723 research outputs found
Integer quantum Hall effect and Hofstadter's butterfly spectra in three-dimensional metals in external periodic modulations
We propose that Hofstadter's butterfly accompanied by quantum Hall effect
that is similar to those predicted to occur in 3D tight-binding systems by
Koshino {\it et al.} [Phys. Rev. Lett. {\bf 86}, 1062 (2001)] can be realized
in an entirely different system -- 3D metals applied with weak external
periodic modulations (e.g., acoustic waves). Namely, an effect of two periodic
potentials interferes with Landau's quantization due to an applied magnetic
field \Vec{B}, resulting generally in fractal energy gaps as a function of
the tilting angle of \Vec{B}, for which the accompanying quantized Hall
tensors are computed. The phenomenon arises from the fact that, while the
present system has a different physical origin for the butterfly from the 3D
tight-binding systems, the mathematical forms are remarkably equivalent.Comment: 4 pages, 2 figure
Duality and integer quantum Hall effect in isotropic 3D crystals
We show here a series of energy gaps as in Hofstadter's butterfly, which have
been shown to exist by Koshino et al [Phys. Rev. Lett. 86, 1062 (2001)] for
anisotropic three-dimensional (3D) periodic systems in magnetic fields
\Vec{B}, also arise in the isotropic case unless \Vec{B} points in
high-symmetry directions. Accompanying integer quantum Hall conductivities
can, surprisingly, take values
even for a fixed direction of \Vec{B}
unlike in the anisotropic case. We can intuitively explain the high-magnetic
field spectra and the 3D QHE in terms of quantum mechanical hopping by
introducing a ``duality'', which connects the 3D system in a strong \Vec{B}
with another problem in a weak magnetic field .Comment: 7 pages, 6 figure
Trigonal warping and Berry’s phase Npi in ABC-stacked multilayer graphene.
The electronic band structure of ABC-stacked multilayer graphene is studied within an effective mass approximation. The electron and hole bands touching at zero energy support chiral quasiparticles characterized by Berry’s phase Nπ for N-layers, generalizing the low-energy band structure of monolayer and bilayer graphene. We investigate the trigonal-warping deformation of the energy bands and show that the Lifshitz transition, in which the Fermi circle breaks up into separate parts at low energy, reflects Berry’s phase Nπ. It is particularly prominent in trilayers, N = 3, with the Fermi circle breaking into three parts at a relatively large energy that is related to next-nearestlayer coupling. For N = 3, we study the effects of electrostatic potentials which vary in the stacking direction, and find that a perpendicular electric field, as well as opening an energy gap, strongly enhances the trigonal-warping effect. In magnetic fields, the N = 3 Lifshitz transition is manifested as a coalescence of Landau levels into triply-degenerate levels
Transport in Bilayer Graphene: Calculations within a self-consistent Born approximation
The transport properties of a bilayer graphene are studied theoretically
within a self-consistent Born approximation. The electronic spectrum is
composed of -linear dispersion in the low-energy region and -square
dispersion as in an ordinary two-dimensional metal at high energy, leading to a
crossover between different behaviors in the conductivity on changing the Fermi
energy or disorder strengths. We find that the conductivity approaches
per spin in the strong-disorder regime, independently of the
short- or long-range disorder.Comment: 8 pages, 5 figure
Magnetic field screening and mirroring in graphene
The orbital magnetism in spatially varying magnetic fields is studied in
monolayer graphene within the effective mass approximation. We find that,
unlike the conventional two-dimensional electron system, graphene with small
Fermi wave number k_F works as a magnetic shield where the field produced by a
magnetic object placed above graphene is always screened by a constant factor
on the other side of graphene. The object is repelled by a diamagnetic force
from the graphene, as if there exists its mirror image with a reduced amplitude
on the other side of graphene. The magnitude of the force is much greater than
that of conventional two-dimensional system. The effect disappears with the
increase of k_F.Comment: 5 pages, 3 figure
Two-photon nonlinearity in general cavity QED systems
We have investigated the two-photon nonlinearity at general cavity QED
systems, which covers both weak and strong coupling regimes and includes
radiative loss from the atom. The one- and two-photon propagators are obtained
in analytic forms. By surveying both coupling regimes, we have revealed the
conditions on the photonic wavepacket for yielding large nonlinearity depending
on the cavity Q-value. We have also discussed the effect of radiative loss on
the nonlinearity.Comment: 8 pages, 5 figure
Quantum Zeno and anti-Zeno effects by indirect measurement with finite errors
We study the quantum Zeno effect and the anti-Zeno effect in the case of
`indirect' measurements, where a measuring apparatus does not act directly on
an unstable system, for a realistic model with finite errors in the
measurement. A general and simple formula for the decay rate of the unstable
system under measurement is derived. In the case of a Lorentzian form factor,
we calculate the full time evolutions of the decay rate, the response of the
measuring apparatus, and the probability of errors in the measurement. It is
shown that not only the response time but also the detection efficiency plays a
crucial role. We present the prescription for observing the quantum Zeno and
anti-Zeno effects, as well as the prescriptions for avoiding or calibrating
these effects in general experiments.Comment: 4 pages, 3 figure
Moir\'{e} phonons in graphene/hexagonal boron nitride moir\'e superlattice
We theoretically study in-plane acoustic phonons of graphene/hexagonal boron
nitride moir\'e superlattice by using a continuum model. We demonstrate that
the original phonon bands of individual layers are strongly hybridized and
reconstructed into moir\'e phonon bands consisting of dispersive bands and flat
bands. The phonon band structure can be effectively described by a spring-mass
network model to simulate the motion of moir\'e domain walls, where the
flat-band modes are interpreted as vibrations of independent, decoupled
strings. We also show that the moir\'e phonon has angular momentum due to the
inversion symmetry breaking by hBN, with high amplitudes concentrated near
narrow gap region. Finally, we apply the same approach to twisted bilayer
graphene, and we find a notable difference between the origins of the flat-band
modes in G/hBN and TBG, reflecting distinct geometric structures of domain
pattern.Comment: 13 pages, 11 figure
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