65 research outputs found
Tunable Electron Interactions and Fractional Quantum Hall States in Graphene
The recent discovery of fractional quantum Hall states in graphene raises the
question of whether the physics of graphene and its bilayer offers any
advantages over GaAs-based materials in exploring strongly-correlated states of
two-dimensional electrons. Here we propose a method to continuously tune the
effective electron interactions in graphene and its bilayer by the dielectric
environment of the sample. Using this method, the charge gaps of prominent FQH
states, including \nu=1/3 or \nu=5/2 states, can be increased several times, or
reduced all the way to zero. The tunability of the interactions can be used to
realize and stabilize various strongly correlated phases in the FQH regime, and
to explore the transitions between them.Comment: 4.2 pages, 5 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
Band mass anisotropy and the intrinsic metric of fractional quantum Hall systems
It was recently pointed out that topological liquid phases arising in the
fractional quantum Hall effect (FQHE) are not required to be rotationally
invariant, as most variational wavefunctions proposed to date have been.
Instead, they possess a geometric degree of freedom corresponding to a shear
deformation that acts like an intrinsic metric. We apply this idea to a system
with an anisotropic band mass, as is intrinsically the case in many-valley
semiconductors such as AlAs and Si, or in isotropic systems like GaAs in the
presence of a tilted magnetic field, which breaks the rotational invariance. We
perform exact diagonalization calculations with periodic boundary conditions
(torus geometry) for various filling fractions in the lowest, first and second
Landau levels. In the lowest Landau level, we demonstrate that FQHE states
generally survive the breakdown of rotational invariance by moderate values of
the band mass anisotropy. At 1/3 filling, we generate a variational family of
Laughlin wavefunctions parametrized by the metric degree of freedom. We show
that the intrinsic metric of the Laughlin state adjusts as the band mass
anisotropy or the dielectric tensor are varied, while the phase remains robust.
In the n=1 Landau level, mass anisotropy drives transitions between
incompressible liquids and compressible states with charge density wave
ordering. In n>=2 Landau levels, mass anisotropy selects and enhances stripe
ordering with compatible wave vectors at partial 1/3 and 1/2 fillings.Comment: 9 pages, 8 figure
Interferometric probes of many-body localization
We propose a method for detecting many-body localization (MBL) in disordered
spin systems. The method involves pulsed, coherent spin manipulations that
probe the dephasing of a given spin due to its entanglement with a set of
distant spins. It allows one to distinguish the MBL phase from a
non-interacting localized phase and a delocalized phase. In particular, we show
that for a properly chosen pulse sequence the MBL phase exhibits a
characteristic power-law decay reflecting its slow growth of entanglement. We
find that this power-law decay is robust with respect to thermal and disorder
averaging, provide numerical simulations supporting our results, and discuss
possible experimental realizations in solid-state and cold atom systems.Comment: 5 pages, 4 figure
Fleet renewal: An approach to achieve sustainable road transport
With more stringent requirements for efficient utilization of energy resources within the transport industry a need for implementation of sustainable development principles has appeared. Such action will be one of competitive advantages in the future. This is especially confirmed within the road transport sector. A methodology implemented in public procurement procedures for fleet renewal regarding the calculation of road vehiclesā operational lifecycle costs has been analyzed in detail in this paper. Afore mentioned calculation comprises the costs for: vehicle ownership, energy, carbon dioxide and pollutants emissions. Implementation of this methodology allows making the choice of energy efficient vehicles and vehicles with notable positive environmental effects. The objective of the research is to assess the influence of specific parameters of vehicle operational lifecycle costs, especially energy costs and estimated vehicle energy consumption, on vehicle choice in the procurement procedure. The case of urban bus fleet in Serbia was analyzed. Their operational lifecycle costs were calculated and differently powered vehicles were assessed. Energy consumption input values were defined. It was proved that defined fleet renewal scenarios could influence unquestionable decrease in energy consumption
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
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Comparative analysis of objective techniques for criteria weighing in two MCDM methods on example of an air conditioner selection
This paper deals with comparative analysis of two different types of objective techniques for criteria weighing: Entropy and CRITIC and two MCDM methods: MOORA and SAW on example of an air conditioner selection. We used six variants for calculation of normalized performance ratings. Results showed that the decision of the best air conditioner was basically independent of the MCDM method used, despite the applied technique for determination of criteria weights. Complete ranking within all of the combinations of methods and techniques with diverse ratio calculation variants showed that the best ranked air conditioner was A7, while the worst ones were A5 and A9. Significant positive correlation was obtained for almost all the pairs of variants in all the combinations except for the MOORA - CRITIC combination with SAW - Entropy combination to have the highest correlations between variants (p < 0.01)
A Method to Evaluate and Compare Two Different Intraoral Radiographs of the Same Patient
Objective of this study was to determine the accuracy of the method of the clinical
intraoral densitometry, to compare differences in the calculation with or without subtraction
of the background adjacent soft-tissues from the stepwedge (SW) and to verify
which regression model best fitted the experimental data in order to express the measured
values in equivalents of SW thickness. Two intraoral radiographs, one after another,
were made for each of 6 patients. A copper SW (6 steps, thickness 0.05ā0.3 mm)
was attached to each radiograph, trying to avoid the superimposition of the bony structures.
Films were processed and digitized. Grey levels were measured on each step of the
SW, on the background of the SW and on the same 3 randomly chosen regions of interest
(ROIs) on each digitized image. The measurement with and without the subtraction of
optical densities of the background around the SW from the optical densities of the SW
was performed. For the calculation of the SW thickness equivalents, the regression
analysis was performed by using different regression models. The best fitting regression
model was the 3rd degree polynomial. The results were more precise when using the subtraction
of the background overlapping the SW
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