Magnetothermal Transport and Elastoresistive Properties of Low-Dimensional Magnetoelectrics and Dichalcogenides

Co₄Nb₂O₉ is a quasi two dimensional material that has been known to exhibit magnetoelectric behavior with an antiferromagnetic transition (T[subscript N]) at ∼ 27 K. Our findings reveal strong evidence of magnon heat transport below T[subscript N] through magnetothermal conductivity measurements. Magnetothermal coupling is strongest below 2 K around 0.2 T suggesting presence of the thermal heat-valve effect causing an increase in its thermal conductivity by nearly 4-fold. Independent of field, we not only see the presence of phonon-magnon resonant scattering but also strong phonon-spin coupling; giving rise to an anisotropic thermal conductivity where within the plane of spins conductivity is greater than inter-plane. Thermal measurements were also carried out on Ba₃Cr₂O₈ which has been shown to undergo Bose-Einstein condensation of magnetic excitations. Through intercalation with copper and palladium atoms between the layers of the transition metal dichalcogenides titanium diselenide (TiSe₂), it has previously been shown that where the intrinsic charge density wave’s (CDW) temperature dependent resistivity peak anomaly occurs (T[subscript cdw] ∼165 K) decreases with increased intercalation and/or hydrostatic pressure. To mimic the chemical pressure caused by intercalation, uniaxial physical pressure (both compression and expansion) is used along with the Poisson effect. In the experiment with compressive pressure perpendicular to the layers (c-axis), resistivity data shows a shift in the peak (T[subscript cdw]) towards lower temperatures; expected for increasing dimensionality. For uniaxial pressure parallel to the layers, the Poisson effect should separate the layers, providing a separation of the layers similar to that of intercalation. Preliminary results show T[subscript cdw] increases initially, and then decreases. A prototype to induce uniaxial tension perpendicular to the layers has recently been developed and preliminary results will be reported. Electrical transport measurements were also executed for the first time on the first FE(II)-TCNQ spin cross-over (SCO) system ever synthesized. In fact, our material is the first structurally defined magnetically bistable semiconductor to be constructed from TCNQ radical ions. With the aid of spider silk fibers (detailed in further sections), our findings show it is a narrow band-gap semiconductor where its activation energy changes from 110 meV to 10 meV as it crosses the SCO transition temperature to lower temperature.