204 research outputs found
Splitting of the Zero-Energy Landau Level and Universal Dissipative Conductivity at Critical Points in Disordered Graphene
We report on robust features of the longitudinal conductivity ()
of the graphene zero-energy Landau level in presence of disorder and varying
magnetic fields. By mixing an Anderson disorder potential with a low density of
sublattice impurities, the transition from metallic to insulating states is
theoretically explored as a function of Landau-level splitting, using highly
efficient real-space methods to compute the Kubo conductivities (both
and Hall ). As long as valley-degeneracy is
maintained, the obtained critical conductivity
is robust upon disorder increase (by almost one order of magnitude) and
magnetic fields ranging from about 2 to 200 Tesla. When the sublattice symmetry
is broken, eventually vanishes at the Dirac point owing to
localization effects, whereas the critical conductivities of pseudospin-split
states (dictating the width of a plateau) change to
, regardless of the splitting strength, superimposed
disorder, or magnetic strength. These findings point towards the non
dissipative nature of the quantum Hall effect in disordered graphene in
presence of Landau level splitting
Efficient Linear Scaling Approach for Computing the Kubo Hall Conductivity
We report an order-N approach to compute the Kubo Hall conductivity for
disorderd two-dimensional systems reaching tens of millions of orbitals, and
realistic values of the applied external magnetic fields (as low as a few
Tesla). A time-evolution scheme is employed to evaluate the Hall conductivity
using a wavepacket propagation method and a continued fraction
expansion for the computation of diagonal and off-diagonal matrix elements of
the Green functions. The validity of the method is demonstrated by comparison
of results with brute-force diagonalization of the Kubo formula, using
(disordered) graphene as system of study. This approach to mesoscopic system
sizes is opening an unprecedented perspective for so-called reverse engineering
in which the available experimental transport data are used to get a deeper
understanding of the microscopic structure of the samples. Besides, this will
not only allow addressing subtle issues in terms of resistance standardization
of large scale materials (such as wafer scale polycrystalline graphene), but
will also enable the discovery of new quantum transport phenomena in complex
two-dimensional materials, out of reach with classical methods.Comment: submitted PRB pape
Unconventional Features in the Quantum Hall Regime of Disordered Graphene: Percolating Impurity States and Hall Conductance Quantization
We report on the formation of critical states in disordered graphene, at the
origin of variable and unconventional transport properties in the quantum Hall
regime, such as a zero-energy Hall conductance plateau in the absence of an
energy bandgap and Landau level degeneracy breaking. By using efficient
real-space transport methodologies, we compute both the dissipative and Hall
conductivities of large size graphene sheets with random distribution of model
single and double vacancies. By analyzing the scaling of transport coefficients
with defect density, system size and magnetic length, we elucidate the origin
of anomalous quantum Hall features as magnetic-field dependent impurity states,
which percolate at some critical energies. These findings shed light on
unidentified states and quantum transport anomalies reported experimentally.Comment: 7 pages, 7 figures. Accepted in PR
Linear Scaling Approach for Optical Excitations Using Maximally Localized Wannier Functions
We present a theoretical method for calculating optical absorption spectra
based on maximally localized Wannier functions, which is suitable for large
periodic systems. For this purpose, we calculate the exciton Hamiltonian, which
determines the Bethe-Salpeter equation for the macroscopic polarization
function and optical absorption characteristics. The Wannier functions are
specific to each material and provide a minimal and therefore computationally
convenient basis. Furthermore, their strong localization greatly improves the
computational performance in two ways: first, the resulting Hamiltonian becomes
very sparse and, second, the electron-hole interaction terms can be evaluated
efficiently in real space, where large electron-hole distances are handled by a
multipole expansion. For the calculation of optical spectra we employ the
sparse exciton Hamiltonian in a time-domain approach, which scales linearly
with system size. We demonstrate the method for bulk silicon - one of the most
frequently studied benchmark systems - and envision calculating optical
properties of systems with much larger and more complex unit cells, which are
presently computationally prohibitive.Comment: submitted to J. Phys. Mate
Polaron Transport in Organic Crystals: Temperature Tuning of Disorder Effects
We explore polaronic quantum transport in three-dimensional models of
disordered organic crystals with strong coupling between electronic and
vibrational degrees of freedom. By studying the polaron dynamics in a static
disorder environment, temperature dependent mobilities are extracted and found
to exhibit different fingerprints depending on the strength of the disorder
potential. At low temperatures and for strong enough disorder, coherence
effects induce weak localization of polarons. These effects are reduced with
increasing temperature (thermal disorder) resulting in mobility increase.
However at a transition temperature, phonon-assisted contributions driven by
polaron-phonon scattering prevail, provoking a downturn of the mobility. The
results provide an alternative scenario to discuss controversial experimental
features in molecular crystals
Magnetoresistance in Disordered Graphene: The Role of Pseudospin and Dimensionality Effects Unraveled
We report a theoretical low-field magnetotransport study unveiling the effect
of pseudospin in realistic models of weakly disordered graphene-based
materials. Using an efficient Kubo computational method, and simulating the
effect of charges trapped in the oxide, different magnetoconductance
fingerprints are numerically obtained in system sizes as large as 0.3
micronmeter squared, containing tens of millions of carbon atoms. In
two-dimensional graphene, a strong valley mixing is found to irreparably yield
a positive magnetoconductance (weak localization), whereas crossovers from
positive to a negative magnetoconductance (weak antilocalization) are obtained
by reducing disorder strength down to the ballistic limit. In sharp contrast,
graphene nanoribbons with lateral size as large as 10nm show no sign of weak
antilocalization, even for very small disorder strength. Our results
rationalize the emergence of a complex phase diagram of magnetoconductance
fingerprints, shedding some new light on the microscopical origin of pseudospin
effects.Comment: 8 pages, 5 figure
Charge Transport in Organic Crystals
The understanding of charge transport is one of the central goals in the research on semiconducting crystals. For organic crystals this is particularly complicated due to the strength of the electron-phonon interaction which requires the description of a seamless transition between the limiting cases of a coherent band-transport mechanism and incoherent hopping.
In this thesis, charge transport phenomena in organic crystals are studied by theoretical means.
A theory for charge transport in organic crystals is developed which covers the whole temperature range from low T, where it reproduces an expression from the Boltzmann equation for band transport, via elevated T, where it generalizes Holstein's small-polaron theory to finite bandwidths, up to high T, for which a temperature dependence equal to Marcus' electron-transfer theory is obtained. Thereby, coherent band transport and thermally induced hopping are treated on equal footing while simultaneously treating the electron-phonon interaction non-perturbatively. By avoiding the approximation of narrow polaron bands the theory allows for the description of large and small polarons and serves as a starting point for computational studies.
The theoretical description is completed by using ab initio material parameters for the selected crystals under study. These material parameters are taken from density functional theory calculations for durene, naphthalene, and guanine crystals. Besides the analysis of the transport mechanism, special focus is put on the study of the relationship between mobility anisotropy and structure of the crystals. This study is supported by a 3D-visualization method for the transport channels in such crystals which has been derived in this thesis
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