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Superparamagnetic elongated Fe-Cu-B alloys were prepared in aqueous solution via sodium borohydride reduction of copper and iron dodecyl sulfate, Cu(DS){sub 2} and Fe(DS){sub 2}. The magnetization reversal of an individual Fe-Cu-B nanoparticle can be described by uniform rotation of magnetization and by thermal activation over a single-energy barrier as originally proposed by Neel and Brown.

We review the mesoscopic transport in a diffusive proximity superconductor made of a normal metal (N) in metallic contact with a superconductor (S). The Andreev reflection of electrons on the N-S interface is responsible for the diffusion of electron pairs in N. Superconducting-like properties are induced in the normal metal. In particular, the conductivity of the N metal is locally enhanced by the proximity effect. A re-entrance of the metallic conductance occurs when all the energies involved (e.g. temperature and voltage) are small. The relevant characteristic energy is the Thouless energy which is $\hbar$ divided by the diffusion time for an electron travelling throughout the sample. In loop-shaped devices, a 1/T temperature-dependent oscillation of the magnetoresistance arises with a large amplitude from the long-range coherence of low-energy pairs., Review paper, 13 pages with 5 included epsf figures, to appear in Superlattices and Microstructures, minor changes

Highly anisotropic sub-100nm InP patterns presenting low roughness have been obtained using SiCl"4 reactive ion etching. Aspect ratios above 20 are reported with low sidewall damage characterized by transmission electron microscopy and conductance measurements. The influence of the material used as an etch mask on the anisotropy and the roughness of the etched structures is investigated.

We present an original method to measure the visibility of interferences in an electronic Mach–Zehnder interferometer in the presence of low frequency phase fluctuations. We studied the visibility of the interferences as a function of the energy and showed a gaussian variation of the decoherence and/or phase averaging at finite energy. This gaussian variation leads to a visibility modulation with a single side lobe.

At low temperature, electronic wave functions in a metal keep their phase coherence on a length Lφ which can be of the order of few microns. Transport and thermodynamic properties of mesoscopic systems whose size are smaller than Lφ exhibit spectacular signatures of this coherence which can be revealed by instance through the sensitivity of the phase of the electrons to an applied vector potential. These quantum effects crucially depend on the way measurements are performed, in this paper we emphasize the difference between: • - Connected open systems, characterized by their transmission properties accessible through conductance measurements. • - Isolated closed systems characterized by their energy level spectra and investigated through thermodynamic (mostly magnetization) and ac conductance (response to an electromagnetic wave) measurements. They correspond to different types of coupling to the measuring apparatus, and present different sensitivities to phase coherence. The amplitude of quantum oscillations of the magneto-conductance on a connected system are indeed only a small fraction of the classical conductance and can much larger on an isolated system.

We report on a study of the spatiotemporal variation of magnetic induction in a superconducting niobium sample during a slow sweep of external magnetic field. A sizable fraction of the increase in the local vortex population occurs in abrupt jumps. We compare the size distribution of these avalanches with the predictions of self-organized-criticality models for vortex dynamics.

In a mesoscopic metal in proximity with a superconductor, the electronic conductance is enhanced in a very energy-sensitive way. In this paper, we discuss the spectral conductance of a proximity superconductor from both the theoretical and experimental point of view. The dependence of the spectral conductance on the phase-breaking length, gap of the superconductor and interface transparency is theoretically investigated. We present experimental data on the reentrance of the normal-state conductance at very low temperature and bias voltage. A complete description of the experimental data needs taking into account heating of the reservoirs by the bias current. In addition, we show that the energy sensitivity of the proximity effect enables one to access the energy distribution of the conduction electrons inside a mesoscopic sample.

The combination of highly sensitive Superconducting Quantum Interference Device (SQUID) with high quality nanoparticles allowed us to check for the first time simple classical models proposed nearly 50 years ago by Neel, Stoner and Wohlfarth. For nanoparticles containing about 10/sup 5/ to 10/sup 6/ /spl mu/B the quantitative agreement with the Neel-Brown theory of thermal activated magnetization reversal allowed us to identify unambiguously the magnetization reversal mechanism as uniform rotation. In the case of insulating barium ferrite nanoparticles containing about 10/sup 5/ /spl mu//sub B/ and below 0.4 K, strong deviations from this model are found which are quantitatively in agreement with the predictions of the theory of macroscopic quantum tunneling of magnetization in the low dissipation regime.

We have measured dissipative (real) and non-dissipative (imaginary) magneto-conductance of arrays of disconnected GaAs-GaAlAs mesoscopic rings and square dots in the MHz-GHz range. The samples are coupled to a highly sensitive electromagnetic multimode superconducting micro-resonator and lead to a perturbation of the resonance frequency and quality factor. This experiment provides a tool for the investigation of the conductance of mesoscopic sytems without any connection to invasive probes. It can be confronted with recent theoretical predictions emphasizing the differences between isolated and connected geometries and the relation between ac conductance, orbital magnetism and electrical polarisability. The magneto-conductance measured is dominated by its imaginary component whose sign (corresponding to diamagnetism in zero field) and amplitude are analysed and compared to theoretical predictions on persistent currents and magneto-polarisability. The real component of the magneto-conductance is very different from the weak localisation signals observed in similar connected samples. It changes sign from positive to negative when increasing electronic density and frequency. We propose an interpretation of this phenomenon in relation to fundamental properties of energy level spacing statistics in the dots.

We have performed magnetotransport measurements in ballistic cavities and obtained the average by small modulations on the shapes and/or on the Fermi level. We work with cavities whose underlying classical dynamics is chaotic (stadia and Sina\"{\i} billiards) and integrable (circles and rectangles). The former show a Lorentzian weak-localization peak, in agreement with semiclassical predictions and other averaging methods that have been used in recent measurements. For integrable cavities our measurements show that the shape of the weak localization is very sensitive to the exact geometry of the sample: a linear magnetoconductance has been observed for rectangles as expected by the theory for integrable cavities, whereas for circles the shape is always Lorentzian. These discrepancies illustrate the nongeneric behavior of scattering through integrable geometries, that we analyze taking into account the interplay of integrability with smooth disorder and geometrical effects. The power spectra of the conductance fluctuations are also analyzed, the deduced typical areas are in good agreement with those obtained from the weak localization. Periodic orbits in nonaveraged Fourier transforms of the magnetoconductance for regular cavities are clearly identified indicating the good quality of our samples.

We measure the subgap conductance across a superconductor-GaAs junction at low temperature. Below T=4 K we observe a zero bias conductance peak. For the first time, we observe that at even lower temperatures (T{le}0.8 K ), the conductance peak is shifted to a finite voltage. Application of a magnetic field restores a zero bias conductance peak. This is consistent with theoretical predictions for superconducting-semiconducting junctions in the regime where the contact conductance is larger than the coherent semiconductor conductance. {copyright} {ital 1997} {ital The American Physical Society}

New experiments are presented on the transmission of electron waves through a 2DEG (2 dimensional electron gas) ring with a gate on top of one of the branches. Magnetoconductance oscillations are observed, and the phase of the Aharanov-Bohm signal alternates between 0 and pi as the gate voltage is scanned. A Fourier transform of the data reveals a dominant period in the voltage which corresponds to the energy spacing between successive transverse modes.A theoretical model including random phase shifts between successive modes reproduces the essential features of the experiment., 4 pages, 6 Postscript figures, TEX, submitted to Physical Review Letters

It is known that the local conditions in an electrical circuit can affect a nearby, but electrically isolated circuit. Such a ‘noninvasive’ measurement technique can be employed to study the screening ability (compressibility) of an electron system. In this work, we use a quasi-one-dimensional ballistic channel to screen an electric field between a gate and lateral quantum dot. Our results clearly show that the screening ability decreases with decreasing number of occupied 1D channel. When there are a large number of 1D subbands occupied in the channel, the screening ability tends to saturate, approaching its 2D limit.

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.

International audience; In recent years, an innovative technique of ion-shaping of matter at the nanoscale has been proposed based on the deformation of embedded metallic nanoparticles (NPs) induced by host amorphous matrix deformation and nanoparticle melting during swift heavy ions irradiation. This technique allows the deformation of spherical metallic particles embedded in a silica matrix into several original classes of ion-shaped nanoparticles: i) facetted-like NPs, ii) nanowires iii) chromosome-like particles. The fundamental aspect of ion-matter interaction has been investigated through the modeling of the temperature profile within the nanoparticle by implementing the thermal-spike model for three-dimensional anisotropic and composite media. A straight correlation has been found between the spatial distribution of molten matter and the deformation path followed by the nanoparticles during the irradiation. Localized Surface Plasmon Resonances (LSPR) are generated along ion-shaped nanoparticles through electron excitation in a Scanning Transmission Electron Microscope. This has been investigated through nanometer-scale Electron Energy Loss Spectroscopy (EELS) analysis. The near field response of nanoparticles was investigated experimentally for different kinds of ion-shaped objects. We confirmed the near field distribution and the identification of LSPR modes of nanoparticles by modeling with a home developed Auxiliary Differential Equations-Finite Difference Time Domain (ADE-FDTD) code. This work demonstrates the possibility to use ion irradiation as a tool for the controllable fabrication of plasmonic nanostructures with topologically tunable optical properties. These ion-beam shaped composite media have potential applications spanning from plasmonic photovoltaics to Surface-Enhanced Raman Scattering (SERS) and Surface enhanced infrared absorption (SEIRA) spectroscopies.

Quasi-particle excitations in graphene exhibit a unique behavior concerning two key phenomena of mesoscopic physics: electron localization and the quantum Hall effect. A direct transition between these two states has been found in disordered two-dimensional electron gases at low magnetic field. It has been suggested that it is a quantum phase transition, but the nature of the transition is still debated. Despite the large number of works studying either the localization or the quantum Hall regime in graphene, such a transition has not been investigated for Dirac fermions. Here we discuss measurements on low-mobility graphene where the localized state at low magnetic fields and a quantum Hall state at higher fields are observed. We find that the system undergoes a direct transition from the insulating to the Hall conductor regime. Remarkably, the transverse magneto-conductance shows a temperature independent crossing point, pointing to the existence of a genuine quantum phase transition.

We have used a quantum antidot (QAD) to detect the oscillation of the Fermi energy in a two-dimensional electron gas (2DEG) as a function of magnetic field. At a magnetic field B 0 = n s h 2 ( e 2 π R 2 ) (where n s is the carrier density and R is the radius of the QAD at the Fermi energy) the Shubnikov-de Haas (SdH) oscillation period becomes greater than the period of the Aharonov-Bohm (AB) oscillations from the QAD. Hence, above this field, the corresponding oscillation of the Fermi energy is superimposed onto the period of the AB oscillations. We have taken measurements of the AB period as a function of magnetic field at a number of different QAD radii. These measurements reveal an average oscillatory period which is consistent with the motion of the Fermi energy through the predicted QAD eigen spectrum.

We review transport measurements in a normal metal (N) in contact with one or two superconducting (S) islands. From the experiment, we distinguish the Josephson coupling, the mesoscopic fluctuations and the proximity effect. In a loop-shaped N conductor, we observe large h/2e-periodic magnetoresistance oscillations that decay with temperature T with a 1/T power-law. This behaviour is the signature of the long-range coherence of the low-energy electron pairs induced by the Andreev reflection at the S interface. At temperature and voltage below the Thouless energy $\hbar D / L^2$, we observe the re-entrance of the metallic resistance. Experimental results agree with the linearized quasiclassical theory., Comment: 8 pages, 6 included epsf figures, Invited paper at the LT21 Conference, Praha, August 1996. To appear in Czech. J. of Phys. 46, Part S6 (1996)

We first present magnetic aftereffect measurements of an array of submicronic amorphous noninteracting Co particles (300 nm \ifmmode\times\else\texttimes\fi{} 200 nm \ifmmode\times\else\texttimes\fi{}30 nm). At low temperatures, the thermal dependence of the magnetic viscosity S=dM/d(ln(t)) exhibits an unusual behavior. Comparison with measurements performed on a single particle allows the effects of the averaging over the statistical ensemble of the array of 2\ifmmode\times\else\texttimes\fi{}${10}^{7}$ Co particles to be studied experimentally. The link between both experiments is then performed by using the linear-response theory. This formalism allows the noise, D(T), which accounts for the thermal dependence of the coupling of switching spins to their environment, to be defined. The consistency of this description is checked by comparing the noise measured in this way on the array of particle with the noise measured on the single particle through the Arrhenius law. In both cases, the thermal dependence of the noise D(T) has a coth(${\mathit{T}}_{0}$/T) form.

We report on the measurement of phase coherence length in a high mobility two-dimensional electron gas patterned in two different geometries, a wire and a ring. The phase coherence length is extracted both from the weak localization correction in long wires and from the amplitude of the Aharonov-Bohm oscillations in a single ring, in a low temperature regime when decoherence is dominated by electronic interactions. We show that these two measurements lead to different phase coherence lengths, namely $L_{\Phi}^\mathrm{wire}\propto T^{-1/3}$ and $L_{\Phi}^\mathrm{ring}\propto T^{-1/2}$. This difference reflects the fact that the electrons winding around the ring necessarily explore the whole sample (ergodic trajectories), while in a long wire the electrons lose their phase coherence before reaching the edges of the sample (diffusive regime)., Comment: LaTeX, 5 pages, 4 pdf figures ; v2: revised version

International audience; Quantum point contacts exhibit mysterious conductance anomalies in addition to well known conductance plateaus at multiples of 2e²/h. These 0.7 and zero-bias anomalies have been intensively studied, but their microscopic origin in terms of many-body effects is still highly debated. Here we use the charged tip of a scanning gate microscope to tune in situ the electrostatic potential of the point contact. While sweeping the tip distance, we observe repetitive splittings of the zero-bias anomaly, correlated with simultaneous appearances of the 0.7 anomaly. We interpret this behaviour in terms of alternating equilibrium and non-equilibrium Kondo screenings of different spin states localized in the channel. These alternating Kondo effects point towards the presence of a Wigner crystal containing several charges with different parities. Indeed, simulations show that the electron density in the channel is low enough to reach one-dimensional Wigner crystallization over a size controlled by the tip position.

The first magnetization measurements of individual single-domain particles (50–150 nm) patterned by electron-beam lithography are presented. The angular dependence of the switching field is measured and the temperature dependence (0.1–6 K) of the reversal process is investigated by two independent methods: (i) by measuring the switching field and the width of the switching field distribution as a function of temperature and field sweeping rate, (ii) by measuring the probability for switching at constant applied magnetic field as a function of time and temperature. We found that mean values like the mean switching field and the mean switching time verify a simple model based on thermal activation between 1 and 6 K. Below 1 K thermal activation is not sufficient anymore to explain their variation. However the width of the switching field distribution and the probability of switching are in disagreement with such a model. The results are discussed in view of Macroscopic Quantum Tunnelling of magnetization.

We have measured the flux dependence of both real and imaginary conductance of $GaAs/GaAlAs$ isolated mesoscopic rings at 310 MHz. The rings are coupled to a highly sensitive electromagnetic superconducting micro-resonator and lead to a perturbation of the resonance frequency and quality factor. This experiment provides a new tool for the investigation of the conductance of mesoscopic systems without any connection to invasive probes. It can be compared with recent theoretical predictions emphasizing the differences between isolated and connected geometries and the relation between ac conductance and persistent currents. We observe $\Phi_0/2$ periodic oscillations on both components of the magnetoconductance. The oscillations of the imaginary conductance whose sign corresponds to diamagnetism in zero field, are 3 times larger than the Drude conductance $G_0$. The real part of the periodic magnetoconductance is of the order of $0.2 G_0$ and is apparently negative in low field. It is thus notably different from the weak localisation oscillations observed in connected rings, which are much smaller and opposite in sign., Comment: 4 pages, revtex, epsf, 4 Postscript files

We present the first magnetization measurements of single submicronic particles at very low temperature made of either Ni, Co, CoZrMoNi or TbFe. As detector we use a micro-bridge-DC-SQUID deposited onto or next to the sample. Sample and detector are patterned by electron-beam lithography. The dynamics and the temperature dependence of the magnetization reversal is studied in view of Macroscopic Quantum Tunneling (MQT).

Magnetic relaxation plateaux observed at low temperature in complex systems, are generally attributed to Quantum Tunneling of the Magnetization (QTM). If the experiments are not performed down to low enough temperatures, alternative interpretations can be given in terms of powerlaw distributions resulting from either switching field distributions or/and coupling between switching blocks leading to self-organized criticality. Besides, the first low-temperature/time-dependent magnetization experiments, performed on single sub-micronic particles and on arrays nanoparticles in molecular crystals, are described.

We demonstrate a direct approach to investigate heat transport in the fractional quantum Hall regime. At filling factor of 4/3, we inject power at quantum point contacts and detect the related heating from the activated current through a quantum dot. The experiment reveals a chargeless heat transport from a significant heating that occurs upstream of the power injection point, in the absence of a concomitant electrical current. By tuning in-situ the edge path, we show that the chargeless heat transport does not follow the reverse direction of the electrical current path along the edge. This unexpected heat conduction demonstrates a novel aspect, yet to be elucidated, of the physics in fractional quantum Hall systems., Manuscript (5 pages, 3 figures) and supplementary material (8 pages, 9 figures); v2 includes additional supplementary material and references; to be published in Phys. Rev. Lett

The ion-beam-shaping mechanism is studied for several classes of nearly monodispersed metallic nanoparticles (Au, Ag, and Au${}_{0.5}$Ag${}_{0.5}$). They are in the range of 3--100 nm and embedded within a silica matrix. Experimentally, we show that depending on their initial sizes, several final morphologies can be obtained: (i) spherical shapes, (ii) facetted nanoparticles, (iii) nanorods, and (iv) nanowires. In parallel, the thermal-spike model was implemented for three-dimensional anisotropic and composite media to study the evolution of the temperature profile within a nanoparticle. This way, a clear correlation is found between the deformation path followed by the nanoparticles during the irradiation and the fraction of the nanoparticle that is molten (vaporized) due to the interaction with a swift heavy ion. This allows the construction of a size-vs-shape diagram relating the initial nanoparticle size to its final morphology. This diagram is used to give a rational description of the ion-beam-shaping process for all nanoparticle dimensions as a function of the irradiation parameters.

In this letter we report on transport measurements of epitaxial graphene on SiC(0001) with oxygen adsorption. In a $50\times 50 \mu\mathrm{m^2}$ size Hall bar we observe the half-integer quantum Hall effect with a transverse resistance plateau quantized at filling factor around $\nu = 2$, an evidence of monolayer graphene. We find low electron concentration of $9\times 10^{11} \textrm{cm}^{-2}$ and we show that a doping of $10^{13}\textrm{cm}^{-2}$ which is characteristic of intrinsic epitaxial graphene can be restored by vacuum annealing. The effect of oxygen adsorption on carrier density is confirmed by local angle-resolved photoemission spectroscopy measurements. These results are important for understanding oxygen adsorption on epitaxial graphene and for its application to metrology and mesoscopic physics where a low carrier concentration is required., Comment: 4 pages, 3 figures

We present an experiment where the quantum coherence in the edge states of the integer quantum Hall regime is tuned with a decoupling gate. The coherence length is determined by measuring the visibility of quantum interferences in a Mach-Zehnder interferometer as a function of temperature, in the quantum Hall regime at filling factor two. The temperature dependence of the coherence length can be varied by a factor of two. The strengthening of the phase coherence at finite temperature is shown to arise from a reduction of the coupling between co-propagating edge states. This opens the way for a strong improvement of the phase coherence of Quantum Hall systems. The decoupling gate also allows us to investigate how inter-edge state coupling influence the quantum interferences' dependence on the injection bias. We find that the finite bias visibility can be decomposed into two contributions: a Gaussian envelop which is surprisingly insensitive to the coupling, and a beating component which, on the contrary, is strongly affected by the coupling., Comment: 4 pages, 5 figures

We report low-temperature transport measurements through a double quantum dot device in a configuration where one of the quantum dots is coupled directly to the source and drain electrodes, and a second (side-coupled) quantum dot interacts electrostatically and via tunneling to the first one. As the interdot coupling increases, a crossover from weak to strong interdot tunneling is observed in the charge stability diagrams that present a complex pattern with mergings and apparent crossings of Coulomb blockade peaks. While the weak coupling regime can be understood by considering a single level on each dot, in the intermediate and strong coupling regimes, the multi-level nature of the quantum dots needs to be taken into account. Surprisingly, both in the strong and weak coupling regimes, the double quantum dot states are mainly localized on each dot for most values of the parameters. Only in an intermediate coupling regime the device presents a single dot-like molecular behavior as the molecular wavefunctions weight is evenly distributed between the quantum dots. At temperatures larger than the interdot coupling energy scale, a loss of coherence of the molecular states is observed., 9 pages, 5 figures

Strong hyperfine coupling in a 0.2% Holmium doped LiYF_4 single crystal induces staircaselike hysteresis loops of the magnetization at very low temperatures. The field sweep rate dependence of hysteresis loops allows the study of two different regimes in the magnetic relaxation of these weakly coupled magnetic moments. At slow field sweep rates, quantum tunneling of the magnetization occurs at avoided level crossings in the low-energy scheme of a single ion Ho^3+. At faster sweep rates, nonequilibrated spin-phonon and spin-spin transitions, mediated by weak dipolar interactions, lead to magnetization oscillations and additional steps., 4 figures, Conference proceeding: 1st Joint European Magnetic Symposia (JEMS01'), Grenoble (France), 28th August - 1st September, 2001

Ion irradiation is an original process to pattern the structural and as a consequence the magnetic properties of ultra-thin films, down to the nanometer scale. Patterns of dots and tracks have been fabricated by focused Ga + ion beam scanned onto a Co layer with perpendicular magnetic anisotropy. Depending on the dose, the magnetic behaviour of the nanometric irradiated lines can be tuned from the ferromagnetic with reduced coercivity to paramagnetic. The larger the fluence, the smaller is the exchange between dots or tracks. These systems enabled investigations of the competition between exchange and dipolar interactions. For arrays designed with high irradiation doses and only coupled by dipolar interactions, the magnetic relaxation proceeds by the magnetization reversal of individual dots and follows a power-law time decay. Monte Carlo simulations reproduce this time dependence.

What are the quantum laws of electricity in mesoscopic circuits? This very fundamental question has also direct implications for the quantum engineering of nanoelectronic devices. Indeed, when a quantum coherent conductor is inserted into a circuit, its transport properties are modified. In particular, its conductance is reduced because of the circuit back-action. This phenomenon, called environmental Coulomb blockade, results from the granularity of charge transfers across the coherent conductor. Although extensively studied for a tunnel junction in a linear circuit, it is only fully understood for arbitrary short coherent conductors in the limit of small circuit impedances and small conductance reduction. Here, we investigate experimentally the strong back-action regime, with a conductance reduction of up to 90%. This is achieved by embedding a single quantum channel of tunable transmission in an adjustable on-chip circuit of impedance comparable to the resistance quantum $R_K=h/e^2$ at microwave frequencies. The experiment reveals important deviations from calculations performed in the weak back-action framework, and matches with recent theoretical results. From these measurements, we propose a generalized expression for the conductance of an arbitrary quantum channel embedded in a linear circuit., 11 pages including supplementary information, to be published in Nature Physics

Periodic modulations in the switching current of niobium mesoscopic thin loops have been observed as a function of a perpendicular magnetic field. These modulations reflect the formation of specific vortices configurations in the arms and in the contacts of the loops. The voltage measured just before the current-driven transition to the normal state takes either zero or nonzero values depending on whether the vortices are efficiently pinned or move under the action of the applied current. This switching voltage varies sharply in the vicinity of the regularly spaced matching fields at which the switching current is minimum. This reflects the rearrangement of the vortices pattern from an unstable configuration into a more stable one. In addition, multimodal switching current distributions obtained at a constant field highlight that the few vortices involved in the field-cooling process freeze into a limited number of configurations. Finally, the spacing between the matching fields allows us to extract the critical field for complete vortex expulsion of the samples. Electrical transport measurements appear to be an efficient tool to determine the superheating field of submicrometer-wide structures, which is not obvious when measured by scanning probe microscopy.

We present an original approach to fabricate single GaAs/AlGaAs core-shell nanowire with robust and reproducible transport properties. The core-shell structure is buried in an insulating GaAs overlayer and connected as grown in a two-probe setup using the highly doped growth substrate and a top diffused contact. The measured conductance shows a non-Ohmic behavior with temperature and voltage-bias dependences following power laws, as expected for a quasi one- dimensional system.

We study the disorder dependence of the phase coherence time of quasi one-dimensional wires and two-dimensional (2D) Hall bars fabricated from a high mobility GaAs/AlGaAs heterostructure. Using an original ion implantation technique, we can tune the intrinsic disorder felt by the 2D electron gas and continuously vary the system from the semi-ballistic regime to the localized one. In the diffusive regime, the phase coherence time follows a power law as a function of diffusion coefficient as expected in the Fermi liquid theory, without any sign of low temperature saturation. Surprisingly, in the semi-ballistic regime, it becomes independent of the diffusion coefficient. In the strongly localized regime we find a diverging phase coherence time with decreasing temperature, however, with a smaller exponent compared to the weakly localized regime., 21 pages, 30 figures

This paper reports on the progress in the development of fabrication process of metrological quantum Hall effect (QHE) devices from GaAs/AlGaAs two-dimensional electron gas (2DEG), including single Hall bar quantum resistance standards, Quantum Hall Arrays Resistance Standards (QHARS) and other specific devices (quantum Wheatstone bridges) dedicated to very high-accuracy quantum Hall resistance comparisons and QHE universality tests. Single Hall bars have been obtained exhibiting very low resistance ohmic contacts (

We investigate the energy exchanges along an electronic quantum channel realized in the integer quantum Hall regime at filling factor $\nu_L=2$. One of the two edge channels is driven out-of-equilibrium and the resulting electronic energy distribution is measured in the outer channel, after several propagation lengths $0.8~\mu$m$\leq L\leq30~\mu$m. Whereas there are no discernable energy transfers toward thermalized states, we find efficient energy redistribution between the two channels without particle exchanges. At long distances $L\geq10~\mu$m, the measured energy distribution is a hot Fermi function whose temperature is lower than expected for two interacting channels, which suggests the contribution of extra degrees of freedom. The observed short energy relaxation length challenges the usual description of quantum Hall excitations as quasiparticles localized in one edge channel., Comment: To be published in PRL, 10 pages including supplementary material

The critical current of a niobium type II superconducting loop has been measured far below the critical temperature using contacts applied symmetrically or asymmetrically around the loop. The magnetic field dependence of the critical current allows a direct observation of the circulating current resulting from the fluxoid quantization and presents a saw-tooth behavior. The periodicity is given by a flux quantum through the outer area of the loop. Despite the presence of many vortices at high magnetic fields, the circulating current finds its way in between them and still has an impact on the total critical current of the loop. Macroscopic quantum coherence effects in such a niobium loop allow observing single quanta changes of the fluxoid up to a magnetic field of 1.2 T.

Graphene is generating a considerable interest in materials science and condensed-matter physics. One crucial technological problem, that will govern future applicability of this material, is related to the patterning of graphene while preserving the exceptionally high crystallinity and electronic properties of this material. This article is aiming at demonstrating that a highly focused beam of gallium ions can be applied for sculpting ultra-thin and high quality suspended graphene nano-ribbons (GNRs).

The authors studied the resistive transition of several 2-D superconducting-wire networks of various coupling strengths, which they characterize in terms of the Kosterlitz-Thouless transition temperature and the ratio {xi}/a of the coherence length to the array period. In the extreme strong-coupling limit where the mesh size is of the order of the zero-temperature coherence length, the superconducting behavior is well described by the mean-field properties of the superconducting wave function. Extending to 2-D array, the 1-D phase-slippage model explains the dissipative regime observed above the Ginzburg-Landau depairing critical current. On the other hand, when the coupling is weak, phase fluctuations below the Ginzburg-Landau transition and vortex depinning dominate the resistive behavior. An activated dissipation is observed even below the depairing critical current. Results obtained in this regime for critical temperature, magnetoresistance, or critical current versus temperature, and magnetic field are shown; their periodic oscillations are discussed in terms of depinning of vortices on the array. A simple periodic pinning potential for a vortex in a wire network is calculated, and compared with the case of pinning in Josephson junction arrays. It is shown that this model explains qualitatively the experimental results observed for small {xi}/a.

We have measured 50 magnetoconductance curves corresponding to 50 microscopic disorder configurations in a single mesoscopic GaAs:Si wire at T=45 mK. This permits to establish that the universal conductance fluctuation decreases by a factor-of-two when one breaks time reversal symmetry by application of the magnetic field which destroys the weak-localisation correction to the mean conductance. We have also observed the same factor of reduction for the 1/f noise power in a gated GaAs:Si wire at T=45 mK, that indicates the quantum interference origin of this noise at least at very low temperature.

We present an experimental study consisting of 0-D and 1-D energy spectroscopy by means of transport and magneto-transport measurements carried out in nanometric double barrier resonant tunnelling heterostructures. Coulomb blockade effects in our structures are discussed.

Experiments performed on mesoscopic systems made of dilute magnetic alloys ( Au Mn and Au Cr) show the effect of magnetic impurities on the Universal Conductance Fluctuations. The magnetic field allows us to switch from a low field state where the impurities are free and destroy the phase coherence to a high field state where the magnetization is saturated and the phase coherence is conserved along the whole sample. The results are interpreted by a molecular field approach in the spin glass state. However, we do not observe any jump of the conductance with time induced by spin flip in the zero field spin glass state. An explanation of the absence of jump is proposed.

At mesoscopic scale, the variable range hopping can be described by a tunnel effect through a single barrier. We have placed a GaALAs/GaAs HEMT in the quantum Hall regime (high magnetic field and very low temperature) and measured the components of the conductance tensor σxx and σxy. We study the regime of the Shubnikov de Haas peaks and compare it with the same regime in large sample, commonly understood as a VRH regime. We analyse the variation in temperature in the range [0.1K-1K] for kT to scale the fundamental energies, and demonstrate that in the mesoscopic scale, the measured longitudinal conductivity corresponds exactly to the tunneling through a single barrier. We deduce the barrier characteristics by fitting our data with the conductivity of a square tunnel barrier, and we retrieve the characteristic height and length of the bare disorder potential.

The magnetoresistance of a low carrier density, disordered GaAs based two-dimensional (2D) electron gas has been measured in parallel magnetic fields up to 32 T. The feature in the resistance associated with the complete spin polarization of the carriers shifts down by more than 20 T as the electron density is reduced, consistent with recent theories taking into account the enhancement of the electron-electron interactions at low densities. Nevertheless, the magnetic field for complete polarization, Bp, remains 2-3 times smaller than predicted for a disorder free system. We show, in particular by studying the temperature dependance of Bp to probe the effective size of the Fermi sea, that localization plays an important role in determining the spin polarization of a 2D electron gas., Published in the Physical Review B

International audience; Coherent x-ray diffractive imaging is a powerful method for studies on nonperiodic structures on the nanoscale. Access to femtosecond dynamics in major physical, chemical, and biological processes requires single-shot diffraction data. Up to now, this has been limited to intense coherent pulses from a free electron laser. Here we show that laser-driven ultrashort x-ray sources offer a comparatively inexpensive alternative. We present measurements of single-shot diffraction patterns from isolated nano-objects with a single 20 fs pulse from a table-top high-harmonic x-ray laser. Images were reconstructed with a resolution of 119 nm from the single shot and 62 nm from multiple shots.