Publications (300)1013.05 Total impact

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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 electronhole continuum, its quality factor decreasing sharply. We find that the value of the critical wave vector is strongly spin polarizationdependent, in a way that has no analogue in ordinary twodimensional electron gases. The origin of this effect is in the coupling between the plasmon and the interband electronhole 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. 
Dataset: PhysRevLett 103 196601

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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.Nature Material 04/2015; 14(4):421425. DOI:10.1038/nmat4169 · 36.43 Impact Factor 
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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)].Physical Review B 02/2015; 91:205426. DOI:10.1103/PhysRevB.91.205426 · 3.66 Impact Factor 
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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)]. 
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ABSTRACT: We analyze the effect known as "spin current swapping" due to electronimpurity scattering in a twodimensional 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 spindependent driftdiffusion 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. 
Acta Physica Polonica Series a 02/2015; 127(2):454456. DOI:10.12693/APhysPolA.127.454 · 0.60 Impact Factor

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ABSTRACT: The experimental availability of ultrahighmobility samples of graphene opens the possibility to realize and study experimentally the "hydrodynamic" regime of the electron liquid. In this regime the rate of electronelectron collisions is extremely high and dominates over the electronimpurity and electronphonon scattering rates, which are therefore neglected. The system is brought to a local quasiequilibrium 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 electronelectron interactions. We show that, in spite of the linear lowenergy 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 NavierStokes equations. 
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ABSTRACT: We study the Kondo effect in threedimensional (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 longrange scalar disorder and Kondo effect. In the presence of disorderinduced longrange 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 nonFermi liquid behavior. Finally we compare the properties of the Kondo effect in 3D Dirac materials and 2D Dirac systems like graphene and topological insulators. 
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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.Physical Review B 10/2014; 91(3). DOI:10.1103/PhysRevB.91.035403 · 3.66 Impact Factor 
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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.Physical Review B 10/2014; 90(16):165408. DOI:10.1103/PhysRevB.90.165408 · 3.66 Impact Factor 
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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.Physical Review B 09/2014; DOI:10.1103/PhysRevB.90.245302 · 3.66 Impact Factor 
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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.Physical Review B 09/2014; 91(7). DOI:10.1103/PhysRevB.91.075109 · 3.66 Impact Factor 
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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.Physical Review B 07/2014; 90(11). DOI:10.1103/PhysRevB.90.115116 · 3.66 Impact Factor 
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. 
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.Physical Review B 06/2014; 90(15). DOI:10.1103/PhysRevB.90.155409 · 3.66 Impact Factor 
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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. 
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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). DOI:10.1103/PhysRevB.89.155402 · 3.66 Impact Factor
Publication Stats
5k  Citations  
1,013.05  Total Impact Points  
Top Journals
Institutions

1988–2015

University of Missouri
 Department of Physics and Astronomy
Columbia, Missouri, United States


2006–2014

Università Degli Studi Roma Tre
 Department of Mathematics and Physics
Roma, Latium, Italy 
Chonnam National University
 Department of Physics
Gwangju, Gwangju, South Korea


2011

Institute of physics china
Peping, Beijing, China


2004

Scuola Normale Superiore di Pisa
 Laboratory NEST: National Enterprise for NanoScience and NanoTechnology
Pisa, Tuscany, Italy


1998

North Dakota State University
 Department of Physics
Fargo, ND, United States


1995

University of California, Santa Barbara
 Kavli Institute for Theoretical Physics
Santa Barbara, CA, United States


1987–1989

University of Tennessee
Knoxville, Tennessee, United States


1985–1987

Max Planck Institute for Solid State Research
Stuttgart, BadenWürttemberg, Germany


1981–1985

Northwestern University
 Department of Physics and Astronomy
Evanston, IL, United States
