CQC2T Seminar (UNSW Sydney): Nagaoka ferromagnetism in a quantum dot plaquette

Speaker – Dr Juan Pablo Dehollain, UTS:

The analytical tractability of Nagaoka ferromagnetism makes it a convenient model to explore the capabilities of quantum simulators of collective electron interactions. However, the small ground-to-excited state energy compared to electron interactions, as well as the difficulty of measuring magnetization in few particle devices, have made the Nagaoka model experimentally unattainable. In this talk, I will describe the experiments leading to the observation of signatures of the ferromagnetic ground state, predicted for 3 electrons in a 4 site square plaquette, engineered using electrostatically defined quantum dots. I will give a short introduction to itinerant magnetism and the discussions behind it, which will lead into Nagaoka ferromagnetism and its manifestation in a 2×2 electron plaquette. I will describe the double-layer technique used to fabricate the quantum dot device, followed by a characterisation of the device features required for this quantum simulation. Finally, I will describe the experimental technique used to prepare the Nagaoka condition and probe the different levels of the spectrum of the system, using singlet-triplet measurements. Apart from observing the ferromagnetic ground state, I will show how we test its robustness under different scenarios of lattice topology, device homogeneity and magnetic flux through the plaquette. This long-sought demonstration of Nagaoka ferromagnetism establishes quantum dot systems as prime candidates for quantum simulators of magnetic phenomena driven by electron-electron interactions.