Giovanni Vignale

University of Missouri, Columbia, Missouri, United States

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Publications (294)1001.25 Total impact

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    ABSTRACT: Graphene plasmons were predicted to possess ultra-strong field confinement and very low damping at the same time, enabling new classes of devices for deep subwavelength metamaterials, single-photon nonlinearities, extraordinarily strong light-matter interactions and nano-optoelectronic switches. While all of these great prospects require low damping, thus far strong plasmon damping was observed, with both impurity scattering and many-body effects in graphene proposed as possible explanations. With the advent of van der Waals heterostructures, new methods have been developed to integrate graphene with other atomically flat materials. In this letter we exploit near-field microscopy to image propagating plasmons in high quality graphene encapsulated between two films of hexagonal boron nitride (h-BN). We determine dispersion and particularly plasmon damping in real space. We find unprecedented low plasmon damping combined with strong field confinement, and identify the main damping channels as intrinsic thermal phonons in the graphene and dielectric losses in the h-BN. The observation and in-depth understanding of low plasmon damping is the key for the development of graphene nano-photonic and nano-optoelectronic devices.
    Nature Material 04/2015; 14(4):421-425. DOI:10.1038/nmat4169 · 36.43 Impact Factor
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    ABSTRACT: We explore the collective density oscillations of a collection of charged massive Dirac particles, in one, two and three dimensions and their one dimensional superlattice. We calculate the long wavelength limit of the dynamical polarization function analytically, and use the random phase approximation to obtain the plasmon dispersion. The density dependence of the long wavelength plasmon frequency in massive Dirac systems is found to be different as compared to systems with parabolic, and gapless Dirac dispersion. We also calculate the long wavelength plasmon dispersion of a 1d metamaterial made from 1d and 2d massive Dirac plasma. Our analytical results will be useful for exploring the use of massive Dirac materials as electrostatically tunable plasmonic metamaterials and can be experimentally verified by infrared spectroscopy as in the case of graphene [Nat. Nanotechnol. 6, 630 (2011)].
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    ABSTRACT: We explore the collective density oscillations of a collection of charged massive Dirac particles, in one, two and three dimensions and their one dimensional superlattice. We calculate the long wavelength limit of the dynamical polarization function analytically, and use the random phase approximation to obtain the plasmon dispersion. The density dependence of the long wavelength plasmon frequency in massive Dirac systems is found to be different as compared to systems with parabolic, and gapless Dirac dispersion. We also calculate the long wavelength plasmon dispersion of a 1d metamaterial made from 1d and 2d massive Dirac plasma. Our analytical results will be useful for exploring the use of massive Dirac materials as electrostatically tunable plasmonic metamaterials and can be experimentally verified by infrared spectroscopy as in the case of graphene [Nat. Nanotechnol. 6, 630 (2011)].
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    Ka Shen, R. Raimondi, G. Vignale
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    ABSTRACT: We analyze the effect known as "spin current swapping" due to electron-impurity scattering in a two-dimensional electron gas. In this effect a primary spin current $J_i^a$ (lower index for spatial direction, upper index for spin direction) generates a secondary spin current $J_a^i$ if $i \neq a$, or $J_j^j$ with $j\ne i$ if $i= a$. By employing microscopic diagrammatic calculations, as well as spin-dependent drift-diffusion equations, we show that, contrary to naive expectation, the homogeneous spin current associated with the uniform drift of the spin polarization in the presence of an electric field does not act a source of spin current swapping. On the other hand, the inhomogeneous spin current associated with spin diffusion is a legitimate source of spin current swapping and does generate a transverse spin current. An experimental setup for the observation of the effect is therefore proposed.
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    Alessandro Principi, Giovanni Vignale
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    ABSTRACT: The experimental availability of ultra-high-mobility samples of graphene opens the possibility to realize and study experimentally the "hydrodynamic" regime of the electron liquid. In this regime the rate of electron-electron collisions is extremely high and dominates over the electron-impurity and electron-phonon scattering rates, which are therefore neglected. The system is brought to a local quasi-equilibrium described by a set of smoothly varying (in space and time) functions, {\it i.e.} the density, the velocity field and the local temperature. In this paper we calculate the charge and spin conductivities of doped graphene due solely to electron-electron interactions. We show that, in spite of the linear low-energy band dispersion, graphene behaves in a wide range of temperatures as an effectively Galilean invariant system: the charge conductivity diverges in the limit $T \to 0$, while the spin conductivity remains finite. These results pave the way to the description of charge transport in graphene in terms of Navier-Stokes equations.
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    Alessandro Principi, Giovanni Vignale, Enrico Rossi
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    ABSTRACT: We study the Kondo effect in three-dimensional (3D) Dirac materials and Weyl semimetals. We find the scaling of the Kondo temperature with respect to the doping $n$ and the coupling $J$ between the moment of the magnetic impurity and the carriers of the semimetal. We find that when the temperature is much smaller than the Kondo temperature the resistivity due to the Kondo effect scales as the $n^{-4/3}$.We also study the effect of the interplay of long-range scalar disorder and Kondo effect. In the presence of disorder-induced long-range carrier density inhomogeneities the Kondo effect is not characterized by a Kondo temperature but by a distribution of Kondo temperatures. We obtain the expression of such distribution and show that its features cause the appearance of strong non-Fermi liquid behavior. Finally we compare the properties of the Kondo effect in 3D Dirac materials and 2D Dirac systems like graphene and topological insulators.
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    I. V. Tokatly, E. E. Krasovskii, Giovanni Vignale
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    ABSTRACT: The broken inversion symmetry at the surface of a metallic film (or, more generally, at the interface between a metallic film and a different metallic or insulating material) greatly amplifies the influence of the spin-orbit interaction on the surface properties. The best known manifestation of this effect is the momentum-dependent splitting of the surface state energies (Rashba effect). Here we show that the same interaction also generates a spin-polarization of the bulk states when an electric current is driven through the bulk of the film. For a semi-infinite jellium model, which is representative of metals with a closed Fermi surface, we prove as a theorem that, regardless of the shape of the confinement potential, the induced surface spin density at each surface is given by ${\bf S} =-\gamma \hbar {\bf \hat z}\times {\bf j}$, where ${\bf j}$ is the particle current density in the bulk, ${\bf \hat z}$ the unit vector normal to the surface, and $\gamma=\frac{\hbar}{4mc^2}$ contains only fundamental constants. For a general metallic solid $\gamma$ becomes a material-specific parameter that controls the strength of the interfacial spin-orbit coupling. Our theorem, combined with an {\it ab initio} calculation of the spin polarization of the current-carrying film, enables a determination of $\gamma$, which should be useful in modeling the spin-dependent scattering of quasiparticles at the interface.
    Physical Review B 10/2014; 91(3). DOI:10.1103/PhysRevB.91.035403 · 3.66 Impact Factor
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    ABSTRACT: Graphene sheets encapsulated between hexagonal Boron Nitride (hBN) slabs display superb electronic properties due to very limited scattering from extrinsic disorder sources such as Coulomb impurities and corrugations. Such samples are therefore expected to be ideal platforms for highly-tunable low-loss plasmonics in a wide spectral range. In this Article we present a theory of collective electron density oscillations in a graphene sheet encapsulated between two hBN semi-infinite slabs (hBN/G/hBN). Graphene plasmons hybridize with hBN optical phonons forming hybrid plasmon-phonon (HPP) modes. We focus on scattering of these modes against graphene's acoustic phonons and hBN optical phonons, two sources of scattering that are expected to play a key role in hBN/G/hBN stacks. We find that at room temperature the scattering against graphene's acoustic phonons is the dominant limiting factor for hBN/G/hBN stacks, yielding theoretical inverse damping ratios of hybrid plasmon-phonon modes of the order of $50$-$60$, with a weak dependence on carrier density and a strong dependence on illumination frequency. We confirm that the plasmon lifetime is not directly correlated with the mobility: in fact, it can be anti-correlated.
    Physical Review B 10/2014; 90(16):165408. DOI:10.1103/PhysRevB.90.165408 · 3.66 Impact Factor
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    Ka Shen, R. Raimondi, G. Vignale
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    ABSTRACT: Spin-orbit interactions in two-dimensional electron liquids are responsible for many interesting transport phenomena in which particle currents are converted to spin polarizations and spin currents and viceversa. Prime examples are the spin Hall effect, the Edelstein effect, and their inverses. By similar mechanisms it is also possible to partially convert an optically induced electron-hole density wave to a spin density wave and viceversa. In this paper we present a unified theoretical treatment of these effects based on quantum kinetic equations that include not only the intrinsic spin-orbit coupling from the band structure of the host material, but also the spin-orbit coupling due to an external electric field and a random impurity potential. The drift-diffusion equations we derive in the diffusive regime are applicable to a broad variety of experimental situations, both homogeneous and non-homogeneous, and include on equal footing "skew scattering" and "side-jump" from electron-impurity collisions. As a demonstration of the strength and usefulness of the theory we apply it to the study of several effects of current experimental interest: the inverse Edelstein effect, the spin-current swapping effect, and the partial conversion of an electron-hole density wave to a spin density wave in a two-dimensional electron gas with Rashba and Dresselhaus spin-orbit couplings, subject to an electric field.
    Physical review. B, Condensed matter 09/2014; DOI:10.1103/PhysRevB.90.245302 · 3.66 Impact Factor
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    ABSTRACT: We investigate the effects of inhomogeneities on spin entanglement in many-electron systems from an ab-initio approach. The key quantity in our approach is the local spin entanglement length, which is derived from the local concurrence of the electronic system. Although the concurrence for an interacting systems is a highly nonlocal functional of the density, it does have a simple, albeit approximate expression in terms of Kohn-Sham orbitals. We show that the electron localization function -- well known in quantum chemistry as a descriptor of atomic shells and molecular bonds -- can be reinterpreted in terms of the ratio of the local entanglement length of the inhomogeneous system to the entanglement length of a homogenous system at the same density. We find that the spin entanglement is remarkably enhanced in atomic shells and molecular bonds.
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    ABSTRACT: Thermoelectric transport in nanoscale conductors is analyzed in terms of the response of the system to a thermo-mechanical field, first introduced by Luttinger, which couples to the electronic energy density. While in this approach the temperature remains spatially uniform, we show that a spatially varying thermo-mechanical field effectively simulates a temperature gradient across the system and allows us to calculate the electric and thermal currents that flow due to the thermo-mechanical field. In particular, we show that, in the long-time limit, the currents thus calculated reduce to those that one obtains from the Landauer-B{\"u}ttiker formula, suitably generalized to allow for different temperatures in the reservoirs, if the thermo-mechanical field is applied to prepare the system, and subsequently turned off at ${t=0}$. Alternately, we can drive the system out of equilibrium by switching the thermo-mechanical field after the initial preparation. We compare these two scenarios, employing a model noninteracting Hamiltonian, in the linear regime, in which they coincide, and in the nonlinear regime in which they show marked differences. We also show how an operationally defined local effective temperature can be computed within this formalism.
    Physical Review B 07/2014; 90(11). DOI:10.1103/PhysRevB.90.115116 · 3.66 Impact Factor
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    A. Principi, G. Vignale
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    ABSTRACT: The Wiedemann-Franz law, connecting the electronic thermal conductivity to the electrical conductivity of a disordered metal, is generally found to be well satisfied even when electron-electron (e-e) interactions are strong. In ultra-clean conductors, however, large deviations from the standard form of the law are expected, due to the fact that e-e interactions affect the two conductivities in radically different ways. Thus, the standard Wiedemann-Franz ratio between the thermal and the electric conductivity is reduced by a factor $1+\tau/\tau_{\rm th}^{\rm ee}$, where $1/\tau$ is the momentum relaxation rate, and $1/\tau_{\rm th}^{\rm ee}$ is the relaxation time of the thermal current due to e-e collisions. Here we study the density and temperature dependence of $1/\tau_{\rm th}^{\rm ee}$ in the important case of doped, clean single layers of graphene, which exhibit record-high thermal conductivities. We show that at low temperature $1/\tau_{\rm th}^{\rm ee}$ is $8/5$ of the quasiparticle decay rate. We also show that the many-body renormalization of the thermal Drude weight coincides with that of the Fermi velocity.
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    ABSTRACT: Collective charge-density modes (plasmons) of the clean two-dimensional unpolarized electron gas are stable, for momentum conservation prevents them from decaying into single-particle excitations. Collective spin-density modes (spin plasmons) possess no similar protection and rapidly decay by production of electron-hole pairs. Nevertheless, if the electron gas has a sufficiently high degree of spin polarization ($P>1/7$, where $P$ is the ratio of the equilibrium spin density and the total electron density, for a parabolic single-particle spectrum) we find that a long-lived spin-plasmon---a collective mode in which the densities of up and down spins oscillate with opposite phases---can exist within a "pseudo gap" of the single-particle excitation spectrum. The ensuing collectivization of the spin excitation spectrum is quite remarkable and should be directly visible in Raman scattering experiments. The predicted mode could dramatically improve the efficiency of coupling between spin-wave-generating devices, such as spin-torque oscillators.
    Physical Review B 06/2014; 90(15). DOI:10.1103/PhysRevB.90.155409 · 3.66 Impact Factor
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    Marco Polini, Giovanni Vignale
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    ABSTRACT: We present a calculation of the quasiparticle decay rate due to electron-electron interactions in a doped graphene sheet. In particular, we emphasize subtle differences between the perturbative calculation of this quantity in a doped graphene sheet and the corresponding one in ordinary parabolic-band two-dimensional (2D) electron liquids. In the random phase approximation, dynamical overscreening near the light cone yields a universal quasiparticle lifetime, which is independent of the dielectric environment surrounding the 2D massless Dirac fermion fluid.
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    Cüneyt Şahin, Giovanni Vignale, Michael E. Flatté
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    ABSTRACT: A general approach is derived for constructing an effective spin-orbit Hamiltonian for nonmagnetic materials, which is useful for calculating spin-dependent properties near an arbitrary point in momentum space with pseudospin degeneracy. The formalism is verified through comparisons with other approaches for III-V semiconductors, and its general applicability is illustrated by deriving the spin-orbit interaction and predicting spin lifetimes for strained SrTiO$_3$ and a two-dimensional electron gas in SrTiO$_3$ (such as at the LaAlO$_3$/SrTiO$_3$ interface). These results suggest robust spin coherence and spin transport properties in SrTiO$_3$-based materials at room temperature.
    Physical Review B 04/2014; 89(155402). DOI:10.1103/PhysRevB.89.155402 · 3.66 Impact Factor
  • Marco Polini, Giovanni Vignale
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    ABSTRACT: We present a calculation of the quasiparticle decay rate due to electron-electron interactions in a doped graphene sheet. In particular, we emphasize subtle differences between the perturbative calculation of this quantity in a doped graphene sheet and the corresponding one in ordinary parabolic-band two-dimensional (2D) electron liquids. In the random phase approximation, dynamical overscreening near the light cone yields a universal quasiparticle lifetime, which is independent of the dielectric environment surrounding the 2D massless Dirac fermion fluid.
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    ABSTRACT: A normal metallic film sandwiched between two insulators may have strong spin-orbit coupling near the metal-insulator interfaces, even if spin-orbit coupling is negligible in the bulk of the film. In this paper we study two technologically important and deeply interconnected effects that arise from interfacial spin-orbit coupling in metallic films. The first is the spin Hall effect, whereby a charge current in the plane of the film is partially converted into an orthogonal spin current in the same plane. The second is the Edelstein effect, in which a charge current produces an in-plane, transverse spin polarization. At variance with strictly two-dimensional Rashba systems, we find that the spin Hall conductivity has a finite value even if spin-orbit interaction with impurities is neglected and "vertex corrections" are properly taken into account. Even more remarkably, such finite value becomes "universal" in a certain configuration. This is a direct consequence of the spatial dependence of spin-orbit coupling on the third dimension, perpendicular to the film plane. The non-vanishing spin Hall conductivity has a profound influence on the Edelstein effect, which we show to consist of two terms, the first with the standard form valid in a strictly two-dimensional Rashba system, and a second arising from the presence of the third dimension. Whereas the standard term is proportional to the momentum relaxation time, the new one scales with the spin relaxation time. Our results, although derived in a specific model, should be valid rather generally, whenever a spatially dependent Rashba spin-orbit coupling is present and the electron motion is not strictly two-dimensional.
    Physical Review B 03/2014; 89(24). DOI:10.1103/PhysRevB.89.245443 · 3.66 Impact Factor
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    Ka Shen, G. Vignale, R. Raimondi
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    ABSTRACT: We provide a precise microscopic definition of the recently observed "Inverse Edelstein Effect" (IEE), in which a non-equilibrium spin accumulation in the plane of a two-dimensional (interfacial) electron gas drives an electric current perpendicular to its own direction. The drift-diffusion equations that govern the effect are presented and applied to the interpretation of the experiments.
    Physical Review Letters 03/2014; 112:096601. DOI:10.1103/PhysRevLett.112.096601 · 7.73 Impact Factor

Publication Stats

5k Citations
1,001.25 Total Impact Points

Institutions

  • 1988–2015
    • University of Missouri
      • Department of Physics and Astronomy
      Columbia, Missouri, United States
  • 2003–2014
    • Università Degli Studi Roma Tre
      • Department of Mathematics and Physics
      Roma, Latium, Italy
  • 2010–2012
    • Universiteit Utrecht
      • Institute for Theoretical Physics
      Utrecht, Provincie Utrecht, Netherlands
  • 2011
    • Institute of physics china
      Peping, Beijing, China
  • 2009–2011
    • Academia Sinica
      • Research Center for Applied Sciences
      Taipei, Taipei, Taiwan
    • Moscow Institute of Electronic Technology
      Moskva, Moscow, Russia
    • Universität Regensburg
      • Intitute of Theoretical Physics
      Ratisbon, Bavaria, Germany
    • Pierre and Marie Curie University - Paris 6
      • Laboratoire de Chimie Théorique (LCT - UMR 7616)
      Paris, Ile-de-France, France
    • University of Wuerzburg
      • Department of Theoretical and Astrophysics
      Würzburg, Bavaria, Germany
  • 2008–2009
    • Los Alamos National Laboratory
      • Theoretical Division
      Los Alamos, NM, United States
    • Yerevan State University
      Ayrivan, Yerevan, Armenia
    • Zhejiang Normal University
      • Department of Physics
      Jinhua, Zhejiang Sheng, China
  • 2007
    • Università degli Studi di Trieste
      Trst, Friuli Venezia Giulia, Italy
    • Universidad del País Vasco / Euskal Herriko Unibertsitatea
      Leioa, Basque Country, Spain
  • 1997–2007
    • Scuola Normale Superiore di Pisa
      • Laboratory NEST: National Enterprise for Nano-Science and Nano-Technology
      Pisa, Tuscany, Italy
  • 2006
    • Chonnam National University
      • Department of Physics
      Gwangju, Gwangju, South Korea
  • 2004
    • Peking University
      • Department of Physics
      Peping, Beijing, China
  • 2001
    • University of São Paulo
      • Instituto de Química de São Carlos (IQSC)
      São Paulo, Estado de Sao Paulo, Brazil
  • 1995–2001
    • University of California, Santa Barbara
      • • Department of Physics
      • • Kavli Institute for Theoretical Physics
      Santa Barbara, California, United States
  • 1998
    • North Dakota State University
      • Department of Physics
      Fargo, ND, United States
  • 1992
    • Oak Ridge National Laboratory
      • Solid State Division
      Oak Ridge, Florida, United States
  • 1987–1989
    • University of Tennessee
      Knoxville, Tennessee, United States
  • 1985–1987
    • Max Planck Institute for Solid State Research
      Stuttgart, Baden-Württemberg, Germany
  • 1981–1985
    • Northwestern University
      • Department of Physics and Astronomy
      Evanston, IL, United States