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Influence of Gd, Fe, Zn, and Al substitutions on the tunneling characteristics of the YBaCuO system.
Solid State Communications, Vol. 92, No.I i, pp. X73-875,1994 Elsevier Science Ltd Printed in Great Britain 1)038-1098(94)OOfi7}g-4 ()038-109g/94 $7.00+.1)0
Pergamon
INFLUENCE OF Gd, Fe, Zn AND AI SUBSTITUTIONS ON THE TUNN~.rNG CHARACTERISTICS OF THE Y-Ba-C'u-O SYSTEM. R. Di Leo, A.M. Cucolo, A. Nigro and P. Romano Dipartim~to di ~ Universi~ di Salerno, via S. Allende - 84081 Ban3~issi, Italy. F. Licci MASPEC, Consiglio Nazionale deUe Ricerche, via Chiavari, 18/A - 43100 Panna,ltaly.
(Received 5 August 1994 by C. Odandra)
We have investigation the effect of Gd, Fc, Zn and AI substitutions on the tunneling characteristics of planar tunnel junctions realized on single crystals of Y-Ba-Cu-O with different concentration doping. No relevant differences have been found in the conductance characteristics vs voltage of undoped 90 K and Gd junctions, so confirming that substitution of Y by other rare-earth does not affect the material superconducting properties. A different behavior has been found for substitutions on the Cu sites. Fe, Zn and AI doped junctions show broadened gap-like structures at about +25 mV and disappearance of the structures at +5 mV, observed for the 90 K Y-Ba-Cu-O phase.
S,mp~ P~'l~mxion, ~ ~ p ~ . , t ~ Tec~oiq,c
1. Introduction
The single crystals used in this study were prepared by throughly mixing appropriate amounts of BaCO3, CuO and R20 powders of high-pmity, for the Y-Ba-Cu-O and Gd-Ba-Cu-O samples. Same melts containing Fe203, ZnO as additional components have been used to obtain Y-Ba-Cu-M-O samplesS. The single crystals were grown in alumina crucibles and afterwards annealed in oxygen auncepbcre. The contamination of the Y-Ba-Cu-O single crystals with alumina crucibles provides the A! doped samples. The specimen composition was determined by energy dispersive spectroscopy (EDS) and the lattice paran~ters were inferred from X-ray analysis. The data arc reported in Table L Superconducting transition tempe~ures were obtained by resistive measurements by means of the standard fourprobe method in an exchange gas cryostat. In Fig. I we relxm the temperature dependence of the resistivity of the Y-Ba-Cu-O single crystals with different substitutions. As seen in the figure, the critical transition temperature is maximum for the Gd-Ba-Cu-O compound, which shows Tc (1~0)--90 K. For the substitution on the Cu sites the Tc shifts toward lower values: the cu~es exhibit Tc (f~0)--80 K for 3% Ai, Tc (9=0)=60 K for 3% Zn, Tc (9=0)=59 K for 4% Pc-doped respectively. These samples have been used m fabricate planar tunnel junctions with Pb thin film as counterelectrodes.
The study of the effect of substitutions on the physical
pmp~es of high-Tcsuperco~uctms (HTS) is important to understand the nature of the superconductive state of these materials. In the last years, particular attention has been payed to the pcrovskite ~Ba2Cu30~.8(Y-Ba-Cu-O) system which crystallizes in a orthorombic cell with t~ansition criti""'~ltemperannc of 92 K. Extensive m u c u n l studies show that the substitution of Y with different u'ivalent rare-atth elements (Eu, Gd, Dy, Ho, Er, Tin, Yb) doc~ not affect the compound Tc and only small changes are t,bsesved in the unitary cell lattice parametersl, 2. Alternatively, the compound can be doped on the Cu sites by 3d atoms or by elements with a similar ionic radium 3.4, i.e.: AI, Zn, Fe, Co, Mo, Ge, etc. In this case the doping causes, with the increase of the atomic concentration, a decrease of the transition critical temperature and in some cases a change of the crystal symmetry from orthorombic to tetragonal structure. These results indicate that, in the layered HTS, the superconducting properties are mostly confined in the
Coo2 p~.s. In this work we have investigated the doping influence in the tunneling spectra of plan-.r junctions realized on RBa2Cu3OT.8 (R-Ba-Cu-O) with R--Y, Gd and "IBa2(CuxMl.x)3OT-s (Y-Ba-Cu-M-O) with M=AI, Zn, Fe -ingle crystals. 873
~74
Vol. 92. No.l I
INFLUENCE OF Gd,Fe.Zn AND AI SUBSTITUTIONS Table I
Gd
5.3
a
b
c
3.815 A
3.890 A
11.665A
AI 3%
3.863 A
3.863 ]~
11.672 ]~
Zn 3%
3.820 A
3.893 A
11.675A
Fe4%
3.867 A
3.867 A
11.728 A
The single crystals were chemically etched in a 1% of Br in methanol for 30 rain and the tunnel barriers were formed by exposing the crystals to the ambient atmosphere for about 30 rain. Finally, the counterelectrode was thermally ev~ted through a metallic mask, to form cross type junctions with -0.1 mm 2 area. The resistance of the junctions, at bias ,,100 mV, ranged between 50 ~ and 10 k ~ . Four terminal measurements of the dynamic resistances were performed using a resistance bridge and a standard low frequency lock-in technique. 3. Experimental Resultsand Discussion
We have measured at least 10 junctions for each different dopant with high reproducibility of the results. The use of Pb superconducting counterelectrode allows rigorous controls on the quality of the tunnel barriers 6. Measurements of the I-V characteristics, in the Pb sub-gap region, showed negligible leakage currents for the Pb/Y-Ba-Cu-O and Pb/Gd-Ba-Cu-O junctions "/. Low amount of sub-gap currents have been found in the Pb/Y-Ba-Cu-M-O, with M=AI, Zn, Fe, that might indicate the presence of impurities in the tunnel barriers 7.8. These results me confirmed by measurements of the Pb energy gap and phonon su'uctures. In Fig. 2 we show the low bias conductance data, at T=4.2 K, for a) Pb/Gd-Ba-Cu-O, b) Pb/Y-Ba-Cu-Zn-O, c) Pb/Y-Ba-Cu-Fe-O and d) Pb/YBa-Cu-AI-O junctions. The curve a) reveals a Pb energy gap and phonon osci.gadons at the correct energies and with the correct amplitudes, indicating a single step tunneling
4.7
7.6 7.2 .
O
10
2.5 Fe 4%
2.0 E
~
0.0
50
6~
2 90 I10 T (K)
15
1.0 0.8 0.6 0.4
> ~3 ,
/ 7O
10
0 5 V (mV)
process through the barrier. An analogous behavior has been reported, in the refs. 6 and 9, for 90K Y-Ba-Cu-O b a u d junctions. In curves b), c) and d) the Pb gap femmes are progre.~ively smeared. We interpretate this fact as a possible indication that interation of tunneling eleclrons with impurities at the interface and/or in the barrier reduce the Pb gap structure in these junctions. In Fig.3 we report the differential conductances versus voltage at T=8 K. for the same systems. The curve a) for a Pb/Gd-Ba-Cu-O junctions, shows gap-like structures at :L5 mV and +19 inV. The curves b). c) and d). for the Pb/YBa-Cu-Z~-O, Pb/'Y-Ba-Cu-Fe-O and Pb/Y-Ba-Cu-A1-O junctions respectively, show only a change of slope at :J:25 mV and no structures at : ~ mV are presenL The slructures at ~ mV have been observed for lower concentrations of AI and Fe doping7.8. A comparison of our results with those obtained on undoped Y-Ba-Cu-O junctionst,9.10 confirms that the Y
v
D
0.5
-5
"U
-
1.5 1.0
O.
~
1.6 1.4 1.2
Fig. 2 Low bias conductance vs voltage, measured at T--4.2 K. for:. a) Pb/C,:l-Ba-Cu-O, b) Pb/Y-Ba-CuZn-O, c) Pb/Y-Ba-Cu-Fe-O and d) Pb/Y-Ba-CuAI-O tunnel junctions.
g
8
c)
-15 -I0
> 3.0
2.6 2.4 2.2 2.0 o
j
130
150
Fig. I Resistivity vs temperature for GdBa2Cu3(:b.s and substituted YBa2(CuAM].x)30~.s
-i00
I
-50
,
I
0 V (mV)
,
I
50
.
100
Fig. 3 Normalized conductance vs voltage, measured at T-8 K. for: a) Pb/Gd-Ba-Cu-O, b) Pb/Y-Ba-CuZn-O, c) Pb/Y-Ba-Cu-Fe-O and d) Pb/Y-Ba-CuAI-O tunnel junctions. The vertical axis has been shifted for clarity.
Vol. 92, No.I I
INFLUENCE OF Gd,Fe,Zn AND AI SUBSTITUTIONS
substitution does not greatly affect the superconducting properties so indicating that the Y sites are substantially isolated from the superconducting region. On the other hand, two component superconducting models 11,12 for high Tc superconductors suggest that the high-energy gap like structure is due to the CuO2 planes and the low-energy structure to the CuO chains. The location of the AI and Fe atoms on the Cu(1) sites of the crystal lattice3-5, seems to be consistent with disappearence of the structure at :/:5 inV. More ambiguous is the role of the Zn substitution. Neutron diffraction studiesl3,14 and structural measurements 15 indicate that Zn replaces preferentially in the Cu(2) planes. Small concentration of
875
Zn in the unitary cell modifies in a complex way the density of the charge carriers 15,16. In this scheme, we speculate that the modification induced in the tunneling spectra is consistent with a degradation of the superconducting properties of the planes. 3. Conclusions In summary, we have compared planar tunnel junctions realized on single crystals with different substitutions on both Y and Cu sites. The results revealed that the gap-like s u ' u c ~ at :1:5mV are heavly depressed in the Fe, AI, Zn doped systen~
References I.
2.
3.
4. 5. 6. 7. 8.
P.H. Hor, R.L. Meng,Y.Q. Wang, L. Gao, Z.J.Huang, J. Bechtold, K. Forster and C.W. Chu, Phys. Rev. Lett. $8, 1891 (1987). Y. Le Page, T. Siegrest, S.A. Sunshine, L.F. Schneemeyer, D.W. Murphy, S.M. Zahurak, J.W. Waszczak, W.R. McKinnon, J.M. Taras on, G.W. Hull and L.H. Greene, Phys. Rev. B 36, 3617 (1987). J.M. Tarascon, P. Barboux, P.F. Miceli, L.H. Greene, G.W. Hull, M. Eibschutz and S.A. Sunshine, Phys. Rev. B 37, 7458 (1988). K. Westerholt, H. J. Wuller, H. Bach and P. Stauche, Phys. Rev. B 39, 11680 (1989). F. Licci, C. Frigeri and H.J. Scheel, J. Crystal Growth 112, 606 (1991). A.M. Cucolo, Int. J. of Mod. Phys. B 7, 2549 (1993). A.M. Cucolo, R. Di Leo, A. Nigro, P. Romano and Schncemeyer, Physica B 194-196, 1743 (1994). A.M. Cucolo, R. Di Leo, A. Nigro and F. Licci, Phys. Rev. B 46, 9250 (1992)
9. A.M. Cucolo, R. Di Leo, P. Romano, L.F. Schneemeyer and J.W. Waszczak, IEEE Trans. on Magn. 27, 1349 (1991). I0. M. Gurvitch, J.M. Valles Jr, A.M. Cucolo, R.C. Dynes, J.P. Garno, L.F. Schneemeyer and J.V. Waszczak, Phys. Rev. Lett. 63, 1008 (1989). 11. S. Takahashi and M. Tachiki, Physica C 170, 505 (1990). 12. A.M. Cucolo, C. Noce and A. Romano, Phys. Rev. B 46, 5864 (1992). 13. M. Mehbod, W. Biberaker, A.G.M. Jansen, P. Wyder, R. Dehour and P.H. Duvigneaud, Phys. Rev. B 16, 11813 (1988). 14. G. Xian, M.Z. Cieplak, D. Mnsser, A. Garvin, F.H. Streitz, C.L. Chien, J.J. Rhyne and J.A. Gotaas, Nature 332, 238 (1988). 15. L. Ratio, F. l.,icci and A. Migliori, Phys. Rev. B 48, 1192 (1993). 16. G. Xiao, M.Z. Cieplak, A. Garvin, F.H. Streitz, A. Bakhshai and C.L. Chien, Phys. Rcv. Lett. 60, 1446 (1988).
Pergamon
INFLUENCE OF Gd, Fe, Zn AND AI SUBSTITUTIONS ON THE TUNN~.rNG CHARACTERISTICS OF THE Y-Ba-C'u-O SYSTEM. R. Di Leo, A.M. Cucolo, A. Nigro and P. Romano Dipartim~to di ~ Universi~ di Salerno, via S. Allende - 84081 Ban3~issi, Italy. F. Licci MASPEC, Consiglio Nazionale deUe Ricerche, via Chiavari, 18/A - 43100 Panna,ltaly.
(Received 5 August 1994 by C. Odandra)
We have investigation the effect of Gd, Fc, Zn and AI substitutions on the tunneling characteristics of planar tunnel junctions realized on single crystals of Y-Ba-Cu-O with different concentration doping. No relevant differences have been found in the conductance characteristics vs voltage of undoped 90 K and Gd junctions, so confirming that substitution of Y by other rare-earth does not affect the material superconducting properties. A different behavior has been found for substitutions on the Cu sites. Fe, Zn and AI doped junctions show broadened gap-like structures at about +25 mV and disappearance of the structures at +5 mV, observed for the 90 K Y-Ba-Cu-O phase.
S,mp~ P~'l~mxion, ~ ~ p ~ . , t ~ Tec~oiq,c
1. Introduction
The single crystals used in this study were prepared by throughly mixing appropriate amounts of BaCO3, CuO and R20 powders of high-pmity, for the Y-Ba-Cu-O and Gd-Ba-Cu-O samples. Same melts containing Fe203, ZnO as additional components have been used to obtain Y-Ba-Cu-M-O samplesS. The single crystals were grown in alumina crucibles and afterwards annealed in oxygen auncepbcre. The contamination of the Y-Ba-Cu-O single crystals with alumina crucibles provides the A! doped samples. The specimen composition was determined by energy dispersive spectroscopy (EDS) and the lattice paran~ters were inferred from X-ray analysis. The data arc reported in Table L Superconducting transition tempe~ures were obtained by resistive measurements by means of the standard fourprobe method in an exchange gas cryostat. In Fig. I we relxm the temperature dependence of the resistivity of the Y-Ba-Cu-O single crystals with different substitutions. As seen in the figure, the critical transition temperature is maximum for the Gd-Ba-Cu-O compound, which shows Tc (1~0)--90 K. For the substitution on the Cu sites the Tc shifts toward lower values: the cu~es exhibit Tc (f~0)--80 K for 3% Ai, Tc (9=0)=60 K for 3% Zn, Tc (9=0)=59 K for 4% Pc-doped respectively. These samples have been used m fabricate planar tunnel junctions with Pb thin film as counterelectrodes.
The study of the effect of substitutions on the physical
pmp~es of high-Tcsuperco~uctms (HTS) is important to understand the nature of the superconductive state of these materials. In the last years, particular attention has been payed to the pcrovskite ~Ba2Cu30~.8(Y-Ba-Cu-O) system which crystallizes in a orthorombic cell with t~ansition criti""'~ltemperannc of 92 K. Extensive m u c u n l studies show that the substitution of Y with different u'ivalent rare-atth elements (Eu, Gd, Dy, Ho, Er, Tin, Yb) doc~ not affect the compound Tc and only small changes are t,bsesved in the unitary cell lattice parametersl, 2. Alternatively, the compound can be doped on the Cu sites by 3d atoms or by elements with a similar ionic radium 3.4, i.e.: AI, Zn, Fe, Co, Mo, Ge, etc. In this case the doping causes, with the increase of the atomic concentration, a decrease of the transition critical temperature and in some cases a change of the crystal symmetry from orthorombic to tetragonal structure. These results indicate that, in the layered HTS, the superconducting properties are mostly confined in the
Coo2 p~.s. In this work we have investigated the doping influence in the tunneling spectra of plan-.r junctions realized on RBa2Cu3OT.8 (R-Ba-Cu-O) with R--Y, Gd and "IBa2(CuxMl.x)3OT-s (Y-Ba-Cu-M-O) with M=AI, Zn, Fe -ingle crystals. 873
~74
Vol. 92. No.l I
INFLUENCE OF Gd,Fe.Zn AND AI SUBSTITUTIONS Table I
Gd
5.3
a
b
c
3.815 A
3.890 A
11.665A
AI 3%
3.863 A
3.863 ]~
11.672 ]~
Zn 3%
3.820 A
3.893 A
11.675A
Fe4%
3.867 A
3.867 A
11.728 A
The single crystals were chemically etched in a 1% of Br in methanol for 30 rain and the tunnel barriers were formed by exposing the crystals to the ambient atmosphere for about 30 rain. Finally, the counterelectrode was thermally ev~ted through a metallic mask, to form cross type junctions with -0.1 mm 2 area. The resistance of the junctions, at bias ,,100 mV, ranged between 50 ~ and 10 k ~ . Four terminal measurements of the dynamic resistances were performed using a resistance bridge and a standard low frequency lock-in technique. 3. Experimental Resultsand Discussion
We have measured at least 10 junctions for each different dopant with high reproducibility of the results. The use of Pb superconducting counterelectrode allows rigorous controls on the quality of the tunnel barriers 6. Measurements of the I-V characteristics, in the Pb sub-gap region, showed negligible leakage currents for the Pb/Y-Ba-Cu-O and Pb/Gd-Ba-Cu-O junctions "/. Low amount of sub-gap currents have been found in the Pb/Y-Ba-Cu-M-O, with M=AI, Zn, Fe, that might indicate the presence of impurities in the tunnel barriers 7.8. These results me confirmed by measurements of the Pb energy gap and phonon su'uctures. In Fig. 2 we show the low bias conductance data, at T=4.2 K, for a) Pb/Gd-Ba-Cu-O, b) Pb/Y-Ba-Cu-Zn-O, c) Pb/Y-Ba-Cu-Fe-O and d) Pb/YBa-Cu-AI-O junctions. The curve a) reveals a Pb energy gap and phonon osci.gadons at the correct energies and with the correct amplitudes, indicating a single step tunneling
4.7
7.6 7.2 .
O
10
2.5 Fe 4%
2.0 E
~
0.0
50
6~
2 90 I10 T (K)
15
1.0 0.8 0.6 0.4
> ~3 ,
/ 7O
10
0 5 V (mV)
process through the barrier. An analogous behavior has been reported, in the refs. 6 and 9, for 90K Y-Ba-Cu-O b a u d junctions. In curves b), c) and d) the Pb gap femmes are progre.~ively smeared. We interpretate this fact as a possible indication that interation of tunneling eleclrons with impurities at the interface and/or in the barrier reduce the Pb gap structure in these junctions. In Fig.3 we report the differential conductances versus voltage at T=8 K. for the same systems. The curve a) for a Pb/Gd-Ba-Cu-O junctions, shows gap-like structures at :L5 mV and +19 inV. The curves b). c) and d). for the Pb/YBa-Cu-Z~-O, Pb/'Y-Ba-Cu-Fe-O and Pb/Y-Ba-Cu-A1-O junctions respectively, show only a change of slope at :J:25 mV and no structures at : ~ mV are presenL The slructures at ~ mV have been observed for lower concentrations of AI and Fe doping7.8. A comparison of our results with those obtained on undoped Y-Ba-Cu-O junctionst,9.10 confirms that the Y
v
D
0.5
-5
"U
-
1.5 1.0
O.
~
1.6 1.4 1.2
Fig. 2 Low bias conductance vs voltage, measured at T--4.2 K. for:. a) Pb/C,:l-Ba-Cu-O, b) Pb/Y-Ba-CuZn-O, c) Pb/Y-Ba-Cu-Fe-O and d) Pb/Y-Ba-CuAI-O tunnel junctions.
g
8
c)
-15 -I0
> 3.0
2.6 2.4 2.2 2.0 o
j
130
150
Fig. I Resistivity vs temperature for GdBa2Cu3(:b.s and substituted YBa2(CuAM].x)30~.s
-i00
I
-50
,
I
0 V (mV)
,
I
50
.
100
Fig. 3 Normalized conductance vs voltage, measured at T-8 K. for: a) Pb/Gd-Ba-Cu-O, b) Pb/Y-Ba-CuZn-O, c) Pb/Y-Ba-Cu-Fe-O and d) Pb/Y-Ba-CuAI-O tunnel junctions. The vertical axis has been shifted for clarity.
Vol. 92, No.I I
INFLUENCE OF Gd,Fe,Zn AND AI SUBSTITUTIONS
substitution does not greatly affect the superconducting properties so indicating that the Y sites are substantially isolated from the superconducting region. On the other hand, two component superconducting models 11,12 for high Tc superconductors suggest that the high-energy gap like structure is due to the CuO2 planes and the low-energy structure to the CuO chains. The location of the AI and Fe atoms on the Cu(1) sites of the crystal lattice3-5, seems to be consistent with disappearence of the structure at :/:5 inV. More ambiguous is the role of the Zn substitution. Neutron diffraction studiesl3,14 and structural measurements 15 indicate that Zn replaces preferentially in the Cu(2) planes. Small concentration of
875
Zn in the unitary cell modifies in a complex way the density of the charge carriers 15,16. In this scheme, we speculate that the modification induced in the tunneling spectra is consistent with a degradation of the superconducting properties of the planes. 3. Conclusions In summary, we have compared planar tunnel junctions realized on single crystals with different substitutions on both Y and Cu sites. The results revealed that the gap-like s u ' u c ~ at :1:5mV are heavly depressed in the Fe, AI, Zn doped systen~
References I.
2.
3.
4. 5. 6. 7. 8.
P.H. Hor, R.L. Meng,Y.Q. Wang, L. Gao, Z.J.Huang, J. Bechtold, K. Forster and C.W. Chu, Phys. Rev. Lett. $8, 1891 (1987). Y. Le Page, T. Siegrest, S.A. Sunshine, L.F. Schneemeyer, D.W. Murphy, S.M. Zahurak, J.W. Waszczak, W.R. McKinnon, J.M. Taras on, G.W. Hull and L.H. Greene, Phys. Rev. B 36, 3617 (1987). J.M. Tarascon, P. Barboux, P.F. Miceli, L.H. Greene, G.W. Hull, M. Eibschutz and S.A. Sunshine, Phys. Rev. B 37, 7458 (1988). K. Westerholt, H. J. Wuller, H. Bach and P. Stauche, Phys. Rev. B 39, 11680 (1989). F. Licci, C. Frigeri and H.J. Scheel, J. Crystal Growth 112, 606 (1991). A.M. Cucolo, Int. J. of Mod. Phys. B 7, 2549 (1993). A.M. Cucolo, R. Di Leo, A. Nigro, P. Romano and Schncemeyer, Physica B 194-196, 1743 (1994). A.M. Cucolo, R. Di Leo, A. Nigro and F. Licci, Phys. Rev. B 46, 9250 (1992)
9. A.M. Cucolo, R. Di Leo, P. Romano, L.F. Schneemeyer and J.W. Waszczak, IEEE Trans. on Magn. 27, 1349 (1991). I0. M. Gurvitch, J.M. Valles Jr, A.M. Cucolo, R.C. Dynes, J.P. Garno, L.F. Schneemeyer and J.V. Waszczak, Phys. Rev. Lett. 63, 1008 (1989). 11. S. Takahashi and M. Tachiki, Physica C 170, 505 (1990). 12. A.M. Cucolo, C. Noce and A. Romano, Phys. Rev. B 46, 5864 (1992). 13. M. Mehbod, W. Biberaker, A.G.M. Jansen, P. Wyder, R. Dehour and P.H. Duvigneaud, Phys. Rev. B 16, 11813 (1988). 14. G. Xian, M.Z. Cieplak, D. Mnsser, A. Garvin, F.H. Streitz, C.L. Chien, J.J. Rhyne and J.A. Gotaas, Nature 332, 238 (1988). 15. L. Ratio, F. l.,icci and A. Migliori, Phys. Rev. B 48, 1192 (1993). 16. G. Xiao, M.Z. Cieplak, A. Garvin, F.H. Streitz, A. Bakhshai and C.L. Chien, Phys. Rcv. Lett. 60, 1446 (1988).