arXiv subjects

Marco Polini

Publications and source records attributed to Marco Polini.

At least 109 records · Page 6Linked to original sources

arXiv2017

Ultra-confined acoustic THz graphene plasmons revealed by photocurrent nanoscopy

Terahertz (THz) fields are widely applied for sensing, communication and quality control. In future applications, they could be efficiently confined, enhanced and manipulated - well below the classical diffraction limit - through the excitation of graphene plasmons (GPs). These possibilities emerge from the strongly reduced GP wavelength, lp, compared to the photon wavelength, l0, which can be controlled by modulating the carrier density of graphene via electrical gating. Recently, GPs in a graphene-insulator-metal configuration have been predicted to exhibit a linear dispersion (thus called acoustic plasmons) and a further reduced wavelength, implying an improved field confinement, analogous to plasmons in two-dimensional electron gases (2DEGs) near conductive substrates. While infrared GPs have been visualised by scattering-type scanning near-field optical microscopy (s-SNOM), the real-space imaging of strongly confined THz plasmons in graphene and 2DEGs has been elusive so far - only GPs with nearly free-space wavelength have been observed. Here we demonstrate real-space imaging of acoustic THz plasmons in a graphene photodetector with split-gate architecture. To that end, we introduce nanoscale-resolved THz photocurrent near-field microscopy, where near-field excited GPs are detected thermoelectrically rather than optically. The on-chip GP detection simplifies GP imaging, as sophisticated s-SNOM detection schemes can be avoided. The photocurrent images reveal strongly reduced GP wavelengths (lp = l0/66), a linear dispersion resulting from the coupling of GPs with the metal gate below the graphene, and that plasmon damping at positive carrier densities is dominated by Coulomb impurity scattering. Acoustic GPs could thus strongly benefit the development of deep subwavelength-scale THz devices.

cond-mat.mes-hall↗

arXiv2016

Modulated phases of graphene quantum Hall polariton fluids

There is growing experimental interest in coupling cavity photons to the cyclotron resonance excitations of electron liquids in high-mobility semiconductor quantum wells or graphene sheets. These media offer unique platforms to carry out fundamental studies of exciton-polariton condensation and cavity quantum electrodynamics in a regime in which electron-electron interactions are expected to play a pivotal role. Focusing on graphene, we present a theoretical study of the impact of electron-electron interactions on a quantum Hall polariton fluid, that is a fluid of magneto-excitons resonantly coupled to cavity photons. We show that electron-electron interactions are responsible for an instability of graphene integer quantum Hall polariton fluids towards a modulated phase. We demonstrate that this phase can be detected by measuring the collective excitation spectra, which soften at a characteristic wave vector of the order of the inverse magnetic length.

cond-mat.mes-hall↗

arXiv2016

Theory of plasmonic effects in nonlinear optics: the case of graphene

We develop a microscopic large-$N$ theory of electron-electron interaction corrections to multi-legged Feynman diagrams describing second- and third-order nonlinear response functions. Our theory, which reduces to the well-known random phase approximation in the linear-response limit, is completely general and is useful to understand all second- and third-order nonlinear effects, including harmonic generation, wave mixing, and photon drag. We apply our theoretical framework to the case of graphene, by carrying out microscopic calculations of the second- and third-order nonlinear response functions of an interacting two-dimensional (2D) gas of massless Dirac fermions. We compare our results with recent measurements, where all-optical launching of graphene plasmons has been achieved by virtue of the finiteness of the quasi-homogeneous second-order nonlinear response of this inversion-symmetric 2D material.

cond-mat.mtrl-sci↗

arXiv2016

Electron hydrodynamics dilemma: whirlpools or no whirlpools

In highly viscous electron systems such as, for example, high quality graphene above liquid nitrogen temperature, a linear response to applied electric current becomes essentially nonlocal, which can give rise to a number of new and counterintuitive phenomena including negative nonlocal resistance and current whirlpools. It has also been shown that, although both effects originate from high electron viscosity, a negative voltage drop does not principally require current backflow. In this work, we study the role of geometry on viscous flow and show that confinement effects and relative positions of injector and collector contacts play a pivotal role in the occurrence of whirlpools. Certain geometries may exhibit backflow at arbitrarily small values of the electron viscosity, whereas others require a specific threshold value for whirlpools to emerge.

cond-mat.mes-hall↗

arXiv2016

Electronic Cooling via Interlayer Coulomb Coupling in Multilayer Epitaxial Graphene

In van der Waals bonded or rotationally disordered multilayer stacks of two-dimensional (2D) materials, the electronic states remain tightly confined within individual 2D layers. As a result, electron-phonon interactions occur primarily within layers and interlayer electrical conductivities are low. In addition, strong covalent in-plane intralayer bonding combined with weak van der Waals interlayer bonding results in weak phonon-mediated thermal coupling between the layers. We demonstrate here, however, that Coulomb interactions between electrons in different layers of multilayer epitaxial graphene provide an important mechanism for interlayer thermal transport even though all electronic states are strongly confined within individual 2D layers. This effect is manifested in the relaxation dynamics of hot carriers in ultrafast time-resolved terahertz spectroscopy. We develop a theory of interlayer Coulomb coupling containing no free parameters that accounts for the experimentally observed trends in hot-carrier dynamics as temperature and the number of layers is varied.

cond-mat.mes-hall↗

arXiv2016

Orbital and spin order in oxide two-dimensional electron gases

We describe a variational theory of multi-band two-dimensional electron gases that captures the interplay between electrostatic confining potentials, orbital-dependent interlayer electronic hopping and electron-electron interactions, and apply it to the d-band two-dimensional electron gases that form near perovskite oxide surfaces and heterojunctions. These multi-band two-dimensional electron gases are prone to the formation of Coulomb-interaction-driven orbitally-ordered nematic ground-states. We find that as the electron density is lowered and interaction effects strengthen, spontaneous orbital order occurs first, followed by spin order. We compare our results with known properties of single-component two-dimensional electron gas systems and comment on closely related physics in semiconductor quantum wells and van der Waals heterostructures.

cond-mat.str-el↗

arXiv2016

Super-Planckian electron cooling in a van der Waals stack

Radiative heat transfer (RHT) between macroscopic bodies at separations that are much smaller than the thermal wavelength is ruled by evanescent electromagnetic modes and can be orders of magnitude more efficient than its far-field counterpart, which is described by the Stefan-Boltzmann law. In this Letter we present a microscopic theory of RHT in van der Waals stacks comprising graphene and a natural hyperbolic material, i.e. hexagonal boron nitride (hBN). We demonstrate that RHT between hot carriers in graphene and hyperbolic phonon-polaritons in hBN is extremely efficient at room temperature, leading to picosecond time scales for the carrier cooling dynamics.

cond-mat.mes-hall↗

arXiv2016

Nonlocal topological valley transport at large valley Hall angles

Berry curvature hot spots in two-dimensional materials with broken inversion symmetry are responsible for the existence of transverse valley currents, which give rise to giant nonlocal dc voltages. Recent experiments in high-quality gapped graphene have highlighted a saturation of the nonlocal resistance as a function of the longitudinal charge resistivity $ρ_{{\rm c}, xx}$, when the system is driven deep into the insulating phase. The origin of this saturation is, to date, unclear. In this work we show that this behavior is fully compatible with bulk topological transport in the regime of large valley Hall angles (VHAs). We demonstrate that, for a fixed value of the valley diffusion length, the dependence of the nonlocal resistance on $ρ_{{\rm c}, xx}$ weakens for increasing VHAs, transitioning from the standard $ρ^3_{{\rm c}, xx}$ power-law to a result that is independent of $ρ_{{\rm c}, xx}$.

cond-mat.mes-hall↗

arXiv2016

Current-induced birefringent absorption and non-reciprocal plasmons in graphene

We present extensive calculations of the optical and plasmonic properties of a graphene sheet carrying a dc current. By calculating analytically the density-density response function of current-carrying states at finite temperature, we demonstrate that an applied dc current modifies the Pauli blocking mechanism and that absorption acquires a birefringent character with respect to the angle between the in-plane light polarization and current flow. Employing the random phase approximation at finite temperature, we show that graphene plasmons display a degree of non-reciprocity and collimation that can be tuned with the applied current. We discuss the possibility to measure these effects.

cond-mat.mes-hall↗

arXiv2016

Theory of third harmonic generation in graphene: a diagrammatic approach

We present a finite-temperature diagrammatic perturbation theory of third harmonic generation (THG) in doped graphene. We carry out calculations of the third-order conductivity in the scalar potential gauge, highlighting a subtle cancellation between a Fermi surface contribution, which contains only power laws, and power-law contributions of inter-band nature. Only logarithms survive in the final result. We conclude by presenting quantitative results for the up-conversion efficiency at zero and finite temperature. Our results shed light on the on-going dispute over the dependence of THG on carrier concentration in graphene.

cond-mat.mes-hall↗

arXiv2015

Resonant tunneling and the quasiparticle lifetime in graphene/boron nitride/graphene heterostructures

Tunneling of quasiparticles between two nearly-aligned graphene sheets produces resonant current-voltage characteristics because of the quasi-exact conservation of in-plane momentum. We claim that, in this regime, vertical transport in graphene/boron nitride/graphene heterostructures carries precious information on electron-electron interactions and the quasiparticle spectral function of the two-dimensional electron system in graphene. We present extensive microscopic calculations of the tunneling spectra with the inclusion of quasiparticle lifetime effects and elucidate the range of parameters (inter-layer bias, temperature, twist angle, and gate voltage) under which electron-electron interaction physics emerges.

cond-mat.mes-hall↗

arXiv2015

Helicons in Weyl semimetals

Helicons are transverse electromagnetic waves propagating in three-dimensional (3D) electron systems subject to a static magnetic field. We present a theory of helicons propagating through a 3D Weyl semimetal. Our approach relies on the evaluation of the optical conductivity tensor from semiclassical Boltzmann transport theory, with the inclusion of certain Berry curvature corrections that have been neglected in the earlier literature (such as the one due to the orbital magnetic moment). We demonstrate that the axion term characterizing the electromagnetic response of Weyl semimetals dramatically alters the helicon dispersion with respect to that in nontopological metals. We also discuss axion-related anomalies that appear in the plasmon dispersion relation.

cond-mat.mes-hall↗

arXiv2015

Quasiparticle Mass Enhancement and Fermi Surface Shape Modification in Oxide Two-Dimensional Electron Gases

We propose a model intended to qualitatively capture the electron-electron interaction physics of two-dimensional electron gases formed near transition-metal oxide heterojunctions containing $t_{2g}$ electrons with a density much smaller than one electron per metal atom. Two-dimensional electron systems of this type can be described perturbatively using a $GW$ approximation which predicts that Coulomb interactions enhance quasiparticle effective masses more strongly than in simple two-dimensional electron gases, and that they reshape the Fermi surface, reducing its anisotropy.

cond-mat.str-el↗

arXiv2015

Non-local transport and the hydrodynamic shear viscosity in graphene

Motivated by recent experimental progress in preparing encapsulated graphene sheets with ultra-high mobilities up to room temperature, we present a theoretical study of dc transport in doped graphene in the hydrodynamic regime. By using the continuity and Navier-Stokes equations, we demonstrate analytically that measurements of non-local resistances in multi-terminal Hall bar devices can be used to extract the hydrodynamic shear viscosity of the two-dimensional (2D) electron liquid in graphene. We also discuss how to probe the viscosity-dominated hydrodynamic transport regime by scanning probe potentiometry and magnetometry. Our approach enables measurements of the viscosity of any 2D electron liquid in the hydrodynamic transport regime.

cond-mat.mes-hall↗

arXiv2015

Bulk and shear viscosities of the 2D electron liquid in a doped graphene sheet

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" $τ_{\rm v}$, which we calculate in this Article. The transverse nature of the excitations contributing to $τ_{\rm v}$ 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 $τ_{\rm v} \sim 1/T^2$.

cond-mat.mes-hall↗

arXiv2015

Accessing phonon polaritons in hyperbolic crystals by ARPES

Recently studied hyperbolic materials host unique phonon-polariton (PP) modes. The ultra-short wavelengths of these modes, which can be much smaller than those of conventional exciton-polaritons, are of high interest for extreme sub-diffraction nanophotonics schemes. Polar hyperbolic materials such as hexagonal boron nitride can be used to realize strong long-range coupling between PP modes and extraneous charge degrees of freedom. The latter, in turn, can be used to control and probe PP modes. Of special interest is coupling between PP modes and plasmons in an adjacent graphene sheet, which opens the door to accessing PP modes by angle-resolved photoemission spectroscopy (ARPES). A rich structure in the graphene ARPES spectrum due to PP modes is predicted, providing a new probe of PP modes and their coupling to graphene plasmons.

cond-mat.mes-hall↗

arXiv2015

Transport and optical properties of an electron gas in a Sierpinski carpet

Recent progress in the design and fabrication of artificial two-dimensional (2D) materials paves the way for the experimental realization of electron systems moving on plane fractals. In this work, we present the results of computer simulations for the conductance and optical absorption spectrum of a 2D electron gas roaming on a Sierpinski carpet, i.e. a plane fractal with Hausdorff dimension intermediate between one and two. We find that the conductance is sensitive to the spatial location of the leads and that it displays fractal fluctuations whose dimension is compatible with the Hausdorff dimension of the sample. Very interestingly, electrons in this fractal display a broadband optical absorption spectrum, which possesses sharp "molecular" peaks at low photon energies.

cond-mat.mes-hall↗

arXiv2015

Persistent Current States in Bilayer Graphene

We argue that at finite carrier density and large displacement fields, bilayer graphene is prone to $\ell =0$ and $\ell = 1$ Pomeranchuk Fermi surface instabilities. The broken symmetries are driven by non-local exchange interactions which favor momentum space condensation. We find that electron-electron interactions lead first to spontaneous valley polarization, which breaks time-reversal invariance and is associated with spontaneous orbital magnetism, and then under some circumstances to a nematic phase with reduced rotational symmetry. When present, nematic order is signaled by reduced symmetry in the dependence of optical absorption on light polarization.

cond-mat.mes-hall↗