Periodic Reporting for period 4 - SPINCAD (Spin correlations by atomic design)
Okres sprawozdawczy: 2021-02-01 do 2021-07-31
Quantum mechanics, while fully worked-out and understood in fundamental terms, gives rise to completely unpredictable material properties on length-scales exceeding tens of atoms. A prime example of such emergent behaviour is found in the field of magnetism. In this research, test structures are composed of tens to hundreds of magnetic atoms coupled to each other in chains or two-dimensional structures. This is done using a technique called scanning tunnelling microscopy (STM). The main objective is to learn how collective spin excitations propagate through such extended structures. In order to do this, we will develop local magnon detectors (also built from individual atoms) that allow excitations to be measured at a different location from where they were created. As such, the dynamics of excitations can be explored.
Once successful, the research will contribute to our understanding of quantum magnetism in materials. In addition, it progresses our capabilities of building and operating spin-based circuitry on the atomic scale.
Conclusion upon completion: a working magnon detector was developed and built. The detector was composed of 11 atoms, and was equipped with sensor, readout, memory and reset functionality. When connected to input chains of various lengths, it became possible to measure the propagation extent of magnons. Due to interference of the magnons (spin waves) with itself, for some locations in the input chain the probability of the magnon reaching the detector was minimized. This interference effect was observed and was rationalized though time-dependent quantum mechanical calculations.
Once successful, the research will contribute to our understanding of quantum magnetism in materials. In addition, it progresses our capabilities of building and operating spin-based circuitry on the atomic scale.
Conclusion upon completion: a working magnon detector was developed and built. The detector was composed of 11 atoms, and was equipped with sensor, readout, memory and reset functionality. When connected to input chains of various lengths, it became possible to measure the propagation extent of magnons. Due to interference of the magnons (spin waves) with itself, for some locations in the input chain the probability of the magnon reaching the detector was minimized. This interference effect was observed and was rationalized though time-dependent quantum mechanical calculations.
Initial focus in this project has been on our abilities to build the atomic scale components needed for our experiments. While previous experiments in this field were often limited in size due to material constraints, we have developed a new preparation technique that allows us to build arrays that may extend, in principle, to hundreds of nanometres in size.
Next, we developed the magnon detector. We diverted from the original design by adding a ‘counter balance’ lead to cancel the effective magnetic field due to the input lead. As a result, the detector became symmetric: it had no intrinsic preference for either of the two magnetization states, making it optimally sensitive to incoming magnons. The counter balance could also be used as a ‘reset button’, allowing the experiment to be re-initialized after the detector had sprung.
Apart from the magnon detector, we also focused on the physics of the magnetic atoms used. In particular, we studied the behavior of individual iron atoms positioned in specific high-symmetry locations on our crystal surface. We discovered that, upon sending a current through the atoms, in addition to well-known spin excitations a new excitation emerged at relatively high energy. After analysis and comparison to theoretical modelling, we managed to identify this excitation as a complete reversal of the atom’s orbital angular momentum. At first it was surprising that a single electron could cause such a big change in angular momentum. Eventually, this could be explained though in terms of the so-called Einstein – de Haas effect.
Next, we developed the magnon detector. We diverted from the original design by adding a ‘counter balance’ lead to cancel the effective magnetic field due to the input lead. As a result, the detector became symmetric: it had no intrinsic preference for either of the two magnetization states, making it optimally sensitive to incoming magnons. The counter balance could also be used as a ‘reset button’, allowing the experiment to be re-initialized after the detector had sprung.
Apart from the magnon detector, we also focused on the physics of the magnetic atoms used. In particular, we studied the behavior of individual iron atoms positioned in specific high-symmetry locations on our crystal surface. We discovered that, upon sending a current through the atoms, in addition to well-known spin excitations a new excitation emerged at relatively high energy. After analysis and comparison to theoretical modelling, we managed to identify this excitation as a complete reversal of the atom’s orbital angular momentum. At first it was surprising that a single electron could cause such a big change in angular momentum. Eventually, this could be explained though in terms of the so-called Einstein – de Haas effect.
Having studied the dynamics of atomic magnetism in space, using the magnon detector, we proceeded to also study the behaviour in time. To this end, we adopted and optimized a recently developed technique, ESR-STM, allowing individual atomic spins to be brought in resonance with a radio signal in the GHz frequency range.
In particular, we focused on two titanium atoms positioned in such close proximity that they could feel each other’s dipolar and exchange magnetic fields. By adjusting the height of the magnetic probe tip over one of the two atoms, we were able to bring the two magnetic moments exactly in tune, meaning that they experienced the exact same local magnetic field. From that point onward, the two atoms were able to entangle. We then inverted the magnetic moment of the atom underneath the tip by means of a current pulse, initializing a coherent flip-flop motion that we could observe by means of a pulsed measurement scheme. This was a surprising result, as it demonstrated that the coherence of atomic spin states was not destroyed by the (incoherent) current pulse.
In particular, we focused on two titanium atoms positioned in such close proximity that they could feel each other’s dipolar and exchange magnetic fields. By adjusting the height of the magnetic probe tip over one of the two atoms, we were able to bring the two magnetic moments exactly in tune, meaning that they experienced the exact same local magnetic field. From that point onward, the two atoms were able to entangle. We then inverted the magnetic moment of the atom underneath the tip by means of a current pulse, initializing a coherent flip-flop motion that we could observe by means of a pulsed measurement scheme. This was a surprising result, as it demonstrated that the coherence of atomic spin states was not destroyed by the (incoherent) current pulse.