L. Forro

École Polytechnique Fédérale de Lausanne, Lausanne, Vaud, Switzerland

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Publications (475)1358.6 Total impact

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    ABSTRACT: Monod and Beuneu [Monod and Beuneu, Phys. Rev. B 19, 911 (1979)] established the validity of the Elliott-Yafet theory for elemental metals through correlating the experimental electron spin resonance line-width with the so-called spin-orbit admixture coefficients and the momentum-relaxation theory. The spin-orbit admixture coefficients data were based on atomic spin-orbit splitting. We highlight two shortcomings of the previous description: i) the momentum-relaxation involves the Debye temperature and the electron-phonon coupling whose variation among the elemental metals was neglected, ii) the Elliott-Yafet theory involves matrix elements of the spin-orbit coupling (SOC), which are however not identical to the SOC induced energy splitting of the atomic levels, even though the two have similar magnitudes. We obtain the empirical spin-orbit admixture parameters for the alkali metals by considering the proper description of the momentum relaxation theory. In addition, we present a model calculation which highlights the difference between the SOC matrix element and energy splitting.
    01/2014; 89(11).
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    ABSTRACT: We developed a topotactic strategy to grow BiTeCl single crystals. Structural characterization by means of X-ray diffraction was performed, and the high crystallinity of the material was proven. Measurements of the thermoelectrical coefficients electrical resistivity, thermoelectric power and thermal conductivity show an enhanced room temperature power factor of 20 μW cm−1 K−2. The high value of the figure of merit (ZT = 0.17) confirms that BiTeCl is a promising material for engineering in thermoelectric applications at low temperature.
    Scripta Materialia 01/2014; 76:69–72. · 2.82 Impact Factor
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    ABSTRACT: We report on the increase of the Young's modulus (E) of chemical vapor deposition (CVD) grown multi-walled carbon nanotubes (MWNTs) upon high temperature heat treatment. The post heat-treatment at 2200–2800 o C in a controlled atmosphere results in a considerable improvement of the microstructure, chemical stability and electro-physical properties of the nanotubes. The Young's modulus of MWNTs of different diameters was measured by the deflection of a single tube suspended across the hole of silicon nitride membrane and loaded by an atomic force microscope tip. Contrary to previous reports, a strong increase of E was feasible due to the improved growth conditions of pristine carbon nanotubes and to the improved heat treatment conditions. However, the elastic modulus of CVD grown MWNTs still shows strong diameter dependence resulting from the remaining structural inhomogeneities in large diameter nanotubes. C 2013 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4829272]
    AIP Advances. 12/2013; 3.
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    ABSTRACT: The synthesis, structural and physical properties of iron lanthanide oxypnictide superconductors, L4Fe2As2Te1-xO4 (L = Pr, Sm, Gd), with transition temperature at ~ 25 K are reported. Single crystals have been grown at high pressure using cubic anvil technique. The crystal structure consists of layers of L2O2 tetrahedra separated by alternating layers of chains of Te and of Fe2As2 tetrahedra: -L2O2-Te-L2O2-Fe2As2-L2O2-Te-L2O2- (space group: I4/mmm, a ~ 4.0, c ~ 29.6 {\AA}). Substitution of oxygen by fluorine increases the critical temperature, e.g. in Gd4Fe2As2Te1-xOyF4-y up to 45 K. Magnetic torque measurements reveal an anisotropy of the penetration depths of ~31.
    Physical review. B, Condensed matter 11/2013; 89(2). · 3.77 Impact Factor
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    ABSTRACT: Thin film gas sensors made of nanocomposite MWCNT·SnO2(1:66), semiconductor compound WO3·SnO2(1:9), and also multicomponent structure MWCNT·SnO2(1:66)/WO3·SnO2(1:9) have been fabricated by high-frequency magnetron sputtering and electron-beam deposition methods. Sensitivity of the prepared sensors to influence of gases, such as hydrogen, methane, butane, and also ethanol vapors, was investigated. Sensors made of MWCNT·SnO2(1:66) and WO3·SnO2(1:9) show appreciable sensitivity to hydrogen and alcohol vapors already at working body temperature 100–150°C. Sensors made of MWCNT·SnO2(1:66)·WO3·SnO2(1:9) can be used for detection of low concentrations of hydrogen and ethanol vapors; besides, monotonous increase in the structure sensitivity with increase in content of the alcohol vapors allows one to apply these sensors also for fast detection of concentration of these vapors in air.
    Journal of Contemporary Physics (Armenian Academy of Sciences) 07/2013; 48(4). · 0.25 Impact Factor
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    ABSTRACT: We demonstrate that the thermopower (S) can be used to probe the spin fluctuations (SFs) in proximity to the quantum critical point (QCP) in Fe-based superconductors. The sensitivity of S to the entropy of charge carriers allows us to observe an increase of S/T in Ba(Fe1−xCox)2As2 close to the spin-density-wave (SDW) QCP. This behavior is due to the coupling of low-energy conduction electrons to two-dimensional SFs, similar to heavy-fermion systems. The low-temperature enhancement of S/T in the Co substitution range 0.02<x<0.1 is bordered by two Lifshitz transitions, and it corresponds to the superconducting region, where a similarity between the electron and nonreconstructed hole pockets exists. The maximal S/T is observed in proximity to the commensurate-to-incommensurate SDW transition, for critical xc≈0.05, close to the highest superconducting Tc. This analysis indicates that low-T thermopower is influenced by critical spin fluctuations which are important for the superconducting mechanism.
    Physical review. B, Condensed matter 06/2013; 87(22). · 3.77 Impact Factor
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    ABSTRACT: We report crystal growth at high pressure, structure determination, and magnetic and transport studies of an oxypnictide superconductor Pr4Fe2As2Te1−xO4. Its structure resembles the known 1111 phase except for the considerably larger c lattice constant and intercalated tellurium atoms and it crystallizes in a tetragonal lattice [a = 4.0165(2) Å, c = 29.8572(16) Å, and space group I4/mmm (no. 139)]. The electrical resistivity ρ(T) and magnetization measurement show a transition at Tc≈25 K. The lower (Hc1) and upper (Hc2) critical fields are 2 mT and 6.5 T, respectively. The Ginzburg-Landau parameter of κ ≈ 80 places this compound in the family of strong type-II superconductors.
    Physical review. B, Condensed matter 05/2013; 87(18). · 3.77 Impact Factor
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    ABSTRACT: We observe the electron spin resonance of conduction electrons in boron-doped (6400 ppm) superconducting diamond (Tc=3.8 K). We clearly identify the benchmarks of conduction electron spin resonance (CESR): the nearly temperature independent electron spin resonance signal intensity and its magnitude, which is in good agreement with that expected from the density of states through the Pauli spin susceptibility. The temperature dependent CESR linewidth weakly increases with increasing temperature, which can be understood in the framework of the Elliott-Yafet theory of spin relaxation. An anomalous and yet unexplained relation is observed between the g-factor, CESR linewidth, and the resistivity using the empirical Elliott-Yafet relation.
    Physical Review B 05/2013; 87:195132. · 3.77 Impact Factor
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    ABSTRACT: Oxygen vacancies created in anatase TiO 2 by UV photons (80–130 eV) provide an effective electron-doping mechanism and induce a hitherto unobserved dispersive metallic state. Angle resolved photo-emission reveals that the quasiparticles are large polarons. These results indicate that anatase can be tuned from an insulator to a polaron gas to a weakly correlated metal as a function of doping and clarify the nature of conductivity in this material.
    Physical Review Letters 05/2013; 110(19):196403. · 7.94 Impact Factor
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    ABSTRACT: The temperature dependent resistivity and thermo-electric power of Cr-N codoped TiO2 were compared with that of single element N and Cr doped and undoped TiO2 using epitaxial anatase thin films grown by pulsed laser deposition on (100) LaAlO3 substrates. The resistivity plots and especially the thermoelectric power data confirm that codoping is not a simple sum of single element doping. However, the negative sign of the Seebeck coefficient indicates electron dominated transport independent of doping. The narrowing distinction among the effects of different doping methods combined with increasing resistivity of the films with improving crystalline quality of TiO2 suggest that structural defects play a critical role in the doping process.
    Applied Physics Letters 05/2013; 102(17):172108. · 3.79 Impact Factor
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    ABSTRACT: The resistivity of a very high quality anatase TiO2 doped with 6% of Nb was measured from 300 K down to 40 mK. No sign of superconductivity was detected. Instead, a minute quantity of cation vacancies resulted in a Kondo scattering. Measurements of thermo-electric power and resistivity were extended up to 600 K. The calculated power factor has a peak value of 14 μW/(K2cm) at 350 K, which is comparable to that of Bi2Te3 [Venkatasubramanian et al., Nature 413, 597 (2001)], the archetype thermolectrics. Taking the literature value for the thermal conductivity of Nb doped TiO2 thin films, the calculated figure of merit (ZT) is in the range of 0.1 above 300 K. This value is encouraging for further engineering of the material in order to reach ZT of 1 suitable for high temperature thermoelectrics.
    Applied Physics Letters 01/2013; 102(1):013901. · 3.79 Impact Factor
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    ABSTRACT: We demonstrate that the thermopower (S) can be used to probe the spin fluctuations (SFs) in proximity to the quantum critical point (QCP) in Fe-based superconductors. The sensitivity of S to the entropy of charge carriers allows us to observe an increase of S/T in Ba(Fe1−xCox)2As2 close to the spin-density-wave (SDW) QCP. This behavior is due to the coupling of low-energy conduction electrons to two-dimensional SFs, similar to heavy-fermion systems. The low-temperature enhancement of S/T in the Co substitution range 0.02<x<0.1 is bordered by two Lifshitz transitions, and it corresponds to the superconducting region, where a similarity between the electron and nonreconstructed hole pockets exists. The maximal S/T is observed in proximity to the commensurate-to-incommensurate SDW transition, for critical xc≈0.05, close to the highest superconducting Tc. This analysis indicates that low-T thermopower is influenced by critical spin fluctuations which are important for the superconducting mechanism.
    Physical Review B. 01/2013; 87(22):224508.
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    ABSTRACT: Photosensitizers may convert molecular oxygen into reactive oxygen species (ROS) including, e.g., singlet oxygen (1O2), superoxide anion (), and hydroxyl radicals (•OH), chemicals with extremely high cyto- and potential genotoxicity. Photodynamic ROS reactions are determinative in medical photodynamic therapy (cancer treatment with externally added photosensitizers) and in reactions damaging the photosynthetic apparatus of plants (via internal pigments). The primary events of photosynthesis take place in the chlorophyll containing reaction center protein complex (RC), where the energy of light is converted into chemical potential. 1O2 is formed by both bacterial bacteriochlorophylls and plant RC triplet chlorophylls in high light and if the quenching of 1O2 is impaired. In plant physiology, reducing the formation of the ROS and thus lessening photooxidative membrane damage (including the RC protein) and increasing the efficiency of the photochemical energy conversion is of special interest. Carbon nanotubes, in artificial systems, are also known to react with singlet oxygen. To investigate the possibility of 1O2 quenching by carbon nanotubes in a biological system, we studied the effect of carbon nanotubes on 1O2 photogenerated by photosynthetic RCs purified from purple bacteria. 1,3-Diphenylisobenzofuran (DPBF), a dye responding to oxidation by 1O2 with absorption decrease at 420 nm was used to measure 1O2 concentrations. 1O2 was produced either from a photosensitizer (methylene blue) or from triplet photosynthetic RCs and the antioxidant capacity of carbon nanotubes was assessed. Less 1O2 was detected by DPBF in the presence of carbon nanotubes, suggesting that these are potential quenchers of this ROS.
    physica status solidi (b) 01/2013; 250(12):2539–2543. · 1.49 Impact Factor
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    ABSTRACT: H2O2 is a product of reactions catalysed by several oxidase enzymes and it is essential in environmental and pharmaceutical analyses. The most commonly used enzyme in understanding the biological behaviour of catalysed oxidation of H2O2 is horseradish peroxidase (HRP). In our experiments HRP was bound to carboxyl-functionalized multiwalled carbon nanotubes (MWNT-COOH) by N-hydroxysuccinimide (NHS) and 1-[3-dimethylaminopropyl]-3-ethyl-carbodiimide (EDC) crosslinkers. The activity of this bio-nanocomposite and the limit of detection (LOD) for H2O2 were determined by measuring the fluorescence of tetraguaiacol (which chemical is the product of guaiacol oxidation after addition of H2O2 to the reaction mixture) as a function of time. The hydrogen peroxide biosensor we developed exhibited a detection limit of 1.2 µM H2O2 s−1 which resolution was better than the one measured in solution by about a factor of eight (it was 10 µM H2O2 s−1 in solution). An attempt has been made to measure the concentration of H2O2 in an electrochemical cell with HRP immobilized on the surface of an electrode made of indium tin oxide (ITO, a transparent conductive oxide) and MWNT.
    physica status solidi (b) 01/2013; 250(12):2559–2563. · 1.49 Impact Factor
  • physica status solidi (b) 12/2012; 249(12):2487-2490. · 1.49 Impact Factor
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    ABSTRACT: A large number of studies have indicated recently that photosynthetic reaction center proteins (RC) bind successfully to nanostructures and their functional activity is largely retained. The major goal of current research is to find the most efficient systems and conditions for the photoelectric energy conversion and for the stability of this bio-nanocomposite. In our studies, we immobilized the RC protein on multiwalled carbon nanotubes (MWNT) through specific chemical binding to amine functional groups and through conducting polymer (poly(3-thiophene acetic acid), PTAA). Both structural (TEM, AFM) and functional (absorption change and conductivity) measurements has shown that RCs could be bound effectively to functionalized CNTs. The kinetics of the light induced absorption change indicated that RCs were still active in the composite and there was an interaction between the protein cofactors and the CNTs. The light generated photocurrent was measured in an electrochemical cell with transparent CNT electrode designed specially for this experiment. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    physica status solidi (b) 12/2012; 249(12):2386-2389. · 1.49 Impact Factor
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    ABSTRACT: We have performed electron spin resonance (ESR) measurements on a large assembly of graphene oxide (GO) and reduced graphene oxide (RGO) flakes. In GO samples the Curie tail is coming from 1.4 × 10 18 cm −3 of localized spins. Although reduction of GO was expected to reestablish the pristine properties of graphene, no Pauli-like contribution was detected and only a low concentration of 1.2 × 10 16 cm −3 spin carrying defects were measured. Our study, completed by resistivity measurements, shows that the carrier transport in RGO samples is dominated by hopping. The incomplete reduction of GO leaves behind a large number of defects, presumably the majority of which are ESR silent, causing the Anderson localization of the electronic states. Slight doping with potassium indicates the appearance of a Pauli contribution in the spin susceptibility.
    Physical Review B 11/2012; 86(19):195139. · 3.77 Impact Factor
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    ABSTRACT: EPL, 99 (2012) 57005 Please visit the new website www.epljournal.org
    EPL (Europhysics Letters) 09/2012; 99(5):57005. · 2.26 Impact Factor
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    ABSTRACT: We present the investigation of a monoclinic compound SeCuO3 using x-ray powder di?raction, magnetization, torque and electron-spin-resonance (ESR). Structurally based analysis suggests that SeCuO3 can be considered as a 3D network of tetramers. The values of intra-tetramer exchange interactions are extracted from the temperature dependence of the susceptibility and amount to ~200 K. The inter-tetramer coupling leads to the development of long-range antiferromagnetic order at TN = 8 K. An unusual temperature dependence of the effective g-tensors is observed, accompanied with a rotation of macroscopic magnetic axes. We explain this unique observation as due to site-selective quantum correlations.
    Physical Review B 08/2012; 86(05):054405. · 3.77 Impact Factor
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    ABSTRACT: We present the microrheological study of the two close human epithelial cell lines: non-cancerous HCV29 and cancerous T24. The optical tweezers tracking was applied to extract the several seconds long trajectories of endogenous lipid granules at time step of 1μs. They were analyzed using a recently proposed equation for mean square displacement (MSD) in the case of subdiffusion influenced by an optical trap. This equation leads to an explicit form for viscoelastic moduli. The moduli of the two cell lines were found to be the same within the experimental accuracy for frequencies 10(2) - 10(5) Hz. For both cell lines subdiffusion was observed with the exponent close to 3/4, the value predicted by the theory of semiflexible polymers. For times longer than 0.1s the MSD of cancerous cells exceeds the MSD of non-cancerous cells for all values of the trapping force. Such behavior can be interpreted as a signature of the active processes and prevents the extraction of the low-frequency viscoelastic moduli for the living cells by passive microrheology.
    The European Physical Journal E 07/2012; 35(7):63. · 1.82 Impact Factor

Publication Stats

8k Citations
1,358.60 Total Impact Points

Institutions

  • 1970–2014
    • École Polytechnique Fédérale de Lausanne
      • • Institute of Condensed Matter Physics
      • • Faculté des Sciences de Base
      • • Laboratoire de systèmes microélectroniques
      Lausanne, Vaud, Switzerland
  • 2013
    • Iowa State University
      • Department of Physics and Astronomy
      Ames, Iowa, United States
    • Eawag: Das Wasserforschungs-Institut des ETH-Bereichs
      Duebendorf, Zurich, Switzerland
  • 2003–2009
    • IPMC
      Akra, Greater Accra, Ghana
    • University of Szeged
      Algyő, Csongrád, Hungary
  • 2001–2009
    • ETH Zurich
      • Laboratory for Solid State Physics
      Zürich, ZH, Switzerland
    • Université Paris-Sud 11
      • Laboratoire de Physique des Solides
      Paris, Ile-de-France, France
    • University of Geneva
      Genève, Geneva, Switzerland
  • 1994–2008
    • Budapest University of Technology and Economics
      • • Institute of Physics
      • • Department of Physics
      Budapeŝto, Budapest, Hungary
  • 2007
    • Université de Neuchâtel
      • Institut de physique (IPH)
      Neuchâtel, NE, Switzerland
  • 1990–2006
    • Stony Brook University
      • Department of Physics and Astronomy
      Stony Brook, NY, United States
    • University of Florida
      • Department of Physics
      Gainesville, Florida, United States
  • 1984–2006
    • Institute of Physics, Zagreb
      Zagrabia, Grad Zagreb, Croatia
  • 2005
    • Polish Academy of Sciences
      Warszawa, Masovian Voivodeship, Poland
  • 2004
    • University of Lausanne
      Lausanne, Vaud, Switzerland
    • Ecole Normale Supérieure de Paris
      • Laboratoire Pierre Aigrain
      Paris, Ile-de-France, France
  • 1997–2001
    • Hungarian Academy of Sciences
      • • Institute of Biophysics
      • • Institute for Solid State Physics and Optics
      Budapest, Budapest fovaros, Hungary
    • Laboratório Nacional de Luz Síncrotron
      Conceição de Campinas, São Paulo, Brazil
  • 1998
    • Georgia Institute of Technology
      • School of Physics
      Atlanta, GA, United States
  • 1996–1997
    • Bavarian Academy of Sciences and Humanities
      Arching, Bavaria, Germany
  • 1993–1994
    • University of Cambridge
      • Department of Physics: Cavendish Laboratory
      Cambridge, England, United Kingdom
  • 1990–1993
    • State University of New York
      New York City, New York, United States
  • 1991
    • CSU Mentor
      Long Beach, California, United States
  • 1989–1991
    • University of Zagreb
      Zagrabia, Grad Zagreb, Croatia
  • 1988
    • Max Planck Institute for Medical Research
      Heidelburg, Baden-Württemberg, Germany
  • 1987
    • Universität Heidelberg
      • Institute of Inorganic Chemistry
      Heidelburg, Baden-Württemberg, Germany
    • University of California, Los Angeles
      Los Angeles, California, United States
  • 1986
    • IT University of Copenhagen
      København, Capital Region, Denmark