Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - ABSOLUTESPIN (Absolute Spin Dynamics in Quantum Materials)

Teaser

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...

Summary

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 electron spin as a potential element in information processing. Using the electron 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. The goal is to use time-resolved (pump-probe) measurements to study the spin dynamics and coherence times of single spin systems as well as the spin interactions involved in the decay mechanisms. This can be done best if the decay channels are reduced to an absolute minimum. This will have direct impact on the feasibility of quantum spin information processing with single spin systems on different decoupling surfaces and their scalability at the atomic level.

Work performed

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 110GHz was developed including the design of an ultrahigh vacuum compatible double ended feedthrough, which is not commercially available, as well as 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. We were able to determine the actual amplitude of the microwave in the tunnel junction as we have originally proposed in the project. This provides valuable information, for example, for estimating the expected Rabi frequency in electron spin resonance measurements. Further, 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. 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. The next step will be to apply the microwaves to single spin systems in a magnetic field in order to measure an electron spin resonance signal and move on with the project as planned.

Final results

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 110GHz. We will use 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.