133 research outputs found
Holonomic Quantum Computing Based on the Stark Effect
We propose a spin manipulation technique based entirely on electric fields
applied to acceptor states in -type semiconductors with spin-orbit coupling.
While interesting in its own right, the technique can also be used to implement
fault-resilient holonomic quantum computing. We explicitly compute adiabatic
transformation matrix (holonomy) of the degenerate states and comment on the
feasibility of the scheme as an experimental technique.Comment: 5 page
Nonlocal edge state transport in the quantum spin Hall state
We present direct experimental evidence for nonlocal transport in HgTe
quantum wells in the quantum spin Hall regime, in the absence of any external
magnetic field. The data conclusively show that the non-dissipative quantum
transport occurs through edge channels, while the contacts lead to
equilibration between the counter-propagating spin states at the edge. We show
that the experimental data agree quantitatively with the theory of the quantum
spin Hall effect.Comment: 13 pages, 4 figure
Supersymmetric Quantum Hall Liquid with a Deformed Supersymmetry
We construct a supersymmetric quantum Hall liquid with a deformed
supersymmetry. One parameter is introduced in the supersymmetric Laughlin
wavefunction to realize the original Laughlin wavefunction and the Moore-Read
wavefunction in two extremal limits of the parameter. The introduced parameter
corresponds to the coherence factor in the BCS theory. It is pointed out that
the parameter-dependent supersymmetric Laughlin wavefunction enjoys a deformed
supersymmetry. Based on the deformed supersymmetry, we construct a
pseudo-potential Hamiltonian whose groundstate is exactly the
parameter-dependent supersymmetric Laughlin wavefunction. Though the SUSY
pseudo-potential Hamiltonian is parameter-dependent and non-Hermitian, its
eigenvalues are parameter-independent and real.Comment: 14 pages, contribution to the proceedings of the Group 27 conference,
Yerevan, Armenia, August 13-19, 2008, published versio
Quantum Blockades and Loop Currents in Graphene with Topological Defects
We investigate the effect of topological defects on the transport properties
of a narrow ballistic ribbon of graphene with zigzag edges. Our results show
that the longitudinal conductance vanishes at several discrete Fermi energies
where the system develops loop orbital electric currents with certain
chirality. The chirality depends on the direction of the applied bias voltage
and the sign of the local curvature created by the topological defects. This
novel quantum blockade phenomenon provides a new way to generate a magnetic
moment by an external electric field, which can prove useful in carbon
electronics.Comment: 6 pages, 7 figure
Insulating behavior in ultra-thin bismuth selenide field effect transistors
Ultrathin (~3 quintuple layer) field-effect transistors (FETs) of topological
insulator Bi2Se3 are prepared by mechanical exfoliation on 300nm SiO2/Si
susbtrates. Temperature- and gate-voltage dependent conductance measurements
show that ultrathin Bi2Se3 FETs are n-type, and have a clear OFF state at
negative gate voltage, with activated temperature-dependent conductance and
energy barriers up to 250 meV
The Quantum Spin Hall Effect: Theory and Experiment
The search for topologically non-trivial states of matter has become an
important goal for condensed matter physics. Recently, a new class of
topological insulators has been proposed. These topological insulators have an
insulating gap in the bulk, but have topologically protected edge states due to
the time reversal symmetry. In two dimensions the helical edge states give rise
to the quantum spin Hall (QSH) effect, in the absence of any external magnetic
field. Here we review a recent theory which predicts that the QSH state can be
realized in HgTe/CdTe semiconductor quantum wells. By varying the thickness of
the quantum well, the band structure changes from a normal to an "inverted"
type at a critical thickness . We present an analytical solution of the
helical edge states and explicitly demonstrate their topological stability. We
also review the recent experimental observation of the QSH state in
HgTe/(Hg,Cd)Te quantum wells. We review both the fabrication of the sample and
the experimental setup. For thin quantum wells with well width
nm, the insulating regime shows the conventional behavior of vanishingly small
conductance at low temperature. However, for thicker quantum wells ( nm), the nominally insulating regime shows a plateau of residual
conductance close to . The residual conductance is independent of the
sample width, indicating that it is caused by edge states. Furthermore, the
residual conductance is destroyed by a small external magnetic field. The
quantum phase transition at the critical thickness, nm, is also
independently determined from the occurrence of a magnetic field induced
insulator to metal transition.Comment: Invited review article for special issue of JPSJ, 32 pages. For
higher resolution figures see official online version when publishe
Evidence of silicene in honeycomb structures of silicon on Ag(111)
In the search for evidence of silicene, a two-dimensional honeycomb lattice
of silicon, it is important to obtain a complete picture for the evolution of
Si structures on Ag(111), which is believed to be the most suitable substrate
for growth of silicene so far. In this work we report the finding and evolution
of several monolayer superstructures of silicon on Ag(111) depending on the
coverage and temperature. Combined with first-principles calculations, the
detailed structures of these phases have been illuminated. These structure were
found to share common building blocks of silicon rings, and they evolve from a
fragment of silicene to a complete monolayer silicene and multilayer silicene.
Our results elucidate how silicene formes on Ag(111) surface and provide
methods to synthesize high-quality and large-scale silicene.Comment: 6 pages, 4 figure
Single valley Dirac fermions in zero-gap HgTe quantum wells
Dirac fermions have been studied intensively in condensed matter physics in
recent years. Many theoretical predictions critically depend on the number of
valleys where the Dirac fermions are realized. In this work, we report the
discovery of a two dimensional system with a single valley Dirac cone. We study
the transport properties of HgTe quantum wells grown at the critical thickness
separating between the topologically trivial and the quantum spin Hall phases.
At high magnetic fields, the quantized Hall plateaus demonstrate the presence
of a single valley Dirac point in this system. In addition, we clearly observe
the linear dispersion of the zero mode spin levels. Also the conductivity at
the Dirac point and its temperature dependence can be understood from single
valley Dirac fermion physics.Comment: version 2: supplementary material adde
Quantum Mechanics Model on K\"ahler conifold
We propose an exactly-solvable model of the quantum oscillator on the class
of K\"ahler spaces (with conic singularities), connected with two-dimensional
complex projective spaces. Its energy spectrum is nondegenerate in the orbital
quantum number, when the space has non-constant curvature. We reduce the model
to a three-dimensional system interacting with the Dirac monopole. Owing to
noncommutativity of the reduction and quantization procedures, the Hamiltonian
of the reduced system gets non-trivial quantum corrections. We transform the
reduced system into a MIC-Kepler-like one and find that quantum corrections
arise only in its energy and coupling constant. We present the exact spectrum
of the generalized MIC-Kepler system. The one-(complex) dimensional analog of
the suggested model is formulated on the Riemann surface over the complex
projective plane and could be interpreted as a system with fractional spin.Comment: 5 pages, RevTeX format, some misprints heve been correcte
New Family of Robust 2D Topological Insulators in van der Waals Heterostructures
We predict a new family of robust two-dimensional (2D) topological insulators
in van der Waals heterostructures comprising graphene and chalcogenides BiTeX
(X=Cl, Br and I). The layered structures of both constituent materials produce
a naturally smooth interface that is conducive to proximity induced new
topological states. First principles calculations reveal intrinsic
topologically nontrivial bulk energy gaps as large as 70-80 meV, which can be
further enhanced up to 120 meV by compression. The strong spin-orbit coupling
in BiTeX has a significant influence on the graphene Dirac states, resulting in
the topologically nontrivial band structure, which is confirmed by calculated
nontrivial Z2 index and an explicit demonstration of metallic edge states. Such
heterostructures offer an unique Dirac transport system that combines the 2D
Dirac states from graphene and 1D Dirac edge states from the topological
insulator, and it offers new ideas for innovative device designs
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