Probing electronic nematicity and anisotropic electron-phonon coupling in strained YBCO nanowires

Abstract

Despite nearly 40 years since its discovery in 1986, the underlying mechanism behind the high-temperature superconductivity (HTS) in cuprates remains a significant enigma in condensed matter physics. The existence of multiple intertwined local orders, originating from the strongly correlated electrons, further complicates the study of these materials. Such complexity of the normal state is depicted in a very intricate temperature-doping phase diagram. A way to advance the knowledge of these materials is to tune the local orders, both in the superconducting and in the normal state, to disentangle them for individual study. One way to achieve such an effect is to apply strain to the cuprates in nm-thick films. Previously, it was discovered that the unidirectional strain, induced by few-unit-cell-thick films deposited on a nanostructured surface, can modify the charge order and cause the in-plane resistivity of the films to become much more anisotropic than in bulk materials. According to the Boltzmann transport model, such anisotropy in the in-plane resistivity is due to the directional modification of the Fermi velocity in which the velocity along one crystallographic in-plane direction is much higher than another. This results in an anisotropic Fermi surface that connects to the presence of an electronic nematicity, wherein the electronic structure retains translational symmetry while spontaneously breaking rotational symmetry. In earlier reports on the archetypal HTS YBa2Cu3O7−δ (YBCO) superconductor, the photoemission and transport measurements appear to show that electron-phonon coupling (EPC) can become directionally suppressed if the Fermi surface becomes nematic (as in our sample). This should strongly affect the heat transport in nm-thick YBCO films. Hence, the focus of this thesis work is to investigate the anisotropic EPC through the study of electrical and heat transport properties of YBCO nanowires oriented along different crystallographic axes. The nanowires are fabricated from YBCO thin films epitaxially grown by the Pulsed Laser Deposition technique, and the strain is tuned by modifying the film thickness.