A. Zimmers

Université Paris-Sud 11, Orsay, Île-de-France, France

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Publications (46)94.92 Total impact

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    ABSTRACT: We report the study of gold-SrTiO3 (STO)-gold memristors where the doping concentration in STO can be fine-tuned through electric field migration of oxygen vacancies. In this tunnel junction device, the evolution of the density of states (DOS) can be followed continuously across the metal-insulator transition (MIT). At very low dopant concentration, the junction displays characteristic signatures of discrete dopant levels. As the dopant concentration increases, the semiconductor band gap fills in but a soft Coulomb gap remains. At even higher doping, a transition to a metallic state occurs where the DOS at the Fermi level becomes finite and Altshuler-Aronov corrections to the DOS are observed. At the critical point of the MIT, the DOS scales linearly with energy N(ϵ)∼ϵ, the possible signature of multifractality.
    Physical Review Letters 02/2014; 112(6):066803. · 7.73 Impact Factor
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    ABSTRACT: The electronic properties of transition metal oxide materials depend on the electronic carrier density, which can be tuned with the oxygen stoichiometry. In binary MOx or ternary perovskite ABOx, it has been shown that upon applying a strong electric field, oxygen vacancies can be created or displaced in the material. This effect is responsible for the memristive behavior recently discovered in TiO2 materials by HP laboratory and launched a worldwide renew interest into ionics. We present a study of oxygen ions vacancies displacement in SrTiO3, the archetype perovskite oxide. For this work, metallic electrodes, separated by distances about 100 -- 300 nm, are deposited on the surface of a STO crystal and ions migration procedures and current-voltage characteristics measurements are done at low temperature, T ˜ 260 mK. Upon applying large voltage up to 30 V, oxygen vacancies migration is identified as the apparition of resistance switching events in current-voltage characteristics. Detailed measurements of the junction show that the switching event led to the formation of a nanosized region of highly doped STO, located within the electrodes where the current-voltage characteristics show the presence of the doped in-gap states. This work was supported by the French ANR grants 10-BLAN-0409-01 and 09-BLAN-0388-01.
    03/2013;
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    ABSTRACT: During this last decade, numerous progresses have been obtained in the chemical synthesis of nanoparticle. Various materials (oxides, chalcogenides) known for their peculiar electronic or magnetic properties -- superconductivity, Mott localization, topological protection -- can now be obtained as nanoparticles through chemical synthesis. These new nano-materials are offering a unique opportunity to study the effect of quantum confinement on unconventional electronic orders. To improve the preparation of samples with single nanoparticles trapped within a nanogap, we developed a new method where nanoparticles are projected in-vacuum on chip circuits covered by nanogap spaced electrodes. Continuous current measurements during the projection allow identifying the trapping of a single nanoparticle within the nanogap. We apply the method for trapping single gold nanoparticles, which led to the observation of Coulomb blockade. We also applied the method to magnetite (Fe3O4) nanoparticles, which allows to study the electric field induced insulator to metal transition in only a few nanoparticles.
    03/2013;
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    ABSTRACT: In our previous study anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) of epitaxial La2∕3Ca1∕3MnO3 (LCMO) thin films grown on SrTiO3(001) (STO) substrates were studied, and a phenomenological model in the high field limit was developed based on the 4/mmm point group. The derived longitudinal resistivity includes a four-fold as well as a two-fold symmetry term of the in-plane field angle, which can fit the experimental results well. In this study, to highlight the effects of misfit strain, AMR and PHE of LCMO thin films epitaxially grown on LaAlO3(001) substrates were studied, along either the [110] or the [100] direction. Both values are around a few percent, comparable to those measured in films on STO. Nevertheless, only tiny four-fold oscillations appear below the metal-insulator transition temperature Tp along the [110] direction, in contrast to the case of STO, where the four-fold term is prominent. The relationship between this four-fold symmetry and the misfit strain is then discussed in terms of the partial recovery of orbital magnetic moment. The mechanism for AMR and PHE in manganites then can be understood as an anisotropic percolation at metal-insulator transition resulting in the peak, and the spin-orbital coupling effect that accounts for the remnant far below Tp.
    Journal of Applied Physics 02/2013; 113(5). · 2.21 Impact Factor
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    ABSTRACT: We show that the main mechanism for the dc voltage or dc current induced insulator-metal transition in vanadium dioxide VO_{2} is due to local Joule heating and not a purely electronic effect. This "tour de force" experiment was accomplished by using the fluorescence spectra of rare-earth doped micron sized particles as local temperature sensors. As the insulator-metal transition is induced by a dc voltage or dc current, the local temperature reaches the transition temperature indicating that Joule heating plays a predominant role. This has critical implications for the understanding of the dc voltage or dc current induced insulator-metal transition and has a direct impact on applications which use dc voltage or dc current to externally drive the transition.
    Physical Review Letters 02/2013; 110(5):056601. · 7.73 Impact Factor
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    ABSTRACT: Starting with a discussion of the percolation problem applied to the trapping of conducting nanoparticles between nanometer spaced electrodes, we show that the best strategy to trap a single nanoparticle between the electrodes is to prepare chips with low coverage of nanoparticles to avoid percolating current paths. To increase the probability of trapping a single nanoparticle, we developed a new method where nanoparticles are projected in-vacuum on the chip, followed by a measure of the tunnel current, in a cycle that is repeated up to a few thousand times until a preset threshold value is reached. A plot of the tunneling current as function of time allows discriminating between the two possible current paths, i.e. a single nanoparticle trapped between the electrodes or a percolating path across many nanoparticles. We applied the method to prepare chips circuits with single gold nanoparticles as demonstrated by the observation of Coulomb blockade. Furthermore, we applied the method to trap single Magnetite nanoparticles for the study of electric-field induced switching from insulator to metal in single nanoparticles.
    ACS Nano 01/2013; · 12.03 Impact Factor
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    ABSTRACT: Electrostatic force microscopy (EFM) allows measurement of tiny changes in tip-sample capacitance. When nanoobjects are studied by EFM, they only contribute a very small fraction of the total capacitance between the tip and the sample. We show that the analysis of 3D maps of the EFM signal allows extracting the contribution of the nanomaterial to the total capacitance. This opens the way to applications of EFM as a measure of the dielectric coefficient of electrically insulating nanomaterials or the quantum capacitance of conducting nanomaterials. We apply this method to study the charge response of magnetite, Fe3O4, nanoparticles. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4790587]
    Applied Physics Letters 01/2013; 102(5). · 3.79 Impact Factor
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    ABSTRACT: Over recent years, the insulator to metal transition (IMT) of the vanadium dioxide (VO2) Mott insulator has been revisited revealing an electric-field-induced resistance switching. Whether this feature is purely due to an electrical field effect or due to some Joule heating is still under debate. Here we report a local temperature measurement in a 10μm and a 20μm VO2 junction while going through the resistance switching. The sample was placed at δT=15K below 340K (the thermally induced insulator to metal transition). When ramping up the voltage across the junction we find that the local heating inside the VO2 junction is close to 15K. This data suggests that in these temperature, current and voltage ranges, the field induced IMT can be explained by local Joule heating. Work supported by the French ANR-09-BLAN-0388-01 and the US DOE and AFOSR.
    02/2012;
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    ABSTRACT: The electronic compressibility is a fundamental property that characterizes the electronic properties of materials submitted to an external electric field. In metals (insulators), the electronic compressibility is large (small) and leads to a small (large) screening length. Variations of the screening length can be observed through measurements of the ``quantum'' capacitance between one material and a metallic counter-electrode. Using an Electrostatic Force Microscope (EFM), we measured maps of the local capacitance of 8 nm magnetite nanoparticles synthesized following the ``benzyl alcohol route'' deposited on a metallic substrate. Magnetite, an inverse spinel structure of composition Fe3O4, is a material with strongly correlated electronic properties and presents a metal-insulator transition at 120 K, the so-called Verwey transition. We present EFM measurements of these nanoparticles as a function of tip-sample distance and temperature.
    02/2012;
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    ABSTRACT: Electrostatic Force Microscopy (EFM) and its variants such as Kelvin Probe Microscopy (KPM) are ordinarily used to image charged states or electrochemical surface potentials. However, EFM can also be used to measure the local capacitance between the tip and the substrate. For perfectly metallic substrates, this capacitance is purely geometrical, i.e. it is set by the tip shape and substrate geometry. In semi-metals with long screening length, the measured capacitance contains a ``quantum'' component, which is set by the electronic compressibility. Using finite element calculations, we demonstrate that this quantum capacitance component can be measured by EFM. We apply these calculations to the analysis of EFM data on magnetite nanoparticles presented by A. Mottaghizadeh during this meeting.
    02/2012;
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    ABSTRACT: We describe current-voltage (I-V) characteristics of alkyl-ligated gold nanocrystals ~5 nm arrays in a long screening length limit. Arrays with different alkyl ligand lengths have been prepared to tune the electronic tunnel coupling between the nanocrystals. For long ligands, electronic diffusion occurs through sequential tunneling and follows activated laws, as a function of temperature σ∝e(-T(0)/T) and as a function of electric field I∝e(-E(0)/E). For better conducting arrays, i.e., with small ligands, the transport properties cross over to the cotunneling regime and follow Efros-Shklovskii laws as a function of temperature σ∝e(-(T(ES)/T)(1/2) and as a function of electric field I∝e(-(E)(ES)/E)(1/2). The data show that electronic transport in nanocrystal arrays can be tuned from the sequential tunneling to the cotunneling regime by increasing the tunnel barrier transparency.
    Physical Review Letters 10/2011; 107(17):176803. · 7.73 Impact Factor
  • Physical Review Letters 08/2011; 107(7). · 7.73 Impact Factor
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    ABSTRACT: We report on laser-excited angle-resolved photoemission spectroscopy in the electron-doped cuprate Sm1.85Ce0.15CuO(4-δ). The data show the existence of a nodal hole-pocket Fermi surface both in the normal and superconducting states. We prove that its origin is long-range antiferromagnetism by an analysis of the coherence factors in the main and folded bands. This coexistence of long-range antiferrmagnetism and superconductivity implies that electron-doped cuprates are two-Fermi-surface superconductors. The measured superconducting gap in the nodal hole pocket is compatible with a d-wave symmetry.
    Physical Review Letters 05/2011; 106(19):197002. · 7.73 Impact Factor
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    ABSTRACT: We report on local probe measurements of current-voltage and electrostatic force-voltage characteristics of electric-field-induced insulator to metal transition in VO2 thin film. In conducting AFM mode, switching from the insulating to metallic state occurs for electric-field threshold E~6.5\times10^7 Vm-1 at 300K. Upon lifting the tip above the sample surface, we find that the transition can also be observed through a change in electrostatic force and in tunneling current. In this noncontact regime, the transition is characterized by random telegraphic noise. These results show that electric field alone is sufficient to induce the transition; however, the electronic current provides a positive feedback effect that amplifies the phenomena.
    Physical Review B 01/2011; 84:241410(R). · 3.66 Impact Factor
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    ABSTRACT: We have investigated the influence of point defect disorder in the electronic properties of manganite films. Real-time mapping of ion irradiated samples conductivity was performed through conductive atomic force microscopy (CAFM). CAFM images show electronic inhomogeneities in the samples with different physical properties due to spatial fluctuations in the point defect distribution. As disorder increases, the distance between conducting regions increases and the metal-insulator transition shifts to lower temperatures. Transport properties in these systems can be interpreted in terms of a percolative model. The samples saturation magnetization decreases as the irradiation dose increases whereas the Curie temperature remains unchanged.
    Journal of Applied Physics 07/2010; · 2.21 Impact Factor
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    ABSTRACT: Over recent years, the insulator to metal transition of the vanadium dioxide (VO2) Mott insulator has been revisited revealing surprising new features: (i) electronic inhomogeneities were mapped out by local infrared spectroscopy near the transition temperature TIM; (ii) electric-field-induced resistance switching has been found in this material. To further investigate the insulator/metal transition, we have performed non-equilibrium transport measurements on 200nm VO2 dots using a variable temperature/magnetic field conducting AFM setup. I(V) spectra of numerous VO2 dots as a function of temperature have enabled us to determine the voltage threshold, the current jump and the the noise spectrum as voltage is swept across the electric-field-induced transition.
    03/2010;
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    ABSTRACT: We have used oxygen ions irradiation to generate controlled structural disorder in thin manganite films. Conductive atomic force microscopy CAFM), transport and magnetic measurements were performed to analyze the influence of the implantation process in the physical properties of the films. CAFM images show regions with different conductivity values, probably due to the random distribution of point defect or inhomogeneous changes of the local Mn3+/4+ ratio to reduce lattice strains of the irradiated areas. The transport and magnetic properties of these systems are interpreted in this context. Metal-insulator transition can be described in the frame of a percolative model. Disorder increases the distance between conducting regions, lowering the observed TMI. Point defect disorder increases localization of the carriers due to increased disorder and locally enhanced strain field. Remarkably, even with the inhomogeneous nature of the samples, no sign of low field magnetoresistance was found. Point defect disorder decreases the system magnetization but doesn t seem to change the magnetic transition temperature. As a consequence, an important decoupling between the magnetic and the metal-insulator transition is found for ion irradiated films as opposed to the classical double exchange model scenario.
    Physical review. B, Condensed matter 12/2009; 81(13). · 3.77 Impact Factor
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    ABSTRACT: Angle-resolved photoemission measurements on the electron-doped cuprate Sm(1.85)Ce(0.15)CuO(4) evidence anisotropic dressing of charge-carriers due to many-body interactions. Most significantly, the scattering rate along the zone boundary saturates for binding energies larger than ~200 meV, while along the diagonal direction it increases nearly linearly with the binding energy in the energy range ~150-500 meV. These results indicate that many-body interactions along the diagonal direction are strong down to the bottom of the band, while along the zone-bounday they become very weak at energies above ~200 meV.
    04/2009;
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    ABSTRACT: We present confined geometry measurements of manganites La0.325Pr0.3Ca0.375MnO3 (LPCMO) and La2/3Sr1/3MnO3 (LSMO). As reported previously, due to electronic phase separation, LPCMO microwires show a step-like metal insulator transition as temperature is lowered and as magnetic field is sweped. We will show how this feature evolves as a function of the width and shape of the microwires. On the contrary, LSMO microwires are found to have a smooth transition in all wire sizes down to a width of 140nm. We have created extra disorder in the LSMO sample by irradiating it using 150keV oxygen ions with a dose of 2 x 10^14 ions/cm^2. As expected, the microwires TMI transitions are lowered by 120K but the transitions remain stepless. The fact that the electronic homogeneity in LSMO sample is robust, even when irradiated, enabled us start an optical study of artificially phase separated highly correlated materials. We will show preliminary results comparing reflectivity spectra of micropatterned samples to effective medium approximation models.
    03/2009;
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    ABSTRACT: As shown recently Proslier et al., Europhys. Lett. 73, 962 2006, it is possible to map the superconducting SC condensate by measuring locally the Josephson tunneling current. We apply this technique to image the vortex lattice in V 3 Si which is used as the simplest example of a spatially varying quantum condensate. The Josephson scanning tunneling microscope JSTM maps revealed the vortex lattice with the Josephson effect being present outside the vortices and disappearing progressively toward the vortex core. The characteristic length scales of a vortex observed in the Josephson regime are compared to the ones obtained in the quasi-particle regime. We demonstrate that the JSTM allows a nanometer scale resolution of the SC condensate that may be applied to inhomogeneous phases such as high-T c superconductors. Scanning tunneling microscopy STM and spectroscopy STS have been key techniques to probe the superconduct-ing SC state on a nanometer scale. Applied to conventional superconductors, STM has allowed, among numerous ex-amples, the study of the vortex lattice. 1 In high-T c supercon-ductors, it revealed i a nanometer variation in the SC gap and ii the presence of pseudogap regions above and below the SC transition temperature. 2 One hypothesis explaining the pseudogap is the existence of noncondensed Cooper pairs in these regions. 3 Unfortunately, STS probes the SC conden-sate indirectly through the quasiparticle QP excitations of the SC condensate, and thus is not a proof of the existence of coherent Cooper pairs. A straightforward manner to probe the SC condensate di-rectly would be to measure locally the Josephson current JC between a surface and a tip, both superconducting. 4 In-deed, if the barrier is sufficiently narrow, the overlap be-tween macroscopic SC wave functions of the tip and the sample may result in a measurable current of Cooper pairs. 5 The JC is therefore a direct consequence of the existence of SC condensates in both tunneling electrodes. The relevant energy scale governing the pair tunneling is the Josephson energy E J . At zero temperature, for identical tip and sample superconductors, the Josephson coupling energy is E J = 2e 2 R N
    Physical Review B 07/2008; · 3.66 Impact Factor

Publication Stats

219 Citations
94.92 Total Impact Points

Institutions

  • 2006–2011
    • Université Paris-Sud 11
      • Laboratoire de Physique des Solides
      Orsay, Île-de-France, France
  • 2005–2009
    • University of Maryland, College Park
      • Department of Physics
      College Park, MD, United States
  • 2007–2008
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France