Effect of K-doping to the weak link behaviour of (Cu0.5Tl0.5−xKx)Ba2Ca2Cu3O10−δ superconductors

Physica C 470 (2010) 51–54

Contents lists available at ScienceDirect

Physica C journal homepage: www.elsevier.com/locate/physc

Effect of K-doping to the weak link behaviour of (Cu0.5Tl0.5xKx)Ba2Ca2Cu3O10d superconductors Nawazish A. Khan *, Safeer Hussain Materials Science Laboratory, Department of Physics, Quaid-i-Azam University, Islamabad 45320, Pakistan

a r t i c l e

i n f o

Article history: Received 28 November 2007 Received in revised form 12 August 2009 Accepted 2 October 2009 Available online 8 October 2009 Keywords: (Cu0.5Tl0.25K0.25)Ba2Ca2Cu3O10d superconductors AC-susceptibility In-field measurements Enhanced inter-grain coupling

a b s t r a c t A systematic study of the weak link behaviour for (Cu0.5Tl0.5xKx)Ba2Ca2Cu3O10d (x = 0, 0.25) superconductors samples has been carried out using electrical resistivity and AC-susceptibility techniques. The Kdoped (Cu0.5Tl0.5xKx)Ba2Ca2Cu3O10d samples were synthesized by solid-state reaction method. In magnetic susceptibility measurements, the real (v0 ) and imaginary (v00 ) parts of the magnetic susceptibility of (Cu0.5Tl0.5xKx)Ba2Ca2Cu3O10d (x = 0, 0.25) samples were measured as a function of temperature under various DC-magnetic fields up to 172 Oe. It is observed from these studies that the magnitude of the diamagnetism is substantially enhanced by K-doping. The possible reasons for the enhanced magnitude of diamagnetism have been investigated. It is observed from in-field magnetic measurements that the inter-grain coupling is improved with the K-doping. It is concluded from these studies that potassium atoms appearing at the crystal boundaries enhance inter-grain coupling and pinning mechanism in Kdoped (Cu0.5Tl0.25K0.25)Ba2Ca2Cu3O10d superconductors. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction In 1962 Josephson published a theoretical paper [1] in which he predicted the two interesting effects in superconductor–insulator– superconductor junctions. In such junctions, the carriers can tunnel across the insulating barrier resulting in a tunnelling current, the critical value of which depends upon an external magnetic field. When the current exceeds its critical value, the junction starts generating high frequency electromagnetic waves, which is known as the second Josephson effect. Both these effects were thoroughly investigated and verified by experiments [2,3], shortly after the publication of their research work [4]. It was soon realized that the Josephson effects exist not only in tunnel junctions, but also in weak links arising among the grains in the polycrystalline [5,6] granular materials. Oxide high temperature superconductors are mostly growned in granular form and these grains are connected by a network of weak links which form an array of Josephson junction [7,8]. The connectivity of weakly coupled grains can reduce the overlap of the wave functions of the carriers in the two adjacent grains enhancing width of tunnel barriers. Some of the reasons for the formation of the weak links are oxygen deficiencies, disorientation at grain boundaries and composition variations in the adjacent grains [9,10]. These weak links promote inferior transport properties in

* Corresponding author. Tel.: +92 51 90642122; fax: +92 51 90642240. E-mail address: [email protected] (N.A. Khan). 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.10.005

the final compound. It is known that the composition variation at the grain boundaries is one of the mechanisms that control the weak link behaviour of polycrystalline high temperature superconductors. Deviations from ideal cation stoichiometry and formation of secondary phases can be considered as a fundamental mechanism that control electronic transport across grain boundaries. On the other hand if there are defects in the oxygen sub-lattice, they may suppress the superconducting order-parameter. In present research article, we have studied the effect of K-doping to the weak link behaviour of (Cu0.5Tl0.5xKx)Ba2Ca2Cu3O10d superconductors. 2. Experimental The samples were prepared by solid-state reaction in a two step procedure. At the first stage (Cu0.5K0.25)Ba2Ca2Cu3O10d precursor material was synthesized using K2CO3, Ba(NO3)2, Ca(NO3)2 and Cu(CN) as starting compounds. These compounds were mixed in appropriate ratios in an agate mortar and pestle. The mixed material was fired in air in a quartz boat at 840 °C for 24 h followed by cooling the furnace to room temperature in 4 h. The precursor material was then ground for about an hour and mixed with Tl2O3 for the preparation of (Cu0.5Tl0.5xKx)Ba2Ca2Cu3O10d. The thallium mixed material was then pelletized under 4.0 tons/cm2 pressure and the pellets were wrapped in a gold capsule. The pellet-containing gold capsule was heat treated at 840 °C for 10 min and quenched to room temperature after the heat treatment. The

N.A. Khan, S. Hussain / Physica C 470 (2010) 51–54

0.45

diamagnetism of the sample was measured (which was accomplished in 2 h) by AC-susceptibility measurements at a lock-in frequency of 270 Hz. The in-field magnetic measurements were performed by applying a constant DC-field parallel to the ambient field of the coil. The superconducting phase was identified by X-ray diffraction (XRD) measurements.

The quality of the K-doped (Cu0.5Tl0.5xKx)Ba2Ca2Cu3O10d (x = 0.25) samples is shown in Fig. 1 of the X-ray diffraction scan. Most of the diffraction lines could be indexed as tetragonal structure followed by a P4/mmm space group; the inclusions of partial derivative phases such as (Cu0.5Tl0.25K0.25)Ba2Ca3Cu4O12d and (Cu0.5Tl0.25K0.25)Ba2Ca1Cu2O8d can also be seen in the diffraction scans. The resistivity measurements of K-doped (Cu0.5Tl0.5xKx) Ba2Ca2Cu3O10d (x = 0, 0.25) samples are shown in Fig. 2. The room temperature resistivity of the samples with a K-doping of 0.25 is much smaller than that of the un-doped sample. Also the metallic decrease of resistivity from room temperature down to the onset of the superconductivity is a salient feature of these samples. The Tc (R = 0) is observed at 109.9 K for the un-doped material and, 105.5 K, for potassium doping concentration of x = 0, 0.25, respectively. The AC-magnetic susceptibility of these samples is shown in inset of Fig. 2; the onset of diamagnetism is observed around 109.9 K for the un-doped material and, 105.5 K for the samples with K-doping of x = 0, 0.25, respectively. The magnitude of the diamagnetism is observed from the in-phase component of the magnetic susceptibility (v0 ) and is found to increase systematically up to a doping level of x = 0.25. The out-of-phase component of the magnetic susceptibility (v00 ) shows a well defined peak behaviour for a K-doping of x = 0.25 demonstrating enhanced inter-grain connectivity. A ceramic HTSC is an array of granular material in which the grains are connected by metallic or an insulating material at their termination ends [11]. It is also possible that material at the termination ends of the grains may be a superconductor (with lower Tc), a normal metal, a semi-conductor or an insulator, which provides Josephson’s coupling among them [12]. Therefore, the nature of material at the grain boundaries would determine the strength of inter-grain connectivity. The magnetic response of granular high temperature superconductors as determined by AC-susceptibility measurements has two components (1) in-phase, (2) out-of-phase component. The in-phase component (real part) of the susceptibility is the response of the crystalline part of the material in the intra-grain regions, whereas the out-of-phase component (imaginary part) is mostly arising from the non-crystalline part of the material

0.35 0.30

ρ(Ω−cm)

3. Results and discussion

K=0 K=0.25

0.40

x10

0.25

2

0.20

0

χ (emu/gram)

52

0.15 0.10

-2

K=0 K=0.25

-4 -6 -8 -10 -12 -14

0.05

80

90

100

110

120

130

Temperature (K)

0.00

100

150

200

250

300

Temperature (K) Fig. 2. Resistivity measurements of (Cu0.5Tl0.5xKx)Ba2Ca2Cu3O10d (x = 0, 0.25) with AC-susceptibility measurements in in-set.

present at the inter-grain sites [12]. At the transition temperature when the intra-grain sites start becoming superconducting, the inter-grain sites with normal material start pinning the current loops and limiting their width to the grain sites. The pinning processes set-in at the inter-grain in turn pin the material within the grain. When the material at the intra-grain sites becomes fully superconducting, the competing out-of-phase component of the magnetic susceptibility turns towards zero and even becomes negative when the material at the inter-grain sites is a strong superconductor. Therefore, these competing effects of in-phase and out-of-phase components of the magnetic susceptibility determine the strength of a superconductor material. However, if the material at the intergrain sites is not pinned, the out-of-phase component of the susceptibility gives an incomplete turning towards zero or a broad peak behaviour or even saturates to a certain positive value. The in-phase component of magnetic susceptibility (v0 ) is related to intra-grain regions associated with the Meissner volume fraction of the diamagnetism within the grains. The out-of-phase component of (v00 ), however, determines the inter-granular contribution of superconductivity arising from the coupling matrix of the grains which determines inter-grain connectivity. The v00 term is strongly effected by the external magnetic field and the shift of it is associated with pinning of the vortices at the grains sites. The peak maximum of v00 occurs at a temperature, at which the shielding current

Fig. 1. X-ray diffraction of (Cu0.5Tl0.5xKx)Ba2Ca2Cu3O10d (x = 0.25) superconductor.

N.A. Khan, S. Hussain / Physica C 470 (2010) 51–54

is equal to the critical current density in that grain. The flux dynamics at this stage is controlled by pinning. When the applied field reaches the centre of the sample, the Jc at temperature Tp corresponds to equation

J c ¼ Ba =l0 R R is the radius of the grain [13]. The increased strength of the external magnetic field decreases the superconducting volume fraction within the grains. The in-field magnetic measurements of K-free (Cu0.5Tl0.5)Ba2Ca2Cu3O10d samples are shown in Fig. 3a. When no external field is applied, these samples have shown an onset of diamagnetism in the in-phase component of the magnetic susceptibility around 110 K and a peak temperature around 104 K in the outof-phase component of magnetic susceptibility. The applied external DC-magnetic field of 26, 54, 99, 141 and 172 Oe shifts the onset of diamagnetism to 108.9, 105, 103.6, 102.6 and 101.6 K and the peak temperature Tp in v00 to 102.1, 98.5, 98.1, 97.5 and 97.2 K, respectively. The magnitude of diamagnetism is decreased and the peak of the out-of-phase component (v00 ) is shifted to lower temperatures with the increase of magnetic field strength. The magnitude of diamagnetism for HDC = 172 Oe becomes half of the value observed in zero applied DC-field measurements. In K-doped (Cu0.5Tl0.5xKx)Ba2Ca2Cu3O10d (x = 0.25) samples, the onset of diamagnetism is observed around 105.5 K with Tp at 103.6 K when no external field is applied, Fig. 3b. The magnitude of diamagnetism is decreased and the onset of diamagnetism is also shifted to lower temperature values with increased applied external magnetic field. The onset of diamagnetism is observed at 103, 102, 101, 99.5 and

Fig. 3a. AC-susceptibility measurements of (Cu0.5Tl0.5xKx)Ba2Ca2Cu3O10d (x = 0) of superconductor in DC-magnetic field of HDC = 0, 26, 54, 99, 141, 172 Oe.

53

96.5 K under applied field HDC of 26, 54, 99, 141 and 172 Oe; the peak temperatures Tp observed from v00 are observed at 100.4, 99.5, 98.03, 96.0 and 93.6 K, respectively. The applied DC-field amplitudes versus peak temperatures Tp can be fitted to a powerlaw of the form Hac  (1  Tp/Tc)n, where the number ‘‘n” is indicative for the nature of the junction. The value n = 2 is typical for superconductor–normal metal–superconductor (SNS) junction [14], n = 1 for superconductor–insulator–superconductor (SIS) junctions [15] and n = 1.5 for superconductor–insulator–normal metalsuperconductor (SINS) junctions [7]. The best fitting of our data for (Cu0.5Tl0.5xKx)Ba2Ca2Cu3O10d (x = 0, 0.25) samples is achieved for n = 2, which indicate that the material at the inter-grain sites is a normal metal, Fig. 4a and b. The normal materials appearing at the crystal boundaries (grain boundaries) are Cu atoms in case of K-free (Cu0.5Tl0.25)Ba2Ca2Cu3O10d samples and a mix of Cu/K atoms in (Cu0.5Tl0.25K0.25)Ba2Ca2Cu3O10d samples [16]. The improved SNS characteristic in K-doped samples is most likely arising from the ability of alkali metals to loose their 3s electron very easily, which most likely increases the normal metal characteristics at the inter-grain sites resulting in an improved coupling of grains. Since the critical current density is related to applied field Ha, therefore, Jc is also enhanced with the K-doping in (Cu0.5Tl0.25)Ba2Ca2Cu3O10d samples. 4. Conclusions We have successfully synthesized K-doped (Cu0.5Tl0.5xKx) Ba2Ca2Cu3O10d (x = 0, 0.25) samples and studied their inter-grain

Fig. 3b. AC-susceptibility measurements of (Cu0.5Tl0.5xKx)Ba2Ca2Cu3O10d (x = 0.25) of superconductor in DC-magnetic field of HDC = 0, 26, 54, 99, 141 and 172 Oe.

54

N.A. Khan, S. Hussain / Physica C 470 (2010) 51–54

K=0

HDC(Oe)

150

100

50

0 0.002

0.003

0.004

0.005

0.006

0.007

0.008

2

(1-TP/Tc)

inset of Fig. 2) which is most likely arising from enhanced carrier’s concentration in conducting CuO2 planes [17]; in Bi-2212 samples, the highest Tc (R = 0) of 93 K was observed in a Bi2Sr1:9K0:1CaCu2Oy samples. It was suggested that K-doping increases the charge state of the Cu atoms in the CuO2 planes which promotes an increase in the density of mobile carrier. Alkali metals are known to loose their outer-most ‘‘s” electron, which enhances the Fermi-vector ½K F ¼ ð3p2 NV Þ1=3  of the carriers, which in turn increases the coherence length nc = ⁄KF/2mD and superconductivity order-parameter kT c ¼ 1:14D sinh ½1=V 0 ZðE0f Þ exp  1=V 0 ZðE0f Þ, where V 0 ZðE0f Þ is coupling constant between 0.2 and 0.4 for conventional superconductors [18]. The superior weak link behaviour in K-doped (Cu0.5Tl0.25K25)Ba2Ca2Cu3O10d samples is most likely arising from the presence of K at grain boundaries, which enhances the SNS characteristics of the junction. The superior SNS characteristics probably help in enhancing the pinning of vortices at the grain boundaries, which not only helps in increasing the magnitude of diamagnetism in K-doped samples but also enhances Jc of the final compound.

Fig. 4a. Variation of field amplitude HDC of (Cu0.5Tl0.5xKx)Ba2Ca2Cu3O10d (x = 0) with (1Tp/Tc)2.

Acknowledgement Higher Education Commission, Pakistan is acknowledged for its financial support through Grant No. 20-259 R&D.

K=0.25 150

HDC(Oe)

References 100

50

0 0.0003

0.0004

0.0005

0.0006

0.0007

0.0008

0.0009

2

(1-TP/Tc)

Fig. 4b. Variation of field amplitude HDC of (Cu0.5Tl0.5xKx)Ba2Ca2Cu3O10d (x = 0.25) with (1  Tp/Tc)2.

coupling by carrying in-field magnetic measurements. The magnitude of diamagnetism is enhanced with K-doping (as shown in the

[1] A.A. Abrikosov, Zh. Eksp. Teor. Fiz. JETP 32 (1957) 1442. [2] I.K. Yanson, V.M. Svistunov, I.M. Dmitrenko, Zh. Eksp.Teor. Fiz. JETP 21 (1965) 650. [3] S. Shapiro, Phys. Rev. Lett. 11 (1963) 80. [4] B.D. Josephson, Phys. Lett. 1 (1962) 251. [5] Nawazish A. Khan, P. Kameli, A.A. Khurram, Supercond. Sci. Technol. 19 (2006) 410. [6] H. Salamati, P. Kameli, Physica C 403 (2004) 60. [7] H. Salamati, P. Kameli, Solid State Commun. 125 (2003) 407. [8] J. Halbritter, Supercond. Sci. Technol. 16 (2003) 47. [9] S.K. Agarwal, B.V. Kumaraswamy, J. Phys. Chem. Solid 66 (2005) 729. [10] M. Tinkham, Introduction to Superconductivity, second ed., McGraw-Hill, New York, 1996. p. 363. [11] A.A. Khurram, Nawazish A. Khan, Supercond. Sci. Technol. 19 (2006) 679. [12] V.V. Schmidt, in: P. Muller, A.V. Ustinov (Eds.), The Physics of Superconductors, Springer-Verlag, Berlin, Heidelberg, New York, 1997, p. 71. [13] F. Gömöry, Supercond. Sci. Technol. 523 (1997) 10. [14] P.G. Degennes, Rev. Mod. Phys. 36 (1964) 225. [15] V. Ambegaokar, A. Baratoff, Phys. Rev. Lett. 10 (1963) 486. [16] A. Veneva, I. Iordanov, L. Toshev, A. Stoyanova-Ivanova, D. Gogova, Physica C 308 (1998) 175. [17] M. Chandra Sekhary, B. GopalaKrishnay, U.V. Varadarajuz, S.V. Suryanarayanay, Supercond. Sci. Technol. 9 (1996) 756. [18] H. Ibach, H. Luth, Solid-State Physics, second ed., Springer, 1995. p. 244.