The Temperature Dependene of Critial Current
in YBa
2 Cu
3 O
7 d
Thin Films Deposited
on MgO by an Elipse PLD
Masato Ohmukai,Takehiro Fujita,and Takayuki Ohno
Department ofEletrialEngineering,
Akashi CollegeofTehnology, Akashi,Hyogo 674-8501,Japan
Reeivedon19April,2001
YBa2Cu3O
7 d
thin lmswere deposited on MgO single rystals by means of an elipse pulsed
laserdeposition(PLD)method. Adepositedlmexhibitedtheritial temperatureof74K.The
dependeneofritialurrentontemperaturewaswellexplainedbytheGinzburg-Landautheory.
Althoughthetheoryisdesribedonlynearritialtemperature,experimentalresultsarewelltted
inawiderangebetween25Kand74K.ItstemsfromthestronglyJosephson-ouplingregime.
I Introdution
Reently the YBa
2 Cu
3 O
7 d
(YBCO) thin lms have
been applied to superonduting devies suh as
Josephsonjuntions[1,2℄,uxowtransistors[3,4℄,
su-peronduting quantum interferene devies (SQUID)
[5℄. YBCOthinlmsaremoreandmoresigniantfor
eletroni deviesnowadays. Itisof importane to
in-vestigate fundamental properties of YBCO thin lms:
theritialtemperatureandtheritialurrent.
Researhershavehallengedavarietyoffabriation
tehniquestodepositYBCOthinlmssinethe
disov-eryofthematerial. Amongthesedepositiontehniques,
sputteringandpulsedlaserdeposition(PLD)are
possi-blythemostpromisingdepositionmethods. In
sputter-ing, there isaproblem in priniple; resputtering. The
resputteringisausedbyoxygenanionsgeneratedina
sputtering hamber. Thegeneration of oxygen anions
is inevitable beauseYBCO is an oxide material. On
theother hand,PLDis freefromresputteringbeause
of theabseneofeletrield inthehamber,but
an-other problem of a droplet should be onsidered. A
droplet, abig luster of atoms or ions, deteriorates a
deposited lm. Inorderto avoidthedeterioration, an
elipse PLDmethodiseetive[6℄.
In this artileYBCO thin lms were deposited on
MgOsubstratesbyanelipsePLD.Thedependeneof
ritial urrent on temperature was investigated after
patterningof theYBCOlmswasperformed.
II Experimental details
A ommerially available stoihiometri single target
wasused;25mmindiameterwithathiknessof2mm.
diation. AKrFeximerlaserwasusedfortheablation,
whihosillatesat246nm. Theoutputenergyand
rep-etitionfrequenywas100mJand8Hz,respetively.
A(100)MgOsinglerystalwasusedasasubstrate.
Thesubstratewaspreparedinthe formofasquare of
100mm 2
with a thiknessof 2mm. MgO havebeen
widely used as a substrate for YBCO thin lms
re-gardlessof large mistof 7.7%, beause the dieletri
propertiesareexellent;lowdieletrionstantandloss
tangent. Thesubstratewasleansedinaetoneandin
ethanoloneafteranotherrepeatedlyusinganultrasoni
leaner. Aleansedsubstratewasdriedinairandputin
thedepositionhamberinsuhawaythatthesubstrate
wasparallelto thetarget(on-axisonguration). The
substrate wasgripped with a sampleholder on whih
an SiC heater was mounted. The substrate
tempera-turewaselevatedupto 740 Æ
Cduring the deposition,
whihwasmonitoredwithathermooupleattahed to
thesampleholder.
The deposition hamber was evauated to 3*10 6
Torrwithaturbopump. Thedepositionwasperformed
underanoxygen atmosphereforanhour. The
deposi-tiontime orresponds to the lm thikness of 90 nm.
Afterthe deposition the hamberwas lled with
oxy-gengas. Thesubstratewasooleddowninthehamber
slowly to 400 Æ
C for 30 min. and then to room
tem-peraturenaturally. Thetreatmentmakesusto omita
post-depositionanneal inanoxygenatmosphere.
Deposited lms were patterned to be in the form
depitedshematially in Fig. 1. The patterning was
performedbyahemialethingwithaphosphoriaid
Figure1. AnillustrationofpatternedYBCOlms.
Chem-ialethingwasperformedwithphosphoriaid.
The resistivity and I-V harateristi of the
de-posited lm were obtained by an ordinary four-port
method. Thetemperaturedependenewasobtained
us-ingaliquidheliumryostatandaomputerontrolled
heaterattahedtothesampleholder. Theritial
tem-peraturewasdened asthat wherethegradientofthe
resistaneto temperatureisatmaximum.
III Results and disussion
An x-ray diration pattern of the deposited YBCO
lmsshowed ten(00I) peaks from YBCO aswell as a
strongpeak(2=42.8degree)orrespondingtoanMgO
substrate. Theresultsindiatedthatthelmis
prefer-ably oriented in the manner that axis is normal to
thesurfae. Thelmthiknesswas90 nm,whihwas
determinedbyaneedle-ontat-typethiknessproler.
The temperature dependene of the resistivity is
showninFig. 2. Theresistivityisalulatedinluding
thedimensionoftheethedlm. Thegureshowsthat
the transition ours steeply at 74 K. It indiates the
existene ofonly one phase. Theoxygen deienyis
estimatedtobe0.37basedonthedatareported
previ-ously[7℄.
Figure 2. A temperature dependene of resistivity. The
TheI-Vharateristiwasmeasuredbetween25K
and 72 K. Typial results (T=25 K, 50 K,65 K,and
72 K) areshown in Figs. 3-6. Eah gureshowsthat
whentheurrentissmallthevoltageiszero. The
fea-tureindiatesthatthesuperondutingurrentours.
Iftheurrentreahesaritialurrent,forexample42
KforT=50K,avoltageinreaseswithouttheurrent
inrease. Theourreneofvoltage dropindiates the
break of thesuperonduting state. The feature is of
theaseregardlessofthediretionoftheurrentfrom
thefatthat thedataissymmetrialattheorigin.
Figure3. TheI-Vharateristi atT=25K.
Figure6. TheI-VharateristiatT=72K.
Asthetemperaturebeomeshigher,theritial
ur-rent beomes smaller as is obtained from the gures.
Therelationbetweentheritialurrentand
tempera-ture isplotted with losed irlesin Fig. 7. It learly
showsthat the ritialtemperaturemonotonially
de-reasewiththetemperature. Asolidlineinthegureis
drawnonthebasisofthe Ginzburg-Landau(GL)
the-ory,whih statesthat theritialurrentisin
propor-tionto(1-T/T) 3=2
. T meanstheritialurrentand
aoeÆientisan adjustableparameter. Theobserved
dataagreewellwiththetheory,whentheoeÆientis
225mA.
Figure 7. Thetemperaturedependeneof theritial
ur-rent. Closed irlesare obtained from the experiment. A
solid line was drawnonthebasis of theGinzburg-Landau
theory.
J.R. Clem et al. have reported that granular
superondutors show the rossover eet between
Ambegaokar-Barato (AB) and GL temperature
de-pendene onritialurrentdensity [8℄. TheAB
tem-peraturedependene,thattheritialurrentisin
pro-portionto(1-T/T),isbasedontheweaklyJosephson
oupledregime andobservedin granular
superondu-tors. TheABdependeneshowsonvexoneinthe
rit-ialurrentdensityasafuntion oftemperature. The
dependene is observed under the rossover
tempera-ture. From the rossover temperature up to the T,
GLdependeneisobservedwhereurrent-induedgap
suppressionisdominant.
Therossoverhasbeenalsoreported in YBCO
e-in the rossovertemperature with the derease of the
T. WhentheTisunder80Ktherossoveroursno
higherthan10K.TheTisdeterminedbytheoxygen
deieny. As theoxygen deieny isstrong, the
o-herenelengthbeomessmallfollowedbythederease
oftherossovertemperature. Forinstane, an YBCO
samplewiththeoxygendeienybetween0.2and0.25
showed the superonduting transition at 80 K where
therossovertemperaturewassituatedbetween10and
20K. The oherene lengthwas 1nm and thesize of
superondutingdomainis around1nm.
The literature treated the YBCO lms deposited
on SrTiO
3
and LaAlO
3
substrates. The lattie
mis-mathwithYBCOlmsislessthan3%. Inthisartile
theMgOsubstratewasusedwhoselattiemismathis
7.7%.Thedisussionintheliteratureisonsistentwith
ourresults that YBCO lms withT of 75 Kshowed
Ginzburg-Landau temperature dependene in a wide
range. ItimpliesastronglyJosephson-ouplingregime
wherethe size of superonduting domain is lessthan
1nm.
IV Conlusions
YBCOthin lmsare deposited onMgO substratesby
means of an elipse PLD method. The YBCO lms
werepatterned bymeansofhemialething by
phos-phori aid with the help of photolithography. The
I-V harateristi was measured, from whih
temper-aturedependene of theritialurrentwasobtained.
Thedependene isin an exellentagreementwith the
Ginzburg-Landautheoryoverarangebetween25Kand
72 K. It stems from the strongly Josephson-oupling
regime.
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