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Anisotropie superconductivity and oxide electronics
ELSEVIER
I. Introduction
Since the discovery of high T, oxide superconductors, nearly ten years ha1.epxsed. The 11~c11anis111s of high T, sitperconduc~i\,ity and their full-scale application to electronic de\%xs ha\,e not yet been clarified. The studies of the pairing symmetry of high T, superconductors ha\~ attracted considerable attention rcccntly. The pairing symmetry of the high T, supcrcondlIetors, whether it is of s-wave nature or of d-\b ;IVL’ nature has been, so far. discussedin terms of the tcmpcraturc dependenceof the magnetic field penetration depth. the nuclear magnetic relaxation time. the Knight shift etc. Quite recently. a more direct determination of the pairing symmetry based on the phase sensitiL.cmeasurement proposed by Sigrist and Rice [I] has been gi\,en by Wollman et al. [3]. Followed by this work, the various types of phase sensiti\,e measurements using the YBCO supcrconductors ha\,e been performed, which may be classified into three different groups; the d.c. SQUID type measurements using s-wa\‘e superconductor (Pb) high T, superconductor junctions [2 4]> the single junction measurementsusing s-wax superconductor (Pb) high T, superconductor junctions [3.5 81 and the topological measurements containing high T, Josephson junctions [9.10]. Most ofthesc measurementssupport4 the d-wa1.e
pairing symmetry (d, 2 , 2). while only a few measurements supported the s-WVL’ one. From the viewpoint of these cxpcrimental results, it is now almost undisputed that the YBCO superconductor hasthe pairing symmetry of ;I d-\va\,e nature. In this paper. we discuss the superconducting oxide electronics based on the concept of an anisotropic d-Leave oxide superconductor.
S
Cc)
Fig. I. Schematics of d-v,;;Ivejunctions(a) s d Junction.(h) d d ,junctlon.(c) s d edgejuncticln.and(d) s d cornc’rjunction. 0911-5107
96 LFIS.00 ( I996
Ftwwx
Science S.A. PI/
All Iright
S002I-.S107~‘)h,0lh?8-S
rescncd
a normal-like behavior I = I, sin 41for the larger value of T/T,. whereas it deforms considerably at low reduced temperatures, indicating the strong appearance of the sin 24 term. This suggeststhat the magnetic field dependence of Josephson critical current will not shovv 21conventional Fraunhofer-like diffraction pattern and some deformed patterns are expected. At the s;mle time, it also suggests the appearance of Shapiro steps :it half-integer voltage values V= 11lrJ‘2~,(12= I 1 2. _+3: 2...) as well 21sinteger ones (it = 0, F 1. ? 2.. .) under microwave irradiation where ,f’ is the microvvav’e frequency. h is Planck’s const;mt and 1’ is the electronic charge. Normalized
Phase
‘9/n
Fig. 2. Josephson current-phaw relation li)~- the junction3 with bicrystal angles of (a) - 0. (b) - 0. ITT, and Cc) - 0.2~. The curves A. B. C and D denote T T, = 0.025. 0. IS. 0.3 and 0.6. rcspcctivcly. (after Ref. [I II).
2. Theoretical
aspects for d-wave
Josephson
junctions
The present picture of a layered oxide superconductor is given by the image that the anisotropic order parameter is realized in the CuO, planes (&-plane) and the Josephson coupling exists between the CuO, sheets along the c-axis direction. Fig. I(a) and (b) depict the schematic pictures for a junction between isotropic s-wave and anisotropic d-wave superconductors (s, I d junction) and a junction between two anisotropic dwave superconductors (d’I:d junction) where two crystals meet at different angles relative to the boundary interfrtce. The former type junctions (Fig. I(a)) were used for testing the pairing symmetry of high T, superconductors. The s I d junctions behaves differently Xcording to the junction geometry shown in Fig. I(c) and (d). i.e. an edge type and a corner type. The d, I,d type junction in Fig. l(b) corresponds to the so-called bicrystal junction or biepitaxial junction. According to a recent calculation for a d-wave d-wave junction by Tanaka and Kashiwaya [I I]. the Josephson current across the grain boundary strongly depends on the right- and left-crystal angles relative to the interface boundary. They found that the Josephsonjunction thus formed would exhibit very strange behaviours which could not be seen in conventional s-wave junctions. Under this circumstance. the zero value of phase difference (4 = 0) does not correspond to the free energy minimum condition any more. Fig. 2 shows the examples of the calculated Josephson current-phase relations at different reduced temperatures. The upper middle and lower figures correspond to the bicrystal ;mgle of 0”. 18”. 36” respectively. A. B, C. D correspond temperatures T: T, = 0.025. 0.15, 0.3 and 0.6, respectively, Especially for the lower figure casewith higher bicrystal angle. the current-phase (I -- J, ) relation nearly exhibits
3. Some experimental
results
III the following. some experimental data for high-T, junctions are presented. As for the Josephson tunneling between s-wave and d-wave superconductors. sev,eral
experiments
have been performed
to test the pairing
symmetry of high T, superconductors :is stated abo1.e [2- IO]. Fig. 3 shows the magnetic-field dependencies of the Josephson critical current f, for Pb MgO YBCO sandwiched type tunnel junctions (MgO - 3 nm) with ir c-axis oriented YBCO film (upper) and with a11 t/-axis oriented YBCO film (lower) [5]. It is uorth noting that I, hltd a dip-like structure around zero flus for the upper figure. while it exhibited a normal-like pattern for the lower figure. The observed behavior in the upper figure is considered to arise from the microscopic surface morphology of the junction interface. III the case of the c-ktxis oriented film. the suppi-essionof Josephson current along the c-axis and the distribution of micrograins at the junction inter-time enable us to provide the different contributions of edge and corner type Josephson current elements according to the grain size of a fihn (for more details. seeRef. [S]). The results are consistent with the properties of s I d junctions between the s-wave and anisotropic d-wave superconductors. As stated above. the anisotropy also affects the junction characteristics of 21d/Ld type junction depending on the detailed bound;try interface condition. For example. in the case of bicrystal junctions. the magnitude of Josephson current strongly depended on bicrystal angles. For YBCO junctions grown on the bicrystal substrate with a 36.8” tilted angle. the Josephson current vvas much smaller than that of those with 21”. For BSCCO junctions with 36.8”, even the Josephson current itself was sometimes not observable. The control of the Josephson critical current for the artificial grain boundary junctions grown on the bicrystal substrates with a certain fixed angle (34Oor 36.8”) is. however, difficult, probably bec;iuse of the presence of microstructures at the interface boundary as noted in
the next paragraph. We also point out that the nature of bicrystal junctions has not yet been clarified. whether they are SIS-like, SNS-like. SNINS-like or something else. They are, however. now considered to be the best among three different types of junctions; bicrystal, biepitaxial and step-edge. Moreover. the junction properties for bicrystal junctions seem to depend on high T, materials (YBCO, BSCCO, BKBO). For noncupratc BKBO superconductors. near-ideal Josephson tunnel characteristics have been reported by two groups rccently [12.13]. The results are very consistent with the s-wave nature of BKBO superconductors as recognized by many researchers. The YBCO bicrystal junctions yielded much better characteristics than those of BSCCO ones. which are
ready to apply to electronic devices [ 14- 181.The I I’ characteristics were conventional RSJ-like ones and the Shapiro steps were observable at least up to 6 mV. The IcRn product generally lies in a few tenth of mV ~ a few mV. at 4.2 K. On the other hand. the properties of BSCCO bicrystal junctions appeared to be rather poor. The good I I’ characteristics were obtained only for ;I certain range of device parameters and the IcRn product at 4.1 K was less than I mV. The results suggest that the microscopic nature of the bicrystal junction would be different between YBCO and BSCCO. For the latter, the effects of the crystal intcrgrowth and the crystal defects might be involved in the junction characteristics.
4. Microstructures at the junction interface I
’
I
1
-6
-3
I
I
1
3
6
0.4
0.3
0.2
0.1
0
0 magnetic
I
I
I
field (10-4T)
I
I
I
I
0
5
10
15
” ’ .
-15
-10
-5 magnetic
Fig.
3. Dependence
magnetic field c-axis oriented YBCO calculated
film
the
K for YBCO film (upper
(lower figure). curbe by assuming
of corner type ratio of ,y’“” on
of the Josephson at 3.2 YBCO
surface
field critical
MgO ligure)
(10“‘T) current
Pb tunnel and \vith
I, on the junctions an taxis
applied with oriented
The solid line ,n the upper liyurc is the the microscopic fractional contribution ) and cdgc lqpe Junctmn (I:") with lhe junction (/:““lc’ :,:‘Jg” = 6.6 due to the presence of rough morphology of YBC‘O tilm (we Ref. [5]).
a
We mentioned that Josephson tunneling strongly depended on the crystal orientations of osidc superconductors relative to the interface boundary in the caseof a d-wave superconductor. In :I real situation. however. is not ideally the junction interface across a film straight [IY]. The upper part of Fig. 4 shows an AFM pictul-e obser\,ed near the grain boundary interface of a Y BCO bicrystal junction (Y BCO: c-axis oriented film). The lower left part of Fig. 4 shows the enlarged picture of the junction interface. The lower right part of Fig. 4 is the AFM image of the bicrystal boundary itself. From these obser\,ntions. it is found that the microscopic grain boundary wiggles back and forth in ;I scale of 1% 200 nm at the interface. The average tilted angle is, however. estimated to be about 24” along the interface boundary. In case of BSCCO junctions. this problem is more serious [Xl. Fig. 5 compares the line scanning image in the .\’ ~3plane at constant : (z: c-axis direction) at the interface boundary for YBCO (upper figure) and BSCCO (lower figure) bicrystal junctions. The microscopic complications at the boundary was much enhanced in the case of a BSCCO junction and some distribution of holes near the interface was also obser\,able. The presence of the zigzag structure will largely affect the Josephson properties since each microscopic interface has ;t different angle with respect to the crystal orientation of. a YBCO thin film, which limits the Josephson current accordingly. Note that. in the case that the pairing symmetry is of s-wave nature. these complexities do not arise. The presence of microscopic boundary structures affects the Josephson properties, i.e. the magnitude of Josephsoncurrent, the spatial distribution of Josephson current. etc. In particular. the Josephson maximum-current versus the applied magnetic-field curves will exhibit complicated behaviors due to the sum of the microscopically different current flows from the \,iewpoint of a d-wave superconductor. In fact. in the measurements
79
Fig. 4. AFM image of the YBCO (lower) of the junction boundary
junction on a MgO( 100) bicrystal (left) and the bicrqatal-substrate
I
Fig. 5. AFM 9~ J* line-scanning (upper) and the BSCCO (loww)
imngcs bicrystal
2w
4
(I: constant) interfaces.
of
the
YBCO
substrate boundary
\%ith a misorientation itself (right).
angle
of 24’
(upper).
The
cnlargeci
pictures
using bicrystal junctions. most of Josephson maximumcurrent versus magnetic-field curves did not exhibit ;t conventional Fraunhofer-like pattern and the obser\,ed patterns appeared to be sometimes quite irregular. On the other hand. the junction properties under microwave irradiation. i.e. the Shapiro steps. are not affected by the presence of micrograin boundaries e~cn for a d-wave superconductor since each microscopic Josephson element responds to microLvave field exactly in the same way as described by the Josephson \,oltagePfrequency relation. The voltages at \+,hich the steps appear are identical for all microscopic junctions. Fig. 6 shows the currenPvoltage characteristics of the YBCO grain boundary junction with a tilted angle of 24” at 4.2 K under microwave irradiation of 22 GHz. Although the microscopic structural AFM images at the junction boundary exhibited a complicated junction structure such as shown in Fig. 4, the observed Shapiro steps appeared very clear and pronounced in consistence with the above arguments. The dependence of Shapiro steps on the microwave power almost agreed with that predicted by the conventional theory for those with integer values. It is remarkable to point out here that in addition to the integer steps as described by the Josephson relation V= &f’ 2~, the half-integer steps (n = k 112, k 3,‘2...) at relatively higher microwave powers. are also clearly Lrisible. The appearance of the subharmonic current steps may be ascribed to the non-
# YBM-25 ix22 GHz O-70dB
irradiation
iIt different
pouel-
lw&
linear effect of the junction in which the current-phase relation is generally expressed by Z LI,, sin tfd) (for example. the Anderson-Daycm bridge using low T, superconductors. etc.). but if this is the case. the subharmonic steps corresponding to sin ttcl) (tt > 3) have also been observable in addition to half-integer steps. Experimentally, only those corresponding to the sin 241 term (half-integer steps) 1x1~ been observed and these steps are highly enhanced even in the 1 1’ characteristics of the junction. The results are very consistent with the recent calculation for ii d-wave superconductor; insulator d-wave superconductor (d ‘I ‘d) junction [I 11.
5. Application
to electronic devices
Now we discuss high T, cuprate superconductors from the viewpoint of application to oxide electronics. Based on the assumption that the unisotropic d-wave superconductivity is the essential nature of high T, superconductors, the control of the Josephson current will not be generally so easy and the complicated magnetic-field dependence of Josephson critical current due to the interferences of many different Josephson microelements across the junction boundary, as described above. may be expected. Although the detailed calculations in the presence of microscopically irregular structures as shown in Fig. 4 have not yet been performed, it may be conceivable that the averaging effect on these Josephson microelements will also be expected provided that some conditions are met. in which case the control of Josephson current will not be difficult and the magnetic-field behavior of Josephson critical current will become a near-normal one. On the other hand, if the fabrication technology of submicron high T, junctions (size: 0.1--0.2 /lrn) is developed, the junction properties will become well-defined. accordingly. Under this circumstance. the anisotropic nature of dwave superconductivity provides a variety of novel
devices different from those of s-wave superconductors. such as those utilizing the bound states of quasiparticles at the boundary interface and the interference of anglcdependent Josephson microjunctions [I I]. In practical use, there is no problem for SQUID devices since the Josephson junctions are used iis a switch of fluxon entry whose behoviour is decided by the size of the SQUID loop, not by the detailed structure of the Josephson junction. The spontaneous magnetization for a d-wave junction geometry may shift the zero flux point but it does not affect the SQUID operation since it serves as an astatic flux meter. In fact. the high T, SQUIDS have demonstrated good performance at 77 K and some SQUIDS are already in commercial market. The application to digital elements using S;I,S Josephson tunnel junctions is still not at hand. In the case of d-wave superconductors, the gap structure is a much smeared one as compared with that of s-wave superconductors due to gapless regime behavior, although appreciable nonlinear conductance is again expected. The expected hysteretic tunnel characteristics may be used again for digital elements, but the problem is how to fabricate and control the junction properties including the magnitude of Josephson current and its magnetic-field dependence. The situation may be different from that of bicrystal junctions as given in Figs. 4 and 5. The fabrication of the almost atomically Rat sandwich structure is necessary to realize this device, 21 part of which has been developed quite recently [?I]. The application of tunnel junctions to SIS mixer is. however. almost hopeless since the nonlinearity of the quasiparticle current portion is not sharp enough to allow high sensitivity detection.
References [I] M. Sigriht end T.M. Rw. J. P11.w. SK. Jp.. hl (I YY2) 3283. [I?] D.A. Wollman, D.J. Vun hrlingen. W.C. Lee. D.M. Ginsburg and .A.J. Lcggett. /%I,\. RCI-. Lctr.. 71 (1003) 1134.
(31 D.A. Brawner and H.R. Ott. P/IV\. RN. R. 50 (199-I) 6530. [4] 4. Mathai. Y. Gim. R.C. Black. ,A. Amar and F.C. Wellstood. [l-l]
/‘/I,Y.\. RN. L~/I., 7-J I 1995) 4573. [5] I. lguchi and Z. Wen. P/I,I,.s, Rcr. H. JY (1994) 12388. [6] A.G. Sun. D.A Cajcwhhi. M.B. Maples and R.C. Dynes. Ret. Lcrr.. 72 (1994) 2267. [7] D.A. Wollmxn. D.J. Van Harlingen. J. Giapintzaki$ and Ginsberg. P/II,\. Ret h/r.. 74 (1995) 747.
/%,I..\.
[I’]
D.M.
[lb]
M.
Kawwki.
P. Chaudh;iri
(1992) 1065. Y.1’. Di\in. PII,~.\. fY//.. T. Amrcin. P/!,l.\. fYI/.. P.A. Nilsson.
J. Mygrind. hl ( 1991) hl. Seitz. 6.1 (1993) Z.G.
[I71
593. [I I] Y. T>unak;l. Kashi~aqa.
[I91
K. Lee
[20]
K. Lee. I. Iychi. T. l\hibashi ( 1996) 99. M. Kawasaki. K. Takaharhi.
P//>.v. Rcr. hr.. to be published
72 (1994)
?871:
Y. Tanaka
and
S.
[?I] [ 131 T. Takami.
K.
Kuroda.
M.
Kntaokx
Y.
Wada.
K.
Tcrada.
J.
E.A. Stepanw 7072.
and
hara. 0. Koinuma.
I. Iguchi.
A. Gupta.
P/II:\.
RN.
L/r..
N.F Pedcrsrn and P. Chaudharl. 3050. D. Uhl. L. Schulz and K. Clrban. 1978. I~anov. H.K. Olsson. D. M’inklcr. and A. Ya Tzalenchuk. /. .+/I/.
[X] J.H. Miller. Jr.. Q.1’. \I’ing. Z.G. Zou. N.Q. Fan. J.H. Xu, M.F. Dabis and J.C. Wolfe. P/I),\. Rc,I. I.csrr.. 7J (1995) 2347. [9] P. Chaudhori and S.-Y. Lin, PI!,,.\. Rrr. Lerr.. 7-7 (1994) 1084. [IO] C.C. Tswi. J.R. Kritlq. C.C. Chi. L.S. Yu-Jahncs. A. Gupta. T. Shaw. J.Z. Sun and M.R. Ketchen. P/I,Ix. Ret,. Lcrr.. 7.7 (1994)
Clac\on. 7.5 ( 1994)
and
.-l/y/.
Ph,v.\.
f.v/r..
and
M.
66 (1995) Ka\\abc.
T. Ma&.
Isl~iyam;~. T. Yoncraua. &wrr( (‘. 3% ( 1993) 1540.
.-l/y/. 1pp1. T. Ph,r.\..
769. P/~~~.\wtr C‘. 237
R. T3uchi)a. M.
h,S
\I’o\himotot
hf. Shinoand
H.
I. Introduction
Since the discovery of high T, oxide superconductors, nearly ten years ha1.epxsed. The 11~c11anis111s of high T, sitperconduc~i\,ity and their full-scale application to electronic de\%xs ha\,e not yet been clarified. The studies of the pairing symmetry of high T, superconductors ha\~ attracted considerable attention rcccntly. The pairing symmetry of the high T, supcrcondlIetors, whether it is of s-wave nature or of d-\b ;IVL’ nature has been, so far. discussedin terms of the tcmpcraturc dependenceof the magnetic field penetration depth. the nuclear magnetic relaxation time. the Knight shift etc. Quite recently. a more direct determination of the pairing symmetry based on the phase sensitiL.cmeasurement proposed by Sigrist and Rice [I] has been gi\,en by Wollman et al. [3]. Followed by this work, the various types of phase sensiti\,e measurements using the YBCO supcrconductors ha\,e been performed, which may be classified into three different groups; the d.c. SQUID type measurements using s-wa\‘e superconductor (Pb) high T, superconductor junctions [2 4]> the single junction measurementsusing s-wax superconductor (Pb) high T, superconductor junctions [3.5 81 and the topological measurements containing high T, Josephson junctions [9.10]. Most ofthesc measurementssupport4 the d-wa1.e
pairing symmetry (d, 2 , 2). while only a few measurements supported the s-WVL’ one. From the viewpoint of these cxpcrimental results, it is now almost undisputed that the YBCO superconductor hasthe pairing symmetry of ;I d-\va\,e nature. In this paper. we discuss the superconducting oxide electronics based on the concept of an anisotropic d-Leave oxide superconductor.
S
Cc)
Fig. I. Schematics of d-v,;;Ivejunctions(a) s d Junction.(h) d d ,junctlon.(c) s d edgejuncticln.and(d) s d cornc’rjunction. 0911-5107
96 LFIS.00 ( I996
Ftwwx
Science S.A. PI/
All Iright
S002I-.S107~‘)h,0lh?8-S
rescncd
a normal-like behavior I = I, sin 41for the larger value of T/T,. whereas it deforms considerably at low reduced temperatures, indicating the strong appearance of the sin 24 term. This suggeststhat the magnetic field dependence of Josephson critical current will not shovv 21conventional Fraunhofer-like diffraction pattern and some deformed patterns are expected. At the s;mle time, it also suggests the appearance of Shapiro steps :it half-integer voltage values V= 11lrJ‘2~,(12= I 1 2. _+3: 2...) as well 21sinteger ones (it = 0, F 1. ? 2.. .) under microwave irradiation where ,f’ is the microvvav’e frequency. h is Planck’s const;mt and 1’ is the electronic charge. Normalized
Phase
‘9/n
Fig. 2. Josephson current-phaw relation li)~- the junction3 with bicrystal angles of (a) - 0. (b) - 0. ITT, and Cc) - 0.2~. The curves A. B. C and D denote T T, = 0.025. 0. IS. 0.3 and 0.6. rcspcctivcly. (after Ref. [I II).
2. Theoretical
aspects for d-wave
Josephson
junctions
The present picture of a layered oxide superconductor is given by the image that the anisotropic order parameter is realized in the CuO, planes (&-plane) and the Josephson coupling exists between the CuO, sheets along the c-axis direction. Fig. I(a) and (b) depict the schematic pictures for a junction between isotropic s-wave and anisotropic d-wave superconductors (s, I d junction) and a junction between two anisotropic dwave superconductors (d’I:d junction) where two crystals meet at different angles relative to the boundary interfrtce. The former type junctions (Fig. I(a)) were used for testing the pairing symmetry of high T, superconductors. The s I d junctions behaves differently Xcording to the junction geometry shown in Fig. I(c) and (d). i.e. an edge type and a corner type. The d, I,d type junction in Fig. l(b) corresponds to the so-called bicrystal junction or biepitaxial junction. According to a recent calculation for a d-wave d-wave junction by Tanaka and Kashiwaya [I I]. the Josephson current across the grain boundary strongly depends on the right- and left-crystal angles relative to the interface boundary. They found that the Josephsonjunction thus formed would exhibit very strange behaviours which could not be seen in conventional s-wave junctions. Under this circumstance. the zero value of phase difference (4 = 0) does not correspond to the free energy minimum condition any more. Fig. 2 shows the examples of the calculated Josephson current-phase relations at different reduced temperatures. The upper middle and lower figures correspond to the bicrystal ;mgle of 0”. 18”. 36” respectively. A. B, C. D correspond temperatures T: T, = 0.025. 0.15, 0.3 and 0.6, respectively, Especially for the lower figure casewith higher bicrystal angle. the current-phase (I -- J, ) relation nearly exhibits
3. Some experimental
results
III the following. some experimental data for high-T, junctions are presented. As for the Josephson tunneling between s-wave and d-wave superconductors. sev,eral
experiments
have been performed
to test the pairing
symmetry of high T, superconductors :is stated abo1.e [2- IO]. Fig. 3 shows the magnetic-field dependencies of the Josephson critical current f, for Pb MgO YBCO sandwiched type tunnel junctions (MgO - 3 nm) with ir c-axis oriented YBCO film (upper) and with a11 t/-axis oriented YBCO film (lower) [5]. It is uorth noting that I, hltd a dip-like structure around zero flus for the upper figure. while it exhibited a normal-like pattern for the lower figure. The observed behavior in the upper figure is considered to arise from the microscopic surface morphology of the junction interface. III the case of the c-ktxis oriented film. the suppi-essionof Josephson current along the c-axis and the distribution of micrograins at the junction inter-time enable us to provide the different contributions of edge and corner type Josephson current elements according to the grain size of a fihn (for more details. seeRef. [S]). The results are consistent with the properties of s I d junctions between the s-wave and anisotropic d-wave superconductors. As stated above. the anisotropy also affects the junction characteristics of 21d/Ld type junction depending on the detailed bound;try interface condition. For example. in the case of bicrystal junctions. the magnitude of Josephson current strongly depended on bicrystal angles. For YBCO junctions grown on the bicrystal substrate with a 36.8” tilted angle. the Josephson current vvas much smaller than that of those with 21”. For BSCCO junctions with 36.8”, even the Josephson current itself was sometimes not observable. The control of the Josephson critical current for the artificial grain boundary junctions grown on the bicrystal substrates with a certain fixed angle (34Oor 36.8”) is. however, difficult, probably bec;iuse of the presence of microstructures at the interface boundary as noted in
the next paragraph. We also point out that the nature of bicrystal junctions has not yet been clarified. whether they are SIS-like, SNS-like. SNINS-like or something else. They are, however. now considered to be the best among three different types of junctions; bicrystal, biepitaxial and step-edge. Moreover. the junction properties for bicrystal junctions seem to depend on high T, materials (YBCO, BSCCO, BKBO). For noncupratc BKBO superconductors. near-ideal Josephson tunnel characteristics have been reported by two groups rccently [12.13]. The results are very consistent with the s-wave nature of BKBO superconductors as recognized by many researchers. The YBCO bicrystal junctions yielded much better characteristics than those of BSCCO ones. which are
ready to apply to electronic devices [ 14- 181.The I I’ characteristics were conventional RSJ-like ones and the Shapiro steps were observable at least up to 6 mV. The IcRn product generally lies in a few tenth of mV ~ a few mV. at 4.2 K. On the other hand. the properties of BSCCO bicrystal junctions appeared to be rather poor. The good I I’ characteristics were obtained only for ;I certain range of device parameters and the IcRn product at 4.1 K was less than I mV. The results suggest that the microscopic nature of the bicrystal junction would be different between YBCO and BSCCO. For the latter, the effects of the crystal intcrgrowth and the crystal defects might be involved in the junction characteristics.
4. Microstructures at the junction interface I
’
I
1
-6
-3
I
I
1
3
6
0.4
0.3
0.2
0.1
0
0 magnetic
I
I
I
field (10-4T)
I
I
I
I
0
5
10
15
” ’ .
-15
-10
-5 magnetic
Fig.
3. Dependence
magnetic field c-axis oriented YBCO calculated
film
the
K for YBCO film (upper
(lower figure). curbe by assuming
of corner type ratio of ,y’“” on
of the Josephson at 3.2 YBCO
surface
field critical
MgO ligure)
(10“‘T) current
Pb tunnel and \vith
I, on the junctions an taxis
applied with oriented
The solid line ,n the upper liyurc is the the microscopic fractional contribution ) and cdgc lqpe Junctmn (I:") with lhe junction (/:““lc’ :,:‘Jg” = 6.6 due to the presence of rough morphology of YBC‘O tilm (we Ref. [5]).
a
We mentioned that Josephson tunneling strongly depended on the crystal orientations of osidc superconductors relative to the interface boundary in the caseof a d-wave superconductor. In :I real situation. however. is not ideally the junction interface across a film straight [IY]. The upper part of Fig. 4 shows an AFM pictul-e obser\,ed near the grain boundary interface of a Y BCO bicrystal junction (Y BCO: c-axis oriented film). The lower left part of Fig. 4 shows the enlarged picture of the junction interface. The lower right part of Fig. 4 is the AFM image of the bicrystal boundary itself. From these obser\,ntions. it is found that the microscopic grain boundary wiggles back and forth in ;I scale of 1% 200 nm at the interface. The average tilted angle is, however. estimated to be about 24” along the interface boundary. In case of BSCCO junctions. this problem is more serious [Xl. Fig. 5 compares the line scanning image in the .\’ ~3plane at constant : (z: c-axis direction) at the interface boundary for YBCO (upper figure) and BSCCO (lower figure) bicrystal junctions. The microscopic complications at the boundary was much enhanced in the case of a BSCCO junction and some distribution of holes near the interface was also obser\,able. The presence of the zigzag structure will largely affect the Josephson properties since each microscopic interface has ;t different angle with respect to the crystal orientation of. a YBCO thin film, which limits the Josephson current accordingly. Note that. in the case that the pairing symmetry is of s-wave nature. these complexities do not arise. The presence of microscopic boundary structures affects the Josephson properties, i.e. the magnitude of Josephsoncurrent, the spatial distribution of Josephson current. etc. In particular. the Josephson maximum-current versus the applied magnetic-field curves will exhibit complicated behaviors due to the sum of the microscopically different current flows from the \,iewpoint of a d-wave superconductor. In fact. in the measurements
79
Fig. 4. AFM image of the YBCO (lower) of the junction boundary
junction on a MgO( 100) bicrystal (left) and the bicrqatal-substrate
I
Fig. 5. AFM 9~ J* line-scanning (upper) and the BSCCO (loww)
imngcs bicrystal
2w
4
(I: constant) interfaces.
of
the
YBCO
substrate boundary
\%ith a misorientation itself (right).
angle
of 24’
(upper).
The
cnlargeci
pictures
using bicrystal junctions. most of Josephson maximumcurrent versus magnetic-field curves did not exhibit ;t conventional Fraunhofer-like pattern and the obser\,ed patterns appeared to be sometimes quite irregular. On the other hand. the junction properties under microwave irradiation. i.e. the Shapiro steps. are not affected by the presence of micrograin boundaries e~cn for a d-wave superconductor since each microscopic Josephson element responds to microLvave field exactly in the same way as described by the Josephson \,oltagePfrequency relation. The voltages at \+,hich the steps appear are identical for all microscopic junctions. Fig. 6 shows the currenPvoltage characteristics of the YBCO grain boundary junction with a tilted angle of 24” at 4.2 K under microwave irradiation of 22 GHz. Although the microscopic structural AFM images at the junction boundary exhibited a complicated junction structure such as shown in Fig. 4, the observed Shapiro steps appeared very clear and pronounced in consistence with the above arguments. The dependence of Shapiro steps on the microwave power almost agreed with that predicted by the conventional theory for those with integer values. It is remarkable to point out here that in addition to the integer steps as described by the Josephson relation V= &f’ 2~, the half-integer steps (n = k 112, k 3,‘2...) at relatively higher microwave powers. are also clearly Lrisible. The appearance of the subharmonic current steps may be ascribed to the non-
# YBM-25 ix22 GHz O-70dB
irradiation
iIt different
pouel-
lw&
linear effect of the junction in which the current-phase relation is generally expressed by Z LI,, sin tfd) (for example. the Anderson-Daycm bridge using low T, superconductors. etc.). but if this is the case. the subharmonic steps corresponding to sin ttcl) (tt > 3) have also been observable in addition to half-integer steps. Experimentally, only those corresponding to the sin 241 term (half-integer steps) 1x1~ been observed and these steps are highly enhanced even in the 1 1’ characteristics of the junction. The results are very consistent with the recent calculation for ii d-wave superconductor; insulator d-wave superconductor (d ‘I ‘d) junction [I 11.
5. Application
to electronic devices
Now we discuss high T, cuprate superconductors from the viewpoint of application to oxide electronics. Based on the assumption that the unisotropic d-wave superconductivity is the essential nature of high T, superconductors, the control of the Josephson current will not be generally so easy and the complicated magnetic-field dependence of Josephson critical current due to the interferences of many different Josephson microelements across the junction boundary, as described above. may be expected. Although the detailed calculations in the presence of microscopically irregular structures as shown in Fig. 4 have not yet been performed, it may be conceivable that the averaging effect on these Josephson microelements will also be expected provided that some conditions are met. in which case the control of Josephson current will not be difficult and the magnetic-field behavior of Josephson critical current will become a near-normal one. On the other hand, if the fabrication technology of submicron high T, junctions (size: 0.1--0.2 /lrn) is developed, the junction properties will become well-defined. accordingly. Under this circumstance. the anisotropic nature of dwave superconductivity provides a variety of novel
devices different from those of s-wave superconductors. such as those utilizing the bound states of quasiparticles at the boundary interface and the interference of anglcdependent Josephson microjunctions [I I]. In practical use, there is no problem for SQUID devices since the Josephson junctions are used iis a switch of fluxon entry whose behoviour is decided by the size of the SQUID loop, not by the detailed structure of the Josephson junction. The spontaneous magnetization for a d-wave junction geometry may shift the zero flux point but it does not affect the SQUID operation since it serves as an astatic flux meter. In fact. the high T, SQUIDS have demonstrated good performance at 77 K and some SQUIDS are already in commercial market. The application to digital elements using S;I,S Josephson tunnel junctions is still not at hand. In the case of d-wave superconductors, the gap structure is a much smeared one as compared with that of s-wave superconductors due to gapless regime behavior, although appreciable nonlinear conductance is again expected. The expected hysteretic tunnel characteristics may be used again for digital elements, but the problem is how to fabricate and control the junction properties including the magnitude of Josephson current and its magnetic-field dependence. The situation may be different from that of bicrystal junctions as given in Figs. 4 and 5. The fabrication of the almost atomically Rat sandwich structure is necessary to realize this device, 21 part of which has been developed quite recently [?I]. The application of tunnel junctions to SIS mixer is. however. almost hopeless since the nonlinearity of the quasiparticle current portion is not sharp enough to allow high sensitivity detection.
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