Fluctuation-induced excess conductivity in the compounds CaREBaCu3OT_y (RE = La and Sin)

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Bull. Mater. Sci., Vl. 14, N. 3, June 1991, pp. 747 752. Printed in India. Fluctuatin-induced excess cnductivity in the cmpunds CaREBaCu3OT_y (RE = La and Sin) 1.

1 Bull. Mater. Sci., Vl. 14, N. 3, June 1991, pp Printed in India. Fluctuatin-induced excess cnductivity in the cmpunds CaREBaCu3OT_y (RE = La and Sin) 1. Intrductin H KRISHNAN t, R SRINIVASAN, V SANKARANARAYANAN, C K SUBRAMANIAM and G V SUBBA RAO Materials Science Research Centre, Department f Physics, Indian Institute f Technlgy, Madras , India Abstract. The cmpunds CaREBaCu307_r (RE = La and Sm) are tetragnal at rm temperature with T~ between 60 and 70 K. The single-phase cmpunds were prepared by slid-state reactin. The resistivity was measured by a fur-prbe technique in a cntinuus flw crystat with the temperature being cntrlled t an accuracy f 10 mk. The resistivity vs temperature shwed a break in slpe arund 180 K in CaLaBaCu30 7 _ r and arund 220 K in CaSmBaCu3OT_r The results were analysed fr fluctuatin cnductivity frm 180K dwnwards. A plt fdp/dt vs T shwed a sharp peak at T,, = K fr La cmpund and K fr the Sm cmpund. Detailed analysis f the resistivity in the regin T. t 180 K was carried ut using the prcedure due t Veira and Vidal. The results are discussed in this paper. Keywrds. High Tc supercnductrs; CaREBaCu30 7 _ r; fluctuatin; excess cnductivity. The shrt cherence length f high Tc materials cmpared t the cnventinal supercnductrs gives rise t a larger excess cnductivity in these materials. The high resistivity f these materials in principle makes it easier t detect the fluctuatin cnductivity. Hwever, there are the fllwing tw majr prblems in analysing fluctuatin cnductivity: (a) The determinatin f the mean field transitin temperature (T,,) and (b) the estimatin f backgrund resistivity. These prblems are discussed by Srinivasan (1990), The mean-field transitin temperature is taken as clse t the temperature at which the peak in dp/dt ccurs in the lw temperature regin. The backgrund cntributin is usually estimated by extraplating the resistivity vs temperature plt frm rm temperature t 2T~. High Tc cmpunds have a shrter cherence length alng the c-axis cmpared t the ab-plane. S it is believed that the excess cnductivity varies as fr a layered cmpund (Lawrence and Dniach 1971), where AO'fl u = (ez/16hd) [e(e + A)]- 1/2, (1) = In(T/T.), (2) and A = (2~c(0)/d) ~. (3) Here d is the distance between adjacent supercnducting layers, Sme wrkers have taken d as the value f c-lattice parameter (Veira and Vidal 1989) and thers as half f the c value (Hagen et al 1988). The cntributin t resistivity frm fluctuatin cnductivity at high temperature will vary as ~-1 and cannt be separated frm the backgrund cnductivity. While the single crystal and riented thin films are the best samples fr study f fluctuatin effects, Veira and Vidal (1989) have shwn that 747

2 748 H Krishnan et al meaningful results can als be btained frm a prper analysis f data frm plycrystalline single phase pellets in which the samples shw a single peak in dp/d T vs temperature plt. Many studies have been carried ut in YBa2Cu3Ov_y and T12CaBa2Cu20 8 cmpunds (see references in Srinivasan 1990). We have studied the fluctuatin cnductivity in a sample each f CaLaBaCu3OT_r and CaSmBaCuaOT_r. These samples were tetragnal at rm temperature. The results f these studies are presented in this paper. 2. Experimental results The CaREBaCu30 7_r cmpunds were prepared by high temperature slid-state reactin using high purity RE20 3 (RE = La and Sm), CaCO3, BaCO 3 and CuO (Varadaraju et al 1990). The characterizatin by XRD shws that the cmpunds were single-phase. The lattice parameters btained by least squares methd were a = 3"86,~ and c = 11 "57,~ fr CaLaBaCu3OT_r and a = 3"85,~ and c = ~ fr CaSmBaCu3OT_ r These values agree with the reprted values in the literature (de Leeuw 1988). The xygen cntent f the samples was fund t be in the range 6" 82 and The resistivity was measured by the fur-prbe technique due t Mntgmery (1971). A cntinuus flw crystat was used t get the temperature variatin. The temperature was cntrlled t better than 10 mk using an electrdynamic valve and Leybld Varitemp temperature cntrller. A Keithley cnstant current surce and a Keithley nanvltmeter were used, t measure resistivity. At each temperature the current was reversed t eliminate therm emf effects. Figure 1 shws the plt f p vs T fr bth the samples. In the case f CaLaBaCuaOT_y a break in the slpe f the resistivity curve is seen clearly arund 180 K and in the case f CaSmBaCu3OT_y the break in the slpe is arund 220 K. A E Y E c- O '; I- a a ao / DDOOC~ l Dn 0 O0 u 0000 _ ry Temperature (K) Figure 1. Variatin f electrical resistivity with temperature fr CaLaBaCuaO7_ r and CaSmBaCuaO 7_y.

3 Fluctuatin-induced excess cnductivity in Ca-REBa-Cu-O x I-- O n Qg 0 0 Or1 D ~ g ~ a ~ g a Cal..a Ba Cu30T_y CaSm Ba Cu3OT_y O O O Q O ~ ~DS~ p l ) TemperQture (K) Figure 2. Variatin f dp/dt with temperature fr samples f CaLaBaCU3OT_r and CaSmBaCu30 7_y. similar change f slpe is bserved in CaLaBaCuO7 _y arund 220 K (Peng et al t989). Because f the break in the slpe, the analysis f the resistivity data is limited t temperatures belw 180 K. Figure 2 shws the plt fdp/dt vs T. It is seen that there is a single sharp peak in dp/d T indicating that the samples are single phase. A rugh analysis f the data was made by estimating the backgrund resistivity by extraplating the resistivity frm 180K t 120K. Such an analysis is nt rigrus because it ignres the fluctuatin cntributin t the resistivity abve 120 K. A mre precise analysis was made using the prcedure due t Veira and Vidal (1989). In this prcedure the resistivity f the pellet at any temperature T can be written as p(t) = P + P(T)/P (4) Here, P is the cntributin frm grain bundaries and is assumed t be temperatureindependent, p is a factr which represents the reductin in the effective area f crsssectin fr cnductin arising frm the mismatch f ab-planes f neighburing grains. p(t) is the resistivity due t the grains and is given by l/p(t ) = AO-fl u -~- 1/(pl + C2T), (5) where Pl is the residual resistivity f the grains and C2g is the linear temperature cefficient. The fluctuatin cnductivity Aaft. is given by equatin (1). The experimental data were fitted t equatin (4) in the range T,, + 1 t 180 K taking int accunt equatins (1) and (5). The parameters btained in this analysis are cllected in table 1. Figure 3 shws the fit t the resistivity data btained bth fr La and Sm cmpunds. The rms deviatins f the fit btained fr CaLaBaCu307 _ r and CaSmBaCu30 7 _y are 0.79% and 1"65% respectively. In figure 4 is shwn the variatin f the fluctuatin cnductivity as a functin f temperature fr CaLaBaCu30 7 _ r. The cntinuus curve shws the fit t equatin (1). The parameters btained are cmpared with the parameters fr YBa2Cu3Ov_y (Veira and Vida11989), T12122 (Krishnan 1990) and riented thin film ft12122 (Kim et al 1989). It is seen that the intragrain parameters C2g are cmparable t the values

4 YBa2Cu307 _~ YBa2Cu307 _ ~ T12CaBa2Cu ' CaLaBaCu307 _y "012 0' "54 CaSmBaCu30 7 _~ " " d J~ References 11"67 ~ Veira and ~ Vidal (1989) Krishnan (1990) ~ Present wrk 11"48 J...a Table 1. The values f intergrain and intragrain parameters btained by a fit t the mdel due t Veira and Vidal t~ Intragrain parameters Intergrain parameter Material p(300) T,.~. Pl C2g ~(0) P P T... mdcrn K mt)cm cm/k A x 102 mt~cm K

5 Fluctuatin-induced excess cnductivity in Ca-REBa-Cu-O ! O Y E - 0 > rr CaSm BaCu3OT_y --Fit t eqn,(4) I I I I I Temperature (K) Figure 3. Electrical resistivity data fr CaLaBaCu30 7_r and CaSmBaCu3OT_y fitted t the mdel due t Veira and Vidal (equatin (4)). a0 I E 60 u c. ~ I CLBQCu 3 07-y x Experiment -- Fit b ~ Temperature (K) Figure 4. Variatin f excess fluctuatin cnductivity with temperature fr CaLaBaCu30 7 _,., btained in single crystal and riented films. The ~c(0) value in CaLaBaCu3OT_y is f the rder f 1 A which is similar t the value btained fr ther materials. The value btained fr the parameter p is This is btained n the assumptin that the parameter d in the LD equatin (3) is the c-axis lattice parameter. If it is taken as the distance between neighburing Cu-O planes (,-~ 5"8.~), then the value f p will cme ut t be half the value given in table 1. It is ntewrthy that in the case f CaSmBaCu307 _y the value f P l is nt zer unlike in the case f ther materials. This high residual resistivity in the grains f CaSmBaCu307 _r needs further investigatin.

6 752 H Krishnan et al 4. Cnclusin The resistivity f a pellet each f CaLaBaCu307 -y and CaSmBaCuaOv _y has been measured frm rm temperature t Tzer. The resistivity shwed a break in slpe arund 180 K in La cmpund and 220 K in Sm cmpund. The resistivity data fr fluctuatin effects were analysed frm 180 K dwnwards using the prcedure suggested by Veira and Vidal (1989). The intragrain parameters btained by this analysis are cmparable in values t the intragrain parameters f YBaCu3OT_ r and T cmpund. Hwever in CaSmBaCu30 7 -r the grain appears t have a high residual resistivity and the reasn fr this needs t be studied further. Acknwledgement The authrs acknwledge gratefully the Prgramme Management Bard n Supercnductivity, Department f Science and Technlgy, Gvernment f India fr financial supprt. References de Leeuw D M, Mutsaers C A H A, Van Hal H A M, Verweij H, Carim A H and Smrenberg H C A 1988 Physica C Hagen S J, Wang Z Z and Ong N P 1988 Phys. Rev. B Kim D H, Gldman A M, Kang J H, Gray K E and Kampwirth R T 1989 Phys. Rev Krishnan H 1990 (unpublished) Lawrence W E and Dniach S 1971 Prc. XII Int. Cnf. Lw Temp. Phys. Kyt (ed.) K Eiz (Tky: Keigaku) p. 361 Mntgmery H C 1971 J. Appl. Phys Peng J L, Klavins P, Sheltn R N, Radusky H B, Hahn P A and Bernardez L Cnstantin 1989 Phys. Rev. B Srinivasan R 1990 Prc. Int. Cnf. High Temp. Supercnductivity, Bangalre, India, (and references therein) Varadaraju U V, Venkateswara Ra C R, Subba Ra G V, Krishnan H, Srinivasan R, Tmy C V and Malik S K 1990 Prc. Int. Cnf. High Temp. Supercnductivity, Bangalre, India Veira J A and Vidal F 1989 Physica C

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