S. Lupi

Sincrotrone Trieste S.C.p.A., Trst, Friuli Venezia Giulia, Italy

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Publications (227)549.71 Total impact

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    ABSTRACT: We present an experimental and theoretical study exploring surface effects on the evolution of the metal-insulator transition in the model Mott-Hubbard compound Cr-doped V$_2$O$_3$. We find a microscopic domain formation that is clearly affected by the surface crystallographic orientation. Using scanning photoelectron microscopy and X-ray diffraction, we find that surface defects act as nucleation centers for the formation of domains at the temperature-induced isostructural transition and favor the formation of microscopic metallic regions. A density functional theory plus dynamical mean field theory study of different surface terminations shows that the surface reconstruction with excess vanadyl cations leads to doped, and hence more metallic surface states, explaining our experimental observations.
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    ABSTRACT: The combination of amino acid anions with the choline cation gives origin to a new and potentially important class of organic ionic liquids that might represent a viable and bio-compatible alternative with respect to the traditional ones. We present here a detailed study of the bulk phase of the prototype system composed of the simplest amino acid (alanine) anion and the choline cation, based on ab initio and classical molecular dynamics. Theoretical findings have been validated by comparing with accurate experimental X-ray diffraction data and infrared spectra. We find that hydrogen bonding (HB) features in these systems are crucial in establishing their local geometric structure. We have also found that these HBs once formed are persistent and that the proton resides exclusively on the choline cation. In addition, we show that a classical force field description for this particular ionic liquid can be accurately performed by using a slightly modified version of the generalized AMBER force field.
    The Journal of Chemical Physics 06/2015; 142(23):234502. DOI:10.1063/1.4922442 · 3.12 Impact Factor
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    ABSTRACT: The control of non-equilibrium phenomena in complex solids is an important research frontier, encompassing new effects like light induced superconductivity. Here, we show that coherent optical excitation of molecular vibrations in the organic conductor K3C60 can induce a non-equilibrium state with the optical properties of a superconductor. A transient gap in the real part of the optical conductivity and a low-frequency divergence of the imaginary part are measured for base temperatures far above equilibrium Tc=20 K. These findings underscore the role of coherent light fields in inducing emergent order.
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    ABSTRACT: Vanadium sesquioxide V2O3 is considered a textbook example of Mott-Hubbard physics. In this paper we present an extended optical study of its whole temperature/doping phase diagram as obtained by doping the pure material with M=Cr or Ti atoms (V1-xMx)2O3. We reveal that its thermodynamically stable metallic and insulating phases, although macroscopically equivalent, show very different low-energy electrodynamics. The Cr and Ti doping drastically change both the antiferromagnetic gap and the paramagnetic metallic properties. A slight chromium content induces a mesoscopic electronic phase separation, while the pure compound is characterized by short-lived quasiparticles at high temperature. This study thus provides a new comprehensive scenario of the Mott-Hubbard physics in the prototype compound V2O3.
    Physical Review B 04/2015; 91(15). DOI:10.1103/PhysRevB.91.155133 · 3.74 Impact Factor
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    ABSTRACT: The great potential of Dirac electrons for plasmonics and photonics has been readily recognized after their discovery in graphene, followed by applications to smart optical devices. Dirac carriers are also found in topological insulators (TI) --quantum systems having an insulating gap in the bulk and intrinsic Dirac metallic states at the surface--. Here, we investigate the plasmonic response of ring structures patterned in Bi$_2$Se$_3$ TI films, which we investigate through terahertz (THz) spectroscopy. The rings are observed to exhibit a bonding and an antibonding plasmon modes, which we tune in frequency by varying their diameter. We develop an analytical theory based on the THz conductivity of unpatterned films, which accurately describes the strong plasmon-phonon hybridization and Fano interference experimentally observed as the bonding plasmon is swiped across the promineng 2\,THz phonon exhibited by this material. This work opens the road for the investigation of plasmons in topological insulators and for their application in tunable THz devices.
    Advanced Optical Materials 04/2015; DOI:10.1002/adom.201400513 · 4.06 Impact Factor
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    ABSTRACT: Amyloid and amyloid-like fibrils are a general class of protein aggregates and represent a central topic in life sciences for their involvement in several neurodegenerative disorders and their unique mechanical and supramolecular morphological properties. Both their biological role and their physical properties, including their high mechanical stability and thermodynamic inertia, are related to the structural arrangement of proteins in the aggregates at molecular level. Significant variations may exist in the supramolecular organization of the commonly termed cross-β structure that constitutes the amyloid core. In this context, a fine knowledge of the structural details in fibrils may give significant information on the assembly process and on possible ways of tuning or inhibiting it. Here we propose a simple method based on the combined use of Fourier transform infrared spectroscopy and Fourier transform Raman spectroscopy to accurately reveal structural details in the fibrillar aggregates, side-chain exposure and intermolecular interactions. Interestingly, coupled analysis of mid-infrared spectra reveals antiparallel β-sheet orientation in ConA fibrils. We also report the comparison between THz absorption spectra of Concanavalin A in its native and fibrillar state at different hydration levels, allowing obtaining corroboration of peaks assignation in this range and information on the effect of amyloid supramolecular arrangement on the network dynamics of hydration water. Copyright © 2015 Elsevier B.V. All rights reserved.
    Biophysical chemistry 04/2015; 199:17-24. DOI:10.1016/j.bpc.2015.02.007 · 2.32 Impact Factor
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    ABSTRACT: We have studied the optical properties of four $({\mathrm{LaNiO}}_{3}{)}_{n}/({\mathrm{LaMnO}}_{3}{)}_{2}$ superlattices (SL) ($n=2,3,4,5$) on ${\mathrm{SrTiO}}_{3}$ substrates. We have measured the reflectivity at temperatures from 20 to 400 K, and extracted the optical conductivity through a fitting procedure based on a Kramers-Kronig consistent Lorentz-Drude model. With increasing ${\mathrm{LaNiO}}_{3}$ thickness, the SLs undergo an insulator-to-metal transition (IMT) that is accompanied by the transfer of spectral weight from high to low frequency. The presence of a broad midinfrared band, however, shows that the optical conductivity of the $({\mathrm{LaNiO}}_{3}{)}_{n}/({\mathrm{LaMnO}}_{3}{)}_{2}$ SLs is not a linear combination of the ${\mathrm{LaMnO}}_{3}$ and ${\mathrm{LaNiO}}_{3}$ conductivities. Our observations suggest that interfacial charge transfer leads to an IMT due to a change in valence at the Mn and Ni sites.
    Physical Review Letters 04/2015; 114(15). DOI:10.1103/PhysRevLett.114.156801 · 7.51 Impact Factor
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    ABSTRACT: Advanced accelerator technology, based on plasma structures, requires high brightness electron beams, which can be used also to drive advanced radiation sources. Indeed, electron beams to be injected into the plasma and accelerated in the plasma channel are characterized by small transverse size and ultra-short time duration, allowing the production of coherent radiation in the THz range. In the present work we report both theoretical and experimental studies on the spatial/angular distribution of Coherent Transition and Diffraction Radiation in the pre-wave zone.
    Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 03/2015; 309. DOI:10.1016/j.nimb.2015.03.047 · 1.12 Impact Factor
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    ABSTRACT: We have studied the optical properties of four (LaNiO$_3$)$_n$/(LaMnO$_3$)$_2$ superlattices (SL) ($n$=2, 3, 4, 5) on SrTiO$_3$ substrates. We have measured the reflectivity at temperatures from 20 K to 400 K, and extracted the optical conductivity through a fitting procedure based on a Kramers-Kronig consistent Lorentz-Drude model. With increasing LaNiO$_3$ thickness, the SLs undergo an insulator-to-metal transition (IMT) that is accompanied by the transfer of spectral weight from high to low frequency. The presence of a broad mid-infrared band, however, shows that the optical conductivity of the (LaNiO$_3$)$_n$/(LaMnO$_3$)$_2$ SLs is not a linear combination of the LaMnO$_3$ and LaNiO$_3$ conductivities. Our observations suggest that interfacial charge transfer leads to an IMT due to a change in valence at the Mn and Ni sites.
    Physical Review Letters 03/2015; 114(15). · 7.51 Impact Factor
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    ABSTRACT: One of the pivotal questions in the physics of high-temperature superconductors is whether the low-energy dynamics of the charge carriers is mediated by bosons with a characteristic timescale. This issue has remained elusive since electronic correlations are expected to dramatically speed up the electron-boson scattering processes, confining them to the very femtosecond timescale that is hard to access even with state-of-the-art ultrafast techniques. Here we simultaneously push the time resolution and the frequency range of transient reflectivity measurements up to an unprecedented level that enables us to directly observe the 16 fs build-up of the effective electron-boson interaction in hole-doped copper oxides. This extremely fast timescale is in agreement with numerical calculations based on the t-J model and the repulsive Hubbard model, in which the relaxation of the photo-excited charges is achieved via inelastic scattering with short-range antiferromagnetic excitations.
    Nature Physics 01/2015; 11(5). DOI:10.1038/nphys3265 · 20.60 Impact Factor
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    ABSTRACT: Terahertz spectroscopy has vast potentialities in sensing a broad range of elementary excitations (e.g., collective vibrations of molecules, phonons, excitons, etc.). However, the large wavelength associated with terahertz radiation (about 300 μm at 1 THz) severely hinders its interaction with nano-objects, such as nanoparticles, nanorods, nanotubes and large molecules of biological relevance, practically limiting terahertz studies to macroscopic ensembles of these compounds, in the form of thick pellets of crystallized molecules or highly concentrated solutions of nanomaterials. Here we show that chains of terahertz dipole nanoantennas spaced by nanogaps of 20 nm allow retrieving the spectroscopic signature of a monolayer of cadmium selenide quantum dots, a significant portion of the signal arising from the dots located within the antenna nanocavities. A Fano-like interference between the fundamental antenna mode and the phonon resonance of the quantum dots is observed, accompanied by an absorption enhancement factor greater than one million. NETS can find immediate applications in terahertz spectroscopic studies of nanocrystals and molecules at extremely low concentrations. Furthermore, it shows a practicable route towards the characterization of individual nano-objects at these frequencies.
    Nano Letters 11/2014; DOI:10.1021/nl503705w · 13.59 Impact Factor
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    ABSTRACT: Extraordinary optical transmission (EOT) peaks mediated by plasmonic excitations can be observed in a variety of subwavelength patterned metallic surfaces. In this paper, we have fabricated and spectroscopically characterized plasmon devices exhibiting EOT peaks at terahertz (THz) frequencies. These devices, which resonate with intermediate and collective modes of macromolecules, can be used for detection of materials of biological interest and their performances have been experimentally determined by measuring the variation of the EOT frequencies for thin sub-micrometric organic layers deposited onto the device surface.
    Plasmonics 10/2014; DOI:10.1007/s11468-014-9775-3 · 2.74 Impact Factor
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    ABSTRACT: We report an infrared spectroscopy study of a 200 nm thick FeSe$_{0.5}$Te$_{0.5}$ film grown on LaAlO$_3$ with T$_c$=13.7 K. We analyze the 20 K normal state absolute reflectance R$_N$ measured over a broad infrared range and the reflectance ratio R$_S$/R$_N$, R$_S$ being the superconducting state reflectance, measured at 6 K in the terahertz range down to 12 cm$^{-1}$. We show that the normal state model conductivity is given by two Drude components, one of which much broader and intense than the other. In the superconducting state, we find that a gap $\Delta$=37$\pm$3 cm$^{-1}$ opens up in the narrow Drude band only, while the broad Drude band results to be ungapped, at least in the explored spectral range. Our results show that only a two-band model can coherently describe both normal and superconducting state data.
    Superconductor Science and Technology 10/2014; 27(12). DOI:10.1088/0953-2048/27/12/125011 · 2.80 Impact Factor
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    ABSTRACT: Plasmons in the terahertz (THz) range have been observed in Bi2Se3 topological insulator films patterned in form of micro-structures. These plasmons have been assigned to collective excitations of massless Dirac carriers located onto the Bi2Se3 topological surfaces. Their lifetime is mainly determined by the single-particle scattering rate and results scarcely dependent on temperature as expected for topological carriers.
    8th International Congress on Advanced Electromagnetic Materials in Microwaves and Optics - Metamaterials 2014, Copenhagen, Denmark; 08/2014
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    ABSTRACT: A plasmonic analogue of electromagnetically induced transparency is activated and tuned in the terahertz (THz) range in asymmetric metamaterials fabricated from high critical temperature (Tc) superconductor thin films. The asymmetric design provides a near-field coupling between a superradiant and a subradiant plasmonic mode, which has been widely tuned through superconductivity and monitored by Fourier transform infrared spectroscopy. The sharp transparency window that appears in the extinction spectrum exhibits a relative modulation up to 50% activated by temperature change. The interplay between ohmic and radiative damping, which can be independently tuned and controlled, allows for engineering the electromagnetically induced transparency of the metamaterial far beyond the current state-of-the-art, which relies on standard metals or low-Tc superconductors.Keywords: superconductivity; electromagnetic-induced transparency; THz; dark mode; Fano resonance; tunable metamaterials
    07/2014; 1(7):570-575. DOI:10.1021/ph500104k
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    ABSTRACT: The optical properties of a V4O7 single crystal have been investigated from the high temperature metallic phase down to the low temperature antiferromagnetic insulating one. The temperature dependent behavior of the optical conductivity across the metal-insulator transition (MIT) can be explained in a polaronic scenario. Charge carriers form strongly localized polarons in the insulating phase as suggested by a far-infrared charge gap abruptly opening at T_MIT = 237 K. In the metallic phase instead the presence of a Drude term is indicative of fairly delocalized charges with a moderately renormalized mass m* = 5m_e. The electronic spectral weight is almost recovered on an energy scale of 1 eV, which is much narrower compared to VO2 and V2O3 cases. Those findings suggest that electron-lattice interaction rather than electronic correlation is the driving force for V4O7 metal-insulator transition.
    Physical Review B 07/2014; 90(11). DOI:10.1103/PhysRevB.90.115149 · 3.74 Impact Factor
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    ABSTRACT: Exploiting the localization and enhancement capabilities of terahertz resonant dipole nanoantennas coupled through nanogaps, we present an effective method to perform terahertz spectroscopy on an extremely small number of nano-objects.
    CLEO: Science and Innovations; 06/2014
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    ABSTRACT: Photosynthetic reaction centers develop a stable charge separated state upon illumination. To investigate the molecular vibrations associated with the illuminated state of a reaction center we recorded terahertz absorption spectra of the photosynthetic reaction center from Rhodobacter sphaeroides in the dark and upon illumination and observed a small, but significant THz absorption increase in the 20 to 130 cm−1 spectral region. Reaction centers show very similar terahertz absorption increase when solubilized in detergents and in a lipidic sponge phase indicating that the nature of the bulk solvent has limited influence on the vibrational spectrum. The absorption change of the isolated LM subunit is very similar to that of the intact reaction center. Through temperature control experiments we show that 89% of the absorption change is likely attributed to the non-thermal activation of the protein molecules. These results indicate that picosecond molecular vibrations change primarily in the cofactors and/or in the evolutionary conserved core of the reaction center upon illumination, whereas the nuclear motions of the H-subunit and the bulk solvent have limited impact on the terahertz spectral changes.
    RSC Advances 05/2014; 4(49):25502-25509. DOI:10.1039/C4RA03787A · 3.84 Impact Factor
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    ABSTRACT: It has been recently reported (S. Lee et al., Nature Materials 12, 392, 2013) that superlattices where layers of the 8% Co-doped BaFe2As2 superconducting pnictide are intercalated with non superconducting ultrathin layers of either SrTiO3 or of oxygen-rich BaFe2As2, can be used to control flux pinning, thereby increasing critical fields and currents, without significantly affecting the critical temperature of the pristine superconducting material. However, little is known about the electron properties of these systems. Here we investigate the electrodynamics of these superconducting pnictide superlattices in the normal and superconducting state by using infrared reflectivity, from THz to visible range. We find that multi-gap structure of these superlattices is preserved, whereas some significant changes are observed in their electronic structure with respect to those of the original pnictide. Our results suggest that possible attempts to further increase the flux pinning may lead to a breakdown of the pnictide superconducting properties.
    Applied Physics Letters 04/2014; 104(22). DOI:10.1063/1.4880939 · 3.52 Impact Factor
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    ABSTRACT: This paper describes the scientific aims and potentials as well as the preliminary technical design of IRIDE, an innovative tool for multi-disciplinary investigations in a wide field of scientific, technological and industrial applications. IRIDE will be a high intensity “particles factory”, based on a combination of high duty cycle radio-frequency superconducting electron linacs and of high energy lasers. Conceived to provid eunique research possibilities for particle physics, for condensed matter physics,chemistry and material science, for structural biology and industrial applications, IRIDE will open completely new research possibilities and advance our knowledge in many branches of science and technology. IRIDE is also supposed to be realized in subsequent stages of development depending on the assigned priorities.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 03/2014; 740:138-146. DOI:10.1016/j.nima.2013.11.040 · 1.32 Impact Factor

Publication Stats

2k Citations
549.71 Total Impact Points

Institutions

  • 2012–2015
    • Sincrotrone Trieste S.C.p.A.
      Trst, Friuli Venezia Giulia, Italy
  • 1989–2015
    • Sapienza University of Rome
      • Department of Physics
      Roma, Latium, Italy
  • 2013
    • AREA Science Park
      Trst, Friuli Venezia Giulia, Italy
  • 1994–1996
    • Université Paris-Sud 11
      Orsay, Île-de-France, France
  • 1990
    • University of Geneva
      • Department of Condensed Matter Physics
      Genève, Geneva, Switzerland