Spin-Orbit induced phase-shift in Bi2_{2}Se3_{3} Josephson junctions

The transmission of Cooper pairs between two weakly coupled superconductors produces a superfluid current and a phase difference; the celebrated Josephson effect. Because of time-reversal and parity symmetries, there is no Josephson current without a phase difference between two superconductors. Reciprocally, when those two symmetries are broken, an anomalous supercurrent can exist in the absence of phase bias or, equivalently, an anomalous phase shift $\varphi_0$ can exist in the absence of a superfluid current. We report on the observation of an anomalous phase shift $\varphi_0$ in hybrid Josephson junctions fabricated with the topological insulator Bi$_2$Se$_3$ submitted to an in-plane magnetic field. This anomalous phase shift $\varphi_0$ is observed directly through measurements of the current-phase relationship in a Josephson interferometer. This result provides a direct measurement of the spin-orbit coupling strength and open new possibilities for phase-controlled Josephson devices made from materials with strong spin-orbit coupling

High Tc Josephson nanoJunctions made by ion irradiation : characteristics and reproducibility

Reproducible High Tc Josephson junctions have been made in a rather simple two-step process using ion irradiation. A microbridge 1 to 5 micrometers wide is firstly designed by ion irradiating a c-axis-oriented YBa2Cu3O7 film through a gold mask such as the unprotected part becomes insulating. A lower Tc part is then defined within the bridge by irradiating with a much lower dose through a 20 nm wide narrow slit opened in a standard electronic photoresist. These planar junctions, whose settings can be finely tuned, exhibit reproducible and nearly ideal Josephson characteristics. Non hysteretic Resistively Shunted Junction (RSJ) like behavior is observed, together with sinc Fraunhofer patterns for rectangular junctions. The IcRn product varies with temperature ; it can reach a few mV. The typical resistance ranges from 0.1 to a few ohms, and the critical current density can be as high as 30 kA/cm2. The dispersion in characteristics is very low, in the 5% to 10% range. Such nanojunctions have been used to make microSQUIDs (Superconducting Quantum Interference Device) operating at Liquid Nitrogen (LN2) temperature. They exhibit a very small asymmetry, a good sensitivity and a rather low noise. The process is easily scalable to make rather complex Josephson circuits.Comment: 4 pages, 5 figures, Applied Superconductivity Conference Seattle 200

Anomalous microwave response in the dissipative regime of topological superconducting devices based on Bi2Te2.3Se0.7

Superconducting proximity junctions based on topological insulators are widely believed to harbor Majorana-like bound states. The latter serves as a paradigm non-local topological quantum computation protocols. Nowadays, a search for topological phases in different materials, perspective for a realization of topological qubits, is one of the central efforts in quantum physics. It is motivated, in particular, by recent observation of anomalous ac Josephson effect, which being a signature of Majorana physics. Its manifestations, such as a fractional Josephson frequency and the absence of the first (or several odd in more rare cases), Shapiro steps, were reported for different materials. Here we study Shapiro steps in Nb/Bi2Te2.3Se0.7/Nb junctions, based on ultrasmall single crystals of a 3D topological insulator synthesized by a physical vapor deposition (PVD) technique. We present evidence that our junctions are ballistic. When subjected to microwave radiation, the junctions exhibit Shapiro steps, but the first step is missing. Typically it is assumed that the missing first step (MFS) effect cannot be observed in the presence of quasiparticle poisoning due to suppression of the 4{\pi}-periodic component. Our findings within the context of the RSJ-model of Josephson junction dynamics show that such behaviour of samples corresponds to a specific condition, requiring a minimum of 5% of the 4{\pi}-component for disappearance of the first Shapiro step.Comment: Keywords: Shapiro step missing, Topological insulator, Superconductivity, Ballistic transport, 4{\pi}-periodic componen

Present and future of high-temperature superconductor quantum-based voltage standards

This paper presents a brief overview of the current state-of-the-art of Josephson junctions for Quantum-based Voltage Standards fabricated with High-Temperature Superconductors (HTS). A short introduction on the history and technical evolution of Low Temperature Superconductors (LTS) technology is provided for non-specialists. Then HTS technology is summarized and discussed in the context of quantum voltage standard applications. Finally, the two most promising technologies: bicrystal and Focused Helium-Ion Beam junctions are discussed with more detail, analyzing strength, limitations and perspectives in both cases

Engineering two-dimensional superconductivity and Rashba spin-orbit coupling in LaAlO/SrTiO quantum wells by selective orbital occupancy

The discovery of two-dimensional electron gases (2DEGs) at oxide interfaces-involving electrons in narrow d -bands-has broken new ground, enabling the access to correlated states that are unreachable in conventional semiconductors based on s - and p - electrons. There is a growing consensus that emerging properties at these novel quantum wells-such as 2D superconductivity and magnetism-are intimately connected to specific orbital symmetries in the 2DEG sub-band structure. Here we show that crystal orientation allows selective orbital occupancy, disclosing unprecedented ways to tailor the 2DEG properties. By carrying out electrostatic gating experiments in LaAlO₃/SrTiO₃ wells of different crystal orientations, we show that the spatial extension and anisotropy of the 2D superconductivity and the Rashba spin-orbit field can be largely modulated by controlling the 2DEG sub-band filling. Such an orientational tuning expands the possibilities for electronic engineering of 2DEGs at LaAlO₃/SrTiO₃ interfaces. Two-dimensional electron gases at oxide interfaces induce exotic behaviours. By studying samples with different crystal orientation, Herranz et al. show that the extension and anisotropy of the oxide quantum well properties can be controlled through selective sub-band filling via orientational tuning

Electronic bandstructure of superconducting KTaO3 (111) interfaces

Two-dimensional electron gases(2DEGs)based on KTaO3 are emerging as a promising platform for spin-orbitronics due to their high Rashba spin-orbit coupling (SOC) and gate-voltage tunability. The recent discovery of a superconducting state in KTaO3 2DEGs now expands their potential towards topological superconductivity. Although the band structure of KTaO3 surfaces of various crystallographic orientations has already been mapped using angle-resolved photoemission spectroscopy(ARPES), this is not the case for superconducting KTaO3 2DEGs. Here, we reveal the electronic structure of superconducting 2DEGs based on KTaO3 (111) single crystals through ARPES measurements. We fit the data with a tight-binding model and compute the associated spin textures to bring insight into the SOC-driven physics of this fascinating system.Comment: 9 pages, 4 figure