Ion-irradiated YBa Cu O Josephson arrays 2 3 7 A.Sharafiev1∗,M.Malnou1,C.Feuillet-Palma1,C.Ulysse2,P.Febvre3,J.Lesueur1,N.Bergeal1 1LaboratoiredePhysiqueetd’EtudedesMate´riaux-UMR8213-CNRS-ESPCIParis-UPMC,PSLResearchUniversity,10RueVauquelin-75005Paris, France. 2LaboratoiredePhotoniqueetdeNanostructuresLPN-CNRS,RoutedeNozay,91460Marcoussis,France. 3IMEP-LAHC-UMR5130CNRS,Universite´SavoieMontBlanc,73376LeBourgetduLaccedex,France. Abstract. We designed, fabricated and tested short one dimensional arrays of masked ion-irradiated YBa Cu O Josephson junctions (JJ) embedded into log-periodic spiral antennas. Our arrays consist of 4 2 3 7 or 8 junctions separated either by 960 nm or 80 nm long areas of undamaged YBCO. Samples with distanced junctions and with closely spaced junctions showed qualitatively different behaviors. Well separated arrays demonstratedgiantShapirostepsinthehundreds-GHzbandat66KandweretestedasJosephsonmixerswith 7 improvedimpedancematching. Allcloselyspacedarraysbehavedasonejunctionwithalowersuperconduct- 1 ing transition temperature, hence forming a single weak link on distances up to 880 nm. Such design opens 0 a new way to increase the I R product of ion-irradiated junctions and we speculate that the phenomena and 2 c N physicsbehinditmightbesimilartotheso-called”giant”Josephsoncouplingobservedincuprates. n a J 9 1 Introduction ] n Ion-irradiation technique is a promising approach to manufacturing high temperature supercon- o c ducting integrated circuits [1,2]. It allows designing a large number of arbitrary located Josephson - r junctions (JJ) on a single superconducting film and therefore offers a natural scalability compared to p other types of high-T junctions. The technique was first demonstrated by Tinchev [3] for rf SQUID u c s experiments and since then many other applications were suggested: dc SQUIDs [4], SQIFs [5], . t Josephson mixers [6,7] and even digital circuits [8]. Study of properties of ion-irradiated junctions a m and further development of the technology attracted considerable attention during the last years. Re- - cent researches in the area focused on ion-irradiated MgB junctions [9], direct junction patterning d 2 n with Focused Ion Beam Irradiation (FIBI) technology [10,11] and study of temperature dependence o ofJosephsongenerationlinewidth[12]. c [ At the same time one of the main drawbacks of the technology restricting its applicability in real systems is the difficulty to produce high-impedance junctions. Normal resistance of single ion- 1 v irradiated junctions is typically of order of a few ohms which makes it difficult to couple it with 0 standard 50-ohm microwave environment. In particular, it has been identified as one of the main is- 2 3 suesformixingapplicationswhereonlymoderateconversionefficiencyhasbeendemonstrated[6,7]. 2 One of possibleways to overcome the problem consistsin replacing single JJ with anarray of coher- 0 . ently operating junctions in combination with planar coupling structures. The interest in Josephson 1 0 arrays originates from theoretical considerations ([13] and associated references) which have shown 7 potentialsuperiorityofarraysbaseddevicesoversingle-junctionones. Operatingprototypesofarray- 1 based devices for different applications have been developed: voltage standards [14,15], wave form : v generators[16],detectorsandmixers[17,18],etc. i X Ionirradiationtechnologyiscapableofdeliveringreliablearraysoflargescale[5,19–21]. Forex- r a ampleintherecentreference[22]serialSQIF-sensorconsistingof36000junctionshasbeenreported. JunctionsuniformityissufficienttoobtaingiantShapirosteps[19,23]. Oneuniquepropertyoftheirradiationtechnologyliesintothepossibilitytofabricateverydensely packed arrays keeping it lumped while benefiting at the same time from planar geometry (unlike e.g. stacked or intrinsic junctions, see review in [24]). Moreover, since many applications require a coherent operation of all junctions in the array, external resonators are usually employed to provide long-range synchronization mechanism (theoretical review [25], experimental results [15,26–29]). In turn, short-range interaction is almost never discussed in the context of planar structures because ∗sharafi[email protected] 1 (a) (b) Figure1: (a)Schematicimageofthesamplewithphotoofitscentralpartincluding2µm-bridgewith8serial Josephsonjunctions. Theexternalturndiameteroftheantennais∼1mm;(b)Typicaltemperaturedependence of the critical current: four samples with 960 nm-spaced junctions (on the right side of the plot) and one with 80 nm spaced (on the left side). Inset: typical temperature dependence of the resistance of 960 nm spaced arraysconsistingof4and8junctions. of technical difficulty of dense packing (except for low-T microbridges, see [25] and associated c references). In this article, we report experimental results on short ion-irradiated Josephson arrays, compare and study their behavior depending on temperature and spacing between junctions. In particular, we address the question of mixing on giant Shapiro steps with an array of well separated junctions and alsodiscussthephysicsofcloselyspacedarrays. 2 Experimental For optimal coupling with external signals and possibility to read rf signal from the sample all our arrays were embedded into spiral log-periodic antennas and a 50 Ω coplanar waveguide (CPW) transmission lines (figure 1(a)). Details of our standard fabrication technique can be found in [7,30]. In short, we start from a commercial 70 nm thick YBa Cu O films grown on a Al O substrate2, 2 3 7 2 3 and covered with a 200 nm gold layer. The spiral antenna and the CPW transmission line are first defined in the gold layer through a MAN e-beam patterned resist, followed by a 500-eV Ar+ Ion BeamEtching. Thena2µmwidechannellocatedatthecenteroftheantennaispatternedina MAN e-beam resist, followed by a 70 keV oxygen ion irradiation at a dose of 2·1015 at/cm2. This process ensures that the regions of the film which are not protected either by the resist or by the gold layer become deeply insulating. Finally, the junction array is defined at the center of the superconducting channel by irradiating through 40-nm wide slits patterned in a PMMA resist, with 110 keV oxygen ionsatadoseof3·1013 at/cm2. The samples were placed inside a closed cycle refrigerator, at the focal point of a Winston cone, and then exposed to the external signals through an optical window. CPW lines of the samples were connectedtomicrowavereadoutlinesthroughPCB.Thesignalsinthe4-8GHzbandwidthwerefirst amplified at cryogenic temperature by a HEMT low noise amplifier before further amplification at room temperature. An isolator was placed between the sample and the first amplifier to minimize the back action noise. Dc output voltage and ac output power at intermediate frequency P were IF measuredasfunctionsofappliedbiascurrent. 2Ceracogmbh. 2 (a) (b) (c) Figure2: (a)MeasuredoutputmixingpowerP at6GHz(colour,redisforhigherP andblueisforlower) IF IF asafunctionoflocaloscillatorpowerP andbiasvoltageforaserialarrayof4ion-irradiated960nmspaced LO junctions at temperature T=66K. Local oscillator frequency f ≈20GHz, signal frequency f ≈14GHz. LO sign OutputpowerfollowsdifferentialresistancedemonstratinggiantShapirostepseach4·20GHzalongbiasvoltage axis; (b) the same for temperature T=60K; (c) section of the plot (a) along P =6dB (black line). Inset: the LO sameforhigherfrequencymixing(f ≈150GHz). LO 3 960nm-separated arrays Thisdistancebetweentheslitsislargeenoughtoensurethatdamagedareasdonotoverlapandthat the junctions are independent. All samples demonstrated qualitatively similar behaviors and typical results are shown on figures 1(b), 2 and 3(b). The temperature dependence of the critical currents of the arrays corresponds to the one of a single junction (see curves on the right side of figure 1(b)). For temperatures just below the Josephson coupling temperature T (see the inset in figure 1(b)) the J parameters spread among junctions in one array was sufficiently small to obtain clear giant Shapiro steps(redlineontheinsetoffigure3(b)). However,furtherloweringofthetemperatureincreasesthe parametersspreadandinsteadofgiantsteps,morechaoticbehaviorisobserved. This behavior explains the mixing patterns for high (66K) and low (60K) temperatures shown on figure 2 (bias voltage is expressed in frequency units in accordance with Josephson equation V = f · Φ , Φ is flux quantum). In this experiment, a comparatively weak signal with frequency 0 0 f =14 GHz was applied to a 4 JJ array along with a local oscillator signal f =20 GHz whose sign LO power was swept. The available power at intermediate frequency P was measured with a spectrum IF analyzeranddepictedincolourscaleonfigures2(a)and(b). JosephsontemperatureT forthissam- J ple was 70 K. At T=66 K the mixing pattern almost coincides with what one would expect from a single junction (figure 2(a) and (c)) but at lower temperatures, the picture becomes more and more fuzzy(figure2(b)forT=60K).InaccordancewithgeneraltheoryofJosephsonmixeroutputpoweris proportionaltodifferentialresistance(seee.g. [31])andshowseithersinglelocalmaximumbetween steps when f is comparatively low (black line on figure 3(c)) or two maxima and minimum when LO f ishigherandstepsarewellseparated(redinsetlineonthesamefigure). LO JosephsonmixerpropertiescanbeimprovedbyemployingJJarrayinsteadofonejunctionintwo ways: better impedance matching and decreasing noise temperature. Sufficient uniformity between junctions (ability to obtain giant Shapiro steps) is necessary to provide equal optimal biasing for all junctions in the array. Providing this condition is satisfied one can fully benefit from better matching and, strictly speaking, mutual phase locking is not a necessity (as it was noticed e.g. in [32]). At the same time, mutual locking would allow to decrease the linewidth of the junctions self-oscillation and therefore decrease the noise temperature which is a substantial problem for Josephson mixers. Indeed, the local oscillator generates additional noise in the output IF band by down conversion of wide Josephson self-oscillations (see e.g. [13]). For the devices directly using Josephson genera- tion to produce output signal (e.g. THz-range generators or devices with internal pumping) mutual phase-locking is a necessity. The fact that short range interaction in some cases can lead to mutual phaselocking([33–36])andthatparametersspreadinourarraysallowsobtaininggiantShapirosteps encouraged us to put slits as close as possible to keep damaged areas still separated but with shorter 3 (a) (b) Figure3: (a)ResultofMonte-Carlo(SRIM)simulationofO+irradiationofa70nmthickYBa Cu O micro- 2 3 7 bridgethroughtwo40nmslitsseparatedby80nmofresist-coveredarea. Colourcorrespondstodisplacements per atom (dpa) level. According to the simulation, two damaged areas almost do not overlap; (b) typical tem- peraturedependenceofresistivityof4-slitsstructureswith960nmand80nmseparation. InsetshowsShapiro stepsof80nm-spacedsamplewhenexternalsignalfrequencyis20GHzandtemperatureissignificantlylower than T . All closely-spaced samples showed single-junction behavior demonstrating Josephson coupling on J distancesaslongas860nm. undamagedareas. 4 80nm-separated arrays Inthisexperiment,thedistancebetweeninternaledgesofneighboringslitsinPMMAwas80nm as shown in figure 3(a). The figure displays the result of Monte-Carlo (SRIM) simulation of lattice damage for two junctions in an array (brighter areas correspond to higher displacement per atom level). Samples with 4 or 8 slits located in the center of micro bridges were used in the study. The rest of geometry as well as all manufacturing parameters were the same as we used for the samples describedintheprevioussection. Allsampleddemonstratedquiteasurprisingbehaviorillustratedonfigure3(b). Normalresistance scaled with number of slits and was essentially the same as for 960 nm-separated arrays. However, Josephsontemperature(T )wassignificantlylowerandthetemperaturedependenceofcriticalcurrent J was less steep (curve on left side of figure 1(b)). Moreover these samples demonstrated sound one- junction behavior as confirmed by Shapiro steps (inset figure 3(b)). The inset shows two IV-curves for distanced and closely spaced arrays of 4 JJ under external 20 GHz-signal. This behavior has been observed for all produced samples over the full temperature range of interest (corresponding to Josephson regime): from T (∼50 K) to 28 K. Note that, as the shown IV-curve of closely-spaced J array has been measured at low temperature (T=28 K), the Shapiro steps are comparatively sharp (in agreement with our previous results on generation linewidth temperature dependence [12]), and normalresistanceR iscomparativelylow. N This indicates that Josephson coupling was established in our structures over large distance be- tweentwo”bulk” undamagedareas(880nm for8-junctionarray). Toourknowledge suchadistance forJosephsoncouplinginYBCOhasneverbeenreportedyetanddeservesaseparatediscussion. 4 (a) (b) Figure 4: (a) R(T) and I (T) characteristics of YBCO 5 µm bridges irradiated with dose 3·1013 ions/cm2 c through slits of different width ([45]); (b) R(T) and IV-curves (with and without applied 20 GHz external signal)ofanarrayformedby8slitsspacedwith80nmofcoveredareas. 5 Discussion Ion-irradiatedjunctionsareusuallyconsideredasSNStype([1,37,38])whereJosephsoncoupling is associated with proximity effect, first theoretically described by de Gennes in [39]. Although this theory was not originally developed for d-wave high-T superconductors, it has been exploited many c timestodescribethedifferentpropertiesofion-irradiatedYBCOjunctions(e.g. [12,37]). To our knowledge there have been only two experimental reports on closely spaced locally dam- aged junctions [40,41] and in both cases, a strong interaction between junctions was observed. Our geometry and manufacturing process is essentially close to the ones described in [41] where authors produced a pair of junctions using masked ion-irradiation technique. By measuring Shapiro steps, they observed the pair operating as a single or two separate junctions depending on the power of appliedacsignal. Our previous experiments with 5 µm wide bridges irradiated through slits of different width with the same dose have shown that the Josephson temperature T strongly depends on the width of the J slit until it reaches a saturating value (compare lines for 20-200 nm and 500-5000 nm wide slits on the figure 4(a)). From the figure, one can conclude that the spatial limit of effective coupling for this irradiation dose is between 200 nm and 500 nm, and therefore the effective coherence length in damaged YBCO (i.e. effective ξ ) is about 100-250 nm. Beyond this value the damaged segment N behavesas”bulk”superconductorofreducedcriticaltemperature(T(cid:48))andT isreducedtoT(cid:48). c J c For our samples, the total length of a 4 JJ array and of a 8 JJ array were 400 nm and 880 nm re- spectively. Forthe4-JJsamplesweobservedsoundShapirostepsindicatingthatJosephsoncoupling was dominating effect for broad temperature range. For the 8-JJ array, steps could be seen (inset to figure 4(b)), though traits of flux-flow regime were more pronounced especially when temperature wascomparativelylow. AtthisstagetheexactreasonforsuchalongrangeofJosephsoncouplinginmulti-slitsystemsis notfullyunderstood. Takingintoaccountresultsfrom[41]whereedge-to-edgeseparationwas75nm wesuggestthatwewereclosetomulti-junctionregime. Howeverinourcase,unlike[41],undamaged islands were not big enough to separate junctions from each other (probably because of 70 nm thick films we used instead of 200 nm in [41]). At the same time, the presence of the islands may have boosted Josephson coupling effect. In this context, we would like to mention certain similarity of our situation with ”giant” Josephson coupling observed along c-axis in cuprates ([42] and references there). Theoretical approach to describe the phenomena was suggested [43,44]. The main idea is that two distanced superconductors (Gor’kov pairing functions F and F do not overlap) separated L R with a chain of superconducting islands (with they own pairing functions F , F ...F ) might form a 1 2 N single Josephson junction if F overlaps with F , F with F ... and F with F . Deeper theoretical L 1 1 2 N R 5 investigationremainsnecessarytoexplainallaspectsofexperimentalresults. 6 Conclusion Josephson mixing on giant Shapiro steps of ion-irradiated Josephson arrays has been demon- strated. Replacing single junction by an array would allow us to improve impedance mismatch of previously reported Josephson mixers ([6,7]). To improve noise temperature of the mixer as well as for many other possible applications mutual phase locking between junctions in an array is neces- sary. We believe that for this particular type of junctions short-range interaction may be a promising approachtoachievesynchronizationinarrays. Ourfirstexperimentsshowedthat80nm-spacedjunctionsbehaveassingleweaklinks. Itoffersan interesting possibility to increase I R product of ion-irradiated junctions: not optimized single-slit c N junctions we used for the reported arrays had a normal resistance R ≈1Ω when the critical current N wasI ≈25µAand8-slitjunctiondemonstratedR ≈3ΩforthesameI . c N c Propertiesofcloselyspacedarraysarethereforeofhighinterestbothfromfundamentalandprac- ticalpointsofviewbutremaintobeinvestigatedindetails. Acknowledgments The authors thank Yann Legall for ion irradiations and Thomas Wolf for picture 4(a). Useful commentsofV.Kresin,Yu. OvchinnickovandV.Kornevaregreatlyappreciated. 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