Also known as the degeneracy valley parameter (Nv) and defined as the number of distinct Fermi surfaces at the Fermi level, is directly related to the thermoelectric performance of a material. Typical values of degeneracy of the band extrema (Nv) for TE materials are in the range of 6 to 8. My question is how can one compute the Nv values? Any suggestion would be highly helpful. Thanks in advance.
Usually the symmetry of the Fermi surface is consistent with the symmetry of the crystal lattice. Therefore, I would increase the number of valleys in accordance with the order of the axes of symmetry in a given direction.
The calculation of electronic and thermoelectric properties from electronic band structure
has received much attention lately with the increasing availability of computational algorithms and
resources. Ab-initio calculations are very important from a materials’ design perspective in that
they provide insight into the underlying electronic states which give rise to the experimentally
measurable thermoelectric properties. However, the correlation between calculated properties
and experimental results is usually qualitative at best. Therefore, it is important to understand the
methods that experimentalists use to characterize the thermoelectric performance, specifically in
the context of what the results indicate about the electronic band structure. In this chapter, I
develop a new metric for determining the viability of thermoelectric materials whose electronic
band structures have been computed using ab-initio techniques. The approach is rooted in the
Boltzmann transport equation, but is based on semi-empirical band engineering models and
techniques which have been discussed in detail in this thesis (Single Parabolic Band model,
Chapter 2). I will also give examples where band engineering (and degeneracy) simply cannot
capture the behavior, which is most often observed in systems where additional topological
features (in addition to individual isolated carrier pockets) arise. I apply the technique over a large
database of compounds to show its validity in high-throughput screening of thermoelectric
materials, allowing computations to overcome the limitations of the constant relaxation time
approximation (CRTA).
Dear Joshi,
The valley degeneracy is determined from the energy band structure of your material. When different valleys are overlapping at their extreme points (e.g., the top of different valence bands).
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