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.


strange metal

High-Tc superconductors





charge density wave

Kollektorn (A423), MC2, Kemivägen 9
Opponent: Dr. Cyril Proust, Laboratoire National des Champs Magnétiques Intenses Toulouse, France


Eric Wahlberg

Chalmers, Microtechnology and Nanoscience (MC2), Quantum Device Physics

Although more than 30 years have passed since the discovery of high critical temperature superconductivity (HTS) in the copper oxides many of their properties remain unexplained. This is especially true for the normal metal state out of which superconductivity emerges. At room temperature the metallic state is commonly referred to as a “strange metal”. One of the defining properties of this state is a simple linear in temperature behavior of the resistivity, which cannot be accounted for by the conventional theories of electric transport in metals. In the temperature range between the strange metal and the superconducting state multiple electronic orders develop, where charge density wave (CDW) order is the most prominent one.  Understanding the relations and intertwining of these different orders can be seen as the key to disclose the mechanism behind high temperature superconductivity.

In this thesis we use YBa2Cu3O7-δ thin films and nanowires to study the evolution of the strange metal, CDW and HTS phases with strain and nanoscale dimensions. The aim is to get new insights about their intertwining by tuning the electronic ground state of the material. We show that the CDW plays an active role in breaking down the strange metal state and that transport measurements in thin films and nanostructures can give new hints about the competing nature of the CDW and superconducting state. These results add an important piece to the puzzle of high temperature superconductivity.

Oxide Nanoelectromechanical Systems for Ultrasensitive and Robust Sensing of Biomagnetic Fields (Oxinems)

European Commission (EC) (EC/H2020/828784), 2019-05-01 -- 2023-04-30.

Functional Dirac Materials

Knut and Alice Wallenberg Foundation (KAW.2013.0096), 2014-07-01 -- 2019-06-30.

Areas of Advance

Nanoscience and Nanotechnology


Basic sciences

Subject Categories

Nano Technology

Condensed Matter Physics


Nanofabrication Laboratory



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5026



Kollektorn (A423), MC2, Kemivägen 9


Opponent: Dr. Cyril Proust, Laboratoire National des Champs Magnétiques Intenses Toulouse, France

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