Raman scattering in La2−xSrxCuO4 single crystals

Volume 140, number 3

PHYSICS LE~FERSA

11 September 1989

RAMAN SCATTERING IN Ia2~Sr~CuO4 SINGLE CRYSTALS

V.N. DENISOV, B.N. MAVRIN, V.B. PODOBEDOV Institute ofSpectroscopy, USSR Academy ofSciences, Troitslç Moscow Region 142092, USSR

A.B. BYKOV, A.F. GONCHAROY, I.P. ZIBROV and O.K. MEL’NIKOV Institute of Crystallography, USSR Academy ofSciences, Leninskii prospect 59, Moscow 117333, USSR Received 4 May 1989; accepted for publication 15 May 1989 Communicated by V.M. Agranovich

The polarized Raman spectra of I 2_~,Sr~CuO4 single crystals (x=0, 0.01, 0.02 and 0.06) excited by different wavelengths (~.,=488.O,514.5 and 647.1 nm) werestudied. All five totally symmetric vibrations active in Raman scattering ofthe La2CuO4 orthorhombic phase were observed and the assignment ofvibrational modes was carriedout. The transformation ofthe zz spectra at Sr doping was studied. The anomalous dependence ofzz spectraon the angle 8 between the Oz axis and the direction ofincident polarization was found. We discuss the reasonof the anomalouslystrong intensity dropofboth softand 01-modes at the Sr doping and increase of the 8 angle. Raman intensity in the xx and zz geometries was measured at the different x.

1. Introduction

The lattice dynamics of the La2_~Sr~CuO4 system is of great interest due to the appearance of high-i’. superconductivity at x~0.15. In spite of a lot of studies on Raman scattering [1—5],IR spectra [6,71, neutron scattering [8,9] and lattice-dynamical calculations [10—121a number of questions are still not clear due to the absence of necessary data. As is known, at decreasing temperature a phase transition from the tetragonal phase (Dc, z— 1) to the orthorhombic one (Dib, z= 2) takes place in the La2_~Sr~CuO4 compounds; besides the transition temperature 7’0 falls with increasing x [13]. The diminution of the symmetry increases the number of vibrational bands in the spectra in the center of the Brillouin zone. For example, the number of totally symmetric vibrations grows from two in the tetragonal phase to five in the orthorhombic one [4]. Besides two symmetrical axial modes of La and apical 01 thatare Raman active also in the tetragonalphase, three additional modes of the Briliouin zone edge of the tetragonal phase become active in the Raman spectra of the orthorhombic phase (one soft mode [4,81 and only one of the other two is observed at

273 cm—’ [41). It should be noted that at x=0.15 and T=8 K (i.e. TcK T0) the soft mode was not observed in the Raman spectra [41 which led to the suggestion of a possible correspondence between this phenomenon and high-T,, superconductivity. Another not understood peculiarity of the La2 _~Sr~CuO4 Raman spectra is the appearance of additional bands at x< 0.04 in the xx scattering geometry [4,51(here and below the designation of tetragonal axes is used) which were not expected from a factor-group analysis forfundamental vibrations as well as the appearance of a broad band at ~ 3200 cm—’ related to the two-maguon excitations [2,5]. In spite of the study of the xx spectrum dependence on both temperature and x content [5] the nature of this anomalous spectrum is still not completely understood. In the present work the polarized Raman spectra ofLa2_~Sr~CuO4 single crystals (x= 0,0.01,0.02 and 0.06) were studied in the frequency region 15—1600 cm-’. The evolution of the spectra at the Sr doping, an anomalous angular dependence of zz spectra as well as the dependence of zz and xx spectra on the different wavelengths of incident radiation (488.0, 514.5 and 647.1 nm) were studied.

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2. Experimental The crystals studied were prepared by the spontaneous crystallization method from nonstoichiometric composition. The Sr content was determined by the X-ray measurements and was found to be about 0.2 of the Sr content in a melt. At room ternperature all the samples had an orthorhombic symmetry and did not transform into the superconductive state at T>4.2 K. The study of Raman spectra was carried out in the grazing incidence geometry from the polished zzplane by the triple-stage multichannel spectrometer [14]. Laser power did not exceed 20 mW and was focused by 2.anTo F=remove 10 cm the lensbackground into a spot from of about the 0.1 x rotational 1 mm pure Raman spectrum of air the samples were placed in a vacuum chamber. The spectral resolution was ~ 5 cm’ and the accumulation time did not exceed 30 mm.

11 September 1989

A

9

g

A9

A9 o0 • Cu 0 La

Fig. 1.Forms of normal modes ofthe Agsymmetry in the La 2CuO4 orthorhombic crystal.

3. Raman spectra at x=O

The results of our measurements of the polarized Rainan spectra of La2CuO4 are very closeto the (1510. reported in refs. [4,5]. Though we were able to study Raman spectra beginning from 9 cm’ at I%j = 647.1 nrn and 15 cm-’ at ,%1=5l4.5 nm no new peculiarities in the low frequency region of the xx and zx spectra were found. Let us discuss the zz-spectra in d ta~l m~T< 1’0 in the zz geometry one can expect five bands corresponding to the ~ totally symmetric modes whose forms are shown in fig. 1. The modes A~and A~are related to the stretch axial vibration of Laand 01 atomsandhaveto be active in theRaman spectra of the tetragonal phase. The modes A~,A and A~are related to the vibrations of the crystal lattice at the edge of the Brillouin zone of the tetragonallattice (pointX). TheA~mode shouldbe related to the rotational vibrationsof the Cu06 octahedron around the 0x,~rhombic axis. It is known that in the orthorhombic phase these octahedra are rotated az d the OXr axis by the angle 6~[15] which tends toO at T-.T0. So one can expect that the order parameter of the phase transition in the La2CuO4 crystal at T= 1’0 may be described by the 142

static component (form-frozen) of the A5 mode and

this mode itself becomes the soft one. The other two modes A~and A are related to vibrations of the La and 01 atoms along the OXr axis. Both these vibrations, as well as the soft mode, become inactive in the Raman spectra of the tetragonal phase. The zz spectrum of the La2CuO4 crystal at room .

temperature is shown in fig. 2a. Unlike the results of refs. [1,3,4], the band at 522 cm— was not observed in our spectra Tins band is usually assigned to the defect-induced mode [1] which is forbidden in Raman scattering. At the same time we observed a weak maximum at ~ 370 cm-’ close to that reported in ref. [101.The intensity of this banddepended on the sample but at excitation by the 647.1 nm radiation it was always absent. We suppose that this band is not relatedto the vibrations of the La2CuO4 crystal. The bands at 102 and 426 cm-’ are the most inten.. sive in the spectrum (fig. 2a). Besides, one can see weaker bands at 160, 227 and 273 cm-’. The bands at 227 and 42~cnr’ are also present in the tetragonal phase (4] and therefore they may be assigned to the A5 and A~modes, respectively. Our measurements confirm the diminution of the band frequency at 102 cm’ observed in ref. [41 with increasing

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11 September 1989

may be explained by the approach to the phase transition point since T0 changes with increasingx [13]. The diminution of the soft mode intensity as well as that of the bands at 160 and 273 cm—’ with increasing x can be easily explained by the approach to the phase transition point and diminution of the degree of orthorhombic symmetry in La2~Sr~CuO4 crystals. However, that may not be the reason of the intensity decrease of the band at 227 cm’in tetragonal phase of La2CuO4. Therefore, we suppose that the intensity with the increasing decrease x isofdue thistoband, the interaction its broadening of a phonon with free carriers generated by the Sr dop-

La

2..~Sr~Cu04

J~J~ J’~JL a

b

d

1I FRfQUENCY (cm

Fig. 2. Raman spectra of the La 2.~Sr~,CuO4 crystals in the ~ ometry: (a) x=0, (b) x=0.01, (c) x=0.02, (d) x=O.06.

~

temperature as well as its assignment to the A~soft mode. Note that recent data on inelastic neutron scattering [8] do not contradictthis suggestion. The spectral position of the other two modes at 160 and 273 cm’, as well as the temperature dependence of the intensity, agree with their assignment to the A~ and A~bending modes, respectively,

ing. One can expect that the same mechanism is responsible also for faster diminution of the soft mode intensity with respect to that ofthe bands at 227 and 160 cm-’. Thus, one can understand why the soft mode was not observed in the spectra atx=O. 15 and T=8 K [4]. As is seen from fig. 2, the interaction ofphonons with carriershas a selective character and this interaction most strongly affects the Raman intensities of the soft mode and variation of the apical oxygen 01.

4. Raman spectra at xO

5. Angular dependence of zz spectra

The total assignment of the A5 vibrations at x=O makes it possible to interpret correctly the changes in the spectra with increasing Sr content in the La2 _~Sr~Cu04 crystals. Even a small concentration of Sr transforms essentially the spectra in the xx and zz geometries. In the spectra studied we, as well as the authors of ref. [6], observed a considerable intensity degradation of the whole xx spectrum with increasing x. At the same time the zz spectra obtamed at room temperature (fig. 2) demonstrate noticeable changes of the relative band intensities with increasing x. As is seen from fig. 2, at x> 0.02 in the zz spectrum the band at 227 cm-’ becomes the most intensive one and its intensity does practically not depend on x. At the same time the intensities of the other bands including the soft mode and the most intensive band at 426 cm-’(at x=0) decrease essentially. At increasing x only the frequency of the soft mode changes considerably. This mode shifts to the low-frequency region and becomes broader than

The Raman bands assigned to the A5 modes are observed only in the zz scattering geometry. In the xx geometry instead of the expected totally symmetric modes one can observe the above mentioned anomalous spectrum (fig. 3c) which contains both relatively narrow bands and the broad background. The zz spectra of the La2CuO4 crystal obtained at sand p-excitation are shown in figs. 3a and 3b, respectively. In the case of s-excitation the polarization of both incident and scattered radiations may be directed exactly along the Oz axis and one observes a typical spectrum of the orthorhombic crystal (fig, 3b). At p-excitation (fig. 3a) not only the spectrum became weaker (for example, the intensityof the soft mode decreases by a factor ~ 10) but the relative band intensities also changed. Note that from geometrical considerations one can expect the same diminutionof intensityfor all the bands which should be proportional to cos~6(6 is the angle between the Ozaxis and the direction of incident polarization, in our case 6~20~). 143

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where Iph: is the phonon intensity, 1~is the incident intensity, y is the constant of the anharmonic interaction of the phonon with the continuous spectrum. The intensity I,, represents a small part of the continuou: ~um its contribution shown m either increase or decrease the intensity ofthe phonon band depending on the relative sign of a,~and bph. In (1) it was taken into account that bp,, ~ aPh, a~hand a ~ a’, b [5]. From (1) follows (2) 1(8)11(0) cos28 ~‘ br,, sin(28) —

—

—

Using the experimental dependence (fig. 5) and the relation (2) one obtains that the yaCS/bPh ratio falls with increasing x (0.45, 0.3 and 0.14 for x=O, 0.01 and 0.02, respectively). We suppose that this diminution is due to degradation of the continuous spectrum intensity, i.e. the a,~component with the x increase [5]. Faster diminution of the 1(6) intensity of the soft mode at 102 cm-’ as compared to that at 426 cm-’ (fig. 4) may indicate a strong anharmonic interaction of the soft mode with the continuous spectrum. At the same time the y constant for the bands at 227 and 273 cm—’ is essentially smaller (the 1(8) dependence for the band at 160 cm—’ was not found) and that to the 1(6) trend for these bands close 28 leads (fig. 4). to In cosconclusion of this section note that the low intensity ofp-spectra of La 2CuO4 at Troom (fig. 3), i.e. above the Noel temperature TN, was observed also at T= 80K~TN when magnetic moments were essentially ordered. Note that at T> TN one could expect the appearance of a continuous magnon spectrum in Raman scattering due to the short life time of the correlated magnetic moments [17]. On the other hand, at T’a~TN the ordering of the magnetic momeats excludes the magnetic excitation asa possible reason of appearance of the continuous spectrum in the frequency range of phonons discussed. Therefore, we consider the xx background scattering to have an electronic nature.

8

scattering

From the Raman spectra of La2CuO4 excited at 144

a

6

2 —

~

2

FREQUENCY (x1O~cm~l Fig. 6. Experimental (points) and calculated (solid curves) dependences of the 426 and 1430 cm—’ band intensity in the La 2_~,Sr~CuO4 spectraon the incident photon energies: (a) x= 0, (b) x=0.Ol, (c) x=0.02. Calculations were carried out in the same approach asin ref. [5] taking into account the damping of the electronic level.

1, = 457.9, 488.0 and 514.5 nm it was found that the intensity of the xx spectrum in the frequency range cv> 800 cm’ (fig. 3c) increased with 2, [5]. We

have additionally studied the Rainan intensity at the excitation by 2, = 647.1 nm. It was found that the 1(8) angular dependence obtained at 2, = 647.1 nm was practically the same as at 2, = 514.5 tim (figs. 4, 5) and the intensity ofthe bands at 426 cm—’(in the zz spectrum) and 1430 cm—’ (in the xx spectrum) increased monotonously with 2, for all x values (fig. 6). The response intensitiesofmeasured were corrected by the spectral the spectrometer via a calibration process including Raman intensities of the CaCO3 spectrum. Due to the absence of reliable optical characteristics of the La2~Sr~CuO4 crystals in the visible region it is difficult to conclude whether the observed intensity increase is due to the corresponding increase of the penetration depth. Nevertheless, if the intensity changes are due to the resonance phenomenon the effective electronic level responsible for the observed intensity increase (fig. 6) must be located near 1.2 eV and has a width ~0.7 eV. Note that according to calculation [181the transition associated with atoms of Cu and planar oxygens must be active in this energy region. 7.

6.~Reseasce ~

11 September 1989

ConclusIons

A study of the polarized zz spectra in La2~Sr~CuO4 orthorhombic crystals has been car-

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in fig. and shows that 1(8)xdrops strongly than 2O 5though at increasing 1(8) more becomes closer to

La

2CuO4

______________________________ 0

11 September 1989

500 1000 1) FREQUENCY (cm

1500

Fig. 3. Reman spectra ofthe La2CuO4 crystal in different scattering geometries: (a) zz (p-excitation), (b) zz (s-excitation, the intensity is reducedby a factor 4), (c) xx

La2CuO4

B deg

rae

cos the cos28 curve. We suppose that the reason of the anomalous dependence of 1(8) for the band at 426 cm’ may be explained by the interaction between this phonon and the continuous xx spectrum (fig. 4c). In fact, this continuous spectrum may be related to the A 5 symmetry due to its polarization properties. Note that this spectrum tries [5] whereappears the components in the xx,ofthe xy and Raman zz geometensor of the A 8 vibrations may be nonzero (in the tetragonal axes this tensor has components xx=yy= a, xy=yx=a’ and zz= b).

The problem of interaction of a discrete spectrum (in our case, phonons) with a continuous one has been discussed many times (see, for example, ref. [16]). Due to the properties of our spectra (the abone may conclude that the interaction of the Ag sence of a considerable band shift with 8 increase) phonons with the continuous spectrum is weak which makes it possible to approximate the dependence of Raman intensity on the 8 angle as 8)=Iph +I~+I~~ I(

~ 0

200

400

FREQUENCY (cm’1)

cos28+a~sin28

(1)

+yaCSbPhsin(28)]Io,

Fig. 4. Raman spectra ofthe La

2CuO4 crystalat different 0angles where 0 is the angle between the Oz axis and the direction ofthe incident light polarization.

To explain the anomaly in the p-spectrum of the La2CuO4 crystal as well as ofthe La2_~Sr~CuO4 crystals we have studied in more detail the dependence of s-spectra on the 0 angle (fig. 4) which was varied by rotating the crystal around the Oy axis (the Oyaxis is perpendicular to the crystal plane as shown in the insert in fig. 4). In this experiment the polarization of the scattered light was not analyzed because the intensity of the zz spectrum of particular interest was at 8<450 more than 30 times higher than and xx spectra intensity which could appear the withzxincreasing 8. The dependence of the normalized intensity 1(8) ofthe band at 426 cm—’ on the Oangle is represented

1.0

,, ~

~

\. ‘0

‘‘

e, deg Fig. 5. Relative intensities of the 426 cm-’ band in the 28 depenLa2_~,Sr~,CuO4 (•) x=0, ((>) dence; (0) theRaman intensityspectra, dependence of the (0) 1430 x=O.0l, cm-’ band in

x= 0.02, at different 8 angles. The solid curve is the cos the xx spectrum of La 2CuO4. Dotted curves are the calculated dependences (1) for ya~,/bPh=0.l4(a), 0.3 (b) and 0.45 (c).

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ned out. All five fundamental modes predicted by factor-group analysis for totally symmetric vibrations were detected and related to the corresponding forms ofnormal modes As long as the Sr doping in creases the anomalous intensity drop occurs for both the soft mode and vibration of the apical oxygen 01. We suppose this drop to be due to the interaction of these phonons with free carriers. The obtained angular intensity dependences of the Ag phonons have shownthat the same vibrations interactmost strongly with the electronic continuum ofthe xx polarization. Therefore, thereis an experimental indication ofthe possible role’ of these phonons in the high-Ta phenomenon. The measurements of some Raman band intensities atdifferentincident lightwavelengthshave shown the resonance character of the Ag phonon spectra as well as of the anomalous xx spectrum.

11 September 1989

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[7] G.L. Doll, J.T. Nicholls, M.S. Dresselhaus, A.M. Rao, J.M. Zhang. G.W. Lehman, P.C. Eklund, G. Dresselhaus and A.J. [8] P. Boni,JD. Axe, G. Shirane, iLl. Birgeneau D.R. Gabbe, H.P. Jenasen, M.A. Kastner, CJ. Peters, P.J. Picone andT.H. Thurston, Phys. Rev. B 38 (1988) 185; Physica B 156/157 (1989) 902. [9]M.J. Rosseinsky, K. Prassides, P. Day and A.J. Dianoux, (101 ~ ReY~B~7, 8)~2~. Gray, R. Bhadra, V. Maroni and C.-K. Loong, Phys. Rev. B 35 (1987) 8837. [11] J. Prade, A.D. Kulkarni, F.W. de Wette, W. Kress, M. Cardona, R. Reigerand U. Schroder, Solid State Commun. 64(1987)1267. [121R.E. Cohen, W.E. Pickett, LL. Boyer and H. Krakauer, Phys. Rev. Lett. 60(1988)817. [131R.J. Birgeneau, D.R. Gabbe, H.P. Jenssen, M.A. Kastner, P.J. Picone, T.R. Thurston, G. Shirane, Y. Endoh,M. Sato, X. Yamada, Y. Hidaka, M. Oda, Y. Enomoto, M. Suzuki and T. Murakami, Phys. Rev. B 38 (1988) 6614. [14]A.F. Goncharov, V.N. Denisov, B.N. Mavrin and V.B. Podobedov, Zh. Eksp. Teor. Fiz. 94 (1988) 321. [15] K. Yamada, E. Kudo, Y. Endoh, K. Tsuda, M. Tanaka, K. Kokusho~IL Asano, F. Izumi, M. Oda, Y. Hidaka, M. Suzuki and T. Murakami, Japan. J. AppI. Phys. 27 (1988) 1132. [16]M.V. Belousov, in: Excitons, eds. E.I. Rashba and M.D. Sturge (North-Holland, Amsterdam, 1982). [171G. Shirane, Y. Endoh, R.J. Birgeneau, M.A. Kastner, Y. Hidaka, M. Oda, M. Suzuki and T. Murakami, Phys. Rev.

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Acknowledgement The authors thank Professor S.M. Stishov for helpful discussions. References

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