N. Bergeal

Engineering two-dimensional superconductivity and Rashba spin–orbit coupling in LaAlO3/SrTiO3 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.

High-quality planar high-Tc Josephson junctions

Reproducible high-Tc Josephson junctions have been made in a rather simple two-step process using ion irradiation. A microbridge (1 to 5μm wide) is firstly designed by ion irradiating a c-axis-oriented YBa2Cu3O7−δ film through a gold mask such as the nonprotected part becomes insulating. A lower Tc part is then defined within the bridge by irradiating with a much lower fluence through a narrow slit (20 nm) opened in a standard electronic photoresist. These planar junctions, whose settings can be finely tuned, exhibit reproducible and nearly ideal Josephson characteristics. This process can be used to produce complex Josephson circuits.

Using ion irradiation to make high-Tc Josephson junctions

In this article we describe the effect of ion irradiation on high-Tc superconductor thin film and its interest for the fabrication of Josephson junctions. In particular, we show that these alternative techniques allow to go beyond most of the limitations encountered in standard junction fabrication methods, both in the case of fundamental and technological purposes. Two different geometries are presented: a planar one using a single high-Tc film and a mesa one defined in a trilayer structure.

High-Tc superconducting Josephson mixers for terahertz heterodyne detection

We report on an experimental and theoretical study of the high-frequency mixing properties of ion-irradiated YBa2Cu3O7 Josephson junctions embedded in THz antennas. We investigated the influence of the local oscillator power and frequency on the device performances. The experimental data are compared with theoretical predictions of the general three-port model for mixers in which the junction is described by the resistively shunted junction model. A good agreement is obtained for the conversion efficiency in different frequency ranges, spanning above and below the characteristic frequencies fc of the junctions.

High Tc superconducting quantum interference devices made by ion irradiation

The authors describe a method to make superconducting quantum interference devices (SQUIDs) formed in a single layer of high Tc superconducting materials. The superconducting loop is patterned using ion beam irradiation through an in situ suitable gold mask. Josephson junctions are defined by a lower fluence irradiation through narrow slits opened in a polymethyl methacrylate resist. The critical current and the resistance of the SQUIDs at a given temperature can be adjusted precisely by choosing the fluence of irradiation to make the device operational at temperatures between 4.2K and the Tc of the superconducting material.

YBCO Josephson Junctions and Striplines for RSFQ Circuits Made by Ion Irradiation

We have investigated the electrodynamic properties of high-Tc Josephson junctions (JJs) and striplines made by ion irradiation in order to evaluate the potential of such a technology for low-power rapid single flux quantum superconductor digital electronics. The process is briefly presented, together with the JJ characteristics in the context of high-speed electronics. Line inductances of different geometries and at different temperatures have been measured through superconducting quantum interference device (SQUID) modulation. A detailed comparison with 3-D numerical calculations has been made, and a quantitative agreement that shows typical values of 4-8 pH per square without a ground plane, depending on linewidth and the gap to the ground line, has been obtained.

Study and optimization of ion-irradiated high Tc Josephson junctions by Monte Carlo simulations

High Tc Josephson junctions (HTc JJ) made by irradiation have remarkable properties for technological applications. However, the spread in their electrical characteristics increases with the ion dose. We present a simple model to explain the JJ inhomogeneities, which accounts quantitatively for experimental data. The spread in the slit’s width of the irradiation mask is the limiting factor. Monte Carlo simulations have been performed using different irradiation conditions to study their influence on the spread of the JJ characteristics. A “universal” behavior has been evidenced, which allows us to propose new strategies to optimize JJ reproducibility.

Improving ion irradiated high Tc Josephson junctions by annealing: The role of vacancy-interstitial annihilation

The authors have studied the annealing effect in the transport properties of high Tc Josephson junctions (JJs) made by ion irradiation. Low temperature annealing (80°C) increases the JJ coupling temperature (TJ) and the IcRn product, where Ic is the critical current and Rn the normal resistance. They have found that the spread in JJ characteristics can be reduced by sufficient long annealing times, increasing the reproducibility of ion irradiated Josephson junctions. The characteristic annealing time and the evolution of the spread in the JJ characteristics can be explained by a vacancy-interstitial annihilation process rather than by an oxygen diffusion one.

Toward terahertz heterodyne detection with superconducting Josephson junctions

We report on the high-frequency mixing properties of ion irradiated YBa2Cu3O7 Josephson junctions. The frequency range, spanning above and below the characteristic frequencies fc of the junctions, permits a clear observation of the transition between two mixing regimes. The experimental conversion gain was found to be in good agreement with the prediction of the three-port model. Finally, we discuss the potential of the junctions to build a Josephson mixer operating in the terahertz frequency range.

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 mum 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 sine 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 spread 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 complex Josephson circuits.