An official website of the United States government
The magnetic-field–tuned superconductor-to-insulator transition was studied in a hybrid system of superconducting indium islands, deposited on an indium oxide (InOx) thin film, which exhibits global superconductivity at low magnetic fields. Vacuum annealing was used to tune the conductivity of the InOx film, thereby tuning the inergrain coupling and the nature of the transition. The hybrid system exhibits a “giant” magnetoresistance above the magnetic-field–tuned superconductor-to-insulator transition (H-SIT), with critical behavior similar to that of uniform InOx films but at much lower magnetic fields, that manifests the duality between Cooper pairs and vortices. A key feature of this hybrid system is the separation between the quantum criticality and the onset of nonequilibrium behavior.
more »
« less
Electrostatic effects and topological superconductivity in semiconductor–superconductor–magnetic-insulator hybrid wires
We investigate the impact of electrostatics on the proximity effect between a magnetic insulator and a semiconductor wire in semiconductor–superconductor–magnetic-insulator hybrid structures. By performing self-consistent Schrödinger-Poisson calculations using an effective model of the hybrid system, we find that large effective Zeeman fields consistent with the emergence of topological superconductivity emerge within a large parameter window in wires with overlapping layers of magnetic insulator and superconductor, but not in nonoverlapping structures. We show that this behavior is essentially the result of electrostatic effects controlling the amplitude of the low-energy wave functions near the semiconductor–magnetic-insulator interface.
more »
« less
- Award ID(s):
- 2014156
- PAR ID:
- 10476242
- Publisher / Repository:
- American Physical Society
- Date Published:
- Journal Name:
- Physical Review B
- Volume:
- 104
- Issue:
- 19
- ISSN:
- 2469-9950
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
We study Andreev bound states in the presence of a magnetic moment in a ferromagnetic topological insulator in superconductor/magnetic topological insulator/superconductor Josephson junctions. We analytically find zero energy states for out-of-plane and in-plane directions of the magnetic moment. In the case of the out-of-plane magnetic moment, the energy is independent of the scattering angle. If both magnetic and nonmagnetic scattering mechanisms are considered, the zero energy state requires the scattering angle to the electrode to be zero as in the case of Majorana fermions. In the presence of an in-plane magnetic moment, the energy band always exhibits a nonvanishing gap if the magnetic moment has a nonzero component, i.e., there are no zero energy states. Here we assume that the electrons tunnel in the direction. If the magnetic moment is aligned with the tunneling direction, the zero energy states always exist and are independent of the scattering angle. Contrary to the Majorana fermion case, the phase shift between two superconductor electrodes is not. This phase difference depends on the system parameters such as the Fermi velocity, the barrier potential magnitude, the exchange coupling between localized and delocalized electrons, and the component of the magnetic moment. We find an anomalous Josepheson current when the magnetic moment has a component in the direction, where the current is nonzero despite. This is due to the violation of time reversal and chiral symmetries in the Josepheson junction. This leads to the observation of the Josephson Diode effect as well. For large scattering magnitudes, we find that the transmission coefficient approaches one at larger barrier magnitudes. This is the main reason why in superconductor/magnetic topological insulator/superconductor Josephson junctions critical current is much higher than in superconductor/normal metal/superconductor junctions. This effect is similar in origin to Klein Tunneling for relativistic Dirac electrons. In the case of nonmagnetic and out-of-plane magnetic scatterings, the current vanishes when the barrier amplitudes are approximately equal and large. This effect cannot be explained by the relativistic nature of the Dirac equation and is specific to the model. We also study temperature dependencies for in- and out-of-plane magnetic moments. We find that current at high temperatures is significantly smaller than at low temperatures. The current approaches a constant value at low temperatures, at approximately. This value depends on the other system parameters. The existence of new zero energy states in magnetic topological insulators in superconductor/magnetic topological insulator/superconductor Josephson junctions opens new opportunities in quantum computing because of the presence of the additional symmetry with respect to the scattering angle.more » « less
-
We fabricate three-terminal hybrid devices consisting of a semiconductor nanowire segment proximitized by a grounded superconductor and having tunnel probe contacts on both sides. By performing simultaneous tunneling measurements, we identify delocalized states, which can be observed from both ends, and states localized near one of the tunnel barriers. The delocalized states can be traced from zero magnetic field to fields beyond 0.5 T. Within the regime that supports delocalized states, we search for correlated low-energy features consistent with the presence of Majorana zero modes. While both sides of the device exhibit ubiquitous low-energy features at high fields, no correlation is inferred. Simulations using a one-dimensional effective model suggest that the delocalized states, which extend throughout the whole system, have large characteristic wave vectors, while the lower momentum states expected to give rise to Majorana physics are localized by disorder. To avoid such localization and realize Majorana zero modes, disorder needs to be reduced significantly. We propose a method for estimating the disorder strength based on analyzing the level spacing between delocalized states.more » « less
-
We address an outstanding problem that represents a critical roadblock in the development of the Majorana-based topological qubit using semiconductor-superconductor hybrid structures: the quantitative characterization of disorder effects generated by the unintentional presence of charge impurities within the hybrid device. Given that disorder can have far-reaching consequences for the Majorana physics but is intrinsically difficult to probe experimentally in a hybrid structure, providing a quantitative theoretical description of disorder effects becomes essential. To accomplish this task, we develop a microscopic theory that (i) provides a quantitative characterization of the effective potential generated by a charge impurity embedded inside a semiconductor wire proximity coupled to a superconductor layer by solving selfconsistently the associated three-dimensional Schrödinger-Poisson problem, (ii) describes the low-energy physics of the hybrid structure in the presence of s-wave superconductivity, spin-orbit coupling, Zeeman splitting, and disorder arising from multiple charge impurities by using the results of (i) within a standard free-fermion approach, and (iii) links the microscopic results to experimentally observable features by generating tunneling differential-conductance maps as a function of the control parameters (e.g., Zeeman field and chemical potential). We find that charge impurities lead to serious complications regarding the realization and observation of Majorana zero modes, which have direct implications for the development of Majorana-based qubits. More importantly, our work provides a clear direction regarding what needs to be done for progress in the field, including specific materials-quality and semiconductor-purity targets that must be achieved to create a topological qubit.more » « less
-
A quantum anomalous Hall (QAH) insulator coupled to an s-wave superconductor is predicted to harbor chiral Majorana modes. A recent experiment interprets the half-quantized two-terminal conductance plateau as evidence for these modes in a millimeter-size QAH-niobium hybrid device. However, non-Majorana mechanisms can also generate similar signatures, especially in disordered samples. Here, we studied similar hybrid devices with a well-controlled and transparent interface between the superconductor and the QAH insulator. When the devices are in the QAH state with well-aligned magnetization, the two-terminal conductance is always half-quantized. Our experiment provides a comprehensive understanding of the superconducting proximity effect observed in QAH-superconductor hybrid devices and shows that the half-quantized conductance plateau is unlikely to be induced by chiral Majorana fermions in samples with a highly transparent interface.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1103/PhysRevB.104.195433
