8 research outputs found
Efficient linear scaling method for computing the thermal conductivity of disordered materials
An efficient order real-space Kubo approach is developed for the
calculation of the thermal conductivity of complex disordered materials. The
method, which is based on the Chebyshev polynomial expansion of the time
evolution operator and the Lanczos tridiagonalization scheme, efficiently
treats the propagation of phonon wave-packets in real-space and the phonon
diffusion coefficients. The mean free paths and the thermal conductance can be
determined from the diffusion coefficients. These quantities can be extracted
simultaneously for all frequencies, which is another advantage in comparison
with the Green's function based approaches. Additionally, multiple scattering
phenomena can be followed through the time dependence of the diffusion
coefficient deep into the diffusive regime, and the onset of weak or strong
phonon localization could possibly be revealed at low temperatures for thermal
insulators. The accuracy of our computational scheme is demonstrated by
comparing the calculated phonon mean free paths in isotope-disordered carbon
nanotubes with Landauer simulations and analytical results. Then, the
upscalibility of the method is illustrated by exploring the phonon mean free
paths and the thermal conductance features of edge disordered graphene
nanoribbons having widths of 20 nanometers and lengths as long as a
micrometer, which are beyond the reach of other numerical techniques. It is
shown that, the phonon mean free paths of armchair nanoribbons are smaller than
those of zigzag nanoribbons for the frequency range which dominate the thermal
conductance at low temperatures. This computational strategy is applicable to
higher dimensional systems, as well as to a wide range of materials
Phonon transport in large scale carbon-based disordered materials: Implementation of an efficient order-N and real-space Kubo methodology
We have developed an efficient order-N real-space Kubo approach for the
calculation of the phonon conductivity which outperforms state-of-the-art
alternative implementations based on the Green's function formalism. The method
treats efficiently the time-dependent propagation of phonon wave packets in
real space, and this dynamics is related to the calculation of the thermal
conductance. Without loss of generality, we validate the accuracy of the method
by comparing the calculated phonon mean free paths in disordered carbon
nanotubes (isotope impurities) with other approaches, and further illustrate
its upscalability by exploring the thermal conductance features in large width
edge-disordered graphene nanoribbons (up to ~20 nm), which is out of the reach
of more conventional techniques. We show that edge-disorder is the most
important scattering mechanism for phonons in graphene nanoribbons with
realistic sizes and thermal conductance can be reduced by a factor of ~10.Comment: Accepted for publication in Physical Review B - Rapid Communication
Topological Signatures in the Electronic Structure of Graphene Spirals
Topology is familiar mostly from mathematics, but also natural sciences have
found its concepts useful. Those concepts have been used to explain several
natural phenomena in biology and physics, and they are particularly relevant
for the electronic structure description of topological insulators and graphene
systems. Here, we introduce topologically distinct graphene forms - graphene
spirals - and employ density-functional theory to investigate their geometric
and electronic properties. We found that the spiral topology gives rise to an
intrinsic Rashba spin-orbit splitting. Through a Hamiltonian constrained by
space curvature, graphene spirals have topologically protected states due to
time-reversal symmetry. In addition, we argue that the synthesis of such
graphene spirals is feasible and can be achieved through advanced bottom-up
experimental routes that we indicate in this work
Graphene: Piecing it together
Graphene has a multitude of striking properties that make it an exceedingly
attractive material for various applications, many of which will emerge over
the next decade. However, one of the most promising applications lie in
exploiting its peculiar electronic properties which are governed by its
electrons obeying a linear dispersion relation. This leads to the observation
of half integer quantum hall effect and the absence of localization. The latter
is attractive for graphene-based field effect transistors. However, if graphene
is to be the material for future electronics, then significant hurdles need to
be surmounted, namely, it needs to be mass produced in an economically viable
manner and be of high crystalline quality with no or virtually no defects or
grains boundaries. Moreover, it will need to be processable with atomic
precision. Hence, the future of graphene as a material for electronic based
devices will depend heavily on our ability to piece graphene together as a
single crystal and define its edges with atomic precision. In this progress
report, the properties of graphene that make it so attractive as a material for
electronics is introduced to the reader. The focus then centers on current
synthesis strategies for graphene and their weaknesses in terms of electronics
applications are highlighted.Comment: Advanced Materials (2011
The peculiar potential of transition metal dichalcogenides for thermoelectric applications : a perspective on future computational research
A bottom-up route to enhance thermoelectric figures of merit in graphene nanoribbons
We propose a hybrid nano-structuring scheme for tailoring thermal and thermoelectric transport properties of graphene nanoribbons. Geometrical structuring and isotope cluster engineering are the elements that constitute the proposed scheme. Using first-principles based force constants and Hamiltonians, we show that the thermal conductance of graphene nanoribbons can be reduced by 98.8% at room temperature and the thermoelectric figure of merit, ZT, can be as high as 3.25 at T = 800 K. The proposed scheme relies on a recently developed bottom-up fabrication method, which is proven to be feasible for synthesizing graphene nanoribbons with an atomic precision