Periodic Reporting for period 1 - PCSV (Point contacts for quantum spin valleytronics (PCSV))
Okres sprawozdawczy: 2021-06-01 do 2023-05-31
Since the discovery of the transistor in 1948, semiconductor electronics have changed our lives in unprecedented ways. The enormous miniaturization of transistors, which has triggered an exponential increase of the world’s computational power, is reaching a fundamental limit. In this context, the achievement of complex operations by alternative approaches is crucial for the future of computation. A recently developed alternative relies on the quantum entanglement between the ultimately small quantum dots (QD) and has already proved to be faster than conventional computations for certain operations. However, the realization of complex operations relies on the transfer of quantum information, which is a bottleneck due to the destructive effects of interactions with the environment.
In this context, van der Waals heterostructures made of bilayer graphene (BLG) and hexagonal boron nitride (hBN), where information can be stored in the spin and valley degrees of freedom, arise as new platforms for quantum coherent transport. Their long spin and valley coherence times make these systems promising for quantum computing but electronic transport in quantum coherent BLG devices needs further experimental studies. For this purpose, PCSV studies electrostatically defined quantum point contacts (QPCs) in BLG-based novel device platforms where charge transport between the QPCs occurs in a ballistic manner. The results show that electron beams can keep their valley coherence even after being reflected by electrostatically-defined edges. In addition, PCSV shows that, under the application of moderate out-of-plane magnetic fields, these QPCs become quarter metals capable of emitting completely spin and valley-polarized currents. These results open the way for new devices where the spin and valley degrees of freedom are used as information carriers and may serve as new interconnects between QDs in future BLG-based quantum computing devices.
In this context, van der Waals heterostructures made of bilayer graphene (BLG) and hexagonal boron nitride (hBN), where information can be stored in the spin and valley degrees of freedom, arise as new platforms for quantum coherent transport. Their long spin and valley coherence times make these systems promising for quantum computing but electronic transport in quantum coherent BLG devices needs further experimental studies. For this purpose, PCSV studies electrostatically defined quantum point contacts (QPCs) in BLG-based novel device platforms where charge transport between the QPCs occurs in a ballistic manner. The results show that electron beams can keep their valley coherence even after being reflected by electrostatically-defined edges. In addition, PCSV shows that, under the application of moderate out-of-plane magnetic fields, these QPCs become quarter metals capable of emitting completely spin and valley-polarized currents. These results open the way for new devices where the spin and valley degrees of freedom are used as information carriers and may serve as new interconnects between QDs in future BLG-based quantum computing devices.
To achieve the goals of PCSV, a fabrication recipe that allowed the fabrication of QPCs and QDs in state-of-the-art van der Waals heterostructures based on BLG was developed. Using this recipe, the researcher defined multiple 50-nm-wide QPCs and QDs on different BLG-based devices where the disorder is so low that the quantum properties of the electrons are preserved even after propagating several micrometers in the channel.
Ballistic transport experiments performed between opposite QPCs show that the propagation of electrical currents depends on the valley degree of freedom. A scientific article explaining these results is currently in preparation.
Ballistic transport experiments performed between parallel QPCs and oriented along different crystallographic directions show clear indications that the reflection of ballistic electron beams against electrostatically-defined smooth edges also preserves this quantum degree of freedom. These results have been published as [J. Ingla-Aynés et al. Nano Lett. 23, 5453 (2023)] and explained in a post on LinkedIn and in the researcher's website.
Spectroscopic studies of the QPCs achieved in PCSV, which are the narrowest reported up to date, unveiled the emergence of a spin and valley-polarized QPC for out-of-plane magnetic fields above 0.6 T. These results, together with the ballistic experiments between QPCs, open the way for devices with new functionalities based on the spin and valley degrees of freedom. The results of these experiments are being analyzed to prepare a publication describing the possible mechanisms leading to the spin-and-valley ferromagnetic ground state of the QPC.
Finally, we have also defined QPCs and QDs in BLG/WSe2 heterostructures where the WSe2 layer is expected to imprint spin-orbit coupling to the neighboring BLG layer and allow for spin manipulation. In these samples we have been able to observe conductance plateaus, indicating the formation of QPCs, and Coulomb oscillations, indicating the formation of QDs. These results are promising for the realization of QD devices with tunable spin-orbit coupling.
Ballistic transport experiments performed between opposite QPCs show that the propagation of electrical currents depends on the valley degree of freedom. A scientific article explaining these results is currently in preparation.
Ballistic transport experiments performed between parallel QPCs and oriented along different crystallographic directions show clear indications that the reflection of ballistic electron beams against electrostatically-defined smooth edges also preserves this quantum degree of freedom. These results have been published as [J. Ingla-Aynés et al. Nano Lett. 23, 5453 (2023)] and explained in a post on LinkedIn and in the researcher's website.
Spectroscopic studies of the QPCs achieved in PCSV, which are the narrowest reported up to date, unveiled the emergence of a spin and valley-polarized QPC for out-of-plane magnetic fields above 0.6 T. These results, together with the ballistic experiments between QPCs, open the way for devices with new functionalities based on the spin and valley degrees of freedom. The results of these experiments are being analyzed to prepare a publication describing the possible mechanisms leading to the spin-and-valley ferromagnetic ground state of the QPC.
Finally, we have also defined QPCs and QDs in BLG/WSe2 heterostructures where the WSe2 layer is expected to imprint spin-orbit coupling to the neighboring BLG layer and allow for spin manipulation. In these samples we have been able to observe conductance plateaus, indicating the formation of QPCs, and Coulomb oscillations, indicating the formation of QDs. These results are promising for the realization of QD devices with tunable spin-orbit coupling.
The reproducible 50-nm-wide QPCs which were created in the state-of-the-art Kavli Nanolab cleanroom in Delft for the ballistic transport experiments showed clear size quantization in all cases. These results represent a new milestone in the field and enable the realization of more complex transport devices using QPCs. The ballistic transport experiments realized between electrostatically-defined QPCs in BLG represent the first realization of such measurements and open the way for new spintronic and valleytronic experiments in the quantum and classic regime. The observation and detailed study of spin and valley-polarized quantum point contacts is promising for the future application of these devices in quantum computing applications.
The fabrication expertise acquired for PCSV has also been transferred to other projects which explore magnetic proximity effects in van der Waals heterostructures using optical and electronic transport measurements. These results will shine new light on magnetic van der Waals heterostructures and lead to new publications.
In addition to the scientific impact created by the results described above, the project has also enabled the training of young researchers performing their bachelor and master projects under the supervision of the researcher on state-of-the-art cleanroom and measurement equipment available in the host group.
The fabrication expertise acquired for PCSV has also been transferred to other projects which explore magnetic proximity effects in van der Waals heterostructures using optical and electronic transport measurements. These results will shine new light on magnetic van der Waals heterostructures and lead to new publications.
In addition to the scientific impact created by the results described above, the project has also enabled the training of young researchers performing their bachelor and master projects under the supervision of the researcher on state-of-the-art cleanroom and measurement equipment available in the host group.