C. Feuillet-Palma

Limit of the electrostatic doping in two-dimensional electron gases of LaXO3(X = Al, Ti)/SrTiO3

In LaTiO3/SrTiO3 and LaAlO3/SrTiO3 heterostructures, the bending of the SrTiO3 conduction band at the interface forms a quantum well that contains a superconducting two-dimensional electron gas (2-DEG). Its carrier density and electronic properties, such as superconductivity and Rashba spin-orbit coupling can be controlled by electrostatic gating. In this article we show that the Fermi energy lies intrinsically near the top of the quantum well. Beyond a filling threshold, electrons added by electrostatic gating escape from the well, hence limiting the possibility to reach a highly-doped regime. This leads to an irreversible doping regime where all the electronic properties of the 2-DEG, such as its resistivity and its superconducting transition temperature, saturate. The escape mechanism can be described by the simple analytical model we propose.

Top-gated field-effect LaAlO3/SrTiO3 devices made by ion-irradiation

We present a method to fabricate top-gated field-effect devices in a LaAlO3/SrTiO3 two-dimensional electron gas (2-DEG). Prior to the gate deposition, the realisation of micron size conducting channels in the 2-DEG is achieved by an ion-irradiation with high-energy oxygen ions. After identifying the ion fluence as the key parameter that determines the electrical transport properties of the channels, we demonstrate the field-effect operation. At low temperature, the normal state resistance and the superconducting Tc can be tuned over a wide range by a top-gate voltage without any leakage. A superconductor-to-insulator quantum phase transition is observed for a strong depletion of the 2-DEG.

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 superconducting quantum interference filters (SQIFs) made by ion irradiation

Superconducting Quantum Interference Filters (SQIFs) are arrays of superconducting loops of different sizes including Josephson Junctions (JJ). For a random distribution of sizes, they present a non-periodic response to an applied magnetic field, with an extended linear regime and a sizable field sensitivity. Such properties make SQIFs interesting devices to detect the magnetic component of electromagnetic waves at microwave frequencies. We have used the highly scalable technique of ion irradiation to make High Tc SQUIDs and SQIFs based on commercial YBa2Cu3O7 films, and studied their properties. Both display optimum performances as a function of temperature and bias current, that can be understood in the frame of numerical simulations that we developed. The role of asymmetries and spread in JJ characteristics (routinely found in HTSc technologies) is described : ion irradiation based devices appear robust against them. We finally present results on SQIF made with 2000 SQUID in series, showing a transfer function dV/dB ~ 1000V/T .

Static and radio frequency magnetic response of high T c superconducting quantum interference filters made by ion irradiation

Superconducting Quantum Interference Filters (SQIF) are promising devices for Radio- Frequency (RF) detection combining low noise, high sensitivity, large dynamic range and wide-band capabilities. Impressive progress have been made recently in the field, with SQIF based antennas and amplifiers showing interesting properties in the GHz range using the well-established Nb/AlOx technology. The possibility to extend these results to High Temperature Superconductors (HTS) is still open, and different techniques to fabricate HTS SQIFs are competing to make RF devices. We report on the DC and RF response of a High Temperature SQIF fabricated by the ion irradiation technique. It is made of 1000 Superconducting QUantum Interference Devices (SQUIDs) in series, with loop areas randomly distributed between 6 micron2 and 60 micron2. The DC transfer factor is around 450 V/T at optimal bias and temperature, and the maximum voltage swing around 2:5 mV . We show that such a SQIF detects RF signals up to 150 MHz. It presents linear characteristics for RF power spanning more than five decades, and non-linearities develop beyond PRF = -35 dBm in our set-up configuration. Second-harmonic generation has been shown to be minimum at the functioning point in the whole range of frequencies. A model has been developed which captures the essential features of the SQIF RF response.

Josephson oscillation linewidth of ion-irradiated YBa2Cu3O7 junctions

We report on the noise properties of ion-irradiated YBa2Cu3O7 Josephson junctions. This work aims at investigating the linewidth of the Josephson oscillation with a detector response experiment at sime132 GHz. Experimental results are compared with a simple analytical model based on the Likharev–Semenov equation and the de Gennes dirty limit approximation. We show that the main source of low-frequency fluctuations in these junctions is the broadband Johnson noise and that the excess

Effect of barrier geometry for HTS ion damage Josephson junctions

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Effect of disorder on superconductivity and Rashba spin-orbit coupling in LaAlO3/SrTiO3 interfaces

noise contribution does not prevail in the temperature range of interest, as reported in some other types of high-T c superconducting Josephson junctions. Finally, we discuss the interest of ion-irradiated junctions to implement frequency-tunable oscillators consisting of synchronized arrays of Josephson junctions.

INHOMOGENEOUS ELECTRON GAS AT OXIDE INTERFACES WITH STRONG RASHBA SPIN–ORBIT COUPLING

The large area (2D) technology for high temperature superconducting (HTS) Josephson junctions and circuits we are developing is based on the ion damage process established at LPEM [1, 2]. The aim of this work was to do a statistical analysis of the effect of geometrical parameters on Josephson junctions properties. Only junction length and barrier thickness are varied; film thickness is kept constant at a nominal 150nm. Statistical analysis of characteristic parameters (Ic, Rn, temperature range of operation...) is presented.