More Like this

1. Magnetization switching using topological surface states

   https://doi.org/10.1126/sciadv.aaw3415  

   Li, Peng; Kally, James; Zhang, Steven S.-L.; Pillsbury, Timothy; Ding, Jinjun; Csaba, Gyorgy; Ding, Junjia; Jiang, J. S.; Liu, Yunzhi; Sinclair, Robert; et al (August 2019, Science Advances)

   Topological surface states (TSSs) in a topological insulator are expected to be able to produce a spin-orbit torque that can switch a neighboring ferromagnet. This effect may be absent if the ferromagnet is conductive because it can completely suppress the TSSs, but it should be present if the ferromagnet is insulating. This study reports TSS-induced switching in a bilayer consisting of a topological insulator Bi 2 Se 3 and an insulating ferromagnet BaFe 12 O 19 . A charge current in Bi 2 Se 3 can switch the magnetization in BaFe 12 O 19 up and down. When the magnetization is switched by a field, a current in Bi 2 Se 3 can reduce the switching field by ~4000 Oe. The switching efficiency at 3 K is 300 times higher than at room temperature; it is ~30 times higher than in Pt/BaFe 12 O 19 . These strong effects originate from the presence of more pronounced TSSs at low temperatures due to enhanced surface conductivity and reduced bulk conductivity. 

   more » « less
2. Examining the electron transport in chalcogenide perovskite BaZrS ₃

   https://doi.org/10.1039/d1tc00374g  

   Osei-Agyemang, Eric; Koratkar, Nikhil; Balasubramanian, Ganesh (March 2021, Journal of Materials Chemistry C)

   null (Ed.)

   Orthorhombic BaZrS 3 is a potential optoelectronic material with prospective applications in photovoltaic and thermoelectric devices. While efforts exist on understanding the effects of elemental substitution and material stability, fundamental knowledge on the electronic transport properties are sparse. We employ first principles calculations to examine the electronic band structure and optical band gap and interrogate the effect of electron transport on electrical and thermal conductivities, and Seebeck coefficient, as a function of temperature and chemical potential. Our results reveal that BaZrS 3 has a band gap of 1.79 eV in proximity of the optimal 1.35 eV recommended for single junction photovoltaics. An absorption coefficient of 3 × 10 5 cm −1 at photon energies of 3 eV is coupled with an early onset to optical absorption at 0.5 eV, significantly below the optical band gap. The carrier effective mass being lower for electrons than holes, we find the Seebeck coefficient to be higher for holes than electrons. A notable (≈1.0 at 300 K) upper limit to the thermoelectric figure of merit, obtained due to high Seebeck coefficient (3000 μV K −1 ) and ultra-low electron thermal conductivity, builds promise for BaZrS 3 as a thermoelectric. 

   more » « less
3. Measured and simulated thermoelectric properties of FeAs _2−x Se _x ( x = 0.30–1.0): from marcasite to arsenopyrite structure

   https://doi.org/10.1039/d0ma00371a  

   Perez, Christopher J.; Devlin, Kasey P.; Skaggs, Callista M.; Tan, Xiaoyan; Frank, Corey E.; Badger, Jackson R.; Kang, Chang-Jong; Emge, Thomas J.; Kauzlarich, Susan M.; Taufour, Valentin; et al (August 2020, Materials Advances)

   null (Ed.)

   FeAs 2−x Se x ( x = 0.30–1.0) samples were synthesized as phase pure powders by conventional solid-state techniques and as single crystals ( x = 0.50) from chemical vapor transport. The composition of the crystals was determined to be Fe 1.025(3) As 1.55(3) Se 0.42(3) , crystallizing in the marcasite structure type, Pnnm space group. FeAs 2−x Se x (0 < x < 1) was found to undergo a marcasite-to-arsenopyrite ( P 2 1 / c space group) structural phase transition at x ∼ 0.65. The structures are similar, with the marcasite structure best described as a solid solution of As/Se, whereas the arsenopyrite has ordered anion sites. Magnetic susceptibility and thermoelectric property measurements from 300–2 K were performed on single crystals, FeAs 1.50 Se 0.50 . Paramagnetic behavior is observed from 300 to 17 K and a Seebeck coefficient of −33 μV K −1 , an electrical resistivity of 4.07 mΩ cm, and a very low κ l of 0.22 W m −1 K −1 at 300 K are observed. In order to determine the impact of the structural transition on the high-temperature thermoelectric properties, polycrystalline FeAs 2−x Se x ( x = 0.30, 0.75, 0.85, 1.0) samples were consolidated into dense pellets for measurements of thermoelectric properties. The x = 0.85 sample shows the best thermoelectric performance. The electronic structure of FeAsSe was calculated with DFT and transport properties were approximately modeled above 500 K. 

   more » « less
4. Deep tuning of photo-thermoelectricity in topological surface states

   https://doi.org/10.1038/s41598-020-73950-z  

   Huang, Shouyuan; Miotkowski, Ireneusz; Chen, Yong P.; Xu, Xianfan (December 2020, Scientific Reports)

   null (Ed.)

   Abstract Three-dimensional topological insulators have been demonstrated in recent years, which possess intriguing gapless, spin-polarized Dirac states with linear dispersion only on the surface. The spin polarization of the topological surface states is also locked to its momentum, which allows controlling motion of electrons using optical helicity, i.e., circularly polarized light. The electrical and thermal transport can also be significantly tuned by the helicity-control of surface state electrons. Here, we report studies of photo-thermoelectric effect of the topological surface states in Bi 2 Te 2 Se thin films with large tunability using varied gate voltages and optical helicity. The Seebeck coefficient can be altered by more than five times compared to the case without spin injection. This deep tuning is originated from the optical helicity-induced photocurrent which is shown to be enhanced, reduced, turned off, and even inverted due to the change of the accessed band structures by electrical gating. The helicity-selected topological surface state thus has a large effect on thermoelectric transport, demonstrating great opportunities for realizing helicity control of optoelectronic and thermal devices. 

   more » « less
5. Topological phase and thermoelectric properties of bialkali bismuthide compounds (Na, K) ₂ RbBi from first-principles

   https://doi.org/10.1088/1361-648X/ac431d  

   Yalameha, Shahram; Nourbakhsh, Zahra; Vashaee, Daryoosh (December 2021, Journal of Physics: Condensed Matter)

   Abstract We report the topological phase and thermoelectric properties of bialkali bismuthide compounds (Na, K) 2 RbBi, as yet hypothetical. The topological phase transitions of these compounds under hydrostatic pressure are investigated. The calculated topological surface states and Z 2 topological index confirm the nontrivial topological phase. The electronic properties and transport coefficients are obtained using the density functional theory combined with the Boltzmann transport equation. The relaxation times are determined using the deformation potential theory to calculate the electronic thermal and electrical conductivity. The calculated mode Grüneisen parameters are substantial, indicating strong anharmonic acoustic phonons scattering, which results in an exceptionally low lattice thermal conductivity. These compounds also have a favorable power factor leading to a relatively flat p-type figure-of-merit over a broad temperature range. Furthermore, the mechanical properties and phonon band dispersions show that these structures are mechanically and dynamically stable. Therefore, they offer excellent candidates for practical applications over a wide range of temperatures. 

   more » « less