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IOP Conf. Ser., No 167, pt. II, pp. 253-256 (2000)

253

Current transport mechanism in YBCO bicrystal junctions on sapphire.
G A Ovsyannikov, I V Borisenko, A D Mashtakov and K Y Constantinian
Institute of Radio Engineering and Electronics RAS, Moscow 103907, Russia ABSTRACT: YBCO junctions were made on r-cut sapphire bicrystal substrates in which the directions <1120> for both parts of the substrate have the angles ±12° to the plane of the interface of electrodes. The junctions were tested at dc and mm waves. The junctions with 5 µm width have high normal resistance RN=10В20 , IcRN=1В2 mV and tolerance of characteristic interface resistance around 30% on a chip. DC, microwave and magnetic characteristics of the junctions have been investigated experimentally. The sinusoidal superconducting current-phase relation and a linear dependence of critical current density vs square root of the barrier transparency was revealed. Experimental results are discussed in framework of predictions for superconducting current transport via Andreev bound surface states in bicrystal junction. 1. INTRODUCTION The high values of normal-state resistance RN and critical frequency fc=(2e/h)IcRN, as well the nonhysteretic I-V curves of high-Tc superconducting (HTSC) Josephson junctions make them appreciably superior to low-TC superconducting junctions at liquid-helium temperature (T=4.2K). The high critical temperature and proper superconducting gap give promising opportunities for applications at frequencies higher than those, corresponded to energy gap of ordinary (say, Nb) superconductor. However, the aspects involved in the reproducible fabrication of high quality HTSC Josephson junctions on one hand, and the mechanism, describing current transport, on the other hand are the problems which have not been solved yet. The most reproducible junctions having a critical current spread of ±12% per chip are fabricated on SrTiO3 bicrystal substrates (Vale 1997), but because of their high dielectric constant >1000 they are unsuitable for high-frequency applications. Sapphire having a relatively low 9-11 and low losses (tan 10-8 at 72 GHz), is the traditional material used in microwave electronics. Here we present the results of fabrication and characterization of HTSC Josephson junctions on sapphire bicrystal substrates in a view of determination of current transport mechanism. The high frequency dynamics of those junctions is discussed. 2. EXPERIMENTAL RESULTS

The Josephson junctions were fabricated on the r-cut sapphire bicrystal substrates for which the directions <1120> Al2O3 for both parts were misoriented at the angles ±12° to the plane of the interface of electrodes. The YBa2Cu3Ox (YBCO) film was deposited by dc sputtering at high oxygen pressure after the CeO2 epitaxial buffer layer rf magnetron sputtering. The following epitaxial relation: (001)YBCO//(001)CeO2//(1102)Al2O3, [110]YBCO// [001]CeO2// [1120]Al2O3 was fulfilled for the deposited films (Fig.1). Thin film YBCO bridges each 5 µm wide and 10 µm long, crossing the bicrystal boundary, were fabricated by rf plasma and Br2-ethanol etching (Mashtakov 1999). The angle between the normal to the interface and current direction was varied from 0° to 54°. The bicrystal junctions (BJ) with current density Ic/S=104В105 A/cm2 at T=4.2K gave the


IOP Conf. Ser., No 167, pt. II, pp. 253-256 (2000)

254

Fig.1. Crystallographic axes orientations of CeO2 and YBCO films in sapphire bicrystal junction with =33°, =-33°. The domain of the film with the direction <100>YBCO misoriented on the angle '=90°- is the twin to YBCO. parameters: RN=5В30 , Ic=50В200 µA with IcRN=1В2 mV. Ic(T) nearly linear increases with T reduction at T<
12 10 8 6 4 2 0 12 0 10 8 6 4 2 0 10 0 8 6 4 2 0
0

n=0



=0

0 0

IS

1

=54

0





n=1

2

4

6

I1, I1/2, µA

1

2

3

4

5

6

7

n=1/2

1

2

3

A/IC

4

5

6

7

Fig.2. Normalized RF current dependence of the critical current (n=0), first Shapiro step (n=1) and half Shapiro steps (n=1/2) for two BJ =0 (filled squares), and =54°(opened circles). Dashed and solid lines show the calculated curves for =0 and =0.2 correspondingly for current-phase relation Is()=(1-)Icsin+Icsin2. The current-phase relation for these two cases are shown in inset.


IOP Conf. Ser., No 167, pt. II, pp. 253-256 (2000)

255

superconducting junction, but at T<2eIcRN/h in the case of Is()=Icsin and Is()=(1-)Icsin+Icsin2 =0.2 are presented on Fig.2. The difference between these two theoretical dependencies of Ic,1(A) is small and in both cases its well fit to experiment. At the same time, a small deviations IS() from sin-type dependence yield subharmonic (fractional n/m) Shapiro steps. The maximum amplitude of subharmonic steps Im/n are proportional to harmonics sin(n) in IS(). The precise measurements of In(A) (n=0,1,2), as well Im/n(A) at T=4.2 (T/Tc0.05) allows us to state the absence of sin(2) components in IS() function for all investigated BJs with symmetrical biasing (=0В36°) with accuracy at least of 5%. For strong asymmetric biasing (>40°) the contribution of the component Sin2 increases monotonously. 3. DISCUSSION The IS() can be determined from the energy of bound Andreev levels EB in the junction since Is()dEB/d (Tanaka 1997, Riedel 1998). For SIS junctions the dependence Eb() =0

1 - D sin

2

(

) 2

(1)

gives IS()=ICsin. For the tunnel junction of two d-wave superconductors with gaps R(L)=0cos (2+2( )) Eb depends on 4 angles: quasiparticle incidence angle -, phase -, misorientation angles (and ). Andreev levels for mirror symmetric d-wave junctions ( =- ) at several are presented at Fig.3a. One can see that in the range =10°В45° Eb() dependence is very close to EB (2) corresponds to =/4 exactly: EB=±Dsin(/2) (2)

Note, proportionality Ic D observed in experiment (Ovsyannikov 1999) directly follows from equation (2).

Fig3. (a)- Andreev levels in DID- junction for several , D =10-4, =/6, T=4,2K. EB() confined with equations (1) and (2) for =0 and /4 correspondingly. (b)-Amplitude of Andreev levels in DID- for several , D =10-4, T=4,2K.


IOP Conf. Ser., No 167, pt. II, pp. 253-256 (2000)

256

Fig.4. Current-phase relation calculated for symmetrical (45°, 45°)-dotted line and mirror symmetrical (45°,-45°)-solid line bicrystal junctions. Dashed line corresponds to the parallel connection of these two junctions. T==4,2K, D =10-4. The behavior of the amplitude of Andreev levels with increasing misorientation angle at several quasiparticle incidence angles () is shown on fig.3b. For =10°В45° the condition max|EB|>0.10 satisfied for small amount of incident quasiparticles in the range =0В10°. Therefore the averaged income of the quasiparticles would be small. The results of Is() calculation for symmetrical and mirror symmetrical junction with misorientation angle 45° using the technique (Reidel 1998) are shown on fig.4. Taking into account the twins in superconducting films, the experinental samples may be considered as a parallel connection of pairs of two similar BJs (in our experiment D33ID-33 and D33ID57). Even the calculated IS() for D45ID45 as well as for D45ID-45 are nonsinusoidal for experimental T==4,2K and D =10-4, the resulting current through the parallel connection of these junctions is Is()Icsin (see fig.4) as we observed in experiment. For larger values of the (36°В54°) the nonsinusoidal parts of the Is() dependence doesn't cancel and subharmonic step appears at the I-V curve under microvawe irradiation due to nonsymmetrical contribution of two types of BJs. 4. ACKNOWLEDGEMENTS

The work was partially support by Russian Foundation of Fundamental Research, Russian State Program "Modern Problems of the Solid State Physics", "Superconductivity" division, and INTAS program of EU. REFERENCES Andreev A V et al 1994 Physica C 226, 17 Barash Yu S, Bukrhardt H and Rainer D 1994 Phys. Rev. Lett. 77, 4070 Barash Yu S, Galaktionov A V and Zaikin A D 1995 Phys. Rev. B52, 661 Likharev K K 1972 Rev. Mod. Phys. 51, 102 Mashtakov A D et al 1999 Technical Physics Letter 25, 249 Ovsyannikov G A et al 1999 Abstract Book of Int. Conf on Physics and Chemistry of Molecular and Oxide Superconductors (Stockholm) p 154 Riedel R A and Bagwell P F 1998 Phys. Rev. B57, 6084 Tanaka Y and Kashiwaya S 1997 Phys. Rev. B56, 892 Vale L R et al 1997 IEEE Tr. Appl. Superconductivity 7, 3193