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Polarized Raman spectra of superconducting and semiconducting YBa2Cu3Ox single crystals
PhysicaC 153-155 (1988) 290-291 North-Holland, Amsterdam
POLARIZED RAMAN SPECTRA OF SUPERCONDUCTINGAND SEMICONDUCTINGYBa2Cu30x SINGLE CRYSTALS V.G. HADJIEV, M.N. ILIEV and P.G. VASSILEV Sofia University, Faculty of Physics, 5 A. Ivanov Blvd., BG-1126 Sofia, Bulgaria The polarization properties of Raman scattering from single crystals of YBagCuoOv are studied at room temperature for both the superconducting(x~7) and semicon~ucting(x~6.2~) ~h~ses. The assignment of the phonon lines at 502, ~35, 339, 148, 118 and 59 cm- ( f o r the orthorhombic phase) and at 482, 450, 341, 208?, and 140 cm-'(for the tetragonal phase) to certain l a t t i c e vibrations is discussed. I . INTRODUCTION The Raman scattering studies of high T~ YBagCu~Ov superconductors have recently b~en extended^to single c r y s t a l s ( l - 4 ) . Nevertheless, discrepancies concerning the symmetry of Raman lines and t h e i r assignment to certain l a t t i c e vibrations s t i l l remain. Moreover, only the f i v e A (I-4) and one(4) of the remaining ten B2,(B3~) m Bdes have unambigously been i d e n t i f i e d . ~ Here we report the spectra of polarized Raman scattering from single microcrystals f o r both the orthorhombic(O, x~7) and tetragonal(T,x~6.25) phases. The assignment of the observed Raman lines is also discussed. 2. MATERIAL, METHODAND EXPERIMENTAL RESULTS The o r i g i n a t i n g media f o r our microcrystals were pellets prepared by the standard solid state reaction method(4). The polished surface of the p e l l e t s contains d i f f e r e n t l y oriented mostly elongated crystals of 10 to 200 ~m in the longest dimension. The Raman spectra were measured at room temperature with an optical multichannel spectrometer Microdil 28(Dilor) equipped with a microscope. Fig.1 shows the Raman spectra of two YBagCuRO~ microcrystals. Curves I-4 correspond to ~ m~c~ocrystal of the O-phase with x ~ 7 whereas the curves 5-8 are related to a T-phase microcrystal(x ~ 6.25). The x-values were determined from the correlation b~tween the position of the peak near 500 cm and the oxigen content(6). The Raman spectra of each microcrystal were measured in X(ZZ)X, X(ZY)X, X(YY)X and X(YZ)X polarization-configuTations. and X cor-respond to the directions of the Tncident and the scattered l i g h t . Z and Y are the directions of the short and long dimentions of the microcrystals. The correlation between the laboratory system XYZ and the c r y s t a l l o graphic axes a,b,c is not obvious. I t was established, ~ e r , that in our case the Z direction is very close to the direction of c and X and Y to the ones of a(or b) and b(or a)
(see Ref.4). I t follows from curves I-4 tha~ the lines at 502, 435, 339, 149 and 118 cm appear in only parallel polarizations which allows to assign them unambigously t~ modes of A~ symmetry. The weak l i n e at 59 cm- is seen in crossed polarization and has to be assigned to one of the B~ (B3 ) modes. I t should be_voted that the photons ~t 502, 435 and 148 cm display only %7 component_~f the Raman tensor, while the phon~n at 339 cm reveals only ~ . . ( ~ x . ) component. In contrast, the A_ mode at YY ~18 cm displays both the ~ . . a~d ~,, ( ~ x ) components. The comparison b~tween c~ves 1-~ and 5-8 shows the differences between the Raman spectra of O-superconducting and T-semiconducting phases. Except fo( the well knowo s h i f t of the l i n e at 502 cm-'(O) ~o 482 cm-'(T 1 (6), two other O-lines at 435 cm-" and 148 cm-' change both t h e i r position and polarization properties. The 435 cm- (0) l i n e s t i f f e n s to 450 cm-'(T) and appears in YY(XX) polarization. The presence of this l i n e in ZZ polarization (curve 5) is not clear, because of overlapping by the ~ine at 482 cm-'(T). Th~ ZZ l i n e at 148 cm-'(O) softens to 140 cm- (T) appearing in YY(XX) polarization. In addition a very weak structure appears at 208 cm-'(T) in ZZ polarization. 3. DISCUSSION The factor-group analysis shows that the Raman active vibrations in O-phase involve only 02, 03, 04, Cu2 and Ba(we follow the notations I of Ref.7). The O-lines at 502, 435 and 339 cme x h i b i t a dependence on the oxigen mass(6) The l i n e at 502 cm- (0) has been assigned to t i e symmetric stretching vibrations of 04 along the Z axis(2-4 and_~efs therein). The both lines at 435 and 339 cm have been attributed to pure bond bending vibrations of 02-Cu2 and 03-Cu2 bonds along the Z axis with 02 and 03 neighbGurs moving in phase(S-mode) and out of phase(A-mode), respectively(2,3). The lowest lines at 118 and
This work has been completed with financial support of the Bulgarian Committe for Science at the Council of Ministers under Contracts N635 and N695 0921-4534/88/$03.50 © Elsevier Science Publishers B.V. (North-HoUand Physics Publishing Division)
V.G. Hadjieu et al. / Superconducting and semiconducting YBaeCu sO~ single crystals
A/ /
~ ~
' G
J1"ex¢: 51Z+5 nm
should be involved in almost denegerate vibrations with both higher frequency an~ i n t e n s i t y . Thus we assign the 502 and 435 cm- O-lines to S- and A-modes, respectively. The reduced s p l i t t i n g in frequencies between the 482 and 450 cm-" lines in the T-phase, where 02 and 03 vibrations are degenerate supports such an assignment. The factor group analysis determines the A~ modes involving the both 02 and 03 in the O-pha~e become A1a and BI~ in the T-phase. The change ~f the polarizatioH properties of the 450 cm- l i n e shows the t r a n s i t i o n to B. mode possessing only ~,,(~vv) components. Th~g rest oxigen-involving : : l i H ~ at 339 cm- has to be a t t r i b u t e d to 04 vibrations. The 01 atoms in proximity of 04 are removed in the T-phase(8). The l a t t e r seems to r e s u l t in increasing of the ~,,,,(%v) components of the Raman tensor as the p6~ar~2ability in X(Y) direction is enhanced. S i m i l a r l y , the p o l a r i z a b i l i t y related to the chain Cu2-O4-Cu1-O4-Cu2 is also changed which explains t~e increasing of the Cu2-1ine i n t e n s i t y at 140 cm in YY(XX) polarization. The Ba lines are not seen in the T-phase. I t is probably due to unsuitable experimental conditions. The very weak defect-induced Raman lines observed near 208 and 600 cm are not discussed here.
' lt,81 '
8,~: A-IZsI "
~3s
339
J ~
Z
--
291
3/+
z_ eY I,LI I'-L~
0
500
~00
300
200 RAHAN SHIFT(cm -I}
100
FIGURE I 59 cm -I are assigned to scattering_~=rom vibrations of Ba(4). The line at 148 cm " displays the axial symmetric stretching of Cu2(3,4). The above assignments have been supported by l a t t i c e dynamic calculations(2,3). Besides, the only l i n e possessing an anomalous behaviour near T is the one at 339 cm- (5) which supposes c vibrations in the superconducting Cu2-02-03 planes. Within above assignments, however, i t is d i f f i c u l t to explain the large s p l i t t i n g between the S-_~#nd A-modes of 02,03. Further, i f the 502 cm " l i n e is caused by 04 vibrations i t shoud s t i f f e n with decreasing x because of reducing the 04-Cui bond lenght(8). Indeed, the valency of Cul is also reduced(8) but i t is not obvious that the l a t t e r e f f e c t is the dominating one. These speculations and the experimental results presented here suggest an a l t e r n a t i v e i n t e r p r e t a t i o n . Even in the O-phase the 02-Cu2 and 03-Cu2 bond lenghts are almost equal(8). Besides, the 02(03)-Cu2 bond lenght are shorter than the one of 04-Cui. Therefore 02 and 03
REFERENCES ( I ) R. Bhadra, T.O. Brun, M.A. Beno, B. Dabrowski, D.G. Hinks, J.Z. Liu, J.D. Jorgensen, L.J. Nowicki, A.P. Paulikas, Y.K. Shuller, S.U. Serge, L. Soderholm, B. Veal, H.H. Wang, J.M. Williams, K. Zhang and M. Grimsditch Phys.Rev.B in print(1987). (2) D°M. Krol, M. Stavola, W. Weber, L.F. Schneemeyer, J.V. Waszczak and S. Kosinski Phys.Rev.B in print(1987). (3) R. Liu, C. Thomsen, W. Kress, M. Cardona, B. Gegenheimer, F.W. Wette, J. Prade, A.D. Kulkarni and U. Schroderet Phys.Rev.Lett. in print(1987). (4) V. Hadjiev and M. l l i e v Solid State Commun. in print(1988). (5) R.M. Macfarlane, H. Rosen and H. Seki Solid State Commun. 63(1987)831. (6) G.A. Korouklis, A. Jayaraman, B. Batlogg, R.J. Cava, M. Stavola, D.M. Krol, E.A. Rietman and L.F. Schneemeyer Phys.Rev.B in print(1987). (7) M.A. Beno: L. Soderholm, D.W. Capone ! I , D.G. Hinks, J.D. Jorgensen, I.K. Schuller, C.U. Serge, K. Zhang and J.D. Grace Appl.Phys.Lett. 51(1987)57. (8) A.W. Hewat, J.J. Capponi, C. Chaillout, M. Marezio and E.A. Hewat Nature, in print(1987).
POLARIZED RAMAN SPECTRA OF SUPERCONDUCTINGAND SEMICONDUCTINGYBa2Cu30x SINGLE CRYSTALS V.G. HADJIEV, M.N. ILIEV and P.G. VASSILEV Sofia University, Faculty of Physics, 5 A. Ivanov Blvd., BG-1126 Sofia, Bulgaria The polarization properties of Raman scattering from single crystals of YBagCuoOv are studied at room temperature for both the superconducting(x~7) and semicon~ucting(x~6.2~) ~h~ses. The assignment of the phonon lines at 502, ~35, 339, 148, 118 and 59 cm- ( f o r the orthorhombic phase) and at 482, 450, 341, 208?, and 140 cm-'(for the tetragonal phase) to certain l a t t i c e vibrations is discussed. I . INTRODUCTION The Raman scattering studies of high T~ YBagCu~Ov superconductors have recently b~en extended^to single c r y s t a l s ( l - 4 ) . Nevertheless, discrepancies concerning the symmetry of Raman lines and t h e i r assignment to certain l a t t i c e vibrations s t i l l remain. Moreover, only the f i v e A (I-4) and one(4) of the remaining ten B2,(B3~) m Bdes have unambigously been i d e n t i f i e d . ~ Here we report the spectra of polarized Raman scattering from single microcrystals f o r both the orthorhombic(O, x~7) and tetragonal(T,x~6.25) phases. The assignment of the observed Raman lines is also discussed. 2. MATERIAL, METHODAND EXPERIMENTAL RESULTS The o r i g i n a t i n g media f o r our microcrystals were pellets prepared by the standard solid state reaction method(4). The polished surface of the p e l l e t s contains d i f f e r e n t l y oriented mostly elongated crystals of 10 to 200 ~m in the longest dimension. The Raman spectra were measured at room temperature with an optical multichannel spectrometer Microdil 28(Dilor) equipped with a microscope. Fig.1 shows the Raman spectra of two YBagCuRO~ microcrystals. Curves I-4 correspond to ~ m~c~ocrystal of the O-phase with x ~ 7 whereas the curves 5-8 are related to a T-phase microcrystal(x ~ 6.25). The x-values were determined from the correlation b~tween the position of the peak near 500 cm and the oxigen content(6). The Raman spectra of each microcrystal were measured in X(ZZ)X, X(ZY)X, X(YY)X and X(YZ)X polarization-configuTations. and X cor-respond to the directions of the Tncident and the scattered l i g h t . Z and Y are the directions of the short and long dimentions of the microcrystals. The correlation between the laboratory system XYZ and the c r y s t a l l o graphic axes a,b,c is not obvious. I t was established, ~ e r , that in our case the Z direction is very close to the direction of c and X and Y to the ones of a(or b) and b(or a)
(see Ref.4). I t follows from curves I-4 tha~ the lines at 502, 435, 339, 149 and 118 cm appear in only parallel polarizations which allows to assign them unambigously t~ modes of A~ symmetry. The weak l i n e at 59 cm- is seen in crossed polarization and has to be assigned to one of the B~ (B3 ) modes. I t should be_voted that the photons ~t 502, 435 and 148 cm display only %7 component_~f the Raman tensor, while the phon~n at 339 cm reveals only ~ . . ( ~ x . ) component. In contrast, the A_ mode at YY ~18 cm displays both the ~ . . a~d ~,, ( ~ x ) components. The comparison b~tween c~ves 1-~ and 5-8 shows the differences between the Raman spectra of O-superconducting and T-semiconducting phases. Except fo( the well knowo s h i f t of the l i n e at 502 cm-'(O) ~o 482 cm-'(T 1 (6), two other O-lines at 435 cm-" and 148 cm-' change both t h e i r position and polarization properties. The 435 cm- (0) l i n e s t i f f e n s to 450 cm-'(T) and appears in YY(XX) polarization. The presence of this l i n e in ZZ polarization (curve 5) is not clear, because of overlapping by the ~ine at 482 cm-'(T). Th~ ZZ l i n e at 148 cm-'(O) softens to 140 cm- (T) appearing in YY(XX) polarization. In addition a very weak structure appears at 208 cm-'(T) in ZZ polarization. 3. DISCUSSION The factor-group analysis shows that the Raman active vibrations in O-phase involve only 02, 03, 04, Cu2 and Ba(we follow the notations I of Ref.7). The O-lines at 502, 435 and 339 cme x h i b i t a dependence on the oxigen mass(6) The l i n e at 502 cm- (0) has been assigned to t i e symmetric stretching vibrations of 04 along the Z axis(2-4 and_~efs therein). The both lines at 435 and 339 cm have been attributed to pure bond bending vibrations of 02-Cu2 and 03-Cu2 bonds along the Z axis with 02 and 03 neighbGurs moving in phase(S-mode) and out of phase(A-mode), respectively(2,3). The lowest lines at 118 and
This work has been completed with financial support of the Bulgarian Committe for Science at the Council of Ministers under Contracts N635 and N695 0921-4534/88/$03.50 © Elsevier Science Publishers B.V. (North-HoUand Physics Publishing Division)
V.G. Hadjieu et al. / Superconducting and semiconducting YBaeCu sO~ single crystals
A/ /
~ ~
' G
J1"ex¢: 51Z+5 nm
should be involved in almost denegerate vibrations with both higher frequency an~ i n t e n s i t y . Thus we assign the 502 and 435 cm- O-lines to S- and A-modes, respectively. The reduced s p l i t t i n g in frequencies between the 482 and 450 cm-" lines in the T-phase, where 02 and 03 vibrations are degenerate supports such an assignment. The factor group analysis determines the A~ modes involving the both 02 and 03 in the O-pha~e become A1a and BI~ in the T-phase. The change ~f the polarizatioH properties of the 450 cm- l i n e shows the t r a n s i t i o n to B. mode possessing only ~,,(~vv) components. Th~g rest oxigen-involving : : l i H ~ at 339 cm- has to be a t t r i b u t e d to 04 vibrations. The 01 atoms in proximity of 04 are removed in the T-phase(8). The l a t t e r seems to r e s u l t in increasing of the ~,,,,(%v) components of the Raman tensor as the p6~ar~2ability in X(Y) direction is enhanced. S i m i l a r l y , the p o l a r i z a b i l i t y related to the chain Cu2-O4-Cu1-O4-Cu2 is also changed which explains t~e increasing of the Cu2-1ine i n t e n s i t y at 140 cm in YY(XX) polarization. The Ba lines are not seen in the T-phase. I t is probably due to unsuitable experimental conditions. The very weak defect-induced Raman lines observed near 208 and 600 cm are not discussed here.
' lt,81 '
8,~: A-IZsI "
~3s
339
J ~
Z
--
291
3/+
z_ eY I,LI I'-L~
0
500
~00
300
200 RAHAN SHIFT(cm -I}
100
FIGURE I 59 cm -I are assigned to scattering_~=rom vibrations of Ba(4). The line at 148 cm " displays the axial symmetric stretching of Cu2(3,4). The above assignments have been supported by l a t t i c e dynamic calculations(2,3). Besides, the only l i n e possessing an anomalous behaviour near T is the one at 339 cm- (5) which supposes c vibrations in the superconducting Cu2-02-03 planes. Within above assignments, however, i t is d i f f i c u l t to explain the large s p l i t t i n g between the S-_~#nd A-modes of 02,03. Further, i f the 502 cm " l i n e is caused by 04 vibrations i t shoud s t i f f e n with decreasing x because of reducing the 04-Cui bond lenght(8). Indeed, the valency of Cul is also reduced(8) but i t is not obvious that the l a t t e r e f f e c t is the dominating one. These speculations and the experimental results presented here suggest an a l t e r n a t i v e i n t e r p r e t a t i o n . Even in the O-phase the 02-Cu2 and 03-Cu2 bond lenghts are almost equal(8). Besides, the 02(03)-Cu2 bond lenght are shorter than the one of 04-Cui. Therefore 02 and 03
REFERENCES ( I ) R. Bhadra, T.O. Brun, M.A. Beno, B. Dabrowski, D.G. Hinks, J.Z. Liu, J.D. Jorgensen, L.J. Nowicki, A.P. Paulikas, Y.K. Shuller, S.U. Serge, L. Soderholm, B. Veal, H.H. Wang, J.M. Williams, K. Zhang and M. Grimsditch Phys.Rev.B in print(1987). (2) D°M. Krol, M. Stavola, W. Weber, L.F. Schneemeyer, J.V. Waszczak and S. Kosinski Phys.Rev.B in print(1987). (3) R. Liu, C. Thomsen, W. Kress, M. Cardona, B. Gegenheimer, F.W. Wette, J. Prade, A.D. Kulkarni and U. Schroderet Phys.Rev.Lett. in print(1987). (4) V. Hadjiev and M. l l i e v Solid State Commun. in print(1988). (5) R.M. Macfarlane, H. Rosen and H. Seki Solid State Commun. 63(1987)831. (6) G.A. Korouklis, A. Jayaraman, B. Batlogg, R.J. Cava, M. Stavola, D.M. Krol, E.A. Rietman and L.F. Schneemeyer Phys.Rev.B in print(1987). (7) M.A. Beno: L. Soderholm, D.W. Capone ! I , D.G. Hinks, J.D. Jorgensen, I.K. Schuller, C.U. Serge, K. Zhang and J.D. Grace Appl.Phys.Lett. 51(1987)57. (8) A.W. Hewat, J.J. Capponi, C. Chaillout, M. Marezio and E.A. Hewat Nature, in print(1987).