Quantum Materials at the Micron Scale - Finite Size Effects and a Twist in Correlations

In the pursuit to develop new and more powerful electronics, semiconductor research has pushed the sample size over many decades to the nanometer scaler now aiming for single electron transistors. By achieving this, new length scales have become more important leading to hydrodynamic, ballistic and quantum transport phenomena. To realize these regimes in strongly correlated electron systems requires samples in the order of a few micrometers even in high quality single crystals. To achieve this, we use focused ion beam (FIB) machining. This gives us control of sample shape, size and current path orientation with nanometer precision.

In the first part of my talk I will show you how finite size confinement allows us to achieve precise current flow in highly anisotropic crystals. With this we can demonstrate the particle-like (ballistic, in-plane) and wave-like (phase coherent, out-of-plane) nature of the electronic transport in the anisotropic, single band metal PdCoO₂ [1,2]. By using angle dependent magnetic field measurements of the out-of-plane transport we can further demonstrate the crossover from finite size to bulk transport regime. I will give an outlook how this finding can open a new route to study the elusive Pseudogap in high purity cuprate superconductors [3].

In the second part I will turn to a topic that has recently attracted much attention - strain tuning in twisted bi-layers of graphene. Translating these types of measurements from thin films to bulk single crystals are challenging in the very least. I will present a novel approach to control higher order strain tuning by means of FIB machining. As candidate material we will look at Cd₃As₂ that like graphene should produce a Pseudomagnetic field when exposed to a strain gradient and at URu₂Si₂ where we have used a twist to tune the electronic correlations.

1: Bachmann, et al. Nat. Comm. 10, 5081 (2019).
2: Putzke, et al. Science 368, 1234 (2020).
3: Putzke, et al. Nature Physics (in print) (2021).