Charge-density waves (CDWs) are electronic modulations that break translational symmetry and can affect thermodynamic, optical and electronic properties. Typically coupled to a periodic lattice distortion (PLD), CDWs are characterised by parameters such as their amplitude, the wave vector, the phase, correlation lengths etc. of the electronic charge modulation. CDWs preferentially occur in low-dimensional systems with weakly coupled one-dimensional chains or in layered materials. When such layered materials host uniaxial modulations, these can locally break rotational symmetry, forming an anisotropic electronic state.
Electron microscopy is a suitable probe to study the PLD associated with CDWs with high resolution. This technique is exciting because cryogenic electron microscopy is progressively advancing to lower and lower temperatures where quantum effects like high temperature superconductivity can be readily observed.
In particular, cuprates are a class of high temperature superconductors that host CDWs. When doping the Mott-insulating antiferromagnetic parent compound of cuprates with charge carriers (holes or electrons), these materials lose their electrical resistivity as they are cooled below relatively high
superconducting critical temperatures, Tc. The carrier doping does not only lead to superconductivity but also generates a number of complex electronic phases that emerge alongside or in competition with superconductivity. The competition between the CDW and superconductivity and their occurrence in a common doping and temperature range suggest that it is the same electrons that either participate in the CDW or form superconducting Cooper pairs. Our aim is to advance the understanding of the CDW phase with electron microscopy which will push the characterisation of the CDW structure down to the atomic scale at cryogenic temperatures. This will help to improve microscopic theories that also have to explain the origin of the other electronic phases of high temperature superconductors.
Unlike other cuprates which only show short-range CDW correlations, YBa2Cu3Oy (YBCO) hosts a long-range CDW phase. Strong competition with superconductivity prevents long-range order in zero field. Conventionally, the long-range CDW is reached by suppressing superconductivity with high magnetic fields, but recently it has been shown that uniaxial pressure can stabilise the long-range CDW even in zero field.This has opened an exciting avenue to study a truly long-ranged phase that is in fierce competition with superconductivity. How exactly superconductivity and charge ordering compete is largely unknown. Information from most techniques is indirect, as, e.g. the CDW correlation lengths from X-ray diffraction are average values that are determined over the beam size of hundreds of square micrometers. Fortunately, electron microscopy allows to map CDW patches and domains down to the atomic scale in real space and thus visualise the competition of superconductivity and CDWs directly.
This motivates our project “TEMCO” which proposes a “Transmission Electron Microscopy study of Charge Ordering in cuprate superconductors”.
Our objective is to study of the short-range CDW phase of YBCO and to induce the long-range CDW phase by tensile stress. Samples with doping close to p=0.12 (y=6.60) are most suitable as they have the highest CDW onset temperature and the largest CDW amplitude. As the CDW amplitude and correlation length grow with cooling, it is of interest to reach the range of 60-70 K, just above the superconducting Tc, which is rather challenging for electron microscopy. Reaching temperatures below Tc would allow visualising the impact of superconductivity on the CDW at the nano scale. We face the challenge that the c-axis coherence length in between different CuO2 bi-layers that host CDWs is only a couple of unit cells in the short-range CDW phase. Thus signals from layers across the specimen do not necessarily add up coherently and it is unclear whether the CDW can be directly seen by TEM. It might be necessary to improve the CDW signal by tuning the competition between the CDW and superconductivity. This is why we develop a technique to apply tensile stress to the sample.
