Search

Proximity effect between superconductors and ferromagnets may become long range due to the generation of triplet pairs. The recent finding of a long one micron-range unconventional Josephson effect between YBa2Cu3O7 High Tc cuprates separated by a half metallic La0.7Sr0.3MnO3 manganite ferromagnet has uncovered a novel unconventional triplet proximity effect. In this paper we examine the temperature dependence of the critical current of planar Josephson junctions. We find that the critical current –normal resistance product follows the predictions of traditional superconductor-normal metal-superconductor junctions which implies that triplet pairs in a ferromagnet are transported in the diffusive limit similarly to singlet pairs in a normal metal. This result calls for theoretical studies of the new triplet Josephson effect and underlines its potential in future superconducting spintronics.

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.

The Josephson effect results from the coupling of two superconductors across a spacer such as an insulator, a normal metal or a ferromagnet to yield a phase coherent quantum state. However, in junctions with ferromagnetic spacers, very long-range Josephson effects have remained elusive. Here we demonstrate extremely long-range (micrometric) high-temperature (tens of kelvins) Josephson coupling across the half-metallic manganite La0.7Sr0.3MnO3 combined with the superconducting cuprate YBa2Cu3O7. These planar junctions, in addition to large critical currents, display the hallmarks of Josephson physics, such as critical current oscillations driven by magnetic flux quantization and quantum phase locking effects under microwave excitation (Shapiro steps). The latter display an anomalous doubling of the Josephson frequency predicted by several theories. In addition to its fundamental interest, the marriage between high-temperature, dissipationless quantum coherent transport and full spin polarization brings opportunities for the practical realization of superconducting spintronics, and opens new perspectives for quantum computing. Josephson coupling over micrometres and at tens of kelvins is demonstrated across the half-metallic manganite La0.7Sr0.3MnO3 combined with the superconducting cuprate YBa2Cu3O7.

The electric-field effect control of two-dimensional electron gases (2-DEGs) has allowed nanoscale electron quantum transport to be explored in semiconductors. Structures based on transition metal oxides have electronic states that favour the emergence of novel quantum orders that are absent in conventional semiconductors and the 2-DEG formed at a LaAlO3/SrTiO3 interface—a structure in which superconductivity and spin–orbit coupling can coexist—is a promising platform to develop devices for spintronics and topological electronics. However, field-effect control of the properties of this interface at the nanoscale remains challenging. Here we show that a quantum point contact can be formed in a LaAlO3/SrTiO3 interface through electrostatic confinement of the 2-DEG using a split gate. Our device exhibits a quantized conductance due to ballistic transport in a controllable number of one-dimensional conducting channels. Under a magnetic field, the direct observation of the Zeeman splitting between spin-polarized bands allows the determination of the Lande g-factor, whose value differs strongly from that of the free electrons. Through source–drain voltage measurements, we also performed a spectroscopic investigation of the 3d energy levels inside the quantum point contact. The LaAlO3/SrTiO3 quantum point contact could potentially be used as a spectrometer to probe Majorana states in an oxide 2-DEG. A quantum point contact formed in the two-dimensional electron gas of a LaAlO3/SrTiO3 interface exhibits quantized conductance due to ballistic transport in a controllable number of one-dimensional conducting channels.

The term tunnel electroresistance (TER) denotes a fast, non-volatile, reversible resistance switching triggered by voltage pulses in ferroelectric tunnel junctions. It is explained by subtle mechanisms connected to the voltage-induced reversal of the ferroelectric polarization. Here we demonstrate that effects functionally indistinguishable from the TER can be produced in a simpler junction scheme—a direct contact between a metal and an oxide—through a different mechanism: a reversible redox reaction that modifies the oxide’s ground-state. This is shown in junctions based on a cuprate superconductor, whose ground-state is sensitive to the oxygen stoichiometry and can be tracked in operando via changes in the conductance spectra. Furthermore, we find that electrochemistry is the governing mechanism even if a ferroelectric is placed between the metal and the oxide. Finally, we extend the concept of electroresistance to the tunnelling of superconducting quasiparticles, for which the switching effects are much stronger than for normal electrons. Besides providing crucial understanding, our results provide a basis for non-volatile Josephson memory devices., The non-volatile switching of tunnel electroresistance in ferroelectric junctions provides the basis for memory and neuromorphic computing devices. Rouco et al. show tunnel electroresistance in superconductor-based junctions that arises from a redox rather than ferroelectric mechanism and is enhanced by superconductivity.

One of the hallmarks of the Berezinskii-Kosterlitz-Thouless (BKT) transition in two-dimensional superconductors is the universal jump of the superfluid density that can be indirectly probed via the nonlinear exponent of the current-voltage $I\text{\ensuremath{-}}V$ characteristics. Here, we compare the experimental measurements of $I\text{\ensuremath{-}}V$ characteristics in two cases, namely NbN thin films and ${\mathrm{SrTiO}}_{3}$-based interfaces. While the former display a paradigmatic example of BKT-like nonlinear effects, the latter do not seem to justify a BKT analysis. Rather, the observed $I\text{\ensuremath{-}}V$ characteristics can be well reproduced theoretically by modeling the effect of mesoscopic inhomogeneity of the superconducting state. Our results offer an alternative perspective on the spontaneous fragmentation of the superconducting background in confined two-dimensional systems.

In this article, we show that the two-dimensional electron gas formed at the ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ interface behaves as a Josephson junction array. In particular, it exhibits a stochastic switching of the superconducting critical current, which qualitatively follows the dynamics of the resistively and capacitively shunted Josephson junction model. The switching current distribution (SCD) has been measured as a function of temperature and back-gate voltage. At low temperatures a clear saturation of the standard deviation of the SCD is observed, possibly indicating the presence of a macroscopic quantum tunneling regime with phase diffusion. Through the gate voltage we modify the damping of the array and compare it to artificial arrays of junctions.

In multi-orbital materials, superconductivity can exhibit several coupled conden- sates. In this context, quantum con nement in two-dimensional superconducting oxide interfaces o ers new degrees of freedom to engineer the band structure and selectively control 3d-orbitals occupancy by electrostatic doping. Here, we use resonant microwave transport to extract the super uid sti ness of the (110)-oriented LaAlO3/SrTiO3 in- terface in the entire phase diagram. We evidence a transition from single-condensate to two-condensate superconductivity driven by continuous and reversible electrostatic doping, which we relate to the Lifshitz transition between 3d-bands based on numerical simulations of the quantum well. We nd that the superconducting gap is suppressed while the second band is populated, challenging the Bardeen-Cooper-Schrie er theory. We ascribe this behavior to the existence of superconducting order parameters with opposite signs in the two condensates, due to a repulsive coupling. Our ndings o er an innovative perspective on the possibility to tune and control multiple-orbitals physics in superconducting interfaces., The authors thank K. Behnia and J. Lorenzana for useful discussions. This work was supported by the Région Ile-de-France in the framework of CNano IdF, OXYMORE and Sesame programmes, by CNRS through a PICS programme (S2S) and ANR JCJC (Nano-SO2DEG). This work was supported by the Spanish MAT2017-85232-R, MAT2014-56063-C2-1-R and Severo Ochoa SEV-2015-0496 grants and the Generalitat de Catalunya (2017 SGR 1377). This work was supported by the Italian MAECI under the Italia–India collaborative project SUPERTOP-PGR04879. The authors acknowledge funding from the project Quantox of QuantERA ERA-NET Cofund in Quantum Technologies (grant agreement no. 731473) implemented within the European Union’s Horizon 2020 Programme. The authors also acknowledge the COST project Nanoscale Coherent Hybrid Devices for Superconducting Quantum Technologies–Action CA16218.

The Josephson junction (JJ) is the corner stone of superconducting electronics and quantum information processing. While the technology for fabricating low Tc JJ is mature and delivers quantum circuits able to reach the “quantum supremacy”, the fabrication of reproducible and low-noise high-Tc JJ is still a challenge to be taken up. Here we report on noise properties at RF frequencies of recently introduced high-Tc Josephson nano-junctions fabricated by mean of a Helium ion beam focused at sub-nanometer scale on a YBa2Cu3O7 thin film. We show that their current-voltage characteristics follow the standard Resistively-Shunted-Junction (RSJ) circuit model, and that their characteristic frequency fc = (2e/h)IcRn reaches ~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 ~ 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.

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 $60\ \mu m^{2}$, 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 ($I=60\ \mu A$) and DC magnetic field ($B=3\ \mu T$), 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.

International audience; We designed, fabricated and measured short one-dimensional arrays of masked ion-irradiated YBa 2 Cu 3 O 7 Josephson junctions embedded into log-periodic spiral antennas. They consist of 4 or 8 junctions separated either by 960 nm or 80 nm long areas of pristine material. Large spacing arrays show " Giant " Shapiro steps in the hundreds-GHz band at 66 K and are tested as Josephson mixers with improved impedance matching. On the contrary, small spacing arrays behave as one junction with a lower superconducting transition temperature, hence forming a single weak link on distances up to 880 nm. Such design opens a new way to increase the I c R n product of the devices, and therefore the efficiency of the Josephson mixers. Hints on the origin of the observed long range proximity effect are proposed.

International audience; In this article, we report on the fabrication of very long YBa2Cu3O7 nanowires in a meander shape patterned in a CeO2-capped thin film by high-energy oxygen ion irradiation. DC and RF characterizations outline the good superconducting properties of the nanowires whose geometryapproaches the one used in single photon detectors. Their inductance, which mainly sets the maximum speed of these devices, has been measured on a wide range of temperature by mean of a resonant method. The extracted values are in agreement with the ones calculated from the geometry of the meanders and from the known London penetration depth in YBa2Cu3O7 thin films.

A rather unique feature of the two-dimensional electron gas formed at the interface between the two insulators LaAlO3 and SrTiO3 is to host both gate-tunable superconductivity and strong spin-orbit coupling. In the present work, we use the disorder generated by Cr substitution of Al atoms in LaAlO3 as a tool to explore the nature of superconductivity and spin-orbit coupling in these interfaces. We analyze the transport properties of three different samples whose only relevant difference is their elastic scattering time tau(e). A reduction of the superconducting T-c is observed with Cr doping consistent with an increase of electron-electron interaction in the presence of disorder. In addition, the evolution of spin-orbit coupling with gate voltage and Cr doping evidences a Dyakonov-Perel mechanism of spin relaxation (tau(SO) proportional to tau(-1)(e)) in the presence of a Rashba-type spin-orbit interaction.

We studied vortex dynamics in a YBa 2 Cu 3 O 7−δ thin film with two different sources of pinning: intrinsic random defects and an artificial square array of defects created by masked ion irradiation. We study commensurability effects between the vortex lattice and the pinning array as a function of the vortex velocity v and the temperature. We find that at low temperatures the commensurability effects (magneto-resistance drop at the matching fields) are stronger at low velocities, in contrast with the behavior previously observed in low-critical-temperature superconductors.

The diverse phenomena associated with the two-dimensional electron gas (2DEG) that occurs at oxide interfaces include, among others, exceptional carrier mobilities, magnetism and superconductivity. Although these have mostly been the focus of interest for potential future applications, they also offer an opportunity for studying more fundamental quantum many-body effects. Here, we examine the magnetic-field-driven quantum phase transition that occurs in electrostatically gated superconducting LaTiO3/SrTiO3 interfaces. Through a finite-size scaling analysis, we show that it belongs to the (2+1)D XY model universality class. The system can be described as a disordered array of superconducting puddles coupled by a 2DEG and, depending on its conductance, the observed critical behaviour is single (corresponding to the long-range phase coherence in the whole array) or double (one related to local phase coherence, the other one to the array). A phase diagram illustrating the dependence of the critical field on the 2DEG conductance is constructed, and shown to agree with theoretical proposals. Moreover, by retrieving the coherence-length critical exponent ν, we show that the quantum critical behaviour can be clean or dirty according to the Harris criterion, depending on whether the phase-coherence length is smaller or larger than the size of the puddles.

We use magnetic flux quanta (superconducting vortices) on artificial energy landscapes (pinning arrays) to create a new type of artificial ice. This vortex ice shows unusual temperature effects that offer new possibilities in the study of ice systems. We have investigated the matching of the flux lattice to pinning arrays that present geometrical frustration. The pinning arrays are fabricated on YBCO films using masked O+ ion irradiation. The details of the magneto-resistance imply that the flux lattice organizes into a vortex ice. The absence of history-dependent effects suggests that the vortex ice is highly ordered. Due to the technique used for the artificial energy landscape fabrication, we have the ability to change the pinning array geometry using temperature as a control knob. In particular we can switch the geometrical frustration on and off, which opens the door to performing a new type of annealing absent in other artificial ice systems. * Work supported by the French ANR “MASTHER”, and the Fundacion Barrie (Galicia, Spain)

International audience; 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.

A fluorescent erbium/ytterbium co-doped fluoride nanocrystal glued at the end of a sharp atomic force microscope tungsten tip was used as a nanoscale thermometer. The thermally induced fluorescence quenching enabled observation of the heating and measurement of the temperature distribution in a Joule-heated 80 nm wide and 2 μm long titanium nanowire fabricated on an oxidized silicon substrate. The measurements have been carried out in an alternating heating mode by applying a modulated current on the device at low frequency. The heating is found to be inhomogeneous along the wire, and the temperature in its center increases quadratically with the applied current. Heat appears to be confined mainly along the wire, with weak lateral diffusion along the substrate and in the lateral metallic pads. The lateral resolution of this thermal measurement technique is better than 250 nm. It could also be used to study thermally induced defects in nanodevices.

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.

The pseudogap state in the high-temperature superconductors may be either a precursor state to superconductivity or a competing state. A direct probe of the Cooper pairs can address this conundrum.

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 ., Comment: 16 pages, 6 figures

Oxide heterostructures are of great interest both for fundamental and applicative reasons. In particular the two-dimensional electron gas at the LaAlO$_3$/SrTiO$_3$ or LaTiO$_3$/SrTiO$_3$ interfaces displays many different physical properties and functionalities. However there are clear indications that the interface electronic state is strongly inhomogeneous and therefore it is crucially relevant to investigate possible intrinsic electronic mechanisms underlying this inhomogeneity. Here the electrostatic potential confining the electron gas at the interface is calculated self-consistently, finding that the electron confinement at the interface may induce phase separation, to avoid a thermodynamically unstable state with a negative compressibility. This provides a generic robust and intrinsic mechanism for the experimentally observed inhomogeneous character of these interfaces., Comment: 8 pages and 4 figures

The field of fluxtronics is based on transmitting and processing information using quanta of magnetic flux in a type-II superconductor. In this study, temperature is used to reversibly tune the energy landscape of flux vortices pinned in a patterned sample of YBa${}_{2}$Cu${}_{3}$O${}_{7}$, providing an avenue to thermally switchable device functionalities. This nanopatterning method could also be applied to other oxide materials, such as ferromagnets or ferroelectrics, to possibly extend the concept of temperature-dependent functionality.

In this paper we investigate the effect of disorder on highly correlated electron systems, which exhibit metal-insulator transition (MIT) and structural-phase transition (SPT). We show that the effect of ion irradiation is strikingly different between ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ and $\mathrm{V}{\mathrm{O}}_{2}$, two otherwise similar materials. Upon irradiation, the MIT and SPT temperatures in ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ decrease drastically at low absolute dosages, much lower than for $\mathrm{V}{\mathrm{O}}_{2}$. At a low threshold dose, the insulating state of ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ drastically collapses into a metallic state. Contrary to this, irradiation of $\mathrm{V}{\mathrm{O}}_{2}$ leads to a much milder reduction of the MIT and SPT temperatures and to a weak, gradual decrease of the insulating state resistivity---not suppressed even at one order of magnitude higher doses than the ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ threshold. These major differences imply that the phase transition in ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ arises from global (rather than local as in $\mathrm{V}{\mathrm{O}}_{2}$) physical mechanisms that are extremely sensitive to disorder. This shows that the MIT and SPT may have substantially different physical origins in different systems, with the consequent major implications for theoretical descriptions of the MIT in highly correlated electron systems.

Hybrid nanostructures not only open new routes for studying the coexistence of superconductivity and ferromagnetism, but they also provide new insights into the fundamentals of quantum electronics. We have investigated, by a series of experiments, the change in the superconducting wavefunction resulting from the interaction between the spins of a Cooper pair and the exchange field. Tunneling spectroscopy reveals an inhomogeneous Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) superconducting state induced in a ferromagnetic thin film by the proximity effect. As a consequence, critical current and macroscopic quantum interference (SQUID) experiments show π -coupling between two superconductors coupled through a ferromagnetic thin film. π -coupling originates a spontaneous half quantum flux in a superconducting ring. This phase transition has been observed by Josephson and Hall magnetometry. To cite this article: M. Aprili et al., C. R. Physique 7 (2006).

Phase sensitive experiments have shown that the gap function of HTC superconductors changes of sign in the way expected for a condensate wave function with d x 2 − y 2 symmetry suggesting unconventional pairing. We present tunneling spectroscopy, Josephson effect and quantum interference measurements on superconducting/ferromagnetic (S/F) nanostructures based on a conventional superconductor: Nb. We observe a change sign oscillating order parameter induced into the ferromagnet by the proximity effect. This new inhomogeneous superconducting state indicates that a negative order parameter and hence π -coupling does not necessarily require unconventional pairing. More generally, it suggests that S/F hybrid nanostructures are an interesting alternative to investigate the interplay between magnetic order and superconductivity.

Recently, numerous experiments have been reported on critical current reductions in thin films cuprates as being due to spin injection from ferromagnetic manganites. However, little theoretical justification for these very strong effects exists, and the necessary spin relaxation length is always larger than predictions. In the present work, we investigate the possibility that these effects are due to a different origin and we report on devices designed such that the temperature of the layer itself can be measured in situ. Our data show that similar reductions of the critical current are quantitatively correlated to heating in the manganite electrode due to dissipation of the polarization current.

YBa 2 Cu 3 O 7−δ /PrBa 2 Cu 3 O 7−δ (YBCO/PBCO) bilayers [1 0 0] and [1 1 0] oriented are in situ grown by a reactive co-deposition technique on SrTiO 3 single-crystal substrates. Atomic force microscopy and transmission electron microscopy images, X-rays diffraction patterns have been used to investigate their structural properties and surface morphology, in order to optimize their characteristics. To obtain high-quality YBCO [1 0 0] and [1 1 0] layers, we first deposit a PBCO template layer at 630°C, followed by a YBCO film at 630–750°C, with a growth rate of 0.063 nm/s. They display a very high crystallinity and a surface roughness as low as 0.5 nm for YBCO/PBCO [1 0 0] bilayer at a 1 μm 2 scale. Reproducible Josephson junctions have been made on these films.

Several experiments reveal the inhomogeneous character of the superconducting state that occurs when the carrier density of the two-dimensional electron gas formed at the LaXO3/SrTiO3 (X=Al or Ti) interface is tuned above a threshold value by means of gating. Re-analyzing previous measurements, that highlight the presence of two kinds of carriers, with low and high mobility, we shall provide a description of multi-carrier magneto-transport in an inhomogeneous two-dimensional electron gas, gaining insight into the properties of the physics of the systems under investigation. We shall then show that the measured resistance, superfluid density, and tunneling spectra result from the percolative connection of superconducting "puddles" with randomly distributed critical temperatures, embedded in a weakly localizing metallic matrix. We shall also show that this scenario is consistent with the characteristics of the superconductor-to-metal transition driven by a magnetic field. A multi-carrier description of the superconducting state, within a weak-coupling BCS-like model, will be finally discussed., Comment: 12 pages 10 figures

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.

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.

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. © 2014 AIP Publishing LLC. Fil: Malnou, M.. Laboratoire de Physique et d’Etude des Materiaux; Francia Fil: Feuillet-Palma, C.. Laboratoire de Physique et d’Etude des Materiaux; Francia Fil: Ulysse, C.. Laboratoire de Photonique et de Nanostructures LPN-CNRS; Francia Fil: Faini, G.. Laboratoire de Photonique et de Nanostructures LPN-CNRS; Francia Fil: Febvre, P.. Universite de Savoie; Francia Fil: Sirena, Martin. Comision Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Resonancias Magnéticas; Argentina Fil: Olanier, L.. Laboratoire de Physique et d’Etude des Materiaux; Francia Fil: Lesueur, J.. Laboratoire de Physique et d’Etude des Materiaux; Francia Fil: Bergeal, N.. Laboratoire de Physique et d’Etude des Materiaux; Francia

The problem of an ensemble of repulsive particles on a potential-energy landscape is common to many physical systems and has been studied in multiple artificial playgrounds. However, the latter usually involve fixed energy landscapes, thereby impeding in situ investigations of the particles' collective response to controlled changes in the landscape geometry. Here, we experimentally realize a system in which the geometry of the potential-energy landscape can be switched using temperature as the control knob. This realization is based on a high-temperature superconductor in which we engineer a nanoscale spatial modulation of the superconducting condensate. Depending on the temperature, the flux quanta induced by an applied magnetic field see either a geometrically frustrated energy landscape that favours an ice-like flux ordering, or an unfrustrated landscape that yields a periodic flux distribution. This effect is reflected in a dramatic change in the superconductor's magneto-transport. The thermal switching of the energy landscape geometry opens new opportunities for the study of ordering and reorganization in repulsive particle manifolds.

We report transport measurements on Superconductor/Ferromagnet/Superconductor (S/F/S) junctions: Nb/Al/Gd/Al/Nb where gadolinium (Gd) is a weakly polarized ferromagnet. A sizeable critical current I c is observed in the I(V) characteristics. This current can be modulated by a weak magnetic field, as expected for a Josephson current. With these experiments, we establish that superconducting coherent transport survives across a small ferromagnetic layer. The penetration depth of Cooper pairs in Gd has been measured. An extensive study of the Josephson critical current in temperature for different thicknesses of magnetic compounds is presented. A comparison of transport measurements with S/N/S junction is given through measurements made on Nb/Al/Y/Al/Nb, where yttrium (Y) is used as non magnetic rare earth metal.

Cu x Mo 6 S 8 thin films have been grown in situ by pulsed laser deposition (PLD) on R-cut Al 2 O 3 single-crystal substrates. Under selected deposition conditions, they are oriented and present epitaxial relationships with the in-plane substrate directions. The influence of deposition parameters on composition or epitaxial quality has been studied in detail. In most samples, an excess of metallic molybdenum has been detected by X-ray diffraction and quantified by Rutherford backscattering spectrometry (RBS). The deposition under argon (8.10 −2 mbar) at 870°C with a target–substrate distance of 38 mm and an average laser fluence of 5 J cm −2 , has entailed a significant reduction of this secondary phase. Moreover, using a small-sized, home made vacuum chamber, single-phased Cu x Mo 6 S 8 films have been in situ grown for the first time. The epitaxial relations have been investigated by X-ray diffraction in various modes (namely θ-2θ, θ-scans and ϕ-scans) and by scanning electron microscopy. Under secondary vacuum, (100) Rh oriented thin films are obtained above 820°C and a progressive improvement of the epitaxial growth has been observed up to 890°C. The superconducting critical temperature T c , measured by the a.c. susceptibility method, appears insensitive to the microstructural modifications. Such deposition conditions have been applied to other substrates like Al 2 O 3 C, (100) MgO or (100) Y: ZrO 2 and have led to new complex epitaxial growth relations.

Pour comprendre la supraconductivite a haute temperature critique, et tester les modeles theoriques proposes, l'effet tunnel et le couplage Josephson sont des outils tres importants. En mesurant la densite d'etats au niveau de Fermi d'une part, et les proprietes intrinseques du parametre d'ordre supraconducteur d'autre part, ces experiences apportent un eclairage indispensable sur la physique de ces composes. Cependant, ces techniques sont tributaires de la qualite de surface des materiaux, ainsi que de leur microstructure (joints de grains, macles, defauts etendus...). Dans un premier temps, nous presentons une vue generale de la supraconductivite a haute Tc aujourd'hui et des questions fondamentales qui restent en suspend. Ensuile, nous analysons les principaux resultats d'effet tunnel et Josepshon. Enfin, nous discutons l'influence des imperfections cristallines sur ces mesures, et sur les conclusions qui peuvent en etre tirees.

We have made in-situ [103] and [110]YBCO/Pb junctions, which display an hysteretic behaviour with both a well-defined Quasi-Particule branch and a Josephson current. A zero energy conductance peak is observed in the tunneling conductance — which is a signature of Andreev Bound States (ABS) at a surface of a d-wave superconductor — which splits at low temperature. This may confirm the existence of a surface state which breaks time-reversal symmetry. However, up to now, we have not seen any evidence of a coupling between Josephson current and the zero-energy ABS, predicted by Y. Tanaka and S. Kashiwaya 1 .

Narrow ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ films, excited by pulses of supercritical current, had their nanosecond electric response monitored in zero applied magnetic field. Delayed voltage steps plus constant differential resistance, characteristic of phase-slip centers (PSC), are observed at all temperatures. The duration of the initial zero voltage state is well fit by Ginzburg-Landau based theories, with a gap relaxation time controlled by phonon escape. At higher levels of excitation, PSC's give birth to slowly spreading normal hot spots.

(0 0 1)CeO 2 thin films and bilayer (0 0 1)YBa 2 Cu 3 O 7 /(0 0 1)CeO 2 films grown in situ by pulsed laser deposition (PLD) on single crystal (1 1 0 2) sapphire have been investigated. CeO 2 buffer layers exhibit single crystal-like quality as proven by the full-width at half-maximum (FWHM) of the 0 0 2 rocking curve (Δ θ ∼0.03°) and the channeling minimum yield measured by baskscattering spectrometry (RBS) ( χ min ∼3%). Therefore, the YBa 2 Cu 3 O 7 /CeO 2 /Al 2 O 3 heterostructures present sharp interfaces as shown by the observation of Laue oscillations on both CeO 2 and YBa 2 Cu 3 O 7 XRD patterns. In agreement with this crystalline perfection, YBa 2 Cu 3 O 7 thin films present excellent and reproducible superconducting properties: T c ( R =0) >88.5 K, transition width ∼0.5 K and microwave surface resistance R s 0.5–2 mΩ at 10 GHz and 77 K.

The 2D electron gas (2DEG) formed at the LaXO 3/ SrTiO 3( X = Al , Ti ) oxide interface appears inhomogeneous in several experiments. In particular, we discuss evidences of electron inhomogeneities provided by the phenomenology of the superconducting (SC) phase, which occurs when the carrier density is tuned above a critical value by means of gating, and of the superconductor-to-metal transition driven by gate voltage or magnetic field. The measured resistance and superfluid density result from the percolative connection of superconducting "puddles" with randomly distributed critical temperatures, embedded in a weakly localizing metallic matrix. We propose a possible intrinsic origin of the electron inhomogeneity, resulting from the strong Rashba spin–orbit coupling (RSOC) measured at these oxide interfaces.

Thin films of Cu 4 Mo 6 S 8 have been deposited in situ by pulsed laser ablation on (110) R sapphire substrate. The high quality of films has enabled us to perform chemical treatments. Desintercalation reactions carried out in hydrochloric acid solution has led to metastable Mo 6 S 8 as shown by X-ray diffraction (XRD) and Rutherford Backscattering Spectroscopy (RBS). In some cases an exchange reaction has been shown to take place between copper and silver, which comes from the silver paint used to stick the substrate and has allowed us to obtain AgMo 6 S 8 film. The copper reinsertion performed on a desintercalated film has succeeded, proving the reversibility of the desintercalation reaction.

We have investigated the scaling effects on the electron interface in inhomogeneous Ni films near the percolation threshold. As a result of the high exchange interaction of the conduction electrons, the conductance-anomaly at low temperature is due to the enhancement of the Coulomb e-e interactions by disorder. However, the magnetoresistance analysis shows that a small contribution of weak electron localization remains, while the enhancement of the e-e interactions is insensitive to the applied magnetic field. The signature of the modified dimensionality introduced by the percolating structure appears at very low temperature (T < 4 K) when the phase coherence scale is of the same order as the percolation length. In this regime, the study of the magnetoresistance renormalization proves that localization and magnetism coexist in itinerant ferromagnetic materials.

Thin films of Chevrel phases CuxMo6S8 (2 ≤ x ≤ 4) have been in-situ deposited by laser ablation. Epitaxial growth of these films have been achieved on suitable substrates. CuxMo6S8 epitaxial layers have been grown on R-cut Al2O3 single-crystal substrates (corresponding to the (1 1 0 2)Hex or (1 1 0)R orientation in the hexagonal or rhombohedral indexing, respectively) and present a (1 0 0)R preferential orientation as evidenced by X-ray diffraction in different modes (i.e. θ−2θ, φ-scans). The film composition has been checked by Rutherford backscattering spectrometry. The superconducting transition temperature has been determined from d.c. resistive measurements and a.c. susceptibility measurements. The critical temperature Tc (R = 0) is typically between 9.5 and 10.3 K for x ∼ 2 and decreases when x increases, in good agreement with the bulk materials behaviour. Moreover the superconducting transition is narrow, the full width at half maximum of the in quadrature inductive signal, χ″, ranging between 0.1 and 0.5 K.

All {ital in situ} YBu{sub 2}Cu{sub 3}O{sub 7}/Pb and YBa{sub 2}Cu{sub 3}O{sub 7}/Ag/Pb planar tunnel junctions have been successfully made on c-axis-oriented YBa{sub 2}Cu{sub 3}O{sub 7} thin films grown by a coevaporation technique, which show very-low-leakage I-V characteristics. The junctions are hysteretic and display a Josephson current I{sub c} below 7.2 K (lead T{sub c}). The only modification introduced by Ag is a substantial lowering of the normal-state resistance R{sub n}, but the I{sub c}R{sub n} product remains roughly constant. The magnetic-field modulation of I{sub c} shows that the current distribution is rather homogeneous over the whole junction area (0.5 mm{sup 2}), and follows the expected behavior for a short junction on a thin film (thickness {lt}{lambda}{sub ab}). The Josephson coupling between a conventional superconductor and the high-T{sub c} compound YBa{sub 2}Cu{sub 3}O{sub 7} along its c axis indicates that an s component is present in the superconducting order parameter of this material. {copyright} {ital 1997} {ital The American Physical Society}

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.