511 research outputs found
Chiral sound waves in strained Weyl semimetals
We show that a strained wire of a Weyl semimetal supports a new type of
gapless excitation, the chiral sound wave (CSW). It is a longitudinal charge
density wave analog to the chiral magnetic wave predicted in the quark-gluon
plasma but driven by an elastic axial pseudo-magnetic field. It involves the
axial-axial-axial contribution to the chiral anomaly which couples the chiral
charge density to the elastic axial gauge field. The chiral sound is
unidirectional: it propagates along the elastic magnetic field and not in the
opposite direction. The CSW may propagate for long distances as it does not
couple directly to quickly dissipating electromagnetic plasmons, while its
damping is controlled by the slow chirality flip rate. We propose an
experimental setup to directly detect the chiral sound, which is excited by
mechanical vibrations of the crystal lattice in the GHz frequency range. Our
findings contribute to a new trend, the chiral acoustics, in strained Weyl
semimetals.Comment: 5 pages, 3 figures; v2: minor changes, published versio
Collective excitations and low temperature transport properties of bismuth
We examine the influence of collective excitations on the transport
properties (resistivity, magneto- optical conductivity) for semimetals,
focusing on the case of bismuth. We show, using an RPA approximation, that the
properties of the system are drastically affected by the presence of an
acoustic plasmon mode, consequence of the presence of two types of carriers
(electrons and holes) in this system. We found a crossover temperature T*
separating two different regimes of transport. At high temperatures T > T* we
show that Baber scattering explains quantitatively the DC resistivity
experiments, while at low temperatures T < T* interactions of the carriers with
this collective mode lead to a T^5 behavior of the resistivity. We examine
other consequences of the presence of this mode, and in particular predict a
two plasmon edge feature in the magneto-optical conductivity. We compare our
results with the experimental findings on bismuth. We discuss the limitations
and extensions of our results beyond the RPA approximation, and examine the
case of other semimetals such as graphite or 1T-TiSe_2
Collective excitations and universal broadening of cyclotron absorption in Dirac semimetals in a quantizing magnetic field
The spectrum of electromagnetic collective excitations in Dirac semimetals
placed in a quantizing magnetic field is considered. We have found the Landau
damping regions using the energy and momentum conservation law for allowed
transitions between one-particle states of electron excitations. Analysis of
dispersion equations for longitudinal and transverse waves near the window
boundaries in the Landau damping regions reveals different types of collective
excitations. We also indicate the features of universal broadening of cyclotron
absorption for a magnetic field variation in systems with linear dispersion of
the electron spectrum. The use of the obtained spectrum also allows us to
predict a number of oscillation and resonance effects in the field of
magneto-optical phenomena.Comment: 7 pages, 4 eps figure
Weyl excitations via helicon-phonon mixing in conducting materials
Quasiparticles with Weyl dispersion can display an abundance of novel
topological, thermodynamic and transport phenomena, which is why novel Weyl
materials and platforms for Weyl physics are being intensively looked for in
electronic, magnetic, photonic and acoustic systems. We demonstrate that
conducting materials in magnetic fields generically host Weyl excitations due
to the hybridisation of phonons with helicons, collective neutral modes of
electrons interacting with electromagnetic waves propagating in the material.
Such Weyl excitations are, in general, created by the interactions of helicons
with longitudinal acoustic phonons. An additional type of Weyl excitation in
polar crystals comes from the interaction between helicons and longitudinal
optical phonons. Such excitations can be detected in X-ray and Raman scattering
experiments. The existence of the Weyl excitations involving optical phonons in
the bulk of the materials also leads to the formation of topologically
protected surface arc states that can be detected via surface plasmon
resonance.Comment: 7+3 pages, and 2 figures. Published versio
Screening of Coulomb interactions in Holography
We introduce Coulomb interactions in the holographic description of strongly
interacting systems, by performing a (current-current) double-trace deformation
of the boundary theory. In the theory dual to a Reissner-Nordstr\"om
background, this deformation leads to gapped plasmon modes in the
density-density response, as expected from conventional RPA calculations. We
further show that by introducing a -dimensional Coulomb interaction in
a boundary theory in spacetime dimensions, we recover plasmon modes whose
dispersion is proportional to , as observed for example in
graphene layers. Moreover, motivated by recent experimental results in layered
cuprate high-temperature superconductors, we present a toy model for a layered
system consisting of an infinite stack of (spatially) two-dimensional layers,
that are coupled only by the long-range Coulomb interaction. This leads to
low-energy `acoustic plasmons'. Finally, we compute the optical conductivity of
the deformed theory in , where a logarithmic correction is present
and we show how this can be related to the conductivity measured in Dirac and
Weyl semimetals.Comment: 39 pages, 15 figures; Published version (small changes according to
referee's suggestions
Resonant plasmon-phonon coupling and its role in magneto-thermoelectricity in bismuth
Using diagrammatic methods we derive an effective interaction between a low
energy collective movement of fermionic liquid (acoustic plasmon) and acoustic
phonon. We show that the coupling between the plasmon and the lattice has a
very non-trivial, resonant structure. When real and imaginary parts of the
acoustic plasmon's velocity are of the same order as the phonon's velocity, the
resonance qualitatively changes the nature of phonon-drag. In the following we
study how magneto-thermoelectric properties are affected. Our result suggests
that the novel mechanism of energy transfer between electron liquid and crystal
lattice can be behind the huge Nernst effect in bismuth.Comment: accepted in EPJB, to appear with a highligh
Hydrodynamic model for electron-hole plasma in graphene
We propose a hydrodynamic model describing steady-state and dynamic electron
and hole transport properties of graphene structures which accounts for the
features of the electron and hole spectra. It is intended for electron-hole
plasma in graphene characterized by high rate of intercarrier scattering
compared to external scattering (on phonons and impurities), i.e., for
intrinsic or optically pumped (bipolar plasma), and gated graphene (virtually
monopolar plasma). We demonstrate that the effect of strong interaction of
electrons and holes on their transport can be treated as a viscous friction
between the electron and hole components. We apply the developed model for the
calculations of the graphene dc conductivity, in particular, the effect of
mutual drag of electrons and holes is described. The spectra and damping of
collective excitations in graphene in the bipolar and monopolar limits are
found. It is shown that at high gate voltages and, hence, at high electron and
low hole densities (or vice-versa), the excitations are associated with the
self-consistent electric field and the hydrodynamic pressure (plasma waves). In
intrinsic and optically pumped graphene, the waves constitute quasineutral
perturbations of the electron and hole densities (electron-hole sound waves)
with the velocity being dependent only on the fundamental graphene constants.Comment: 11 pages, 6 figure
Optical interface states protected by synthetic Weyl points
Weyl fermions have not been found in nature as elementary particles, but they
emerge as nodal points in the band structure of electronic and classical wave
crystals. Novel phenomena such as Fermi arcs and chiral anomaly have fueled the
interest in these topological points which are frequently perceived as
monopoles in momentum space. Here we report the experimental observation of
generalized optical Weyl points inside the parameter space of a photonic
crystal with a specially designed four-layer unit cell. The reflection at the
surface of a truncated photonic crystal exhibits phase vortexes due to the
synthetic Weyl points, which in turn guarantees the existence of interface
states between photonic crystals and any reflecting substrates. The reflection
phase vortexes have been confirmed for the first time in our experiments which
serve as an experimental signature of the generalized Weyl points. The
existence of these interface states is protected by the topological properties
of the Weyl points and the trajectories of these states in the parameter space
resembles those of Weyl semimetal "Fermi arcs surface states" in momentum
space. Tracing the origin of interface states to the topological character of
the parameter space paves the way for a rational design of strongly localized
states with enhanced local field.Comment: 36 pages, 9 figures. arXiv admin note: text overlap with
arXiv:1610.0434
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