J. Electromagnetic Analysis & Applications, 2010, 2: 63-74 63
doi:10.4236/jemaa.2010.22010 Published Online February 2010 (www.SciRP.org/journal/jemaa)
A Search for a Low Anisotropic Superconductor
A. A. Khurram1, Nawazish A. Khan2
Experimental Physics Labs, National Centre for Physics, Quaid-i-Azam University, Islamabad, Pakistan; 2Materials Science Labo-
ratory, Department of Physics, Quaid-i-Azam University, Islamabad, Pakistan.
Received July 27th, 2009; revised September 12th, 2009; accepted September 17th, 2009.
In this article, we have given a review of thallium based (TlmBa2Can-1CunO2n+2) and copper based (CuBa2Can-1 CunO2n+4)
high temperature superconductors. This review article is based on the methods of preparation in the form of bulk and
thin films and their physical properties. We have also studied the sources of anisotropy and methods of lowering it via
changing the composition of charge reservoir layer and developing a better inter-plane coupling.
Keywords: Review Article, TlmBa2Can-1CunO2n+2 and CuBa2Can-1CunO2n+4 Superconductors, Synthesis, Physical
1. Introduction prepared at normal pressure by substituting thallium at
copper site in CumBa2Can-1CunO2n+2 superconductors.
One of the general features of high temperature super- These superconductors have anisotropy parameter
conductors is the anisotropy parameter [ = c/ ab= ab/ c] ( =3-11) in between that of Cu and Tl based supercon-
which comes from their layered structure. The techno- ductors. In this article we have first reviewed the meth-
logical critical temperature and irreversibility field which ods of preparation and physical properties of thallium
play very important role in the use of high Tc supercon- based superconductors and then the copper based su-
ductors in various applications are strongly related to the perconductor. We have also shown that the Tl-1223 as
anisotropy parameter as Z==Tc/ and Hirr=Hc2/ respec- well Cu-1223 superconductors is more promising mem-
tively The understanding of the sources of anisotropy in bers of their families. At the end the end of the article the
these layered cuprates is essential to accomplish the dir- superconducting properties of (Cu1-xTlx) Ba2Ca2Cu3O10-
ely needed isotropic superconductor; i.e. like BCS su- are given.
perconductor. Therefore, it is very important to search for
a superconducting material which has higher critical 2. Thallium Based Superconductors
temperature, Tc, critical current density and as well as
The thallium based Tl-Ba-Ca-Cu-O (TBCCO) supercon-
lower anisotropy. The thallium based superconductors
ductor was discovered in 1988 by Sheng and Hermann
TlmBa2Can-1CunO2n+2 (m=1, 2; n=1, 2, 3, 4, 5) are superior
[1,2–6] which is proven to be the best among all other
over other cuprates due to their higher Tc, Jc, Hirr and low
microwave surface resistances. The maximum critical cuprates due to their higher transition temperature,
temperature achieved in both the families of thallium [Tc(0)], transport critical current densities, [Jc] and low
based superconductors is 123K and 127K and current microwave surface resistances, [Rs]. Since the discovery
densities are of the order of 106 A/cm2. Another family of of superconductivity in this system, several new phases
oxides which has comparable superconducting character- of TlmBa2Can-1CunO2n+2 (m=1, 2; n=1, 2, 3, 4, 5) have
istics as thallium based compounds but with lower ani- been synthesized [7,8–10]. In the two-layer thallium
sotropy is CumBa2Can-1CunO2n+2 (m=1 n=1, 2, 3, 4, 5). compounds, the Tl2Ba2Cu1O6, Tl2Ba2Ca1Cu2O8, Tl2Ba2
The major disadvantage associated with these supercon- Ca2Cu3O10, Tl2Ba2Ca3Cu4O10, and Tl2Ba2Ca4Cu5O8 are
ductors is their preparation under very high pressures i.e. major phases, which can be simply represented as, Tl-2201,
4GPa, which makes their large scale production difficult. Tl-2212, Tl-2223, Tl-2224 and Tl-2245 with critical
However, the superconducting properties of TlmBa2Can-1 temperatures of 95, 118, 127, 112 and 105K respectively.
CunO2n+2 and CumBa2Can-1CunO2n+2 are combined in a The maximum critical temperature of ~127K is found in
new superconductor family which is represented as Tl-2223 superconductor [11,12]. On the other hand the
(Cu1-xTlx)Ba2Can-1CunO2n+2. These superconductors are monolayer Tl-based system includes TlBa2Ca1Cu2O7,
Copyright © 2010 SciRes JEMAA
64 A Search