

-"

" - ...... " "~ " "

.

;

":

t.u > Z

":

i

zu_

"

"..'"

, ,

~.

-

<~

z

,

~L~

x~

".. . . . . . . . . ."

" ............"

-

400 FREOUENCY

3.5>.~

.

...

200

,.,.~

;

...*

600 (cm "1)

Fig. 4 - Same as legend to Fig. 3 for Eu-type modes.

-

0

362

POLAR PHONON MODES IN YBa2Cu306. 4

Table 1. Experimental TO and LO phonon mode frequencies in cm "1 unit, followed by mode dampings inside parentheses, compared to results of recent dynamical calculations. experiment this work

TO modes calculations Ref.10 Ref14

LO modes experiment calculations this work Ref lO 14 E//c

103 (18) 148 (18) 216(10) 353 (57) 430 (21) 575 (80) 652(20)

109 161 199 319 375 502 555

66 92 137 145 208 427 550

127 (14) 183 (20) 223 (6) 420 (23) 465 (23) 640 (70) 660(19)

130 208 199 319 423 518 555

86 97 140 208 347 499 622

63 (15) 129 (11) 201 (11) 270 (17) 418 (30) 636 (45)

62 119 240 337 454 555

84 137 179 257 414 559

EZc 56 (15) 102 (11) 185 (14) 239 (17) 340 (32) 560 (56)

62 116 238 291 361 555

84 135 170 223 276 527

have been underestimated. However present experiments concerns the semiconducting variety of Y-Ba-Cu-O whereas calculations of Chaplot refer explicitly to the 'metallic" orthorhombie compound

Vol. 69, No. 4

YBa2Cu~O 7, as well as those of Liu et al, for which there is no explicit indication of oxygen content. Cation-anion distances are renormalized by typically 1% from the tetragonal to the orthorhombic struet~e. This is very small and hardly explains the discrepancies, at least from the lattice dynamics point of view. On the other hand one should be careful on utilizing infrared data to fit a lattice dynamical model in the orthorhombie metallic phase, not to take the observed phonon values which are screened by the plasmon, but rather data decoupled from the plasma background. Otherwise apparent TO-LO splittings will be minimized. If we sum over the LO mode frequencies squared and substract a similar term for TO modes, the result is found nearly independent on polarization, contrary to what was found in our previous studies.5,6 The anisotropy found in lanthanum copper (or nickel) oxide was found consistent with shorter bond lengths within the basal a-b plane compared to parallel to the c axis. Here in YBa-Cu-O, short bond lengths exist in the direction parallg~ to c within the chains, even shorter than within the a-b plane, t-' It is therefgre not surprising that Coulombic terms of the form (Ze)k~/mk summed over all atoms of the unit cell along c or perpendicular to it do not show much difference in Y-Ba-Cu-O. This is a straightforward application of the relationship

~'j(I~jLO2 - IIjTO2)~ = (EvV)-l~k (Ze)k$/mk

(3)

where the summation is over all atoms in the volume V, v is the dielectric constant of vacuum and t~. denotes a direction of polarization. Conversely, an anisotropy factor as large as 4 was found in La2NiO4, where short Ni-O bond lengths are limited to the a-b plane, like in its isostructural copper parent.

REFERENCES 1. 2.

3. 4. 5. 6. 7.

Z. Schlesinger, R.T. Collins and M.W. Schaffer, Phys. Rev. B 35, 7232 (1987). D.A. Bonn, J.E. Greedan, C.V. Stager, T. Timnsk, M.G. Doss, S.L. Herr, K. Kamaras, C.D. Porter, D.B. Tanner, J.M. Taraseon, W.R. McKinnon and L.H. Greene, Phys. Rev. B 35, 8843 (1987). S.L. Herr, K. Kamaras, C.D. Porter, M.G. Doss, D.B. Tanner, D.A. Bonn, J.E. Greedan, C.V. Stager and T. Timnsk, Phys. Rev. B 36, 733 (1987). G.A. Thomas, A.J. Millis, R.N. Bhatt, R.F. Cava and E.A. Rietman, Phys. Rev. B 36, 736 (1987). J.M. Bassat, P. Odier and F. Gervais, Phys. Rev. B 35, 7136 (1987). F. Gervais, P. Echegut, J.M. Bassat and P. Odier, Phys. Rev. B 37, 9364 (1988). F. Gervais, P. Simon, P. Echegut and B. CalSs, Jap. J. Appl. Phys. 24, 24 (1985).

8. 9. 10. 11. 12. 13. 14. 15.

S. Sugai, Phys. Rev. B 36, 7133 (1988). Z. Schlesinger, R.T. Collins, D.L. Kaiser and F. Holtzberg, Phys. Rev. Letters 59, 1958 (1987). R. Lin, C. Thomsen, W. Kress, M. Cardona, B. Gegenheimer, F.W. de Wette, J. Prade, A.D. Kulkarni and U. Schr6der, Phys. Rev. B 37, 7991 (1988). L.F. Schneemeyer, j;v; Waszczak, T. Siegrist, R.B. Van Dover, L.N. Rupp, B. Batlogg, R.J. Cava and D.W. Murphy, Nature 328, 601 (1987). R.A. Laudise, LF. Schneemeyer and R.L. Barns, J. Cryst. Growth 85, 569 (1987). H.G. Scheel and F. Licci, J. Cryst. Growth 85, 607 (1987). S.L. Chaplot, Phys. Rev. B 37, 7435 (1988). M. Cardona, L. Genzel, R. Liu, A. Wittlin and H.J. Mattausch, Solid St. Commun. 64, 727 (1987).