629 research outputs found
Charge and Spin Reconstruction in Quantum Hall Strips
We study the effect of electron-electron interactions on the charge and spin
structures of a Quantum Hall strip in a triangularly confined potential. We
find that the strip undergoes a spin-unpolarized to spin-polarized transition
as a function of magnetic field perpendicular to the strip. For sharp
confinements the spin-polarization transition is spontaneous and first develops
at the softer side of the triangular potential which shows up as an
"eye-structure" in the electron dispersion. For sufficiently weak confinements
this spin-polarization transition is preceded by a charge reconstruction of a
single spin species, which creates a spin-polarized strip of electrons with a
width of the order of the magnetic length detached from the rest of the system.
Relevance of our findings to the recent momentum resolved tunneling experiments
is also discussed.Comment: 4+ page
Stability of the k=3 Read-Rezayi state in chiral two-dimensional systems with tunable interactions
The k=3 Read-Rezayi (RR) parafermion quantum Hall state hosts non-Abelian
excitations which provide a platform for the universal topological quantum
computation. Although the RR state may be realized at the filling factor
\nu=12/5 in GaAs-based two-dimensional electron systems, the corresponding
quantum Hall state is weak and at present nearly impossible to study
experimentally. Here we argue that the RR state can alternatively be realized
in a class of chiral materials with massless and massive Dirac-like band
structure. This family of materials encompasses monolayer and bilayer graphene,
as well as topological insulators. We show that, compared to GaAs, these
systems provide several important advantages in realizing and studying the RR
state. Most importantly, the effective interactions can be tuned {\it in situ}
by varying the external magnetic field, and by designing the dielectric
environment of the sample. This tunability enables the realization of RR state
with controllable energy gaps in different Landau levels. It also allows one to
probe the quantum phase transitions to other compressible and incompressible
phases.Comment: 12 pages, 5 figures; to appear in New Journal of Physics, Focus on
Topological Quantum Computatio
Spin - Phonon Coupling in Nickel Oxide Determined from Ultraviolet Raman Spectroscopy
Nickel oxide (NiO) has been studied extensively for various applications
ranging from electrochemistry to solar cells [1,2]. In recent years, NiO
attracted much attention as an antiferromagnetic (AF) insulator material for
spintronic devices [3-10]. Understanding the spin - phonon coupling in NiO is a
key to its functionalization, and enabling AF spintronics' promise of
ultra-high-speed and low-power dissipation [11,12]. However, despite its status
as an exemplary AF insulator and a benchmark material for the study of
correlated electron systems, little is known about the spin - phonon
interaction, and the associated energy dissipation channel, in NiO. In
addition, there is a long-standing controversy over the large discrepancies
between the experimental and theoretical values for the electron, phonon, and
magnon energies in NiO [13-23]. This gap in knowledge is explained by NiO
optical selection rules, high Neel temperature and dominance of the magnon band
in the visible Raman spectrum, which precludes a conventional approach for
investigating such interaction. Here we show that by using ultraviolet (UV)
Raman spectroscopy one can extract the spin - phonon coupling coefficients in
NiO. We established that unlike in other materials, the spins of Ni atoms
interact more strongly with the longitudinal optical (LO) phonons than with the
transverse optical (TO) phonons, and produce opposite effects on the phonon
energies. The peculiarities of the spin - phonon coupling are consistent with
the trends given by density functional theory calculations. The obtained
results shed light on the nature of the spin - phonon coupling in AF insulators
and may help in developing innovative spintronic devices.Comment: 16 pages; 2 figure
Numerical studies of the fractional quantum Hall effect in systems with tunable interactions
The discovery of the fractional quantum Hall effect in GaAs-based
semiconductor devices has lead to new advances in condensed matter physics, in
particular the possibility for exotic, topological phases of matter that
possess fractional, and even non-Abelian, statistics of quasiparticles. One of
the main limitations of the experimental systems based on GaAs has been the
lack of tunability of the effective interactions between two-dimensional
electrons, which made it difficult to stabilize some of the more fragile
states, or induce phase transitions in a controlled manner. Here we review the
recent studies that have explored the effects of tunability of the interactions
offered by alternative two-dimensional systems, characterized by non-trivial
Berry phases and including graphene, bilayer graphene and topological
insulators. The tunability in these systems is achieved via external fields
that change the mass gap, or by screening via dielectric plate in the vicinity
of the device. Our study points to a number of different ways to manipulate the
effective interactions, and engineer phase transitions between quantum Hall
liquids and compressible states in a controlled manner.Comment: 9 pages, 4 figures, updated references; review for the CCP2011
conference, to appear in "Journal of Physics: Conference Series
Quantum Hall Effects in Graphene-Based Two-Dimensional Electron Systems
In this article we review the quantum Hall physics of graphene based
two-dimensional electron systems, with a special focus on recent experimental
and theoretical developments. We explain why graphene and bilayer graphene can
be viewed respectively as J=1 and J=2 chiral two-dimensional electron gases
(C2DEGs), and why this property frames their quantum Hall physics. The current
status of experimental and theoretical work on the role of electron-electron
interactions is reviewed at length with an emphasis on unresolved issues in the
field, including assessing the role of disorder in current experimental
results. Special attention is given to the interesting low magnetic field limit
and to the relationship between quantum Hall effects and the spontaneous
anomalous Hall effects that might occur in bilayer graphene systems in the
absence of a magnetic field
Tunable interactions and phase transitions in Dirac materials in a magnetic field
A partially filled Landau level (LL) hosts a variety of correlated states of
matter with unique properties. The ability to control these phases requires
tuning the effective electron interactions within a LL, which has been
difficult to achieve in GaAs-based structures. Here we consider a class of
Dirac materials in which the chiral band structure, along with the mass term,
gives rise to a wide tunability of the effective interactions by the magnetic
field. This tunability is such that different phases can occur in a single LL,
and phase transitions between them can be driven in situ. The incompressible,
Abelian and non-Abelian, liquids are stabilized in interaction regimes
different from GaAs. Our study points to a realistic method of controlling the
correlated phases and studying the phase transitions between them in materials
such as graphene, bilayer graphene, and topological insulators.Comment: 4 pages, 3 figures; supersedes earlier versio
Energy gaps at neutrality point in bilayer graphene in a magnetic field
Utilizing the Baym-Kadanoff formalism with the polarization function
calculated in the random phase approximation, the dynamics of the
quantum Hall state in bilayer graphene is analyzed. Two phases with nonzero
energy gap, the ferromagnetic and layer asymmetric ones, are found. The phase
diagram in the plane , where is a
top-bottom gates voltage imbalance, is described. It is shown that the energy
gap scales linearly, $\Delta E\sim 14 B[T]K, with magnetic field.Comment: 5 pages, 3 figures, title changed, references added, JETP Letters
versio
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