105 research outputs found
Hydrodynamics and plasmonics in two-dimensional materials
This Thesis is devoted to the study of two different aspects of electron behavior in two-dimensional
materials, namely hydrodynamic electron transport and plasmon propagation. The Thesis is structured
as follows. In Chapter 1 the main experimental facts that motivated our work on electron hydrodynamics
and plasmonics are presented and critically discussed. Chapter 2 contains our main results on
hydrodynamic electron transport. After deriving the basic equations of the electron hydrodynamics and
discussing their limit of applicability, we use them to quantify the impact of two different transport
coffcients, the shear and Hall viscosities of the electron liquid, on steady-state transport. Our results
are used to propose experimental protocols that allow an experimental determination of these transport
coeffcients. Chapter 3 deals with plasmon propagation through inhomogeneous media. We consider
three dfferent geometries: an interface between two dfferent materials, a one dimensional perturbation,
and a zero dimensional perturbation in an otherwise uniform electron system. We calculate scattering
observables for plasmons in these geometries. For the interface geometry we also investigated the presence
of plasmonic bound states localized near the interface, while for the second and third geometries we
quantify the impact of non-local fects. Chapter 4 presents a theory of chiral plasmons in materials with
a non-trivial Berry curvature in the electronic band structure. We firstly employ the results of Chapter 3
to obtain a semi-classical theory of Chiral Berry Plasmons (CBPs) at a generic interface between two
materials having different Berry uxes across the Fermi surface. We then test the impact of different
types of screened electron-electron interaction, and of a finite damping rate on the dispersion and lifetime
of CBPs
Edge modes and Fabry-Perot Plasmonic Resonances in anomalous-Hall Thin Films
We study plasmon propagation on a metallic two-dimensional surface partially
coated with a thin film of anomalous-Hall material. The resulting three
regions, separated by two sharp interfaces, are characterised by different Hall
conductivities but identical normal conductivities. A single bound mode is
found, which can localise to either interface and has an asymmetric potential
profile across the region. For propagating modes, we calculate the reflection
and transmission coefficients through the magnetic region. We find Airy
transmission patterns with sharp maxima and minima as a function of the plasmon
incidence angle. The system therefore behaves as a high-quality filter.Comment: 11 pages, 7 figures. Upon revision, the basic content and analysis of
the paper is unchanged but the emphasis on topological insulators has been
removed. The title and abstract has been changed to reflect this. Damping of
the bound mode has been include
Non-local transport and the Hall viscosity of 2D hydrodynamic electron liquids
In a fluid subject to a magnetic field the viscous stress tensor has a
dissipationless antisymmetric component controlled by the so-called Hall
viscosity. We here propose an all-electrical scheme that allows a determination
of the Hall viscosity of a two-dimensional electron liquid in a solid-state
device.Comment: 12 pages, 4 figure
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.Comment: 12 pages, 4 multi-panel figure
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
Optical and plasmonic properties of twisted bilayer graphene: Impact of interlayer tunneling asymmetry and ground-state charge inhomogeneity
We present a theoretical study of the local optical conductivity, plasmon
spectra, and thermoelectric properties of twisted bilayer graphene (TBG) at
different filling factors and twist angles . Our calculations are based
on the electronic band structures obtained from a continuum model that has two
tunable parameters, and , which parametrize the intra-sublattice
inter-layer and inter-sublattice inter-layer tunneling rate, respectively. In
this Article we focus on two key aspects: i) we study the dependence of our
results on the value of , exploring the whole range ;
ii) we take into account effects arising from the intrinsic charge density
inhomogeneity present in TBG, by calculating the band structures within the
self-consistent Hartree approximation. At zero filling factor, i.e. at the
charge neutrality point, the optical conductivity is quite sensitive to the
value of and twist angle, whereas the charge inhomogeneity brings about
only modest corrections. On the other hand, away from zero filling, static
screening dominates and the optical conductivity is appreciably affected by the
charge inhomogeneity, the largest effects being seen on the intra-band
contribution to it. These findings are also reflected by the plasmonic spectra.
We compare our results with existing ones in the literature, where effects i)
and ii) above have not been studied systematically. As natural byproducts of
our calculations, we obtain the Drude weight and Seebeck coefficient. The
former displays an enhanced particle-hole asymmetry stemming from the
inhomogeneous ground-state charge distribution. The latter is shown to display
a broad sign-changing feature even at low temperatures ()
due to the reduced slope of the bands, as compared to those of single-layer
graphene.Comment: 28 pages, 16 figures, 6 appendice
GW Theory of Magic-Angle Twisted Bilayer Graphene
Strong correlations occur in magic-angle twisted bilayer graphene (MATBG)
when the octet of flat moir\'e minibands centered on charge neutrality (CN) is
partially occupied. The octet consists of a single valence band and a single
conduction band for each of four degenerate spin-valley flavors. Motivated by
the importance of Hartree electrostatic interactions in determining the
filling-factor dependent band structure, we use a time-dependent Hartree (GW)
approximation to gain insight into electronic correlations. We find that the
electronic compressibility is dominated by Hartree interactions, that
paramagnetic states are stable over a range of density near CN, and that the
dependence of energy on flavor polarization is strongly overestimated by
mean-field theory
Quantitative scattering theory of near-field response for 1D polaritonic structures
Scattering-type scanning near-field optical microscopy is a powerful imaging
technique for studying materials beyond the diffraction limit. However,
interpreting near-field measurements poses challenges in mapping the response
of polaritonic structures to meaningful physical properties. To address this,
we propose a theory based on the transfer matrix method to simulate the
near-field response of 1D polaritonic structures. Our approach provides a
computationally efficient and accurate analytical theory, relating the
near-field response to well-defined physical properties. This work enhances the
understanding of near-field images and complex polaritonic phenomena. Finally,
this scattering theory can extend to other systems like atoms or nanoparticles
near a waveguide
Electrical plasmon detection in graphene waveguides
We present a simple device architecture that allows all-electrical detection
of plasmons in a graphene waveguide. The key principle of our electrical
plasmon detection scheme is the non-linear nature of the hydrodynamic equations
of motion that describe transport in graphene at room temperature and in a wide
range of carrier densities. These non-linearities yield a dc voltage in
response to the oscillating field of a propagating plasmon. For illustrative
purposes, we calculate the dc voltage arising from the propagation of the
lowest-energy modes in a fully analytical fashion. Our device architecture for
all-electrical plasmon detection paves the way for the integration of graphene
plasmonic waveguides in electronic circuits.Comment: 9 pages, 3 figure
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