Reshaping the phase diagram of YBa2Cu3O7-δ through strain in ultrathin films and nanowires

Doctoral thesis, 2021

This thesis aims at adding important pieces to the puzzle of understanding the physics of the High critical Temperature Superconductor (HTS) cuprates, where despite over 30 years of intense research many open questions remain. The HTS cuprates are characterized by an incredibly complex phase diagram with multiple intertwined local orders, such as charge density waves (CDW). The superconducting state originates from an enigmatic state called "strange metal". One of the defining properties of the strange metal is a surprisingly simple linear temperature dependence of the resistivity, a possible consequence of the strong electron-electron correlations in these materials. This behavior cannot be accounted for by the conventional theories of electric transport and calls for new theoretical and experimental approaches that can give more hints about its true nature.

This thesis describes new experiments that are able to highlight the physics of the strange metal by studying the physical mechanisms that lead to its breakdown, using nanoscale \newline YBa2Cu3O7-δ (YBCO) thin films and devices.

So far, a detailed temperature-doping phase diagram describing the complex properties of the HTS cuprates was available only for single crystals. The first part of the thesis shows that we can reproduce all the main features of the HTS phase diagram for YBCO thin films and nanowires. By reconstructing the surface of the substrates, with high temperature annealing, we are able to grow highly strained, untwinned films. These films are instrumental for studying anisotropic transport properties of both the strange metal and the superconducting state.

In the second part of the thesis we have studied the evolution of the $T$-linear resistivity in the strained films as a function of the thickness and of the doping. In ultrathin and underdoped YBCO films the strange metal phase is restored when the CDW order, detected by resonant inelastic X-ray scattering, is suppressed. This observation points towards an intimate connection between the onset of CDW and the breakdown of the $T$-linear resistivity in underdoped cuprates, a link that was missing until now.

Finally, the thesis describes how the phase diagram of YBCO is reshaped for thin films and devices at the nanoscale, and in particular how the superconducting transition is enhanced by the suppression of CDW order in the underdoped regime. We also show that the dynamics of the phase-slip phenomenon in ultrathin nanowires becomes very different in the direction where the CDW order is suppressed. These results highlight the competing nature of superconductivity and charge order.

Overall, the research presented in the thesis work, demonstrates how strain control and nanoscale dimensions allow to manipulate the ground state of HTS which is an important step to disclose the mechanism for high critical temperature superconductivity.