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BoltzTraP. A code for calculating band-structure dependent quantities

日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

Physical Review B

Herein we cover the key concepts in the field of thermoelectric materials research, present the current understanding, and show the latest developments. Current research is aimed at increasing the thermoelectric figure of merit (ZT) by maximizing the power factor and/or minimizing the thermal conductivity. Attempts at maximizing the power factor include the development of new materials, optimization of existing materials by doping, and the exploration of nanoscale materials. The minimization of the thermal conductivity can come through solid-solution alloying, use of materials with intrinsically low thermal conductivity, and nanostructuring. Herein we describe the most promising bulk materials with emphasis on results from the last decade. Single-phase bulk materials are discussed in terms of chemistry, crystal structure, physical properties, and optimization of thermoelectric performance. The new opportunities for enhanced performance bulk nanostructured composite materials are examined and a look into the not so distant future is attempted.

New and Old Concepts in Thermoelectric Materials

A quantum critical point (QCP) develops in a material at absolute zero when a new form of order smoothly emerges in its ground state. QCPs are of great current interest because of their singular ability to influence the finite temperature properties of materials. Recently, heavy-fermion metals have played a key role in the study of antiferromagnetic QCPs. To accommodate the heavy electrons, the Fermi surface of the heavy-fermion paramagnet is larger than that of an antiferromagnet1,2,3. An important unsolved question is whether the Fermi surface transformation at the QCP develops gradually, as expected if the magnetism is of spin-density-wave (SDW) type4,5, or suddenly, as expected if the heavy electrons are abruptly localized by magnetism6,7,8. Here we report measurements of the low-temperature Hall coefficient (RH)—a measure of the Fermi surface volume—in the heavy-fermion metal YbRh2Si2 upon field-tuning it from an antiferromagnetic to a paramagnetic state. RH undergoes an increasingly rapid change near the QCP as the temperature is lowered, extrapolating to a sudden jump in the zero temperature limit. We interpret these results in terms of a collapse of the large Fermi surface and of the heavy-fermion state itself precisely at the QCP.

https://arxiv.org/pdf/cond-mat/0411074.pdf

Hall-effect evolution across a heavy-fermion quantum critical point.

${\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ is the only clathrate known so far where the guest positions are fully occupied by a rare-earth element. Our investigations show that, in addition to the previously synthesized ${\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ modification with clathrate-I structure, there exists a second modification with clathrate-VIII structure. Polycrystalline samples of both phases behave as local-moment ferromagnets with relatively low Curie temperatures (10.5 and 36 K). The charge-carrier concentrations are rather small (3.8 and $12.5\ifmmode\times\else\texttimes\fi{}{10}^{20} {\mathrm{cm}}^{\ensuremath{-}3}$ at 2 K) and, together with the low Curie temperatures, point to a semimetallic behavior. Both the specific heat and the thermal conductivity are consistent with the concept of guest atoms ``rattling'' in oversized host cages, leading to low thermal conductivities (``phonon glasses''). However, the electron mobilities are quite low, which, if intrinsic, would question the properties of an ``electron crystal'', commonly presumed in ``filled-cage'' materials. The dimensionless thermoelectric figure of merit reaches values of 0.01 at 100 K.

/pdf/structural-transport-magnetic-and-thermal-properties-of-eu-8-4seprzbq4f.pdf

Structural, transport, magnetic, and thermal properties of Eu 8 Ga 16 Ge 30

We report thermodynamic measurements in a magnetic-field-driven quantum critical point of a heavy fermion metal, YbRh2Si2. The data provide evidence for an energy scale in the equilibrium excitation spectrum that is in addition to the one expected from the slow fluctuations of the order parameter. Both energy scales approach zero as the quantum critical point is reached, thereby providing evidence for a new class of quantum criticality.

https://science.sciencemag.org/content/sci/315/5814/969.full.pdf

Multiple Energy Scales at a Quantum Critical Point

This paper addresses some important questions concerning hopping conduction in doped conducting polymers, i.e., dimensionality, homogeneity, adiabaticity, Coulomb interactions, and multiphonon character. It will be shown that electron transport in these materials is due to correlated hopping between polaronic clusters. We propose a model which describes quantitatively this process. Conductivity and magnetoresistance measurements performed on disordered polypyrroles doped with polyelectrolytes are presented in support of these ideas. This model emphasizes the role of the dopant as an attractive potential and as a tunneling bridge between neighboring chains. The dopant ions are the source of both the polaronic clusters stability and of the transverse conduction.

Hopping in disordered conducting polymers

Transport properties of ${\mathrm{Ba}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ single-crystalline samples prepared with an excess of Ga are presented. The excess Ga does not only produces p-type transport properties but also a thermal conductivity (\ensuremath{\kappa}) that, unlike in any other ${\mathrm{Ba}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ samples reported in the literature, is similar to that of the iso-structural ${\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ and ${\mathrm{Sr}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}.$ These observations disagree with the commonly made assumption that \ensuremath{\kappa} of ${\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ and ${\mathrm{Sr}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ at very low temperatures is determined by phonons scattered from guest atom tunneling states, since such states are believed to be absent in ${\mathrm{Ba}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}.$ Instead we propose that phonon charge carrier scattering must be considered in order to explain \ensuremath{\kappa} at the low temperatures. The transport data also suggest that the resonant scattering, which dominates at intermediate temperatures, strongly depends on the charge carrier concentration.

/pdf/thermal-conductivity-of-thermoelectric-clathrates-27784xik35.pdf

Silke Paschen

Papers

Hall-effect evolution across a heavy-fermion quantum critical point.

Structural, transport, magnetic, and thermal properties of Eu 8 Ga 16 Ge 30

Multiple Energy Scales at a Quantum Critical Point

Hopping in disordered conducting polymers

Thermal conductivity of thermoelectric clathrates