|Ph.D Student||Amit Eran|
|Subject||Investigating the Mechanism of High Temperature|
Superconductivity by Oxygen Isotope Substitution
|Department||Department of Physics||Supervisor||Professor Amit Keren|
|Full Thesis text|
The phenomenon of high temperature superconductivity, discovered in cuprates more than 20 years ago, attracts many experimental and theoretical efforts. One of the big challenges is isolating the properties related directly to superconductivity from the unique material properties. In this work we approach this challenge by measuring the physical response of materials to relatively small chemical changes.
In the first part of this work we explore the critical doping variations in cuprates. These are the doping levels at which the compound ground state changes its nature (from an antiferromagnet to a spin glass to superconductor to metal), and they were found to be non-universal. We investigate the origin of these variations by measuring the in-plane oxygen pσ hole density in the CuO2 layers as a function of the oxygen density y in (CaxLa1-x)(Ba1.75-xLa0.25)Cu3Oy (CLBLCO). This is done using the oxygen 17 nuclear quadrupole resonance parameter νQ. We compare compounds with x=0.1 and 0.4 which have significant critical y variations and find that these variations can be explained by a change in the efficiency of hole injection into the pσ orbital.
The second part of this work is the measurement of the way oxygen isotope substitution affects the Néel temperature. Since isotope substitution is probably the smallest perturbation that can be applied in condensed matter physics its results are expected to provide strong experimental constrains on superconductivity theories. We choose to measure the isotope effect in (Ca0.1La0.9)(Ba1.65La0.35)Cu3Oy which allows to exclude the contribution of a change in the number of holes to the effect. We use ?SR and find the absence of oxygen isotope effect on the Néel temperature.
Our first finding, that the critical doping levels are global when the right physical parameter is used, has two direct implementations. The first conclusion is that fictitious effects can be caused if the doping is described by non-physical parameter. This conclusion is also common to our isotope effect measurement: The isotope effect in cuprates can be described as a change in the number of holes, and therefore should not be used to explain the superconductivity mechanism.
The second implementation comes from a better understanding of the CLBLCO phase diagram. In the transition from four distinct phase diagrams into one universal curve we use only measured parameters. The scaling process shows that the maximum TC is determined by the planar antiferromagnetic energy scale of the parent compound.