IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 9, No. 2, JUNE 1999 4530

Bi2SrzCaCu208+6 Intrinsic Junction and its Application in Harmonic Mixing at the Millimeter Waveband

W.L.Slian. G.D.Zlion. Y.J.Feng. Z.M.Ji. W.W.Xn. S.Z.Yang. P.H.Wu DeIxutineiit of Electronic Science aiiiil Eligllieerhg, Nanjllig 1 Jiuversi&, Nanjing, 2 10003, P. K. Cllillia

Y.H.Zliang Structure Research IAoratoy, 1 huversity of Science & Technology of China,

Hefei, Ailhui 230026, P. R. Clhuia

Ab.@wct-€n this paper, we report on the successdul fabrication and characterisation of the BizSrzCaCuzOx+fi intiinsic junctions. To study the microwave influence of these intrinsic junctions, harmonic mixing was carried out in the millimeter waveband at temperatures ranging from 20K to 77K Up to 68'h harmonic mising in the 8 millimeter wivehand ilnd S6* in the 3 millimeter waveband were clrr!rly obsenetl. The dependence of the intermediate frequency output on the temperature, the signal power and the local oscillator power were studied, and some unexpected feiitures were obsenetl.

Tlie layered structure of the oxide superconductors. \vlucli behaves like a series array of Joseplison junctions. has been extensively studied in niany ways including its lugli freqnency respoiise[l].[2]. A number of experiments have bcen made on the intrinsic Josep1ison.junctioii stacks naturall!. fornicd in the BilSr2CaCu20x. single cnstals in order to investigate tlie nucrowa\~e induced steps in the I-V curves, when the junctions werc irradiated by aii external nicrowave source. In some of tlie experiments. 110 i1licronm-e induced steps (Sliapiro steps) Iuve been observed. wide in tlie other experiments. some steps have been found nhicli should be explained by coupled flux-flow instead of nucrow:i\.e induced Sliapiro steps [3]-[5]. These experiments indicate that tlie intrinsic junctions may liave some new phenomena tliat do not exist in tlie single Joseplison .iuuction 3-stem. Since tlie Sliapiro steps 1m.e not becn found in tlic intrinsic Joseplison junctions. tlie question n.lictlicr tlie devices based 011 the ac Josephson effect could be realized bj, iiiirinsic jnnctions remains iinsoh.ed.

From a technical point of \;iew. the we of intrinsic junction StiiCkS as nisers is promising because of its larger energ! gap. For the Bi2Sr2CaCn20x-r materials. tlie characteristic frequency ,/:. is almost IOTHz. sliowing tliat thus nmterial niaiy offer opportniuties for tlie application in tlie THz range. Fundamental iiusing experinient nsing Bi2Sr2CaCu2Oxt7, intrinsic junctions

have already sliown niany new plienoniena [GI. In this paper. ixe report a detailed investigation of the lianiic~nic inixi tig experiments using tlie Bi2Sr2CaCn20xLr. intrinsic junctions at inillinieter wave band. wlucli reveal more iilfonriation of tlic interaction of tlie intrinsic junction with tlie microwave signals.

Small inesa-&pe strnctnre with l~tcrril dirncnsions of lcss t11,;ni

50 niicronietcr \vas realizcd on top of tlic high qu;ilit~, Bi2Sr2CaCu20x.~- single cn~stal by two diKerent metliods. Tlic first one is a combination of tlie argon ion milling and the standard pliotolitliograpliy. The second one nses a set of metal niasks instead of tlie photolitliograpliy [ 71. Tlic iiiesa lieiglit 1% ;is controlled by vaqing tlie intensity of tlic ;irgon ion t~emi and thc etclung time. fornurig a junction stack of about 1-10 -junctions. Insulating layer and silver electrodes were finally alpplied to t lic sample to allow a real four-point Iiieasnrenient.

I n order to study the lianncnic niixing propcrtics. tlie sainplc was mounted in a W-band wwgnidc inixer block (Fig. 1). An HPX362 1 A qntliesizcd swcper was nsed as a l0c:;il oscil1;ilor (LO) with frequency ranging from 45MHz to 20GHz. Tlic F1F signal was generated by two tnncablc Gunn oscillalors with lllic center frcquenq of 1 OXGHz and 3OGHz rcspectivcl!.. Tlhc intennediatc freqiienq (IF) signal was dctccted by a low noisc amplifier with the centre frequency of 320MHz and bandwidth of 4OMHz. Then the signal was picked up by ;in

HPX5OOA spectniiii anal!zer. Tlie Ineasnrenicnts lime bccn made at teniperahires range from 20K to 77K.

'-GIJ Mixing

~ 1051-8223/99$10.00 0 1999 IEEE

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Multi-branch structures have been clearly observed in the I-V curves of the Bi2Sr2CaCuQ,?, intrinsic junctions. which reveals tlie occurrence of a series array of tlie hysteretic Josephson junctions. Two different types of the I- J' curves were obtained and shown in Fig. l(a) and 01). It is still not clear for us why dserent transport characteristics were observed on the samples fibricated with the Same process. Fig. 2(a) shows the more common case for our intrinsic junctions, which has been reported in most of the research papers. It is characterized by its

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Fig. 2. Two d i m a t types ofI-V mrvm ofour slunplzs. (a) The I-V cune with ii zmo voltage k x q h x m current. The inslt shows the shuLzurZ ofthe mwsa. (h) The I-V curve with ;i large dynamic resistmce at zmo voltage region. The inset show the d u g & wule ofthe I-v clllve.

zero voltage Josephson current and equally separated resistive branches. The voltage jump between two adjacent branches Vg is about 20mV at 20K and strongly depends on the teinperature. We also got the Z-J,'curve as shown in Fig 2.01). where no zero voltage Josephson current is found. The first resistive branch appears at about 5OmV wluch is far larger than expected. It looks like that some branches are missing. resulting a lugldy nonlinear characteristic. Contrast to tlie linear behaviour at zero voltagc point of tlie first case. in this kind of tlie I-J- curves a 1;irgc dynamic resistance occurs at the zero voltagc. which is supposcd to be helpful for mixing.

The mixing experiment was carried out on the simples with the I-J'curves of the second case. To eliminate the shot noise and heating effect induced by the dc bias current. tlie intrinsic junction is unbiased during the mixing experiment. With the optimum LO power, up to the 6 ~ * lianiionic mixing in the 8 nini waveband and the 56"' in the 3 nun wvaveband at zero-bias current were observed. At some hannonic number the signal-to- noise ratio reaches 35dI3. wluch meets tlie demand of tlic application of the inixer in a phasdocking loop [X I .

At 31nm waveband only the even nurnber lannonic mixing generates a detectable IF signal. It is almost the snnic casc uhcn tlie signal is in thc Xinin ivavcband, bcsidcs occasionally wc got \ay weak IF output of the 3"' lixmonic mixing. This phenomenon. that even number llannonic Josephson inking allows unbiased operation while odd harmonic ones do not. haic also be observed in the mixing experiment using a traditional single Josephson junction. and was phenomenallp explained on the basis of Josephson relation[9]. It should be mentioned that the Josephson relation is not a necessary condition to get the result. so this explanation means that the above results may be a consequence of tlie Joscplison nuxing. but it is not a proof. Further experiments with the magnetic field applied to tlic intrinsic junctions are nccded to probe whcthcr Joscplison cffcct causes tlie mixing results.

Fig. 3 shows tlie IF output as a fiinction of the temperature for tlie intrinsic junction shown in Fig. 2b. The IF output declines slowly when increasing the temperature below the critical temperature T,. At T, the IF output drops dramatically and disappears in the noise.

Fig. 4 shows tlie dependence of the IF output on the LO polvcr of the 28"' inking at 3nun waveband using the junction showed in Fig. 2b. The IF output increases as the LO powcr increases. and reaches its niaxinium at P:,, = 13dl3m. A further increase of the LO power causes a decrease of the IF po\vcr. It should be noted that tlie LO power value is tlie available powr at the output port of the oscillator. The power coupled to tlic junction should be several orders of magnitude lowcr because of tlie poor impedmce matching and low radiation eficiency for no special antenna was used in our experiment.

Fig. 5 shows the IF signal intensity as a fiinction of tlie applied RF power of the 36" mixing at 3mm waveband using the sanlc junction. The IF signal intensity depends linearly on tlie RF power. similar to the case of the mixing using a singlc Josephson junction.

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Fix. 3 . The IF output :LV ii kindion ofthe temperature lixth? niwi showi in Fig. 2h.

I!i suiiuna~. we have realized the hamionic mixing with mesas patterned on top of Bi,Sr2CaCii20x-, single cn;stals. A non-linear region has been found in the 1-1 ' cun'e around zero voltage. Even number hantionic niixing is obtained at zero bias current. Up to OXd' hannonic mixing in Xitun wave range and 3(? in inun wive range have been observed, which implies that the intrinsic juiiction iniser lias a high non-linear region around zero voltage. The IF output switches off a t the critical

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Fig. 4. The dqxndence of'the IF s i g d on the L() power (28"' h;mnoiuc mixing id 3nmi hand, T=20I;).

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Fig. 5 .IF signal intensity i ~ q hiidion ot'.4ttenuation ot'the applied W hiumonic mising at 3nun hand. T-2OIi).

(3(?

teniperahire, nlucli reveals that tlie nu.xing effecl inust be :I

consequence of tlie supercondiicting state. while to prove it to be a Josephson lianiionic inking effect needs further cs~xriiiient s.

ACLW )Imww/IE:x-r'

The authors would like to thank Jin Hao for liclps in niakii;ig the Bi2Sr2CaCu20x- - single crystals.

REFERENCES

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