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24 Chinese Science Bulletin Vol. 48 No. 1 January 2003

Chinese Science Bulletin 2003 Vol. 48 No.1 24�27

Fabrication andcharacteristics of intrinsicJosephson junctions inBi2Sr2CaCu2O8+x singlecrystalsYOU Lixing1, Wu Peiheng1, Cai Weixing1,Yang Senzu1, Wang Huabing2 & Kang Lin1

1. Department of Electronic Science and Engineering, Nanjing Univer-sity, Nanjing 210093, China;

2. Research Institute of Electrical Communication, Tohoku University,Sendai 980-8577, Japan

Correspondence should be addressed to You Lixing (e-mail: lxyou@nju.edu.cn)

Abstract Mesa-structured intrinsic Josephson junctionsare fabricated in Bi2Sr2CaCu2O8+x single crystals. Typicalcurrent-voltage characteristics of intrinsic Josephson junc-tions are observed, which include multiple quasi-particlebranches, surface junction with critical current lower thanthose of inner junctions, and subgap structures on quasi-particle branches. The corresponding physical explanationsare also given. The energy gap voltage of the intrinsicJosephson junctions at 30 K is about 20 mV. Besides, themeasured Ic-T relationship agrees quite well with thetheoretical computations based on -2 2- wavex yd supercon-

ductor. Our measured dI/dV−−−−V relationship shows theV-shaped gap structure, obviously differing from theU-shaped gap structure of the s-wave superconductor.

Keywords: intrinsic Josephson junctions, surface junction, multi-branch structure, subgap structure, pairing symmetry.

As one of main high-Tc superconducting materials,Bi2Sr2CaCu2O8+x (BSCCO) is widely studied in materialand device areas. Like other high-Tc superconducting cu-prates, BSCCO has perovskite-type structure, which maybe also described as ‘sandwich’ structure. The supercon-ducting Cu-O layers, each 0.3 nm thick, couple to eachother through the intermediate insulating or semiconduct-ing Bi-O and Sr-O layers, which are 1.2 nm thick in all,thus forming intrinsic S-I-S junction stacks in series.Kleiner et al. first observed intrinsic Josephson effects inBSCCO single crystals in 1992[1]. Compared to intrinsicJosephson junctions (IJJs) in thin films, IJJs in singlecrystals are easier to fabricate, and have better layer-structure and consistency. As the superconducting layersare very thin, plasma oscillation in any of the SIS junc-tions of the stack is not confined to itself but can form acollective oscillation along the c-axis. Thus IJJs can be agood candidate for oscillator with high power output at

high frequencies up to terahertz region[2]. Many efforts arefocused on it. Besides, they can also be used to detect highfrequency signals at 100 GHz or even a few terahertz[3,4],or to develop quantum voltage standards[5]. With highstability and excellent layered structures, IJJs in BSCCOsingle crystals are most widely adopted in the research ofIJJs.

In this report, we shall describe the fabrication ofIJJs in BSCCO single crystals, the detailed measurementson their electrical transport, the temperature dependenceof the current-voltage (I-V) curves, and the implication ofthe measured dI/dV−V relationship.

1 Experimental results and discussions

A slice with the sizes of 2 mm�2 mm and thicknessof 0.3 mm is cleaved from a BSCCO single crystal. Thenthe slice is glued onto a Si substrate by polyimide for fur-ther processing. Very anisotropic along the c-axis and inthe a-b plane, the cleaved surface of the slice always runsparallel to the a-b plane. Fig. 1 shows the X-ray diffrac-tion pattern of the cleaved slice fixed on Si substrate. Ex-cept for (100) peak of the Si substrate, all the peaks are(002n) peaks of BSCCO, indicating that the BSCCO slicehas consistent direction and the c-axis is strictly perpen-dicular to the surface.

Fig. 1. XRD spectrum of cleaved BSCCO slice fixed on Si (100) sub-strate.

To begin with, a gold thin film, 70 nm thick, is de-posited onto the surface of the sample to prevent BSCCOfrom possible contamination in further fabrication proc-esses. Then the sample is annealed in oxygen flow at600� to reduce the contact resistance between BSCCOand Au and to make the contact resistance linear. Usingconventional photolithography and argon ion milling, amesa with the sizes of 6 µm�6 µm is formed in BSCCOslice. A SiO layer is evaporated onto the samples to isolateand protect the mesa. The photoresist and SiO on the mesa

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Chinese Science Bulletin Vol. 48 No. 1 January 2003 25

are removed by supersonic cleaning in acetone. The junc-tion number in the mesa is controlled by ion etching rateand time. Finally an electric lead is glued onto the ex-posed top gold of the mesa by silver paste. Two moreelectrodes are connected to the base of the mesa. Fig. 2shows the cross section of the sample. All the measure-ments are carried out in liquid helium using the three-terminal method.

Fig. 2. Cross section of IJJs in BSCCO single crystals.

Fig. 3. I-V characteristics of BSCCO IJJs, the temperature is 15 K, thesurface junction and subgap structure are also observed.

Fig. 3 shows typical I-V characteristics of the sampleat 15 K without magnetic field. By continuous exposuresduring current scanning, we obtained nearly all thequasi-particle branches in IJJs. There are 19 junctions inthe stack as indicated by the number of the branches. Thecritical currents of all the junctions in IJJs range from 1 to1.5 mA except that the critical current of the topmostjunction is only 0.1 mA. Kim et al. pointed out that thelower critical current originates from a surface junction ofBSCCO[6], because the topmost Cu-O layer is in contactwith Au layer and its superconductivity is weakened dueto proximity effect[7,8]. Thus the critical current of the sur-face junction is much lower than those of other junctions.

It decreases with the increase of temperature and disap-pears when the temperature is above 30 K. Fig. 4 showsthe I-V curves of the IJJs when this happens. With thetopmost Cu-O layer being a normal metal now, the surfacejunction is an N-I-S heterogeneous junction, which pro-duces strong non-linearity in the first branch of IJJs. Theenergy gap voltages determined by the intervals betweenquasi-particle branches are nearly the same, about 20 mV,which is quite close to the results previously reported[9].When the current I is about 0.2 mA, small but clear volt-age jumps are observed in figs. 3 & 4 indicated by thearrows. This can be attributed to the existence of subgapwhich is well explained by Helm et al. using a modifiedRSJ model[10].

Fig. 4. I-V characteristics of BSCCO IJJs. The temperature is 30 K, andthe surface junction disappeared. The first branch shows strong nonlineardue to the N-I-S junction degraded from the surface junction.

Theoretical researches as well as experiments havefirmly established that Cooper pairs of high-Tc cupratesuperconductors have different pairing symmetry fromthat of low-Tc superconductors[11]. Low-Tc superconduc-tors have isotropic s-wave pairing symmetry, while high-Tc superconductors have predominant anisotropic 2 2x y

d −-

wave pairing symmetry. The Cu-O layers in BSCCO showanisotropy along different directions in a-b plane. On theother hand, due to 2 2x y

d −-wave pairing symmetry, the

Josephson junctions along c-axis in BSCCO show differ-ent characteristics from those of low-Tc Josephson junc-tions. Fig. 5 shows the temperature dependence of thecritical current of IJJs with a maximum of Ic = 1.52 mA at4.2 K. The square dots are experimental results, and thesolid line represents the theoretical results by Tanaka et al.who derived the temperature dependence in two-dimen-sional limit for 2 2x y

d −-wave superconductor[12], where

we suppose the maximum Ic = 1.52 mA at 0 K and Tc =

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26 Chinese Science Bulletin Vol. 48 No. 1 January 2003

86.7 K. Our experimental data agree quite well with thetheoretical results. The critical currents of IJJs decreasewith the increase of temperature, and so do the hysteresesof IJJs (McCumber constants β ). Furthermore, at hightemperatures the quasi-particle branches turn to be linear,indicating that the junction resistances of IJJs tend to belinear with the increase of temperature. Fig. 6 shows theI-V characteristics when T = 74 K. The critical current ofIJJs has decreased to 0.5 mA. The inset of Fig. 6 showsthe I-V characteristics under high current bias when T = 78K. The envelope is observed after all the junctions arebiased. Clear energy gap structure is observable on theenvelope. However, it is difficult to observe when the

Fig. 5. Ic-T relationship of BSCCO IJJs. The square dots are experi-mental results, the solid line represents theoretical results for 2 2 -x yd −

wave superconductor along c-axis in two-dimensional limit[12], wherewe suppose the maximum Ic = 1.52 mA at 0 K and Tc = 86.7 K.

Fig. 6. I-V characteristics of BSCCO IJJs when T = 74 K. The quasi-particle branches are linear. The inset shows I-V characteristics ofBSCCO IJJs under high current bias when T = 78 K.

hysteresis is high at low temperatures, because the energygap structure hides in the multi-branch structures. Ne-glecting the supercurrent, we have calculated the dI/dV-Vcurve from the quasi-particle branch in the inset of Fig. 6,and the result is shown in Fig. 7. The curve differs fromthat obtained for s-wave superconductor in the sense thatthe dI/dV-V curve of IJJs along c-axis is V-shaped, whilethat of s-wave superconductor is U-shaped. The tunnelingspectrum of superconductor is given by

( )eVσ ∝ ( )Eρ∞

−∞∫ ( ) d ,f E eV EeV

∂ − + ∂ where ρ (E) and f(E) are density of states (DOS) andFermi distribution function respectively. For BSCCO with

2 2x yd −

-wave pairing symmetry, DOS is given by

π

0 2 2 2

1( ) d ,

cos (2 )

EE

x Eρ θ

θ=

− ∆∫

where θ is the angle between the lattice axis and the di-rection of Cooper pairs. Substituting DOS of BSCCO intothe above relationship, we can derive V-shaped tunnelingspectrum for 2 2x y

d −-wave superconductor[13].

Fig. 7. dI/dV-V relationship corresponding to the I-V curve in the insetof Fig. 6, which shows a V-shaped structure.

2 Conclusions

In summary, we have fabricated IJJs along c-axis inBSCCO single crystals. Typical and clear multi-branchstructures are observed in the I-V curves. The gap voltageis about 20 mV at 30 K. The surface junction weakenedby proximity effect and the subgap on quasi-particlebranches are also observed. The temperature dependenceof critical current of IJJs agrees with the theoretical com-putations based on 2 2x y

d −-wave superconductor. Strong

nonlinear quasi-particle branches turn to be linear with theincrease of temperature. The dI/dV-V curve is V-shapedand differs from that of low-Tc superconductor which

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Chinese Science Bulletin Vol. 48 No. 1 January 2003 27

supports the 2 2x yd −

-wave pairing symmetry of BSCCO.

The fabrication process is simple and the experiments arereproducible. Further researches will be concentrated onthe high frequency responses of IJJs, on the properties ofsurface junction, and on the implications of pairing sym-metry.

Acknowledgements The authors would thank Dr. Zhu for helpfuldiscussions. This work was partially done in Research Institute of Elec-trical Communication, Tohoku University, Japan, and supported by theMinistry of Science and Technology of China (Grant No. G19990646).

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(Received August 7, 2002)