Aluminum nitride (AlN) plays a key role in modern power electronics and deep-ultraviolet photonics, where an understanding of its thermal properties is essential. Here we measure the thermal conductivity of crystalline AlN by the 3ω method, finding it ranges from 674 ± 56 Wm -1 K -1 at 100 K to 186 ± 7 Wm -1 K -1 at 400 K, with a value of 237 ± 6 Wm -1 K -1 at room temperature. We compare these data with analytical models and first principles calculations, taking into account atomic-scale defects (O, Si, C impurities, and Al vacancies). We find Al vacancies play the greatest role in reducing thermal conductivity because of the largest mass-difference scattering. Modeling also reveals that 10% of heat conduction is contributed by phonons with long mean free paths (MFPs), over ~7 μm at room temperature, and 50% by phonons with MFPs over ~0.3 μm. Consequently, the effective thermal conductivity of AlN is strongly reduced in sub-micron thin films or devices due to phonon-boundary scattering.

matter arising from spin-orbit interaction that is strong enough to change the band topology. [8][9][10][11][12][13] The polarized spins on the topological surface states (TSS) in TIs are topologically protected from nonmagnetic backscattering by time-reversal symmetry, offering great potential of larger SOT and higher switching efficiency. [14][15][16][17][18][19] However, though various techniques such as spin-torque ferromagnetic resonance (ST-FMR), [14,20] spin pumping, [21] and second harmonic measurement [15,22] have been used to determine the spin Hall angle θ SH in TIs, there are still large discrepancies among the reported values of θ SH . Such discrepancies exist even when the same measurement setup is employed for different TI samples. [23] Besides, a fundamental difference between TIs and heavy metals is that the spin polarization in TIs arises from the topologically protected TSS, which is reported to be quite temperature-sensitive, [24][25][26] but the temperature dependence of SOT from TSS still remains unclear.In this work, we comprehensively determined θ SH in a Crdoped Bi x Sb 2-x Te 3 /undoped Bi x Sb 2-x Te 3 (Cr-BST/BST) bilayer structure by both hysteresis loop shift measurement and SOT magnetometer based on the magneto-optic Kerr effect (MOKE), where consistent results were obtained by these two approaches. Moreover, θ SH from hysteresis loop shift measurements exhibits a drastic increase as the temperature decreases when the temperature is below 12 K. The sharp increase of θ SH could be attributed to a higher spin polarization ratio in TSS at lower temperatures. Measurement of SOT was also carried out in a uniformly doped Cr-BST film, where the devices were fabricated into a field-effect transistor (FET) geometry to tune the top TSS Fermi level (E F ). While the carrier concentration of the top TSS is tuned by electrostatic gating, the current-induced SOT effective field can be manipulated, revealing a competition between the top TSS and bottom TSS in the contribution to SOT. The strongly temperature-and carrier concentration-dependent SOT as well as the competition between the top and bottom TSS may help elucidate the wide discrepancies among the reported values of θ SH .The modulation-doped Cr-BST/BST bilayer film illustrated in Figure 1a was grown on an insulating GaAs (111) B substrate by molecular beam epitaxy (MBE). The bottom layer of the TI film is not magnetically doped and the thickness is 2 quintuple layers (QLs), while the top layer is doped with Cr and has a thickness of 5 QLs. When a DC current passes through the The topological surface states (TSS) in topological insulators (TIs) can exert strong spin-orbit torque (SOT) on adjacent magnetization, offering great potential in implementing energy-efficient magnetic memory devices. However, there are large discrepancies among the reported spin Hall angle values in TIs, and its temperature dependence still remains elusive. Here, the spin Hall angle in a modulation-doped Cr-Bi x Sb 2−x Te 3 (Cr-BST) film is quantitatively determin...

More efficient thermoelectric devices would revolutionize refrigeration and energy production, and low-dimensional thermoelectric materials are predicted to be more efficient than their bulk counterparts. But nanoscale thermoelectric devices generate thermal gradients on length scales that are too small to resolve with traditional thermometry methods. Here we fabricate, using singlecrystal bismuth telluride (Bi 2 Te 3 ) and antimony/bismuth telluride (Sb 2−x Bi x Te 3 ) flakes exfoliated from commercially available bulk materials, functional thermoelectric coolers (TECs) that are only 100 nm thick. These devices are the smallest TECs ever demonstrated by a factor of 10 4 . After depositing indium nanoparticles to serve as nanothermometers, we measure the heating and cooling produced by the devices with plasmon energy expansion thermometry (PEET), a high-spatial-resolution, transmission electron microscopy (TEM)-based thermometry technique, demonstrating a ΔT = −21 ± 4 K from room temperature. We also establish proof-of-concept for condensation thermometry, a quantitative temperature-change mapping technique with a spatial precision of ≲300 nm.

Thermoelectrics have a wide variety of applications, but their efficiency, typically stated in terms of the figure of merit ZT, must be improved before they become economical for non-niche applications. According to theory [1,2], ZT can be improved relative to the bulk by constructing devices that feature nanometer-scale confinement in one or more dimensions. Here we describe transmission electron microscopy (TEM) observations of cooling in heterojunctions constructed from 2D flakes of exfoliated bismuth telluride and bismuth-antimony telluride.