7,096 research outputs found
Observation of cooper pair splitting and andreev bound states in carbon nanotubes
Get PDFIn this thesis, we investigate Cooper pair splitting in double quantum dot devices made from carbon nanotubes. We present transport measurements in which the controlled splitting of Cooper pairs dominates the current through such a device, reaching unprecedented splitting efficiencies of up to 90%.
In another experiment we use Cooper pair splitting (and a related non-local transport processes) as a tool to investigate Andreev bound states. Andreev bound states are a key concept in mesoscopic superconductivity. They can form due to the penetration of Cooper pairs into a quantum dot, leading to a new type of energy level that differs drastically from conventional particle-in-a-box states
Microwave spectroscopy of interacting Andreev spins
No full textAndreev bound states are fermionic states localized in weak links between superconductors which can be occupied with spinful quasiparticles. Microwave experiments using superconducting circuits with InAs/Al nanowire Josephson junctions have recently enabled probing and coherent manipulation of Andreev states but have remained limited to zero or small magnetic fields. Here, we use a flux-tunable superconducting circuit compatible in magnetic fields up to 1T to perform spectroscopy of spin-polarized Andreev states up to ∼250mT, beyond which the spectrum becomes gapless. We identify singlet and triplet states of two quasiparticles occupying different Andreev states through their dispersion in magnetic field. These states are split by exchange interaction and couple via spin-orbit coupling, analogously to two-electron states in quantum dots. We also show that the magnetic field allows to drive a direct spin-flip transition of a single quasiparticle trapped in the junction. Finally, we measure a gate- and field-dependent anomalous phase shift of the Andreev spectrum, of magnitude up to ∼0.7π. Our observations demonstrate alternative ways to manipulate Andreev states in a magnetic field and reveal spin-polarized triplet states that carry supercurrent.QRD/Kouwenhoven LabQRD/Goswami LabQN/Kouwenhoven LabAndersen La
Andreev Modes from Phase Winding in a Full-Shell Nanowire-Based Transmon
Get PDFWe investigate transmon qubits made from semiconductor nanowires with a fully surrounding superconducting shell. In the regime of reentrant superconductivity associated with the destructive Little-Parks effect, numerous coherent transitions are observed in the first reentrant lobe, where the shell carries 2π winding of superconducting phase, and are absent in the zeroth lobe. As junction density was increased by gate voltage, qubit coherence was suppressed then lost in the first lobe. These observations and numerical simulations highlight the role of winding-induced Andreev states in the junction.BUS/Quantum Delf
Microwave spectroscopy of interacting Andreev spins
Get PDFAndreev bound states are fermionic states localized in weak links between superconductors which can be occupied with spinful quasiparticles. Microwave experiments using superconducting circuits with InAs/Al nanowire Josephson junctions have recently enabled probing and coherent manipulation of Andreev states but have remained limited to zero or small magnetic fields. Here, we use a flux-tunable superconducting circuit compatible in magnetic fields up to 1T to perform spectroscopy of spin-polarized Andreev states up to ∼250mT, beyond which the spectrum becomes gapless. We identify singlet and triplet states of two quasiparticles occupying different Andreev states through their dispersion in magnetic field. These states are split by exchange interaction and couple via spin-orbit coupling, analogously to two-electron states in quantum dots. We also show that the magnetic field allows to drive a direct spin-flip transition of a single quasiparticle trapped in the junction. Finally, we measure a gate- and field-dependent anomalous phase shift of the Andreev spectrum, of magnitude up to ∼0.7π. Our observations demonstrate alternative ways to manipulate Andreev states in a magnetic field and reveal spin-polarized triplet states that carry supercurrent
Andreev reflection spectroscopy in transition metal oxides
Get PDFHere we review the literature concerning measurement of the Andreev reflection between a superconductor (S) and ferromagnet (F), with particular attention to the case where the ferromagnet is a transition metal oxide. We discuss the practicality of utilisation of the current models for determination of the transport current spin polarisation and examine the evidence for Andreev bound states
Tunable Crossed Andreev Reflection and Elastic Cotunneling in Hybrid Nanowires
No full textA short superconducting segment can couple attached quantum dots via elastic cotunneling (ECT) and crossed Andreev reflection (CAR). Such coupled quantum dots can host Majorana bound states provided that the ratio between CAR and ECT can be controlled. Metallic superconductors have so far been shown to mediate such tunneling phenomena, albeit with limited tunability. Here, we show that Andreev bound states formed in semiconductor-superconductor heterostructures can mediate CAR and ECT over mesoscopic length scales. Andreev bound states possess both an electron and a hole component, giving rise to an intricate interference phenomenon that allows us to tune the ratio between CAR and ECT deterministically. We further show that the combination of intrinsic spin-orbit coupling in InSb nanowires and an applied magnetic field provides another efficient knob to tune the ratio between ECT and CAR and optimize the amount of coupling between neighboring quantum dots.QRD/Kouwenhoven LabQRD/Wimmer GroupQRD/Goswami LabBUS/Quantum DelftQN/Wimmer GroupQN/Kouwenhoven LabQubit Research Divisio
A New General Derandomization Method
No full textWe show that quick hitting set generators can replace quick pseudorandom generators to derandomize any probabilistic two-sided error algorithms. Up to now quick hitting set generators have been known as the general and uniform derandomization method for probabilistic one-sided error algorithms, while quick pseudorandom generators as the generators as the general and uniform method to derandomize probabilistic two-sided error algorithms.Our method is based on a deterministic algorithm that, given a Boolean circuit C and given access to a hitting set generator, constructs a discrepancy set for C. The main novelty is that the discrepancy set depends on C, so the new derandomization method is not uniform (i.e., not oblivious).The algorithm works in time exponential in k(p(n)) where k(*) is the price of the hitting set generator and p(*) is a polynomial function in the size of C. We thus prove that if a logarithmic price quick hitting set generator exists then BPP = P
Directional point-contact Andreev-reflection spectroscopy of Fe-based superconductors: Gap symmetry and Fermi surface topology
No full textThe number and the symmetry of the order parameter(s) (OPs) in Fe-based superconductors, as well as the occurrence of anisotropic or nodal OP symmetries are important topics at the center of the present scientific debate. We show here that when accurate point-contact Andreev-reflection (PCAR) experiments in good-quality samples are carried out by different groups they do provide surprisingly consistent results. Moreover, the analysis of directional PCAR results in complex materials requires enhanced theoretical tools. We recently extended the theoretical model for Andreev reflection to the full 3D case accounting for Fermi surfaces of (almost) arbitrary shape. This combined theoretical/experimental approach allows us to conclude that, in the most studied La 1111, Sm 1111, Co-doped and K-doped Ba 122 compounds around optimal doping, the available high-quality PCAR results agree rather well on the presence of two isotropic gaps. On the other hand, our recent directional PCAR results on single crystals of the Ca 122 family clearly show the presence of at least one nodal (or strongly anisotropic) OP, possibly in agreement with recent theoretical prediction
Superconducting order parameter and bosonic mode in hydrogen-substituted NdFeAsO0.6H0.36 revealed by multiple-Andreev-reflection spectroscopy
No full textUsing intrinsic multiple-Andreev-reflection-effect spectroscopy, we studied ballistic superconductor-normal metal-superconductor (SnS) contacts in layered oxypnictide superconductors NdFeAsO0.6H0.36 with critical temperatures Tc=45−48 K. We directly determined the magnitude of two bulk superconducting order parameters, the large gap ΔL≈10.4 meV, a possible small gap ΔS≈1.8 meV, and their temperature dependence. Additionally, a resonant coupling with a characteristic bosonic mode was observed—the boson energy at 4.2 K, ɛ0=10.5−11.0 meV being less than the indirect gap (ΔL<ɛ0<ΔL+ΔS)
Spin-filtered measurements of Andreev bound states in semiconductor-superconductor nanowire devices
No full textSemiconductor nanowires coupled to superconductors can host Andreev bound states with distinct spin and parity, including a spin-zero state with an even number of electrons and a spin-1/2 state with odd-parity. Considering the difference in spin of the even and odd states, spin-filtered measurements can reveal the underlying ground state. To directly measure the spin of single-electron excitations, we probe an Andreev bound state using a spin-polarized quantum dot that acts as a bipolar spin filter, in combination with a non-polarized tunnel junction in a three-terminal circuit. We observe a spin-polarized excitation spectrum of the Andreev bound state, which can be fully spin-polarized, despite strong spin-orbit interaction in the InSb nanowires. Decoupling the hybrid from the normal lead causes a current blockade, by trapping the Andreev bound state in an excited state. Spin-polarized spectroscopy of hybrid nanowire devices, as demonstrated here, is proposed as an experimental tool to support the observation of topological superconductivity.QRD/Kouwenhoven LabQRD/Wimmer GroupBUS/Quantum DelftQN/Kouwenhoven La
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