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Topological insulators and semimetals have been shown to possess intriguing thermoelectric properties promising for energy harvesting and cooling applications. However, thermoelectric transport associated with the Fermi arc topological surface states on topological Dirac semimetals remains less explored. This work systematically examines thermoelectric transport in a series of topological Dirac semimetal Cd₃As₂thin films grown by molecular beam epitaxy. Surprisingly, significantly enhanced Seebeck effect and anomalous Nernst effect are found at cryogenic temperatures when the Cd₃As₂layer is thin. In particular, a peak Seebeck coefficient of nearly 500 µV K⁻¹and a corresponding thermoelectric power factor over 30 mW K⁻² m⁻¹are observed at 5 K in a 25‐nm‐thick sample. Combining angle‐dependent quantum oscillation analysis, magnetothermoelectric measurement, transport modeling, and first‐principles simulation, the contributions from bulk and surface conducting channels are isolated and the unusual thermoelectric properties are attributed to the topological surface states. The analysis showcases the rich thermoelectric transport physics in quantum‐confined topological Dirac semimetal thin films and suggests new routes to achieving high thermoelectric performance at cryogenic temperatures.

 

(Bi,Sb)2(Te,Se)3 tetradymite materials are among the most efficient for thermoelectric energy conversion, and most robust for topological insulator spintronic technologies, but should possess rather disparate doping properties to be useful for either technology. In this work, we report results on the molecular beam epitaxy growth of p-type (Bi0.43Sb0.57)2Te3 and n-type Bi2(Te0.95Se0.05)3 that can contribute to both technology bases, but are especially useful for topological insulators where low bulk doping is critical for devices to leverage the Dirac-like topological surface states. Comprehensive temperature, field and angular dependent magnetotransport measurements have attested to the superior quality of these ternary tetradymite films, displaying low carrier density on the order of 1018 cm–3 and a record high mobility exceeding 104 cm2 V–1 s–1 at 2 K. The remarkable manifestation of strong Shubnikov–de Haas (SdH) quantum oscillation under 9 T at liquid helium temperatures, as well as the analyses therein, has allowed direct experimental investigation of the tetradymite electronic structure with optimized ternary alloying ratio. Our effort substantiates tetradymites as a critical platform for miniaturized thermoelectric cooling and power generation in wearable consumer electronics, as well as for futuristic topological spintronics with unprecedented magnetoelectric functionalities. 

In this study, we report a large magneto-thermal conductivity effect, potentially usable in heat flow switches and thermoelectric devices, in Ag2Te over a wide temperature range, including room temperature. When a magnetic field of μ0H = 9 T is applied to Ag2Te at 300 K along the direction perpendicular to the heat and electric currents, the thermal conductivity κ decreases by a remarkable 61%. This effect is mainly caused by the suppressed electronic thermal conductivity in association with a significant magnetoresistance effect, but the suppression of the thermal conductivity is larger than that of the electrical conductivity, presumably due to a field-induced decrease in the Lorenz ratio. Its very low lattice thermal conductivity, as low as 0.5 W m−1 K−1, also greatly contributes to the large relative magneto-thermal conductivity effect. The significant decrease in thermal conductivity and the 18% increase in the Seebeck coefficient S lead to a nearly 100% increase in the thermoelectric figure of merit zT = S2σTκ−1 despite the 43% decrease in electrical conductivity σ.

 

This article shows experimentally that an external electric field affects the velocity of the longitudinal acoustic phonons (v_LA), thermal conductivity (κ), and diffusivity (D) in a bulk lead zirconium titanate–based ferroelectric. Phonon conduction dominates κ, and the observations are due to changes in the phonon dispersion, not in the phonon scattering. This gives insight into the nature of the thermal fluctuations in ferroelectrics, namely, phonons labeled ferrons that carry heat and polarization. It also opens the way for phonon-based electrically driven all-solid-state heat switches, an enabling technology for solid-state heat engines. A quantitative theoretical model combining piezoelectric strain and phonon anharmonicity explains the field dependence ofv_LA, κ, andDwithout any adjustable parameters, thus connecting thermodynamic equilibrium properties with transport properties. The effect is four times larger than previously reported effects, which were ascribed to field-dependent scattering of phonons.

 

The exploration of quantum materials in which an applied thermo/electrical/magnetic field along one crystallographic direction produces an anisotropic response has led to unique functionalities. Along these lines, KMgBi is a layered, narrow gap semiconductor near a critical state between multiple Dirac phases due to the presence of a flat band near the Fermi level. The valence band is highly anisotropic with minimal cross‐plane dispersion, which, in combination with an isotropic conduction band, enables axis‐dependent conduction polarity. Thermopower and Hall measurements indicate dominant p‐type conduction along the cross‐plane direction, and n‐type conduction along the in‐plane direction, leading to a significant zero‐field transverse thermoelectric response when the heat flux is at an angle to the principal crystallographic directions. Additionally, a large Ordinary Nernst effect (ONE) is observed with an applied field.  It arises from the ambipolar term in the Nernst effect, whereby the Lorentz force on electrons and holes makes them drift in opposite directions so that the resulting Nernst voltage becomes a function of the difference between their partial thermopowers, greatly enhancing the ONE. It is proven that axis‐dependent polarity can synergistically enhance the ONE, in addition to leading to a zero‐field transverse thermoelectric performance.