Palladium gates for reproducible quantum dots in silicon

We replace the established aluminium gates for the formation of quantum dots in silicon with gates made from palladium. We study the morphology of both aluminium and palladium gates with transmission electron microscopy. The native aluminium oxide is found to be formed all around the aluminium gates, which could lead to the formation of unintentional dots. Therefore, we report on a novel fabrication route that replaces aluminium and its native oxide by palladium with atomic-layer-deposition-grown aluminium oxide. Using this approach, we show the formation of low-disorder gate-defined quantum dots, which are reproducibly fabricated. Furthermore, palladium enables us to further shrink the gate design, allowing us to perform electron transport measurements in the few-electron regime in devices comprising only two gate layers, a major technological advancement. It remains to be seen, whether the introduction of palladium gates can improve the excellent results on electron and nuclear spin qubits defined with an aluminium gate stack

Anisotropic Pauli spin blockade in hole quantum dots

We present measurements on gate-defined double quantum dots in Ge-Si core-shell nanowires, which we tune to a regime with visible shell filling in both dots. We observe a Pauli spin blockade and can assign the measured leakage current at low magnetic fields to spin-flip cotunneling, for which we measure a strong anisotropy related to an anisotropic g-factor. At higher magnetic fields we see signatures for leakage current caused by spin-orbit coupling between (1,1)-singlet and (2,0)-triplet states. Taking into account these anisotropic spin-flip mechanisms, we can choose the magnetic field direction with the longest spin lifetime for improved spin-orbit qubits

Professionalization of student teachers for inclusive science teaching

Durch die Ratifizierung der UN-Behindertenrechtskonvention (United Nations 2006, Artikel 24) hat sich Deutschland verpflichtet »Bildung für alle« (UNESCO 2015) im deutschen Schulsystem möglich zu machen. In der Lehrer*innenbildung stellen sich zwei Herausforderungen für die Fachdidaktiken: Erstens ist der Schulunterricht in verschiedene Fächer aufgegliedert, die jeweils eigene Fachinhalte aufweisen. Diese Inhalte müssen entlang fachdidaktischer Prinzipien inklusiv aufbereitet werden. Zweitens steigt die Komplexität der Fachinhalte vom Übergang der Grundschule in die Sekundarstufe deutlich an, sodass es für Lehrkräfte immer schwieriger wird, auf heterogene Lernvoraussetzungen angemessen einzugehen (Musenberg und Riegert 2015, 5). Die Professionalisierung von Lehrkräften muss gezielt auf diese Herausforderungen reagieren und Lehramtsstudierende nachhaltig auf ihr Tätigkeitsfeld vorbereiten. Wie diese Professionalisierung von Lehramtsstudierenden in Forschung und Lehre umgesetzt werden kann, wird exemplarisch an einem Projektseminar zum inklusiven naturwissenschaftlichen Unterricht vorgestellt. Das BMBF Projekt »Nawi-In« vereint Forschung und Lehre, indem es die Kompetenzentwicklung Studierender im Projektseminar beforscht. Dies wird durch videobasierte Kompetenzforschung begleitet (Riegel 2013).Germany ratified the UN Convention on the Rights of Persons with Disabilities (United Nations 2006, Article 24), and is therefore bound by contract to make »Education for All« possible (UNESCO 2015) in the German school system. Now there are two challenges for science education: first, the school education is divided in different subjects with different subject contents. The contents have to be prepared inclusively and subject-related. Second, the complexity of the subject contents rises noticeably from primary to secondary school, so it gets more difficult to regard all needs of a heterogeneous group of students adequately (Musenberg and Riegert 2015, 5). Therefore, a professionalization of teacher students should target these challenges to prepare them effectively for their working field. How the implementation of teacher students’ professionalization can be orchestrated is shown exemplary by a project seminar designed for inclusive science teaching. Research and teaching is united through the »Nawi-In«-project – funded by the Federal Ministry of Education and Research – by investigating the students’ development of competencies. This development is monitored by video-based competency research (Riegel 2013)

Hard superconducting gap and diffusion-induced superconductors in Ge-Si nanowires

We show a hard induced superconducting gap in a Ge-Si nanowire Josephson transistor up to in-plane magnetic fields of $250$ mT, an important step towards creating and detecting Majorana zero modes in this system. A hard induced gap requires a highly homogeneous tunneling heterointerface between the superconducting contacts and the semiconducting nanowire. This is realized by annealing devices at $180$ $^\circ$C during which aluminium inter-diffuses and replaces the germanium in a section of the nanowire. Next to Al, we find a superconductor with lower critical temperature ($T_\mathrm{C}=0.9$ K) and a higher critical field ($B_\mathrm{C}=0.9-1.2$ T). We can therefore selectively switch either superconductor to the normal state by tuning the temperature and the magnetic field and observe that the additional superconductor induces a proximity supercurrent in the semiconducting part of the nanowire even when the Al is in the normal state. In another device where the diffusion of Al rendered the nanowire completely metallic, a superconductor with a much higher critical temperature ($T_\mathrm{C}=2.9$ K) and critical field ($B_\mathrm{C}=3.4$ T) is found. The small size of diffusion-induced superconductors inside nanowires may be of special interest for applications requiring high magnetic fields in arbitrary direction

Hole spins in Ge-Si nanowires

In a universal quantum computer, coherent control over the state of a quantum mechanical two-level system is needed. This requires interactions of the quantum state with its environment. Inherently, such interactions also lead to decoherence and thus limit the performance of the quantum computer. A profound knowledge of the relevant interaction mechanisms is therefore key to the realization of a quantum computer.\ud In this thesis we use Ge-Si core-shell nanowires to investigate holes confined to one dimension. Mixing of heavy and light hole states leads to a strong, anisotropic spin-orbit interaction in this system. We define highly stable quantum dots of different lengths in the nanowire and controllably split up longer quantum dots into double quantum dots. The effective g-factor in these one-dimensional hole quantum dots is found to be highly anisotropic with respect to the nanowire axis as well as the electric-field axis. In double quantum dots, we observe shell filling of new orbitals and Pauli spin blockade of the second hole entering the orbital. The leakage current in the spin-blocked state is highly anisotropic with spin-flip cotunnelling as the dominant leakage mechanism. At finite magnetic fields, we also find signatures of leakage current induced by spin-orbit coupling and anisotropic Coulomb effects