Massimo Rontani

INO - Istituto Nazionale di Ottica, Florens, Tuscany, Italy

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Publications (87)327.99 Total impact

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    ABSTRACT: The control of orbitals and spin states of single electrons is a key ingredient for quantum information processing and novel detection schemes and is, more generally, of great relevance for spintronics. Coulomb and spin blockade in double quantum dots enable advanced single-spin operations that would be available even for room-temperature applications with sufficiently small devices. To date, however, spin operations in double quantum dots have typically been observed at sub-kelvin temperatures, a key reason being that it is very challenging to scale a double quantum dot system while retaining independent field-effect control of individual dots. Here, we show that the quantum-confined Stark effect allows two dots only 5 nm apart to be independently addressed without the requirement for aligned nanometre-sized local gating. We thus demonstrate a scalable method to fully control a double quantum dot device, regardless of its physical size. In the present implementation we present InAs/InP nanowire double quantum dots that display an experimentally detectable spin blockade up to 10 K. We also report and discuss an unexpected re-entrant spin blockade lifting as a function of the magnetic field intensity.
    Nature Nanotechnology 11/2014; 9(12). DOI:10.1038/nnano.2014.251 · 33.27 Impact Factor
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    K. A. Guerrero Becerra, Massimo Rontani
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    ABSTRACT: We show that Dirac electrons in a graphene quantum dot with a mass gap localize a la Wigner for realistic values of device parameters. Our theoretical evidence relies on many-body energies, one-body densities, and pair correlation functions obtained through the exact diagonalization of the interacting Hamiltonian, which allows us to take all many-body correlations into account. We predict that the experimental signatures of Wigner localization are the suppression of the fourfold periodicity of the filling sequence and the quenching of excitation energies, which may be both accessed through Coulomb blockade spectroscopy.
    Physical Review B 05/2014; 90(12). DOI:10.1103/PhysRevB.90.125446 · 3.74 Impact Factor
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    Massimo Rontani
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    ABSTRACT: We show theoretically that an undoped carbon nanotube might be an excitonic insulator---the long-sought phase of matter proposed by Keldysh, Kohn and others fifty years ago. We predict that the condensation of triplet excitons, driven by intervalley exchange interaction, spontaneously occurs at equilibrium if the tube radius is sufficiently small. The signatures of exciton condensation are its sizeable contributions to both the energy gap and the magnetic moment per electron. The increase of the gap might have already been measured, albeit with a different explanation [Deshpande et al., Science 323, 106 (2009)]. The enhancement of the quasiparticle magnetic moment is a pair-breaking effect that counteracts the weak paramagnetism of the ground-state condensate of excitons. This property could rationalize the anomalous magnitude of magnetic moments recently observed in different devices close to charge neutrality.
    Physical Review B 05/2014; 90(19). DOI:10.1103/PhysRevB.90.195415 · 3.74 Impact Factor
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    Pino D'Amico, Massimo Rontani
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    ABSTRACT: We study a few Fermi atoms interacting through attractive contact forces in a one-dimensional trap by means of numerical exact diagonalization. From the combined analysis of energies and wave functions of correlated ground and excited states we find evidence of BCS-like pairing even for very few atoms. For moderate interaction strength, we reproduce the even-odd oscillation of the separation energy observed in [G. Zuern, A. N. Wenz, S. Murmann, A. Bergschneider, T. Lompe, and S. Jochim, Phys. Rev. Lett. 111, 175302 (2013)]. For stronger interactions nonlinear, co-operative effects emerge, including non trivial spatial arrangement of atomic Cooper pairs in the trap.
    Physical Review A 04/2014; 91(4). DOI:10.1103/PhysRevA.91.043610 · 2.99 Impact Factor
  • Massimo Rontani
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    ABSTRACT: We show theoretically that an undoped carbon nanotube might be an excitonic insulator---the long-sought phase of matter proposed by Keldysh, Kohn and others fifty years ago. We predict that the condensation of triplet excitons, driven by intervalley exchange interaction, spontaneously occurs at equilibrium if the tube radius is sufficiently small. The signatures of exciton condensation are its sizeable contributions to both the energy gap and the magnetic moment per electron. The increase of the gap might have already been measured, albeit with a different explanation [Deshpande et al., Science 323, 106 (2009)]. The enhancement of the quasiparticle magnetic moment is a pair-breaking effect that counteracts the weak paramagnetism of the ground-state condensate of excitons. This property could rationalize the anomalous magnitude of magnetic moments recently observed in different devices close to charge neutrality.
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    Pino D'Amico, Massimo Rontani
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    ABSTRACT: We provide an accurate calculation of the energy spectrum of three atoms interacting through a contact force in a one-dimensional harmonic trap, considering both spinful fermions and spinless bosons. We use fermionic energies as a benchmark for exact-diagonalization technique (also known as full configuration interaction), which is found to slowly converge in the case of strong interatomic attraction.
    Journal of Physics B Atomic Molecular and Optical Physics 10/2013; 47(6). DOI:10.1088/0953-4075/47/6/065303 · 1.92 Impact Factor
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    Massimo Rontani
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    ABSTRACT: A simple theory for the tunneling of two cold atoms out of a trap in the presence of an attractive contact force is developed. Two competing decay channels, respectively for single-atom and bound-pair tunneling, contribute independently to the decay law of the mean atom number in the trap. The single-atom tunneling rate is obtained through the quasiparticle wave function formalism. For pair tunneling an effective equation for the center-of-mass motion is derived, so the calculation of the corresponding tunneling rate is again reduced to a simpler one-body problem. The predicted dependence of tunneling rates on the interaction strength qualitatively agrees with a recent measurement of the two-atom decay time [G. Zuern, A. N. Wenz, S. Murmann, T. Lompe, and S. Jochim, arXiv:1307.5153].
    Physical Review A 08/2013; 88(4). DOI:10.1103/PhysRevA.88.043633 · 2.99 Impact Factor
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    ABSTRACT: Two electrons on a string form a simple model system where Coulomb interactions are expected to play an interesting role. In the presence of strong interactions, these electrons are predicted to form a Wigner molecule, separating to the ends of the string. This spatial structure is believed to be clearly imprinted on the energy spectrum, yet so far a direct measurement of such a spectrum in a controllable one-dimensional setting is still missing. Here we use an ultraclean carbon nanotube to realize this system in a tunable potential. Using tunnelling spectroscopy we measure the addition spectra of two interacting carriers, electrons or holes, and identify seven low-energy states characterized by their exchange symmetries. The formation of a Wigner molecule is evident from a tenfold quenching of the fundamental excitation energy as compared with the non-interacting value. Our ability to tune the two-carrier state in space and to study it for both electrons and holes provides an unambiguous demonstration of this strongly interacting quantum ground state.
    Nature Physics 07/2013; 9(9):576-581. DOI:10.1038/NPHYS2692 · 20.60 Impact Factor
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    Andrea Secchi, Massimo Rontani
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    ABSTRACT: We develop a theory of inter-valley Coulomb scattering in semiconducting carbon-nanotube quantum dots, taking into account the effects of curvature and chirality. Starting from the effective-mass description of single-particle states, we study the two-electron system by fully including Coulomb interaction, spin-orbit coupling, and short-range disorder. We find that the energy level splittings associated with inter-valley scattering are nearly independent of the chiral angle and, while smaller than those due to spin-orbit interaction, large enough to be measurable.
    Physical Review B 06/2013; 88(12). DOI:10.1103/PhysRevB.88.125403 · 3.74 Impact Factor
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    Massimo Rontani, L. J. Sham
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    ABSTRACT: We review the topic of Bose-Einstein condensation of excitons in semiconductors, focusing on the signatures of the macroscopic order of the exciton condensate.
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    Dimitrios Toroz, Massimo Rontani, Stefano Corni
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    ABSTRACT: Scanning tunneling spectroscopy (STS) allows us to image single molecules decoupled from the supporting substrate. The obtained images are routinely interpreted as the square moduli of molecular orbitals, dressed by the mean-field electron-electron interaction. Here we demonstrate that the effect of electron correlation beyond the mean field qualitatively alters the uncorrelated STS images. Our evidence is based on the ab initio many-body calculation of STS images of planar molecules with metal centers. We find that many-body correlations alter significantly the image spectral weight close to the metal center of the molecules. This change is large enough to be accessed experimentally, surviving to molecule-substrate interactions.
    Physical Review Letters 01/2013; 110(1):018305. DOI:10.1103/PhysRevLett.110.018305 · 7.51 Impact Factor
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    Massimo Rontani
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    ABSTRACT: A theory for the tunneling of one atom out of a trap containing two interacting cold atoms is developed. The quasiparticle wave function, dressed by the interaction with the companion atom in the trap, replaces the noninteracting orbital at resonance in the tunneling matrix element. The computed decay time for two ^{6}Li atoms agrees with recent experimental results [G. Zürn, F. Serwane, T. Lompe, A. N. Wenz, M. G. Ries, J. E. Bohn, and S. Jochim, Phys. Rev. Lett. 108, 075303 (2012)], unveiling the "fermionization" of the wave function and a novel two-body effect.
    Physical Review Letters 03/2012; 108(11):115302. DOI:10.1103/PhysRevLett.108.115302 · 7.51 Impact Factor
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    ABSTRACT: We study by means of exact-diagonalization techniques the ground state of a few-fermion system with strong short-range repulsive interactions trapped by a harmonic potential in one spatial dimension. Even when the ground-state density profile displays at strong coupling very well pronounced Friedel oscillations with a `4k_F periodicity', the pair correlation function does not show any signature of Wigner-molecule-type correlations. For the sake of comparison, we present also numerical results for few-electron systems with Coulomb interactions, demonstrating that their ground state at strong coupling is, on the contrary, a Wigner molecule.
    Physical review. B, Condensed matter 02/2012; 86(7). DOI:10.1103/PhysRevB.86.075110 · 3.66 Impact Factor
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    ABSTRACT: We observe a low-lying sharp spin mode of three interacting electrons in an array of nanofabricated AlGaAs/GaAs quantum dots by means of resonant inelastic light scattering. The finding is enabled by a suppression of the inhomogeneous contribution to the excitation spectra obtained by reducing the number of optically probed quantum dots. Supported by configuration-interaction calculations we argue that the observed spin mode offers a direct probe of Stoner ferromagnetism in the simplest case of three interacting spin one-half fermions.
    Physical review. B, Condensed matter 09/2011; 85(3). DOI:10.1103/PhysRevB.85.033307 · 3.66 Impact Factor
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    Andrea Secchi, Massimo Rontani
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    ABSTRACT: We demonstrate that the profile of the space-resolved spectral function at finite temperature provides a signature of Wigner localization for electrons in quantum wires and semiconducting carbon nanotubes. Our numerical evidence is based on the exact diagonalization of the microscopic Hamiltonian of few particles interacting in gate-defined quantum dots. The minimal temperature required to suppress residual exchange effects in the spectral function image of (nanotubes) quantum wires lies in the (sub-) Kelvin range.
    Physical review. B, Condensed matter 09/2011; 85(12). DOI:10.1103/PhysRevB.85.121410 · 3.66 Impact Factor
  • Massimo Rontani, Dimitrios Toroz, Stefano Corni
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    ABSTRACT: Scanning tunnelling spectroscopy (STS) visualizes electron states in both extended systems and nano-objects, as quantum dots and molecules. Whereas bulk quantum states are insensitive to electron number fluctuations, an energy gap opens each time a new electron is injected by the STS tip into a sufficiently small system. This gap originates from the interaction of the next incoming electron with the others already present in the system. In this Coulomb blockade regime a fundamental question is whether the wave function of the "quasi-particle" added to the system -imaged by the STS tip- is sensitive to electron-electron interaction. Here we show that the STS images of single planar molecules with metal centres predicted by ab initio many-body calculations differ qualitatively from their uncorrelated counterparts. We find in the maps resolved at the Fermi energy that correlation significantly removes spectral weight from the metal atom, as well as the overall weight is remarkably reduced. This change may be measured and compared with STS images of molecules without the metal center, whose many-body and uncorrelated versions are alike.
  • Andrea Secchi, Massimo Rontani
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    ABSTRACT: The paradigm of few-electron complexes in quantum dots (QDs) relies on the idea that the lowest quantized levels are filled according to Pauli's exclusion principle. If Coulomb repulsion is sufficiently strong to overcome the kinetic energy cost of localization, a different scenario is predicted: a "Wigner" molecule (WM) forms, made of electrons frozen in space according to a geometrical pattern. Despite considerable experimental effort, evidence of the WM in semiconductor QDs has been elusive so far. Here we demonstrate theoretically that WMs occur in gate-defined QDs embedded in typical semiconducting carbon nanotubes (CNTs). The unambiguous signatures of the WM state must be searched in the scanning tunneling microscopy (STM) images of the electrons. Through exact diagonalisation (ED) calculations, we unveil the inherent features of the electron molecular states. We show that, like nuclei in a usual molecule, electrons have localized wave functions and hence negligible exchange interactions. ED results for single and double QDs provide a simple interpretation for transport experiments in ultraclean CNTs.
  • Massimo Rontani
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    ABSTRACT: Tunnelling and capacitance spectroscopies are able to image the wavefunctions of electrons in atom-like solid-state systems as they are shaped by an external magnetic field.
    Nature Material 03/2011; 10(3):173-5. DOI:10.1038/nmat2970 · 36.43 Impact Factor
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    Dimitrios Toroz, Massimo Rontani, Stefano Corni
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    ABSTRACT: Scanning tunneling microscopy (STM) has been a fundamental tool to characterize many-body effects in condensed matter systems, from extended solids to quantum dots. STM of molecules decoupled from the supporting conductive substrate has the potential to extend STM characterization of many-body effects to the molecular world as well. In this paper, we describe a many-body tunneling theory for molecules decoupled from the STM substrate, and we report on the use of standard quantum chemical methods to calculate the quantities necessary to provide the "correlated" STM molecular image. The developed approach has been applied to 18 different molecules to explore the effects of their chemical nature and of their substituents, as well as to verify the possible contribution by transition metal centers. Whereas the bulk of calculations has been performed with the configuration interaction method with single and double excitations (CISD), because of the computational cost some tests have been also performed with the more accurate coupled cluster with single and double excitations (CCSD) method to quantify the importance of the computational level on many-body STM images. We have found that correlation induces a remarkable squeezing of the images, and that correlated images are not derived from Hartree-Fock HOMO or LUMO alone, but include contributions from other orbitals as well. Although correlation effects are too small to be resolved by present STM experiments for the studied molecules, our results provide hints for seeking out other species with larger, and possibly experimentally detectable, correlation effects.
    The Journal of Chemical Physics 01/2011; 134(2):024104. DOI:10.1063/1.3520567 · 3.12 Impact Factor