Year of Publication

2004

Document Type

Dissertation

College

Arts and Sciences

Department

Physics and Astronomy

First Advisor

Kwok-Wai Ng

Abstract

One of the challenges facing condensed matter physics nowadays is to understand the electronic structure of high temperature superconductors. This dissertation compiles our contribution to the experimental information concerning this subject. Tunneling conductance spectroscopy a technique capable of probing the electronic density of states in hybrid structures was used to study the current and spin transport properties across junctions between metallic counterelectrodes and Bi2Sr2CaCu2O8- (BSCCO) crystals. Since in these structures the transport is mediated by transmission channels depending on superconductive characteristics, the energy resolved density of states is a signature of the mechanism of superconductivity. For instance, one can observe the superconductive energy gap and the behavior of subgap bound states due to phase sensitive Andreev reflections at the junction interface. In particular, tunneling spectroscopy makes possible the observation of the LOFF state characterized by the coexistence of superconductivity and magnetism. Cuprates like BSCCO are highly anisotropic materials and their superconductivity is almost two dimensional, being confined in the CuO2 planes. Therefore, our junctions combine monocrystals of underdoped samples of BSCCO with various thin film counterelectrodes normal metal (Ag), conventional superconductor (Pb) and ferromagnetic metal (Fe) deposited perpendicular onto the cuprate ab-plane (CuO2 plane). We performed measurements on Ag/BSCCO junctions for two current injection directions into the same crystal. We observed that, near the 110 crystal surface, the conductance spectra show a high zero bias peak (ZBCP) which is a manifestation of zero energy Andreev bound states due to an anisotropic superconductive order parameter. Near the 100 surface, the ZBCP is largely suppressed. This is consistent with a predominantly 2 2 x y d - -wave pairing symmetry. In some cases, the ZBCP splits or decreases in amplitude at low temperatures. This is consistent with the existence of a subdominant s-wave (or xy d ) resulting in a mixed d is + state which breaks time reversal symmetry (BTRS). Since we observe this phenomenon in the underdoped case, we do not confirm the possibility of a quantum critical point close to the optimal doping. Our Pb/BSCCO spectra contradict the theory explaining the BTRS by proximity effect. The Fe/BSCCO junctions measure the effect of spin polarization. We explain the recorded 4-peak asymmetric structure by the combined effect of a spin independent BTRS state and a spin filtering exchange energy in the barrier responsible for a large ZBCP splitting. The LOFF state was observed in the proximity region induced on the ferromagnetic side of multilayered-Fe/Ag/BSCCO structures. As expected for the LOFF order parameter, the spectra develops coherent damped oscillations with the Fe layer thickness probing different regions. The magnitude and sign of the oscillation depends on the energy. The conductances at energy zero or equal to the superconductive gap are modulated in antiphase proving that the order parameters takes successively positive and negative values. Changing the junction orientation with 4 p , results in an opposite behavior for the same distance. The maximal amplitudes in one direction is replaced by minima, showing that, besides space, the LOFF state modulation depends on the phase of the high temperature order parameter inducing the proximity

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