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1. Thermal conductivity of hexagonal BC 2 P – a first-principles study

   https://doi.org/10.1039/d0ra08444a  

   Muthaiah, Rajmohan ; Tarannum, Fatema ; Annam, Roshan Sameer ; Nayal, Avinash Singh ; Danayat, Swapneel ; Garg, Jivtesh ( November 2020 , RSC Advances)

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   In this work, we report a high thermal conductivity ( k ) of 162 W m −1 K −1 and 52 W m −1 K −1 at room temperature, along the directions perpendicular and parallel to the c -axis, respectively, of bulk hexagonal BC 2 P (h-BC 2 P), using first-principles calculations. We systematically investigate elastic constants, phonon group velocities, phonon linewidths and mode thermal conductivity contributions of transverse acoustic (TA), longitudinal acoustic (LA) and optical phonons. Interestingly, optical phonons are found to make a large contribution of 30.1% to the overall k along a direction perpendicular to the c -axis at 300 K. BC 2 P is also found to exhibit high thermal conductivity at nanometer length scales. At 300 K, a high k value of ∼47 W m −1 K −1 is computed for h-BC 2 P at a nanometer length scale of 50 nm, providing avenues for achieving efficient nanoscale heat transfer. 

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2. Field tunable magnetic transitions of CsCo 2 (MoO 4 ) 2 (OH): a triangular chain structure with a frustrated geometry

   https://doi.org/10.1039/d2qm01272c  

   Sanjeewa, Liurukara D. ; Garlea, V. Ovidiu ; Fishman, Randy S. ; Foroughian, Mahsa ; Yin, Li ; Xing, Jie ; Parker, David S. ; Smith Pellizzeri, Tiffany M. ; Sefat, Athena S. ; Kolis, Joseph W. ( March 2023 , Materials Chemistry Frontiers)

   The sawtooth chain compound CsCo 2 (MoO 4 ) 2 (OH) is a complex magnetic system and here, we present a comprehensive series of magnetic and neutron scattering measurements to determine its magnetic phase diagram. The magnetic properties of CsCo 2 (MoO 4 ) 2 (OH) exhibit a strong coupling to the crystal lattice and its magnetic ground state can be easily manipulated by applied magnetic fields. There are two unique Co 2+ ions, base and vertex, with J bb and J bv magnetic exchange. The magnetism is highly anisotropic with the b -axis (chain) along the easy axis and the material orders antiferromagnetically at T N = 5 K. There are two successive metamagnetic transitions, the first at H c 1 = 0.2 kOe into a ferrimagnetic structure, and the other at H c 2 = 20 kOe to a ferromagnetic phase. Heat capacity measurements in various fields support the metamagnetic phase transformations, and the magnetic entropy value is intermediate between S = 3/2 and 1/2 states. The zero field antiferromagnetic phase contains vertex magnetic vectors (Co(1)) aligned parallel to the b -axis, while the base vectors (Co(2)) are canted by 34° and aligned in an opposite direction to the vertex vectors. The spins in parallel adjacent chains align in opposite directions, creating an overall antiferromagnetic structure. At a 3 kOe applied magnetic field, adjacent chains flip by 180° to generate a ferrimagnetic phase. An increase in field gradually induces the Co(1) moment to rotate along the b -axis and align in the same direction with Co(2) generating a ferromagnetic structure. The antiferromagnetic exchange parameters are calculated to be J bb = 0.028 meV and J bv = 0.13 meV, while the interchain exchange parameter is considerably weaker at J ch = (0.0047/ N ch ) meV. Our results demonstrate that the CsCo 2 (MoO 4 ) 2 (OH) is a promising candidate to study new physics associated with sawtooth chain magnetism and it encourages further theoretical studies as well as the synthesis of other sawtooth chain structures with different magnetic ions. 

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3. Chiral-phonon-activated spin Seebeck effect

   https://doi.org/10.1038/s41563-023-01473-9  

   Kim, Kyunghoon ; Vetter, Eric ; Yan, Liang ; Yang, Cong ; Wang, Ziqi ; Sun, Rui ; Yang, Yu ; Comstock, Andrew H. ; Li, Xiao ; Zhou, Jun ; et al ( February 2023 , Nature Materials)

   Utilization of the interaction between spin and heat currents is the central focus of the field of spin caloritronics. Chiral phonons possessing angular momentum arising from the broken symmetry of a non-magnetic material create the potential for generating spin currents at room temperature in response to a thermal gradient, precluding the need for a ferromagnetic contact. Here we show the observation of spin currents generated by chiral phonons in a two-dimensional layered hybrid organic–inorganic perovskite implanted with chiral cations when subjected to a thermal gradient. The generated spin current shows a strong dependence on the chirality of the film and external magnetic fields, of which the coefficient is orders of magnitude larger than that produced by the reported spin Seebeck effect. Our findings indicate the potential of chiral phonons for spin caloritronic applications and offer a new route towards spin generation in the absence of magnetic materials. 

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4. Magneto-thermal conductivity effect and enhanced thermoelectric figure of merit in Ag2Te

   https://doi.org/10.1063/5.0131326  

   Hirata, Keisuke ; Kuga, Kentaro ; Matsunami, Masaharu ; Zhu, Minyue ; Heremans, Joseph P ; Takeuchi, Tsunehiro ( January 2023 , AIP Advances)

   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.

    

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5. Orientation-Controlled Anisotropy in Single Crystals of Quasi-1D BaTiS 3

   https://doi.org/10.1021/acs.chemmater.2c01046  

   Zhao, Boyang ; Hoque, Md Shafkat ; Jung, Gwan Yeong ; Mei, Hongyan ; Singh, Shantanu ; Ren, Guodong ; Milich, Milena ; Zhao, Qinai ; Wang, Nan ; Chen, Huandong ; et al ( June 2022 , Chemistry of Materials)

   Low-dimensional materials with chain-like (one-dimensional) or layered (two-dimensional) structures are of significant interest due to their anisotropic electrical, optical, and thermal properties. One material with a chain-like structure, BaTiS3 (BTS), was recently shown to possess giant in-plane optical anisotropy and glass-like thermal conductivity. To understand the origin of these effects, it is necessary to fully characterize the optical, thermal, and electronic anisotropy of BTS. To this end, BTS crystals with different orientations (a- and c-axis orientations) were grown by chemical vapor transport. X-ray absorption spectroscopy was used to characterize the local structure and electronic anisotropy of BTS. Fourier transform infrared reflection/transmission spectra show a large in-plane optical anisotropy in the a-oriented crystals, while the c-axis oriented crystals were nearly isotropic in-plane. BTS platelet crystals are promising uniaxial materials for infrared optics with their optic axis parallel to the c-axis. The thermal conductivity measurements revealed a thermal anisotropy of ∼4.5 between the c- and a-axis. Time-domain Brillouin scattering showed that the longitudinal sound speed along the two axes is nearly the same, suggesting that the thermal anisotropy is a result of different phonon scattering rates. 

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