Point contact tunneling study of Bi2Sr2CaCu2O8 single crystals

Pli'YSICA

Physica C 183 (1991) 39-50 North-Holland

Point contact tunneling study of Bi2SreCaCu208 single crystals M . H . J i a n g t, C.A. V e n t r i c e Jr. 2, K.J. S t o l e s 1, S. T y a g i 2, N.J. D i N a r d o 2 a n d A. R o t h w a r f 1 Department of Electrical and Computer Engineering, 2 Department of Physics and Atmospheric Science, Drexel University, Philadelphia, PA 19104, USA

Received 17 June 1991 Revised manuscript received 12 September 1991

Point-contact tunneling spectroscopy measurements ofa high-T, superconducting Bi2Sr2CaCu2Ossingle crystal have been performed. Resonances in the current-voltage curves have been observed. A model based on the layered structure of the crystal using the modified formalism of de Gennes et al., Tomasch, and Rowell et al. has been developed for their interpretation. From the proposed model, a superconducting energy gap parameter of,J ~ 26 meV, corresponding to a BCS ratio 2d/kaTc ~ 7.4, has been obtained. In addition, the Fermi velocity of the quasiparticles in the crystal has been estimated to be rE~ 1.5 × 106 cm/s. The coherence length ~ has also been evaluated and is found to be ~ l A.

1. Introduction

Over the past five years, there has been a tremendous number of experimental and theoretical efforts devoted to understanding the properties o f the new class o f high-To superconducting cuprates. However, the underlying mechanism producing the high transition temperature in these materials is still unresolved. One o f the most controversial issues has been whether the general framework o f the BCS theory is valid in describing the new high-To oxides. Specifically, the question to be settled is how one can reliably verify the existence and value o f the superconducting energy gap, 2/1, in these materials. On the other hand, the layered structure o f the high-To superconductors and the existence o f squareplanar C u - O layers, which is the major difference from conventional superconductors, are widely considered to be related to their remarkable superconducting properties. For instance, both the number of C u - O planes in a unit cell [ 1 ] and the spacing o f the C u - O sheets [2 ] have been correlated with the transition temperature T~. These data strongly suggest that the quasi-two-dimensional nature o f these materials is intimately related to the origin o f the high transition temperature. Yet this correlation is not understood.

Historically, tunneling spectroscopy has been a very important tool for measuring the energy related properties o f superconductors. Therefore, it is natural to explore the origin of the new high-To superconductivity by electron tunneling measurements. Significant efforts have been made by many groups to produce tunnel junctions based on high-To materials, and tunneling data for the layered B i - S r - C a C u - O compounds have already been reported. Ikuta et al. [ 3 ] found a superconducting energy gap parameter,/1= 30 meV, with tunnel junctions made by depositing an oxidized amorphous silicon barrier and a silver counter-electrode on bulk superconductor samples. Lee et al. [4] o b t a i n e d / 1 = 2 5 meV with a tunnel junction made by depositing a Pb counterelectrode on a single crystal B i - S r - C a - C u - O sample. Recently, Kussmaul et al. [ 5 ], reported tunneling measurements with a tunnel junction fabricated on a thin film o f B i - S r - C a - C u - O containing both 110 and 85 K phases with A 1 as a counter-electrode. They obtained/11 = 18-21 meV for the 85 K phase and/12--25-28 meV for the 110 K phase. Tunneling measurements performed with a scanning tunneling microscope on a polished bulk sample o f composition 4 : 3 : 3 : 4 by Vieira et al. [6] found a distribution of/1 values between 15 and 23 meV, with maxima at 16 and 21 meV. They suggest that the two

0921-4534/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.

40

M.H. Jiang et al. / Point contact tunneling study of Bi2Sr2CaCu208single crystals

maxima are the gap parameters of the two superconducting phases present in their samples. By the point contact tunneling technique, even more scattered values of LJhave been reported: Zhao et al. [7 ] obtained A=23 meV for the 84 K phase. Escudero et al. [8] found/1=25.5 meV for their 80 K phase of Bi-based material. Johnson [ 9 ] has claimed that the energy gap for Bi2Sr2CaCu208 obtained in his measurements is 2A=25 meV. Obviously, there is still an uncertainty regarding the value of the superconducting energy gap for this compound. Very recent work on YBa2Cu307 oriented films by Greene et al. [ 20] and on Bi2Sr2CaCu208 ultra thin single crystals by Mandrus et al. [ 11 ] have revealed strong anisotropies in the current-voltage results, depending upon the orientation of the crystals and the direction of travel of the tunneling carriers. Greene et al. used junctions formed by the YBa2Cu307 and Pb, with the barrier provided by a "natural barrier" of surface material, while Mandrus et al. used break junctions formed at low temperatures in vacuum. Both groups found that tunneling along the c-axis showed less structure than that parallel to the C u - O planes. The precise reasons for the extreme anisotropies seen by the above methods is not certain; variations in effective barriers and density of states are likely causes. In our work since the tunneling tip penetrates into the Bi2Sr2CaCu208 and causes damage, the precise direction of the dominant tunneling is unknown. However, the several types of current-voltage curves seen in point contact studies may eventually be related to dominant tunneling directions. The difference in the point contact method relative to the above approaches, is that in penetrating into the superconductor it avoids the uncertainties associated with the unknown surface barrier, while introducing the problem of how damage to the tip and sample affect the results. At this stage of our understanding input from all sources are important to obtaining insight into the nature of these materials. While most tunneling experiments have concentrated on demonstrating the existence of the superconducting energy gap and comparing the ratio 2A/ kBTc to the BCS value of 3.53, few groups have considered other spectral details besides the gap. Tao et al. [ 12 ], reported multipeak structures in their d I / d V versus V characteristics. They explained the harmonic structure in d I / d V c u r v e s in terms of the well-

known Rowell-McMillan oscillation [ 13 ] and the Tomasch oscillation [ 14,15 ]. Several models based on the charging effect related to the granularity of the material have also been proposed for the oscillation of the dynamic conductance [ 16-18 ]. Oshio et al. [ 19 ] also observed similar structures other than the fundamental gap structure. They argued that these structures are not intrinsic to the material but depend on the nature of the junctions. Both experiments of ref. [ 12 ] and ref. [ 19 ] observed a large current around the zero bias as well, which they attributed to Josephson junctions that had formed across the grain boundary. Nevertheless, the origin of these spectral details is not yet well-explained. In this paper, we present a set of tunneling spectra on a Bi2Sr2CaCu208 single crystal obtained by point contact tunneling measurements. Pronounced structure in these spectra are indicative of bound states existing in the superconducting crystals. A model based on the nature of the layered-structure of the crystal has been developed to interpret the tunneling observation of the bound states. From this model and the experimental data, a superconducting energy gap of d ~ 26 meV has been obtained. The energy levels of the bound states have also been estimated, and values of the Fermi velocity of quasiparticles in the crystal, VF, and the coherence length, ~, have been calculated and found to be in agreement with values obtained by alternate techniques. Such an approach to obtaining these parameters is reported here for the first time.

2. Experimental procedures The point contact tunnel junctions were obtained using a scanning tunneling microscope (STM) in which mechanically-extruded P t - I r tips were utilized. The STM design is based on that of Fein et al. [20]. An IBM-PC AT microcomputer was used to control spectral sweeps with dynamical conductance ( d I / d V ) calculated numerically from I - V spectra. The STM was placed in a Janis liquid helium research dewar which was evacuated to < 100 milliTorr prior to each experimental run and was operated in gaseous He admitted through a throttling valve. Vibration isolation was accomplished by mounting the

M.H. Jiang et al. / Point contact tunneling study of BieSr2CaCu208 single crystals research dewar on a frame set upon an optical bench with pneumatic isolation supports. The samples used in these experiments were single crystals of the B i - S r - C a - C u - O compound. Microwave absorption measurements showed a Tc at 82 K, and both microwave absorption and X-ray diffractometry measurements showed that the compound is predominantly a 2212 phase. The crystal was cleaved with adhesive tape to expose a fresh surface immediately prior to mounting in the STM unit. The crystal was mounted on a sample holder with silver paint on the back of the crystal, so that the tip was perpendicular to the a-b plane of the surface. The STM was operated in a point-contact mode (no scanning), and the tip was embedded into the sample by a fine approach micro-actuator with an approach-to-turn ratio of ~ 2 I~m/turn. Tunneling data were taken at temperatures ranging from 4.2 to 77 K.

41

neling spectrum as illustrated in fig. 1 (a) with current on the order of a nA was observed when the tip initially made contact with the freshly-cleaved surface. As the pressure on the embedded tip is increased, the tunneling current will increase until the pure metallic bridge is obtained. After penetrating several layers of the crystal, the end of the tip will begin to curl upon itself, as shown in fig. 2, progressively widening the area of contact. Once the pure-metallic-bridge tunnel junction was obtained, tunneling characteristics as shown in figs. l ( b ) to (d) were observed. There is a high conductance region about the origin in each of these curves. The width of the high conductance region ranges from a few mV to about 10 mV, depending upon the specific contact conditions. The spectral details in figs. l ( b ) to (d) are discussed in the following sections. 3. 2. Spectral indication of bound states

3. Results and discussion 3.1. Point contact tunneling spectra of BieSr2CaCu208 single crystal The shape of the I - V curves and the dynamic conductance ( d I / d V) was strongly dependent upon how contact was made with the superconducting sample. The current values that were observed range from several nA to several laA, corresponding to contact resistances from ~ 100 Mf~ to ~ 10 kfL Figures 1 ( a d) show the wide variety o f / - V and d l / d V- Vcurves that were obtained. Each of these spectra was taken at 4.2 K, using a new P t - I r tip on a freshly-cleaved surface of the single crystal. Since the tip was pushed into the surface of the sample, and some data were obtained even after the tip was pressed in and pulled out several times, it is very difficult to define the exact contact condition for each measurement. It is known theoretically and experimentally that when the resistance of a point-contact is decreased, the I V characteristic changes from that of an insulating, tunneling barrier to that of a pure metallic bridge [ 21,22 ]. Therefore, it can be inferred from the tunneling spectra what type of junction was made from the specific I - V characteristics of individual spectra. Examples of both types of contacts are seen in spectra of fig. 1. Generally, a Schottky-barrier-like tun-

As seen in figs. 1 (b) to (d), there are current steps beyond the high conductance region in the I - V curves, corresponding to conductance troughs which are either zero or negative in the d I / d V characteristics. The number of the current steps also changes from spectrum to spectrum, again depending upon the contact conditions as seen in figs. l (b) to (d). These multi-step features in the I - V curves of figs. l ( b ) to (d) are clearly-observed, and they imply pronounced structure indicative of bound states existing in Bi2Sr2CaCuEOy single crystal, which are similar to the well-known McMillan-Rowell oscillations [ 13 ] and Tomasch oscillations [ 14,15 ]. The question to be addressed is whether such bound states have their intrinsic origin resulting from the fact that the crystal is a layered-structure, with the carriers primarily confined in the CuO2 planes, and hence have the nature of quasi two-dimensionality, or that the point-contact junction itself has similar effects to that of planar junctions of the conventional superconductors which results in the tunneling bound states. A model based on the nature of the layeredstructure of the crystal is proposed in section 4. 3.3. Temperature dependence o f the energy gap When the current-voltage ( I - V ) spectra exhibits the features of a pure-metallic bridge, it still shows

42

M.H. Jiang et al. / Point contact tunneling study of Bi:Sr2CaCuzOs single crystals

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V (mY) Fig. 1. Point contact tunneling spectra of a Bi=Sr=CaCu=O8 single crystal at 4.2 K: (a) Schottky-barrierqike tunneling characteristics; (b), (c) and (d) pure-metallic-bridge tunneling characteristics with resonant effect indicative of bound states.

the superconducting energy gap, because of the discontinuity in the electron potential energy which causes Andreev reflections [ 23 ]. If the energy gap is extracted based on the crossover voltage at which the high conductance region switches to the normal con-

ductance (junction resistance equal to normal state value), as in the case of fig. 1 (b), the value of the energy gap parameter is found to be d ~ 26 meV. This criterion for estimating the energy gap, however, is hard to apply to the spectra of figs. 1(c) and (d),

43

M.H. Jiang et al. I Point contact tunneling study of Bi2Sr2CaCu20s single crystals

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V (mV) Fig. 1. Continued. since there are several current steps exhibited in these I - V curves. Identification o f the energy gap from tunneling spectra in which the signature o f b o u n d states occurs will be discussed in section 4. F o r Andreev reflection, the theoretical ratio o f the conductances o f low a n d high biases is 2, with a crossover near the energy gap [ 22 ]; however, the ratios mea-

sured from the d a t a presented above were much higher. It is likely that tunneling into the a - b plane from the sides o f the e m b e d d e d probe contributes parallel current flow which gives rise to this higher conductance. T e m p e r a t u r e - d e p e n d e n c e o f the tunneling spectra o f fig. l ( b ) has also been measured. Instead o f plot-

44

i¸i¸~;

M.H. Jiang et al. / Point contact tunneling study of Bi2Sr2CaCu20s single crystals

~ i~ !~!~¢~ i!~¸

I i

tact measurements is that, due to differential thermal expansion of the tip and the sample, the probe may change its position as the temperature is varied. This does not seem to be the cause of the abrupt drop in A. Another possible cause is the strong inelastic scattering which gives rise to a normal state resistivity at high bias and at elevated temperature, and suppresses Tc [24,25], which may influence the temperature dependence of the energy gap. Other effects such as the degree of sensitivity of the measurement can also limit the accuracy of the observation of the energy gap as well.

4. Model for the tunneling spectra

Fig. 2. SEM pictures of platinum-iridium tip: (a) mechanicallyextruded fresh up; (b) Pt-lr tip after repeatedly touching the sample surface. ring I - V characteristics, the reduced conductances Rn ( d I / d V) as a function of voltage for various temperatures are presented in fig. 3. Rn corresponds to the resistances at 80 mV bias. Taking the crossover voltage at which the high conductances switch to the normal conductances (equal to one) as the criterion for estimating the energy gap, the value o f d remains at ~ 26 meV until 53 K where no gap energy was resolved, as illustrated in fig. 3. The temperature dependence of the reduced conductances at zero bias and the energy gap as a function of temperature are shown in fig. 4. As one can see in fig. 4, the features, which were extracted based on Andreev reflection, disappear above 53 K. One drawback of point con-

A novel form of potential well was first considered by de Gennes and St-James [26], who pointed out that in a normal metal-superconductor (NS) sandwich, electrons with energies less than the superconducting energy gap, LJ, will be confined to the normal metal by the pair potential barrier. They calculated the excitation spectrum in the N layer and predicted the existence of bound states within this potential well, the level spacing depending on the inverse thickness of normal metal. The observation of electron interference effects was first reported by Tomasch [ 14,15 ] for tunneling into the S side of the sandwich, and then by Rowell and McMiUan [27] for tunneling into the N side of a N-S sandwich; they observed resonant tunneling for energies greater than the superconducting energy gap, zl. Later, bound states at energies less than zl, as predicted by de Gennes and St-James, were observed in both the Z n Pb [ l 1 ] and Cu-Pb [28] systems. Geometric resonance and boundary effects in tunneling from conventional superconductors have only been observed on sandwich junctions [ 14,15,27,28 ]. However, the tunneling observation of bound states reported here is obtained by point contact tunneling measurements. The question is, arising from the difficulty of specifying the boundary conditions for point-contact junction, whether the interference effects take place within a region of thickness, d, in the normal metal probe or in the superconductor. In order to answer the question so as to interpret the data presented in the previous sections, a model based on

45

M.H. Jiang et aL / Point contact tunneling study of BieSr2CaCu208 single crystals (a) T=4.2 K. (f) T=33 K.

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boundary effects of the layered-structure of the crystal is proposed as follows. The ideal structure of the Bi2Sr2CaCu2Os single crystal is composed of two CuO2 planes separated by one Ca layer and intercalated by two SrO and two BiO layers resulting in c / 2 ~ 15 A [29]. Because of the non-metallic a n d / o r insulating nature of the BiO and SrO layers [30 ], the electronic carriers are effectively confined to the CuO2 planes. Thus, the layered-structure of the crystal can be considered as an accumulation of the CaCuO2 slabs, in which the electronic carriers are confined, separated by barrier boundaries due to the BiO and SrO layers. The tunneling structure proposed in this model is illustrated

in fig. 5. A normal-metal probe ( P t - I r ) is in intimate contact with the first CaCuO slab. The first CaCuO slab is then separated from the second CaCuO slab by a barrier boundary. When a normal metal probe is brought into contact with the superconductor, the region of the first CaCuO slab near the contact will be reduced to a low Tc phase with smaller gap parameter/1~, as shown in the figure, or even completely reduced to the normal state. The reason for such a reduction is that the work function difference between the metal and superconductor causes charge transfer in order to have a unique Fermi level in the equilibrium state when they are in contact [ 31 ]. This charge transfer can result in local

M.H. Jiang et al. / Point contact tunneling study of BieSrzCaCuzOs single crystals

46

1.2 1.0 0.8 0.6

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0.0 -0.2 0.0

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|

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0.4

0.6

0.8

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1.0

T/Tc Fig. 4. Temperature dependence of the energy gap and the reduced conductances (at zero bias) obtained from point contact tunneling spectra ofa Bi2Sr2CaCu2Os single crystal.

variation in carrier density in the superconducting CaCuO slab, which in turn can possibly destroy the superconductivity in that slab. When the normal metal is in intimate contact with a superconductor, the proximity effect takes place which also depresses the superconductivity of the contact region. The second CaCuO slab may or may not be affected by the charge transfer taking place in the first slab. The assumption has been made here that the second slab is not affected by what happens in the first one so that it is still in the original superconducting state with energy gap parameter/12, while the first slab is reduced to a low Tc phase with smaller gap parameter /1~, again depending upon the contact conditions, as shown in fig. 5. Based upon such a model, the tunneling spectra shown in figs. 1 (b) to (d) are interpreted. If an external voltage positive with respect to the normal metal is applied, the electrons from the normal metal, travelling in the z direction, will attempt to tunnel into the first CaCuO slab. When the energy is less than/1,, the pair potential in the first slab, there is no density of states for quasiparticles available, and electrons from normal metal can tunnel through the pair potential only as electron pairs, hence produc-

ing Andreev reflections. This corresponds to an initially sharp rising of current and a high conductance dI/d V curve around the origin. As the external voltage passes A~, but is less than the energy gap A2, electrons can be injected into the first slab as quasiparticles. Once the tunneled quasi-electrons get to the first slab, they propagate towards the interface of the first and second slabs where they are reflected by the pair potential A2. The reflected wave is a quasihole state which travels back through the first slab and is reflected again by the interface of the contact. The wave then returns to the interface of the first and second slabs where a further hole-electron reflection takes place. At this point, electron-electron interference occurs, giving rise to the discrete energy levels that are seen in tunneling spectra as shown in figs. 1 (b) to (d). When the applied bias is increased to greater than Az, electrons from the normal metal can tunnel not only into the first slab region but also into the second slab region where the density of states for quasiparticles are available. However, when an electron crosses the interface between the first and second slabs, it still experiences the discontinuity of the pair potential, and thus suffers partial reflection as a hole which in turns results in an interference effect. A resonance feature of the tunneling spectra at energy beyond the gap, therefore, is also expected. The situation of point-contact tunneling in layered-structure of Bi2Sr2CaCu208 single crystal is different from that of tunneling from conventional superconductor sandwich junctions. As discussed previously, oscillations in the density of states of the N - S sandwich occur for definite values of the energy as a function of the thickness, d, of the normal-metal film where electron interferences exists. Instead, in the case of the present model, electron interference takes place in a region within the superconductor (the first CaCuO slab) where the superconductivity has been largely degraded by the contact. As mentioned previously, that situation in the first slab is strongly contact-dependent. It can be seen that the oscillation broadens to different cases as the contact condition is changed, as seen in figs. 1 (b) to (d). It is not impossible but very difficult to specify each contact situation in each case. However, we have calculated the energy levels of bound states using the tunneling structure of fig. 5, and the quantities of the energy gap, Fermi velocity, and coherence length can be ob-

47

M.H. Jiang et aL t Point contact tunnefing study of Bi2Sr2CaCuzOssingle crystals

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Fig. 5. Schematic representation of Pt-Ir point-contact tunneling into the layer-structured superconducting Bi2Sr2CaCu208single crystal. tained from the resonant tunneling spectra as demonstrated as follows. A quasiparticle is incident from the normal side (z < 0 ) to the superconducting slabs (z > 0 ), and interference effect takes place in the first slab. The excitations in slab 1, instead of being one-electron states, are now a mixture of quasi-electron and quasihole states, which can be described by two-component wave functions [ 21 ] ~,¢=[U = O l-I re i k .

at

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The interference between the incoming (towards z=d) and outgoing (towards z = 0 ) quasiparticle waves produces a term proportional to sin [ 2 (E 2 - d 2 ) t/2z/hVF] from which the energy levels can be obtained,

2dl (E 2-d 2) 1i2 =n/t =~ hVF E r ~ ['nnhvE'~2-]'/2

=L

J

'

(6)

M.H. Jiang et aL / Point contact tunneling study of Bi2Sr2CaCu20s single crystals

48

E E = d z + { n x h v F / 2 d l } 2 = d 2 +n2~ 2 XhVF EO= 2dt '

(7)

where n is an integer, dt is the width of the first slab, and Eo is the spacing of the energy-levels. It is argued that eq. (6) is also valid for energy (E) slightly greater than A2 because the quasiparticles in the first slab will be nearly perfectly reflected at the interface of the first and second slabs [23]. If ( q V ) 2 = E 2 is plotted versus n 2 as indicated by eq. (6), the intercept is d 2, from which the superconducting energy gap can be obtained; the slope is e2 from which the interference-levels and the Fermi velocity can be calculated. This has been done by taking the data points (En) from tunneling spectra in fig. 1 (c) and (d), as seen in table l, and the results are shown in fig. 6. As seen from fig. 6, there is a discontinuity between n = 2 and 3 in E 2 vs. r/x curve for the spectra fig. l (c), and a discontinuity between n = 3 and 4 for the spectra fig. l (d), corresponding to a transition of energy regime from At < E l /1x. Thus, the energy gaps, At and dE, are found from the intercepts of the linear portions of E2-vs.-n 2 curves in energy regime Et and E2, respectively, and the intercepts for determining/12 of spectra fig. l (c) and spectra fig. 1 (d) have been indicated in fig. 6. The spacings of the energy-levels, given by eq. (7), are found from the slopes of the linear portions of E 2vs.-n 2 curves. For each spectrum (fig. 1 (c) or l ( d ) ) , two energy spacings, Cot and ~o2, have been obtained corresponding to energy regimes Et and E2, respectively, and the slopes of the linear function in energy regime E2 have been also indicated in fig. 6. The Fermi velocity of quasiparticle in Bi2SrECaCu208 single crystal can be estimated by eq. (7), provided

dl is known. A reasonable value of d~ = 15 ]~, which is the length of c / 2 of the crystal, has been taken as the width of the first slab. Finally, the coherence length, ~, was calculated by using the well-known BCS relation hVF

¢(o)= ~ ( o ~

(8)

The values of these parameters are summarized in table 2. The values of d2 and VF2have been used in eq. (8) to calculate coherence length, ~. It is evident that the values of At are just about the width of the high conductances regions near the origin as illustrated in fig. l (c) and (d), corresponding to the smaller gap of the first lower Tc slab as discussed previously. Thus, /t2 has been attributed to be the intrinsic superconducting energy gap parameter. If taking the gap value obtained from fig. 1 (b) into consideration, we determine that the superconducting energy gap parameter for the Bi2Sr2CaCu208 single crystal is /l~ 26 meV, resulting in a BCS ratio of 2d/kaTc,~7.4. It should be noted that the values of the energy gap, the Fermi velocity and the coherence length presented here are in fairly good agreement with those reported in the literature [32-34] by alternate techniques. For example, an energy gap parameter of around 30 meV for Bi2Sr2CaCu208 single crystals has been obtained by high-resolution electron-energy-loss measurement [ 32 ]. Kresin [ 33 ] has reported that the Fermi velocity of electrons in La-based superconductors is 8 X 106 cm s - i. The coherence length in the c-direction, ?_,.,has been found to be 0.5 A for Bi2Sr2CaCu208 single crystals by upper critical field measurements [34].

5. Conclusions

Table 1 Energy levelsin spectra fig. 1(c) and (d) E,

Spectrum lc

Spectrum ld

1 2 3 4 5 6 7

20 meV 31 meV 43 meV 53 meV 63 meV

11 meV 20 meV 30 meV 45 meV 53 meV 61 meV 69 meV

The fact that the same superconducting energy gap parameter (26 meV) was calculated for three seemingly very different I - V spectra from the same crystal, gives strong evidence for the validity of our model based on the layer-structure of single crystal Bi2Sr2CaCu2Os. Our model also provides a means of calculating the energy levels of the bound states induced at the interface. The agreement of our values of the superconducting energy gap, Fermi velocity

49

M.H. Jiang et al. /Point contact tunnelingstudy of Bi2Sr2CaCu20s single crystals 5000

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I

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10

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30

40

50

n2 Fig. 6. Plot of E 2 as a function of n 2 by taking E, data from table 1, from which the energy gap and the Fermi velocity of electronic carriers have been obtained. Table 2 Values of superconducting parameters of Bi2Sr2CaCu2Os

d~ (meV) A2 (meV) ~ol (meV) %2 (meV) VFI (cms -I) VF2(cms -l) ~(A.)

Spectrum fig. 1(c)

Spectrum fig. 1(d)

13 ~26 15 11 2.2X106 1.6XI06 1.3

3 ~26 10 9 1.4×106 1.3XI06 1.0

and coherence length with measurements performed by alternate techniques lends further credence to our model. The large value o f the BCS ratio (7.4 versus the theoretical BCS ratio o f 3.53) and lack o f temperature dependence o f the superconducting energy gap for temperature up to 53 K are indications o f strong deviations from the simple BCS model.

Acknowledgements This work was supported in part by the U.S. Army

Electronics Technology and Laboratory (Dr. Gerald J. Iafrate) under the auspices o f the U.S. Army Research Office Scientific Services Program administered by Battelle (Delivery Order 1604, Contract DAAL03-D-0001 ), and the Ben Franklin Superconductivity Center. We would like to thank Z.X. Zhao, the Institute o f Physics, Academy o f Sciences, People's Republic of China, for providing the single crystal Bi2Sr2CaCu2Os samples. The contributions o f Mr. Thomas Mercer in data collection are also greatly appreciated.

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