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Planar tunnel junctions on 90 K and 60 K YBCO single crystals
Physica C 179 ( 1991 ) 69-74 North-Holland
Planar tunnel junctions on 90 K and 60 K YBCO single crystals Superconducting and normal state properties A.M. Cucolo Dipartimento di Fisica, Universita" di Salerno, 84081 Baronissi (Sa), Italy
R.C. Dynes t, J.M. VaUes Jr. 2 and L.F. S c h n e e m e y e r AT&T Bell Laboratories, Murray Hill, NJ 07974, USA Received 18 May 1991
We have studied the tunneling characteristics of planar junctions made on YBCO single crystals with Tc= 90 K and Tc= 60 K. Natural barriers of good quality have been obtained on both compounds. In comparison with the 90 K phase, the superconducting conductance curves of the 60 K phase show gap*like structures reduced in amplitude and shifted towards higher energies. For the higher Tc material, a gap opening, at about 90 K, is observed. Normal state conduetances, measured at T> To, are highly linear in voltage on both compounds and have steeper slopes on the 60 K phase. The slopes are quite insensitive to temperature variations, and do not depend on the junction resistances nor on the materials used as counterelectrodes.
1. Introduction
Historically electron tunneling measurements have been one of the most powerful tools for probing the nature of superconductivity in conventional superconductors. Since the discovery of the high-T¢ superconducting oxides, numerous attempts have been made to exploit this technique to obtain quantitative information on the quasiparticle density of states, the energy gap and the mechanism of pairing in these materials. Unfortunately, making tunnel junctions on high-T¢ superconductors has proved non-trivial. Tunneling probes superconductivity within the materiars coherence length ~, that in these compounds is anisotropic and of the order of a few angstroms. More complications arise due to oxygen loss from the surface, at least in the 1 : 2: 3 materials. The role of the oxygen in these materials is in fact critical and decisively affects the electronic properties and especially the superconducting transition temperature Permanent address: Department of Physics, UCSD, La Jolla, CA 92093, USA. 2 Permanent address: University of Oregon, Eugene, OR 97403, USA.
To. For the Y B a 2 C u 3 0 7 _ ~ (YBCO) compound, as the oxygen content is varied from 7 to 6.5, structural transitions occur corresponding to superconducting phases with Tc = 90 K and Tc = 60 K [ 1 ]. One expects that tunneling spectroscopy should be sensitive to these variations and it is clearly of interest to determine the variation of the superconducting density of states with the oxygen concentration. For weakcoupled conventional superconductors, at T = 0 K, the BCS theory fixes the ratio 2A(O)/kTc at 3.5, implying smaller energy gaps, 2A(0), for lower T¢ materials. This quantity has been found to be > 4 in strong-coupling superconductors. Owing to the difficulty in the preparation of tunnel junctions on highT¢ superconductors, few reports of such studies can be found in the literature [ 2 ]. In this paper we report on tunneling measurements carried out on planar junctions fabricated on YBCO single crystals with T~= 90 K and T¢= 60 K. Reproducibility of the data and quality of the tunnel junctions have been carefully tested. We have divided the paper in two main sections devoted to the superconducting and normal state properties. We believe that the normal state tunneling is also of interest since it reveals some unique features of this compound that might help in
0921-4534/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.
70
A.M. Cucolo et al. / Planar tunnel junctions on YBCO single crystals
the understanding of the superconducting properties.
2. Junction preparation The YBCO single crystals were grown by the flux method [ 3 ]. After the growth process they underwent different 02 annealing treatments to produce T~= 90 K and T¢= 60 K. Critical temperatures were measured by AC and DC susceptibility. These samples were etched for 30 min in a 1% Br solution in methanol and the tunnel barriers were formed naturally by exposing the crystals to the ambient air for about 30 min. The counterelectrode, generally a Pb film, was then thermally evaporated through a metallic mask. This fabrication procedure has been reported elsewhere [4 ] and gives good quality tunnel junctions and highly reproducible tunneling data on YBCO single crystals [5,6] and films [7]. Junction dimensions were about 0.1 × 1.0 mm 2. The resistances of the junctions at 100 mV, at room temperature, ranged between a hundred and a few thousand Ohm. Four terminal measurements of the differential resistances were performed by using standard low frequency AC lock-in techniques.
3. Superconducting tunnel In a superconductor/insulator/normal metal (SI - N ) junction, at finite temperature the conductance as function of the voltage Gs(V) is given by: Gs(V) = N o P o j N , ( E ) [ - af(E --
o o
+ eV) /O( eV) l d E ,
(1)
where No is the normal counterelectrode density of states, Po is the tunnel probability, both assumed constant in the considered energy range (few meV), Ns (E) is the quasiparticle density of states of the superconducting material and f ( E ) is the Fermi distribution function. In the limit of T = 0 K, expression ( 1 ) reduces to: as(V) =NoPoN,(E )
( 1' )
and the conductance measured at T<< T¢ is propor-
tional to the superconducting density of states. In the weak coupled BCS approximation, for E < A , N s ( E ) = 0 , while for E > A , N s ( E ) = N N [E/ (E2-A2)~/2], where NN is the normal density of states of the superconductor. The latter relation implies that to compare the experimentally measured conductances Gs(V) with the BCS density of states N s ( E ) , one needs to know the normal density of states NN of the superconductor at the given temperature. In conventional S - I - N junctions the conductances in the superconducting state, Gs(V), are normalized to the conductances in the normal state, GN(V), obtained by applying a magnetic field to drive normal the superconducting material at low temperature. This normalization procedure eliminates the contributions due to barrier effects and to the normal densities of states of both materials. Due to the high nc2 value for the YBCO comPound, the GN(V) of this kind of junctions cannot be measured at low temperatures. So, to analyze the tunneling characteristics of this material, different normalization procedures have to be used. To normalize our data we have used the GN (V) measured above the YBCO T¢, since at a first approximation they can replace the GN (V) at low temperatures. In fig. 1 we report the Gs(V) at 1.2 K normalized to the GN(V) at T = 9 8 K and T = 6 4 K for the YBCO/Pb junction 90/39, (T¢=90 K), (a), and junction 60/ 48, (To= 60 K), (b). These data are representative respectively of conductance curves found on single crystals with T¢=90 K and T¢=60 K. A magnetic field was applied to quench the Pb superconductivity so that the curves can be analyzed in terms of SI-N junctions. Differences and similarities of conductance curves with BCS superconductors, have been extensively discussed in ref. [ 4 ]. Here we want to focus only on the similarities and the relative changes observed in YBCO single crystals with different T¢. We have found that both types of samples show, at low T, Gs (0) ~ 0, well pronounced maxima and minima and highly linear background conductances at V> 50 mV. Referring to fig. 1 we observe that: (1) The maximum at about 20 mV on curve (a), probably related to the gap-like structure of stoichiometric YBCO, is shifted to about 25 mV and reduced in amplitude for junctions fabricated on single crystals with T~=60 K, curve (b).
A.M. Cucolo et al. / Planar tunnel junctions on YBCO single crystals
I
I
I
I
I
IA1.0
~" 0.9
~
1.o
0.9 tl
.~_
I
I
I
I
I
10
20
30
40
50
V (mV) Fig. 1. G,( V ) / G N ( V ) as a function o f voltage at T = 1.2 K, in 1 T magnetic field, for Y B C O / P b junction 90/39 ( T ¢ = 9 0 K ) (a), and junction 60/48 ( T~= 60 K ) (b). The GN (V) have been measured at T = 9 8 K and T = 6 4 K respectively. The solid line is a guide for the eye.
(2) The shoulder at about 5 mV, on curve (a), perhaps related to gap anisotropy or to proximity effects, is always present in the 60 K junctions. We cannot report systematic behavior of this structure that sometime appears to be very broadened. (3) The secondary m a x i m u m at about 36 mV on curve (a), is completely lost on curve (b). These results appear quite intriguing, and point ( l ) seems to indicate that the simple BCS result relating smaller gaps in lower T¢ materials is not observed. In order to verify that we are free of spurious effects in our measurements, a number of tests on the quality of the tunnel barriers have been carried out at low temperatures, with Pb in the superconducting state. At 1.2 K, in zero magnetic field, the conductance curves of junctions made on 90 K crystals showed the Pb gap and phono structures at the correct energies and of correct amplitudes, indicating a single step tunneling process. The same results have been found in the majority of the 60 K junctions, but in some cases (about 30%), we have found reduced amplitudes of the Pb structures. Sub-gap leakage currents,
71
at low temperatures, were less than a few percent, indicating good quality tunnel barriers, continuous and pin-hole free on both phases of the YBCO compound. As a corollary of the previous results, in fig. 2 (a) and (b), we show the temperature dependences of the zero-bias conductances G(0 mV), normalized to the conductances at V= 100 mV, of junctions 90/55 and 60/47, representative of what we have found respectively on To= 90 K and To= 60 K single crystals. For completely oxygenated YBCO crystals, fig. 2 (a), there is an evidence, at about T = 90 K, of the opening of an energy gap, though less pronounced than what expected from the BCS theory. For the 60 K compound, fig. 2(b), we do not see a similar discontinuity when passing through the T~ of the single crystal. If one considers these data as a measure of the local T¢ by tunneling spectroscopy, and one assumes that the peak at 20 mV is the YBCO energy gap, it is possible to evaluate the ratio 2A/kTc only
a4 T C =
0.80
L
i
•
0.70
0.6(
;1
0.5o
b) ,~ 0.45
0.40
0.35
0
, /~ , 1 , , , , I J , ~ j I I 50
100 T{K)
150
Fig. 2. Temperature dependence of G(0 mV)/G( 100 mV) for junction 90/55 (To=90 K), (a), and junction 60/47 (T¢=60 K) (b). The solid line is a guide for the eye.
72
A.M. Cucolo et aL /Planar tunnel junctions on YBCO single crystals
for the 90 K phase: with A = 2 0 mV and T~=90 K, we find 2d/kTc= 5.0. For the 60 K phase we are facing the puzzling result that the data show no clear evidence for a local Tc but we do observe structures at 5 mV and 25 mV on the conductance curves.
•
'
I
'
30
40
'
[
'
'
25
2,.0
4. Normal tunneling As we have already mentioned, one of the remarkable features of our tunnel junctions is the linear dependence of the background conductances with voltages until about 200 mV both for 90 K and 60 K YBCO single crystals. This peculiarity has been routinely found in junctions fabricated on copper oxides, YBCO [4,8], LSCO [9], and TBCCO [ 10], as well as in the bismuth oxides BPBO [ 11 ], and recently in BKBO [ 11,12 ]. Since the high bias conductance of S - I - N junctions is related to the normal state properties, one expects that more insight can be achieved from the study of the tunneling characteristics measured at T> T~ of the YBCO samples. In a normal metal-insulator-high-T~ superconductor junction, at T > To, the conductance as function of the voltage can be written: -boo
GN(V) =No j P(E)NN(E) [ --oo
-Of(E+eV)/O(eV)]dE,
(2)
where No and NN(E) are the normal densities of states of the counterelectrode and of the high-T¢ material, P(E) is the tunneling probability a n d f ( E ) is the usual Fermi distribution function. In the considered energy range (few hundred meV), in fact, both P(E) and NN(E) might depend on energy. In N - I - N ' junctions with ordinary metals, the density of states of both electrodes varies slowly and the conductances are analyzed in terms of P(E). In conventional junctions the tunneling probability P(E) depends on the barrier height, thickness and symmetry and according to this model, the GN (V) are found proportional to V 2 and continuous through the zero-bias. In fig. 3 we show, at T = 98 K and T = 64 K, the normal conductances of junctions 90/39 ( T¢ = 90 K),
1.01 0
10
20
50 60 V (rnV)
70
80
90
Fig. 3. Voltage dependence of the GN(V)/GN(V=0 mY) at T=98 K, for junction 90/39 (T¢=90 K) (a) and at T=64 K, for junction 60/48, (T¢=60 K) (b). The full lines have been calculated taking into account the effect o f the Fermi function on the linear conductances at the given temperatures.
(a), and 60/48 (To= 60 K), (b). The data have been normalized to the zero-bias conductance values GN (0) measured at the same temperatures. The full lines in the figure have been calculated taking into account the effect of the Fermi function on the linear conductances at the given temperatures. We observe that: ( 1 ) Except for thermal rounding at low biases, the normal conductances appear to have a striking linear dependence on both phases. (2) The lower T¢ compound shows a steeper variation in voltage of the conductance. We have found a = 0.005 m V - l for the slope of the conductance of sample 90/39 (T¢=90 K), and a = 0 . 0 2 0 mV -~ for sample 60/48 (To=60 K). These a values are highly reproducible with variations of less than 15% on samples with the same To. They do not depend on the junction resistance nor on the nature of the counterelectrode (Pb/Bi, Au, Bi, Sb have been also used). We have found that the conductanceslopes are quite insensitive to temperature variations, as shown in fig. 4 referring to junction 90/88, To=90 K, ( a ) , and junction 60/46, To=60 K, (b). Intermediate a values have been measured for YBCO single crystals oxygen deficient
A.M. Cucolo et aL / Planar tunnel junctions on YBCO single crystals
i
i
i
A
.~ 0.7
~
//
120K
f
295K
OJ
0.6 \ \
/
78K
62 K 0.3 -150
I
-75
~I" 0
I
75
151
V(mV) Fig. 4. Voltage dependence of the GN( V)/GN (100 mY) at the indicated temperatures, for junction 90/88 ( Tc= 90 K) (a), and junction 60/46 (To=60 K) (b).
with T¢=80 K and Ni doped with T~=75 K. The junction preparation method has been the same in all the cases.
5. Discussion and conclusions From expression (2), deviations of the conductance curves from the usual V 2 dependence, can be understood in terms of density of states effects NN (E) and tunneling probabilities P(E). In fact several explanations for the linear conductance have been proposed including barrier effects [ 13 ], inelastic tunneling [9 ], and density of states effects as in the resonating-valence-bond (RVB) model [ 14 ]. At the moment no one of the proposed mechanisms has been unambiguously confirmed. However, the presence of the linear background conductances in point contacts and in planar junctions made on different high-T¢ materials, single crystals and films, independently from the junction formation
73
method, seems to be strong evidence that this behavior is intrinsic in these compounds. In the YBCO 60 K phase, we have found normal conductances steeper and apparently in contrast with the BCS theory; the gap-like structures are reduced in amplitude and shifted towards higher energies. These results are consistent with the presence of BCS structures superimposed on linear background conductances [ 15 ]. In the lower Tc compound, with steeper slope, the gap-like structures, in fact, are reduced and smeared out and both the opening and the "true" value of the energy gap can be masked by the same effect [15]. The results presented in this paper are consistent with what has been recently found on YBCO single crystals with different oxygen surface stoichiometry [ 16 ] and indicate that, at the moment, little can be said about the value of the 2A(O)/kTc ratio in the lower Tc compound. At the same time, our data seem to be another manifestation of effects recently found by N M R spectroscopy and specific heat measurements. In fact, N M R studies [ 17 ] have revealed a temperature-varying spin susceptibility at the planar sites and, in the 60 K phase, the absence of any prominent feature at Tc strongly contrasts with the behavior of the 90 K phase. Similar contrasts have been found in specific heat behavior where the lower Tc compound exhibits at T = T c a peak diminished compared to the 90 K phase [ 18 ]. To conclude, the normal state properties seem to be an important key for a better understanding of the superconducting properties of the high-T~ superconductors. We believe that measurements of the normal state conductance as well as rigorous controls on the quality of the junctions, in all cases, complement the information obtained by superconducting tunneling data.
Acknowledgements We wish to thank, with pleasure, many colleagues for their suggestions and support. These include M. Gurvitch, G.A. Thomas, T.T.M. Palstra, R.E. Walstedt, R. Vaglio and R.D. Parmentier. We thank J.V. Waszczak, J. Garno and G. Perna for technical as-
74
A.M. Cucolo et al. / Planar tunnel junctions on YBCO single crystals
sistance. O n e o f us, ( A M C ) , has b e e n s u p p o r t e d by a N A T O / C N R grant.
References [ 1 ] See for example: R.J. Cava, A.W. Hewat, E.A. Hewat, B. Batlogg, M. Marezio, K.M. Rab¢, J.J. Krajewski, W.F. Peck Jr. and L.W. Rupp Jr., Physica C 165 (1990) 419. [2] J.S. Tsai, I. Takeuchi, J. Fujita, S. Miura, T. Terashima, Y. Bando, K. Iijima and K. Kamamoto, Physica C 157 ( 1989 ) 538; A.P. Volodin, B. Ya. Kotyuzhanskii and G.A. Stefanyan, JEPT Lett. 48 (1988) 502. [3] L.F. Schneemeyer, J.V. Waszczak, T. Siegrit, R.B. Van Dover, L.W. Rupp, B. Batlogg, R.J. Cava and D.W. Murphy. Nature 601 (1987) 601. [4 ] M. Gurvitch, J.M. Valles Jr., A.M. Cucolo, R.C. Dynes, J.P. Garno, L.F. Sehneemeyer and J.V. Waszczak, Phys. Rev. Lett. 63 (1989) 1008; ibid., Phys. Rev. B, submitted. [5] M. Gurvitch, J.M. VaUes, A.M. Cucolo, R.C. Dynes, J. Garno, L.F. Schneemeyer and J.V. Waszczak, Physica C 162-164 (1989) 1067.
[6] A.M. Cucolo, R. Di Leo, P. Romano, L.F. Schneemeyer, J.V. Waszczak, IEEE Trans. Magn. 27 ( 1991 ) 1349. [7] A.M. Cucolo, J.M. Valles, R.C. Dynes, M. Gurvitch, J.M. Phillips and J.P. Garno, Physiea C 161 (1989) 351. [8] M. Lee, M. Naito, A. Kapitulnik and M.R. Beasley, Solid State Commun. 70 (1989) 449. [9] J.R. Kirtley and D.J. Scalapino, Phys. Rev. Lett. 65 (1990) 798. [ 10] I. Takeuchi, J.S. Tsai, Y. Shimakawa, T. Manako and Y. Kubo, Physica C 158 (1989) 83. [ 11 ] K Sharifi, A. Pargellis, R.C. Dynes, B. Miller, E.S. Hellman, J. Rosamilia and E.H. Hartford Jr., Phys. Rev. B, submitted. [ 12] J.F. Zasadizinski, N. Tralshawala, Q. Huang, K.E. Gray and D.G. Hinks, IEEE Trans Magn. 27 ( 1991 ) 833. [ 13] J.R. Kirtley, Int. J. Mod. Phys. B 4 (1990) 201. [ 14] P.W. Anderson and Z. Zou, Phys. Rev. Left. 60 (1988) 132. [15] A.M. Cucolo, C. Note and A. Romano, Phys. Lett., submitted. [ 16 ] A.M. Cucoto, R. Di Leo, P. Romano, L.F. Schneemeyer and J.V. Waszczak, Phys. Rev. B, to be published. [ 17] R.E. Walstedt, W.W. Warren Jr., R.F. Bell, R.J. Cava, G.P. Espinosa, L.F. Schneemeyer and J.V. Waszczak, Phys. Rev. B41 (1990) 9574. [ 18 ] A. Junod, D. Eckert, T. Graf, G. Triscone and J. Muller, Physica C 162-164 (1989) 1401.
Planar tunnel junctions on 90 K and 60 K YBCO single crystals Superconducting and normal state properties A.M. Cucolo Dipartimento di Fisica, Universita" di Salerno, 84081 Baronissi (Sa), Italy
R.C. Dynes t, J.M. VaUes Jr. 2 and L.F. S c h n e e m e y e r AT&T Bell Laboratories, Murray Hill, NJ 07974, USA Received 18 May 1991
We have studied the tunneling characteristics of planar junctions made on YBCO single crystals with Tc= 90 K and Tc= 60 K. Natural barriers of good quality have been obtained on both compounds. In comparison with the 90 K phase, the superconducting conductance curves of the 60 K phase show gap*like structures reduced in amplitude and shifted towards higher energies. For the higher Tc material, a gap opening, at about 90 K, is observed. Normal state conduetances, measured at T> To, are highly linear in voltage on both compounds and have steeper slopes on the 60 K phase. The slopes are quite insensitive to temperature variations, and do not depend on the junction resistances nor on the materials used as counterelectrodes.
1. Introduction
Historically electron tunneling measurements have been one of the most powerful tools for probing the nature of superconductivity in conventional superconductors. Since the discovery of the high-T¢ superconducting oxides, numerous attempts have been made to exploit this technique to obtain quantitative information on the quasiparticle density of states, the energy gap and the mechanism of pairing in these materials. Unfortunately, making tunnel junctions on high-T¢ superconductors has proved non-trivial. Tunneling probes superconductivity within the materiars coherence length ~, that in these compounds is anisotropic and of the order of a few angstroms. More complications arise due to oxygen loss from the surface, at least in the 1 : 2: 3 materials. The role of the oxygen in these materials is in fact critical and decisively affects the electronic properties and especially the superconducting transition temperature Permanent address: Department of Physics, UCSD, La Jolla, CA 92093, USA. 2 Permanent address: University of Oregon, Eugene, OR 97403, USA.
To. For the Y B a 2 C u 3 0 7 _ ~ (YBCO) compound, as the oxygen content is varied from 7 to 6.5, structural transitions occur corresponding to superconducting phases with Tc = 90 K and Tc = 60 K [ 1 ]. One expects that tunneling spectroscopy should be sensitive to these variations and it is clearly of interest to determine the variation of the superconducting density of states with the oxygen concentration. For weakcoupled conventional superconductors, at T = 0 K, the BCS theory fixes the ratio 2A(O)/kTc at 3.5, implying smaller energy gaps, 2A(0), for lower T¢ materials. This quantity has been found to be > 4 in strong-coupling superconductors. Owing to the difficulty in the preparation of tunnel junctions on highT¢ superconductors, few reports of such studies can be found in the literature [ 2 ]. In this paper we report on tunneling measurements carried out on planar junctions fabricated on YBCO single crystals with T~= 90 K and T¢= 60 K. Reproducibility of the data and quality of the tunnel junctions have been carefully tested. We have divided the paper in two main sections devoted to the superconducting and normal state properties. We believe that the normal state tunneling is also of interest since it reveals some unique features of this compound that might help in
0921-4534/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.
70
A.M. Cucolo et al. / Planar tunnel junctions on YBCO single crystals
the understanding of the superconducting properties.
2. Junction preparation The YBCO single crystals were grown by the flux method [ 3 ]. After the growth process they underwent different 02 annealing treatments to produce T~= 90 K and T¢= 60 K. Critical temperatures were measured by AC and DC susceptibility. These samples were etched for 30 min in a 1% Br solution in methanol and the tunnel barriers were formed naturally by exposing the crystals to the ambient air for about 30 min. The counterelectrode, generally a Pb film, was then thermally evaporated through a metallic mask. This fabrication procedure has been reported elsewhere [4 ] and gives good quality tunnel junctions and highly reproducible tunneling data on YBCO single crystals [5,6] and films [7]. Junction dimensions were about 0.1 × 1.0 mm 2. The resistances of the junctions at 100 mV, at room temperature, ranged between a hundred and a few thousand Ohm. Four terminal measurements of the differential resistances were performed by using standard low frequency AC lock-in techniques.
3. Superconducting tunnel In a superconductor/insulator/normal metal (SI - N ) junction, at finite temperature the conductance as function of the voltage Gs(V) is given by: Gs(V) = N o P o j N , ( E ) [ - af(E --
o o
+ eV) /O( eV) l d E ,
(1)
where No is the normal counterelectrode density of states, Po is the tunnel probability, both assumed constant in the considered energy range (few meV), Ns (E) is the quasiparticle density of states of the superconducting material and f ( E ) is the Fermi distribution function. In the limit of T = 0 K, expression ( 1 ) reduces to: as(V) =NoPoN,(E )
( 1' )
and the conductance measured at T<< T¢ is propor-
tional to the superconducting density of states. In the weak coupled BCS approximation, for E < A , N s ( E ) = 0 , while for E > A , N s ( E ) = N N [E/ (E2-A2)~/2], where NN is the normal density of states of the superconductor. The latter relation implies that to compare the experimentally measured conductances Gs(V) with the BCS density of states N s ( E ) , one needs to know the normal density of states NN of the superconductor at the given temperature. In conventional S - I - N junctions the conductances in the superconducting state, Gs(V), are normalized to the conductances in the normal state, GN(V), obtained by applying a magnetic field to drive normal the superconducting material at low temperature. This normalization procedure eliminates the contributions due to barrier effects and to the normal densities of states of both materials. Due to the high nc2 value for the YBCO comPound, the GN(V) of this kind of junctions cannot be measured at low temperatures. So, to analyze the tunneling characteristics of this material, different normalization procedures have to be used. To normalize our data we have used the GN (V) measured above the YBCO T¢, since at a first approximation they can replace the GN (V) at low temperatures. In fig. 1 we report the Gs(V) at 1.2 K normalized to the GN(V) at T = 9 8 K and T = 6 4 K for the YBCO/Pb junction 90/39, (T¢=90 K), (a), and junction 60/ 48, (To= 60 K), (b). These data are representative respectively of conductance curves found on single crystals with T¢=90 K and T¢=60 K. A magnetic field was applied to quench the Pb superconductivity so that the curves can be analyzed in terms of SI-N junctions. Differences and similarities of conductance curves with BCS superconductors, have been extensively discussed in ref. [ 4 ]. Here we want to focus only on the similarities and the relative changes observed in YBCO single crystals with different T¢. We have found that both types of samples show, at low T, Gs (0) ~ 0, well pronounced maxima and minima and highly linear background conductances at V> 50 mV. Referring to fig. 1 we observe that: (1) The maximum at about 20 mV on curve (a), probably related to the gap-like structure of stoichiometric YBCO, is shifted to about 25 mV and reduced in amplitude for junctions fabricated on single crystals with T~=60 K, curve (b).
A.M. Cucolo et al. / Planar tunnel junctions on YBCO single crystals
I
I
I
I
I
IA1.0
~" 0.9
~
1.o
0.9 tl
.~_
I
I
I
I
I
10
20
30
40
50
V (mV) Fig. 1. G,( V ) / G N ( V ) as a function o f voltage at T = 1.2 K, in 1 T magnetic field, for Y B C O / P b junction 90/39 ( T ¢ = 9 0 K ) (a), and junction 60/48 ( T~= 60 K ) (b). The GN (V) have been measured at T = 9 8 K and T = 6 4 K respectively. The solid line is a guide for the eye.
(2) The shoulder at about 5 mV, on curve (a), perhaps related to gap anisotropy or to proximity effects, is always present in the 60 K junctions. We cannot report systematic behavior of this structure that sometime appears to be very broadened. (3) The secondary m a x i m u m at about 36 mV on curve (a), is completely lost on curve (b). These results appear quite intriguing, and point ( l ) seems to indicate that the simple BCS result relating smaller gaps in lower T¢ materials is not observed. In order to verify that we are free of spurious effects in our measurements, a number of tests on the quality of the tunnel barriers have been carried out at low temperatures, with Pb in the superconducting state. At 1.2 K, in zero magnetic field, the conductance curves of junctions made on 90 K crystals showed the Pb gap and phono structures at the correct energies and of correct amplitudes, indicating a single step tunneling process. The same results have been found in the majority of the 60 K junctions, but in some cases (about 30%), we have found reduced amplitudes of the Pb structures. Sub-gap leakage currents,
71
at low temperatures, were less than a few percent, indicating good quality tunnel barriers, continuous and pin-hole free on both phases of the YBCO compound. As a corollary of the previous results, in fig. 2 (a) and (b), we show the temperature dependences of the zero-bias conductances G(0 mV), normalized to the conductances at V= 100 mV, of junctions 90/55 and 60/47, representative of what we have found respectively on To= 90 K and To= 60 K single crystals. For completely oxygenated YBCO crystals, fig. 2 (a), there is an evidence, at about T = 90 K, of the opening of an energy gap, though less pronounced than what expected from the BCS theory. For the 60 K compound, fig. 2(b), we do not see a similar discontinuity when passing through the T~ of the single crystal. If one considers these data as a measure of the local T¢ by tunneling spectroscopy, and one assumes that the peak at 20 mV is the YBCO energy gap, it is possible to evaluate the ratio 2A/kTc only
a4 T C =
0.80
L
i
•
0.70
0.6(
;1
0.5o
b) ,~ 0.45
0.40
0.35
0
, /~ , 1 , , , , I J , ~ j I I 50
100 T{K)
150
Fig. 2. Temperature dependence of G(0 mV)/G( 100 mV) for junction 90/55 (To=90 K), (a), and junction 60/47 (T¢=60 K) (b). The solid line is a guide for the eye.
72
A.M. Cucolo et aL /Planar tunnel junctions on YBCO single crystals
for the 90 K phase: with A = 2 0 mV and T~=90 K, we find 2d/kTc= 5.0. For the 60 K phase we are facing the puzzling result that the data show no clear evidence for a local Tc but we do observe structures at 5 mV and 25 mV on the conductance curves.
•
'
I
'
30
40
'
[
'
'
25
2,.0
4. Normal tunneling As we have already mentioned, one of the remarkable features of our tunnel junctions is the linear dependence of the background conductances with voltages until about 200 mV both for 90 K and 60 K YBCO single crystals. This peculiarity has been routinely found in junctions fabricated on copper oxides, YBCO [4,8], LSCO [9], and TBCCO [ 10], as well as in the bismuth oxides BPBO [ 11 ], and recently in BKBO [ 11,12 ]. Since the high bias conductance of S - I - N junctions is related to the normal state properties, one expects that more insight can be achieved from the study of the tunneling characteristics measured at T> T~ of the YBCO samples. In a normal metal-insulator-high-T~ superconductor junction, at T > To, the conductance as function of the voltage can be written: -boo
GN(V) =No j P(E)NN(E) [ --oo
-Of(E+eV)/O(eV)]dE,
(2)
where No and NN(E) are the normal densities of states of the counterelectrode and of the high-T¢ material, P(E) is the tunneling probability a n d f ( E ) is the usual Fermi distribution function. In the considered energy range (few hundred meV), in fact, both P(E) and NN(E) might depend on energy. In N - I - N ' junctions with ordinary metals, the density of states of both electrodes varies slowly and the conductances are analyzed in terms of P(E). In conventional junctions the tunneling probability P(E) depends on the barrier height, thickness and symmetry and according to this model, the GN (V) are found proportional to V 2 and continuous through the zero-bias. In fig. 3 we show, at T = 98 K and T = 64 K, the normal conductances of junctions 90/39 ( T¢ = 90 K),
1.01 0
10
20
50 60 V (rnV)
70
80
90
Fig. 3. Voltage dependence of the GN(V)/GN(V=0 mY) at T=98 K, for junction 90/39 (T¢=90 K) (a) and at T=64 K, for junction 60/48, (T¢=60 K) (b). The full lines have been calculated taking into account the effect o f the Fermi function on the linear conductances at the given temperatures.
(a), and 60/48 (To= 60 K), (b). The data have been normalized to the zero-bias conductance values GN (0) measured at the same temperatures. The full lines in the figure have been calculated taking into account the effect of the Fermi function on the linear conductances at the given temperatures. We observe that: ( 1 ) Except for thermal rounding at low biases, the normal conductances appear to have a striking linear dependence on both phases. (2) The lower T¢ compound shows a steeper variation in voltage of the conductance. We have found a = 0.005 m V - l for the slope of the conductance of sample 90/39 (T¢=90 K), and a = 0 . 0 2 0 mV -~ for sample 60/48 (To=60 K). These a values are highly reproducible with variations of less than 15% on samples with the same To. They do not depend on the junction resistance nor on the nature of the counterelectrode (Pb/Bi, Au, Bi, Sb have been also used). We have found that the conductanceslopes are quite insensitive to temperature variations, as shown in fig. 4 referring to junction 90/88, To=90 K, ( a ) , and junction 60/46, To=60 K, (b). Intermediate a values have been measured for YBCO single crystals oxygen deficient
A.M. Cucolo et aL / Planar tunnel junctions on YBCO single crystals
i
i
i
A
.~ 0.7
~
//
120K
f
295K
OJ
0.6 \ \
/
78K
62 K 0.3 -150
I
-75
~I" 0
I
75
151
V(mV) Fig. 4. Voltage dependence of the GN( V)/GN (100 mY) at the indicated temperatures, for junction 90/88 ( Tc= 90 K) (a), and junction 60/46 (To=60 K) (b).
with T¢=80 K and Ni doped with T~=75 K. The junction preparation method has been the same in all the cases.
5. Discussion and conclusions From expression (2), deviations of the conductance curves from the usual V 2 dependence, can be understood in terms of density of states effects NN (E) and tunneling probabilities P(E). In fact several explanations for the linear conductance have been proposed including barrier effects [ 13 ], inelastic tunneling [9 ], and density of states effects as in the resonating-valence-bond (RVB) model [ 14 ]. At the moment no one of the proposed mechanisms has been unambiguously confirmed. However, the presence of the linear background conductances in point contacts and in planar junctions made on different high-T¢ materials, single crystals and films, independently from the junction formation
73
method, seems to be strong evidence that this behavior is intrinsic in these compounds. In the YBCO 60 K phase, we have found normal conductances steeper and apparently in contrast with the BCS theory; the gap-like structures are reduced in amplitude and shifted towards higher energies. These results are consistent with the presence of BCS structures superimposed on linear background conductances [ 15 ]. In the lower Tc compound, with steeper slope, the gap-like structures, in fact, are reduced and smeared out and both the opening and the "true" value of the energy gap can be masked by the same effect [15]. The results presented in this paper are consistent with what has been recently found on YBCO single crystals with different oxygen surface stoichiometry [ 16 ] and indicate that, at the moment, little can be said about the value of the 2A(O)/kTc ratio in the lower Tc compound. At the same time, our data seem to be another manifestation of effects recently found by N M R spectroscopy and specific heat measurements. In fact, N M R studies [ 17 ] have revealed a temperature-varying spin susceptibility at the planar sites and, in the 60 K phase, the absence of any prominent feature at Tc strongly contrasts with the behavior of the 90 K phase. Similar contrasts have been found in specific heat behavior where the lower Tc compound exhibits at T = T c a peak diminished compared to the 90 K phase [ 18 ]. To conclude, the normal state properties seem to be an important key for a better understanding of the superconducting properties of the high-T~ superconductors. We believe that measurements of the normal state conductance as well as rigorous controls on the quality of the junctions, in all cases, complement the information obtained by superconducting tunneling data.
Acknowledgements We wish to thank, with pleasure, many colleagues for their suggestions and support. These include M. Gurvitch, G.A. Thomas, T.T.M. Palstra, R.E. Walstedt, R. Vaglio and R.D. Parmentier. We thank J.V. Waszczak, J. Garno and G. Perna for technical as-
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A.M. Cucolo et al. / Planar tunnel junctions on YBCO single crystals
sistance. O n e o f us, ( A M C ) , has b e e n s u p p o r t e d by a N A T O / C N R grant.
References [ 1 ] See for example: R.J. Cava, A.W. Hewat, E.A. Hewat, B. Batlogg, M. Marezio, K.M. Rab¢, J.J. Krajewski, W.F. Peck Jr. and L.W. Rupp Jr., Physica C 165 (1990) 419. [2] J.S. Tsai, I. Takeuchi, J. Fujita, S. Miura, T. Terashima, Y. Bando, K. Iijima and K. Kamamoto, Physica C 157 ( 1989 ) 538; A.P. Volodin, B. Ya. Kotyuzhanskii and G.A. Stefanyan, JEPT Lett. 48 (1988) 502. [3] L.F. Schneemeyer, J.V. Waszczak, T. Siegrit, R.B. Van Dover, L.W. Rupp, B. Batlogg, R.J. Cava and D.W. Murphy. Nature 601 (1987) 601. [4 ] M. Gurvitch, J.M. Valles Jr., A.M. Cucolo, R.C. Dynes, J.P. Garno, L.F. Sehneemeyer and J.V. Waszczak, Phys. Rev. Lett. 63 (1989) 1008; ibid., Phys. Rev. B, submitted. [5] M. Gurvitch, J.M. VaUes, A.M. Cucolo, R.C. Dynes, J. Garno, L.F. Schneemeyer and J.V. Waszczak, Physica C 162-164 (1989) 1067.
[6] A.M. Cucolo, R. Di Leo, P. Romano, L.F. Schneemeyer, J.V. Waszczak, IEEE Trans. Magn. 27 ( 1991 ) 1349. [7] A.M. Cucolo, J.M. Valles, R.C. Dynes, M. Gurvitch, J.M. Phillips and J.P. Garno, Physiea C 161 (1989) 351. [8] M. Lee, M. Naito, A. Kapitulnik and M.R. Beasley, Solid State Commun. 70 (1989) 449. [9] J.R. Kirtley and D.J. Scalapino, Phys. Rev. Lett. 65 (1990) 798. [ 10] I. Takeuchi, J.S. Tsai, Y. Shimakawa, T. Manako and Y. Kubo, Physica C 158 (1989) 83. [ 11 ] K Sharifi, A. Pargellis, R.C. Dynes, B. Miller, E.S. Hellman, J. Rosamilia and E.H. Hartford Jr., Phys. Rev. B, submitted. [ 12] J.F. Zasadizinski, N. Tralshawala, Q. Huang, K.E. Gray and D.G. Hinks, IEEE Trans Magn. 27 ( 1991 ) 833. [ 13] J.R. Kirtley, Int. J. Mod. Phys. B 4 (1990) 201. [ 14] P.W. Anderson and Z. Zou, Phys. Rev. Left. 60 (1988) 132. [15] A.M. Cucolo, C. Note and A. Romano, Phys. Lett., submitted. [ 16 ] A.M. Cucoto, R. Di Leo, P. Romano, L.F. Schneemeyer and J.V. Waszczak, Phys. Rev. B, to be published. [ 17] R.E. Walstedt, W.W. Warren Jr., R.F. Bell, R.J. Cava, G.P. Espinosa, L.F. Schneemeyer and J.V. Waszczak, Phys. Rev. B41 (1990) 9574. [ 18 ] A. Junod, D. Eckert, T. Graf, G. Triscone and J. Muller, Physica C 162-164 (1989) 1401.