G. Vignale

Università Degli Studi Roma Tre, Roma, Latium, Italy

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Publications (281)910.46 Total impact

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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). · 3.77 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. · 7.94 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. · 3.77 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. · 7.94 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; · 3.77 Impact Factor
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    ABSTRACT: We introduce a non-equilibrium Density-Functional Theory of local temperature and associated local energy density that is suited for the study of thermoelectric phenomena. The theory rests on a local temperature field coupled to the energy-density operator. We identify the excess-energy density as the basic variable, in addition to the particle density. These densities are reproduced by an effective non-interacting Kohn-Sham system. A novel Kohn-Sham-like equation emerges featuring a spatially varying mass representing local temperature variations. The adiabatic contribution to the Kohn-Sham potentials is related to entropy, viewed as a functional of the particle and energy density. Dissipation can be taken into account by employing linear response theory and the thermoelectric transport coefficients of the electron gas.
    08/2013;
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    ABSTRACT: An asymmetric triangular potential well provides the simplest model for the confinement of mobile electrons at the interface between two insulating oxides, such as LaAlO3 and SrTiO3 (LAO/STO). These electrons have been recently shown to exhibit a large spin-orbit coupling of the Rashba type, i.e., linear in the in-plane momentum. In this paper we study the intrinsic spin Hall effect due to Rashba coupling in an asymmetric triangular potential well. This is the minimal model that captures the asymmetry of the spin-orbit coupling on opposite sides of the interface. Besides splitting each subband into two branches of opposite chirality, the spin-orbit interaction causes the transverse wave function (i.e., the wave function in the z direction, perpendicular to the plane of the quantum well) to depend on the in-plane wave vector k. At variance with the standard Rashba model, the triangular well supports a nonvanishing intrinsic spin Hall conductivity, which is proportional to the square of the spin-orbit coupling constant and, in the limit of low carrier density, depends only on the effective mass renormalization associated with the k dependence of the transverse wave functions. The origin of the effects lies in the nonvanishing matrix elements of the spin current between subbands corresponding to different states of quantized motion perpendicular to the plane of the well.
    Physical Review B. 08/2013; 88(7).
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    ABSTRACT: Recent scattering-type scanning near-field optical spectroscopy (s-SNOM) experiments on single-layer graphene have reported Dirac plasmon lifetimes that are substantially shorter than the dc transport scattering time \tau_{tr}. We highlight that the plasmon lifetime is fundamentally different from \tau_{tr} since it is controlled by the imaginary part of the current-current linear response function at finite momentum and frequency. We first present the minimal theory of the extrinsic lifetime of Dirac plasmons due to scattering against impurities. We then show that a very reasonable concentration of charged impurities yields a plasmon damping rate which is in good agreement with s-SNOM experimental results.
    Physical Review B 07/2013; 88(12). · 3.77 Impact Factor
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    ABSTRACT: We present a unified Boltzmann-transport theory for the drag resistivity in two-component systems close to a second-order phase transition. We find general expressions for the drag resistivity in two and three spatial dimensions, for arbitrary population and mass imbalance, for particle- and hole-like bands, and show how to incorporate, at the Gaussian level, the effect of fluctuations close to a phase transition. We find that the proximity to the phase transition enhances the drag resistivity upon approaching the critical temperature from above, and we qualitatively derive the temperature dependence of this enhancement for various cases. In addition, we present numerical results for two concrete experimental systems: i) three-dimensional cold atomic Fermi gases close to a Stoner transition and ii) two-dimensional spatially-separated electron and hole systems in semiconductor double quantum wells.
    06/2013;
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    John F. Dobson, Tim Gould, Giovanni Vignale
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    ABSTRACT: Undoped graphene sheets at low temperatures have previously been predicted, via Random Phase Approximation (RPA) calculations, to exhibit unusual van der Waals interactions. Here we investigate the modifications to these interactions resulting from the inclusion of terms beyond the RPA in the density response of a graphene sheet. For asymptotically large separations, $D\rightarrow\infty$, the interaction is found to be substantially reduced from the RPA prediction. Its qualitative behavior depends strongly on the form of the many-body enhancement of the velocity of the massless Dirac fermions and may provide independent confirmation of the latter.
    06/2013;

Publication Stats

4k Citations
910.46 Total Impact Points

Institutions

  • 2014
    • 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