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Low-Temperature Thermoelectric Materials
Published in Zhifeng Ren, Yucheng Lan, Qinyong Zhang, Advanced Thermoelectrics, 2017
Koen Vandaele, Joseph P. Heremans, Yiming Zhou, Li-Dong Zhao, Huaizhou Zhao, Zhifeng Ren, Machhindra Koirala, Stephen R. Boona
Many of the largest S values observed within this class of materials are found in solid solutions of homogeneously intermediate valence (IV) compounds (e.g., CePd3117 and Ce(Sn1−xInx)3118), in which the valence state of each Ce atom rapidly fluctuates in time. The origin of these anomalously large S values is associated with free carriers scattering in and out of the Ce 4f states, which nearly overlap with the Fermi level EF. This overlap results in an enormous DOS over a small energy range for free carriers, which generally corresponds with anomalously large electronic heat capacity (and therefore entropy) per carrier in these systems,119 along with large charge-carrier effective mass (“heavy fermions”). The complexity of these electron–electron interactions make it almost impossible to search reliably for promising new RE intermetallic materials using ab initio computational approaches. In some materials, s–d and s–f hybridization can cause EF to fall within a semiconductor-like energy gap. This results in the formation of a Kondo insulator120 state that behaves more like a conventional semiconductor at low temperatures, as seen in Ce3Pt3Sb4121 and CeRu4P12.122 While these materials can display S values significantly larger than 130 μV/K, they do not display metallic ρ.
Possible observation of Kondo screening cloud in Yb14MnSb11 *
Published in Philosophical Magazine, 2020
Brian C. Sales, V. O. Garlea, M.B. Stone, M. D. Lumsden, S. E. Nagler, D. Mandrus, M. A. McGuire
For over 50 years there has been sustained interest in how conduction electrons screen local magnetic moments, a process normally referred to as the Kondo effect [1,2]. In metals with a dilute concentration of magnetic impurities the problem is well understood and models that capture the essence of the many body physics have been solved exactly [3–5]. The Kondo effect is at the heart of heavy fermion physics and is related to many problems in the physics of strongly correlated materials. Recent interest has focussed on combining Kondo physics and topology. There have been proposals of a topological Kondo effect with Majorana fermions [6] and examples of using spin–orbit coupling to tune a Kondo insulator into a correlated Weyl semimetal [7,8].
Crystal growth, low-temperature specific heat, and electronic structure of the filled skutterudite compound ThOs4As12
Published in Philosophical Magazine, 2020
J. Juraszek, K. Rola, M. Daszkiewicz, M. Samsel-Czekała, T. Cichorek, Z. Henkie
As seen in Figure 3(b), the tops of valence bands are created predominantly by the As-4p and Os-5d orbitals, while the partial DOS originating from thorium is minor in this energy region. Interestingly, a hybridisation between transition-metal d states and pnictogen p states causes not only a strong covalent bonding in the skutterudite CoAs, but also a small density of states around the Fermi level due to a single band crossing the pseudogap [4]. Such a predicted valence-band electronic structure is in good agreement with the results of x-ray photoelectron spectroscopy that show experimental evidence for the intrinsic contribution of the single band to the DOS at [5]. And lastly, we note that the ab initio calculations for CeOsAs have predicted this cerium-filled skutterudite as a zero-gap material. While Yan et al. [26] have argued that CeOsAs may become topological Kondo insulator at low temperatures, Shankar et al. [47] have shown that an implementation of an advanced Becke and Johnson exchange potential results in opening a very narrow gap near due to the hybridisation of Ce-4f states with the conduction electrons. The Ce-4f states are localised and form a sharp peak just above . Similarly to ThOsAs, the valence band maximum is located at the Γ point and is formed by As-4p and Os-5d as well as Ce-5d states.