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1. Designing Spin‐Crossover Systems to Enhance Thermopower and Thermoelectric Figure‐of‐Merit in Paramagnetic Materials

   https://doi.org/10.1002/eem2.12822  

   Polash, Md_Mobarak Hossain; Stone, Matthew; Chi, Songxue; Vashaee, Daryoosh (August 2024, ENERGY & ENVIRONMENTAL MATERIALS)

   Thermoelectric materials, capable of converting temperature gradients into electrical power, have been traditionally limited by a trade‐off between thermopower and electrical conductivity. This study introduces a novel, broadly applicable approach that enhances both the spin‐driven thermopower and the thermoelectric figure‐of‐merit (zT) without compromising electrical conductivity, using temperature‐driven spin crossover. Our approach, supported by both theoretical and experimental evidence, is demonstrated through a case study of chromium doped‐manganese telluride, but is not confined to this material and can be extended to other magnetic materials. By introducing dopants to create a high crystal field and exploiting the entropy changes associated with temperature‐driven spin crossover, we achieved a significant increase in thermopower, by approximately 136 μV K⁻¹, representing more than a 200% enhancement at elevated temperatures within the paramagnetic domain. Our exploration of the bipolar semiconducting nature of these materials reveals that suppressing bipolar magnon/paramagnon‐drag thermopower is key to understanding and utilizing spin crossover‐driven thermopower. These findings, validated by inelastic neutron scattering, X‐ray photoemission spectroscopy, thermal transport, and energy conversion measurements, shed light on crucial material design parameters. We provide a comprehensive framework that analyzes the interplay between spin entropy, hopping transport, and magnon/paramagnon lifetimes, paving the way for the development of high‐performance spin‐driven thermoelectric materials. 

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2. Magnon-drag thermopower in antiferromagnets versus ferromagnets

   https://doi.org/10.1039/C9TC06330G  

   Polash, Md. Mobarak; Mohaddes, Farzad; Rasoulianboroujeni, Morteza; Vashaee, Daryoosh (March 2020, Journal of Materials Chemistry C)

   The extension of magnon electron drag (MED) to the paramagnetic domain has recently shown that it can create a thermopower more significant than the classical diffusion thermopower resulting in a thermoelectric figure-of-merit greater than unity. Due to their distinct nature, ferromagnetic (FM) and antiferromagnetic (AFM) magnons interact differently with the carriers and generate different amounts of drag-thermopower. The question arises of whether the MED is stronger in FM or in AFM semiconductors. Two material systems, namely MnSb and CrSb, which are similar in many aspects except that the former is FM and the latter AFM, were studied in detail, and their MED properties were compared. Three features of AFMs compared to FMs, namely double degeneracy of the magnon modes, higher magnon group velocity, and longer magnon relaxation time, can lead to enhanced first-order MED thermopower. One effect, magnon–electron relaxation, leads to a higher second-order effect in AFMs that reduces the MED thermopower. However, it is generally expected that the first-order effect dominates and leads to a higher drag thermopower in AFMs, as seen in this case study. 

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3. Time-resolved magneto-optical effects in the altermagnet candidate MnTe

   https://doi.org/10.1063/5.0244878  

   Gray, Isaiah; Deng, Qinwen; Tian, Qi; Chilcote, Michael; Dodge, J Steven; Brahlek, Matthew; Wu, Liang (November 2024, Applied Physics Letters)

   α -MnTe is an antiferromagnetic semiconductor with above room temperature TN = 310 K, which is promising for spintronic applications. Recently, it was reported to be an altermagnet, containing bands with momentum-dependent spin splitting; time-resolved experimental probes of MnTe are, therefore, important both for understanding novel magnetic properties and potential device applications. We investigate ultrafast spin dynamics in epitaxial MnTe(001)/InP(111) thin films using pump-probe magneto-optical measurements in the Kerr configuration. At room temperature, we observe an oscillation mode at 55 GHz that does not appear at zero magnetic field. Combining field and polarization dependence, we identify this mode as a magnon, likely originating from inverse stimulated Raman scattering. Magnetic field-dependent oscillations persist up to at least 335 K, which could reflect coupling to known short-range magnetic order in MnTe above TN. Additionally, we observe two optical phonons at 3.6 and 4.2 THz, which broaden and redshift with increasing temperature. 

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4. Doping molecular organic semiconductors by diffusion from the vapor phase

   https://doi.org/10.1039/D0QM00442A  

   Peterson, Kelly A.; Patterson, Ashlea; Vega-Flick, Alejandro; Liao, Bolin; Chabinyc, Michael L. (January 2020, Materials Chemistry Frontiers)

   Thin films of amorphous small molecule semiconductors are widely used in organic light emitting displays and have promising applications in solar cells and thermoelectric devices. Adding dopants increases the conductivity of organic semiconductors, but high concentrations of dopants can disrupt their structural ordering, alter the shape of the electronic density of states in the material, and increase the effects of Coulomb interactions on charge transport. Electrical doping of the solution processable hole-transport material 2,2′,7,7′-tetrakis[ N , N -di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD) was studied with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ) as a p-type dopant. Infiltration of F 4 TCNQ from the vapor phase into films of spiro-OMeTAD provided a route to highly doped films with up to 39 ± 2 mol% doping. Structural characterization confirmed that the films remain amorphous even at the highest doping levels with no apparent phase separation. We quantitatively determined the carrier concentration using UV-Vis spectroscopy to interpret the evolution of the electrical conductivity. Over the range of carrier concentrations (10 19 –10 20 1 cm −3 ), the electrical conductivity increased no more than linearly with carrier concentration, while the thermopower had a small increase with carrier concentration. The trends in conductivity and thermopower were related to the unique electronic structure of spiro-OMeTAD, which is able to support two carriers per molecule. Temperature-dependent conductivity measurements were used to further analyze the transport mechanism. 

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5. Anomalous quantum criticality in the electron-doped cuprates

   https://doi.org/10.1073/pnas.1817653116  

   Mandal, P. R.; Sarkar, Tarapada; Greene, Richard L. (March 2019, Proceedings of the National Academy of Sciences)

   In the physics of condensed matter, quantum critical phenomena and unconventional superconductivity are two major themes. In electron-doped cuprates, the low critical field (H C2 ) allows one to study the putative quantum critical point (QCP) at low temperature and to understand its connection to the long-standing problem of the origin of the high- T C superconductivity. Here we present measurements of the low-temperature normal-state thermopower ( S ) of the electron-doped cuprate superconductor La 2− x Ce x CuO 4 (LCCO) from x = 0.11–0.19. We observe quantum critical S / T versus l n ( 1 / T ) behavior over an unexpectedly wide doping range x = 0.15–0.17 above the QCP ( x = 0.14), with a slope that scales monotonically with the superconducting transition temperature ( T C with H = 0). The presence of quantum criticality over a wide doping range provides a window on the criticality. The thermopower behavior also suggests that the critical fluctuations are linked with T C . Above the superconductivity dome, at x = 0.19, a conventional Fermi-liquid S ∝ T behavior is found for T ≤ 40 K. 

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