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Superconductivity induced by irradiation in La2CuO4
Physica C 156 ( 1988) 225-229 North-Holland, Amsterdam
SUPERCONDUCTIVITY
INDUCED
BY IRRADIATION
IN La2Cu04
Hiroyuki YOSHIDA and Kozo ADOBE Research Reactor Institute. Kyoto University, Kumatori-cho. OS& 590-04, Japan Received 14 June 1988 Revised manuscript received 27 July 1988
Superconducting transition below a resistivity peak is observed in LazCu04 irradiated with low flux reactor radiation at 350 K. The transition temperature increases with increasing irradiation dose up to T, (onset)=48.9 K at 4.8x lOI6 n/cm* (E>O.I MeV). The result is compared with previous irradiation experiments.
Sincethe discoveryof high-Tcsuperconductivity in LaBaCu04[ 11, greatinteresthasbeenfocussed to theoriginof superconductivity in Laz-xMxCuO~--p compoundswith the K,NiF, type structure,where M is Ba,Sr, Caandsomeotherdivalentions [ 2-6 1. Non-bulk and filamentary superconductivitywas found in LazCuOr?, and local compositionand oxygendeficiencyhasbeendiscussedasan origin of superconductivity [6-l! ]. Recently, bulk superconductivitywasreportedin highpressureoxygenloadedLa$ZuO,[ 121.Herewe presenta result obtainedby resistivity measurements, which shows a possibility of filamentary superconductivityin La,CuO, by reactorirradiation at 350 K. Two specimenswith nominally stoicbiometric La&uO, compositionwerepreparedby solid state reaction.Powdersof LaJ.I~and CuO (cation ratio, La: Cu= 2 : 1) weremixed and calcinedfor 10hours at 800°C in air, and pressedinto small plates.The platesWeresinteredfor 30hoursat 900”C in air and thenslowlycooledin a timace. The tetragonalstrutturewasconfirmedby the powderX-ray diffraction method for an unirradiatedspecimenpreparedsimultaneously.The specimens(LCOl and LC02) wereset at two positionsof different neutronflux (fast neutrons3-7 and ?:!I>!1O’On/cm2 s; thermal neutrons3.0and5.5x 10” n/cm* s;gamma-rays3.6 and 5.7~ lo6 R/cm* h, respectively)in the irradiation cryostatof the horizontallow temperatureloop (LTL) at KUR (Kyoto University Reactor).In-pile measurements of resistances werecontinuouslyper-
formed by the four terminal methodusinga computer control system during cooling down and warmingup the loop by controllingthe helium gas flow from a refrigerator.The specimenswereexposedduringshortandlong irradiationsfor several weeks, Beforethe irradiationtheresistances of LCOl and LC02 showeda negativetemperaturedependence from room-temperatureresistances(at 300 K) of 0.98 Q (resistivity p=O.104 &-cm) and 0.48 R (~~0.073 Q-cm), respectively.The resistances graduallyincreasedwith decreasing temperature. The valuesconsiderablyrosein temperaturerangebelow 150K and an exponentialincreaseappearedbelow 100K, showingsemiconductortype behavior.For LCOl the resistancereacheda peakvalue (R,) of 43.88Q (p,=4.64 Q-cm) at 17.4K andthenslightly ~~~~ !&5~ thepeaktemperature( T,). For LCO2, however,it reachedan R, of 42.260 (p,,=6.45S& cm) at 14.4K but showedno drop. The latter core respondsto the semiconductingbehaviorobserved in stoichiometricLa2Cu0.,[ 51.The formeris shown asa resistancevs temperaturecurveof LCOl before irradiation in fig. 1, which is similar to the filamentary superconductivityobservedfor the oxygende&tent La&uOl-, [7-113. Figure 1 showsthe typical resistancevs temperature curvesmeasuredfor LCOl beforeand after reactorirradiation.After the irradiationat the reactor ambienttemperature(about 350K) the resistance of LCO1slightlyincreasedat almostall temperatures
0921-4534/88/$03.508 ElsevierSciencePublishersB.V. ( North-HollandPhysicsPublishingDivision )
226
H. Yoshida 50
and K. Atobe /Superconductivity
induced
bv irradiation
-
-
40-
0
;
30-
E 2 m
20-
0
II
0 6.5 L 36.5 QA . $
*
-__--_
ta2Cu04-y)
LCOl(
2 00
in La&‘uO,
0
n/cm" 14
h
8.7 x 10
A 122.5
h h
4.9 x 1o15 1.5 x 10 16
189.5
h
2.5 x 1o16
“i.OA.* la*
l
tr. t
50
0
150
100
TEMPERATURE
[ Kl
Fig. 1.Resistancevstemperaturecurveof LazCu04 (LCOI ) obtainedby in-pilemeasurementsbeforeand after the 350 K reactorirradiation. Irradiation conditionsare asfollows: Fastneutrons(E~0.1 MeV) IxlO”n/cm*) 0 u A
A a
Thrrmal neutrons (X10’6n/cm’)
0 0.81
0 0.1
4.9 15.0 25.2
3.9 12.2 20.4
Gammay-rays (x IO”R/cm*) 0
0.23 1.3 4.1 6.8
aboveT,,. Its temperaturedependence is similar to ature ( T,,) becomeshighershowinglowervaluesof that beforethe irradiation. The resistancevs temthe peak resistance(R,). Below Tp the resistance peraturecurvesshowlargepeaks,and a sharpdrop dropstowardzero after exposureover a critical irbelow Tp' The TPgenerallyroseafter eachinterval radiationdose,andthetemperatureto reachzeroreof the irradiationandthe peakresistances(R,) apsistance( To) increasesdependingon the irradiation pearto reducedependingon accumulationof theindose.Tbetransitionwidth from T,,to Tois constant. tervals,altbougbthereis anexception.BelowTpsmall The highesttemperatureof 41.7and 48.9K for To dropsin resistanceappearfor a low dose,and relaand T,, respectively,which appearedat the maxitively largedropsfor a higherdose.The minimum mum irradiationdose[fastneutrons4.8x IO4n/cm* valueof resistance,which was detectedwithin the (EB 0.1 MeV); thermal neutrons3.7~ 10’ n/cm*; presentmeasuringcondition,generallyreducttddegamma~rays1.I x LO9R/cm21 in the presenteqefpending on the irradiation dose, and then epiment, are not yet saturated. proachedzerofor the h&bestirradiationdosein the The variation of the resistanceas a function of figure (LCOl ). temperatureobservedin the irradiatedLCOl and Figure2 showsanotherexampleof the resistance LC02 is very similar to that of LatCuG4,, reported vs temperaturecurvesfor LC02irradiatedat 350K. in previouspapers[ 7-101. Such a featureof temThe curvesshowthesametemperaturedependence, peraturedependenceon resistivity was first preWith increasingirradiation dose,the peaktempersensedby Grant et al. [7] as an evidence for
H. Yoshida
and k-. Atobe
/Superconductivity
induced
by irradiation
TEMPERATURE Fig. 2. Resistance vs temperature at 350 K are as follows: Fast neutrons (k-0.1 ( X IW n/cm*) 0 A ::
1.6 9.2 28.4 47.8
curves
MeV)
obtained
for La#ZuO.,
(LCO2)
227
t Kl
set on a different
position
Thermal neutrons ( x IO’* n/cm’)
Gamma-rays ( x lOa R/cm2)
1.3 7.2 22.3 37.4
0.31 2.1 6.4 10.8
filamentarysuperconductivityin LazCuO,,wherea curiouskink near40 K (just below T,) corresponds to the actual T, (onset) of superconductivityand smallregions(about 1%of the sample) of excessoxygenoccupationarespeculatedto be a sourceof superconductivity.It wasalsopointedout by Johnston et al. [8] that superconductingtransitionoccursat Tp(39 K) andonly 0.03%volume fractionbecomes superconducting, and that both resistivityand thermopowerare very sensitiveto oxygen-defectconcentration. The same feature of filamentary superconductivitywasalso reportedfor lanthanumdeficient, stoichiometric J.a$u04 [ 9,lO] . Structural transition to the orthorhombicallydistorted K&GF, structure[ 6,7,1O,111andantiferromagnetic transition [6- 101hasalsobeendiscussedconceming this type of superconductivity.It is recently pointedout by Schirberet al. [ 121that bulk superconductivityappearedin La$uO.,+, after annealing
in LaguO,
from
fig. I. The irradiation
conditions
in highpressureoxygenatmosphereandis relatedto excessoxygenatomssituatedasO,- superoxideions with thechargebalancerequiringCu’+ to beformed. Throughcollision sequence light oxygenatomsin LazCuO,arepreferentiallydisplacedto form oxygen interstitialsandvacancies,and runningknocked-on ions additionally contribute to make chargeexe changefor the interstitialsand vacancies.VacancybS. rich regionsin the cascadedamagesproducedby neutronirradiation may be appropriatefor formation of small regionswith orderedoxygenvacancies actingasfilamentarysuperconductors. On the other hand,the oxygeninterstitialsmay produce0: superoxideionsat theendof a collisionsequence or in the local oxygen-richregionswith similar condition to high pressureoxygenatmosphere.Thesearepassible mechanismsfor the superconductivityto appearclearlyin the presentexperimentevenat such low doseneutronirradiations.It canbe understood
228
H. Yoshida md A”.Atobe /Sqxwo~d&~~tii,::;~ induced bv irradiation in La.XuO,
asfeaturesof filamentarysupercsnductivity,asdiscussedin previouspapers[ 3-I 11.that thesharpdrop towardszerooccurredjust belowtheresistance peak corresponding to thesuperconducting transition,and someresistivity (about2 x lW4 G-cm) retainedin the “zero resistance”state.The presentdataclearly showthe appearance of the sharpresistivity transit;~n afterthe neutronirradiationwithoutanychange in bulk composition.The increases of Tpand Todependingon the irradiationdosemight correspondto the increaseof defectconcentrationin La$Zu@. Accordingto a recentexperimentby Groult et al. [ 131,T, (midpomt ) in superconductingLa&NI, increasesfrom 32.5to 35.0 K by 2.9 GeV krypton irradiation.They discussthe result in relationwith similarity of radiation-induceddefectsto strontium substitutionand pressureeffect,as well as with the presenceof a spin-density-wave phase.There is no broadstepnear250 a<,in the presentresistancevs temperaturecurves,which is relatedto a stepin susceptibilitycurvesdueto the antiferromagnetic or the spin-density-wavetransition [8,10] for both the unirradiatedand irradiatedspecimens.The present highest‘a, is slightly higherthan the T, (40 K) in orthorhombicallydistortedLa2Cu04and seemsto be not saturatedyet. However,thereis uncertainty whetherthe TPwill achievehighertemperatures,for exampIe the T, (90 K) of orthorhombic LaBazCu@, at a higherdose.irradiation. In the previous experimentsusing the samefaciiity (KURLTL) the 200 K resistivitytransitionwasobserved in YBazCu@,afterlow temperature irradiation(fast neutrondose%9X 1Or6n/cm*) (141,and the resistivity transition at higher temperaturesalso appearedin LaBazCusO,after the 360 K irradiation ( I X 1Or7n/cm’ ) [ 151.Thesephenomenaappearing in relativelylow doseirradiationsseemto be understandable only as a filamentarysuperconductivity, which may be causedby the rearrangement of radiation-induced defectsandthelocal formationof a newp&e or structure.An earlyobservationof T, improvement(from 2 to 5.7 K) in A15 Nb,lt after the 600kV Kr* irradiation is explainedby the formationof an amorphousphasewith T,ZS5.7K [ 161. The increaseof T, (onset)to 25 K, which appeared afterannealingof heavilyirradiatedNbs(Al, Ge), is understoodby reorderingof heavily disordered darnage+zgions [ 17I. ‘it is weillknown in bulk su-
perconductorswith orderedstructurethat T, decreasesby the radiation-induceddisorderingwhen theirradiationdoseincreasesabovea certaincritical dose(for example,fastneutronirradiationof 5x I 01* n/em* at 85“C for Al 5-typesuperconductors [ 181, fast neutronirradiation of 2X lOI n/cm’ at 4.6 K for YBa2Cu30,[ 191,helium ion irradiationof lo-’ displacementper atom for Lal.sSr0.2Cu04 [ 201,and electronirradiationof 5x lOI e/cm2at 20 K [ 131). Thepresentresultin La2Cu04diffea rrorntheabove bulk effectsand is a particularphenomenonof ftlamentarysuperconductivity.However,it leadsto a generalconsiderationon theorigin of superconductivity in the oxides,which is closelyrelatedto concentrationand arrangementof oxygeninterstitials and vacancies. Acknowledgements The authorswould like to expresstheir thanksto Mr. M. Okadaandothermembersof KUR for their supportto theLTL experiments.Thanksarealsodue to Mr. S.Ueharafor his assistance in dataprocessing. References [ 1 ] G. Bednorz and K.A. MiIller, 2. Phys. B 64 ( 1986 ) 189. [ 2) H. Takagai, S. Uchida, K. Kitazawa and S. Tanaka, Jpn J. Appt. Phys. 26 (1987) L218. [ 31 J.M. Tarascon, L.M. Greene, W.R. McKinnon, G.W. Hull and T.H. Geballe, Science 235 ( 19R7) 1373. [4] J.D. Jorgensen, H.-B. Schuttler, D.J. Hinks, D.W. Capone IL Z. Zhang, M.B. Brodsky and D.J. Scalapino, Phys. Rev. Lett. 58 (1987) 1024. [ 51 S. Uchida, H. Takagi, H. Yanagisawa, K. Kishio, K. Kitazawa, K. Fueki and S. Tanaka, Jpn J. Appl. Phys. 26 ( 1987) L445. [6] R.L. Greene, H. Maletta, T.S. Plaskett, Cl. Bednotz and #A. Mllller, Solid State Commun. 63 ( 1987) 379. [7] P.M. Grant, S.S. Parkin, V.Y. Lee, E.M. En8ler, M.L. Ramire& J.E. Vazquez, 0. Lim. R.D. Jacowitz and R.L. Greene, Phys, Rev. Lett. 58 (1987) 2482. [ 8 J DC. Johnston, J.P. Stokes, D.P. Goshorn, and J.T. Lewandowski, Phys. Rev. B 36 (1987) 4oC’ [ 91 S.M. Fine, M. Greenblatt, S. Shimizu and S.A. Friedberg, Phys. Rev. B 36 (1987) 5716. [IO] S.A. Shaheen, N. Jisrawi, Y-H. Lee, YE. Zhang, M. Croft, W.L. McLean, H. Zhen, L. Rebelsky and S. Horn, Phys. Rev. B 36 (1987) 7214.
H. Yoshida and K. Atobe /Superconductivity induced by irradiation in L.u2Cu04 [ 111 E.F. Skelton, W.T. Elam, D.U. Gubser, V. Letoumeau, MS. Osofsky, S.B. Quadri, L.E. Toth and S.A. Wolf, Phys. Rev. B36(1987)5713. [ 121 J.E. Schirber, B. Morosin, R.M. Nerrill. P.F. Hlava, E-L. Venturini, J.F. Kwak. P.J. Nigrey, R.J. Baughan and D.S. Ginley,PhysicaC152(1988) 121. [ 131 Rullier-Albenque. private communication. [ 141 K Atobe and H. Yoshida, Phys. Rev. B 36 ( 1987) 7194. [ IS] H. Yoshida, K. Atobe, K. Miyata and H. Kodaka, in: Proc. Sintering’87, ed. M. Somiya ( 1988), in press. [ 1610. Meyer, R. Kaufmann and R. Flukiger, in: Superconductivity in d- and f-Band Metals, eds. W. Buckel and W. Weber ( Kemforschungsaentrum, Karlsruhe, I982 ) p. I I I.
229
[ 171 H. Yoshida, H. Kodaka and Y. Hayashi, Scripta Metallurg. 22 (1988) 1. [ 181 H. Yoshida, H. Kodaka, M. Takeda and T. Okada, in: Materials Sci. Forum, Vols. 15-18, eds. C. Abromeit and H. Wollenberger, p. I 141. [ 191 P. Mtiller, H. Gerstenbery. M. Fischer, W. Schindler, J. Strobel, G. Saemann-Ischenko and H. Kammermeier, Solid State Commun. 65 (1987) 223. [20] J. Greek, G. Linker, 0. Meyer, C. Politis, F. Ratzel, R. Smithey, B. Strehlau and G.C. Xiong, 2. Phys. B 67 ( 1987) 507.
SUPERCONDUCTIVITY
INDUCED
BY IRRADIATION
IN La2Cu04
Hiroyuki YOSHIDA and Kozo ADOBE Research Reactor Institute. Kyoto University, Kumatori-cho. OS& 590-04, Japan Received 14 June 1988 Revised manuscript received 27 July 1988
Superconducting transition below a resistivity peak is observed in LazCu04 irradiated with low flux reactor radiation at 350 K. The transition temperature increases with increasing irradiation dose up to T, (onset)=48.9 K at 4.8x lOI6 n/cm* (E>O.I MeV). The result is compared with previous irradiation experiments.
Sincethe discoveryof high-Tcsuperconductivity in LaBaCu04[ 11, greatinteresthasbeenfocussed to theoriginof superconductivity in Laz-xMxCuO~--p compoundswith the K,NiF, type structure,where M is Ba,Sr, Caandsomeotherdivalentions [ 2-6 1. Non-bulk and filamentary superconductivitywas found in LazCuOr?, and local compositionand oxygendeficiencyhasbeendiscussedasan origin of superconductivity [6-l! ]. Recently, bulk superconductivitywasreportedin highpressureoxygenloadedLa$ZuO,[ 121.Herewe presenta result obtainedby resistivity measurements, which shows a possibility of filamentary superconductivityin La,CuO, by reactorirradiation at 350 K. Two specimenswith nominally stoicbiometric La&uO, compositionwerepreparedby solid state reaction.Powdersof LaJ.I~and CuO (cation ratio, La: Cu= 2 : 1) weremixed and calcinedfor 10hours at 800°C in air, and pressedinto small plates.The platesWeresinteredfor 30hoursat 900”C in air and thenslowlycooledin a timace. The tetragonalstrutturewasconfirmedby the powderX-ray diffraction method for an unirradiatedspecimenpreparedsimultaneously.The specimens(LCOl and LC02) wereset at two positionsof different neutronflux (fast neutrons3-7 and ?:!I>!1O’On/cm2 s; thermal neutrons3.0and5.5x 10” n/cm* s;gamma-rays3.6 and 5.7~ lo6 R/cm* h, respectively)in the irradiation cryostatof the horizontallow temperatureloop (LTL) at KUR (Kyoto University Reactor).In-pile measurements of resistances werecontinuouslyper-
formed by the four terminal methodusinga computer control system during cooling down and warmingup the loop by controllingthe helium gas flow from a refrigerator.The specimenswereexposedduringshortandlong irradiationsfor several weeks, Beforethe irradiationtheresistances of LCOl and LC02 showeda negativetemperaturedependence from room-temperatureresistances(at 300 K) of 0.98 Q (resistivity p=O.104 &-cm) and 0.48 R (~~0.073 Q-cm), respectively.The resistances graduallyincreasedwith decreasing temperature. The valuesconsiderablyrosein temperaturerangebelow 150K and an exponentialincreaseappearedbelow 100K, showingsemiconductortype behavior.For LCOl the resistancereacheda peakvalue (R,) of 43.88Q (p,=4.64 Q-cm) at 17.4K andthenslightly ~~~~ !&5~ thepeaktemperature( T,). For LCO2, however,it reachedan R, of 42.260 (p,,=6.45S& cm) at 14.4K but showedno drop. The latter core respondsto the semiconductingbehaviorobserved in stoichiometricLa2Cu0.,[ 51.The formeris shown asa resistancevs temperaturecurveof LCOl before irradiation in fig. 1, which is similar to the filamentary superconductivityobservedfor the oxygende&tent La&uOl-, [7-113. Figure 1 showsthe typical resistancevs temperature curvesmeasuredfor LCOl beforeand after reactorirradiation.After the irradiationat the reactor ambienttemperature(about 350K) the resistance of LCO1slightlyincreasedat almostall temperatures
0921-4534/88/$03.508 ElsevierSciencePublishersB.V. ( North-HollandPhysicsPublishingDivision )
226
H. Yoshida 50
and K. Atobe /Superconductivity
induced
bv irradiation
-
-
40-
0
;
30-
E 2 m
20-
0
II
0 6.5 L 36.5 QA . $
*
-__--_
ta2Cu04-y)
LCOl(
2 00
in La&‘uO,
0
n/cm" 14
h
8.7 x 10
A 122.5
h h
4.9 x 1o15 1.5 x 10 16
189.5
h
2.5 x 1o16
“i.OA.* la*
l
tr. t
50
0
150
100
TEMPERATURE
[ Kl
Fig. 1.Resistancevstemperaturecurveof LazCu04 (LCOI ) obtainedby in-pilemeasurementsbeforeand after the 350 K reactorirradiation. Irradiation conditionsare asfollows: Fastneutrons(E~0.1 MeV) IxlO”n/cm*) 0 u A
A a
Thrrmal neutrons (X10’6n/cm’)
0 0.81
0 0.1
4.9 15.0 25.2
3.9 12.2 20.4
Gammay-rays (x IO”R/cm*) 0
0.23 1.3 4.1 6.8
aboveT,,. Its temperaturedependence is similar to ature ( T,,) becomeshighershowinglowervaluesof that beforethe irradiation. The resistancevs temthe peak resistance(R,). Below Tp the resistance peraturecurvesshowlargepeaks,and a sharpdrop dropstowardzero after exposureover a critical irbelow Tp' The TPgenerallyroseafter eachinterval radiationdose,andthetemperatureto reachzeroreof the irradiationandthe peakresistances(R,) apsistance( To) increasesdependingon the irradiation pearto reducedependingon accumulationof theindose.Tbetransitionwidth from T,,to Tois constant. tervals,altbougbthereis anexception.BelowTpsmall The highesttemperatureof 41.7and 48.9K for To dropsin resistanceappearfor a low dose,and relaand T,, respectively,which appearedat the maxitively largedropsfor a higherdose.The minimum mum irradiationdose[fastneutrons4.8x IO4n/cm* valueof resistance,which was detectedwithin the (EB 0.1 MeV); thermal neutrons3.7~ 10’ n/cm*; presentmeasuringcondition,generallyreducttddegamma~rays1.I x LO9R/cm21 in the presenteqefpending on the irradiation dose, and then epiment, are not yet saturated. proachedzerofor the h&bestirradiationdosein the The variation of the resistanceas a function of figure (LCOl ). temperatureobservedin the irradiatedLCOl and Figure2 showsanotherexampleof the resistance LC02 is very similar to that of LatCuG4,, reported vs temperaturecurvesfor LC02irradiatedat 350K. in previouspapers[ 7-101. Such a featureof temThe curvesshowthesametemperaturedependence, peraturedependenceon resistivity was first preWith increasingirradiation dose,the peaktempersensedby Grant et al. [7] as an evidence for
H. Yoshida
and k-. Atobe
/Superconductivity
induced
by irradiation
TEMPERATURE Fig. 2. Resistance vs temperature at 350 K are as follows: Fast neutrons (k-0.1 ( X IW n/cm*) 0 A ::
1.6 9.2 28.4 47.8
curves
MeV)
obtained
for La#ZuO.,
(LCO2)
227
t Kl
set on a different
position
Thermal neutrons ( x IO’* n/cm’)
Gamma-rays ( x lOa R/cm2)
1.3 7.2 22.3 37.4
0.31 2.1 6.4 10.8
filamentarysuperconductivityin LazCuO,,wherea curiouskink near40 K (just below T,) corresponds to the actual T, (onset) of superconductivityand smallregions(about 1%of the sample) of excessoxygenoccupationarespeculatedto be a sourceof superconductivity.It wasalsopointedout by Johnston et al. [8] that superconductingtransitionoccursat Tp(39 K) andonly 0.03%volume fractionbecomes superconducting, and that both resistivityand thermopowerare very sensitiveto oxygen-defectconcentration. The same feature of filamentary superconductivitywasalso reportedfor lanthanumdeficient, stoichiometric J.a$u04 [ 9,lO] . Structural transition to the orthorhombicallydistorted K&GF, structure[ 6,7,1O,111andantiferromagnetic transition [6- 101hasalsobeendiscussedconceming this type of superconductivity.It is recently pointedout by Schirberet al. [ 121that bulk superconductivityappearedin La$uO.,+, after annealing
in LaguO,
from
fig. I. The irradiation
conditions
in highpressureoxygenatmosphereandis relatedto excessoxygenatomssituatedasO,- superoxideions with thechargebalancerequiringCu’+ to beformed. Throughcollision sequence light oxygenatomsin LazCuO,arepreferentiallydisplacedto form oxygen interstitialsandvacancies,and runningknocked-on ions additionally contribute to make chargeexe changefor the interstitialsand vacancies.VacancybS. rich regionsin the cascadedamagesproducedby neutronirradiation may be appropriatefor formation of small regionswith orderedoxygenvacancies actingasfilamentarysuperconductors. On the other hand,the oxygeninterstitialsmay produce0: superoxideionsat theendof a collisionsequence or in the local oxygen-richregionswith similar condition to high pressureoxygenatmosphere.Thesearepassible mechanismsfor the superconductivityto appearclearlyin the presentexperimentevenat such low doseneutronirradiations.It canbe understood
228
H. Yoshida md A”.Atobe /Sqxwo~d&~~tii,::;~ induced bv irradiation in La.XuO,
asfeaturesof filamentarysupercsnductivity,asdiscussedin previouspapers[ 3-I 11.that thesharpdrop towardszerooccurredjust belowtheresistance peak corresponding to thesuperconducting transition,and someresistivity (about2 x lW4 G-cm) retainedin the “zero resistance”state.The presentdataclearly showthe appearance of the sharpresistivity transit;~n afterthe neutronirradiationwithoutanychange in bulk composition.The increases of Tpand Todependingon the irradiationdosemight correspondto the increaseof defectconcentrationin La$Zu@. Accordingto a recentexperimentby Groult et al. [ 131,T, (midpomt ) in superconductingLa&NI, increasesfrom 32.5to 35.0 K by 2.9 GeV krypton irradiation.They discussthe result in relationwith similarity of radiation-induceddefectsto strontium substitutionand pressureeffect,as well as with the presenceof a spin-density-wave phase.There is no broadstepnear250 a<,in the presentresistancevs temperaturecurves,which is relatedto a stepin susceptibilitycurvesdueto the antiferromagnetic or the spin-density-wavetransition [8,10] for both the unirradiatedand irradiatedspecimens.The present highest‘a, is slightly higherthan the T, (40 K) in orthorhombicallydistortedLa2Cu04and seemsto be not saturatedyet. However,thereis uncertainty whetherthe TPwill achievehighertemperatures,for exampIe the T, (90 K) of orthorhombic LaBazCu@, at a higherdose.irradiation. In the previous experimentsusing the samefaciiity (KURLTL) the 200 K resistivitytransitionwasobserved in YBazCu@,afterlow temperature irradiation(fast neutrondose%9X 1Or6n/cm*) (141,and the resistivity transition at higher temperaturesalso appearedin LaBazCusO,after the 360 K irradiation ( I X 1Or7n/cm’ ) [ 151.Thesephenomenaappearing in relativelylow doseirradiationsseemto be understandable only as a filamentarysuperconductivity, which may be causedby the rearrangement of radiation-induced defectsandthelocal formationof a newp&e or structure.An earlyobservationof T, improvement(from 2 to 5.7 K) in A15 Nb,lt after the 600kV Kr* irradiation is explainedby the formationof an amorphousphasewith T,ZS5.7K [ 161. The increaseof T, (onset)to 25 K, which appeared afterannealingof heavilyirradiatedNbs(Al, Ge), is understoodby reorderingof heavily disordered darnage+zgions [ 17I. ‘it is weillknown in bulk su-
perconductorswith orderedstructurethat T, decreasesby the radiation-induceddisorderingwhen theirradiationdoseincreasesabovea certaincritical dose(for example,fastneutronirradiationof 5x I 01* n/em* at 85“C for Al 5-typesuperconductors [ 181, fast neutronirradiation of 2X lOI n/cm’ at 4.6 K for YBa2Cu30,[ 191,helium ion irradiationof lo-’ displacementper atom for Lal.sSr0.2Cu04 [ 201,and electronirradiationof 5x lOI e/cm2at 20 K [ 131). Thepresentresultin La2Cu04diffea rrorntheabove bulk effectsand is a particularphenomenonof ftlamentarysuperconductivity.However,it leadsto a generalconsiderationon theorigin of superconductivity in the oxides,which is closelyrelatedto concentrationand arrangementof oxygeninterstitials and vacancies. Acknowledgements The authorswould like to expresstheir thanksto Mr. M. Okadaandothermembersof KUR for their supportto theLTL experiments.Thanksarealsodue to Mr. S.Ueharafor his assistance in dataprocessing. References [ 1 ] G. Bednorz and K.A. MiIller, 2. Phys. B 64 ( 1986 ) 189. [ 2) H. Takagai, S. Uchida, K. Kitazawa and S. Tanaka, Jpn J. Appt. Phys. 26 (1987) L218. [ 31 J.M. Tarascon, L.M. Greene, W.R. McKinnon, G.W. Hull and T.H. Geballe, Science 235 ( 19R7) 1373. [4] J.D. Jorgensen, H.-B. Schuttler, D.J. Hinks, D.W. Capone IL Z. Zhang, M.B. Brodsky and D.J. Scalapino, Phys. Rev. Lett. 58 (1987) 1024. [ 51 S. Uchida, H. Takagi, H. Yanagisawa, K. Kishio, K. Kitazawa, K. Fueki and S. Tanaka, Jpn J. Appl. Phys. 26 ( 1987) L445. [6] R.L. Greene, H. Maletta, T.S. Plaskett, Cl. Bednotz and #A. Mllller, Solid State Commun. 63 ( 1987) 379. [7] P.M. Grant, S.S. Parkin, V.Y. Lee, E.M. En8ler, M.L. Ramire& J.E. Vazquez, 0. Lim. R.D. Jacowitz and R.L. Greene, Phys, Rev. Lett. 58 (1987) 2482. [ 8 J DC. Johnston, J.P. Stokes, D.P. Goshorn, and J.T. Lewandowski, Phys. Rev. B 36 (1987) 4oC’ [ 91 S.M. Fine, M. Greenblatt, S. Shimizu and S.A. Friedberg, Phys. Rev. B 36 (1987) 5716. [IO] S.A. Shaheen, N. Jisrawi, Y-H. Lee, YE. Zhang, M. Croft, W.L. McLean, H. Zhen, L. Rebelsky and S. Horn, Phys. Rev. B 36 (1987) 7214.
H. Yoshida and K. Atobe /Superconductivity induced by irradiation in L.u2Cu04 [ 111 E.F. Skelton, W.T. Elam, D.U. Gubser, V. Letoumeau, MS. Osofsky, S.B. Quadri, L.E. Toth and S.A. Wolf, Phys. Rev. B36(1987)5713. [ 121 J.E. Schirber, B. Morosin, R.M. Nerrill. P.F. Hlava, E-L. Venturini, J.F. Kwak. P.J. Nigrey, R.J. Baughan and D.S. Ginley,PhysicaC152(1988) 121. [ 131 Rullier-Albenque. private communication. [ 141 K Atobe and H. Yoshida, Phys. Rev. B 36 ( 1987) 7194. [ IS] H. Yoshida, K. Atobe, K. Miyata and H. Kodaka, in: Proc. Sintering’87, ed. M. Somiya ( 1988), in press. [ 1610. Meyer, R. Kaufmann and R. Flukiger, in: Superconductivity in d- and f-Band Metals, eds. W. Buckel and W. Weber ( Kemforschungsaentrum, Karlsruhe, I982 ) p. I I I.
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