Periodic Reporting for period 1 - QMOLESR (Addressing molecular spin qubits by ESR-STM)
Berichtszeitraum: 2022-09-01 bis 2024-08-31
The ultimate goal of the project is to develop methods to prepare, manipulate and read the state of molecular spin qubits and explore the fundamental aspects of quantum information processing and quantum sensing.
The entire project is based on the commissioning and use of Electron Spin Resonance Scanning Tunneling Microscopy (ESR-STM), a tool that enables the active spin manipulation. This technique combines the extreme energy resolution of traditional ESR with the spatial resolution, i.e. at the atomic level, of STM. It thus allows coherent manipulation of individual atomic/molecular spin qubits on surfaces.
With this background, the long-term goal of the project is to extend the use of ESR-STM and address spin qubits in an arbitrary environment, e.g. a metal surface. In fact, the majority of ESR-STM studies available in literature are restricted to individual magnetic atoms laying on thin films of MgO grown on Ag(100). The insulating layer of MgO seems to be an essential ingredient to obtain addressable and functional atomic spin qubits. 1) MgO acts as an active material mediating the coupling between microwave radiation and atomic spin qubits. 2) MgO establishes magnetic anisotropy, through the ligand field, providing atoms with the electronic structure such that they can act as spin qubits. 3) It decouples magnetic atoms from the metallic substrate and improves the qubit behaviour. The key idea to achieve the goal of the project and make robust spin qubits is to use metal-organic complexes whose organic structure is designed in such a way to play the role of the MgO layer. The availability of ESR-STM active molecules will open the possibility of realizing new local and non-destructive magnetic sensing methods.
The entire project is based on the commissioning and use of Electron Spin Resonance Scanning Tunneling Microscopy (ESR-STM), a tool that enables the active spin manipulation. This technique combines the extreme energy resolution of traditional ESR with the spatial resolution, i.e. at the atomic level, of STM. It thus allows coherent manipulation of individual atomic/molecular spin qubits on surfaces.
With this background, the long-term goal of the project is to extend the use of ESR-STM and address spin qubits in an arbitrary environment, e.g. a metal surface. In fact, the majority of ESR-STM studies available in literature are restricted to individual magnetic atoms laying on thin films of MgO grown on Ag(100). The insulating layer of MgO seems to be an essential ingredient to obtain addressable and functional atomic spin qubits. 1) MgO acts as an active material mediating the coupling between microwave radiation and atomic spin qubits. 2) MgO establishes magnetic anisotropy, through the ligand field, providing atoms with the electronic structure such that they can act as spin qubits. 3) It decouples magnetic atoms from the metallic substrate and improves the qubit behaviour. The key idea to achieve the goal of the project and make robust spin qubits is to use metal-organic complexes whose organic structure is designed in such a way to play the role of the MgO layer. The availability of ESR-STM active molecules will open the possibility of realizing new local and non-destructive magnetic sensing methods.
Intermediate goals have been defined to implement the project: 1) demonstrate ESR-STM on magnetic molecules on MgO; 2) demonstrate ESR-STM on magnetic molecules in direct contact with a metallic surface/electrode and use molecules as magnetic sensors.
Most of the work carried out involves the commissioning of the ESR-STM measurement technique, fundamental for the future development of the project. This includes preparation of the necessary ingredients for the execution of the experiments, performing calibrations and familiarizing with the ESR-STM measurement methods. All these steps were performed on a reference system, i.e. individual Ti atoms on MgO/Ag(100).
MgO/Ag(100) is essential for achieving functional atomic spin qubits on surfaces and to date seems to be the only system that allows measurement of the ESR-STM signal. Thus, the growth of a bilayer of MgO on Ag(100) was addressed and successfully accomplished. The deposition of Fe and Ti atoms on MgO islands constitutes further progress. Indeed, the measurement of ESR-STM signal requires 1) a magnetic atom/molecule to be measured having a spin doublet ground state, i.e. a spin qubit behaviour, and 2) a spin-polarized tip for the detection of the resonant transition induced by microwaves at appropriate frequency. Previously reported studies have shown that the residual hydrogen present in the vacuum chamber hydrogenates the Ti atoms leading to the formation of Ti-H molecules. Ti-H on MgO has a spin-1/2 and therefore is the simplest magnetic system on which to measure ESR-STM. Spin-polarized ESR-active tips typically have an apex made up of a bunch of iron atoms (~10 atoms). The Fe atoms deposited on MgO were then used to prepare ESR-active tips. STM atomic manipulation was used to pick up/release atoms and assemble a spin-polarized ESR-active tip: precise experimental protocols were established. Calibration of the microwave amplitude at the tunneling junction and determination of the radiofrequency line transfer function are unavoidable steps for calibrating an ESR-STM setup: methods for doing this have been developed and successfully implemented. Preliminary ESR-STM measurements were successfully achieved on Ti atoms on MgO/Ag(100).
Most of the work carried out involves the commissioning of the ESR-STM measurement technique, fundamental for the future development of the project. This includes preparation of the necessary ingredients for the execution of the experiments, performing calibrations and familiarizing with the ESR-STM measurement methods. All these steps were performed on a reference system, i.e. individual Ti atoms on MgO/Ag(100).
MgO/Ag(100) is essential for achieving functional atomic spin qubits on surfaces and to date seems to be the only system that allows measurement of the ESR-STM signal. Thus, the growth of a bilayer of MgO on Ag(100) was addressed and successfully accomplished. The deposition of Fe and Ti atoms on MgO islands constitutes further progress. Indeed, the measurement of ESR-STM signal requires 1) a magnetic atom/molecule to be measured having a spin doublet ground state, i.e. a spin qubit behaviour, and 2) a spin-polarized tip for the detection of the resonant transition induced by microwaves at appropriate frequency. Previously reported studies have shown that the residual hydrogen present in the vacuum chamber hydrogenates the Ti atoms leading to the formation of Ti-H molecules. Ti-H on MgO has a spin-1/2 and therefore is the simplest magnetic system on which to measure ESR-STM. Spin-polarized ESR-active tips typically have an apex made up of a bunch of iron atoms (~10 atoms). The Fe atoms deposited on MgO were then used to prepare ESR-active tips. STM atomic manipulation was used to pick up/release atoms and assemble a spin-polarized ESR-active tip: precise experimental protocols were established. Calibration of the microwave amplitude at the tunneling junction and determination of the radiofrequency line transfer function are unavoidable steps for calibrating an ESR-STM setup: methods for doing this have been developed and successfully implemented. Preliminary ESR-STM measurements were successfully achieved on Ti atoms on MgO/Ag(100).
To pursue the main objective of the project, i.e. extend the use of ESR-STM to spin qubits in arbitrary environment (not necessarily on MgO), part of the activity has been devoted to the search for molecular systems that can encompass MgO functionality and behave as good spin qubits even on a bare metal surface.
The first considered molecule was Fe(OEP)Cl. Some of the properties that make it a good candidate include: 1) the large molecular ligands decouple the central Fe atom from the surrounding environment by preventing direct interaction with the (metallic) surface; 2) the ligand field generated by the chemical environment in which the Fe atom is embedded gives this molecule spin qubit character, i.e. it has spin S=3/2 with ground state Sz=1/2; 3) the internal structure of the molecule provides an electric dipole enhancing coupling with external microwaves fields. A major difficulty encountered with these molecules was their instability on MgO: the interaction with the STM tip, e.g. while scanning or taking a spectrum, makes them move on the surface very easily or jump on the tip itself. This makes the conditions to achieve ESR-STM difficult at least when Fe(TPP)Cl molecules lie on MgO. One strategy for the future is to measure Fe(TPP)Cl in direct contact with Ag(100).
The second molecule considered was CoCp2. These molecules, similarly to Fe(TPP)Cl, meet the project requirements. However, it was found that CoCp2 molecules decompose on the Ag(100) surface. This makes their use on MgO/Ag(100) or directly on Ag(100) impossible. These results show that molecules to be used as spin qubits must be chosen carefully. They also confirm the need to overcome the MgO/Ag(100) system, which seems to be the limiting factor for molecular systems.
The first considered molecule was Fe(OEP)Cl. Some of the properties that make it a good candidate include: 1) the large molecular ligands decouple the central Fe atom from the surrounding environment by preventing direct interaction with the (metallic) surface; 2) the ligand field generated by the chemical environment in which the Fe atom is embedded gives this molecule spin qubit character, i.e. it has spin S=3/2 with ground state Sz=1/2; 3) the internal structure of the molecule provides an electric dipole enhancing coupling with external microwaves fields. A major difficulty encountered with these molecules was their instability on MgO: the interaction with the STM tip, e.g. while scanning or taking a spectrum, makes them move on the surface very easily or jump on the tip itself. This makes the conditions to achieve ESR-STM difficult at least when Fe(TPP)Cl molecules lie on MgO. One strategy for the future is to measure Fe(TPP)Cl in direct contact with Ag(100).
The second molecule considered was CoCp2. These molecules, similarly to Fe(TPP)Cl, meet the project requirements. However, it was found that CoCp2 molecules decompose on the Ag(100) surface. This makes their use on MgO/Ag(100) or directly on Ag(100) impossible. These results show that molecules to be used as spin qubits must be chosen carefully. They also confirm the need to overcome the MgO/Ag(100) system, which seems to be the limiting factor for molecular systems.