G. Vignale

Donostia International Physics Center, San Sebastián, Basque Country, Spain

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Publications (286)902.88 Total impact

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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.
    10/2014;
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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. · 3.66 Impact Factor
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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.
    arXiv. 09/2014;
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    K. 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.
    09/2014;
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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.
    09/2014;
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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.
    07/2014;
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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.
    06/2014;
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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.
    06/2014;
  • 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.
    04/2014;
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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). · 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.
    03/2014;
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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.
    03/2014;
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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. · 7.73 Impact Factor
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    Andrea Tomadin, Giovanni Vignale, Marco Polini
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    ABSTRACT: The shear viscosity of a variety of strongly interacting quantum fluids, ranging from ultracold atomic Fermi gases to quark-gluon plasmas, can be accurately measured. On the contrary, no experimental data exist, to the best of our knowledge, on the shear viscosity of two-dimensional quantum electron liquids hosted in a solid-state matrix. In this Letter we propose a Corbino disk device, which allows a determination of the viscosity of a quantum electron liquid from the dc potential difference that arises between the inner and the outer edge of the disk in response to an oscillating magnetic flux.
    01/2014;
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    ABSTRACT: The charge density relaxation propagator of a two dimensional electron system, which is the slope of the imaginary part of the polarization function, exhibits singularities for bosonic momenta having the order of the spin-orbit momentum and depending on the momentum orientation. We have provided an intuitive understanding for this non-analytic behavior in terms of the inter chirality subband electronic transitions, induced by the combined action of Bychkov-Rashba (BR) and Dresselhaus (D) spin-orbit coupling. It is shown that the regular behavior of the relaxation propagator is recovered in the presence of only one BR or D spin-orbit field or for spin-orbit interaction with equal BR and D coupling strengths. This creates a new possibility to influence carrier relaxation properties by means of an applied electric field.
    Physical Review B 11/2013; 88(19):195402. · 3.66 Impact Factor
  • 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.
    11/2013;
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    Ka Shen, G Vignale
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    ABSTRACT: We show that an electric field parallel to the wave fronts of an electron-hole grating in a GaAs quantum well generates, via the electronic spin Hall effect, a spin grating of the same wave vector and with an amplitude that can exceed 1% of the amplitude of the initial density grating. We refer to this phenomenon as the "collective spin Hall effect." A detailed study of the coupled spin-charge dynamics for quantum wells grown in different directions reveals rich features in the time evolution of the induced spin density, including the possibility of generating a helical spin grating.
    Physical Review Letters 09/2013; 111(13):136602. · 7.73 Impact Factor
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    F. G. Eich, S. Pittalis, G. Vignale
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    ABSTRACT: We derive the gradient expansion for the exchange energy of a spin-polarized electron gas by perturbing the uniformly spin polarized state and thus inducing a small non-collinearity that is slowly varying in space. We show that the exchange-energy contribution due to the induced longitudinal gradient of the spin polarization to the exchange energy differs from the contribution due to the transverse gradient. The difference is present at any non-vanishing spin polarization and becomes larger with increasing spin polarization. We argue that improved generalized gradient approximations of Spin-Density-Functional Theory must account for the difference between the longitudinal and transverse spin stiffness.
    Physical Review B 09/2013; · 3.66 Impact Factor

Publication Stats

4k Citations
902.88 Total Impact Points

Institutions

  • 2014
    • Donostia International Physics Center
      San Sebastián, Basque Country, Spain
    • Università Degli Studi Roma Tre
      • Department of Mathematics and Physics
      Roma, Latium, Italy
  • 1988–2014
    • University of Missouri
      • Department of Physics and Astronomy
      Columbia, Missouri, United States
  • 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
    • Pierre and Marie Curie University - Paris 6
      • Laboratoire de Chimie Théorique (LCT - UMR 7616)
      Paris, Ile-de-France, France
    • Universität Regensburg
      • Intitute of Theoretical Physics
      Ratisbon, Bavaria, Germany
    • Moscow Institute of Electronic Technology
      Moskva, Moscow, Russia
  • 2008–2009
    • Los Alamos National Laboratory
      • Theoretical Division
      Los Alamos, NM, United States
    • Yerevan State University
      Ayrivan, Yerevan, Armenia
    • Zhejiang Normal University
      Jinhua, Zhejiang Sheng, China
  • 1987–2009
    • University of Wuerzburg
      • • Department of Theoretical and Astrophysics
      • • Institute of Physics
      Würzburg, Bavaria, Germany
  • 2007
    • Universitätsklinikum Erlangen
      Erlangen, Bavaria, Germany
  • 2003–2007
    • Scuola Normale Superiore di Pisa
      • Laboratory NEST: National Enterprise for Nano-Science and Nano-Technology
      Pisa, Tuscany, Italy
  • 2005
    • University of California, San Diego
      • Department of Physics
      San Diego, CA, United States
  • 2004
    • Università degli Studi di Modena e Reggio Emilia
      Modène, Emilia-Romagna, Italy
    • Purdue University
      • Department of Physics
      West Lafayette, IN, United States
  • 1995–2001
    • University of California, Santa Barbara
      • Kavli Institute for Theoretical Physics
      Santa Barbara, California, United States
  • 1998
    • North Dakota State University
      • Department of Physics
      Fargo, ND, United States
  • 1987–1989
    • University of Tennessee
      Knoxville, Tennessee, United States
  • 1985–1987
    • Max Planck Institute for Solid State Research
      Stuttgart, Baden-Württemberg, Germany
    • Northwestern University
      • Department of Physics and Astronomy
      Evanston, IL, United States