J Supercond Nov Magn (2012) 25:2259–2263DOI 10.1007/s10948-012-1614-1

O R I G I NA L PA P E R

An Effective Method for Improving Emissions from IntrinsicJosephson Junction Arrays of Misaligned Tl2Ba2CaCu2O8+δ

Thin Films

Z. Wang · B. Fan · P. Wang · L. Ji · X.J. Zhao · S.L. Yan

Received: 23 March 2012 / Accepted: 6 May 2012 / Published online: 17 May 2012© Springer Science+Business Media, LLC 2012

Abstract In this paper, an effective method for improvingemissions from Josephson junction arrays was proposed. Wetook the substrate as a dielectric resonance antenna to im-prove the coupling of intrinsic junctions to electromagneticwaves. Electromagnetic simulations were performed in par-allel with experiment investigations. Sharp radiation peaksat 76.9 GHz were detected from intrinsic Josephson junc-tion arrays in misaligned Tl2Ba2CaCu2O8+δ thin films at78 K. Experimental results proved the effectiveness of thismethod, which offered a competitive choice for coherentemission from intrinsic Josephson junction arrays.

Keywords Intrinsic Josephson junction arrays · Dielectricresonance antenna · Electromagnetic emission ·Electromagnetic coupling

1 Introduction

According to the AC Josephson effect, Josephson junctionarrays (JJAs) can be ideal high frequency source candidates.

Z. Wang · B. Fan · P. Wang · L. Ji (�) · X.J. Zhao · S.L. YanDepartment of Electronics, Nankai University, Tianjin, 300071,Chinae-mail: luji@nankai.edu.cn

Z. WangPurple Mountain Observatory, CAS, Nanjing, Jiangsu, 210008,Chinae-mail: zhwang8405@gmail.com

Z. WangKey Lab of Radio Astronomy, CAS, Nanjing, Jiangsu, 210008,China

B. FanDepartment of Electrical and Computer Engineering, MichiganState University, East Lansing, MI, 48823, USA

However, uniform junction array and effective electromag-netic coupling are two of the most important prerequisites.Since the intrinsic Josephson effect in high-Tc superconduc-tors allows a relatively easy access to fabricate series ar-rays of hundreds of identical Josephson junctions, intrinsicJosephson junctions (IJJs) have been investigated for manyyears as potential high frequency sources [1–6]. Batov et al.reported the detection of electromagnetic radiation at about500 GHz from current-biased intrinsic Bi2Sr2CaCu2O8+δ

single crystal Josephson junctions [3]; Ozyuzer et al. real-ized intense, coherent, continuous, and monochromatic elec-tromagnetic emission at terahertz frequencies from a sin-gle crystalline mesa structure of Bi2Sr2CaCu2O8+δ intrin-sic Josephson junction system through considering the mesastructure as an electromagnetic cavity [4–6].

In this paper, we report an effective method to strengthenthe coupling of intrinsic Josephson junctions to electro-magnetic system. The intrinsic Josephson junction array(IJJA) was fabricated from epitaxial Tl2Ba2CaCu2O8+δ

(Tl-2212) thin films [7]. This kind of intrinsic Josephsoneffect devices is different from the intrinsic Josephson junc-tion system based on single crystalline mesa structure. It iseasier to fabricate. To guide and explain the experiment,three-dimensional (3D) numerical simulations were per-formed in parallel with experiments. Sharp emission peaksat 76.9 GHz were observed at 78 K, which verified the ef-fectiveness of this method.

2 Sample Preparation and Microwave MeasurementSystem

The intrinsic Josephson junction arrays were made fromTl-2212 thin films grown epitaxially on miscut LaAlO3 sub-strates with dimensions of 10 × 3 × 0.5 mm. The sub-strate surface was cut at a small angle α = 20◦ relative

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to the LaAlO3 (001) plane. Tl-2212 superconducting thinfilms were epitaxially grown on the LAO substrates by DCmagnetron sputtering and Ar post-annealing [8] to keep theCu–O plane of the Tl-2212 films at a slanting angle α tothe substrate surface. The thickness of the films was 0.5 µm,and the transition critical temperatures Tc were 100–103 K.A microbridge integrated with a bow-tie antenna and cur-rent leads was fabricated using standard photolithographyand Ar+-ion milling techniques. The layout of the circuit isshown in Fig. 1(a). The microbridge of 6 µm in length and3 µm in width is aligned along the slanting direction of theCu–O plane to the film surface. The schematic structure ofthe microbridge is shown in Fig. 1(b). The solid lines in thefilm indicate Cu–O layers. Along the microbridge, we get astack of slanting Cu–O layers which are separated by inter-

Fig. 1 (a) A photomicrograph of the sample; (b) A model of the IJJA

planar layers (i.e., Tl–O and Ba–O layers). Thus, each pairof adjacent Cu–O layers in the microbridge acts as an intrin-sic Josephson junction, and the microbridge forms a seriallyconnected junction array. The number of intrinsic Joseph-son junctions N in the microbridge can be estimated fromthe following equation [9]:

N= l sinα − t cosα

s + b(1)

where N is the number of junctions, l and t are length andthickness of the microbridge, s and b are the thickness of theCu–O layer and the interplanar layer, and α is the misalignedangle between the Cu–O plane and the substrate surface. Ac-cording to (1), the number of junctions in the microbridgecan be calculated to be circa 1055 in our case.

The microwave measurement system mainly included alocal oscillator, isolators, attenuators, amplifiers, a mixer,a square-law detector, a measuring system of I–V charac-teristics, and a quasi-optical resonator in which the hemi-spherical mirror was replaced by a horn antenna [10]. Theblock diagram is shown in Fig. 2. The sample was mountedin the quasioptical resonator. The coupling between elec-tromagnetic waves and the sample was performed via thehorn antenna. To obtain optimum coupling to the sample, thewaveguide with the horn antenna and the plane mirror canbe displaced with respect to the sample. The Josephson ra-diation was detected by a superheterodyne radiometer withintermediate frequency (IF) bandwidth of 1.25 ± 0.05 GHzand sensitivity of 16.4 mV/µW. The voltage of IF signal ver-sus the voltage applied to the array was monitored by thesquare-law detector.

3 Model and Simulations

We proposed to take the substrate as a dielectric resonateantenna (DRA) [11]. Thus the coupling of IJJA to the elec-tromagnetic system was expected to be strengthened largely

Fig. 2 A schematic diagram ofthe measurement system

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Fig. 3 (a) x-Axis component ofelectric field E distribution inthe substrate at 76.9 GHz;(b) E-plane and (c) H -planeradiation power patterns of theDRA

through high-frequency electromagnetic coupling mecha-nism [12]. 3D numerical simulations were performed toguide the experiments. The real parameters of the LAO sub-strate were employed. Considering here we just needed tocalculate and analyze the distribution of the electromagneticfield in the DRA, the HTS thin films were simply mod-eled by perfect electric conductors (PEC) and the IJJA weremodeled to be alternating current (AC) source [13]. Stim-ulated by the bow-tie antenna, the substrate can resonate at76.9 GHz. Figure 3(a) shows the electric field E distributionin the substrate. Standing wave feature can be observed ob-viously. And the junction array is located in a position of theantinode of the standing wave in the DRA, which is crucialfor strong electromagnetic coupling [6, 12]. Figure 3(b) and(c) present E-plane and H -plane power patterns of the DRAplaced in front of the plane mirror. From the figures, the ma-jor lobe is oriented perpendicularly to the substrate plane. Inradial phase, the side lobes are small. Consequently, most of

the power radiated by the DRA can be collected by the hornantenna and transmitted to the receiver.

In order to make a comparison, we simulated the caseat 74.2 GHz at the same time. Figure 4(a) shows the distri-bution of electric field E in the substrate at this frequency.No resonance mode can be observed from it. In addition,the major lobe departs largely from the normal direction ofthe substrate (shown in Fig. 4(b) and (c)), leading to a largedissipation of radiation power along radial directions.

4 Experiment Results and Discussions

Based on above numerical simulations, experimental inves-tigations were performed. Figure 5(a) shows the current-voltage characteristic (IVC) of the junctions array withoutexternal irradiation at 78 K. At this temperature, the hys-teresis is very weak. There are several large voltage jumpsto several hundred millivolts at higher bias voltage, which is

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Fig. 4 (a) Distribution ofx-axis component of electricfield E in the substrate at74.2 GHz; (b) E-plane and(c) H -plane radiation powerpatterns of the DRA

Fig. 5 IV curves of the junction array without (a) and with externalirradiations at 76.9 GHz (b) and 74.2 GHz (c), respectively

probably caused by a large number of junctions switching si-multaneously [9]. With an external irradiation at 76.9 GHz,

Fig. 6 Voltage V dropped across the junction array vs. detected radi-ation power Pd at two different frequencies

the critical current could be nearly suppressed to zero byfinely adjusting the horn antenna and the distance betweensample and plane mirror, suggesting that an optimal cou-

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pling between the junction array and the microwave systemwas achieved. The IV curve is shown in Fig. 5(b). While theIVC showed only a moderate suppression when the junc-tion array was irradiated at 74.2 GHz (shown in Fig. 5(c)).Considering the same irradiation power, we supposed thatthe resonance of the substrate (refer to Figs. 3(a) and 4(a))played an important role in coupling of electromagneticwave and junction arrays.

To further explore the coupling mechanism, emissions ofthe junction array were measured. Fixing the positions ofthe sample and the horn antenna after the optimal couplingwas achieved and tuning the local oscillator to 78.1 GHz, webiased the IJJA with a sweeping current and recorded theIF signals using the square-law detector at the same time.Figure 6(a) shows the detected emission power Pd vs. thevoltage V dropped across the IJJA. Two symmetrical peakscan be observed in it. Subsequently, we repeated the exper-iments with the local oscillator fixed to 75.4 GHz. Figure 6(b) shows the result. Two emission peaks which are similarwith previous reports can also be observed [2, 9]. However,compared with the result shown in Fig. 6 (a), the radiationpeaks are much broader and the radiation power is severaltimes lower, which implies a better coupling in the formercase. Once again, the simulations explain the observed fea-tures nicely.

On the basis of above comparative investigations, weconclude that the resonance mode of the substrate plays avery important role in coupling of the IJJA to electromag-netic waves. Therefore, we can improve the coupling ofIJJA through choosing suitable substrate material and di-mensions, and optimal choices may lead to further optimiza-tions of the coupling.

In addition, for the emission peak shown in Fig. 6 (a), themaximum detected power Pd was about 24 pW and the max-imum occurred at V ≈ 127 mV. According to the modifiedJosephson equation fJ = V/Na�0, the number of activatedjunctions Na was estimated to be about 797. This numberis less than that in the IJJA estimated from the microbridgegeometrical structure. Similar results were also observed byother research groups [3, 6]. In our opinion this was mainlycaused by the spread of junctions’ parameters especially forthose junctions at the end of the array (refer to the photomi-crograph of the microbridge in Fig. 1). Furthermore, withthe broad radiation peaks (the linewidth at half-maximum�V ≈ 30 mV) and low radiation power, it is hard to arguethat the emission is coherent. However, considering the highoperation temperature 78 K, the results have been improvedconsiderably [2, 9]. For a large number of serially connectedjunctions, many factor influence the mutual phase lockingof the junctions, e.g., uniformity of the junction parame-ters, heating effects, operation temperature, number of acti-vated junctions, and external magnetic field [10, 12, 14, 15].Therefore, it is very hard to realize synchronization of junc-tions even in very uniform arrays. In our case, we guess thatthe noncoherent emissions are mainly caused by the spread

of junction parameters, high operation temperature, and toofew junctions activated.

5 Conclusions

To improve the coupling of IJJA, the dielectric substrate wasconsidered as a dielectric resonance antenna. Comparativestudies were performed experimentally. Numerical simula-tions were done at the same time to guide and explain theexperiments. The results verified the effectiveness of thismethod. Although the emission from the intrinsic Josephsonjunction array of misaligned Tl-2212 films was not coherent,our success in realizing sharp radiation from this device at78 K offered a competitive method for coherent emissionfrom IJJA.

Acknowledgements The project was supported in part by the Na-tional Natural Science Foundation of China (Grant Nos. 51002081,61101018, 61171028, 61176119) and the Fundamental ResearchFunds for the Central Universities. The author, Z. Wang, thanks Dr. Shiof the Purple Mountain Observatory of China for his valuable discus-sions.

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