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We study the low frequency admittance of a quantum Hall bar of size much larger than the electronic coherence length. We find that this macroscopic conductor behaves as an ideal quantum conductor with vanishing longitudinal resistance and purely inductive behavior up to f, 4 pages article with 4 figures, submitted to Physical Review B Letters, concatenated with 12 pages supplementary information (having 15 figures) in a 17 pages article with concantenated bibliography

International audience; The giant negative magnetoresistance (GNMR), observed in two-dimensional electron gases of high mobility, is studied by controlled definition of additional short-range scatterers in form of two-dimensional Lorentz arrays of varying obstacle density, as well as in the presence of edge scattering. The results support models, which ascribe the temperature-independent regime of the GNMR to strong, classical scattering and the temperaturedependent regime to electron-electron interactions under the influence of mixed disorder. The threshold magnetic field, which separates the two regimes, is in rough agreement with the lower percolation transition of the Lorentz array. At large obstacle densities, interaction corrections are suppressed and memory effects become more relevant. Shape, amplitude, and width of the GNMR depend sensitively on the time scales of the contributing scattering mechanisms. This can lead to qualitatively similar shapes for quite different parameter values.

International audience; Magnetic skyrmions are deemed to be the forerunners of novel spintronic memory and logic devices. While their observation and their current-driven motion at room temperature have been demonstrated, certain issues regarding their nucleation, stability, pinning and skyrmion Hall effect still need to be overcome to realise functional devices. Here we demonstrate that focused He+-ion-irradiation can be used to create and guide skyrmions in racetracks. We show that the weakening of the perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction in the track defined by ion-irradiation leads to the formation of stable isolated skyrmions. Current-driven skyrmion motion experiments and simulations reveal that the skyrmions move along the irradiated track, resulting in a suppression of the skyrmion Hall effect, and that the skyrmion velocity and confinement can be enhanced by tuning the magnetic properties. These results open up a new path to nucleate and guide skyrmions in racetrack devices.

Novel physical properties appear when the size of a superconductor is reduced to the nanoscale, in the range of its superconducting coherence length (ξ0). Such nanosuperconductors are being investigated for potential applications in nanoelectronics and quantum computing. The design of three-dimensional nanosuperconductors allows one to conceive novel schemes for such applications. Here, we report for the first time the use of a He+ focused-ion-beam-microscope in combination with the W(CO)6 precursor to grow three-dimensional superconducting hollow nanowires as small as 32 nm in diameter and with an aspect ratio (length/diameter) of as much as 200. Such extreme resolution is achieved by using a small He+ beam spot of 1 nm for the growth of the nanowires. As shown by transmission electron microscopy, they display grains of large size fitting with face-centered cubic WC1–x phase. The nanowires, which are grown vertically to the substrate, are felled on the substrate by means of a nanomanipulator for their electrical characterization. They become superconducting at 6.4 K and show large critical magnetic field and critical current density resulting from their quasi-one-dimensional superconducting character. These results pave the way for future nanoelectronic devices based on three-dimensional nanosuperconductors., This work was supported by the financial support from Spanish Ministry of Economy and Competitiveness through the projects MAT2014-51982-C2-1-R, MAT2014-51982-C2-2-R, MAT2017-82970-C2-2-R, and from regional Gobierno de Aragon through project E26 and E81 with European Social Fund. R.C. acknowledges Juan de la Cierva-Incorporacion 2014 program. This project has received funding from the EU-H2020 research and innovation programme under Grant Agreement No. 654360 NFFA-Europe.

International audience

International audience; The magnetotransport of an electron gas in a two-dimensional, random arrangement of overlapping retrore-flective crosses is studied using a combination of experiment and classical event-driven molecular dynamics simulation. The experimentally measured magnetoconductivity σ xx as a function of the magnetic field B displays an anomalous behavior at low B fields, accompanied with an increase of σ xx with increasing temperature at B = 0. The simulations show that at B = 0 the magnetoconductivity does not exist and is associated with anomalous diffusion in the asymptotic long-time limit that depends on the orientational order of the obstacles. At any finite B field, the motion of the tracer particle is asymptotically diffusive but the diffusion coefficient vanishes with a power law in the limit B → 0. Introduction. Anomalous diffusion processes such as sub-and superdiffusion are ubiquitous in heterogeneous media [1-3]. However, there are only a few particle-based model systems that exhibit anomalous diffusion as an asymptotic process in the long-time limit. A celebrated example in this context is the disordered Lorentz gas (LG) [4] where, in its simplest version, a tracer particle probes the void space of a random arrangement of overlapping spherical objects. In this model, a conductor-insulator transition at a critical obstacle density n c coincides with the percolation transition of the obstacles, implying subdiffusion due to the fractal structure of the free space at n c [3,5-16]. Evidence for a completely different type of asymptotic subdiffusion has been found for Ehrenfest's wind-tree (EWT) model [17]. Here, a point tracer particle moves through the void space of a two-dimensional (2D) random arrangement of overlapping or nonoverlapping squares. The flat boundaries of the squares induce retracing events (or retroreflections) where the tracer particle is reflected back such that even in short-time windows it returns several times to a region where it has been earlier. For overlapping squares, kinetic theory for low densities [18-20] predicts that the multiple backscattering events of the tracer particle result in a mean-squared displacement (MSD) with an asymptotic sublinear growth in the long-time limit δr 2 (t) ∝ t 1−4n /3 (with n = na 2 a dimensionless obstacle density, where n is the number density of the squares and 2a the length of their diagonals). An early simulation study [21] has qualitatively confirmed the existence of anomalous diffusion in the EWT model with overlapping squares. These results for the EWT model raise many unsolved questions about anomalous transport in heterogeneous 2D

Currently, the patterning of innovative three-dimensional (3D) nano-objects is required for the development of future advanced electronic components. Helium ion microscopy in combination with a precursor gas can be used for direct writing of three-dimensional nanostructures with a precise control of their geometry, and a significantly higher aspect ratio than other additive manufacturing technologies. We report here on the deposition of 3D hollow tungsten carbide nanowires with tailored diameters by tuning two key growth parameters, namely current and dose of the ion beam. Our results show the control of geometry in 3D hollow nanowires, with outer and inner diameters ranging from 36 to 142 nm and from 5 to 28 nm, respectively; and lengths from 0.5 to 8.9 µm. Transmission electron microscopy experiments indicate that the nanowires have a microstructure of large grains with a crystalline structure compatible with the face-centered cubic WC1−x phase. In addition, 3D electron tomographic reconstructions show that the hollow center of the nanowires is present along the whole nanowire length. Moreover, these nanowires become superconducting at 6.8 K and show high values of critical magnetic field and critical current density. Consequently, these 3D nano-objects could be implemented as components in the next generation of electronics, such as nano-antennas and sensors, based on 3D superconducting architectures.

The magnetotransport of two-dimensional Lorentz gases is studied in the presence of two types of additional disorder. We observe that at small magnetic fields, dynamic background potentials cause an increase of the conductivity, while it decreases in response to static background potentials, implemented by a secondary Lorentz gas with large mean free path. At intermediate and large magnetic fields, the effect of the additional disorder is independent of its character but changes sign at a threshold magnetic field. The observed phenomenology is interpreted in terms of the influence the background scattering exerts on classical memory effects, in particular on retroreflection, transient superdiffusive motion and drift along the edges of obstacle clusters.

Magnetic domain walls are topological solitons whose internal structure is set by competing energies which sculpt them. In common ferromagnets, domain walls are known to be of either Bloch or N\'eel types. Little is established in the case of Mn$_3$Sn, a triangular antiferromagnet with a large room-temperature anomalous Hall effect, where domain nucleation is triggered by a well-defined threshold magnetic field. Here, we show that the domain walls of this system generate an additional contribution to the Hall conductivity tensor and a transverse magnetization. The former is an electric field lying in the same plane with the magnetic field and electric current and therefore a planar Hall effect. We demonstrate that in-plane rotation of spins inside the domain wall would explain both observations and the clockwise or anticlockwise chirality of the walls depends on the history of the field orientation and can be controlled., Comment: 8 pages, 5 figures, Supplemental Material is included

Novel schemes based on the design of complex three-dimensional (3D) nanoscale architectures are required for the development of the next generation of advanced electronic components. He+ focused-ion-beam (FIB) microscopy in combination with a precursor gas allows one to fabricate 3D nanostructures with an extreme resolution and a considerably higher aspect ratio than FIB-based methods, such as Ga+ FIB-induced deposition, or other additive manufacturing technologies. In this work, we report the fabrication of 3D tungsten carbide nanohelices with on-demand geometries via controlling key deposition parameters. Our results show the smallest and highest-densely packed nanohelix ever fabricated so far, with dimensions of 100 nm in diameter and aspect ratio up to 65. These nanohelices become superconducting at 7 K and show a large critical magnetic field and critical current density. In addition, given its helical 3D geometry, fingerprints of vortex and phase-slip patterns are experimentally identified and supported by numerical simulations based on the time-dependent Ginzburg–Landau equation. These results can be understood by the helical geometry that induces specific superconducting properties and paves the way for future electronic components, such as sensors, energy storage elements, and nanoantennas, based on 3D compact nanosuperconductors.

International audience; We study the evolution of the surface-plasmon resonances of individual ion-beam-shaped prolate gold nanoparticles embedded in a dielectric SiO 2 environment by electron-energy-loss spectroscopy mapping in a scanning transmission electron microscope. The controlled symmetric dielectric environment obtained through the ion-beam-shaping method allows a direct quantitative comparison with numerical results obtained through simulations (auxiliary differential-equation finite-difference time-domain and boundary-element method) and with theoretical results obtained through analytical models (quasistatic model for prolate nanoellipsoids and waveguide model for infinite one-dimensional plasmonic waveguides), with which our experimental results are in very good agreement. We confirm the accuracy of state-of-the-art numerical tools and analytical theories that establish ion-beam shaping as a very promising method to design metal-dielectric nanocomposites with well-predicted optical properties, and with many possible applications in surface-enhanced Raman spectroscopy and second-harmonic generation, as well as in conventional applications of metamaterials like negative refraction, superimaging, and invisibility cloaking.

International audience; Single-digit nanometer patterning by nanoimprint lithography is a challenging task, which requires optimum stamp fabrication technique. In the current work, we present different strategies for technology of hard master stamps to make intermediate working stamps with sub-10 nm features. Methods of both negative and positive master stamps fabrication, based on EBL, RIE and ALD are described and compared. A single-step copying of negative master stamps using a polymer material is a preferred strategy to reach the ultra high-resolution. Lines as small as 5.6 nm are demonstrated in a resist using a combined thermal and UV-imprint with OrmoStamp material as a working stamp.

We introduce a new scanning probe technique derived from scanning gate microscopy (SGM) in order to investigate thermoelectric transport in two-dimensional semiconductor devices. The thermoelectric scanning gate Microscopy (TSGM) consists in measuring the thermoelectric voltage induced by a temperature difference across a device, while scanning a polarized tip that locally changes the potential landscape. We apply this technique to perform interferometry of the thermoelectric transport in a quantum point contact (QPC). We observe an interference pattern both in SGM and TSGM images, and evidence large differences between the two signals in the low density regime of the QPC. In particular, a large phase jump appears in the interference fringes recorded by TSGM, which is not visible in SGM. We discuss this difference of sensitivity using a microscopic model of the experiment, based on the contribution from a resonant level inside or close to the QPC. This work demonstrates that combining scanning gate microscopy with thermoelectric measurements offers new information as compared to SGM, and provides a direct access to the derivative of the device transmission with respect to energy, both in amplitude and in phase., Accepted for publication in Physical Review Applied

International audience; Electron beam lithography (EBL) at sub-10 nm resolution is mainly limited by resist contrast and proximity effects. In this work, we investigate the use of a recently developed alumina-based resist as a negative-tone resist for electron EBL at 100 keV and focused helium ion beam lithography (FHIBL). The resist is synthesized using a sol-gel method and turns into a near completely inorganic alumina system when exposed to the electron/ion beam. We first investigate the effect on the resist contrast curve on i) development temperature; ii) stability of the resist after exposure and before post-baking and development; and iii) aging of the resist solution. We demonstrate the patterning of isolated features as small as 6.5 nm using an EBL and 5 nm using FHIBL and a resolution down to 10 nm for FHIB exposed films. Finally, we demonstrate the pattern transfer of 10 nm lines with an aspect ratio of 10 in silicon, using an optimized reactive ion etching process.

Conference on Precision Electromagnetic Measurements (CPEM), Paris, FRANCE, JUL 08-13, 2018; International audience; A few years ago, we demonstrated that graphene can markedly surpass GaAs/AlGaAs semiconductor heterostructures for the implementation of the quantum Hall resistance standard in relaxed experimental conditions, while preserving a state-of-the-art accuracy [1]. This paper describes supplementary quantum transport experiments performed on a significant number of graphene based quantum Hall devices obtained from graphene grown by CVD on SiC, like the one used for the above-mentioned demonstration, and with similar process and preparation. The objectives were to investigate the sample-to-sample reproducibility of the electronic properties and device performance, the structural key control parameter and the underpinning physics.

Two-dimensional Lorentz gases formed by obstacles in the shape of circles, squares, and retroreflectors are reported to show a pronounced linear negative magnetoresistance at small magnetic fields. For circular obstacles at low number densities, our results agree with the predictions of a model based on classical retroreflection. In extension to the existing theoretical models, we find that the normalized magnetoresistance slope depends on the obstacle shape and increases as the number density of the obstacles is increased. The peaks are furthermore suppressed by in-plane magnetic fields as well as by elevated temperatures. These results suggest that classical retroreflection can form a significant contribution to the magnetoresistivity of two-dimensional Lorentz gases, while contributions from weak localization cannot be excluded, in particular for large obstacle densities.

We present a study of the lower critical field, ${H}_{c1}$, of ${\mathrm{SrTi}}_{1\ensuremath{-}x}{\mathrm{Nb}}_{x}{\mathrm{O}}_{3}$ as a function of carrier concentration with the aim of quantifying the superfluid density. At low carrier concentration (i.e., the underdoped side), superfluid density and the carrier concentration in the normal state are equal within experimental margin. A significant deviation between the two numbers starts at optimal doping and gradually increases with doping. The inverse of the penetration depth and the critical temperature follow parallel evolutions as in the case of cuprate superconductors. In the overdoped regime, the zero-temperature superfluid density becomes much lower than the normal-state carrier density before vanishing all together. We show that the density mismatch and the clean-to-dirty crossover are concomitant. Our results imply that the discrepancy between normal and superconducting densities is expected whenever the superconducting gap becomes small enough to put the system in the dirty limit. A quantitative test of the dirty BCS theory is not straightforward, due to the multiplicity of the bands in superconducting strontium titanate.

International audience; In a two dimensional electron gas, low energy transport in presence of a magnetic field occurs in chiral 1D channels located on the edge of the sample. In the AC description of quantum transport, the emittance determines the amplitude of the imaginary part of the admittance, whose sign and physical meaning are determined by the sample topology: a Hall bar is inductive while a Corbino ring is capacitive. In this article, the perfect capacitive character of Corbino samples in the quantum Hall effect regime is shown. A vanishing conductance and an electrochemical capacitance which depends on the density of states of 1D channels are measured. Our samples have no gate, neither on the side nor on the top, and the inner capacitances are measured. The transit time of electrons across the device is obtained and the drift velocity of carriers is deduced.

In this work, we have focused on the elaboration and characterization of nanostructured L10-CoPt magnetic dot arrays prepared by depositing Co50Pt50 nanoparticles preformed in the gas phase on Si substrates patterned by electron beam lithography (EBL). The MFM observations have revealed an out-of-plane single domain state. The VSM measurements have indicated a correlated super-spin glass state (CSSG) collective behavior. Consequently, the magnetic properties are mainly governed by a correlated super-spin glass state with strong inter-dot dipolar interactions.

Ion beam shaping is a novel and powerful tool to engineer nanocomposites with effective three-dimensional (3D) architectures. In particular, this technique offers the possibility to precisely control the size, shape and 3D orientation of metallic nanoparticles at the nanometer scale while keeping the particle volume constant. Here, we use swift heavy ions of xenon for irradiation in order to successfully fabricate nanocomposites consisting of anisotropic gold nanoparticle that are oriented in 3D and embedded in silica matrix. Furthermore, we investigate individual nanorods using a nonlinear optical microscope based on second-harmonic generation (SHG). A tightly focused linearly or radially-polarized laser beam is used to excite nanorods with different orientations. We demonstrate high sensitivity of the SHG response for these polarizations to the orientation of the nanorods. The SHG measurements are in excellent agreement with the results of numerical modeling based on the boundary element method.

Heat transport has large potentialities to unveil new physics in mesoscopic systems. A striking illustration is the integer quantum Hall regime, where the robustness of Hall currents limits information accessible from charge transport. Consequently, the gapless edge excitations are incompletely understood. The effective edge states theory describes them as prototypal one-dimensional chiral fermions - a simple picture that explains a large body of observations and calls for quantum information experiments with quantum point contacts in the role of beam splitters. However, it is in ostensible disagreement with the prevailing theoretical framework that predicts, in most situations, additional gapless edge modes. Here, we present a setup which gives access to the energy distribution, and consequently to the energy current, in an edge channel brought out-of-equilibrium. This provides a stringent test of whether the additional states capture part of the injected energy. Our results show it is not the case and thereby demonstrate regarding energy transport, the quantum optics analogy of quantum point contacts and beam splitters. Beyond the quantum Hall regime, this novel spectroscopy technique opens a new window for heat transport and out-of-equilibrium experiments., Comment: 13 pages including supplementary information, Nature Physics in print

Commissioned in May 2004 on the SLS machine, the LUCIA beamline was moved to the synchrotron SOLEIL during the summer of 2008. To take advantage of this new setting several changes to its design were introduced. Here, a review of the various improvements of the mechanics and, mostly, of the optics is given. Described in detail are the results of a new multilayer grating monochromator implemented on the Kohzu vessel already holding the two-crystal set-up. It consists of a grating grooved onto a multilayer (replacing the first crystal) associated to a multilayer (as a second crystal). It allows a shift of the low-energy limit of the beamline to around 500 eV with an energy resolution and a photon flux comparable with those of the previous couples of crystals (KTP and beryl).

In this paper we present an experiment where we measured the quantum coherence of a quasiparticle injected at a well-defined energy above the Fermi sea into the edge states of the integer quantum Hall regime. Electrons are introduced in an electronic Mach-Zehnder interferometer after passing through a quantum dot that plays the role of an energy filter. Measurements show that above a threshold injection energy, the visibility of the quantum interferences is almost independent of the energy. This is true even for high energies, up to 130~$\mu$eV, well above the thermal energy of the measured sample. This result is in strong contradiction with our theoretical predictions, which instead predict a continuous decrease of the interference visibility with increasing energy. This experiment raises serious questions concerning the understanding of excitations in the integer quantum Hall regime.

We present study of edge channels in high magnetic field by direct contrast interference measurement. We model our measures and we estimate geometrical characteristic for edge states. We find relevant parameter, even in plateaus regime, for the existence or not of the dialogue between opposite edge states which allows interference effect.

Well-ordered arrays of magnetic nanodots have been proposed as a new generation of ultra-high density recording media in the range 100 Gbits/in2 to 1 Tbits/in2, typically. To prepare such systems, we present an approach based on the deposition of magnetic nanoclusters preformed in the gas phase on functionalized surfaces. In this paper, we focus on the synthesis and characterization of 2D-organized arrays of Co50Pt50 nanocluster assemblies on HOPG surfaces patterned by Focused Ion Beam technique (FIB). After annealing treatment, the mixed CoPt nanoparticles exhibit the L10-type ordered structure which is well known for its characteristic high magnetic-anisotropy leading to a rather high blocking temperature required for applications.

In this paper, we present our work aiming to explore the nano-structuring potential of high resolution focused ion beams (FIB), a technology capable of overcoming some basic limitations of current nano-fabrication techniques and to propose new patterning schemes for nanoscience. To demonstrate this, we present some of our recent results based on ion-induced local property modifications or localised damage injection. We first detail the very high resolution FIB system we have developed. Then FIB nano-fabrication experiments are presented and discussed. We show that stable artificial surface nano-defects can be created with very low ion dose. The properties of stable magnetic structures (dots and lines) fabricated on ultrathin magnetic films using extremely low surface ion dose (10^1^2-10^1^5ions/cm^2) are presented. In a last example, we show that the promising direction of ''bottom-up'' or ''organisation'' processes can be envisaged with FIB. This can be achieved using defects capable of pinning the diffusion and of promoting organisation of nanometre-sized clusters (gold or cobalt-platinum) on a graphite crystalline surface.

Controlled and reproducible fabrication of nano-structured materials will be one of the main industrial challenges in the next few years. We have recently proposed exploitation of the nano-structuring potential of a high resolution Focused Ion Beam Tool, to overcome basic limitations of current nano-fabrication techniques. The aim of this article is to present some new routes for material patterning, which benefit from ion-induced local property modifications or damage. In the experiments we describe hereafter an ultra-sharp pencil of 30 keV gallium ions is used to tailor the characteristics of several materials at a scale of a few nanometres. The experimental results are then compared to simulations. First, we simulate the control of collisional defects generated in a thin magnetic layer under FIB irradiation. The results explain the stable magnetic structures we have obtained experimentally. This was achieved with a low surface ion dose (1012 to 1014 ions/cm2). In addition we have explored the promising direction of “Bottom-up” or “self-organization” processes using a FIB instrument. We have defined artificial surface defects. These defects created by the impact of an 8-nm FWHM probe were used to pin the diffusion and to organize nanometre-sized gold clusters on a graphite surface.

We report on low temperature magneto-conductance measurements of a single modulation-doped GaAs/AlGaAs core–shell nanowire in transverse magnetic field. Information on the phase coherence length l ϕ is obtained from quantum conductivity corrections due to weak localization effects or universal conductance fluctuation. The nanowires are totally coherent at very low temperature and l ϕ could be larger than 250 nm at 4 K. The temperature dependence of l ϕ shows that the main dephasing mechanism is the one-dimensional electron–electron scattering mechanism.

Tunneling of the total angular momentum of Ho 3+ in LiYF 4 (insulating matrix) is demonstrated. Due to strong hyperfine interactions, electronic angular momentum and nuclear spin tunnel simultaneously (two-body entanglement) and due to weak dipolar interactions, pairs of Ho 3+ ions also tunnel simultaneously (four-body entanglement). Similar results obtained with a metallic matrix (YRu 2 Si 2 ) prove the persistence of tunneling in the presence of free carriers.

Sr 2 RuO 4 is an unconventional superconductor with a superconducting transition temperature of ∼1.5 K. The formation of vortex chains in Sr 2 RuO 4 is observed, they have been directly imaged by μSQUID force microscopy at low temperature. Ginzburg–Landau theory for anisotropic superconductors is in qualitative agreement with the observed dependencies of the vortex chains on the variation of the tilting angle or the amplitude of the external magnetic field.

In this paper, we report on the production of periodic arrangements of nanometric size gold islands. Samples are realized by depositing gold clusters on organized nanodefects created by focused ion beam (FIB) technique on a graphite surface. We probe the effect of ion dose on the morphology of defects as well as the evolution of the cluster organization with artificial defect nature. This study points out the potentialities of this technique to organize nanostructures on patterned surfaces. Moreover, the high sensitivity of clusters to ultrasmall topographic defects and the good sensitivity of graphite towards ion bombardment reveal the FIB as an ultrafast and ultrahigh-resolution functionalization process for our approach.

Magnetization reversal in magnetic particles is one of the fundamental issues in magnetic data storage. Technological improvements require the understanding of dynamical magnetization reversal processes at nanosecond time scales. New strategies are needed to overcome current limitations. For example, the problem of thermal stability of the magnetization state (superparamagnetic limit) can be pushed down to smaller particle sizes by increasing the magnetic anisotropy. High fields are then needed to reverse the magnetization that are difficult to achieve in current devices. Here we propose a new method to overcome this limitation. A constant applied field, well below the switching field, combined with a radio-frequency (RF) field pulse can reverse the magnetization of a nanoparticle. The efficiency of this method is demonstrated on a 20 nm cobalt particle by using the micro-SQUID technique. Other applications of this method might be nucleation or depinning of domain walls., 11 pages, 5 figures

From AC susceptibility, magnetization and Mossbauer spectroscopy measurements, we show the dominant role of the surface properties and surface-to-core exchange coupling on the static and dynamic properties of γ-Fe 2 O 3 particles (2.7, 4.6 and 8.7 nm) dispersed in a polymer or powdered. Measurements of the switching field of individual particles are presented, demonstrating basic surface effects.

This letter reports on the synthesis and observation of periodic arrangements of nanometric size gold islands. Samples are produced by deposition of preformed gold clusters on a functionalized graphite surface. The evolution of the cluster organization with artificial defects nature and periodicity is studied experimentally and analyzed in the framework of Kinetic Monte Carlo (KMC) simulations. This study points out the potentialities of the technique to organize nanostructures on patterned surfaces.

We report on the dependence on field and temperature of the critical current of micro superconducting quantum interference devices (SQUIDs): SQUIDs with diameters as small as 1 μm using Dayem bridges as weak links. We model these SQUIDs by solving the Ginzburg–Landau equations with appropriate boundary conditions to obtain the supercurrent–phase relationships. These solutions show that the phase drops and depression of the order parameter produced by supercurrent flow are often distributed throughout the micro-SQUID structure, rather than being localized in the bridge area, for typical micro-SQUID geometries and coherence lengths. The resultant highly nonsinusoidal current–phase relationships Ic(φ) lead to reduced modulation depths and triangular dependences of the micro-SQUID critical currents on applied magnetic flux Ic(Φ). Our modeling agrees well with our measurements on both Al and Nb micro-SQUIDs.

The conductance through a constriction containing an antidot often exhibits paired Aharonov–Bohm oscillations when the two spins of the lowest Landau level form quantised states. Peaks and dips of the resonances evolve in various ways as the magnetic field or the gate voltages are varied. We show that no existing models, which attribute each resonance to one or other spin, can explain the behaviour, implying the involvement of electron–electron interactions between states of opposite spin.

The magnetic properties of Co/Pt films can be modified and even controlled by ion beam irradiation. For example, the coercivity and magnetic anisotropy of a 1.4nm thick Co layer are reduced with increasing the Ga+ ion dose. Images of the demagnetized states and remanent hysteresis loops are obtained for arrays of dots or tracks patterned by the focused ion beam technique. 20 nm wide sharp non-magnetic irradiated lines separate the magnetic dots or the tracks, the size of the dots or the width of the tracks varying from 1.8 μm down to 70 nm. The efficiency of the dipolar interaction is studied, and the use of such dot arrays for magnetic or magneto-optical recorrding discussed.

We investigate the magneto-transport properties of epitaxial graphene single-layer on 4H-SiC(0001), grown by atmospheric pressure graphitization in Ar, followed by H2 intercalation. We directly demonstrate the importance of saturating the Si dangling bonds at the graphene/SiC(0001) interface to achieve high carrier mobility. Upon successful Si dangling bonds elimination, carrier mobility increases from 3 000 cm2V−1s−1 to >11 000 cm2V−1s−1 at 0.3 K. Additionally, graphene electron concentration tends to decrease from a few 1012 cm−2 to less than 1012 cm−2. For a typical large (30 × 280 μm2) Hall bar, we report the observation of the integer quantum Hall states at 0.3 K with well developed transversal resistance plateaus at Landau level filling factors of ν = 2, 6, 10, 14… 42 and Shubnikov de Haas oscillation of the longitudinal resistivity observed from about 1 T. In such a device, the Hall state quantization at ν = 2, at 19 T and 0.3 K, can be very robust: the dissipation in electronic transport can stay very low, with the longitudinal resistivity lower than 5 mΩ, for measurement currents as high as 250 μA. This is very promising in the view of an application in metrology.

Controlled and reproducible fabrication of nano-structured materials will constitute one of the main industrial challenges for the next 10 years. To overcome the severe limitations of existing nano-fabrication techniques, we have developed and improved the ultimate usable resolution of an innovative focused ion beam instrument (FIB). In this work we demonstrate that FIB techniques allow quite simple, direct, clean and reproducible material nano-structuring close to or below the 10 nm level.

We report on magnetoresistance transport measurements performed on a bipartite tiling of rhombus in the GaAs/GaAlAs system. We observe for the first time large amplitude h/e oscillations in this network as compared to the one measured in square lattices of similar size. These oscillations are the signature of a recently predicted localization phenomenon induced by Aharonov-Bohm interferences in this peculiar network.

Using a new micro-SQUID setup, we investigate magnetic anisotropy in a single 1000-atom cobalt cluster. This system opens new fields in the characterization and understanding of the origin of magnetic anisotropy in such nanoparticles. For this purpose, we report three-dimensional switching field measurements performed on a 3 nm cobalt cluster embedded in a niobium matrix. We are able to separate the different magnetic anisotropy contributions and evidence the dominating role of the cluster surface.

We consider single-electron tunneling through antidot states using a Coulomb-blockade model, and give an explanation for $h/2e$ Aharonov-Bohm oscillations, which are observed experimentally when the two spins of the lowest Landau level form bound states. We show that the edge channels may contain compressible regions, and using simple electrostatics, that the resonance through the outer spin states should occur twice per $h/e$ period. An antidot may be a powerful tool for investigating quantum Hall edge states in general, and the interplay of spin and charging effects that occurs in quantum dots.

We present a new instrument for local magnetization measurements: the micro-Superconducting Quantum Interference Device (microSQUID). The microSQUID consists of a small (∼1 μm diameter) superconducting loop with two microbridge Josephson junctions. The SQUID loop itself serves as the input coil for the magnetic signal. In placing the SQUID very close to the sample, the magnetization measurement of signals as small as 10 000 μB has been demonstrated. In this limit of close proximity, the spatial resolution of the SQUID is a fraction of the width of the SQUID ring (0.2 μm). We are now building a scanning SQUID microscope. The SQUID is placed by electron beam lithography at the apex of a silicon cantilever. The lever is attached to a force sensor, allowing us to image magnetically, as well as topographically. We expect to obtain a spatial resolution of 50 nm and a flux resolution of about 10−4 Φ0. The ensemble is mounted inside an inverted dilution refrigerator. We will discuss the application of this technique to the imaging of vortices in artificial networks.

Using an array of miniature Hall probes, we monitored the spatiotemporal variation of the internal magnetic induction in a superconducting niobium sample during a slow sweep of external magnetic field. We found that a sizable fraction of the increase in the local vortex population occurs in abrupt jumps. The size distribution of these avalanches presents a power-law collapse on a limited range. In contrast, at low temperatures and low fields, huge avalanches with a typical size occur and the system does not display a well-defined macroscopic critical current., 5 pages including 5 figures

Graphene, a one-atom-thick layer of carbon represents a new class of materials which offers new inroads into low-dimensional physics. Nowadays a crucial problem is related to the patterning of graphene structures while keeping intact the unusual physical properties of this material. In this work, we present a way to suspend graphene sheets above electrical contacts and perform direct FIB patterning of freely suspended nanostructures like wires or rings.

We present magnetotransport properties of a multiwalled carbon nanotube. We have developed a technique using electron beam lithography to connect a carbon nanotube with a small contact resistance. The temperature dependence of the nanotube is thermally activated down to 30 mK but with multiple energy gap. At low temperature, the magnetoconductance can be analysed in terms of quantum interferences. Both weak localisation and universal conductance fluctuations have been observed. The phase coherence length extracted from the weak localisation is consistent with the amplitude of the conductance fluctuations.