Classification and Prediction of Skyrmion Material Based on Machine Learning.

The discovery and study of skyrmion materials play an important role in basic frontier physics research and future information technology. The database of 196 materials, including 64 skyrmions, was established and predicted based on machine learning. A variety of intrinsic features are classified to optimize the model, and more than a dozen methods had been used to estimate the existence of skyrmion in magnetic materials, such as support vector machines, k-nearest neighbor, and ensembles of trees. It is found that magnetic materials can be more accurately divided into skyrmion and non-skyrmion classes by using the classification of electronic layer. Note that the rare earths are the key elements affecting the production of skyrmion. The accuracy and reliability of random undersampling bagged trees were 87.5% and 0.89, respectively, which have the potential to build a reliable machine learning model from small data. The existence of skyrmions in LaBaMnO is predicted by the trained model and verified by micromagnetic theory and experiments.

Dramatically enhanced Seebeck coefficient in GeMnTe2-NaBiTe2 alloys by tuning the Spin's thermodynamic entropy.

The emerging material GeMnTe2 provides a rare example to study the spin degree of freedom in thermoelectric transport, as it exhibits an anomalous Seebeck coefficient driven by the spin's thermodynamic entropy. This work presents an unconventional strategy to optimize the thermoelectric performance of GeMnTe2 by manipulating the spin degree of freedom. NaBiTe2 is alloyed into GeMnTe2 to disorder the spin orientation under finite temperature, and the obtained Seebeck coefficient is confirmed to be dramatically enhanced by more than 150%. The measurements of XRD and magnetic susceptibility indicate that the increased Seebeck coefficient is due to the increase of the spin's thermodynamic entropy. Finally, the maximum ZT of 1.06 at 820 K is obtained in Ge0.8Na0.1Bi0.1MnTe2. This work enriches the physical picture of spin degree of freedom in thermoelectric materials.

Anomalous Thermopower and High ZT in GeMnTe2 Driven by Spin's Thermodynamic Entropy.

Na x CoO2 was known 20 years ago as a unique example in which spin entropy dominates the thermoelectric behavior. Hitherto, however, little has been learned about how to manipulate the spin degree of freedom in thermoelectrics. Here, we report the enhanced thermoelectric performance of GeMnTe2 by controlling the spin's thermodynamic entropy. The anomalously large thermopower of GeMnTe2 is demonstrated to originate from the disordering of spin orientation under finite temperature. Based on the careful analysis of Heisenberg model, it is indicated that the spin-system entropy can be tuned by modifying the hybridization between Te-p and Mn-d orbitals. As a consequent strategy, Se doping enlarges the thermopower effectively, while neither carrier concentration nor band gap is affected. The measurement of magnetic susceptibility provides a solid evidence for the inherent relationship between the spin's thermodynamic entropy and thermopower. By further introducing Bi doing, the maximum ZT in Ge0.94Bi0.06MnTe1.94Se0.06 reaches 1.4 at 840 K, which is 45% higher than the previous report of Bi-doped GeMnTe2. This work reveals the high thermoelectric performance of GeMnTe2 and also provides an insightful understanding of the spin degree of freedom in thermoelectrics.

Understanding the effect of thickness on the thermoelectric properties of Ca3Co4O9 thin films.

We are reporting the effect of thickness on the Seebeck coefficient, electrical conductivity and power factor of Ca3Co4O9 thin films grown on single-crystal Sapphire (0001) substrate. Pulsed laser deposition (PLD) technique was employed to deposit Ca3Co4O9 films with precisely controlled thickness values ranging from 15 to 75 nm. Structural characterization performed by scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed that the growth of Ca3Co4O9 on Sapphire (0001) follows the island growth-mode. It was observed that in-plane grain sizes decrease from 126 to 31 nm as the thickness of the films decreases from 75 to 15 nm. The thermoelectric power measurements showed an overall increase in the value of the Seebeck coefficient as the films' thickness decreased. The above increase in the Seebeck coefficient was accompanied with a simultaneous decrease in the electrical conductivity of the thinner films due to enhanced scattering of the charge carriers at the grain boundaries. Because of the competing mechanisms of the thickness dependence of Seebeck coefficient and electrical conductivity, the power factor of the films showed a non-monotonous functional dependence on thickness. The films with the intermediate thickness (60 nm) showed the highest power factor (~ 0.27 mW/m-K2 at 720 K).

Enhanced Thermoelectric Properties of p-Type Bi0.48Sb1.52Te3/Sb2Te3 Composite.

Constructing a nanocomposite to introduce a coherent interface is an effective way to improve the property of thermoelectric material. Here, a series composites of Bi0.48Sb1.52Te3-x wt % Sb2Te3 (x = 0, 0.3, 0.5, 0.8, and 1.0) were synthesized, where the hydrothermally prepared Sb2Te3 nanosheets were intimately wrapped in the solid-state-reacted Bi0.48Sb1.52Te3 matrix. The formation of a coherent interface was observed and confirmed by the scanning electron microscopy characterization. As the Sb2Te3 content was over 0.5 wt %, the carrier mobility could increase by 26%, while the carrier concentration decreased by 9% compared to those of the pure matrix at 300 K, enhancing the power factor to 40.1 µW/cm K2. Moreover, the Bi0.48Sb1.52Te3-0.5 wt % Sb2Te3 sample exhibited a reduced lattice thermal conductivity of 0.83 W/m K at room temperature, resulting from the strengthened phonon scattering by interfaces. Combined with the manipulations of both the electronic and thermal transport by constructing a coherent interface, a maximum ZT of 1.05 was obtained in the x = 0.5 composite at 300 K, and it was improved by 20% compared with the result of the Bi0.48Sb1.52Te3 matrix.

Phonon Engineering for Thermoelectric Enhancement of p-Type Bismuth Telluride by a Hot-Pressing Texture Method.

Phonon engineering is a core stratagem to improve the thermoelectric performance, and multi-scale defects are expected to scatter a broad range of phonons and compress the lattice thermal conductivity. Here, we demonstrate obviously enhanced thermoelectric properties in Bi0.48Sb1.52Te3 alloy by a hot-pressing texture method along the axial direction of a zone-melted ingot. It is found that a plastic deformation of grain refinement and rearrangement occurs during the textured pressing process. Although the obtained power factor is slightly decreased, a large amount of grain boundaries emerges in the textured samples and dense dislocations are observed around the boundaries and inside the grains. These additional phonon scattering centers can effectively scatter the low- and mid-frequency phonons, and the corresponding lattice thermal conductivity is significantly reduced to only 50% of that of zone-melted samples. Consequently, the maximum figure of merit (ZT) reaches 1.44 at 330 K and the average ZT (300-380 K) reaches 1.38. This study suggests that the simple hot-pressing texture technique is a promising method to significantly optimize the cooling capacity of Bi0.48Sb1.52Te3-based thermoelectric refrigeration.

Terbium Ion Doping in Ca3Co4O9: A Step towards High-Performance Thermoelectric Materials.

The potential of thermoelectric materials to generate electricity from the waste heat can play a key role in achieving a global sustainable energy future. In order to proceed in this direction, it is essential to have thermoelectric materials that are environmentally friendly and exhibit high figure of merit, ZT. Oxide thermoelectric materials are considered ideal for such applications. High thermoelectric performance has been reported in single crystals of Ca3Co4O9. However, for large scale applications single crystals are not suitable and it is essential to develop high-performance polycrystalline thermoelectric materials. In polycrystalline form, Ca3Co4O9 is known to exhibit much weaker thermoelectric response than in single crystal form. Here, we report the observation of enhanced thermoelectric response in polycrystalline Ca3Co4O9 on doping Tb ions in the material. Polycrystalline Ca3-xTbxCo4O9 (x = 0.0-0.7) samples were prepared by a solid-state reaction technique. Samples were thoroughly characterized using several state of the art techniques including XRD, TEM, SEM and XPS. Temperature dependent Seebeck coefficient, electrical resistivity and thermal conductivity measurements were performed. A record ZT of 0.74 at 800 K was observed for Tb doped Ca3Co4O9 which is the highest value observed till date in any polycrystalline sample of this system.