Indian Journal of Pure & Applied Physics Vol. 40, Jul y 2002, pp. 476-48 1

Effect of Cu-site occupancy on superconducting order parameter dimensionality and related correlations in low-disorder

Fe and Zn doped YBCO U C Upre ti & A V Narlikar

National Physical Laboratory, Dr K S Kri shnan Road, New Delhi II 0 012

Received I March 2002; accepted I 3 May 2002

The preliminary investigations, on fl uctuation effects due to low chemical disorders in planes and chai ns in Y I 23 system as a comparative study. The possible effects of changing the low concentration planar Cu-si te subst ituent Zn(0.33%) by a chai ner Cu-si te substituent Fe of almost the same concentrati on (0.30%) on the order parameter dimens ionality in YBCO epi tax ial th in fi lms have been investigated. Related correlati ons have also been discussed. In c-axis oriented laser ablated epitaxial thin fi lms of pure and doped YBCO superconductor samples the order parameter fluctuation effects observed in the mean-field-region, are correlated wi th the mean fi eld cri tical temperature (T, mr), the resistivi ty, and the excess conductivi ty (L'lcr). Aslamazov-Larkin (AL) theory wit h Anderson-Zou (AZ) model fo r background resist ivity shows an order parameter dimensionality (OPD) cross-over in the pure film which changes to 3-dimensional (2-d imensional) superconducting behaviou r by substitut ing low disorder of Fe (Zn). Compared to Fe dopi ng, the dimensionali ty behaviour at such low-level dopi ng of Zn suggests fast suppression of superconductivity. The sample parameters suggest correlations such that, the dimensionality is directl y proportional to T, mr. and L'lcr has weak dependence on the resistivity of a system and it (or the fluctuation) has slow suppression to low disorder of Fe.

1 Introduction

T he cuprate superconductors 1 are characteri sed by unique intrinsic ani sotropic properties, the upper critical fi e ld (Hc2) anisotropy2, short coherence length and low carri er densities vis-a-v is high critical temperatures. These properties are connected with observed excess conducti on due to thermodynamical fluctuati ons in hi gh T, superconductors (HTSC). The degree of anisotropy is assoc iated with the crysta lline structure having weakly coupled Cu02 planes. Because of layered structure, cuprate superconductors may exhibit both two-dimensional (20) and three-dimensional (30) behav iour depending on the c-ax is coherence length

l and the layer spacing d. F luctuati on dimensional ity or cross-over depends on the temperature rangeJ. In HTSC, it is be li eved that, the fluctuat ion effects are larger and the reg ion over which these have significant in fluence is much larger compared to the conventi onal superconductors, thus allowing us to study the fluctuati ons in greater detail. The spin gap~, the magnet ic fie ld res ist ive transition broadening and the magneto-res istance' above the critical te mperature as we ll as the irreversibi lity line are relevant to thermodynamic fl uctuat ions. In un-doped

HTSC systems, there ex is t numerous studies on excess conducti vity, but the authors do not find any in doped systems. Excess conducti vity studies have been carried out in polycrystall ine samples6

•10

, sing le crystals 11

.16 and also in thin fi lms 1n °. In YBCO

polycrystalline samples, 30 fl uc tuation-induced conducti vity is suggested. C ross-over has also been reported. Lobb21 suggests a 30 AL formul ation in the mean-fi e ld region (M FR) and pred ic ts an OPO change from -2/3 to - I /3 w ith te mperature lowering in the critica l dynamic region near T,. Veira et al.22

verify thi s in the ir study.

In pure and Zn doped (low-d isorder) Y 123 thin films, compari son wi th other studi es showsn very interesting and important resul ts: a high T,mr does

not impl y low p(7) ; in pure fi lms cross-over corre lates more with p(7) than with T/'r; and the OPO lowers with increase in res istivity; if seen according to T,mr the OPO is d irectl y proporti onal to it and the T,mr and res isti vi ty in the pure fil m are hav ing in verse corre lati on which does not seem to ho ld good for the Zn doped sample; the Zn doped

sample, OPO has uncerta in correlation wi th p(7). Tempted by these observati ons in pure and Zn doped films, here, the authors explore the possible

UPRETI & NARLIKAR: SUPERCONDUCTING ORDER PARAMETER CORRELATIONS IN DOPED YBCO

corre lations by comparing the fluctuati on effects in Y 123 superconductors due to low chemical di sorders by Zn and Fe vi s-a-vi s a pure Y 123 film . The important feature of the present study is that, simil ar to the low concentration of Co in YBC02

\

the 0.30%Fe doped film shows an increase in the di sorder-dependent critical temperature, Tix), and the 0.33% Zn doped Y 123 film has a resistivity lower than that of the undoped Y 123 film similar to that observed in Ni doped YBC025 and in 0.1% and 0.2% Pr doped Gd 123 samples2

r,.

Consistency of results by the authors has been confirmed by comparing the vari ation of the degree of ani sotropy (~,t1/;., and p/ p,h), as observed in other studies on coherence length and res isti vity data for Fe and Zn doped samples.

2 Experimental Details

The pure and Cu-site doped (0.3%Fe, 0.33%Zn) Y- 123 thin films of about 3000A were prepared by laser abl ation technique. To maintain the same oxygen stoichiometry, the target materials were synthes ized by the usua l so lid-state reaction process27 in a s ingle batch. The phase purity of these

477

materials was checked by X-ray diffractometry. These were then mounted in a depositi on chamber evacuated to a base pressure of I x I 0·7 Torr, in each case. During each deposition, the films were deposited on ( I 00) Y -Zr02 (YSZ) substrate mounted at 6 em from the target and maintained at 650 °C in oxygen at a contro lled parti a l pressure of I 00-200 mT. An excimer laser of 248 nm wavelength , a 20 ns pul se-width and a fi xed energy density of 2J/cm was made to be inc ident at 45° to the pe ll et surface. To avo id texturing of its surface, the pe ll et was slowl y rotated during irradi ation. The sampl es were then cooled to the ambient temperature under the same oxygen pressure. The phase purity and the c-axi s orientation of the films was confirmed by X­ray diffractometry. Lakeshore 7000AC susceptometer was used to acquire res ist ivity­temperature data (Fig. I), in the range of 70-300 K, under temperature-controlled conditi ons.

3 Results and Discussion

Generall y, the background res tst tvtty (p11 ) is estimated either from its linear T-dependent form or the AZ form2x Pn(T)=AT+BIT. Eithe r of these T-

140 .-----------------------------~--------------------------------,

120 . . ···

U"l lo

X 100 ... ·· .·

-Pu r e

E u

- - -Y-123 (Zn :o . ~3%) I 80 E

·· ··Y-123 (Fe: 0.30%)

.c 0 E ~ ...... > ..... Ill

Ill QJ

cc::

0 L_ _ _ ~L__L _ _ _ ___ L_ ______ ~.--------J-------~

50 100 150 200 250 300

T(K)

Fig. 1 - Resistivity versus temperature for pure, Zn doped, and Fe doped Y 123 thin film~

478 INDIAN J PURE & APPL PHYS, VOL 40, JULY 2002

2~------~--------~--------~--

0

0

0

-2

8 Y-123 Thin film

b' 1- Pure ~Do

6 2-0.33% Zn -4 3-0.30% Fe ~

<l A c: ;'\ ....,

-6 a IDl

-8 0

0

-10 ~------~-------L------~--------L--------L------~ -4.0 -- 3 .5 -3 .0 -2 .5 -2.0 -1 .5 -1.0

Fig. 2 -ln(t.<J/<J3m) versus In£ plots for a) pure, b) Zn doped, and c) Fe doped Y 123 thin fi lms

dependences does not have any important influence on the extracted ~a. For p" the authors have used the AZ form . ~a is extracted as a function of reduced temperature~. IX £{ =(T-Tcm~/7~mr, as given by Aslamzov-Larkin2~ for fluctuations to BCS superconductors311, Tc"'r IS the dp/dT peak temperature (d2p/dT- = 0) as discussed by the authors for Y 12431

:

for 2-dimensions(2D): ~a(T)= (e2/ 16!7 d)£-1

••• (I)

for 3-dimensions(3D): ~a(T)= (e2/32fif;(0))£·112 .•. (2)

where d is the superconducting layer thickness , ~(0) is the coherence length at T=OK and his the reduced Planck's constant. The excess conductivity as ln(~a/a31x l) versus In£ variation has been discussed.

Within the range -3.5 s In£ s -1.5, (the MFR), close to T/'1, the excess conductivity shows an OPD

cross-over in the un-doped Y 123 film (Fig. 2a). It shows a 20 behaviour in Zn doped sample (Fig. 2b), and a pronounced lowering of OPD with the lowering of temperature in the Fe doped samples (Fig. 2c). This conforms to the prediction of Lobb2 1

and the observation of Viera et af.2' . However, away from the T" the Fe doping shows 3D OPD.

For Zn doping, very close to the MFR lov. temperature end, a cross-over is indicated if we use data points outside the MFR also. But, thi s cross­over, being around the critical fluctuation region. where no mean field theory is applicable, i~

anomalous. Thus, Zn doping has caused suppression of 30 fluctuation s and the system correspond~

mainly to 20 fluctuations. Within the MFR, in the Fe doped system, although, no cross-over i~

observed in AL theory, nevertheless, it is indicatec close to the MFR high temperature end. Whereas

UPRETI & NARLIKAR: SUPERCONDUCTING ORDER PARAMETER CORRELATIONS IN DOPED YBCO 479

Lawrence-Doniach (LD) theory-12 does not provide a cross-over- the conductivity fluctuation s being 3D

indicating thereby, slow suppression of superconductivity . The possibility of a cross-over in low concentration Fe doped film suggests that fluctuation s are not well suppressed by low concentration Fe doping. Because of chain site occupancy and slow superconductivity suppress ion, there are adjacent layers with unsuppressed or rather partly suppressed fluctuations, which seem to affect the interlayer coupling along out of the plane direction . Thus, a strong coupling between the Cu-0 networks is thus di splayed near to T,"'1 whereas, in the high temperature region, the OPD lowering (30 to 20) shows a poor coupling. The deviation of cross-over temperature T* from T,(R=O) is more for the Fe doped film compared to the Zn doped film, which suggests that Zn doping causes a cross-over nearer the critical fluctuation region . Since compared to Fe doping, the T, depression rate for Zn doping is higher, it is ex pected to attain the cross­over faster. Thus, in Zn doped Y 123 film, the fluctuation dimensionality change is comparati vely sharper than the Fe doped Y 123 film and hence the superconductivity suppress ion to reach the cross­over is also fast for Zn doping. The planar occupancy of Zn has a pronounced effect on critical temperature and transition width, only, it is not likely to affect the interlayer coupling. The authors observe that , Zn and Fe doping shift their cross­over temperatures in opposite direc ti ons, relati ve to the pure film, and therefore have large cross-over separati on.

The log-log plots show a large deviation from linear fit beyond the MFR high temperature end, since, away from the T)"1

, the AL formulation does not take into account the copper pair interac ti ons. In some cases, the curve shows an upward turn near to T,.1111 also. Thi s was found to be a sample-dependent behav iour, spec ifica ll y, due to change in the

sharpness of the transition by doping and is evident in a ilcr2 versus T plot. For Fe doped system, thi s plot has a variation, different from that for pure and Zn doped samples and this, it is understood, is because of less sharp transition with an extended tail extremely close to T.(R=O) , an amplified p-T plot makes it evident. The LD formula transformation I /E(ilcr)2=( I /C)£+41/C yields 1:::0 .041 (0.00078) and ~:::2.4 (0.24) for pure (Zn doped) sample23

,

which conform to observed higher (cross-over) and lower (20) OPD. Here, C=e2/l6fl is a constant and 1=(~(0)/d'/ . For Fe doping, I /E(ilcr)" is almost constant in the MFR, therefore, the slope is almost zero and hence 1/C being indeterminate ~(0) cannot be estimated. The pure film cross-over temperature T* obtained from LD and AL formulae are presented in Table I . For cross-over determination , LD theory is more suitable and accurate since, in AL theory T* depends on the points used to obtain the linear fits that decide the OPO. It is to be noted that reduced Y 123 system shows a 20 variable range hopping (VRH) in the normal state, but with Fe doping the VRH process becomes 3-dimensional31

. Thi s corroborates the observed fluctuati on dimensionality.

The authors assess the consistency of thei r results by comparing the variation in the degree of anisotropy, using the data of other workers30

" . For Fe doped Y 123 system, the coherence length data of Lan et aU4 give ~atl~= 4.0, 3.8, and 4.4 for x=O.O, 0.04, and 0.13 , respectively . Thi s shows that, for x = 0.04, the degree of anisotropy of these Fe doped Y I 23 samples tends towards 30 . The results of the authors agree with this, since, in their case, x = 0.0 I . It is seen from the above considerati ons that, for x = 0. I 3, the degree of anisotropy tends towards 20. Similar to Fe, Co also has a chain-s ite occupancy. The coherence length data of Zheng et a/.35 for Co doping give ~,.t/~=6 .66 (9.0) for x=O.O (0.04) . This shows that, the degree of ani sot ropy in Co doped

Table I - Vari ous parameters for pure and doped Y 123 thin film s

Yl 23 T)"1(K) T*( K) PIIXI L'.<J( mncm)' 1 Order parameter sample dp/dl peak (milcm) dimensionality

AL LD lOOK II OK 0.30%Fc 8R .2 0.67 400 :no 3 Pure 88.7 104.5 95 0 .15 450 185 Cross-ove r 0.33% z 81.4 0. 14 80 17 2

480 INDIAN J PURE & APPL PHYS, VOL 40, JULY 2002

Y 123 system tends towards 2D-behaviour just opposite to that of the Fe doped Y 123 system.

The Zn doped Y 123 system coherence length data of Semba et a f. Y• show a tendency towards 3D

behavi our. On the basi s of res istivity ratio pJp .. h

also, thi s is true up to about 200K. In fact, pJp .. h = 31.6(30.4) for x = 0 .0(0.03) at T = 150K, while

pJp .. h = 27 . I (27 .5) for x = 0.0(0.03) at T = 200K. On the other hand , at T = 300K, pJ p .. h= 21.4(22.1) for x = 0.0(0.03). Thi s shows that, at T = 300K Zn doped Y 123 sampl e acquires a tendency towards 2D. The dimensionality behav iour of Zn doped Y 123 samples of Semba et aUr. is opposite to that of the authors', perhaps due to the ir Zn doped system having res ist ivity less than that of the pure system, forT> 87 K.

For more ins ight into the role of Fe and Zn on

t.cr, the authors compare the corresponding Tc"'' , the resistivity PT (at a fixed T), the 2D-3D cross-over, and t.cr, at a fixed temperature, say I 00 K, within the MFR. The authors observe that : ( I ) OPD increases with T/ '1• The 0.30%Fe (0.33%Zn) is the ex treme case of hi ghest (lowest) T/'1 and OPD; (2) since, at I 00 K, the 33 % Zn sample res istivity P HKl is similar to that of the pure sys tem and since t.cr is

quite sma ll , a decrease in T" causes a decrease in t.cr a lso. If thi s result is true, the 0.30% Fe system should show high t.cr. But, as seen in Table I , it is

not so. Since P HKl of 0.30% Fe is much larger than that of pure and 0.33% Zn doped YBCO, it may be said that, fluctu ation suppress ion in Fe doped sys tem is due to its much higher res istivity . Combining these results the authors find that , T/'1

increases t.cr, while, p dec reases it. However, above about I 07 K the increased res isti vity of Fe doped sample does not seem to have an effect on fluctuati on suppress ion , s ince t.cr is the hi ghest, (Table I ) in thi s region .

4 Conclusions

Effects of order parameter fluctuations in pure, low concentration Zn and Fe doped Y -123 epitaxial thin films are di scussed. In the MFR, the OPD shows cross-over in the pure film, which changes to 3D by low concentration Fe substitution at the Cu­site with c ross-over poss ibility in high temperature reg ion whereas, the OPD is mainly 20 in Cu-site Zn substituted film. Zn doping causes a cross-over nearer the c ritical fluctuation region. It is expected

to attain the cross-over faster. In Zn doped Y 12: film the fluctuation dimensionality change i. comparatively sharper than that in the Fe doped filn and hence the super-conduct ivi ty suppression t< reach the cross-ove r is also fast, fo r Zn doping. Th1 authors observe that, Zn and Fe dopings shift thei cross-over temperatures in oppos ite direct ion re lative to the pure film, and, therefo re, have larg1 cross-over separation. The OPD suggests that, lov concentration Zn (Fe for T < I 07 K) causes fas (s low) super-conductivity suppres ·ion. For Fe (lov concentration) substitution the fluctuati ons enhanct above === I 07 K. It is also suggested that, the OPD i proportional to Tc"'' and simultaneously, the exces conductivity is weakly dependent on res istivity; an< t.cr disorder dependence suggests th at, fluctuati ot (or t.cr) suppress ion is s low forT < 107 K, in th1 case of low (0.30 %) Fe disorder.

Acknowledgements

The authors thank ProfS B Ogale, University o Pune, Pune, for provid ing the films and colleague Dr R La! and DrS K Agarwal for useful di scuss ion and help.

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