Increased resistance below the superconducting transition in granular Sm1.83Ce0.17CuO4−y compounds

ELSEVIER

Physica C 289 (1997) 265-274

Increased resistance below the superconducting transition in granular Sml.83Ce0.17CuO4_ycompounds M.J.R. Sandim a, P.A. Suzuki a, S. Spagna b, S.C. Tripp b, R.E. Sager b, R.F. Jardim c,, a Faenquil-Demar, Departamento de Engenharia de Materiais, C.P. 116, 12600-000, Lorena SP, Brazil b Quantum Design Inc., 11578 Sorrento Valley Road, San Diego, CA 92121-1311, USA c lnstituto de Fisica, Universidade de S~to Paulo, C.P. 66318, 05315-970, S~to Paulo, Brazil

Received 4 December 1996; revised 1 May 1997; accepted 2 July 1997

Abstract We have studied the magnetic and transport properties of granular samples of the electron-doped superconductor Sml.s3Ceo.17CuO4_y prepared under certain conditions. Measurements of magnetic susceptibility, electrical resistance and magnetoresistance in applied magnetic fields up to 8.8 T reveal that superconductivity develops below T~i = 17.5 K in these compounds. We also observed an abrupt increase of the electrical resistance below the superconducting transition. It was found that the resistance excess, A R ( T , H ) , is proportional to (T/T~i) -4 close to T~i, 0.6 < (T/T~i)< 1, and shows a crossover to a ( T / T c i ) - J/4 dependence at lower temperatures. We argued that the increase of AR close to Tci is due to the depletion of the concentration of normal carriers as predicted by the semi-phenomenological two-fluid theory of superconductivity. The behaviour of AR at lower temperatures is also discussed. © 1997 Elsevier Science B.V. Keywords: High-T~ oxides; Electrical resistance; Two-fluid theory; Granularity

1. Introduction The superconducting properties of the classical discontinuous systems, including thin metallic films and thicker samples of granular metals, are strongly related to the so-called characteristic disorder length [1]. In the superconductor-insulator (SI) transition a wide range of behaviours in the electrical resistance measurements as a function of the temperature have been observed varying the film thickness. Several investigations have been performed on a wide range

* Corresponding author. Tel.: +55 11 8186891; fax: +55 11 8186984.

of two-dimensional (2D) and three-dimensional (3D) systems, including In films (2D) [2,3], ultrathin Sn films (2D) [4], granular A1-Ge films (2D) [5], Pb and Bi films (2D) [6], MoC thin films (2D) [7], pressed metallic particles (3D) [8], and granular A1 (3D) [9]. Granular systems display a wealth of physical behaviours manifested in t h e electrical transport properties across the SI transition, such as insulator, local superconductivity, metallic behaviour, and global superconductivity. In these systems the superconducting critical temperature T~i is not affected by the superconductor-insulator transition, suggesting that superconductivity is suppressed when the long

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range order in the phase coherence of the order parameter is lost. One of the most interesting features of granular systems is observed in samples belonging to the so-called dielectric side of the superconductor-insulator transition [2,4,9]. In this regime, a drastic increase of the resistance, R(T), due to development of superconductivity is frequently observed below T~i. This increase in R(T) can be explained by the semi-phenomenological two-fluid theory of superconductivity, as suggested elsewhere [2,10]. According to this theory, the portion of charge carriers which remains in the normal fluid below the transition temperature T~i varies as (Z/Tci)4 resulting in a gradual increase in R(T) by a factor of (T/T~i) -4. In this work we focus on the effect of superconductivity on the macroscopic conduction behaviour of polycrystalline samples of the electron-doped superconductor Sml.83Ce0.17CuO4_y [11]. In general, these materials are best described by being com-

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prised of small superconducting islands embedded in an insulating matrix [12,13], a structure similar to a granular superconductor. We have performed electrical resistance, magnetoresistance and magnetization measurements in these compounds and have found an abrupt increase of R(T) below the superconducting transition Tci. This increase in the magnitude of R(T) is rapidly suppressed by the application of large magnetic fields [14]. By considering the excess of resistance A R below Tci, we have found that, for 0.6 < (T/Tci) < 1, A R ( T , H ) is proportional to (T/Tci) -4. We discuss this behaviour within the framework of a superconducting granular model.

2. Experimental procedure Polycrystalline samples of Sm1.83Ce0A7CuO4_y were prepared using the sol-gel route [15]. Stoichiometric powders were pressed into pellets under three

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TEMPERATURE (K) Fig. 1. Temperature dependence of the electrical resistivity, p(T), performed in polycrystalline samples of Sml.83Ce0.~7CuO4-y. The figure shows the results of three different samples: S-147, S-116 and S-245, as discussed in the text. The inset shows the p(T), 4.2 < 20 K, of the sample S-245. Two temperatures are displayed: Tci, which corresponds to the development of superconductivity, and Tcj, the Josephson coupling temperature of the system.

M.J.R. Sandim et al. / Physica C 289 (1997) 265-274

revealed single phase materials with the T'-structure. Conventional four-wire electrical resistance and magnetoresistance measurements were performed in applied magnetic fields H as high as 8.8 T and temperatures as low as 4.2 K using a general purpose commercial variable-temperature and magnetic field cryostat (Physical Properties Measurements System-Quantum Design) employing excitation current densities ranging between 6 X 10 . 6 _
different pressures of: 116, 147 and 245 MPa (referred hereafter S-166, S-147 and S-245). By varying the pressure prior to sintering an important aspect of this work was to produce identical stoichiometric compounds with different underlying morphologies. All the pellets were sintered in air and at 980°C for 48 h, cut in parallelepiped-shape 2 × 2 × 8 mm 3 bars, reduced in Ar at 950°C for 18 h and quenched to room temperature in 2 h. Powder X-ray diffraction, using a Phillips PW1710 diffractometer employing Cu K a radiation,

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TEMPERATURE (K) Fig. 2. T e m p e r a t u r e d e p e n d e n c e o f the electrical resistance R(T) t o g e t h e r w i t h the s e c o n d o r d e r p o l y n o m i a l fit o f the d a t a in the interval 30 < T < 65 K (a), a n d m a g n e t i z a t i o n , M(T) (b) for the s a m p l e S-147. T h e m a g n e t i z a t i o n d a t a s h o w a p p r e c i a b l e d i a m a g n e t i s m b e l o w the s u p e r c o n d u c t i n g critical t e m p e r a t u r e Tci ~ 17.5 K.

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tained in a custom-made vibrating sample magnetometer from 4.2 to 30 K and typical magnetic field of 2 0 e . Meissner fractions were estimated from the theoretical density of the SmLssCe0.15CuOn_y unit cell with no demagnetizing corrections.

perconductivity develops at T~i within superconducting islands embedded in an insulating matrix. However, the p ( T ) data suggest that these samples have a very small superconducting volume fraction since the zero resistance state is not observed even at low temperatures. On the contrary, with decreasing temperature the development of a rapid increase of p(T) is observed well below Tcr Particularly interesting is the curve for sample S-245 where a definite onset of a second transition at temperatures close to T~j --~ 7 K can be observed (see inset of Fig. 1). In this sample, prepared under higher pressure, a more closely packed grain morphology enables the superconducting islands to couple together at low temperatures resulting in a second drop in p(T). Such a coupling can be attributed to a Josephson effect between superconducting regions. It is also important to note that the superconducting transition temperature T~ of these Sm-based compounds is always found to be close to 20 K [12-14]. In these samples, Tci is

3. Results a n d d i s c u s s i o n Fig. 1 displays the temperature dependence of the resistivity, p(T), for three samples, S-116, S-147 and S-245, subjected to an excitation current of 1 ~ A corresponding to current densities of 4.7, 5.6 and 3 × 10 -2 A / c m 2, respectively. A number o f important features can be identified in these curves. First, all samples display a semiconductor-like behaviour for temperatures above ~ 20 K. Secondly, p(T) exhibits a local minimum in its magnitude below Tel -~ 17.5 K, without any evidence of a zero resistance state at low temperatures. These features provide evidence that with decreasing temperature su-

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TEMPERATURE (K) Fig. 3. Temperature dependence of electrical resistance R(T) obtained by increasing the excitation current, 10 < l~x < 10 3 I~A, for the sample S-147. The inset shows the behaviour of R(T) in the temperature interval 4.2 < 12 K.

M.J.R. Sandim et al. / Physica C 289 (1997) 265-274

slightly lower probably due to the highly disordered nature of the materials. We have also identified a partial drop in the R(T,H = 0) curve at T~i = 17.5 K as the onset of the superconductivity in these compounds, as shown in Fig. 2a for the sample S-147. This figure also shows the computed resistance, AR(T,H), exceeding the normal state resistance value expected from a simple polynomial extrapolation of R(T,H--O) down to low temperatures. Such an extrapolation, obtained by a second order polynomial fit of the R(T,H= O) data from 65 K to 30 K, as well as the definition of A R are displayed in Fig. 2a. Further evidence for superconductivity below Tci = 17.5 K is provided by the temperature dependence of the magnetization M(T) shown in Fig. 2b. Samples S-245 and S-147 display appreciable diamagnetism below Tci -~ 17.5 K with comparable screening (ZFC) and Meissner (FC) signals. This feature suggests that superconducting islands or clusters are essentially isolated in the insulating matrix. Also, we have inferred the

269

superconducting fraction of these samples by computing the values of M(T = 5 K) in applied magnetic field of 2 0 e and found that superconducting volume fractions are 16.3 and 6% for samples S-245 and S-147, respectively. These values of superconducting volume are rather small, probably below the percolation threshold (e.g. Ref. [16]), and further suggests that these samples are comprised of isolated superconducting islands embedded in a non-superconducting host. For a better understanding of the underlying mechanism responsible for the increase in R(T) below Tci, we have performed measurements of electrical resistance as a function of the excitation current in the sample S-147. Typical curves, for excitation currents Iex varying from 10 to 10 3 hi,A, are show in Fig. 3. It is observed that the R(T) behaviour is insensitive to changes in the excitation current for temperatures down to = 11 K. At lower temperatures, the curves significantly deviate each other and the magnitude of R(T) increases with

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increasing excitation current. The strong dependence on the transport excitation current below 11 K is indicative of a rapid suppression of Josephson coupling between superconducting islands which for lex = 10 ixA shows the onset of second transition associated with Josephson coupling between grains. With increasing excitation current there is no clear evidence of Josephson coupling and the magnitude of R(T) increases monotonically with decreasing temperature. Quantitatively the increase in the magnitude of R(T) corresponds to ~ 10 12, at 5 K, when the current is increased from 10 to 103 ~zA (see inset of Fig. 3). This is a remarkable increase in the magnitude of R(T) and indicates that Josephson coupling develops at temperatures below Tc~ in these compounds. Consistent with this qualitative behaviour is the temperature dependence of R(T) for different applied magnetic fields H, as shown in Fig. 4. For temperatures above Tel -- 17.5 K, we have found that R(T) is almost insensitive to the applied magnetic

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field. However, below Tci , appreciable negative magnetoresistance is observed for increasing applied magnetic field. A careful inspection of these results also shows that such decrease in the magnitude of R(T) is particularly pronounced at low temperatures. For example, at T = 5 K, increasing magnetic field from H = 0 to H = 8.8 T causes a decrease in R(T) from ~ 70 to ~ 50 12. All the transport properties described above can be combined with the expected morphology of these samples resulting in a simple picture involving single particle tunnelling and Josephson coupling. These granular samples can be described by a collection of isolated metallic islands which undergo a superconducting transition at Tci. As the superconducting volume fraction is below the percolation threshold, the zero resistance state is not achieved in these granular samples even at low temperatures. Instead, an abrupt increase in R(T) is observed below Tci. However, the development of superconductivity within isolated islands depletes the concentration of

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(T/Tci)-4 Fig. 5. The resistance excess A R as a function of (T/T~i) -4 in the range of temperature 0.6 < (T/Tci) < 1. The figure shows data for the sample S-147 subjected to several applied magnetic fields 0 < 8.8 T. The inset shows an expanded view of the data close to AR ~ 0, 0
M.J.R. Sandim et al. / Physica C 289 (1997) 265-274

normal charge carriers, as predicted by the semi-phenomenological two-fluid theory [2,10]. According with this theory, electrons start to condense out into a superfluid at the superconducting transition temperature T~i. The proportion of superfluid increases as the temperature is lowered and no normal fluid would remain at very low temperatures. It is also well known that the proportion of electrons which remains in the normal fluid below T~i must vary as (T//Tci)4 [2,10]. Thus, the excess of electrical resistance of the system below T~i would be increased by the factor of (T//Tci)-4 . Hence, one would expect AR(T,H) to increase gradually by a factor of (T/T~i) -4 just below the superconducting transition. To test and gain insight into these important mechanisms, we have analyzed the AR(T,H) vs. (T//Tci)-4 curves for several applied magnetic fields and in the temperature range 0.6 < ( T / T ~ i ) < 1, as shown in Fig. 5. At the first glance, it seems that the behaviour of AR(T,H) vs. (Z//Tci)-4 is rather complicated for several reasons. One of them concerns

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the negative value of A R for zero applied magnetic field in the temperature range 0.8 <(T/Tci)< 1. However, this is the expected behaviour of these granular samples since a fractional drop in the magnitude of R(T) at Tci is always observed (see Fig. 1). Indeed, the deviation of AR from t h e ( Z / / T c i ) - 4 behaviour, 0.8 < ( T / T ~ i ) < 1, is observed for all curves obtained in applied magnetic fields below 1 T, as shown in the inset of Fig. 5. Such a deviation of A R from the (Z//Tci)-4 behaviour occurs because coupling between superconducting islands would form superconducting clusters just below Tci. This kind of short-range phase coherence of the superconducting order parameter would be affected by the application of magnetic fields leading a different temperature dependence of AR(T,H). Such a feature is evident from the data displayed in the inset of Fig. 5 in the range 0.8 ___(T/Tci) < 1. We have also observed that AR(T,H) ct (T/T~i) -4 in a larger temperature range 0.6_< (T/Tci) _< 1, as shown in Fig. 6. In fact, from a

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(T/Tci)-4 Fig. 6. The resistance excess AR vs. (T/T~i)-4; 0.6 < (T/Tci) < 1. The figure shows data obtained in the sample S-147 subjected to several applied magnetic fields 3 < 8.8 T, as well as their respective linear fits. For these fields we have found that AR cc (T//Tci)-4 in the temperature range 0.6 < 1, as discussed in the text.

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careful analysis of these curves we have found that this temperature range increases monotonically with increasing applied magnetic field. For higher applied magnetic fields, 3 5 HI 8.8 T, AI?R(T,H) can be written as AR(T,H) = A -t-B(T/Tc.)-4 in the temperature range 0.6 d (T/T,,) _< 1, as expected by the two-fluid theory. The discussion here involves some additional aspects since superconductivity close to qi is suppressed by a large magnetic field. By taking this into account, AR(T,H) would strongly decrease, approaching zero for fields exceeding the upper critical field Hc2. This would occur essentially due to Cooper pair breaking effects which increase the density of states and lower the sample resistance, resulting in a gradual increase of the number of normal charge carriers available for the transport properties. Such a behaviour has been also observed in our experiments. However, it is important to stress here that, even for H = 8.8 T, we have found that AR is not strictly zero. An explanation for this behaviour is twofold:

(11 It is well known that the upper critical magnetic fields of electron-doped superconductors are very anisotropic [17-191, For example, it has been found that Nd,,85Ce,,,Cu0,_, has dfi,,/dT = - 8.85 T/K for H I c and d H,,/dT = - 0.4 T/K for Hllc [18]. Similar values were found in Sm,,,,Ce,,,,CuO,_, [17,19]. The value of dH,,/dT parallel to the &-plane is rather high and combined with the misorientation of the crystallites within the granular material would cause a large distribution of Hc2 in polycrystalline samples. Thus, an application of a magnetic field of H = 8.8 T wouid not be enough to completely suppress the supe~onductivity close to Tci in all crystallites of our samples. (2) It is also known that superconducting properties, mostly the superconducting critical temperature qi, of electron-doped su~rconductors are very sensitive to small fluctuations of either cationic (Ce) and anionic (0) distributions f 12- 151.In fact, Gerber and co-authors [t4] have estimated the width of such distribution in polycrystalline Ln,,,,Ce,,,CuO,_ i‘

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Fig. 7. The (T/Tci)-‘f4 dependence of the resistance excess AR obtained in the sample S-147 in the temperature range 0.3 < (T/T,,) < 0.6, under applied magnetic fields up to 8.8 T. Linear fits of the data are also shown.

M.J.R. Sandim et al. / Physica C 289 (1997) 265-274 ( L n = P r , Nd, Sm, Eu) to be of order of ~ 1 0 K. Indeed, it is likely that our samples have a very large distribution of Tci. Such a large distribution in T~ would imply in a correlated dispersion in H~2 values, generating superconducting regions in the neighbourhood of T~i even in applied magnetic fields as high as 8.8 T. Thus, by considering these two features of electron-doped superconductors, one is lead to conclude that A R would decrease with increasing applied magnetic field without approaching zero in the high field limit, as observed in our experiments. It is important to note that the (T/Tci)-4 dependence of A R, even in applied magnetic fields as high as 8.8 T, is limited to the 0.6 < (T/T~)<1 temperature range. For lower temperatures, (T/T~i) < 0.6, fits of the data revealed that AR(T,H)= A Ro(HXT/Tci )-1/4. These results, for curves up to 8.8 T, are shown in Fig. 7. Thus, for values of (T/Tci) < 0.6, other mechanisms are likely to contribute to changes in the magnitude of A R. In fact, it seems that there is a competition between these mechanisms, either one of them becoming more or less evident dependening on the magnitude of the applied magnetic field. In the low field limit, a crossover to an excitation current dependent rate of increasing resistance below T ~ 11 K (i.e. (T/T~i) ~ 0.6) (see Fig. 3) is indicative that Josephson coupling takes place at lower temperatures, since it is very sensitive to small changes in the excitation current and applied magnetic field. It seems that this kind of coupling contributes to changes in the A R behaviour at low temperatures and low applied magnetic fields. On the other hand, we have observed that the (T/Tci) 1/~ dependence of A R is much more pronounced in applied magnetic fields higher than 1.5 T, as one can see in the Fig. 7. Thus, a progressive increase of Josephson coupling as the separation between superconducting grains is decreased seems to be very important to the A R behaviour in the low field limit. However, it is not convincing that coupling between superconducting islands would determine the behaviour of A R in the high field limit. In this limit, it is seems that two mechanisms are likely to contribute to changes in the A R behaviour. One of them certainly would be associated with the intrinsic properties of the superconducting islands. By taking this into account, it is reasonable to assume that intrinsic

273

transport and magnetic properties of these cuprates are somewhat mirrored in the so-called irreversibility line IL [20,21]. This line is defined in the H vs. T phase diagram and is usually written as H < x ( 1 T/T~) 8. Irrespective of the origin of this irreversibility line, most of these superconducting cuprates show a crossover in the IL from /3~ 1.5 to higher /3 values at T/T~i ~ 0.6 [20]. From our results it seems that such a crossover observed in the behaviour of AR(T,H) would be correlated with changes in the dissipation associated with the flux lattice of these granular superconductors. It is also important to note that the data shown in Fig. 7 reveal that increasing applied magnetic field results in a gradual decrease in the magnitude of AR0(H), though the (T/Tel)-1/4 dependence of A R(T, H) is still preserved. This observation strongly suggests that the full determination of A R would involve at least two mechanisms in the range of applied magnetic field discussed here: one that is field dependent and seems to lower the magnitude of AR0(H), and a second mechanism that seems to be responsible for the (T/T~i) -1/4 dependence. It is plausible, assuming that increasing magnetic field would suppress superconductivity in some regions of the material with no significant effect on the normal matrix. Thus, changes in the magnitude of A R 0 ( H ) would be associated with changes in the relationship between superconducting and normal volume fractions of the material. On the other hand, the basic physical mechanism behind the (T/Tci)-1/4 dependence of A R is still unknown. In any event, experiments under applied magnetic fields up to 20 T are under way in order to clarify this point.

4. Conclusions

In conclusion, we have observed an abrupt increase in the magnitude of the electrical resistance below the superconducting transition Tci = 17.5 K in granular samples of Sml.s3Ce0.17CuO4_y. By computing the resistance excess A R(T,H), 0 < H < 8.8 T, below the superconducting transition temperature, we have found that AR is proportional to (T/T~i) -4 close to T~i. We argued that the increase of A R close to Tci is mostly due to the depletion of the concentration of normal charge carriers as predicted by the

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s e m i - p h e n o m e n o l o g i c a l two- fluid theory o f superconductivity. W e also found a c r o s s o v e r o f A R to a ( T / T c i ) - 1 / 4 d e p e n d e n c e at l o w e r temperatures and have discussed this b e h a v i o u r in the f r a m e w o r k of a superconducting granular scenario.

Acknowledgements W e have benefited f r o m fruitful discussions with D. Stroud. This w o r k was supported in part by the Brazilian agencies Funda~ao de A m p a r o h Pesquisa do Estado de S~o P a u | o F A P E S P under contract Nos. 9 3 / 4 2 0 4 - 4 , 9 5 / 4 4 9 1 - 9 and 9 6 / 8 4 1 6 - 4 and Conselho N a c i o n a l de Pesquisa e D e s e n v o l v i m e n t o C N P q under contract N o 4 0 0 8 9 6 / 9 3 - 1 . O n e o f us (RFJ) is C N P q f e l l o w under contract No. 3 0 4 6 4 7 / 9 0 - 0 .

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