A Superconductor Without Fixed Tc



16 March 2010
Superconductors.ORG


       Since the first high temperature superconductor was discovered in 1986, more evolved compounds and structures have been discovered. Those discoveries have taken us far beyond the 30K transition temperature that Müller and Bednorz found in LBCO. However, applying planar weight disparity to the elemental components of LBCO (lanthanum, barium, copper and oxygen) has produced something unexpected - a superconductor without fixed Tc.

       Atomic lattices like the 1212C structure of YBCO offer a unique opportunity to test lanthanum and barium in a proven high-temperature lattice. Through substitution and addition of two calcium atoms and a copper plane, the 1212C unit cell increases in size to a 1223C (shown at left). Planar weight disparity (PWD) now exists between the upper copper chain and lower copper plane. PWD also exists between the upper barium/lanthanum atoms and lower calcium atoms. In theory this should increase Tc, so long as the insulating layer continues to be hole-doped. However, with the substitution of La, the insulating layer is now hole-deficient.

       So what results when PWD is high, but the Mott transition to a metallic state is undermined by electron doping of the insulating layer?


       Multiple resistance tests of LaBaCa2Cu4Ox revealed two things: First, quasi-reentrant behavior below Tc, similar to that of TmNi2B2C. A sharp resistive transition is followed by a slow rise in resistance as the temperature goes lower. Then a second feature that's even more surprising: Tc is not fixed. In a series of four tests, Tc presented at 134K, 131K, 138K, and 148K. This is evidenced in the above four plots which show a sharp resistive transition shifting relative to the stationary "hump" near 106K. Since the test conditions remained the same throughout the four tests, this shift in Tc cannot be explained.

       Since this structure has 8 layers, an attempt was made to create a 6-layer analog with comparable PWD, but without a CuO2 plane (below left). The result was LaBaSrCu3Ox. Its resistance plot also contained a resistance "hump" near 106K (below right). But, there was no indication of superconductivity.


        

       Clearly the mechanism that facilitates high-temperature superconductivity is complex. This discovery of a superconductor without fixed Tc, suggests that yet another variable is at work.

       Synthesis of LaBaCa2Cu4Oxwas by the solid state reaction method. Stoichiometric amounts of the below precursors were mixed and pressed at 70,000 psi and sintered for 24 hours at 775C; then for another 11 hours at 890C. Lastly, the pellet was annealed for 10 hours at 500C in flowing O2.

La2O3  99.99%   (Alfa Aesar)  3.26 grains
BaCuOx   99.9%   (Alfa Aesar)  4.34 grains
CaCO3   99.95%   (Alfa Aesar)  4.00 grains
CuO   99.995%   (Alfa Aesar)  4.77 grains

RESEARCH NOTE: Copper-oxides have been found to be strongly hygroscopic. All tests should be performed immediately after annealing.

- E. Joe Eck
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