138 research outputs found
Electrical plasmon injection in double-layer graphene heterostructures
It is by now well established that high-quality graphene enables a
gate-tunable low-loss plasmonic platform for the efficient confinement,
enhancement, and manipulation of optical fields spanning a broad range of
frequencies, from the mid infrared to the Terahertz domain. While
all-electrical detection of graphene plasmons has been demonstrated, electrical
plasmon injection (EPI), which is crucial to operate nanoplasmonic devices
without the encumbrance of a far-field optical apparatus, remains elusive. In
this work, we present a theory of EPI in double-layer graphene, where a
vertical tunnel current excites acoustic and optical plasmon modes. We first
calculate the power delivered by the applied inter-layer voltage bias into
these collective modes. We then show that this system works also as a
spectrally-resolved molecular sensor.Comment: 10 pages, 6 figure
Electron density distribution and screening in rippled graphene sheets
Single-layer graphene sheets are typically characterized by long-wavelength
corrugations (ripples) which can be shown to be at the origin of rather strong
potentials with both scalar and vector components. We present an extensive
microscopic study, based on a self-consistent Kohn-Sham-Dirac
density-functional method, of the carrier density distribution in the presence
of these ripple-induced external fields. We find that spatial density
fluctuations are essentially controlled by the scalar component, especially in
nearly-neutral graphene sheets, and that in-plane atomic displacements are as
important as out-of-plane ones. The latter fact is at the origin of a
complicated spatial distribution of electron-hole puddles which has no evident
correlation with the out-of-plane topographic corrugations. In the range of
parameters we have explored, exchange and correlation contributions to the
Kohn-Sham potential seem to play a minor role.Comment: 13 pages, 13 figures, submitted. High-quality figures can be
requested to the author
Plasmons in realistic graphene/hexagonal boron nitride moiré patterns
van der Waals heterostructures employing graphene and hexagonal boron nitride (hBN) crystals have emerged as a promising platform for plasmonics thanks to the tunability of their collective modes with carrier density and record values for plasmonics figures of merit. In this paper we investigate theoretically the role of moiré-pattern superlattices in nearly aligned graphene on hBN by using a continuum-model Hamiltonian derived from ab initio calculations. We calculate the system's energy-loss function for a range of chemical potential values that are accessible in gated devices. Our calculations reveal that the electron-hole asymmetry of the moiré bands leads to a remarkable asymmetry of the plasmon dispersion between positive and negative chemical potentials, showcasing the intricate band structure and rich absorption spectrum across the secondary Dirac point gap for the hole bands. © 2019 American Physical Society
Signatures of the super fluid-insulator phase transition in laser driven dissipative nonlinear cavity arrays
We analyze the non-equilibrium dynamics of a gas of interacting photons in an
array of coupled dissipative nonlinear cavities driven by a pulsed external
coherent field. Using a mean-field approach, we show that the system exhibits a
phase transition from a Mott-insulator-like to a superfluid regime. For a given
single-photon nonlinearity, the critical value of the photon tunneling rate at
which the phase transition occurs increases with the increasing photon loss
rate. We checked the robustness of the transition by showing its insensitivity
to the initial state prepared by the the pulsed excitation. We find that the
second-order coherence of cavity emission can be used to determine the phase
diagram of an optical many-body system without the need for thermalization.Comment: 4 pages, 4 figure
Nonlocal Spin Transport as a Probe of Viscous Magnon Fluids
Magnons in ferromagnets behave as a viscous fluid over a length scale, the
momentum-relaxation length, below which momentum-conserving scattering
processes dominate. We show theoretically that in this hydrodynamic regime
viscous effects lead to a sign change in the magnon chemical potential, which
can be detected as a sign change in the nonlocal resistance measured in spin
transport experiments. This sign change is observable when the
injector-detector distance becomes comparable to the momentum-relaxation
length. Taking into account momentum- and spin-relaxation processes, we
consider the quasiconservation laws for momentum and spin in a magnon fluid.
The resulting equations are solved for nonlocal spin transport devices in which
spin is injected and detected via metallic leads. Because of the finite
viscosity we also find a backflow of magnons close to the injector lead. Our
work shows that nonlocal magnon spin transport devices are an attractive
platform to develop and study magnon-fluid dynamics
Nonequilibrium Phase Diagram of a Driven-Dissipative Many-Body System
We study the nonequilibrium dynamics of a many-body bosonic system on a
lattice, subject to driving and dissipation. The time-evolution is described by
a master equation, which we treat within a generalized Gutzwiller mean field
approximation for density matrices. The dissipative processes are engineered
such that the system, in the absence of interaction between the bosons, is
driven into a homogeneous steady state with off-diagonal long range order. We
investigate how the coherent interaction affects qualitatively the properties
of the steady state of the system and derive a nonequilibrium phase diagram
featuring a phase transition into a steady state without long range order. The
phase diagram exhibits also an extended domain where an instability of the
homogeneous steady state gives rise to a persistent density pattern with
spontaneously broken translational symmetry. In the limit of small particle
density, we provide a precise analytical description of the time-evolution
during the instability. Moreover, we investigate the transient following a
quantum quench of the dissipative processes and we elucidate the prominent role
played by collective topological variables in this regime.Comment: 23 pages, 15 figure
Nonequilibrium dynamics of photoexcited electrons in graphene: Collinear scattering, Auger processes, and the impact of screening
We present a combined analytical and numerical study of the early stages
(sub-100fs) of the non-equilibrium dynamics of photo-excited electrons in
graphene. We employ the semiclassical Boltzmann equation with a collision
integral that includes contributions from electron-electron (e-e) and
electron-optical phonon interactions. Taking advantage of circular symmetry and
employing the massless Dirac Fermion (MDF) Hamiltonian, we are able to perform
an essentially analytical study of the e-e contribution to the collision
integral. This allows us to take particular care of subtle collinear scattering
processes - processes in which incoming and outgoing momenta of the scattering
particles lie on the same line - including carrier multiplication (CM) and
Auger recombination (AR). These processes have a vanishing phase space for two
dimensional MDF bare bands. However, we argue that electron-lifetime effects,
seen in experiments based on angle-resolved photoemission spectroscopy, provide
a natural pathway to regularize this pathology, yielding a finite contribution
due to CM and AR to the Coulomb collision integral. Finally, we discuss in
detail the role of physics beyond the Fermi golden rule by including screening
in the matrix element of the Coulomb interaction at the level of the Random
Phase Approximation (RPA), focusing in particular on the consequences of
various approximations including static RPA screening, which maximizes the
impact of CM and AR processes, and dynamical RPA screening, which completely
suppresses them
Collapse and revival in inter-band oscillations of a two-band Bose-Hubbard model
We study the effect of a many-body interaction on inter-band oscillations in
a two-band Bose-Hubbard model with external Stark force. Weak and strong
inter-band oscillations are observed, where the latter arise from a resonant
coupling of the bands. These oscillations collapse and revive due to a weak
two-body interaction between the atoms. Effective models for oscillations in
and out of resonance are introduced that provide predictions for the system's
behaviour, particularly for the time-scales for the collapse and revival of the
resonant inter-band oscillations.Comment: 10 pages, 5 figure
Reservoir engineering and dynamical phase transitions in optomechanical arrays
We study the driven-dissipative dynamics of photons interacting with an array
of micromechanical membranes in an optical cavity. Periodic membrane driving
and phonon creation result in an effective photon-number conserving non-unitary
dynamics, which features a steady state with long-range photonic coherence. If
the leakage of photons out of the cavity is counteracted by incoherent driving
of the photonic modes, we show that the system undergoes a dynamical phase
transition to the state with long-range coherence. A minimal system, composed
of two micromechanical membranes in a cavity, is studied in detail, and it is
shown to be a realistic setup where the key processes of the driven-dissipative
dynamics can be seen.Comment: 16 pages, 9 figure
Negative local resistance caused by viscous electron backflow in graphene
Graphene hosts a unique electron system in which electron-phonon scattering
is extremely weak but electron-electron collisions are sufficiently frequent to
provide local equilibrium above liquid nitrogen temperature. Under these
conditions, electrons can behave as a viscous liquid and exhibit hydrodynamic
phenomena similar to classical liquids. Here we report strong evidence for this
transport regime. We find that doped graphene exhibits an anomalous (negative)
voltage drop near current injection contacts, which is attributed to the
formation of submicrometer-size whirlpools in the electron flow. The viscosity
of graphene's electron liquid is found to be ~0.1 m /s, an order of
magnitude larger than that of honey, in agreement with many-body theory. Our
work shows a possibility to study electron hydrodynamics using high quality
graphene
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