Giovanni Vignale

University of Missouri, Columbia, Missouri, United States

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Publications (314)1057.74 Total impact

  • Steven S. -L. Zhang · Giovanni Vignale
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    ABSTRACT: The anomalous Hall effect is deemed to be a unique transport property of ferromagnetic metals, caused by the concerted action of spin polarization and spin-orbit coupling. Nevertheless, recent experiments have shown that the effect also occurs in a nonmagnetic metal (Pt) in contact with a magnetic insulator (yttrium iron garnet (YIG)), even when precautions are taken to ensure there is no induced magnetization in the metal. We propose a theory of this effect based on the combined action of spin-dependent scattering from the magnetic interface and the spin Hall effect in the bulk of the metal. At variance with previous theories, we predict the effect to be of first order in the spin-orbit coupling, just as the conventional anomalous Hall effect -- the only difference being the spatial separation of the spin orbit interaction and the magnetization. For this reason we name this effect \textit{nonlocal anomalous Hall effect} and predict that its sign will be determined by the sign of the spin Hall angle in the metal. The AH conductivity that we calculate from our theory is in good agreement with the measured values in Pt/YIG structures.
    No preview · Article · Dec 2015
  • Jianmin Tao · Giovanni Vignale · I. V. Tokatly
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    ABSTRACT: This corrects the article DOI: 10.1103/PhysRevLett.100.206405.
    No preview · Article · Nov 2015 · Physical Review Letters
  • Matthew D. Mower · G. Vignale
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    ABSTRACT: We study spin relaxation in dilute magnetic semiconductors near a ferromagnetic transition, where spin fluctuations become strong. An enhancement in the scattering rate of itinerant carriers from the spin fluctuations of localized impurities leads to a change in the dominant spin relaxation mechanism from Dyakonov-Perel to spin flips in scattering. On the ferromagnetic side of the transition, we show that due to the presence of two magnetic components -- the itinerant carriers and the magnetic impurities -- with different gyromagnetic ratios, the relaxation rate of the total magnetization can be quite different from the relaxation rate of the spin. Following a disturbance of the equilibrium magnetization, the spin is initially redistributed between the two components to restore the equilibrium magnetization. It is only on a longer time scale, controlled by the spin-orbit interaction, that the total spin itself relaxes to its equilibrium state.
    No preview · Article · Sep 2015
  • Alessandro Principi · Giovanni 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 ultraclean conductors in the hydrodynamic regime, 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+τ/τ_{th}^{ee}, where 1/τ is the momentum relaxation rate and τ_{th}^{ee} is the relaxation time of the thermal current due to e-e collisions. Here we study the density and temperature dependence of 1/τ_{th}^{ee} of two-dimensional electron liquids. We show that at low temperature 1/τ_{th}^{ee} is 8/5 of the quasiparticle decay rate; remarkably, the same result is found in doped graphene and in conventional electron liquids in parabolic bands.
    No preview · Article · Jul 2015 · Physical Review Letters
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    V. U. Nazarov · G. Vignale
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    ABSTRACT: The localized Hartree-Fock potential has proven to be a computationally efficient alternative to the optimized effective potential, preserving the numerical accuracy of the latter and respecting the exact properties of being self-interaction free and having the correct −1/r asymptotics. In this paper we extend the localized Hartree-Fock potential to fractional particle numbers and observe that it yields derivative discontinuities in the energy as required by the exact theory. The discontinuities are numerically close to those of the computationally more demanding Hartree-Fock method. Our potential enjoys a “direct-energy” property, whereby the energy of the system is given by the sum of the single-particle eigenvalues multiplied by the corresponding occupation numbers. The discontinuities c ↑ and c ↓ of the spin-components of the potential at integer particle numbers N ↑ and N ↓ satisfy the condition c ↑ N ↑ + c ↓ N ↓ = 0. Thus, joining the family of effective potentials which support a derivative discontinuity, but being considerably easier to implement, the localized Hartree-Fock potential becomes a powerful tool in the broad area of applications in which the fundamental gap is an issue.
    Full-text · Article · Jul 2015 · The Journal of Chemical Physics
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    Giovanni Vignale · I. V. Tokatly
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    ABSTRACT: It is well known that a current driven through a two-dimensional electron gas with Rashba spin-orbit coupling induces a spin polarization in the perpendicular direction (Edelstein effect). This phenomenon has been extensively studied in the linear response regime, i.e., when the average drift velocity of the electrons is a small fraction of the Fermi velocity. Here we investigate the phenomenon in the nonlinear regime, meaning that the average drift velocity is comparable to, or exceeds the Fermi velocity. This regime is realized when the electric field is very large, or when electron-impurity scattering is very weak. The quantum kinetic equation for the density matrix of noninteracting electrons is exactly and analytically solvable, reducing to a problem of spin dynamics for "unpaired" electrons near the Fermi surface. The crucial parameter is $\gamma=eEL_s/E_F$, where $E$ is the electric field, $e$ is the absolute value of the electron charge, $E_F$ is the Fermi energy, and $L_s = \hbar/(2m\alpha)$ is the spin-precession length in the Rashba spin-orbit field with coupling strength $\alpha$. If $\gamma\ll1$ the evolution of the spin is adiabatic, resulting in a spin polarization that grows monotonically in time and eventually saturates at the maximum value $n(\alpha/v_F)$, where $n$ is the electron density and $v_F$ is the Fermi velocity. If $\gamma \gg 1$ the evolution of the spin becomes strongly non-adiabatic and the spin polarization is progressively reduced, and eventually suppressed for $\gamma\to \infty$. We also predict an inverse nonlinear Edelstein effect, in which an electric current is driven by a magnetic field that grows linearly in time. The "conductivities" for the direct and the inverse effect satisfy generalized Onsager reciprocity relations, which reduce to the standard ones in the linear response regime.
    Full-text · Article · Jun 2015
  • J. Borge · C. Gorini · G. Vignale · R. Raimondi

    No preview · Article · Jun 2015 · Physical Review B
  • Alessandro Principi · Giovanni Vignale · Matteo Carrega · Marco Polini
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    ABSTRACT: Hydrodynamic flow occurs in an electron liquid when the mean free path for electron-electron collisions is the shortest length scale in the problem. In this regime, transport is described by the Navier-Stokes equation, which contains two fundamental parameters, the bulk and shear viscosities. In this Article we present extensive results for these transport coefficients in the case of the two-dimensional massless Dirac fermion liquid in a doped graphene sheet. Our approach relies on microscopic calculations of the viscosities up to second order in the strength of electron-electron interactions and in the high-frequency limit, where perturbation theory is applicable. We then use simple interpolation formulae that allow to reach the low-frequency hydrodynamic regime where perturbation theory is no longer directly applicable. The key ingredient for the interpolation formulae is the "viscosity transport time" $\tau_{\rm v}$, which we calculate in this Article. The transverse nature of the excitations contributing to $\tau_{\rm v}$ leads to the suppression of scattering events with small momentum transfer, which are inherently longitudinal. Therefore, contrary to the quasiparticle lifetime, which goes as $-1/[T^2 \ln(T/T_{\rm F})]$, in the low temperature limit we find $\tau_{\rm v} \sim 1/T^2$.
    No preview · Article · Jun 2015
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    Amit Agarwal · Giovanni Vignale
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    ABSTRACT: We study the collective charge excitations (plasmons) in spin polarized graphene, and derive explicit expressions for their dispersion in the undamped regime. From this, we are able to calculate the critical wave vector beyond which the plasmon enters the electron-hole continuum, its quality factor decreasing sharply. We find that the value of the critical wave vector is strongly spin polarization-dependent, in a way that has no analogue in ordinary two-dimensional electron gases. The origin of this effect is in the coupling between the plasmon and the inter-band electron-hole pairs of the minority spin carriers. We show that the effect is robust with respect to the inclusion of disorder and we suggest that it can be exploited to experimentally determine the spin polarization of graphene.
    Full-text · Article · May 2015 · Physical Review B
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    Samvel Michael Badalyan · G. Vignale

    Full-text · Dataset · Apr 2015
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    Steven S. -L. Zhang · Giovanni Vignale · Shufeng Zhang
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    ABSTRACT: In a bilayer consisting of an insulator (I) and a ferromagnetic metal (FM), interfacial spin orbit scattering leads to spin mixing of the two conducting channels of the FM, which results in an unconventional anisotropic magnetoresistance (AMR). We theoretically investigate the magnetotransport in such bilayer structures by solving the spinor Boltzmann transport equation with generalized Fuchs-Sondheimer boundary condition that takes into account the effect of spin orbit scattering at the interface. We find that the new AMR exhibits a peculiar angular dependence which can serve as a genuine experimental signature. We also determine the dependence of the AMR on film thickness as well as spin polarization of the FM.
    Preview · Article · Apr 2015 · Physical Review B
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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.
    Full-text · Article · Apr 2015 · Nature Materials
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    Rashi Sachdeva · Anmol Thakur · Giovanni Vignale · Amit Agarwal
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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)].
    Full-text · Article · Feb 2015 · Physical Review B
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    Rashi Sachdeva · Anmol Thakur · Giovanni Vignale · Amit Agarwal
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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)].
    Full-text · Article · Feb 2015
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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.
    Full-text · Article · Feb 2015 · Physical Review B
  • R. Raimondi · Ka Shen · G. Vignale
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    ABSTRACT: We provide a heuristic derivation of the "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 derived and applied to the interpretation of recent experiments. A brief analysis based on the Kubo formula shows that the result is valid also outside the diffusive regime, i.e. when spin and momentum relaxation become comparable.
    No preview · Article · Feb 2015 · Acta Physica Polonica Series a
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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.
    Preview · Article · Jan 2015 · Physical Review B
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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.
    Preview · Article · Oct 2014 · Physical Review B
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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.
    Full-text · Article · Oct 2014 · Physical Review B
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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.
    Full-text · Article · Oct 2014 · Physical Review B

Publication Stats

6k Citations
1,057.74 Total Impact Points

Institutions

  • 1988-2015
    • University of Missouri
      • Department of Physics and Astronomy
      Columbia, Missouri, United States
  • 2014
    • Università Degli Studi Roma Tre
      • Department of Mathematics and Physics
      Roma, Latium, Italy
  • 2004
    • Scuola Normale Superiore di Pisa
      • Laboratory NEST: National Enterprise for Nano-Science and Nano-Technology
      Pisa, Tuscany, Italy
  • 2001
    • University of São Paulo
      San Paulo, São Paulo, Brazil
  • 1998
    • North Dakota State University
      • Department of Physics
      Fargo, ND, United States
  • 1988-1989
    • University of Tennessee
      Knoxville, Tennessee, United States
  • 1987-1988
    • The University of Tennessee Medical Center at Knoxville
      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