878 research outputs found
Dirac Theory and Topological Phases of Silicon Nanotube
Silicon nanotube is constructed by rolling up a silicene, i.e., a monolayer
of silicon atoms forming a two-dimensional honeycomb lattice. It is a
semiconductor or an insulator owing to relatively large spin-orbit interactions
induced by its buckled structure. The key observation is that this buckled
structure allows us to control the band structure by applying electric field
. When is larger than a certain critical value , by
analyzing the band structure and also on the basis of the effective Dirac
theory, we demonstrate the emergence of four helical zero-energy modes
propagating along nanotube. Accordingly, a silicon nanotube contains three
regions, namely, a topological insulator, a band insulator and a metallic
region separating these two types of insulators. The wave function of each zero
mode is localized within the metallic region, which may be used as a quantum
wire to transport spin currents in future spintronics. We present an analytic
expression of the wave function for each helical zero mode. These results are
applicable also to germanium nanotube.Comment: 5 pages, 5 figure
Anomalous Stability of nu=1 Bilayer Quantum Hall State
We have studied the fractional and integer quantum Hall (QH) effects in a
high-mobility double-layer two-dimensional electron system. We have compared
the "stability" of the QH state in balanced and unbalanced double quantum
wells. The behavior of the n=1 QH state is found to be strikingly different
from all others. It is anomalously stable, though all other states decay, as
the electron density is made unbalanced between the two quantum wells. We
interpret the peculiar features of the nu=1 state as the consequences of the
interlayer quantum coherence developed spontaneously on the basis of the
composite-boson picture.Comment: 5 pages, 6 figure
Quasi-Phase Transition and Many-Spin Kondo Effects in Graphene Nanodisk
The trigonal zigzag nanodisk with size has localized spins. We
investigate its thermodynamical properties with and without external leads.
Leads are made of zigzag graphene nanoribbons or ordinary metallic wires. There
exists a quasi-phase transition between the quasi-ferromagnet and
quasi-paramagnet states, as signaled by a sharp peak in the specific heat and
in the susceptability. Lead effects are described by the many-spin Kondo
Hamiltonian. A new peak emerges in the specific heat. Furthermore, the band
width of free electrons in metallic leads becomes narrower. By investigating
the spin-spin correlation it is argued that free electrons in the lead form
spin-singlets with electrons in the nanodisk. They are indications of many-spin
Kondo effects.Comment: 5 pages, 5 figure
Interlayer Coherence in the and Bilayer Quantum Hall States
We have measured the Hall-plateau width and the activation energy of the
bilayer quantum Hall (BLQH) states at the Landau-level filling factor
and 2 by tilting the sample and simultaneously changing the electron density in
each quantum well. The phase transition between the commensurate and
incommensurate states are confirmed at and discovered at . In
particular, three different BLQH states are identified; the compound
state, the coherent commensurate state, and the coherent incommensurate state.Comment: 4 pages including 5 figure
Quasi-Topological Insulator and Trigonal Warping in Gated Bilayer Silicene
Bilayer silicene has richer physical properties than bilayer graphene due to
its buckled structure together with its trigonal symmetric structure. The
buckled structure arises from a large ionic radius of silicon, and the trigonal
symmetry from a particular way of hopping between two silicenes. It is a
topologically trivial insulator since it carries a trivial
topological charge. Nevertheless, its physical properties are more akin to
those of a topological insulator than those of a band insulator. Indeed, a
bilayer silicene nanoribbon has edge modes which are almost gapless and
helical. We may call it a quasi-topological insulator. An important observation
is that the band structure is controllable by applying the electric field to a
bilayer silicene sheet. We investigate the energy spectrum of bilayer silicene
under electric field. Just as monolayer silicene undergoes a phase transition
from a topological insulator to a band insulator at a certain electric field,
bilayer silicene makes a transition from a quasi-topological insulator to a
band insulator beyond a certain critical field. Bilayer silicene is a metal
while monolayer silicene is a semimetal at the critical field. Furthermore we
find that there are several critical electric fields where the gap closes due
to the trigonal warping effect in bilayer silicene.Comment: 8 pages, 11 figures, to be published in J. Phys. Soc. Jp
PseudoSkyrmion Effects on Tunneling Conductivity in Coherent Bilayer Quantum Hall States at
We present a mechamism why interlayer tunneling conductivity in coherent
bilayer quantum Hall states at is anomalously large, but finite in the
recent experiment. According to the mechanism, pseudoSkyrmions causes the
finite conductivity, although there exists an expectation that dissipationless
tunneling current arises in the state. PseudoSkyrmions have an intrinsic
polarization field perpendicular to the layers, which causes the dissipation.
Using the mechanism we show that the large peak in the conductivity remains for
weak parallel magnetic field, but decay rapidly after its strength is beyond a
critical one, Tesla.Comment: 6 pages, no figure
Topological Phase Transition and Electrically Tunable Diamagnetism in Silicene
Silicene is a monolayer of silicon atoms forming a honeycomb lattice. The
lattice is actually made of two sublattices with a tiny separation. Silicene is
a topological insulator, which is characterized by a full insulating gap in the
bulk and helical gapless edges. It undergoes a phase transition from a
topological insulator to a band insulator by applying external electric field.
Analyzing the spin Chern number based on the effective Dirac theory, we find
their origin to be a pseudospin meron in the momentum space. The peudospin
degree of freedom arises from the two-sublattice structure. Our analysis makes
clear the mechanism how a phase transition occurs from a topological insulator
to a band insulator under increasing electric field. We propose a method to
determine the critical electric field with the aid of diamagnetism of silicene.
Diamagnetism is tunable by the external electric field, and exhibits a singular
behaviour at the critical electric field. Our result is important also from the
viewpoint of cross correlation between electric field and magnetism. Our
finding will be important for future electro-magnetic correlated devices.Comment: 4 pages,5 figure
On the Canonical Formalism for a Higher-Curvature Gravity
Following the method of Buchbinder and Lyahovich, we carry out a canonical
formalism for a higher-curvature gravity in which the Lagrangian density is given in terms of a function of the salar curvature as . The local Hamiltonian is obtained by a
canonical transformation which interchanges a pair of the generalized
coordinate and its canonical momentum coming from the higher derivative of the
metric.Comment: 11 pages, no figures, Latex fil
Noncommutative Geometry, Extended W(infty) Algebra and Grassmannian Solitons in Multicomponent Quantum Hall Systems
Noncommutative geometry governs the physics of quantum Hall (QH) effects. We
introduce the Weyl ordering of the second quantized density operator to explore
the dynamics of electrons in the lowest Landau level. We analyze QH systems
made of -component electrons at the integer filling factor .
The basic algebra is the SU(N)-extended W. A specific feature is
that noncommutative geometry leads to a spontaneous development of SU(N)
quantum coherence by generating the exchange Coulomb interaction. The effective
Hamiltonian is the Grassmannian sigma model, and the dynamical field
is the Grassmannian field, describing complex Goldstone
modes and one kind of topological solitons (Grassmannian solitons).Comment: 15 pages (no figures
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