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We show a hard superconducting gap in a Ge-Si nanowire Josephson transistor up to in-plane magnetic fields of 250 mT, an important step toward creating and detecting Majorana zero modes in this system. A hard gap requires a highly homogeneous tunneling heterointerface between the superconducting contacts and the semiconducting nanowire. This is realized by annealing devices at 180 °C during which aluminum interdiffuses and replaces the germanium in a section of the nanowire. Next to Al, we find a superconductor with lower critical temperature (TC = 0.9 K) and a higher critical field (BC = 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 (TC = 2.9 K) and critical field (BC = 3.4 T) is found. The small size of these diffusion-induced superconductors inside nanowires may be of special interest for applications requiring high magnetic fields in arbitrary direction., QuTech, QRD/Kouwenhoven Lab

We show a hard superconducting gap in a Ge-Si nanowire Josephson transistor up to in-plane magnetic fields of 250 mT, an important step toward creating and detecting Majorana zero modes in this system. A hard gap requires a highly homogeneous tunneling heterointerface between the superconducting contacts and the semiconducting nanowire. This is realized by annealing devices at 180 °C during which aluminum interdiffuses and replaces the germanium in a section of the nanowire. Next to Al, we find a superconductor with lower critical temperature (TC = 0.9 K) and a higher critical field (BC = 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 (TC = 2.9 K) and critical field (BC = 3.4 T) is found. The small size of these diffusion-induced superconductors inside nanowires may be of special interest for applications requiring high magnetic fields in arbitrary direction., QRD/Kouwenhoven Lab

We present a Josephson junction based on a Ge-Si core-shell nanowire with transparent superconducting Al contacts, a building block which could be of considerable interest for investigating Majorana bound states, superconducting qubits, and Andreev (spin) qubits. We demonstrate the dc Josephson effect in the form of a finite supercurrent through the junction and establish the ac Josephson effect by showing up to 23 Shapiro steps. We observe multiple Andreev reflections up to the sixth order, indicating that charges can scatter elastically many times inside our junction and that our interfaces between superconductor and semiconductor are transparent and have low disorder., QN/Bakkers Lab, QuTech

Structural optimization for crashworthiness in composite structures has become an important topic of research in aerospace, attributable to its proven benefits to the occupants' safety in an aircraft during a crash event. This thesis provides a comprehensive investigation on LS-DYNA modeling and design optimization of composite square tubes for crashworthiness. The objective of this thesis is to construct a optimization framework specialized for the preliminary design of composite structures with a balance of performance and efficiency. Three primary components are involved: firstly, coupon-level simulations in LS-DYNA are performed to characterize the material properties for carbon/epoxy composite material IM7/8552; secondly, a square tube is modeled by a single-layer approach in three mainstream material cards (MAT-54, MAT-58 and MAT-262) with calibrated parameters for crush simulation in LS-DYNA. Meanwhile, detailed sensitivity analysis of influential parameters in different material models is also be performed to have a more comprehensive understanding of the complex failure mechanism. The simulation results indicate good correlation to the experiments in terms of energy absorption and maximum peak load, with high computational efficiency and low-cost calibration. Lastly, a two-stage single-objective optimization is performed, which incorporates the fiber orientation for each layer as design variables and design/manufacturing rules as constraints. Two surrogate models are created to formulate the mapping between input design variables and output crashworthiness metrics, including Deep Neural Networks (DNN) and Gradient Boosting Regression Trees (GBRT) ensemble. Followed by Mix-Discrete Particle Swarm Optimization (MDPSO) algorithm, the optimal set of design variables are obtained for each surrogate model. The first-stage optimization results demonstrate significant improvement in the crashworthiness performance compared with the baseline value, while the second, Aerospace Engineering

We present a Josephson junction based on a Ge-Si core-shell nanowire with transparent superconducting Al contacts, a building block which could be of considerable interest for investigating Majorana bound states, superconducting qubits, and Andreev (spin) qubits. We demonstrate the dc Josephson effect in the form of a finite supercurrent through the junction and establish the ac Josephson effect by showing up to 23 Shapiro steps. We observe multiple Andreev reflections up to the sixth order, indicating that charges can scatter elastically many times inside our junction and that our interfaces between superconductor and semiconductor are transparent and have low disorder., QN/Bakkers Lab

Structural optimization for crashworthiness in composite structures has become an important topic of research in aerospace, attributable to its proven benefits to the occupants' safety in an aircraft during a crash event. This thesis provides a comprehensive investigation on LS-DYNA modeling and design optimization of composite square tubes for crashworthiness. The objective of this thesis is to construct a optimization framework specialized for the preliminary design of composite structures with a balance of performance and efficiency. Three primary components are involved: firstly, coupon-level simulations in LS-DYNA are performed to characterize the material properties for carbon/epoxy composite material IM7/8552; secondly, a square tube is modeled by a single-layer approach in three mainstream material cards (MAT-54, MAT-58 and MAT-262) with calibrated parameters for crush simulation in LS-DYNA. Meanwhile, detailed sensitivity analysis of influential parameters in different material models is also be performed to have a more comprehensive understanding of the complex failure mechanism. The simulation results indicate good correlation to the experiments in terms of energy absorption and maximum peak load, with high computational efficiency and low-cost calibration. Lastly, a two-stage single-objective optimization is performed, which incorporates the fiber orientation for each layer as design variables and design/manufacturing rules as constraints. Two surrogate models are created to formulate the mapping between input design variables and output crashworthiness metrics, including Deep Neural Networks (DNN) and Gradient Boosting Regression Trees (GBRT) ensemble. Followed by Mix-Discrete Particle Swarm Optimization (MDPSO) algorithm, the optimal set of design variables are obtained for each surrogate model. The first-stage optimization results demonstrate significant improvement in the crashworthiness performance compared with the baseline value, while the second, Aerospace Engineering

The aim of this thesis is to evaluate the influence between the two counter-rotating VAWT rotors with the Double Actuator Cylinder (DAC) model which will be validated against the panel code. The DAC model is developed from the original Actuator Cylinder (AC) model which includes the Modified-Linear (Mod-Lin) correction. The original AC model for modelling single rotor is investigated, modified and validated against the panel code to be sufficient which is the premise for a sufficient DAC model. The original AC model is modified to become a dynamic AC model including Cheng’s modification, flow curvature effect and unsteady aerodynamics. The unsteady aerodynamics is included by coupling the modified Risø dynamic stall model with the AC model. An investigation with Migliore’s conformal mapping method as well as panel code revealed that the modified Risø dynamic stall model implicitly included the flow curvature effects. The derived dynamic AC model is validated to show good agreements with the panel code. It is also revealed that further improvements could be made to the Mod-Lin correction method. The DAC model is finally derived, and the performance on predicting the influence between the two rotors is then investigated., Aerospace Engineering

The aim of this thesis is to evaluate the influence between the two counter-rotating VAWT rotors with the Double Actuator Cylinder (DAC) model which will be validated against the panel code. The DAC model is developed from the original Actuator Cylinder (AC) model which includes the Modified-Linear (Mod-Lin) correction. The original AC model for modelling single rotor is investigated, modified and validated against the panel code to be sufficient which is the premise for a sufficient DAC model. The original AC model is modified to become a dynamic AC model including Cheng’s modification, flow curvature effect and unsteady aerodynamics. The unsteady aerodynamics is included by coupling the modified Risø dynamic stall model with the AC model. An investigation with Migliore’s conformal mapping method as well as panel code revealed that the modified Risø dynamic stall model implicitly included the flow curvature effects. The derived dynamic AC model is validated to show good agreements with the panel code. It is also revealed that further improvements could be made to the Mod-Lin correction method. The DAC model is finally derived, and the performance on predicting the influence between the two rotors is then investigated., European Wind Energy Masters (EWEM)

We define single quantum dots of lengths varying from 60 nm up to nearly half a micron in Ge-Si core-shell nanowires. The charging energies scale inversely with the quantum dot length between 18 and 4 meV. Subsequently, we split up a long dot into a double quantum dot with a separate control over the tunnel couplings and the electrochemical potential of each dot. Both single and double quantum dot configurations prove to be very stable and show excellent control over the electrostatic environment of the dots, making this system a highly versatile platform for spin-based quantum computing., QN/Bakkers Lab

We present angle-dependent measurements of the effective g factor g in a Ge-Si core-shell nanowire quantum dot. g is found to be maximum when the magnetic field is pointing perpendicularly to both the nanowire and the electric field induced by local gates. Alignment of the magnetic field with the electric field reduces g significantly. g is almost completely quenched when the magnetic field is aligned with the nanowire axis. These findings confirm recent calculations, where the obtained anisotropy is attributed to a Rashba-type spin-orbit interaction induced by heavy-hole light-hole mixing. In principle, this facilitates manipulation of spin-orbit qubits by means of a continuous high-frequency electric field., QN/Bakkers Lab

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., QN/Bakkers Lab

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., QN/Bakkers Lab

We present angle-dependent measurements of the effective g factor g in a Ge-Si core-shell nanowire quantum dot. g is found to be maximum when the magnetic field is pointing perpendicularly to both the nanowire and the electric field induced by local gates. Alignment of the magnetic field with the electric field reduces g significantly. g is almost completely quenched when the magnetic field is aligned with the nanowire axis. These findings confirm recent calculations, where the obtained anisotropy is attributed to a Rashba-type spin-orbit interaction induced by heavy-hole light-hole mixing. In principle, this facilitates manipulation of spin-orbit qubits by means of a continuous high-frequency electric field., QN/Bakkers Lab

We define single quantum dots of lengths varying from 60 nm up to nearly half a micron in Ge-Si core-shell nanowires. The charging energies scale inversely with the quantum dot length between 18 and 4 meV. Subsequently, we split up a long dot into a double quantum dot with a separate control over the tunnel couplings and the electrochemical potential of each dot. Both single and double quantum dot configurations prove to be very stable and show excellent control over the electrostatic environment of the dots, making this system a highly versatile platform for spin-based quantum computing., QN/Bakkers Lab