Tunneling spectroscopy on high-Tc superconductors: Certainties clues and ambiguities

Physica C 153-155 (1988) 1712-1717 North-Holland, Amsterdam

TUNNELING SPECTROSCOPY ON HIGH-Tc SUPERCONDUCTORS: CERTAINTIES CLUES AND AMBIGUITIES.*

Antonio BARONE Dipartimento di Scienze Fisiche, Universith di Napoli, P. Tecchio, Napoli, Italy and Istituto di Cibernetica, CNR, Arco Felice, Napoli, Italy. Since about 30 years tunneling spectroscopy has played a fundamental r61e in determining significant aspects of superconductivity. Recently, high-Tc superconductive ceramics have been the subject of intensive studies also in such a context. However, the results collected so far can hardly be cast in an unambiguous frame. E x p e r i m e n t a l data are discussed in connection with different junction configurations. 1. INTRODUCTION Superconducting Tunneling Spectroscopy is a widely recognized diagnostic tool in the field of superconductivity. Accordingly, since the discovery of high critical temperature superconducting ceramics (1-3) such techniques have been largely e m p l o y e d to cast some light on various aspects of these new materials which are more and more in the limelight of the scientific c o m m u n i t y . However, the amount of results obtained in many laboratories all over the world and in some cases even in a single laboratory, show signatures often not consistent with each other. I shall try to make some comments to point out, as stated in the title, certainties and ambiguities of such results. In so doing, I shall try to avoid the temptation of being certain a b o u t a m b i g u i t i e s and a m b i g u o u s about certainties. Although the literature I shall consider is quite large, it will be by necessity not exaustive nor fully updated. I apologize with the colleagues for that. Furthermore, I shall focus the attention on some aspects of the work carried out at the Universities of N a p l e s (Department of Physical Sciences) and Salerno (Department of Physics), and at the Istituto di Cibernetica of the CNR in Arco Felice, (Naples).

*work partially supported by the INFN, Napoli 0921-4534/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

2. JUNCTION CONFIGURATIONS As it is well k n o w n interesting information on the superconductivity can be drawn by referring to single particle and Cooper pairs (Josephson effect) tunneling. To this purpose a spectroscopy study in a wide sense, may include barrier junction structures as well as other weakly coupled superconductor configurations. In the case of ceramic superconductors the canonical sandwich type junctions are by far the most c o m p l i c a t e d to be realized. Another weak link configuration of interest is a bridge type film junction, whose realization implies the availability of very reliable superconducting c e r a m i c films. Such p o s s i b i l i t y h o w e v e r appears to be only a necessary condition since in principle link dimensions, L, should be smaller than the coherence length, ~o • For these materials the condition L < ~o is ruled out by the small values of ~o (order of 10 A). A well established procedure to realize weak links in conventional superconductors is the creation of a link on a film by a suitable selective s u p p r e s s i o n of s u p e r c o n d u c t i v i t y by ion implantation. This was successfully realized by the IBM group (4) on YBCO film. In that case a link 17 Izm wide was realized and in spite of the experimental condition L >> ~o the Josephson weak link was obtained.

A. Barone / Tunneling spectroscopy on high-Tc superconductors

This circumstance was correctly ascribed to the granularity of the superconductor ceramic. Point contact structures represent the class of junctions more widely employed for the tunneling spectroscopy in the high T c superconductors (e.g. ref.5). The reason lies in the r e l a t i v e l y e a s y t e c h n o l o g y and "adjustability" of these d e v i c e s . These structures however have to be regarded as a parallel combination of tunnel and bridge type junctions. In the next sections we shall focus our attention mainly on these structures and on "bulk junctions"(6) configurations. The letter category includes junctions made by bulk ceramic (namely YBCO pellet) overlayed by a suitable superconducting film such as Pb or Nb. These planar junctions approach to some extent the sandwich film type junctions (7). 3. ENERGY GAP DETERMINATION. Since the beginning of the i n v e s t i g a t i o n s on h i g h - T c c e r a m i c , p o i n t contact appeared to be the more s t r a i g h t f o r w a r d t e c h n o l o g y to detect and evaluate from the current-voltage c h a r a c t e r i s t i c s and their d e r i v a t i v e s the energy gap (or gaps) if any. A large number of experiments both on La and Y superconductors were performed. More frequently a point made by a conventional superconductor (e.g. Nb, Pb) was employed in contact with a ceramic pellet. Although "symmetric" structures, point contact (e.g. Y B C O - Y B C O ) (8) or "break j u n c t i o n " ( 9 ) configurations were also investigated. A large variety of gap values were reported leading consequently to different values of the ratio 7 = 2A(0) /kTc which is the well known fundamental parameter underlying the theoretical interpretation of the mechanism of superconductivity. Lack of uniformity in the results and lack of space in this article suggests to resort to the statistical sketch in Fig.1. (10) Looking at the YBCO we can see that in spite of the wide zoology of the data these tend to collect around a value of ~/ = 4-5.

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KBTc Fig. 1 - Rough indication of the values of the parameter ~/ reported in the literature. Let us try to understand to what extent these results can be reliable. Usually these e x p e r i m e n t s do not show very clean I-V curves with a well defined gap structure, rather one has to resort to dI/dV curves,(or high order derivatives). However often also such curves are not suitable of unambiguous interpretation. Let us summarize a variety of processes and circumstances that can originate additional finite voltage structures in the I-V and dI/dV curves which can lead to a different degree of uncertainty in the determination of the energy gap. 1) Step structures arising f r o m higher order tunneling processes ( n a m e l y to the order T 2m, T being the tunneling matrix element) 2) Lifetime e f f e c t s of q u a s i - p a r t i c l e s resulting in the smearing of the density of states and therefore on the gap discontinuity. 3) Leakage currents which can add to the tunneling current especially for N - N or N-S junctions. 4) Finite voltage structures due coupling in Josephson junctions.

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A. Barone / Tunneling spectroscopy opt h igh-T c superconductors

5) Proximity effects which o c c u r at s u p e r c o n d u c t o r - n o r m a l interface present in different structures (e.g. Andreev reflection process in point contacts ). 6) Giavier-Zeller effect occurring in granular superconductors and resulting in a substantial linear dependence of the dynamic conductance vs. voltage (except in the smallest voltage region). 7) In c o n n e c t i o n with the p r e c e e d i n g point, subgap structure (A/n) for a small size of granular layer distribution. 8) G a p a n i s o t r o p y which leads to a measure of a certain average gap (at least in polycrystalline samples). 9) S t o i c h i o m e t r i c variation in different grains in granular s u p e r c o n d u c t o r s which would lead to local changes of all superconducting parameters. 10) Pressure effects occurring locally in point contact structures as well as non uniform strain even in planar junctions.

In this light let us now consider the implications on the study of the high Tc materials. A distinction should of course be made between La-Sr and Y-Ba compounds. For instance, the effect of the pressure on the f o r m e r m a t e r i a l has s h o w n a d r a m a t i c i n c r e a s e in the e n e r g y gap (2-3 times) whereas pressure independent g a p was observed for the YBCO (8). Let us confine ourselves in the following to this latter compound. 4. POINT CONTACT TUNNELING In Fig. 2 is represented dV/dI vs. V curves for a Y B C O - N b point contact at different temperatures (5). This is to our k n o w l e d g e one of the few reported data showing both J o s e p h s o n e f f e c t and gap structure on the same sample. In addition to the YBCO-Nb junction the marked signature (Nb/N) of superconductor-normal path, indicates the actual contact of the Nb with both superconducting and normal regions of the YBCO. A value 7 -= 2A(0)/kT = 4.8_+0.5 can be estimated.

1 1) Edge effects which can occur for a too naive electrode patterning technology adopted. 12) Magnetic flux which can be trapped making occasionally a region of the electrode normal. lOK

13) Heating effect which can act locally against superconductivity. Point contacts or narrow bridges can be effected by high current density in the s u p e r c o n d u c t i n g constriction. M o r e o v e r effect of depairing and fluctuations. All these aspects should p r o v i d e a rather (not fully) complete picture of the difficulties underlying the diagnostic use of tunneling spectroscopy. Obviously some of these items are related to the specific experimental procedure, some are of a more intrinsic nature.

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A. Barone / Tunneling spectroscopy on high- Tc superconductors

Superconductor-insulator-normal tunneling has also been investigated with good results (e.g. ref. 11,12). Also symmetric (YBCO-YBCO) point contacts have been successfully realized (e.g. 8). Of interest are the investigations of point contact on single crystal of YBCO. In ref.(13) experiments have been performed using a Ptlr tip on a single crystal obtaining different values of ~/ depending on whether the tip was approaching parallel or perpendicular to the CuO planes. In any case the values of the 2A/kTc ratio were rather close to that of reference (5). On the other hand, as stated by the authors, some ambiguity remains due to the necessity of driving the tip deeply into the sample to guarantee superconducting coupling. In contrast with tunneling experiments, far infrared spectroscopy on randomly oriented ceramics (both La-Sr and Y-Ba) gives values of "~ smaller than that predicted by BCS (see for instance the IBM work review (14) and r e f e r e n c e s cited therein). However, subsequent results (15) on infrared reflectivity from epitaxial films of YBCO give a value of ~/ = 4.7*_1.2 in agreement with most c o m m o n tunneling results (ref. 5). It is also of interest considering the agreement found for the gap value by internal friction and f r e q u e n c y measurements (16). There is a large number of papers which report only on the Josephson effect considering either or both d.c. and a.c. phenomena. Among the first experiments in this context, I wish to recall early work by de Waele et al (17) and Likharev et al. (18) on symmetric YBCO-YBCO Josephson structures. A large variety of interesting results obtained in Japan part of which are collected in (19). Analogously there is a wide literature on this topic on work carried out in the Republic of China (e.g. 20). It is worth recalling that from the temperature d e p e n d e n c e of the m a x i m u m J o s e p h s o n current Ij, informations on the gap can also be inferred. Point contacts as well as bridge junctions have been successfully employed.

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Very little is reported, at least explicitly, on the m a g n e t i c field d e p e n d e n c e of the Josephson current. This represents in my view a rather serious lack of information for the understanding of a Josephson junction. Detailed experiments and good results have been obtained on the a.c. Josephson effect showing the current behaviour when exposed to a microwave field, namely the induced steps in the current-voltage characteristics (e.g. 21). Effect of noise should also deserve some attention (22). Looking at the results obtained by point contacts as a whole, it appears that the possibility of using bulk material toghether with the intrinsic adjustability of these structures allowed an impressive amount of interesting results. However, as stated in Section 3, various sources of ambiguities arise which should be carefully taken into account. It can also likely o c c u r that additional natural tunnel junctions betwen the grains of the ceramic can alter the results. From this point of view the changing in the contact pressure without observable change in the location of the gap (5) in the current-voltage characteristics could guarantee against such a circumstance. 5. PLANAR JUNCTIONS I shall include in such a non very rigorous definition both 'bulk junctions" and thin films sandwich type j u n c t i o n s . As previously discussed, the former category is represented by devices consisting of bulk ceramic material as base electrode and a c o n v e n t i o n a l s u p e r c o n d u c t i n g film as a c o u n t e r e l e c t r o d e (6,23). O b v i o u s l y such structures, differently from point contact, cannot be adjusted "a posteriori" however they represent an intermediate step toward classical fully thin films sandwich junctions device. We realized (University of Naples) good Josephson junctions (6) with magnetic field modulation of the supercurrent typical of a double j u n c t i o n . S u r p r i s i n g l y no significant change were observed in the sample b e h a v i o r after three months of storage at room temperature. A variety of bulk junctions were realized also (Istituto di Cibernetica CNR) (24) using also

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A. Barone / Tunneling spectroscopy on high-T c superconductors

semiconductor barriers as summarized in Fig. 3. Further studies in this context have been developed at the University of Salerno (25) where T a 2 0 5 barriers were employed. Back Sputtering

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All these structures were characterized by r e a s o n a b l e J o s e p h s o n type behavior shown in the I-V curves. However no success was obtained in trying to e v a l u a t e the energy gap either by I - V or d I / d V c h a r a c t e r i s t i c s . It was always present a background which can be ascribed to the Giaever-Zeller effect (see Section 2) and a number of peaks not suitable for a satisfying interpretation. Such peaks structure is in any case related to the c e r a m i c since it disappears only at high temperature (60 to 90K). Initial attemps in c o m p a r i n g the behavior of the ceramic with sintered pellets of granular Nb (25) provides additional evidence that such "spurious" effects should be strictly connected to the intrinsic granularity of the ceramic. It is interesting to observe the close analogy between the v o l t a g e - c u r r e n t and dV/dI curves obtained for the superconducting ceramic bulk junctions and the result discussed many years ago (26) concerning 2 and 3-dimensional arrays of point contact J o s e p h s o n j u n c t i o n s realized by a large number of Nb spheres. Indeed compressed superconducting powders were investigated. Since many years giving similar results (see refs. reported in 26). Thus bulk junctions although very interesting show limits inherently related to the sintered materials.

As far as intergrain tunneling the reader is referred to a recent paper (27) where Andreev reflection is also discussed. As far as the f i l m - f i l m j u n c t i o n s c o n f i g u r a t i o n is c o n c e r n e d the uncertain structure of I-V and dI/dV vs. V characteristics reproduced essentially that of the bulk j u n c t i o n s . O b v i o u s l y the great interest of f i l m j u n c t i o n s remains of paramount interest (7). CONCLUSIONS A brief discussion has been given on the tunneling spectroscopy as a diagnostic tool for the new high T c c e r a m i c s . The attention has been mainly confined to the Y B C O c o m p o u n d . Although a n u m b e r of important informations can be inferred, it is unrealistic to consider the m e a s u r e m e n t s performed so far as conclusive. Far from being pessimistic it is important, in my opinion, to continue on these investigations within the natural coexistence of enthusiasm and caution.

REFERENCES (1) (2)

(3)

(4)

(5)

J.G. Bednorz and K.A. M/Jller, Z. Phys. 64,189 (1986). M . K . Wu, J. R. Ashburn, C.J. Torn, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q Zang, and C.Z. Chu, Phys.Rev.Lett. 58, 908 (1987). R.J. Cava, R.B. Van Dover, B. Batlog and E. A. Rietman, Phys. Rev. Lett. 58, 408 (1987). R.H. Koch, C..P. Umbach, G.J. Clark, P. Chaudari and R.B. Laibowitz, Appl. Phys. Lett. 51, 200 (1987). A. Barone, A. Di Chiara, G. Peluso, U. Scotti di Uccio, A.M. Cucolo, R. Vaglio, F.C. Matacotta, E. Olzi, Proc. International Workshop on "Novel Mechanisms of Superconductivity", June 22-26, Berkeley, (Ca.) (S. A. Wolf and V.Z. Kresin Eds. Plenum 1987), same authors, Phys.Rev. B36, 235 (1987).

A. Barone / Tunneling spectroscopy opt high- T c superconductors

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A. Barone, A. Di Chiara, G. Peluso, U. Scotti di Uccio, F. C. Matacotta, E. Olzi, Rivista del Nuovo Cimento D9, 727 (1987). M.G. Blamire, G.W. Morris, R.E. Somekh, J.E. Evetts , J. Phys. D Appl. Phys. 20, 1330 (1987) H. Koch, R. Cantor, J.F. March, H. Eickenbush, R. Sch611horn, Phys. Rev. B36, 722 (1987). J. Moreland, A.F. Clark, H.C. Ku and R.N. Shelton, Cryogenics 27, 227 (1987). The results are taken from the current literature. Due to a lack of space detailed bibliography will be reported elsewhere. M.F. Crommie, L.C. Bourne, A.Zettl, M.L.Cohen, A. Stacy, Phys.Rev.B35, 8853 (1987). P.J.M. Bentum, L.E.C. van de Leemput, L.W.M. Schevrs, P.A.A. Teunissen, H. van Kempen, Phys.Rev. B36, 843 (1987). J.R. Kirtley, C.C. Tsuei, S.I. Park, C.C. Chi, J. Rozen, M.W. Shafer, W.J. Gallegher, R.L. Sandstrom, T.R. Dinger, D.A. Chance, Japan. J. Appl. Phys. 997, (1987). A.P. Malozemoff and P.M. Grant, Z. Phys.B Condensed Matter 67, 275 (1987) (see also recent far infrared study on several ceramics by A. Withlin et al, Phys. Rev. B37,652 (1987). R.T. Collins, Z. Schlesinger, R.H. Koch, R. B.Laibowitz, T.S. Plaskett, P. Freitas, W.J. Gallagher, R.L. Sandstrom and T.R. Dinger Phys. Rev. Lett. 59, 704 (1987).

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G. Cannelli, R. Cantelli, F. Cordero, G.A. Costa, M. Ferretti, G.L. Olcese, Phys. Rev. B36, 8907 (1987). A. Th. A.M. de Waele, R.T.M. Smokers, R.W. van der Heijden, K. Kadowaki, Y.K. Huang, M. van Sprang, A.A. Menovsky (preprint March 1987) and Phys. Rev. B35, 8858 (1987) K. K. Likharev et al (Private Communication March 1987). See Japanese J. Appl. Phys. vol. 26, no.5 (1987) and Proc. of LT19 Int. Conf. on Low Temperature Physics, Kyoto 1987, (Japan. J. Appl.Phys. 26, Suppl.26, (1987). High Temperature Superconductivity, Beijing, P.R. China, June 1987 (Z.Z. Gau, G.J. Cui, G.Z. Yang, Q.S. Yang Eds., World Scientific, 1987). J. Niemeyer, M.R. Dietrich, C. Politis, Z. Phys. B, Condensed Matter, 67, 155 (1987). See for instance J. Kuznik, M. Odehnal, S. Safrata, J. Endal, J. Low Temp. Phys. 69, 313 (1987). H. Watanabe, Y. Kasai, T. Mochiku, A. Sugishita, I. Igushi, E. Yamaka, Japan. J. of Appl. Phys. 26, 120 (1987). C. Camerlingo, R. Cristiano, M. Russo, P. Silvestrini (to be published) Private Communication. T.D. Clark, Phys. Rev. B8, 137 (1987). H.F.C. Hoevers, P.J.M. van Bentum, L.E.C. van de Leemput, H. van Kempen, A.I.G. Schellingerhout, D. van der Maral, Physica C152,105,(1988).