Yung-Kang Kuo

We present a comparative study of the thermoelectric (TE) properties of single-crystalline quasi-skutterudites M5Rh6Sn18 (M = Sc, Y, Lu), based on temperature-dependent in-plane measurements of electrical resistivity ρ(T), Seebeck coefficient S(T), and thermal conductivity κ(T). X-ray diffraction analysis confirms a tetragonal structure and suggests the presence of structural disorder. The ρ(T) data reveal poor metallic behavior for M = Sc, nonmetallic-like behavior for M = Y, and intermediate characteristics for M = Lu. The nonmetallic-like ρ(T) in M = Y and Lu can be phenomenologically described by the Mott variable-range hopping model over limited temperature ranges, suggesting disorder-induced carrier localization. Negative S(T) and its nearly linear high-temperature variation indicate electron-dominated diffusive TE transport. The κ(T) data reveal a balance between electronic and lattice contributions, with the lattice κ remaining low (≤2 W m−1 K−1 at 300 K). The modified Debye–Callaway analysis of the lattice component indicates significant contributions from point-defect scattering and effective scattering associated with internal structural disorder. Despite the low lattice thermal conductivity, the overall TE performance is modest, with a maximum power factor (PF) of ∼0.9 μW cm−1 K−2 and a figure of merit (ZT) ∼0.005 at 300 K for M = Lu. These findings highlight the impact of disorder and defects on both charge and phonon transport in this material family.

The magnetic Weyl semimetal Co 3 Sn 2 S 2 has garnered significant experimental and theoretical interest, particularly regarding its electronic structure and transport properties. In this study, we comprehensively investigated the effects of Fe and Ni substitution in Co 3 Sn 2 S 2 . The temperature-dependent Seebeck coefficient ( S ) implies that the reduction of the band gap and the shift in the Fermi level ( E F ) due to Fe and Ni substitution, which was supported by angle-resolved photoemission spectroscopy results. These band-structure modifications influence not only thermoelectric (TE) performance but also the anomalous Hall conductivity ( σ xy A ). Specifically, Fe substitution increases σ xy A via skew scattering, while Ni substitution increases σ xy A via intrinsic deflection with minor skew scattering. Additionally, the Fe and Ni dopant-induced changes in the c / a ratio were demonstrated using high-resolution powder synchrotron x-ray diffraction, which explains the different modulation in the magnetic anisotropy of Co 3 Sn 2 S 2 due to the magnetostriction effect. Notably, the oxidation states of Co, Fe, and Ni in each single crystal were confirmed as 0 using x-ray absorption near-edge structure analysis, further explaining the dopant-induced suppression in ferromagnetism of Co 3 Sn 2 S 2 . These findings provide deeper insight into the band-filling effects on both TE and anomalous Hall transport in Co 3 Sn 2 S 2 , offering perspectives for topological spintronics and TE applications.

The limitation of the single crystal melt growth method to tune the microstructure of the materials in a controlled way and the need for enhancing the thermoelectric properties of single crystal grown Bismuth telluride (Bi ₂ Te ₃ ), through defect and microstructural engineering, has motivated this work. In this work, we address this limitation through a controlled doping strategy using antimony (Sb) and selenium (Se) to introduce targeted defects and microstructural modifications within single-crystalline Bi ₂ Te ₃ . Sb and Se substitutions create atomic scale strain, point defects, and micro-grain structures, enhancing phonon scattering without significantly disrupting the crystalline order. The resulting defect-engineered single crystals exhibit improved thermoelectric performance, with a notable reduction in lattice thermal conductivity and retention of excellent electrical properties. The co-doped compositions, Bi ₂ Te _2.7 Se _0.3 and (Bi0.98Sb _0.02 ) ₂ Te _2.7 Se _0.3 , exhibited significantly enhanced thermoelectric performance, with Seebeck coefficients reaching ~ 253 μV/K and − 211 μV/K, respectively, over the 10–400 K range. The power factor improved remarkably, showing a ~ 30-fold increase for Bi ₂ Te _2.7 Se _0.3 and ~ 20-fold for the Sb-doped variant, while the figure of merit (ZT) improved by ~ 28.5 and ~ 14 times, respectively. Further, a flexible thermoelectric device fabricated from these optimized materials generated output power of 2.7 nW and 3.35 nW at ambient temperature. The non-monotonic variation of the Seebeck coefficient with Sb content, showing an optimal enhancement at x = 0.04, highlights the delicate balance between carrier concentration and band structure modification, emphasizing moderate Sb substitution achieves the most favorable conditions for thermoelectric performance. Our results present a scalable strategy for bridging the performance gap between pristine single crystals and heavily nanostructured thermoelectrics, opening new avenues for high-efficiency energy harvesting devices.

Summary of the methodology and interdependency results of thermoelectric properties of Bi _1–8 Sb _0.2 Te ₃ / x % Ni ( x = 0–5) compounds.

Kagome metal ScV6Sn6 has been a subject of interest due to the emergence of a first-order structural phase transition with intriguing charge density wave behavior below the transition temperature Tc ∼ 92 K. To explore the thermoelectric properties and provide experimental insights into the nature of the phase transition, we have carried out a combined study by means of the electrical resistivity, Seebeck coefficient, and thermal conductivity measurements on single crystalline ScV6Sn6. Pronounced features near Tc have been characterized by all measured physical quantities. In particular, the Seebeck coefficient exhibits a marked reduction as lowering temperature across Tc, attributed to an imbalance of the contribution from different type of carriers induced by the structural phase transition. From the examination of the electronic and lattice thermal conductivities, we obtained a confirmation that the observed enhancement at Tc is essentially caused by the change of the lattice thermal conductivity, demonstrating the primary importance of lattice distortions for the heat transport of ScV6Sn6. In addition, the lattice thermal conductivity above Tc was found to increase monotonically with temperature. We associated the peculiar phenomenon with lattice fluctuations, highlighting the essence of structural instability in the kagome lattice ScV6Sn6. These results add to the knowledge about the thermal transport properties in kagome materials with a hexagonal HfFe6Ge6-type structure.

We investigated the thermal-history dependence of physical properties in a quenched Fe5−xGeTe2 (x ∼ 0.16) single crystal by measuring magnetization (M), electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) as a function of temperature (T). The results reveal anomalies in these physical quantities around various transition points: ferromagnetic (TC ∼ 310–300 K), helimagnetic (TH ∼ 275 K), charge ordering (TCO ∼ 165 K), spin-reorientation (T* ∼ 100–120 K), and a Fermi-liquid (FL) phase below TL ∼ 35 K. Using power-law fitting, the M(T) analysis near TC shows that Fe moments become primarily itinerant after thermal cycling. The ρ(T) results indicate inherent residual stresses in the crystal that alter with thermal cycling, influencing ferromagnetic domain formations within grain boundaries. The system exhibits a strongly correlated FL behavior at low temperatures, which ceases above TL due to spin fluctuations. In the T-range of T* ≤ T ≤ TCO, ρ(T) and S(T) analyses suggest significant electronic band structure modifications with multiband effects. The κ(T) data indicate phonon-dominated heat transport in the crystal, with a phonon behavior influenced by inherent lattice strains following initial thermal cycles, as evidenced by the decreased phonon peak height at low temperatures. In addition, there is evidence of phonon localization and electron–phonon coupling at higher temperatures.