321 research outputs found
Theory of non-retarded ballistic surface plasma waves in metal films
We present a theory of surface plasma waves in metal films with arbitrary
electronic collision rate . Both \textit{tangential} and \textit{normal}
modes are investigated. A universal self-amplification channel for these waves
is established as a result of the unique interplay between ballistic electronic
motions and boundary effects. The channel is shown to be protected by a general
principle and its properties independent of . The effects of film
thickness and surface roughness are also calculated. Experimental implications,
such as Ferrel radiation, are discussed.Comment: 14 pages, 3 figs. arXiv admin note: text overlap with
arXiv:1701.0106
A Theory of Electrodynamic Responses for Bounded Metals: Surface Capacitive Effects
We report a general macroscopic theory for the electrodynamic response of
semi-infinite metals (SIMs). The theory includes the hitherto overlooked
capacitive effects due to the finite spatial extension of a surface. The basic
structure of this theory is independent of the particulars of electron
dynamics. Analytical expressions have been obtained of the charge
density-density response function, which is naturally parsed into two parts.
One of them represents a bulk property while the other a pure surface property.
We apply the theory to study the responses according to several electronic
dynamics models and provide a unified view of their validity and limitations.
The models studied include the local dielectric model (DM), the dispersive
hydrodynamic model (HDM) and specular reflection model (SRM), as well as the
less common semi-classical model (SCM) based on Boltzmann's transport equation.
We show that, in terms of their basic equations, the SRM is an extension of the
HDM, just as the HDM is an extension of the DM. The SCM improves over the SRM
critically through the inclusion of translation symmetry breaking and surface
roughness effects. We then employ the response function to evaluate the
so-called dynamical structure factor, which plays an important role in particle
scattering. As expected, this factor reveals a peak due to the excitation of
surface plasma waves (SPWs). Surprisingly, however, the peak is shown to be
considerably sharper in the SCM than in other models, indicating an incipient
instability of the system according to this model. We also study the
distribution of charges induced by a charged particle grazing over a SIM
surface at constant speed. This distribution is shown to contain model-specific
features that are of immediate experimental interest.Comment: 24 pages, 10 figures, a few more references are added and discussed,
abstract and introduction modified to improve presentation over older
versions, more experimental aspects are discusse
A universal macroscopic theory of surface plasma waves and their losses
Recently, we have revealed an intrinsic instability of metals due to surface
plasma waves (SPWs) and raised the prospect of using it to create lossless
SPWs. The counter-intuitive nature of this finding prompts one to ask, why had
not this instability been disclosed before, given the long history of this
subject? If this instability does exist, how far is it from reality? The
present work is devoted to answering these questions. To this end, we derive a
unified macroscopic theory of SPWs that applies to any type of electron
dynamics, be they local or non-local, classical or quantum-mechanical. In light
of this theory, we analyze the behaviors of SPWs according to several electron
dynamics models, including the widely used local dielectric model (DM), the
hydrodynamic model (HDM) and the specular reflection model (SRM), in addition
to the less common semi-classical model (SCM). We find that, in order to unveil
the instability, one must (i) self-consistently treat surface effects without
any of the usually imposed auxiliary conditions and (ii) include translation
symmetry breaking effects in electron dynamics. As far as we are concerned,
none existing work had fulfilled both (i) and (ii). To assess the possibility
of realizing the instability, we analyze two very important factors: the
dielectric interfacing the metal and inter-band transitions, which both were
ignored in our recent work. Whereas inter-band absorption -- together with
Landau damping -- is shown adverse to the instability, a dielectric brings it
closer to occurrence. One may even attain it in common plasmonic materials such
as silver under not so tough conditions.Comment: 16 pages, 6 figures, title changed, rewrite of the Introduction and
abstract, restructuring, Fig. 2 deleted, Appendix B added, more references
added, material in Sec. VI has some overlap with arXiv:1706.03404 (not to be
submitted
Helical Topological Edge States in a Quadrupole Phase
Topological electric quadrupole is a recently proposed concept that extends
the theory of electric polarization of crystals to higher orders. Such a
quadrupole phase supports topological states localized on both edges and
corners. In this work, we show that in a quadrupole phase of honeycomb lattice,
topological helical edge states and pseudo-spin-polarized corner states appear
by making use of a pseudo-spin degree of freedom related to point group
symmetry. Furthermore, we argue that a general condition for emergence of
helical edge states in a (pseudo-)spinful quadrupole phase is mirror or
time-reversal symmetry. Our results offers a way of generating topological
helical edge states without spin-orbital couplings
On the electrical conductivity of metals with a rough surface
We discuss surface roughness effects on the conduction of electrons in metals
using both the quantal Kubo-Greenwood formalism and the semi-classical
Fuchs-Sondheimer method. The main purpose here is to compare these methods and
clarify a few subtle conceptual issues. One of such issues is concerned with
the conditions under which the broken translation symmetry along a rough
surface may be restored. This symmetry has often been presumed in existing work
but not always with proper justifications. Another one relates to the physical
meaning of a phenomenological parameter (denoted by ) intuitively introduced
in the semi-classical theory. This parameter, called the specularity parameter
or sometimes the \textit{Fuchs} parameter, plays an important role in the
experimental studies of surface roughness but has so far lacked a rigorous
microscopic definition. The third issue arises as to the domain of validity for
the electrical conductivity obtained in those methods. A misplacement of the
domain may have resulted in erroneous analysis of surface effects in a variety
of electrodynamic phenomena including surface plasma waves.Comment: 10 pages, 1 figur
Strong mechanically-induced effects in DC current-biased suspended Josephson junctions
Superconductivity is a result of quantum coherence at macroscopic scales. Two
superconductors separated by a metallic or insulating weak link exhibit the AC
Josephson effect - the conversion of a DC voltage bias into an AC supercurrent.
This current may be used to activate mechanical oscillations in a suspended
weak link. As the DC voltage bias condition is remarkably difficult to achieve
in experiments, here we analyse theoretically how the Josephson effect can be
exploited to activate and detect mechanical oscillations in the experimentally
relevant condition with purely DC current bias. We unveil for the first time
how changing the strength of the electromechanical coupling results in two
qualitatively different regimes showing dramatic effects of the oscillations on
the DC current-voltage characteristic of the device. These include the
apperance of Shapiro-like plateaux for weak coupling and a sudden
mechanically-induced retrapping for strong coupling. Our predictions,
measurable in state of the art experimental setups, allow the determination of
the frequency and quality factor of the resonator using DC only techniques.Comment: 10 pages, 6 figure
Decomposition into Propagating and Evanescent Modes of Graphene Ribbons
Bulk modes (BM) are basis solutions to the Schr\"{o}dinger equation and they
are useful in a number of physical problems. In the present work, we establish
a complete set of BMs for graphene ribbons at arbitrary energy. We derive
analytical expressions for these modes and systematically classify them into
propagating or evanescent mode. We also demonstrate their uses in efficient
electronic transport simulations of graphene-based electronic devices within
both the mode-matching method and the Green's function framework. Explicit
constructions of Green's functions for infinite and semi-infinite graphene
ribbons are presented
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