261 research outputs found
Exact solution for driven oscillations in plasmonic field-effect transistors
High-mobility field effect transistors can serve as resonant detectors of
terahertz radiation due to excitation of plasmons in the channel. The modeling
of these devices previously relied either on approximate techniques, or complex
full-wave simulations. In this paper, we obtain an exact solution for driven
electrical oscillations in plasmonic field-effect transistor with realistic
contact geometry. The obtained solution highlights the importance of evanescent
plasma waves excited near the contacts, which qualitatively modify the detector
responsivity spectra. We derive the boundary condition on the ac floating
electrodes of plasmonic FET which interpolates between open-circuit
(Dyakonov-Shur) and short-circuit (clamped voltage) boundary conditions. In
both limits, the FET photovoltage possesses resonant fringes, however, the
absolute value of voltage is greater in the open-circuit regime.Comment: 5 pages, 4 figure
Hydrodynamic-to-ballistic crossover in Dirac fluid
We develop an exactly solvable classical kinetic model of transport in Dirac
materials accounting for strong electron-electron (e-e) and electron-hole (e-h)
collisions. We use this model to track the evolution of graphene conductivity
and properties of its collective excitations across the
hydrodynamic-to-ballistic crossover. We find the relaxation rate of electric
current by e-e collisions that is possible due to the lack of Galilean
invariance, and introduce a universal numerical measure of this non-invariance
in arbitrary dimension. We find the two branches of collective excitations in
the Dirac fluid: plasmons and electron-hole sound. The sound waves have small
viscous damping at the neutrality point both in the hydrodynamic and ballistic
regimes, but acquire large damping due to e-h friction even at slight doping.
On the contrary, plasmons acquire strong frictional damping at the neutrality
point and become well-defined in doped samples.Comment: 6 pages, 2 figure
Comment on "Negative Landau damping in bilayer graphene"
In [Phys. Rev. Lett. vol. 119, p. 133901 (2017)] it was argued that two
parallel graphene layers in the presence of electron drift support unstable
plasmon modes. Here we show that the predicted plasmon instability is an
artifact of errors upon evaluation of graphene polarizability in the presence
if electron drift. Crucial role of broken Galilean invariance and spatial
dispersion of conductivity for suppression of plasmon instabilities is
highlighted.Comment: 2 pages, 1 figur
Quantum single electron solitons near metal surface
A possibility of a quantum single electron soliton (QSES) formation in
structures with different dimensionality (0, 1, 2, and 3D) and spectrum
(parabolic and linear) placed near metal surface is discussed. These solitons
originate as solutions of the nonlinear Schrodinger equation allowing for
interaction with image charges in metal. The binding energy of those
quasi-particles could exceed the thermal energy at room temperature.Comment: 4 pages, 4 figure
Asymmetry-driven plasmon instabilities in confined hydrodynamic electron flows
Direct current in confined two-dimensional (2d) electron systems can become
unstable with respect to the excitation of plasmons. Numerous experiments and
simulations hint that structural asymmetry somehow promotes plasmon generation,
but a constitutive relation between asymmetry and instability has been missing.
We provide such relation in the present paper and show that bounded perfect 2d
electron fluids in asymmetric structures are unstable under arbitrarily weak
drive currents. To this end, we develop a perturbation theory for hydrodynamic
plasmons and evaluate corrections to their eigenfrequency induced by carrier
drift, scattering, and viscosity. We show that plasmon gain continuously
increases with degree of plasmon mode asymmetry until it surrenders to viscous
dissipation that also benefits from asymmetry. The developed formalism allows
us to put a lower bound on the instability threshold current, which corresponds
to the Reynolds number for one-dimensional plasmons in
2d channel under constant voltage bias.Comment: 9 pages, 4 figure
Plasmon-assisted resonant tunneling in graphene-based heterostructures
We develop a theory of electron tunneling accompanied by carrier-carrier
scattering in graphene - insulator - graphene heterostructures. Due to the
dynamic screening of Coulomb interaction, the scattering-aided tunneling is
resonantly enhanced if the transferred energy and momentum correspond to those
of surface plasmons. We reveal the possible experimental manifestations of such
plasmon-assisted tunneling in current-voltage curves and plasmon emission
spectra of graphene-based tunnel junctions. We find that inelastic current and
plasmon emission rates have sharp peaks at voltages providing equal energies,
momenta and group velocities of plasmons and interlayer single-particle
excitations. The strength of this resonance, which we call plasmaronic
resonance, is limited by interlayer twist and plasmon lifetime. The onset of
plasmon-assisted tunneling can be also marked by a cusp in the junction
-curve at low temperatures, and the threshold voltage for such tunneling
weakly depends on carrier density and persists in the presence of interlayer
twist
Negative dynamic Drude conductivity in pumped graphene
We theoretically reveal a new mechanism of light amplification in graphene
under the conditions of interband population inversion. It is enabled by the
indirect interband transitions, with the photon emission preceded or followed
by the scattering on disorder. The emerging contribution to the optical
conductivity, which we call the interband Drude conductivity, appears to be
negative for the photon energies below the double quasi-Fermi energy of pumped
electrons and holes. We find that for the Gaussian correlated distribution of
scattering centers, the real part of the net Drude conductivity (interband plus
intraband) can be negative in the terahertz and near-infrared frequency ranges,
while the radiation amplification by a single graphene sheet can exceed 2.3%.Comment: 5 pages, 5 figure
Emission of plasmons by drifting Dirac electrons: where hydrodynamics matters
Direct current in clean semiconductors and metals was recently shown to obey
the laws of hydrodynamics in a broad range of temperatures and sample
dimensions. However, the determination of frequency window for hydrodynamic
phenomena remains challenging. Here, we reveal a phenomenon being a hallmark of
high-frequency hydrodynamic transport, the Cerenkov emission of plasmons by
drifting Dirac electrons. The effect appears in hydrodynamic regime only due to
reduction of plasmon velocity by electron-electron collisions below the
velocity of carrier drift. To characterize the Cerenkov effect quantitatively,
we analytically find the high-frequency non-local conductivity of drifting
Dirac electrons across the hydrodynamic-to-ballistic crossover. We find the
growth rates of hydrodynamic plasmon instabilities in two experimentally
relevant setups: parallel graphene layers and graphene covered by subwavelength
grating, further showing their absence in ballistic regime. We argue that the
possibility of Cerenkov emission is linked to singular structure of non-local
conductivity of Dirac materials and is independent on specific dielectric
environment.Comment: 5 pages + 6 pages supporting informatio
Relativistic suppression of Auger recombination in Weyl semimetals
Auger recombination (AR) being electron-hole annihilation with
energy-momentum transfer to another carrier is believed to speed up in
materials with small band gap. We theoretically show that this rule is violated
in gapless three-dimensional materials with ultra-relativistic electron-hole
dispersion, Weyl semimetals (WSM). Namely, AR is prohibited by energy-momentum
conservation laws in prototypical WSM with a single Weyl node, even in the
presence of anisotropy and tilt. In real multi-node WSM, the geometric
dissimilarity of nodal dispersions enables weak inter-node AR, which is further
suppressed by strong screening due to large number of nodes. While partial AR
rates between the nodes of the same node group are mutually equal, the
inter-group processes are non-reciprocal, so that one of groups is
geometrically protected from AR. Our calculations show that geometrical
protection can help prolonging AR lifetime by the two orders of magnitude, up
to the level of nanoseconds.Comment: 6 pages + 10 pages of supporting informatio
Abrupt current switching in graphene bilayer tunnel transistors enabled by van Hove singularities
In a continuous search for the energy-efficient electronic switches, a great
attention is focused on tunnel field-effect transistors (TFETs) demonstrating
an abrupt dependence of the source-drain current on the gate voltage. Among all
TFETs, those based on one-dimensional (1D) semiconductors exhibit the steepest
current switching due to the singular density of states near the band edges,
though the current in 1D structures is pretty low. In this paper, we propose a
TFET based on 2D graphene bilayer which demonstrates a record steep
subthreshold slope enabled by van Hove singularities in the density of states
near the edges of conduction and valence bands. Our simulations show the
accessibility of 3.5 x 10 ON/OFF current ratio with 150 mV gate voltage
swing, and a maximum subthreshold slope of (20 {\mu}V/dec) just above
the threshold. The high ON-state current of 0.8 mA/{\mu}m is enabled by a
narrow (~ 0.3 eV) extrinsic band gap, while the smallness of the leakage
current is due to an all-electrical doping of the source and drain contacts
which suppresses the band-tailing and trap-assisted tunneling
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