Publications (282)899.56 Total impact
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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 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.08/2014;  [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  [Show abstract] [Hide abstract]
ABSTRACT: We introduce a nonequilibrium DensityFunctional 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 energydensity operator. We identify the excessenergy density as the basic variable, in addition to the particle density. These densities are reproduced by an effective noninteracting KohnSham system. A novel KohnShamlike equation emerges featuring a spatially varying mass representing local temperature variations. The adiabatic contribution to the KohnSham 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; 
Article: Intrinsic spin Hall effect at asymmetric oxide interfaces: Role of transverse wave functions
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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 spinorbit coupling of the Rashba type, i.e., linear in the inplane 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 spinorbit coupling on opposite sides of the interface. Besides splitting each subband into two branches of opposite chirality, the spinorbit 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 inplane 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 spinorbit 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).  [Show abstract] [Hide abstract]
ABSTRACT: Recent scatteringtype scanning nearfield optical spectroscopy (sSNOM) experiments on singlelayer 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 currentcurrent 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 sSNOM experimental results.Physical Review B 07/2013; 88(12). · 3.66 Impact Factor 
Article: Unified Boltzmanntransport theory for the drag resistivity close to a secondorder phase transition
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ABSTRACT: We present a unified Boltzmanntransport theory for the drag resistivity in twocomponent systems close to a secondorder 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 holelike 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) threedimensional cold atomic Fermi gases close to a Stoner transition and ii) twodimensional spatiallyseparated electron and hole systems in semiconductor double quantum wells.06/2013;
Publication Stats
4k  Citations  
899.56  Total Impact Points  
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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, 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
