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Superconductors is the materials that have no resistance to the flow of electric current when it is cooled below a certain temperature called the critical temperature, Tc [Omar, 1975]. Superconductor also exhibits perfect diamagnetism where when the weak magnetic fields applied to superconductor, the magnetic line are expelled from the interior of superconductor and this phenomenon is called meissner effect [Christman, 1988]. The phenomenon of superconductivity was first discovered by Onnes (1911) after he successfully discovered the helium gas in 1908. Mercury was the first material that observed for superconductivity behavior and it's superconducting at 4.2 K. Later, many of other metals are discovered to superconducting at low temperature. Not only are those, the superconducting behavior on severe compound also discovered. Until now, compound of cuprate oxide based are the compound that having the highest Tc and those compound are classified as high temperature superconductor (HTSC).
In 1986, Bednorz and Muller discovered superconductivity in cuprates oxides (LaBaCuO ceramics) which superconducting at ~30 K. A year later, YBCO compound was discovered with Tc is 92 K which the first materials that to superconduct at temperature above the boiling point of liquid nitrogen (77 K) [Wu et al., 1987]. Since that, many other investigation on various type of HTSC based on cuprate superconductor had been investigated. Now, high temperature superconductor (HTSC) materials can be classify into four main classes; yttrium based (or other rare earths based superconductor) [Wu et al., 1987; Siegrist et al., 1987], bismuth based [Xu et al., 1990; Tallor et al., 1998], thallium based [Sheng, & Hermann, 1998; Parkin et al., 1988; Kaneko et al, 1992], mercury based [Meng et al., 1993; Schilling et al., 1993; Putlin et al., 1993] superconductor with critical temperature of up to 95, 110, 127, and 134 K respectively [Park & Synder, 1995]. These cuprate superconductors excluding Yttrium based family contains two dimensional CuO2 planes and separating by insulating layers. The concentration of charge carriers in CuO2 planes are crucial part in seeking the optimum superconducting transition temperature.
Formation of Tl1212 phase
Thallium based superconductors are one of the interesting family of HTSC as its have advantage of higher Tc value than the more widely studied YBCO superconductor. Amongst the Tl-based superconductor, TlM2CaCu2O7-Î´ (Tl1212) and TlM2Ca2Cu3O9-Î´ (Tl1223) where M= Ba or Sr, are unique materials to study as it showed better behavior in magnetic field as compared to other series of Tl-based superconductor such as Tl2M2Ca2Cu3O10-Î´, Tl2223 and Tl2M2CaCu2O8-Î´, Tl2212 (M= Ba or Sr) superconductors [Jergel et al., 1996]. Tl1212 is an important member of Tl-based superconductor as its structure is analogous to the structure of the most widely studied YBa2Cu3O7-Î´ (Y123) compound [Nakajima et al., 1990]. Therefore, the similarities and differences between YBa2Cu3O7-Î´ (Y123) and Tl1212 superconductor can be discussed. Moreover, the Tl1212 has a shorter insulating distance between the superconducting Cu-O layers than Tl1223 [Lao et al., 2000]. The short insulating distance is expected to produce higher critical current density, Jc and better performances in magnetic fields due to reduced anisotropy through stronger interlayer coupling and less severe thermally activated flux motion [Lao et al., 2000].
The TlSr2CaCu2O7-Î´ superconductor was reported to be superconducting with Tc of around 70-80 K [Martin et al, 1989]. However, the compound is found to be difficult to synthesize in pure form due to high average copper valence and overdoping of hole carriers [Subramaniam et al, 1998]. Interestingly, the high average Cu valence of this unsubstituted Tl1212 superconductor can be lowered to an optimal value of around +2.3 by partial elemental substitutions of higher valence ions [Shukor and Arulsamy, 2000; Hamid et al., 2004] which then result to a decrease in hole concentration to optimal value. Reduction of the overdoped holes carrier concentration to the optimal value results in stabilization of the Tl1212 phase [Hamid et al., 2004]. In Tl1212 layered structure, other layers beside the CuO2 layers are considered as a charge reservoir. These reservoirs are used to accept or donate electrons into the CuO2 planes. This transfer of the charge, controls the hole concentration in superconducting plane. Most reports of elemental substitution in Tl1212 system were on the various elemental substitutions at Ca site [Sheng et al., 1989a; Li & Greenblatt, 1989]. However, other sites of elemental substitution such as at Sr-site also play an important role in the superconductivity of Tl1212.
On the other hand, elemental substitution in place of copper may go into the CuO2 planes and caused drastic decrease in critical temperature measurement (Tc) [Kandyel et al., 2005; Enengl et al., 2002; Yang Li et al., 1999; Kuhberger & Gritzner, 2003]. For the Tl1212 system, previous report on Cu site substitution in Tl0.5Pb0.5Sr2CaCu2-xMxO7 (M= Co, Ni and Zn) showed strong suppression of Tc which indicates that superconducting properties of the Tl1212 superconductor are sensitive to chemical doping of the CuO2 planes [Kandyel et al., 2005]. Intriguingly, previous studies on (Cu,Tl)-based superconductors (Cu0.5Tl0.5)Ba2Ca2Cu3-yGeyO10-Î´ and (Cu0.5Tl0.5)Ba2Ca3Cu4-yGeyO10-Î´ series [Khan & Irfan, 2008] reported an increase in Tc as a result of Ge4+ substitution. The Tc enhancement was suggested to be due to the development of mixed planes of CuO2/GeO2 which could be responsible for formation of a more effective superconducting layer [Khan & Irfan, 2008]. A similar substitution of Ge4+ in Tl1212 may contribute to the understanding of the effect of mixed CuO2/GeO2 planes on superconductivity, and such work has not yet been reported. Therefore, it would be interesting to see the effect of Ge4+ substitution at Sr- and Cu- site in Tl1212 superconductor.
Superconducting Fluctuation Behavior of HTSC
The electric conduction in a superconductor is due to the flow of pairing of charge carriers called Cooper pairs [Glover, 1967; Crow et al., 1970; Johnson & Tsuei, 1976]. The formation of Cooper pairs in conventional superconductor is below the critical temperature, Tc and is well described by the BCS theory. HTSC have short coherence length (Î¾c(0)). Coherence length here is defined as the distance between the two electrons of the Cooper pair [Cyrot & Pavuna, 1992]. HTSCs also are electrically anisotropic in structure. In contrast with conventional superconductor, the presence of this short coherence length as well as highly anisotropic in HTSC provides possibility to study fluctuating superconducting pairs above Tc [Khan et al., 2010]. Thus, in HTSC the formations of Cooper pairs are exist at even above Tc, however these Cooper pairs are simultaneously formed and broken as approach to Tc, which then result of fluctuation of conductivity in normal state properties of HTSC.
The temperature dependence of electrical resistivity for a HTSC samples (Figure 1.1) shows two difference behavioral states. The first state corresponds to the normal state that shows a metallic behavior and the second state is the state characterized by the contribution of induced fluctuation Cooper pairs to the conductivity above Tc [Christman, 1988; Sharma et al., 1995]. This second state region is in temperature range from the temperature where the resistivity curve starts to deviate away from the projected metallic normal state resistivity curve upon cooling until to the point of temperature of peak, Tcp that given by the graph of (dÏ/dT) versus temperature in Figure 1.1 [Cardona, 1999]. The excess conductivity is clearly visible in this region. This excess conductivity region is then analyzed based on the AL theory in conjunction with LD theory for obtained excess conductivity properties. Figure 1.1 shows the resistivity measurement and its derivative curve of superconducting sample.
Linear metallic line
Point where resistivity curve start to deviate from linear metallic line
Resistivity measurement curve
Figure 1.1: The resistivity measurement and its derivative curve of superconducting sample. The linear fit curves show the background normal state resistivity projection (linear metallic line). The data range from the temperature where the curve start to deviate from linear metallic line until to the temperature peak (Tcp), taken for analyses of excess conductivity.
HTSC materials have much shorter coherence length as compared to conventional superconductors and are electrically anisotropic in structure. A convenient model that can be used to evaluate the layered anisotropic structure and characteristically shorter c-axis coherence length in HTSC materials is the Aslamazov Larkin (AL) model which is used in conjunction with the Lawrence-Doniach (LD) model to analyze fluctuation induced conductivity properties of superconducting samples. Excess conductivity by Aslamazov Larkin (AL) model, its validity to polycrystalline samples (such as compound that used in this study) is limited. However, by using AL model in conjunction with LD model may allow one to describe the excess conductivity properties of this polycrystalline samples [Khan & A. Mumtaz, 2010; Khan et al, 2010]. The models analyze intrinsic information on high temperature superconductivity such as the dimensionality of the conduction channel and also coherence length (Î¾c), interplane coupling (J), and anisotropy (Î³) of the superconducting sample. Thus, it is interesting that, if the influencing of Î¾c, J and Î³ by element substitution at Sr and Cu site in Tl1212 structure to be investigates. However, study of excess conductivity by using Aslamazov Larkin (AL) model in conjunction with Lawrence Doniach (LD) model in Tl1212 has not been reported.
Infrared Absorption Properties
The mechanism of superconducting behavior of the conventional superconductors can be successfully explained by the Bardeen-Cooper-Schrieffer (BCS) theory. This BCS theory was explained based on the electron-phonon interaction at low temperature. As compared to the conventional superconductors, the involvement of electron-phonon interaction mechanism in HTSC is still in debate. However, up until now, there are several groups of researcher investigated the possible electron-phonon interaction and related studies of the phonon and oxygen vibration modes in HTSC [Devereaux et al., 2004; Abd-Shukor, 1992; Hoen et al., 1988]. One of the interesting phonon modes related studies in HTSC are the infrared absorption properties using fourier transform infrared (FTIR) measurement. FTIR study focus on phonon modes related to vibrations of various oxygen atoms in the unit cell. In thallium-based oxide superconductors, absorption peaks connected to oxygen related phonons are observed in the wave number range 400-700 cm-1 [Kulkarni et al., 1990]. The oxygen related phonon modes are specially significant as the possibility of electron phonon interaction in the formation of the Cooper pairs has not been completely ruled out. Furthermore, it has been suggested that the oxygen which is lighter and having the largest vibrational amplitude, is possible to bring about this electron phonon interaction [Khan & Husnain, 2006]. Analyses of these infrared absorption characteristic reveal detail of molecular structure of the sample such as the changes in interplanar distance between CuO2 planes as a result of elemental substitution.
The phonon mode vibration in Tl1212 superconductor (Tl1-xCux)Sr1.6Yb0.4CaCu2O7-âˆ‚ and (Tl0.5Pb0.5)(Sr2-yMgy)(Ca0.8Yb0.2)Cu2O7-Î´ in conjunction with XRD results revealed the elemental substitution strongly affect the inter-plane coupling of the superconductor structure [Ahmad et al., 2009]. The enhanced inter-plane coupling is suggested to lead to a longer Î¾c(0) and thus resulted to an improved superconducting properties. Thus, it is interesting to see the intercoupling effect of elemental substitution at Sr-site or other site such as Cu-site in Tl1212 superconductor and to observe the likelihood of the same phenomenon as has been reported previously. Additionally, there are still very limited studies on the effects of elemental substitution on this oxygen related phonon modes in Tl1212 superconductor.
Thus, in this thesis the effect of Ge4+ substitution at Cu2+ and Sr2+ on superconductivity of Tl0.85Cr0.15Sr2CaCu2-xGexO7-Î´ (x= 0, 0.1, 0.2, 0.3, 0.4 and 0.6) and Tl0.85Cr0.15Sr2-yGeyCaCu2O7-Î´ (y= 0.03, 0.05, 0.08, 0.10, 0.15, 0.20, 0.30 and 0.40) superconductors respectively was investigated. Excess conductivity analysis was carried out using the Aslamazov Larkin (AL) theory as a framework in conjunction with Lawrence-Donaich (LD) theory. FTIR absorption spectra of the compounds were analyzed to investigate changes in phonon modes as a result of the substitutions.
1.4 Objectives of Study
The objectives of this study are:
1) To synthesize the Tl0.85Cr0.15Sr2CaCu2-xGexO7-Î´ (x= 0, 0.1, 0.2, 0.3, 0.4 and 0.6) and Tl0.85Cr0.15Sr2-yGeyCaCu2O7-Î´ (y= 0.03, 0.05, 0.08, 0.10, 0.15, 0.20, 0.30 and 0.40) using the conventional solid state method.
2) To investigate the effect of Ge substitution at Cu-site and Sr-site on superconducting properties of Tl1212 compound.
3) To investigate superconducting fluctuation behavior of Tl0.85Cr0.15Sr2CaCu2-xGexO7 and Tl0.85Cr0.15Sr2-yGeyCaCu2O7-Î´ series by excess conductivity analyses using AL and LD theory.
1.5 Significance of Study
Firstly, the study involves substitution of an element at Cu-site in Tl1212. As has been known, CuO2 planes are the crucial part in the superconductivity of the cuprate superconductor. The substitution of an element at Cu site may introduce an impurity to the CuO2 plane which may result in reduction of hole concentration and thus decrease the superconducting properties. However, the result of this study has showed Ge substitution at Cu-site of Tl1212 incredibly increase in Tc zero for x= 0.1 before decreasing as concentration of Ge substitution increase. The study of excess conductivity behavior and infrared absorption properties provide intrinsic information as these give possible explanation to the above phenomenon.
Secondly, the effect of elemental substitution may introduce a new superconducting series. Substitution of higher valency element to various sites of Tl1212 compound may enhance the value of Tc to a higher value [Sheng et al., 1989b]. The parent compound (Tl0.85Cr0.15)Sr2CaCu2O7 is slightly overdoped with hole carriers [Sheng et al, 1991] and substitution of higher valence Ge4+ to the superconductor sample caused decrease in hole concentration. This study is interesting because it involve substitution of ion, Ge4+ to different sites of Tl1212 series which are the Cu-site and Sr-site. Ge is qualified for these substitution because Ge has higher valency and smaller spacer ion, Ge4+ (ionic radius= 53 pm) compare to the ions at substitution sites. Thus, it is expected to substitute the larger spacer ion Cu2+ (ionic radius= 73 pm) and Sr2+ (ionic radius= 118 pm). These elemental substitutions can be an effective way to stabilize 1212 phase formation and reduce the high average copper oxidation state of 2.5+ to the optimum average copper valence of around 2.25+ [Shukor & Arulsamy, 2000; Hamid et al., 2004]. An optimum average Cu valence between 2.20+ to 2.30+ is expected to exhibit maximum critical transition temperature in this phase [Sheng et al., 1991].
Thirdly, the layered structure of Tl1212 has a small value of coherence length and a large anisotropy as compared to conventional superconductor [Cyrot & Pavuna, 1992]. Due to these properties, the framework of Aslamazov Larkin model [Aslamazov & Larkin, 1968] in conjunction with Lawrence Doniach model [Lawrence & Doniach, 1970; 1971] was used to determine the intrinsic information of this HTSC such as coherence length (Î¾c(0)), interlayer coupling (J) and anisotropy (Î³) of superconducting samples as a result of element substitution. In addition, FTIR studies on layered structure of HTSC are interesting as its providing opportunities to determine changes in bond length and structures of superconducting samples. The result of these studies of normal state behavior on layered structure of Tl1212 superconductor may contribute in understanding of HTSC properties.