טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentShahar Levy
SubjectDevelopment of a Nuclear Quadrupole Based Technique for
Measuring Charge Homogeneity, and its Application
for YBCO
DepartmentDepartment of Physics
Supervisors Full Professor Keren Amit
Professor Emeritus Polturak Emil


Abstract

The phenomenon of high temperature superconductivity (HTSC) has focused a huge research effort for the last two decades. Yet the mechanism for HTSC is not clear yet. The experimental evidence points to the fact that the CuO2 planes play a major role in this mechanism, therefore much of the theoretical work concentrates on the contribution of those planes to superconductivity. One of the dominant theories bases the superconductivity mechanism on the creation of charge stripes in those planes (the stripes theory). The theory is supported by experimental results, obtained first by neutron scattering, and latter supported by many other techniques, like NQR, STM and mSR, and their existence is no longer doubted. The important question is whether they are crucial for the mechanism of superconductivity.

Other theories differ from the theory of stripes in that they don't take the inhomogeneity inside the planes to be involved in the mechanism of HTSC. A way to check the validity of those theories is by measuring Tc, the superconductivity phase transition temperature, as a function of the charge inhomogeneity in the planes. The dependence of Tc on doping is known experimentally. Therefore, by measuring the inhomogeneity as a function of doping we can achieve important information regarding the relation between Tc and homogeneity.

However, there are some experimental problems. With all those techniques we've mentioned it is still difficult (if possible) to obtain data on the inhomogeneity level in the planes for the entire doping range. We chose to attack the problem with a new NQR technique we developed especially for this purpose. We prove, experimentally and theoretically, that this method can addresses the above question.

This work deals with two aspects. Firstly is the development of the new technique, which we call angle dependent NQR (ADNQR), and checking experimentally that the method indeed works for known cases. Secondly, we applied the technique for YBCO, a type of high temperature superconductor, and measured the dependence of the charge inhomogeneity on doping. We found that as doping increases inhomogeneity decreases, namely, there is anti correlation between Tc and inhomogeneity. This result contradicts the importance of stripes for high temperature sperconductivity.