Effect of surface on the conductance characteristics of AuBi2Sr2CaCu2O8+δ(single crystal) point contact junctionsH. Srikanth and A. K. Raychaudhuri Citation: Journal of Applied Physics 70, 7478 (1991); doi: 10.1063/1.349744 View online: http://dx.doi.org/10.1063/1.349744 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/70/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Broadly tunable, high-power terahertz radiation up to 73K from a stand-alone Bi2Sr2CaCu2O8+ δ mesa Appl. Phys. Lett. 105, 202603 (2014); 10.1063/1.4902336 Improved characteristics of mesa-type intrinsic Josephson junctions by vacuum cleavage process forBi2Sr2CaCu2O8+δ/Au contacts J. Vac. Sci. Technol. A 30, 031101 (2012); 10.1116/1.4707151 Resistive switching behaviors in Bi2Sr2CaCu2O8+d (Bi-2212) stacks J. Appl. Phys. 109, 07E147 (2011); 10.1063/1.3562521 Magnetic flux oscillations in partially irradiated Bi 2 Sr 2 CaCu 2 O 8 + δ crystals J. Appl. Phys. 105, 07E310 (2009); 10.1063/1.3068647 Peak Effect as Precursor to Lockin State in Bi2Sr2CaCu2O8+δ Single Crystal AIP Conf. Proc. 850, 799 (2006); 10.1063/1.2354946

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

209.183.185.254 On: Tue, 25 Nov 2014 03:09:49

Effect of surface on the conductance c aracteristics of Au-Bi&CaCu 2 0 a+6 (single crystal) point contact junctions

H. Srikanth and A. K. Raychaudhuri Department of Physics, Indian Institute of Science, Bangalore-12, India

(Received 22 July 1991; accepted for publication 1 September 1991)

We report the effect of surface treatments on the dynamic conductance curves (G=dUdV- V) of Au-BizSrzCaCuz08 + 6 (single crystal) point contact junctions of variable

junction conductances ( 100 mS > G > 100 ,uS). We find that if the crystal surface is cleaved freshly just prior to making contacts, all irreproducible sharp multiple features often observed in tunneling data of Bi(2212) oxide superconductors disappear. If the cleaved crystal surfaces are left under ambient conditions for a few days and the tunneling experiments are repeated, these multiple features reappear. We also find that if the current in the junction is made to pass predominantly through the bulk (and not along the surface), gap features are sharper. The observed conductance curves are fitted to a modified model [G. E. Blonder et al., Phys. Rev. B 25, 4515 (1982)] and estimated gap values are A.-28 to 30 meV corresponding to the ratio 2A/kBTc N 7.5 with lifetime broadening r/A-0.2. We conclude that the sharp multiple features observed in Bi( 2212) tunneling curves has no intrinsic origin in the bulk and they arise from the surface only.

I. INTRODUCTION

Point contact spectroscopy (PCS) and tunneling spec- troscopy have been widely used to study basic properties like the superconducting energy gap (A), density of states near the Fermi level, and weak link behavior in conven- tional as well as high temperature oxide superconductors. Energy resolved information can be obtained from the dy- namic conductance (dI/dV-V) curves of point contact and tunnel junctions formed on oxide superconductors.‘*2 However, the dynamic conductance curves of high T, su- perconductors often show a variety of unusual features that cannot be explained by available theories.“” Prominent among them are a linear variation of the (dI/dV- V) curve at high bias voltages, a conductance peak of width ~-8 to 10 mV near zero bias, and multiple peaks occurring at certain characteristic energies in addition to the main gap structure (corresponding to the ratio ZA/kT, 9 4-8). Very often these multiple peaks are rather sharp and they make a clear identification of the superconducting gap (A) some- what ambiguous. They are particularly prominent in Bi(2212) or Bi(2223) phases of the high T, superconductors.‘“” The origin of these sharp peaks is a point being actively investigated by various groups. The intrinsic nature of these materials like short coherence lengths and large anisotropy may account for some of the new features seen in the (dI/d V- V) curves but they hardly explain the sharp multiple features. Another aspect is that these features are often rather irreproducible which points to an extrinsic origin. In addition to these sharp features, the (dI/dV-V) curves of Bi(2212) junctions often show reproducible shallow features which may be ascribed to phonons. 13-15 In this paper we specifically address this is- sue and show the effect of surface treatment on the dy- namic conductance curves.

A number of point contact studies using cryogenic scanning tunneling microscopes (CSTM) have been re-

ported on Y1BazCu307 _ 6 ( 123) and BilSr,?Ca1C!uz08 + 6 (2212). 2#j~*2*1c19 Some of these claimed to have achieved vacuum tunneling and also obtained surface images after cleaving. LU’-‘~ A number of studies on tunnel junctions have also been reported after various types of mechanical, chemical, and in situ surface treatments.21-“4 These studies indicate that in all likelihood an insulating top surface ox- ide layer prevents proper tunneling and in most cases the tip had to be driven into the surface to obtain gaplike features. More recent reports have clearly demonstrated that surface treatment of the crystals (chemical etching or cleaving) vastly improves the dynamic conductance (cfl/ dV- V) curves and yields reproducible results.23*24 (How- ever, note that some chemical etching procedures and in situ surface treatments have led to degradation of the sur- face. >

In this article we report systematic investigations of dynamic conductance (dI/o!V-V) curves of Au-Bi2Sr2CaCu20s + 6 (single crystal) point contact junc- tions (for various junction resistances) when the crystal surface is (a) untreated and (b) cleaned and freshly cleaved. The purpose of this article is to show (i) the role of cleaving the surface on the dynamic conductance curves; (ii) the specific dependence of multiple features on junc- tion resistance; and (iii) that the reproducible dynamic conductance curves in clean point contact junctions can be quantitatively fitted to the model suggested by Blonder, Tinkham, and Klapwijk (BTK), (see Ref. 27) with some modification.

II. EXPERIMENT

Point contact junctions were made in liquid helium by pressing an electrochemically etched Au wire (diameter -0.5 mm, RRR~L30) against the surface (a-b plane) of the Bi( 2212) single crystal. A differential screw arrange- ment was employed to vary the junction resistance so that

7478 .I. Appl. Phys. 70 (12), 15 December 1991 0021-8979/91/127478-08$03.00 @ 1991 American institute of Physics 7478

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

209.183.185.254 On: Tue, 25 Nov 2014 03:09:49

Mechanical assembly for /fine positioning of tip

- Au Tip

Bi (2212) crystal

L Copper foil

FIG. 1. Schematic showing the two junction configurations in which the dI/dV-V were measured. 1,,Z2,Z3 and V,, C’,, V, are the current and volt- age terminals. Junction (l-2) corresponds to the current flowing through the point contact and along the surface of the crystal. Junction (l-3) corresponds to the current flowing through the point contact and the bulk of the crystal. The mechanical assembly for fine positioning of the tip consists of a differential screw and a piezoelectric tube.

the I-V and dI/d V- V characteristics for different junction resistances can be measured. Details about the cryostat design and instrunientation have been reported elsewhereT5

The Bi(2212) single crystals were grown by the flux melting technique. The quality of typical crystals used in this work were tested both by resistivity (p) and’suscepti- bility (x) and they showed a typical T, = 80 K (zero re- sistance). No tailing or multiple steps were observed in the transition region either in p or x curves.

The sample crystal was polished with a fine grade em- ery paper and fixed on a clean copper foil with conductive silver paint. Thin copper wires (#48 gauge) were used as leads for electrical connections. Pairs of leads were at- tached to the Au tie, the crystal surface, and the bottom copper foil, as shown in the schematic in Fig. 1. This ar- rangement enabled us to compare the (dI/dV- V) curves obtained when current predominantly passes along the path 1-2 (parallel to the surface) and along the path 1-3 (through the bulk of the crystal). In a previous paper we had compared the-point contact tunneling curves and pla- nar junction tunneling curves using a similar cotiguration.26

The sequence in which the surface treatment of the sample was done before making the point contact is as follows:

(i) The Bi( 2212) crystal was cleaned with methanoi but the surface on which the point contact was made was not subjected to any treatment like cleaving or mechanical polishing. The opposite face of the crystal was polished with fine grade emery paper and was mounted on a copper foil with silver paint.

(ii) The same crystal was cleaned with methanol and the surface on which the junction was formed was polished with a fine grade emery paper. The top surface of the crys- tal was cleaved by sticking an adhesive tape and peeling it away j&t prior to making contact. In this method the freshly cleaved surface was exposed to air for less than a couple of minutes. During the experiment, the surface was

7479 J. Appl. Phys., Vol. 70, No. 12, 1.5 December 1991 H. Srikanth and A. K. Raychaudhuri 7479

2.5

2.0

8 51.5 (3

1.0

0.5

0.0 -1E

Tip Voltage (mV)

FIG. 2. A set of dynamic conductance (G-v) curves for different junction resistances obtained when the crystal surface was not subjected to me- chanical polishing or cleaving. The data have been normalized with re- spect to G (100 mV) and the curves have been shifted for clarity. The junction resistance varies over two decades from top to bottom. The G (100 mV) values from top to bottom are 30, 10, 7, 4, and 0.2 mS, re- spectively. The arrows indicate the sharp muitiple features or extra fea- tures.

exposed to gaseous or liquid helium. (iii) The same Bi( 2212) crystal was left in a desiccator

for about seven days and the surfaces were neither chem- ically cleaned nor cleaved.

After each step of surface treatment, we obtained a set of dynamic conductance curves by varying the junction resistance. The motivation behind the above sequence of surface treatments was to check if the “dead” oxide layer on the crystal was responsible for the irreproducibility of the tunneling data, in particular the sharp features.

In all cases the dynamic conductance. (dI/d V- V) for different junction resistances were measured by an ac mod- ulation technique.25

Ill. RESULTS AND DISCUSSION

Figures 2, 3, and 4 show a collection of dI/dV- V curves that correspond to the three surface treatment con- ditions stated in the preceding section. The data were taken by varying the resistance of the junctions. In all cases, the data have been normalized with respect to the dynamic conductance G( =dI/dV) at a tip bias of 100 mV and the curves have been shifted for the sake of clarity. Figure 2 shows the data for the surface cleaned with methanol but not subjected to any mechanical polishing or cleaving. Fig- ure 3 shows the data for the same crystal but with a freshly cleaved surface. Figure 4 shows the data for the same crys- tal but exposed to air for seven days after cleaving. These figures in a nutshell sum up the essential observation that the fresh cleaving is a necessary precondition to obtain good and reproducible point contact data. The shtirp mul- tiple features (marked by arrow in Fig. 2) appear only in point contacts on uncleaved or improperly cleaved.surface. &visual inspection of the figures shows us the following features:

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

209.183.185.254 On: Tue, 25 Nov 2014 03:09:49

2.5 l___i

I------

I I I

0.0 I. .- --.- .I -- - .-._ 1L1 -150 -100 -50 0 50 100 1

Tip Voltage (mV) 150

FIG. 3. Dynamic conductance (G-k’) curves for different junction resis- tances obtained when the crystal surface is freshly cleaved by peeling an adhesive stuck to the surface just before making the point contact. ,The data are normalized with respect to G( 100 mV) and shifted. The G( 100 mV) values for the curves from top to bottom are 33, 19, 5.3, 1, and 0.2 mS, respectively. Note that the clear gap features are seen in the low junction resistance curves. The zero bias peak decreases with increasing junction resistance and eventually disappears for G < 1 mS.

(i) A peak in G-V at zero bias of width approx. S-10 mV exists in all the cases. As the junction resistance in- creases, this zero bias peak gradually diminishes in height and vanishes as G(0) becomes less than 1 mS. These rel- atively high resistance junctions [G(O) < 1 mS] show a lin- ear variation of G with V.

(ii) Comparison of the three figures shows that one can obtain clean conductance curves with freshly cleaved surfaces that do not show any sharp multiple features ob- served in the case of untreated surfaces. In fact, reappear- ance of these features on leaving the freshly cleaved surface for seven days in air proves convincingly that these features

-150 -100 -50 0 50 100 150 Tip Voltage (mV)

FIG. 4. The (G-y) curves obtained when the crystal is left at ambient conditions for seven days after cleaving and the experiment is repeated. Data normalized with respect to G( 100 mV) and shifted for clarity. The G( 100 mV) values for the curves from top to bottom are 9,6, 1.8,0.5, and 0.1 mS. Note the reappearance of the sharp extra features making iden- tification of gap features ambiguous.

7480 J. Appl. Phys., Vol. 70, No. 12, 15 December 1991 H. Srikanth and A. K. Raychaudhuri 7480

1.2

0.8 s

5 0.6

0.4

0.2

0.0

G (5OmV) mS

I I / I , 1 -40 -20 0 20 40 60

Tip Voltage (mV1

FIG. 5. The G-V curves showing the most prominent sharp features seen in high resistance junctions. The data are normalized with respect to G( 50 mV) and shifted for clarity. The G(50 mV) values from top to bottom are 0.09, 0.15, 0.28, and 0.9 mS. Note that the sharp features are quite sym- metric about zero bias. The inset shows that the characteristic voltages at which the sharp features occur 2Y* cc l/G(50 mV). For G(50 mV) > 1 mS, the zero bias peak develops and (G-v) curves show finer structure.

(whatever their cause) are associated not with the bulk but with the surface. And over days, the’cleaved surface dete- riorates so that the effect of cleaving is lost.

(iii) Clear gaplike features can be seen in the dynamic conductance of junctions made on freshly cleaved surfaces with 2A/kBTc f?: 7.5. (A quantitative estimation of the gap has been done later on.)

We stress that these conductance curves obtained on freshly cleaved surface are reproducible. The irreproduc- ibility of the multiple sharp features (which we call extra features) comes if the point contact is made on uncleaved surface. Even mechanical cleaning by polishing with fine abrasive may not lead to clean data and one observes these irreproducible extra features as can be seen in Fig. 5. (The data for junctions shown in Fig. 5 have conductances lower than N- 1 mS and as a result do not show the zero bias peaks.) The occurrence of these extra features leads to ambiguity in gap determination as is evident from Figs. 2 and 4. The importance of freshly cleaved surfaces in mak- ing point contact junctions cannot be overemphasized [re- cent studies done on Bi (22 12) crystals by using scanning tunneling microscopes support our observation].r2

To determine the energy gap (A) we have used a slightly modified version of the BTK mode127 for fitting the conductance curves of Fig. 3. The BTK model is very use- ful particularly in small area junctions (as those obtained in point contact junctions) because in this model the bar-

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

209.183.185.254 On: Tue, 25 Nov 2014 03:09:49

3.01 1 :: :: cr: 1:: 3’ ,.,I

. . . .._............ l.~~ : :

IYS),.Y . . . . . . + . . . . . . . .lw.. 0.5 -

0.01 I _J I I I I -150 -100 -50 0 50 100 150

Tip Voltage (mV)

FIG. 6. The experimental (G-v) curves fitted with a modified BTK model from which the superconducting gap (A) was estimated. The solid curve (-) represents the normalized data, the dotted curve (. . . ) shows the fit with the unmodified BTK model, and the dashed curve (- -) shows the fit with the modified BTK model with a lifetime broadening term (r) incorporated in it. The values for A, r, and z for curves are: curve (i) A = 28 meV, r = 5.6 meV, z=O.9 and curve (ii) A = 30 meV, r = 4.7 meV, z=O.7. Curves (i) and (ii) correspond to the third and second curves (from the top), respectively, shown in Fig. 3.

rier is approximated by a 6 function. This model uses a simple generalized semiconductor scheme to match the wave functions at the N-S interface and the current across the junction is expressed in terms of reflection and trans- mission coefficients which in turn are obtained by solving the Bogulibov equations. A dimensionless parameter 2 = H/Xuf [where Ha(x) is a repulsive potential located at the interface and uf is the Fermi velocity] represents the barrier strength. While the BTK model describes the 1-Y curves of point contacts made on conventional supercon- ductors, we find that some modification is needed to apply the same for high T, oxides. We have incorporated a life- time broadening28 term (r) in this model by taking the modified density of states as

p,(E,r)=Re{(E--ir)/[(E--ir)‘- A2]1’2). (1)

Following the notation-in Ref. 27 and using the Bogulibov formalism along with Rq. ( 1) for the modified density of states, we obtain the modified expressions for Andreev re- flection coefficients A (E,l?) and normal reflection coeffi- cients B(E,l?). The current across the junction is given as

I=C s

m -9-W - eV> --f(E))11 +A(E,r) -02

- &U-‘) Id& (2) where C depends on the density of states on the N side, the junction area, and the Fermi velocity, and f(E) is the Fermi function. The dI/dV curves are numerically simu- lated from the I-V curves generated by this model.

Figure 6 shows the data and the fit for two junctions from which the values obtained for A, l?, and z are (i) A = 28 meV, l? = 5.6 meV, z=O.9, and (ii) A = 30 meV,

I = 4.7 meV, 2=0.7. It also shows the fit with l? = 0 in which case there is an overshoot in G at the gap value. (z=O corresponds to the barrierless N-S contact. z> 5 im- plies a strong barrier and the conductance curve is then identical to that of tunnel junctions with a barrier.)

While the BTK model can be used to fit the data on junctions with low to moderate resistances [G( 100 mV) > 1 mS], it cannot fit the data for junctions with higher resistances [G( 100 mV) < 1 mS] as for these junctions the clear gaplike features are absent and once more the extra- neous features start to show up (see arrows in Fig. 3) even in clean point contacts. In these junctions the tip is almost “scratching” the top surface and we feel that in this case, though the junction current originates from the clean bulk, most of the carriers suffer collisions in the few defective surface layers and lose the memory of their initial states. As a result, no clear gaplike features are observable. In the case of uncleaved surface, the defective surface oxide layer is so thick (>g the coherence length) that scattering in this layer makes the current carriers (at least a major part of them) lose all information about the bulk superconductor and the junction characteristics refer to the property of this surface layer only. In this context it should be mentioned that different oxygen stoichiometry at the surface of high T, oxides (123) affects the tunneling curves, and tunnel

junctions made on oxygen deficient surfaces show smeared out gaplike features.29

The peak in G at zero bias has been reported by many groups and is generally associated with critical currents due to weak links present in the sample.2P30 In polycrystal- line samples, the grain boundaries act as weak links and in single crystals, defects and microcracks could be the origin of weak links. The fact that the Bi(2212) crystals cleave easily and are prone to cracking favors this suggestion. An interpretation for the sharp dips in G-V curves at specific voltages has also been given along those lines.18 Tachiki et al. have also proposed a model showing that multiple fea- tures could be generated in the dI/d V- V curves depending on the specific layer against which the tunneling barrier is formed.20 But our observation that the zero bias peak di- minishes as the junction resistance is increased suggests that these weak links occur in the vicinity of the contact area itself and weak links in the bulk of the crystal may not contribute. In other words, critical current due to proxim- ity induced weak links between the normal .metal tip and the superconductor most likely give rise to the peak in dI/dV at zero voltage.

The extra features, whose cause is most likely the sur- face near the region where the point contact is made, can- not be associated with any bulk characteristics such as phonons. However, the small modulations at 1 VI > A (see Fig. 3) can be associated with phonon structure as sug- gested before16’17 although no independent and confirma- tory proof exists for that. The sharp features that are prom- inent for junctions made on the uncleaved surface (and which also appear in high resistance junctions on the cleaved surface) seem to have a dependence on the junc- tion resistance. This is shown in Fig. 5. The sharpest fea- tures occur rather symmetrically around the zero bias.

7481 J. Appl. Phys., Vol. 70, No. 12, 15 December 1991 H. Srikanth and A. K. Raychaudhuri 7481 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

209.183.185.254 On: Tue, 25 Nov 2014 03:09:49

0.01 I I I I I -150 -100 -50 0 50 100 150

Tip Voltage (mV)

FIG. 7. The dynamic conductance (G-v) curves for different junction resistances when the crystal is cleaved and the current flows through the point contact and along the crystal surface [junction configuration (l-Z)]. Data normalized with respect to G( 100 mV) and G( 100 mV) values from top to bottom are 32, 18.8, 8, 3.2, and 0.35 mS. Note that clear gap features (as seen in Fig. 3) are absent.

(Note that the background G follows an almost linear re- lation with V.) The separation of these sharp features (de- noted by 2V*) is plotted in the inset ‘of Fig. 5 as a function of the junction conductance [G( 50 mV)]. Regarding these extra features (which are not gaps), we have the following observations to make. Fist, these spikes do not come from homogeneous bulk superconductors. Second, the voltage where the spikes occur ( V*> is sensitive either to the con- tact pressure or the junction area, and finally, it must be originating from the area around the tip. Otherwise the variation of the junction resistance cannot lead to the vari- ation of V*. Our present observations do not lead to any firm conclusion about its origin, but they unambiguously point to the fact that it originates in the area near the contact of the surface layer.

Before we end we would like to point out one more observation that shows the role of surface states on the junction conductance. The dI/d V- V curves in Fig. 3 cor- respond to the junction configuration (1-3) (see Fig. 1) when the current passes through the junction and through the bulk of the crystal. We show in Fig. 7 a set of tunneling curves that correspond to the situation when the current passes through the junction and along the surface of the crystal. Data in Figs. 3 and 7 were collected one after the other on the same junction made on a freshly cleaved sur- face. Figure 3 shows much better data with clear gaplike features. The curves in Fig. 7 display finer structure in dI/dV at low bias voltages and a clear gap structure is absent. This difference most likely originates from the large anisotropy in Bi(2212) oxide superconductors. The mate- rial within a length scale of a few 6 around the point of physical contact is generally probed by the point contact method. Since 6 in these materials is very small, the direc- tion of the current flow should not matter much unless c itself is anisotropic, as is the case here. When the current Sow is in the (l-2) configuration, the predominant eleo

7482 J. Appl. Phys., Vol. 70, No. 12, 15 December 1991

tronic states that take part in the junction conduction are the surface states of the defective surface layer. As a result the gap features are suppressed in this configuration com- pared to those seen in the (l-3) configuration where the states are predominantly in the bulk. This observation lends direct support to our notion that clean and reproduc- ible point contact spectra can only be obtained if we ensure that the electrons that take part in the conduction at the junction originate in the bulk and reach the counterelec- trode without much scattering at the “dead” surface layer.

It is now well established that the surface of the high T, superconductor, if anything, is different from the bulk superconductor. Any surface sensitive probe (including tunneling and point contact spectroscopy) will be affected by the surface defects. It is generally believed that the sur- face of the 2212 single crystals are less prone to be defective than the surface of the 123 single crystals. Taking into consideration the vast body of tunneling and point contact spectra reported both on 2212 and 123 single crystals, it appears that the irreproducible sharp features in the con- ductance curves are a more common occurrence in 2212 single crystals. If these features indeed arise from the sur- face, as our experiment suggests, we have to reach the conclusion that there are certain qualitative differences be- tween the surfaces of these two crystals in the microstruc- ture level which lead to these peaks in Bi(2212) single crystals but not in ( 123).

IV. CONCLUSlON

In conclusion, we have investigated the effect of sur- face treatment on the tunneling characteristics of a Au- Bi(2212) point contact junction. The sharp multiple fea- tures often seen in the dI/dV-V characteristics are associated with the surface oxide layer and it is concluded that they are a result of the surface condition around the tip area. It has also been observed that in the junction configuration with current passing through the bulk of the superconductor the best tunneling curves are generally ob- tained. The extra features disappear once the crystal sur- faces are freshly cleaved.

The energy gap ,of Bi(2212) has been determined by fitting the data to a BTK model with lifetime broadening incorporated in it and we find 2A/kBTc _N 7.5 and r/A Z-0.2.

ACKNOWLEDGMENTS

We thank the DA& Government of India for the sponsored project and Professor C. N. R. Rao for the Bi(2212) single crystals. One of us (H.S.) wants to thank the UGC, Government of India for the award of a senior research fellowship.

’ E. L. Wolf, Principles of Electron TunneIing Specfroscopy (-Oxford Uni- versity Press, New York, 1985).

‘J. R. Kirtley, Int. J. Mod. Phys. B 4, 201 (1990). ‘J. M. Valles, Jr. and R. C Dynes, Mater. Res. Sot. Bull. 15.44 (1990). ‘Qiang Huang, J. F. Zasadzinski, K. E. Gray, E. D. Bukowski, and D.

M. Ginsberg, Physica C 161, 141 (!989).

H. Srikanth and A. K. Raychaudhurl 7482

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

209.183.185.254 On: Tue, 25 Nov 2014 03:09:49

sK. E. Gray, Mod. Phys. Lett. B 2, 1125 (1988). 6H. Srikanth and A. K. Raychaudhuri, Phys. Rev. B (to be published). ‘T. Ekino and J. Akimitsu, Phys. Rev. B 40, 6902 (1989). *Qiang Huang, J. F. Zasadzinski, K. E. Gray, J. Z. Liu, and H. Claus, Phys. Rev. B 40, 9366 (1989).

‘A. Kussmaul, J. S. Moodera, G. M. Roesler, Jr., and P. M. Teclrow, Phys. Rev. B 41, 842 (1990).

lo A. Andreone, A. Di Chiara, G. Peluso, U. Scotti di Uccio, F. Tafuri, A. Barone, G. Pepe, and F. Fontana, 11 Nuovo Cimento 12D, 863 ( 1990).

” A. Barone, Earlier and Recent Aspects of Superconductivity, edited by J. G. Bednorz and K. A. Miiller, Springer Series in Solid State Sciences (Springer, Berlin, 1990), Vol. 90.

“T. Hasegawa, M. Nantoh, and K. Kitazawa, Jpn. J. Appl. Phys. 30, L276 (1981).

13N. Miyakawa, D. Shimada, T. Kido, and N. Tsuda, J. Phys. Sot. Jpn. 58, 383 (1989).

“N. Miyakawa, D. Shimada, T. Kido and N. Tsuda, J. Phys. Sot. Jpn. 58, 1141 (1989).

“R. Koltun, M. Hoffmann, P. C Splittgerber-Hiinnekes, Ch. Jarchow, G. Giintherodt, V. V. Moshchalkow, and L. I. Leonyuk, Z. Phys. B (Condensed Matter) 82, 53 (1991).

16R Wilkins, M. Amman, R. E. Soltis, Phys. Rev. B 41, 8904 (1990).

E. Ben-Jacob, and R. C. Jaklevic,

“M. D. Kirk, J. Nogami, A. A. Baski, D. B. Mimi, A. Kapitulnik, T. H. Geballc, and C. F. Quate, Science 242, 1673 (1988).

‘*C K Shih, R. M. Feenstra, J. R. Kirtley, and G. V. Chandrasekhar, Phys: Rev. B 40, 2682 (1989).

i9T Hasegawa and K. Kitazawa, Jpn. J. Appl. Phys. 29, L434 (1990). “M Tachiki, S. Takahashi, F. Steglich and H. Adrian, Z. Phys. B (Con-

densed Matter) 80, 161 (1990). 2’D. J. Werder C. H. Chen, M. Gurvitch, B. Miller, L. F. Schneemeyer,

and J. V. Waszczak, Physica C 160, 411 (1989). 22D M Kroeger, A. Choudhury, J. Brynestad, R. K. Williams, R. A.

Padgett, and W. A. Coghan, J. Appl. Phys. 64, 331 (1988). 23M Gurvitch J M. Valles, Jr., A. M. Cucolo, R. C. Dynes, J. P. Garno,

L.‘F. Schnekmeyer, and J. V. Waszczak, Phys. Rev. Lett. 63, 1008 (1989).

“A. M. Cucolo, J. M. Valles, Jr., R. C. Dynes, M. Gurvitch, I. M. Phillips, and J. P. Garno, Physica C 161, 351 (1989).

25H. Srikanth and A. K. Raychaudhuri, Cryogenics 31, 421 ( 1991). “H. Srikanth and A. K. Raychaudhuri, Pramana-J. Phys. 36, 621

(1991). *‘G E Blonder, M. Tinkham, and T. M. Klapwijk, Phys. Rev. B 25,

4515’(1982). 28R C Dynes, V. Narayanamurti, and J. P. Garno, Phys. Rev. Lett. 41,

1509’( 1978). 29A. M. Cucolo, R. Di Leo, P. Romano, L. F. Schneemeyer, and J. V.

Waszczak preprint. “T. Walsh, J. Moreland, R. H. Ono, and T. S. Kalkur, Phys. Rev. B 66,

516 (1991).

7403 J. Appl. Phys., Vol. 70, No. 12, 15 December 1991 H. Srikanth and A. K. Raychaudhuri 7483 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

209.183.185.254 On: Tue, 25 Nov 2014 03:09:49