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Josephson tunnelling in [1 0 0]-tilt YBa

₂

Cu

₃

O

_7x

bicrystal junctions

Yury Y. Divin

^a,*

, Matvei V. Liatti

^a,b

, Dmitry A. Tkachev

^a,b

, Ulrich Poppe

^a

aInstitute of Solid State Research, Research Center Juelich, Leo-Brandt Str. 1, 52425 Juelich, Germany

bInstitute of Radio Engineering and Electronics, Russian Academy of Sciences, Mokhovaya 11-7, 125009 Moscow, Russia Available online 2 April 2007

Abstract

[1 0 0]-tilt YBa2Cu3O_7xbicrystal junctions have been fabricated and their Josephson behaviour have been studied. Due to highIcRn- values, the ac Josephson effect in the [1 0 0]-tilt junctions has been observed up to the frequency of 5.3 THz. The absorption of Josephson radiation by IR optical phonon mode at the frequency of 4.6 THz in YBa2Cu3O_7xhas been found to be reflected in theI–Vcurve of the [1 0 0]-tilt junctions, thus giving us a possibility to suggest a new kind of near-field THz spectroscopy. The [1 0 0]-tilt bicrystal junctions are very promising both for fundamental study of low-energy excitations in high-Tcsuperconductors and applications in THz electronics.

1. Introduction

In spite of intensive eﬀorts, the mechanisms of electrical transport through grain boundaries (GBs) in high-temperature superconductors are still far from understanding[1].

Conventional artiﬁcial [0 0 1]-tilt GB junctions, due to an island growth of the c-axis high-Tc ﬁlms, demonstrate a facetted meandering of their GB around the substrate GB, inhomogeneous current transport, large parameter spread and low characteristic voltagesIcRn.

Recently, [1 0 0]-tilt high-TcGB junctions with mutually tiltedc-axis’s have been fabricated with an order of magni- tude lower GB meandering and up to a threefold increase of the IcRn-values [2]. A complete antiphase correlation between low-frequency ﬂuctuations of the resistance Rn

and the critical currentIchas been observed in these junctions, thus showing that both quasiparticles and Cooper pairs tunnel directly through the same regions of the bar- rier[3]. Here, we report on our study of Josephson tunnel- ing in these junctions.

2. Experimental

The [1 0 0]-tilt YBa₂Cu₃O_7xGB junctions with characteristic voltagesIcRn up to 8 mV have been fabricated on NdGaO3bicrystal substrates. The junctions were patterned with widths in the range of 1–2lm from 60 nm thick YBa2Cu3O_7x ﬁlms. Meandering of the YBa2Cu3O_7x grain-boundary in AFM image was less than 20 nm. Each junction was supplied with an integrated sinuous log-peri- odic Ag-antenna. Solid-state oscillators and an optically- pumped far-infrared gas laser were used as sources of monochromatic radiation in the frequency range 15 GHz to 6.3 THz. The voltage responsesDV(V) have been mea- sured and the normalized responsesDI(V)/DIc, containing information on the amplitude and linewidth of Josephson oscillations, have been calculated from the experimental data.

Normalized responses DI(V)/DIc of low-resistance [1 0 0]-tilt YBa2Cu3O7x Josephson junction to THz laser radiation are shown inFig. 1. The responses demonstrate odd-symmetric resonance’s at the voltages V=hf/2e, due to a frequency pulling of Josephson oscillations by THz radiation. The amplitude of the normalized response at Vﬃhf/2e is proportional to the amplitude of Josephson oscillations at the frequency f. As it is seen from Fig. 1, the amplitude of the Josephson oscillations is falling down in the THz range. Nevertheless, we succeeded in measure-

doi:10.1016/j.physc.2007.03.310

* Corresponding author. Tel.: +49 2461 612394; fax: +49 2461 616444.

E-mail address:Y.Divin@fz-juelich.de(Y.Y. Divin).

www.elsevier.com/locate/physc Physica C 460–462 (2007) 1270–1271

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Author's personal copy

ments of a response to laser radiation with the frequency of 5.2 THz (inset in Fig. 1), which is above the frequency fph= 4.57 THz of the strongest IR optical phonon in YBa2Cu3O_7x[4]. The result of these measurements is that the ac Josephson eﬀect is observed at the frequency range, where the dynamic conductivityr(f) of the YBa2Cu3O_7x [4]changes radically.

Fine structures of theI–Vcurves of two [1 0 0]-tilt junctions are observed on the dependences of differential resistances dV/dI vs. junction voltage V(Fig. 2). Two curves show the same fine structures, but their visibility increases with the decrease of the junction resistance. The most prominent structures in both curves are situated at the volt- agesVffi9.5 mV, which gives the corresponding Josephson frequency of 4.6 THz in good agreement with the frequency of the strongest IR active optical phonon mode in YBa2Cu3O7x[4]. This mode appears in the infrared spec- tra, when the polarisation of electromagnetic wave is paral- lel to thec-axis of the YBa₂Cu₃O_7x. Also, some in-plane polarized modes together with broadband free-carrier response in YBa2Cu3O_7xhave been observed[5], but none of them is close to the frequency of 4.6 THz, which can give the peculiarity on the dI/dV–Vdependence at 9.5 mV.

We might interpret the peculiarity at Vﬃ9.5 mV as a result of shunting of Josephson oscillations by enhanced dynamic conductivity of YBa2Cu3O_7x electrodes at the frequencies close to the phonon frequency. For low-resistance junctions the strength of the main peculiarity in the dV/dI-curves is larger (Fig. 2), that is in an accordance with Volkov’s results [6], where the eﬀect of shunting of a

Josephson junction by external frequency-dependent admittance was considered. In low-resistance junctions we reach the case of strong coupling of THz Josephson oscillations with optical phonons, which is reflected in the appearance of subharmonic structures at Vnffihfph/2en, where n= 1, 2, 3, in the dV/dI–V curves (Fig. 2). Tenta- tively, we can assign the features in the dV/dI–V curves atVffi7.4 mV and 11.8 mV with two adjacent optical phonon modes in YBa2Cu3O_7x at 3.4 THz and 5.7 THz [4], correspondingly.

3. Conclusions

The [1 0 0]-tilt YBa2Cu3O_7x grain-boundary junctions demonstrate the ac Josephson eﬀect up to frequencies above the frequency of the strongest optical phonon in YBa2Cu3O_7x. Low-resistance [1 0 0]-tilt YBa2Cu3O_7x grain-boundary Josephson junction might be a good instrument to study the THz dynamic conductivity in the YBa2Cu3O_7x ﬁlms and their environment.

References

[1] H. Hilgenkamp, J. Mannhart, Rev. Mod. Phys. 74 (2002) 485.

[2] Y.Y. Divin, U. Poppe, C.L. Jia, P.M. Shadrin, K. Urban, Physica C 372–376 (2002) 115.

[3] M.V. Liatti, U. Poppe, Y.Y. Divin, Appl. Phys. Lett. 88 (2006) 152504.

[4] L. Genzel, in: G. Gru¨ner (Ed.), Millimeter and Submillimeter Wave Spectroscopy of Solids, Springer, Berlin, 1998 (Chapter 5).

[5] C. Bernhard et al., Solid State Commun. 121 (2002) 93.

[6] A.F. Volkov, Radiotekh. Electron. 17 (1972) 2581.

Fig. 1. Normalized responses DI(V)/DIc of [1 0 0]-tilt YBa2Cu3O_7x Josephson junctions to THz radiation. Main graph: Rn= 6X, IcRn= 4.8 mV,T= 30 K. Inset:Rn= 1X,IcRn= 6 mV,T= 10 K.

Fig. 2. Diﬀerential resistances dV/dIvs. voltageVfor two 2·11[1 0 0]- tilt YBa2Cu3O_7xJosephson junctions at the temperature of 5 K:Rn= 0.8X, IcRn= 6.0 mV (lower curve); Rn= 2X, IcRn= 6.8 mV (upper curve). Phonon-related structures atVﬃhfph/2en(n= 1, 2, 3) are marked by arrows.

Y.Y. Divin et al. / Physica C 460–462 (2007) 1270–1271 1271