Periodic Reporting for period 4 - ABSOLUTESPIN (Absolute Spin Dynamics in Quantum Materials)
Période du rapport: 2021-01-01 au 2022-06-30
The world of nanoscience has the objective to explore structures and phenomena at the smallest dimensions. With the continuing trend to miniaturize devices, an obvious question is regarding its limits. How small can we go? This is a principal question that has to be answered before applications can be designed. Richard Feynman said that there is plenty of room at the bottom, referring to the nearly endless possibilities at the nanoscale. However, his statement also implies that there are limits to miniaturization, which have to be thoroughly explored. Knowing the limits will give insight into possible miniaturization processes. At the same time, reducing a system to its essential minimum provides a scientific opportunity to study different phenomena in their purest form, i.e. without interference from other effects. In this project, we address the spin as a potential element in information processing. Using the spin for storing and processing information is challenging due to principal limits in lifetime and decoherence times. The idea is to address single spin systems (atoms or molecules) in an isolated environment in order to assess these principal limits. However, new concepts for measuring single, individual spin systems are needed. In this sense, the main objective of this project is to combine the two well-established techniques electron-spin resonance spectroscopy as well as time- and spin-resolved scanning tunneling microscopy (STM) to explore the dynamics of spins in individual atoms and molecules. We exploit the capabilities of the STM to locally resolve at the atomic scale and to manipulate and move atoms and molecules. This provides excellent control over the spin interactions allowing us to isolate single spin systems, but also induce specific interactions through proximity between different spin systems. We have found that the bias voltage in the STM can be exploited as an additional degree of freedom to tune the transitions of the spin system through spin electric coupling. This provides a hitherto unknown mechanism for fast electrical tuning of spin transitions and interactions at the atomic scale and a pathway for an electrical manipulation of spins.
This project is both scientifically and technically challenging, such that the first half of this period was dedicated to installing and commissioning a new scanning tunneling microscope that is optimized for high magnetic fields to create the Zeeman splitting in the single spin system as well as radiating microwaves into the tunnel junction in order to excite the spin system. A new concept for introducing microwaves with frequencies up to 100GHz was developed including a focused antenna that radiates the microwaves towards the tunnel junction for most efficient coupling. The introduction of microwaves into the tunnel junction has turned out to be very efficient such that we have, in addition to the project's goals, explored the potential of microwave assisted tunneling in the STM. The microwave source extends the capabilities of the experiment and gives access to sample and junction properties, which are unavailable in conventional STM setups, in particular concerning coherent processes addressing the behavior of the phase during the tunneling process. The tunable frequencies of the microwaves also provide a new energy axis for spectroscopic measurements with the potential to reach a much higher energy resolution than along the conventional bias voltage axis. We have investigated the intricate interplay of different coherent processes during quasiparticle tunneling between superconductors. We found strong interference effects between coherent processes such as multiple Andreev reflections and microwave assisted tunneling, which cannot be modeled as a superposition between different independent processes, but have to be modeled in a one-step model combining both processes at the same time. Going beyond the ESR-STM concept and the outlines of this project, we devised a spin-off STM experiment to directly manipulate different atomic scale systems with microwaves. More specifically, we are manipulating Yu-Shiba-Rusinov states with microwaves to test their suitability as a few-level system for direct manipulation. After having demonstrated the principal capabilities of our ESR-STM in the so far unique frequency range between 60GHz and 100GHz, in the second half of the project, we have entered the scientific part of the project. We have found that the applied bias voltage in the STM induces an electric field that directly impacts the properties of the spin system. The dipole of a TiH molecule feels an electric force, which changes the molecule-substrate coupling, which in turn changes the g-factor of the spin system. In this way, we have identified a new mechanism of spin-electric coupling at the atomic scale, which provides the future prospective for direct electrical manipulation of spin transitions, one of the chief goals in spintronics. In general, electric fields are much easier to handle (fast switching, spatial containment) than magnetic fields, so that we see this as the achievement of this project with the highest potential impact in the near future. Furthermore, we have explored the suitability of Yu-Shiba-Rusinov (YSR) states for spin-based few level systems, where we can report two major achievements. The first one is concerned with transport between two YSR states, where we have shown that a very efficient tunneling current (YSR-YSR tunneling) can be passed through two YSR states even at extremely high tunnel junction resistances. As the YSR states present single energy levels, this YSR-YSR tunneling constitutes a minimal tunnel junction. The second achievement is concerned with the identification of the YSR ground state as it is a priori not easily possible to see if the system is in the free spin regime or the screened spin regime. We have exploited the Josephson effect to create essentially the smallest SQUID and use that to identify the YSR ground state through the presence or absence of a supercurrent reversal (π-junction).
We have developed a unique scanning tunneling microscope at a base temperature of 310mK that allows for illuminating the tunnel junction with microwave radiation at high frequencies up to 100GHz. We are using this setup to do high field electron spin resonance spectroscopy on single spin systems such that the Zeeman splitting is much larger than the thermal excitation energy. In this way we create self-initializing systems that are ideally suited for pump-probe measurements to study the spin dynamics, spin interactions, and coherence times on a local scale. We have demonstrated that the bias voltage can be exploited as a new degree of freedom for tuning and manipulation of spin transitions thus enabling the long standing goal in spintronics of spin-electric control at the atomic scale. Furthermore, we have demonstrated the smallest (minimal) tunnel junction through tunneling between two single energy levels and we have demonstrated a supercurrent reversal in an atomic scale YSR state by constructing the smallest SQUID allowing interference between two transport channels in the same tunnel junction. This introduces a rudimentary phase sensitivity in the STM, which was previously not possible.