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 spinorbit interaction on the surface properties. The best known manifestation of this effect is the momentumdependent splitting of the surface state energies (Rashba effect). Here we show that the same interaction also generates a spinpolarization of the bulk states when an electric current is driven through the bulk of the film. For a semiinfinite 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 materialspecific parameter that controls the strength of the interfacial spinorbit coupling. Our theorem, combined with an {\it ab initio} calculation of the spin polarization of the currentcarrying film, enables a determination of $\gamma$, which should be useful in modeling the spindependent scattering of quasiparticles at the interface.10/2014;  [Show abstract] [Hide abstract]
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 highlytunable lowloss 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 semiinfinite slabs (hBN/G/hBN). Graphene plasmons hybridize with hBN optical phonons forming hybrid plasmonphonon (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 plasmonphonon 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 anticorrelated.Physical Review B 10/2014; 90(16):165408. · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Graphene plasmons were predicted to possess ultrastrong field confinement and very low damping at the same time, enabling new classes of devices for deep subwavelength metamaterials, singlephoton nonlinearities, extraordinarily strong lightmatter interactions and nanooptoelectronic switches. While all of these great prospects require low damping, thus far strong plasmon damping was observed, with both impurity scattering and manybody 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 nearfield microscopy to image propagating plasmons in high quality graphene encapsulated between two films of hexagonal boron nitride (hBN). 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 hBN. The observation and indepth understanding of low plasmon damping is the key for the development of graphene nanophotonic and nanooptoelectronic devices.arXiv. 09/2014;  [Show abstract] [Hide abstract]
ABSTRACT: Spinorbit interactions in twodimensional 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 electronhole 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 spinorbit coupling from the band structure of the host material, but also the spinorbit coupling due to an external electric field and a random impurity potential. The driftdiffusion equations we derive in the diffusive regime are applicable to a broad variety of experimental situations, both homogeneous and nonhomogeneous, and include on equal footing "skew scattering" and "sidejump" from electronimpurity 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 spincurrent swapping effect, and the partial conversion of an electronhole density wave to a spin density wave in a twodimensional electron gas with Rashba and Dresselhaus spinorbit couplings, subject to an electric field.09/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the effects of inhomogeneities on spin entanglement in manyelectron systems from an abinitio 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 KohnSham 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;  [Show abstract] [Hide abstract]
ABSTRACT: Thermoelectric transport in nanoscale conductors is analyzed in terms of the response of the system to a thermomechanical 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 thermomechanical field effectively simulates a temperature gradient across the system and allows us to calculate the electric and thermal currents that flow due to the thermomechanical field. In particular, we show that, in the longtime limit, the currents thus calculated reduce to those that one obtains from the LandauerB{\"u}ttiker formula, suitably generalized to allow for different temperatures in the reservoirs, if the thermomechanical 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 thermomechanical 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; 
Dataset: PhysRevB.79.205305
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ABSTRACT: The WiedemannFranz law, connecting the electronic thermal conductivity to the electrical conductivity of a disordered metal, is generally found to be well satisfied even when electronelectron (ee) interactions are strong. In ultraclean conductors, however, large deviations from the standard form of the law are expected, due to the fact that ee interactions affect the two conductivities in radically different ways. Thus, the standard WiedemannFranz 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 ee 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 recordhigh 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 manybody renormalization of the thermal Drude weight coincides with that of the Fermi velocity.06/2014; 
Dataset: PHYSICAL REVIEW B 75, 125321 (2007)
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ABSTRACT: Collective chargedensity modes (plasmons) of the clean twodimensional unpolarized electron gas are stable, for momentum conservation prevents them from decaying into singleparticle excitations. Collective spindensity modes (spin plasmons) possess no similar protection and rapidly decay by production of electronhole 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 singleparticle spectrum) we find that a longlived spinplasmona collective mode in which the densities of up and down spins oscillate with opposite phasescan exist within a "pseudo gap" of the singleparticle 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 spinwavegenerating devices, such as spintorque oscillators.06/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We present a calculation of the quasiparticle decay rate due to electronelectron 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 parabolicband twodimensional (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;  [Show abstract] [Hide abstract]
ABSTRACT: A general approach is derived for constructing an effective spinorbit Hamiltonian for nonmagnetic materials, which is useful for calculating spindependent properties near an arbitrary point in momentum space with pseudospin degeneracy. The formalism is verified through comparisons with other approaches for IIIV semiconductors, and its general applicability is illustrated by deriving the spinorbit interaction and predicting spin lifetimes for strained SrTiO$_3$ and a twodimensional 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  [Show abstract] [Hide abstract]
ABSTRACT: We present a calculation of the quasiparticle decay rate due to electronelectron 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 parabolicband twodimensional (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;  [Show abstract] [Hide abstract]
ABSTRACT: A normal metallic film sandwiched between two insulators may have strong spinorbit coupling near the metalinsulator interfaces, even if spinorbit 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 spinorbit 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 inplane, transverse spin polarization. At variance with strictly twodimensional Rashba systems, we find that the spin Hall conductivity has a finite value even if spinorbit 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 spinorbit coupling on the third dimension, perpendicular to the film plane. The nonvanishing 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 twodimensional 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 spinorbit coupling is present and the electron motion is not strictly twodimensional.03/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We provide a precise microscopic definition of the recently observed "Inverse Edelstein Effect" (IEE), in which a nonequilibrium spin accumulation in the plane of a twodimensional (interfacial) electron gas drives an electric current perpendicular to its own direction. The driftdiffusion 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  [Show abstract] [Hide abstract]
ABSTRACT: The shear viscosity of a variety of strongly interacting quantum fluids, ranging from ultracold atomic Fermi gases to quarkgluon plasmas, can be accurately measured. On the contrary, no experimental data exist, to the best of our knowledge, on the shear viscosity of twodimensional quantum electron liquids hosted in a solidstate 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;  [Show abstract] [Hide abstract]
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 spinorbit momentum and depending on the momentum orientation. We have provided an intuitive understanding for this nonanalytic behavior in terms of the inter chirality subband electronic transitions, induced by the combined action of BychkovRashba (BR) and Dresselhaus (D) spinorbit coupling. It is shown that the regular behavior of the relaxation propagator is recovered in the presence of only one BR or D spinorbit field or for spinorbit 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 
Article: Inverse Edelstein Effect
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ABSTRACT: We provide a precise microscopic definition of the recently observed "Inverse Edelstein Effect" (IEE), in which a nonequilibrium spin accumulation in the plane of a twodimensional (interfacial) electron gas drives an electric current perpendicular to its own direction. The driftdiffusion equations that govern the effect are presented and applied to the interpretation of the experiments.11/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We show that an electric field parallel to the wave fronts of an electronhole 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 spincharge 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 
Article: Transverse and longitudinal gradients of the spin magnetization in SpinDensityFunctional Theory
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ABSTRACT: We derive the gradient expansion for the exchange energy of a spinpolarized electron gas by perturbing the uniformly spin polarized state and thus inducing a small noncollinearity that is slowly varying in space. We show that the exchangeenergy 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 nonvanishing spin polarization and becomes larger with increasing spin polarization. We argue that improved generalized gradient approximations of SpinDensityFunctional 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  
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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, IledeFrance, 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 NanoScience and NanoTechnology
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, EmiliaRomagna, 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, BadenWürttemberg, Germany 
Northwestern University
 Department of Physics and Astronomy
Evanston, IL, United States
