Ph.D Thesis


Ph.D StudentRinat Ofer
SubjectUnderdoped Cuprates: Detection of Charge Inhomogeneity
and the Transition to Magnetic Order
DepartmentDepartment of Physics
Supervisor Full Professors Keren Amit
Full Thesis textFull thesis text - English Version


Abstract

This work consists of two separate parts. In the first part we measure charge inhomogeneity in the CuO2 planes of the cuprate compound YBa2Cu3Oy (YBCO) at different doping levels. A key challenge in the cuprates study is determining whether the charge inhomogeneity found in some of these compound, is an essential part of the mechanism of superconductivity. Our objective in this part is to detect possible charge inhomogeneity in underdoped YBCO, and see if there is a correlation between the electronic structure of the CuO2 planes and the dopant atoms. This will allow us to tell if models that predict inhomogeneity without taking into account the dopant, are relevant to high temperature superconductors or not.

We perform Nuclear Magnetic Resonance (NMR) and Quadrupole Magnetic Resonance (NQR) measurements, on fully 63Cu enriched YBCO. We compare a new method, Angel Dependent NQR (ADNQR), with the nutation spectroscopy NQR and the conventional NMR. We show that the ADNQR is most sensitive to the inhomogeneity. Finally, we conclude that any charge inhomogeneity in the CuO2 planes is found only in conjunction with oxygen deficiency in the chains.

Upon further underdoping, the cuprates lose their inhomogeneity and become magnetically ordered. In the second part of this thesis we use muon spin rotation to determine the zero temperature staggered antiferromagnetic order parameter M0 versus hole doping measured from optimum Dpm, in the (CaxLa1-x)(Ba1.75-xLa0.25+x)Cu3Oy system. In this system the maximum TC and the superexchange J vary by 30% between families (x). M0(x, Dpm)  is found to be x-independent. Using neutron diffraction we determine the lattice parameters variations for all x and doping, and estimate the hopping rate t and J from simple structure considerations.

We show that the origin of different energy scales between the CLBLCO families is mainly the different in-plane buckling angles. Comparison between the t/J ratio extracted from the neutron measurements and the doping dependence of the order parameter, suggests that at zero temperature for this compound the order parameter as a function of mobile holes is independent of t/J.

We offer two possible explanations. One is that at low temperatures the effective Hamiltonian is given by a t-J model but with an effective t that is proportional to J. The second is that the destruction of the AFM order parameter is not a result of single holes hopping and should be described by a completely different Hamiltonian, perhaps hopping of boson pairs.