Superconductors is the stuffs that have no opposition to the flow of electric current when it is cooled below a certain temperature called the critical temperature, Tc [ Omar, 1975 ] . Superconductor besides exhibits perfect diamagnetism where when the weak magnetic Fieldss applied to superconductor, the magnetic line are expelled from the inside of superconductor and this phenomenon is called Meissner consequence [ Christman, 1988 ] . The phenomenon of superconductivity was foremost discovered by Onnes ( 1911 ) after he successfully discovered the He gas in 1908. Mercury was the first stuff that observed for superconductivity behaviour and it ‘s superconducting at 4.2 K. Later, many of other metals are discovered to superconducting at low temperature. Not merely are those, the superconducting behaviour on terrible compound besides discovered. Until now, compound of cuprate oxide based are the compound that holding the highest Technetium 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 twelvemonth subsequently, YBCO compound was discovered with Tc is 92 K which the first stuffs that to superconduct at temperature above the boiling point of liquid N ( 77 K ) [ Wu et al. , 1987 ] . Since that, many other probe on assorted type of HTSC based on cuprate superconductor had been investigated. Now, high temperature superconductor ( HTSC ) stuffs can be classify into four chief categories ; Y based ( or other rare Earths based superconductor ) [ Wu et al. , 1987 ; Siegrist et al. , 1987 ] , Bi based [ Xu et al. , 1990 ; Tallor et al. , 1998 ] , Tl based [ Sheng, & A ; Hermann, 1998 ; Parkin et al. , 1988 ; Kaneko et Al, 1992 ] , quicksilver 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 severally [ Park & A ; Synder, 1995 ] . These cuprate superconductors excepting Yttrium based household contains two dimensional CuO2 planes and dividing by insulating beds. The concentration of charge bearers in CuO2 planes are important portion in seeking the optimal superconducting passage temperature.
Formation of Tl1212 stage
Thallium based superconductors are one of the interesting household of HTSC as its have advantage of higher Tc value than the more widely studied YBCO superconductor. Amongst the Tl-based superconductor, TlM2CaCu2O7-I? ( Tl1212 ) and TlM2Ca2Cu3O9-I? ( Tl1223 ) where M= Ba or Sr, are alone stuffs to analyze as it showed better behaviour in magnetic field as compared to other series of Tl-based superconductor such as Tl2M2Ca2Cu3O10-I? , Tl2223 and Tl2M2CaCu2O8-I? , Tl2212 ( M= Ba or Sr ) superconductors [ Jergel et al. , 1996 ] . Tl1212 is an of import member of Tl-based superconductor as its construction is correspondent to the construction of the most widely studied YBa2Cu3O7-I? ( Y123 ) compound [ Nakajima et al. , 1990 ] . Therefore, the similarities and differences between YBa2Cu3O7-I? ( Y123 ) and Tl1212 superconductor can be discussed. Furthermore, the Tl1212 has a shorter insulating distance between the superconducting Cu-O beds than Tl1223 [ Lao et al. , 2000 ] . The short insulating distance is expected to bring forth higher critical current denseness, Jc and better public presentations in magnetic Fieldss due to reduced anisotropy through stronger interlayer yoke and less terrible thermally activated flux gesture [ Lao et al. , 2000 ] .
The TlSr2CaCu2O7-I? superconductor was reported to be superconducting with Tc of around 70-80 K [ Martin et Al, 1989 ] . However, the compound is found to be hard to synthesise in pure signifier due to high mean Cu valency and overdoping of hole bearers [ Subramaniam et Al, 1998 ] . Interestingly, the high mean Cu valency of this unsubstituted Tl1212 superconductor can be lowered to an optimum value of around +2.3 by partial elemental permutations of higher valency ions [ Shukor and Arulsamy, 2000 ; Hamid et al. , 2004 ] which so result to a lessening in hole concentration to optimum value. Decrease of the overdoped holes bearer concentration to the optimum value consequences in stabilisation of the Tl1212 stage [ Hamid et al. , 2004 ] . In Tl1212 layered construction, other beds beside the CuO2 beds are considered as a charge reservoir. These reservoirs are used to accept or donate negatrons into the CuO2 planes. This transportation of the charge, controls the hole concentration in superconducting plane. Most studies of elemental permutation in Tl1212 system were on the assorted elemental permutations at Ca site [ Sheng et al. , 1989a ; Li & A ; Greenblatt, 1989 ] . However, other sites of elemental permutation such as at Sr-site besides play an of import function in the superconductivity of Tl1212.
On the other manus, elemental permutation in topographic point of Cu may travel into the CuO2 planes and caused drastic lessening in critical temperature measuring ( Tc ) [ Kandyel et al. , 2005 ; Enengl et al. , 2002 ; Yang Li et al. , 1999 ; Kuhberger & A ; Gritzner, 2003 ] . For the Tl1212 system, old study on Cu site permutation in Tl0.5Pb0.5Sr2CaCu2-xMxO7 ( M= Co, Ni and Zn ) showed strong suppression of Tc which indicates that superconducting belongingss of the Tl1212 superconductor are sensitive to chemical doping of the CuO2 planes [ Kandyel et al. , 2005 ] . Intriguingly, old surveies on ( Cu, Tl ) -based superconductors ( Cu0.5Tl0.5 ) Ba2Ca2Cu3-yGeyO10-I? and ( Cu0.5Tl0.5 ) Ba2Ca3Cu4-yGeyO10-I? series [ Khan & A ; Irfan, 2008 ] reported an addition in Tc as a consequence of Ge4+ permutation. The Tc sweetening was suggested to be due to the development of assorted planes of CuO2/GeO2 which could be responsible for formation of a more effectual superconducting bed [ Khan & A ; Irfan, 2008 ] . A similar permutation of Ge4+ in Tl1212 may lend to the apprehension of the consequence of assorted CuO2/GeO2 planes on superconductivity, and such work has non yet been reported. Therefore, it would be interesting to see the consequence of Ge4+ permutation at Sr- and Cu- site in Tl1212 superconductor.
Superconducting Fluctuation Behavior of HTSC
The electric conductivity in a superconductor is due to the flow of coupling of charge bearers called Cooper braces [ Glover, 1967 ; Crow et al. , 1970 ; Johnson & A ; Tsuei, 1976 ] . The formation of Cooper braces in conventional superconductor is below the critical temperature, Tc and is good described by the BCS theory. HTSC have short coherency length ( I?c ( 0 ) ) . Coherence length here is defined as the distance between the two negatrons of the Cooper brace [ Cyrot & A ; Pavuna, 1992 ] . HTSCs besides are electrically anisotropic in construction. In contrast with conventional superconductor, the presence of this short coherency length every bit good as extremely anisotropic in HTSC provides possibility to analyze fluctuating superconducting braces above Tc [ Khan et al. , 2010 ] . Therefore, in HTSC the formations of Cooper braces are exist at even above Tc, nevertheless these Cooper braces are at the same time formed and broken as attack to Tc, which so consequence of fluctuation of conduction in normal province belongingss of HTSC.
The temperature dependance of electrical electric resistance for a HTSC samples ( Figure 1.1 ) shows two difference behavioural provinces. The first province corresponds to the normal province that shows a metallic behaviour and the 2nd province is the province characterized by the part of induced fluctuation Cooper pairs to the conduction above Tc [ Christman, 1988 ; Sharma et al. , 1995 ] . This 2nd province part is in temperature scope from the temperature where the electric resistance curve starts to divert off from the projected metallic normal province electric resistance curve upon chilling until to the point of temperature of extremum, Tcp that given by the graph of ( dI?/dT ) versus temperature in Figure 1.1 [ Cardona, 1999 ] . The extra conduction is clearly seeable in this part. This extra conduction part is so analyzed based on the AL theory in concurrence with LD theory for obtained extra conduction belongingss. Figure 1.1 shows the electric resistance measuring and its derivative curve of superconducting sample.
Linear metallic line
Point where electric resistance swerve start to divert from additive metallic line
Resistivity measuring curve
Transmission control protocol
Figure 1.1: The electric resistance measuring and its derivative curve of superconducting sample. The additive fit curves show the background normal province electric resistance projection ( additive metallic line ) . The information scope from the temperature where the curve start to divert from additive metallic line until to the temperature extremum ( Tcp ) , taken for analyses of extra conduction.
HTSC stuffs have much shorter coherency length as compared to conventional superconductors and are electrically anisotropic in construction. A convenient theoretical account that can be used to measure the superimposed anisotropic construction and characteristically shorter c-axis coherency length in HTSC stuffs is the Aslamazov Larkin ( AL ) theoretical account which is used in concurrence with the Lawrence-Doniach ( LD ) theoretical account to analyse fluctuation induced conduction belongingss of superconducting samples. Excess conduction by Aslamazov Larkin ( AL ) theoretical account, its cogency to polycrystalline samples ( such as compound that used in this survey ) is limited. However, by utilizing AL theoretical account in concurrence with LD theoretical account may let one to depict the extra conduction belongingss of this polycrystalline samples [ Khan & A ; A. Mumtaz, 2010 ; Khan et Al, 2010 ] . The theoretical accounts analyze intrinsic information on high temperature superconductivity such as the dimensionality of the conductivity channel and besides coherency length ( I?c ) , interplane yoke ( J ) , and anisotropy ( I? ) of the superconducting sample. Therefore, it is interesting that, if the influencing of I?c, J and I? by element permutation at Sr and Cu site in Tl1212 construction to be investigates. However, survey of extra conduction by utilizing Aslamazov Larkin ( AL ) theoretical account in concurrence with Lawrence Doniach ( LD ) theoretical account in Tl1212 has non been reported.
Infrared Absorption Properties
The mechanism of superconducting behaviour 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 engagement of electron-phonon interaction mechanism in HTSC is still in argument. However, up until now, there are several groups of research worker investigated the possible electron-phonon interaction and related surveies of the phonon and O quiver manners in HTSC [ Devereaux et al. , 2004 ; Abd-Shukor, 1992 ; Hoen et al. , 1988 ] . One of the interesting phonon manners related surveies in HTSC are the infrared soaking up belongingss utilizing fourier transform infrared ( FTIR ) measuring. FTIR survey focal point on phonon manners related to quivers of assorted O atoms in the unit cell. In thallium-based oxide superconductors, soaking up extremums connected to oxygen related phonons are observed in the moving ridge figure scope 400-700 cm-1 [ Kulkarni et al. , 1990 ] . The O related phonon manners are specially important as the possibility of negatron phonon interaction in the formation of the Cooper brace has non been wholly ruled out. Furthermore, it has been suggested that the O which is lighter and holding the largest vibrational amplitude, is possible to convey about this negatron phonon interaction [ Khan & A ; Husnain, 2006 ] . Analysiss of these infrared soaking up characteristic reveal item of molecular construction of the sample such as the alterations in interplanar distance between CuO2 planes as a consequence of elemental permutation.
The phonon manner quiver in Tl1212 superconductor ( Tl1-xCux ) Sr1.6Yb0.4CaCu2O7-a?‚ and ( Tl0.5Pb0.5 ) ( Sr2-yMgy ) ( Ca0.8Yb0.2 ) Cu2O7-I? in concurrence with XRD consequences revealed the elemental permutation strongly impact the inter-plane yoke of the superconductor construction [ Ahmad et al. , 2009 ] . The enhanced inter-plane yoke is suggested to take to a longer I?c ( 0 ) and therefore resulted to an improved superconducting belongingss. Therefore, it is interesting to see the intercoupling consequence of elemental permutation at Sr-site or other site such as Cu-site in Tl1212 superconductor and to detect the likeliness of the same phenomenon as has been reported antecedently. Additionally, there are still really limited surveies on the effects of elemental permutation on this O related phonon manners in Tl1212 superconductor.
Therefore, in this thesis the consequence of Ge4+ permutation at Cu2+ and Sr2+ on superconductivity of Tl0.85Cr0.15Sr2CaCu2-xGexO7-I? ( x= 0, 0.1, 0.2, 0.3, 0.4 and 0.6 ) and Tl0.85Cr0.15Sr2-yGeyCaCu2O7-I? ( y= 0.03, 0.05, 0.08, 0.10, 0.15, 0.20, 0.30 and 0.40 ) superconductors severally was investigated. Excess conduction analysis was carried out utilizing the Aslamazov Larkin ( AL ) theory as a model in concurrence with Lawrence-Donaich ( LD ) theory. FTIR soaking up spectra of the compounds were analyzed to look into alterations in phonon manners as a consequence of the permutations.
1.4 Aims of Study
The aims of this survey are:
1 ) To synthesise the Tl0.85Cr0.15Sr2CaCu2-xGexO7-I? ( x= 0, 0.1, 0.2, 0.3, 0.4 and 0.6 ) and Tl0.85Cr0.15Sr2-yGeyCaCu2O7-I? ( y= 0.03, 0.05, 0.08, 0.10, 0.15, 0.20, 0.30 and 0.40 ) utilizing the conventional solid province method.
2 ) To look into the consequence of Ge permutation at Cu-site and Sr-site on superconducting belongingss of Tl1212 compound.
3 ) To look into superconducting fluctuation behaviour of Tl0.85Cr0.15Sr2CaCu2-xGexO7 and Tl0.85Cr0.15Sr2-yGeyCaCu2O7-I? series by extra conduction analyses utilizing AL and LD theory.
1.5 Significance of Study
First, the survey involves permutation of an component at Cu-site in Tl1212. As has been known, CuO2 planes are the important portion in the superconductivity of the cuprate superconductor. The permutation of an component at Cu site may present an dross to the CuO2 plane which may ensue in decrease of hole concentration and therefore diminish the superconducting belongingss. However, the consequence of this survey has showed Ge permutation at Cu-site of Tl1212 improbably increase in Tc nothing for x= 0.1 earlier diminishing as concentration of Ge permutation addition. The survey of extra conduction behaviour and infrared soaking up belongingss provide intrinsic information as these give possible account to the above phenomenon.
Second, the consequence of elemental permutation may present a new superconducting series. Substitution of higher valency component to assorted sites of Tl1212 compound may heighten the value of Tc to a higher value [ Sheng et al. , 1989b ] . The parent compound ( Tl0.85Cr0.15 ) Sr2CaCu2O7 is somewhat overdoped with hole bearers [ Sheng et Al, 1991 ] and permutation of higher valency Ge4+ to the superconductor sample caused lessening in hole concentration. This survey is interesting because it involve permutation of ion, Ge4+ to different sites of Tl1212 series which are the Cu-site and Sr-site. Ge is qualified for these permutation because Ge has higher valency and smaller spacer ion, Ge4+ ( ionic radius= 53 autopsy ) comparison to the ions at permutation sites. Therefore, it is expected to replace the larger spacer ion Cu2+ ( ionic radius= 73 autopsy ) and Sr2+ ( ionic radius= 118 autopsy ) . These elemental permutations can be an effectual manner to stabilise 1212 stage formation and cut down the high mean Cu oxidization province of 2.5+ to the optimal mean Cu valency of around 2.25+ [ Shukor & A ; Arulsamy, 2000 ; Hamid et al. , 2004 ] . An optimal mean Cu valency between 2.20+ to 2.30+ is expected to exhibit maximal critical passage temperature in this stage [ Sheng et al. , 1991 ] .
Third, the superimposed construction of Tl1212 has a little value of coherency length and a big anisotropy as compared to conventional superconductor [ Cyrot & A ; Pavuna, 1992 ] . Due to these belongingss, the model of Aslamazov Larkin theoretical account [ Aslamazov & A ; Larkin, 1968 ] in concurrence with Lawrence Doniach theoretical account [ Lawrence & A ; Doniach, 1970 ; 1971 ] was used to find the intrinsic information of this HTSC such as coherency length ( I?c ( 0 ) ) , interlayer yoke ( J ) and anisotropy ( I? ) of superconducting samples as a consequence of element permutation. In add-on, FTIR surveies on superimposed construction of HTSC are interesting as its supplying chances to find alterations in bond length and constructions of superconducting samples. The consequence of these surveies of normal province behaviour on superimposed construction of Tl1212 superconductor may lend in apprehension of HTSC belongingss.