arXiv subjects

François Couëdo

Publications and source records attributed to François Couëdo.

7 recordsLinked to original sources

arXiv2025

Robust quantum Hall resistance standard from uniform wafer-scale epitaxial graphene on SiC

We report high-precision resistance measurements on quantum Hall resistance devices fabricated from uniform epitaxial graphene grown by propane-hydrogen chemical vapor deposition on a two-inch silicon carbide substrate. Through molecular doping, we achieve a low carrier density regime ($n_\mathrm s < $ 1.5 \textperiodcentered 10$^{11}$ cm$^{-2}$) combined with high mobility ($\upmu \geq$ 6000 cm$^2$ V$^{-1}$ s$^{-1}$) at low temperature. Accurate quantization of the Hall resistance is demonstrated at magnetic flux densities as low as 3.5 T, temperatures up to 8 K, and measurement currents up to 325 $\upmu$A, with relative measurement uncertainties of a few parts per billion. A stability diagram mapping dissipation as a function of temperature and current provides insight into optimal doping conditions that maximize the breakdown current. All measurements were carried out in a pulse-tube-based cryomagnetic system, enabling simplified and continuous operation of the quantum Hall resistance standard without liquid helium consumption.

cond-mat.mes-hall↗

arXiv2020

Dynamic properties of high-Tc superconducting nano-junctions made with a focused helium ion beam

The Josephson junction (JJ) is the corner stone of superconducting electronics and quantum information processing. While the technology for fabricating low $T_{c}$ JJ is mature and delivers quantum circuits able to reach the "quantum supremacy", the fabrication of reproducible and low-noise high-$T_{c}$ JJ is still a challenge to be taken up. Here we report on noise properties at RF frequencies of recently introduced high-$T_{c}$ Josephson nano-junctions fabricated by mean of a Helium ion beam focused at sub-nanometer scale on a $\mathrm{YBa}_2\mathrm{Cu}_3\mathrm{O}_7$ thin film. We show that their current-voltage characteristics follow the standard Resistively-Shunted-Junction (RSJ) circuit model, and that their characteristic frequency $f_{c}=(2e/h)I_{c}R_{n}$ reaches $\sim 300$ GHz at low temperature. Using the "detector response" method, we evidence that the Josephson oscillation linewidth is only limited by the thermal noise in the RSJ model for temperature ranging from $T\sim 20$ K to $75$ K. At lower temperature and for the highest He irradiation dose, the shot noise contribution must also be taken into account when approaching the tunneling regime. We conclude that these Josephson nano-junctions present the lowest noise level possible, which makes them very promising for future applications in the microwave and terahertz regimes.

cond-mat.supr-con↗

arXiv2019

High-Tc superconducting detector for highly-sensitive microwave magnetometry

We have fabricated arrays of High-T$_c$ Superconducting Quantum Interference Devices (SQUIDs) with randomly distributed loop sizes as sensitive antennas for Radio-Frequency (RF) waves. These sub-wavelength size devices known as Superconducting Quantum Interference Filters (SQIFs) detect the magnetic component of the electromagnetic field. We use a scalable ion irradiation technique to pattern the circuits and engineer the Josephson junctions needed to make SQUIDs. Here we report on a 300 SQUIDs series array with loops area ranging from $6$ to $60\ μm^{2}$, folded in a meander line covering a $3.5\ mm\times 8\ mm$ substrate area, made out of a $150$-nm-thick $\mathrm{YBa}_2\mathrm{Cu}_3\mathrm{O}_7$ film. Operating at a temperature $T=66\ K$ in a un-shielded magnetic environment, under low DC bias current ($I=60\ μA$) and DC magnetic field ($B=3\ μT$), this SQIF can detect a magnetic field of a few $pT$ at a frequency of $1.125\ GHz$, which corresponds to a sensitivity of a few hundreds of $fT/\sqrt{Hz}$, and shows linear response over 7 decades in RF power. This work is a promising approach for the realization of low dissipative sub-wavelength GHz magnetometers.

cond-mat.supr-con↗

arXiv2018

Static and Radio-frequency magnetic response of high Tc 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.

physics.app-ph↗

arXiv2016

Engineering quantum spin Hall insulators by strained-layer heterostructures

Quantum spin Hall insulators (QSHIs), also known as two-dimensional topological insulators, have emerged as an unconventional class of quantum states with insulating bulk and conducting edges originating from nontrivial inverted band structures, and have been proposed as a platform for exploring spintronics applications and exotic quasiparticles related to the spin-helical edge modes. Despite theoretical proposals for various materials, however, experimental demonstrations of QSHIs have so far been limited to two systems--HgTe/CdTe and InAs/GaSb--both of which are lattice-matched semiconductor heterostructures. Here we report transport measurements in yet another realization of a band-inverted heterostructure as a QSHI candidate--InAs/In$_{x}$Ga$_{1-x}$Sb with lattice mismatch. We show that the compressive strain in the In$_{x}$Ga$_{1-x}$Sb layer enhances the band overlap and energy gap. Consequently, high bulk resistivity, two orders of magnitude higher than for InAs/GaSb, is obtained deep in the band-inverted regime. The strain also enhances bulk Rashba spin-orbit splitting, leading to an unusual situation where the Fermi level crosses only one spin branch for electronlike and holelike bands over a wide density range. These properties make this system a promising platform for robust QSHIs with unique spin properties and demonstrate strain to be an important ingredient for tuning spin-orbit interaction.

cond-mat.mes-hall↗

arXiv2016

Dissipative phases across the superconductor-to-insulator transition

Competing phenomena in low dimensional systems can generate exotic electronic phases, either through symmetry breaking or a non-trivial topology. In two-dimensional (2D) systems, the interplay between superfluidity, disorder and repulsive interactions is especially fruitful in this respect although both the exact nature of the phases and the microscopic processes at play are still open questions. In particular, in 2D, once superconductivity is destroyed by disorder, an insulating ground state is expected to emerge, as a result of a direct superconductor-to-insulator quantum phase transition. In such systems, no metallic state is theoretically expected to survive to the slightest disorder. Here we map out the phase diagram of amorphous NbSi thin films as functions of disorder and film thickness, with two metallic phases in between the superconducting and insulating ones. These two dissipative states, defined by a resistance which extrapolates to a finite value in the zero temperature limit, each bear a specific dependence on disorder. We argue that they originate from an inhomogeneous destruction of superconductivity, even if the system is morphologically homogeneous. Our results suggest that superconducting fluctuations can favor metallic states that would not otherwise exist.

cond-mat.supr-con↗

arXiv2016

Single-edge transport in an InAs/GaSb quantum spin Hall insulator

We report transport measurements in a single edge channel of an InAs/GaSb quantum spin Hall insulator, where the conduction occurs through only one pair of counterpropagating edge modes. By using a specific sample design involving highly asymmetric current paths, we electrically isolate a single edge channel of the two-dimensional topological insulator from the other edge. This enables us to probe a single edge by multiterminal measurements. Both two-terminal and four-terminal resistances show a nearly quantized plateau around $h/e^2$ for a 4-$μ$m-long edge, indicating quasiballistic transport. Our approach is advantageous in that it allows us to gain insight into a microscopic region from local measurements.

cond-mat.mes-hall↗