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<jats:p>Strontium titanate (SrTiO<jats:sub>3</jats:sub>) is a thermoelectric material with large Seebeck coefficient that has potential applications in high-temperature power generators. To simultaneously achieve a low thermal conductivity and high electrical conductivity, polycrystalline SrTiO<jats:sub>3</jats:sub> with a multi-scale architecture was designed by the co-doping with lanthanum, cerium, and niobium. High-quality nano-powders were synthesized via a hydrothermal method. Nano-inclusions and a nano/micro-sized second phase precipitated during sintering to form mosaic crystal-like and epitaxial-like structures, which decreased the thermal conductivity. Substituting trivalent Ce and/or La with divalent Sr and substituting pentavalent Nb with tetravalent Ti enhanced the electrical conductivity without decreasing the Seebeck coefficient. By optimizing the dopant type and ratio, a low thermal conductivity of 2.77 W⋅m<jats:sup>−1</jats:sup>⋅K<jats:sup>−1</jats:sup> and high <jats:italic>PF</jats:italic> of 1.1 mW⋅m<jats:sup>−1</jats:sup>⋅K<jats:sup>−2</jats:sup> at 1000 K were obtained in the sample co-doped with 5-mol% La, 5-mol% Ce, and 5-mol% Nb, which induced a large <jats:italic>ZT</jats:italic> of 0.38 at 1000 K.</jats:p>

<jats:p>The operating frequencies of surface plasmons in pristine graphene lie in the terahertz and infrared spectral range, which limits their utilization. Here, the high-frequency plasmons in doped graphene nanostructures are studied by the time-dependent density functional theory. The doping atoms include boron, nitrogen, aluminum, silicon, phosphorus, and sulfur atoms. The influences of the position and concentration of nitrogen dopants on the collective stimulation are investigated, and the effects of different types of doping atoms on the plasmonic stimulation are discussed. For different positions of nitrogen dopants, it is found that a higher degree of symmetry destruction is correlated with weaker optical absorption. In contrast, a higher concentration of nitrogen dopants is not correlated with a stronger absorption. Regarding different doping atoms, atoms similar to carbon atom in size, such as boron atom and nitrogen atom, result in less spectral attenuation. In systems with other doping atoms, the absorption is significantly weakened compared with the absorption of the pristine graphene nanostructure. Plasmon energy resonance dots of doped graphene lie in the visible and ultraviolet spectral range. The doped graphene nanostructure presents a promising material for nanoscaled plasmonic devices with effective absorption in the visible and ultraviolet range.</jats:p>

<jats:p>AlGaN/GaN high-electron-mobility transistors with Au-free ohmic contacts are fabricated by selective laser annealing and conventional rapid thermal annealing. The current transport mechanism of ohmic contacts is investigated. High-temperature annealing can be avoided in the isolated region and the active region by selective laser annealing. The implanted isolation leakage current is maintained 10<jats:sup>−6</jats:sup> mA/mm even at 1000 V after selective laser annealing. On the contrary, high-temperature annealing will cause obvious degradation of the isolation. The morphology of AlGaN surface is measured by atomic force microscope. No noticeable change of the AlGaN surface morphology after selective laser annealing, while the root-mean-square roughness value markedly increases after rapid thermal annealing. The smaller frequency dispersion of capacitance–voltage characteristics confirms the lower density of surface states after selective laser annealing. Thus, dynamic on-resistance is effectively suppressed.</jats:p>

<jats:p>Extremely large magnetoresistance (XMR) has been explored in many nonmagnetic topologically nontrivial/trivial semimetals, while it is experimentally ambiguous which mechanism should be responsible in a specific material due to the complex electronic structures. In this paper, the magnetoresistance origin of single crystal CaAl<jats:sub>4</jats:sub> with <jats:italic>C</jats:italic>2/<jats:italic>m</jats:italic> structure at low temperature is investigated, exhibiting unsaturated magnetoresistance of ∼ 3000% at 2.5 K and 14 T as the fingerprints of XMR materials. By the combination of ARPES and the first-principles calculations, we elaborate multiband features and anisotropic Fermi surfaces, which can explain the mismatch of isotropic two-band model. Although the structural phase transition from <jats:italic>I</jats:italic>4/<jats:italic>mmm</jats:italic> to <jats:italic>C</jats:italic>2/<jats:italic>m</jats:italic> has been recognized, the subtle impact on electronic structure is revealed by our ARPES measurements. Considering that both charge compensation and potential topologically nontrivial band structure exist in CaAl<jats:sub>4</jats:sub>, our findings report CaAl<jats:sub>4</jats:sub> as a new reference material for exploring the XMR phenomena.</jats:p>

<jats:p>We investigate the spin-related currents and tunnel magnetoresistance through a quantum dot, which is side-coupled with a Majorana fermion zero mode and two thermal-driven ferromagnetic electrodes. It is found that the interplay of Majorana fermion and electrodes’ spin polarization can induce a nonlinear thermal-bias spin current. This interplay also decreases the total magnitude of spin or charge current, in either parallel or antiparallel configuration. In addition, a thermal-driven negative tunnel magnetoresistance is found, which is an unique feature to characterize Majorana fermion. With large temperature difference, a step phenomenon is observed in gate tuned spin-up current. When the coupling between quantum dot and topological superconductor is strong enough, this step will evolve into a linear relation, revealing Majorana fermion’s robustness.</jats:p>

<jats:p>Grain boundaries (GBs), as extremely anisotropic pinning defects, have a strong impact on vortex motion in type-II superconductors, and further on the macro level dominates the superconductivity for example the critical current density. Many previous studies indicated that mostly GB plays the role of a strong barrier for vortex motion, while an easy-flow channel just under some certain conditions. In order to thoroughly make clear of the questions of what is exactly the role of GB on vortex motion and how it works, in this article we developed a large scale molecular dynamic model and revealed the action of GB on vortex motion in type-II superconductors. The most significant finding is that the role of GB on vortex motion can be changeable from a barrier to an easy-flow channel, and which is intrinsically determined by the competition effect correlated with its action on vortex between in the GB and no-GB regions. Such the competition effect essentially depends on the attributes of both the GB (described by the GB strength and angle <jats:italic>θ</jats:italic>) and no-GB pining regions (by the relative disorder strength <jats:italic>α</jats:italic> <jats:sub>p</jats:sub>/<jats:italic>α</jats:italic> <jats:sub>v</jats:sub>). Specifically, for a YBa<jats:sub>2</jats:sub>Cu<jats:sub>3</jats:sub>O<jats:sub>7 – <jats:italic>x</jats:italic> </jats:sub> (YBCO) sample, to obtain a clear knowledge of vortex motion in GB region, we visualized the three typical trajectories of vortices during the three vortex movement stages. Further, in order to understand how GB results in the macro current-carrying property, corresponding to the current–voltage relation of the YBCO conductor, we obtained the average velocity <jats:italic>v<jats:sub>y</jats:sub> </jats:italic> of vortices varying with their driving force, which is nearly identical with the previous observations.</jats:p>

<jats:p>The driving mechanism of nematicity and its twist with superconductivity in iron-based superconductors are still under debate. Recently, a dominant B<jats:sub>1g</jats:sub>-type strain effect on superconductivity is observed in underdoped iron-pnictides superconductors Ba(Fe<jats:sub>1–<jats:italic>x</jats:italic> </jats:sub>Co<jats:sub> <jats:italic>x</jats:italic> </jats:sub>)<jats:sub>2</jats:sub>As<jats:sub>2</jats:sub>, suggesting a strong interplay between nematicity and superconductivity. Since the long-range spin order is absent in FeSe superconductor, whether a similar strain effect could be also observed or not is an interesting question. Here, by utilizing a flexible film as substrate, we successfully achieve a wide-range-strain tuning of FeSe thin flake, in which both the tensile and compressive strain could reach up to ∼0.7%, and systematically study the strain effect on both superconducting and nematic transition (<jats:italic>T</jats:italic> <jats:sub>c</jats:sub> and <jats:italic>T</jats:italic> <jats:sub>s</jats:sub>) in the FeSe thin flake. Our results reveal a predominant A<jats:sub>1g</jats:sub>-type strain effect on <jats:italic>T</jats:italic> <jats:sub>c</jats:sub>. Meanwhile, <jats:italic>T</jats:italic> <jats:sub>s</jats:sub> exhibits a monotonic anti-correlation with <jats:italic>T</jats:italic> <jats:sub>c</jats:sub> and the maximum <jats:italic>T</jats:italic> <jats:sub>c</jats:sub> reaches to 12 K when <jats:italic>T</jats:italic> <jats:sub>s</jats:sub> is strongly suppressed under the maximum compressive strain. Finally, in comparison with the results in the underdoped Ba(Fe<jats:sub>1–<jats:italic>x</jats:italic> </jats:sub>Co<jats:sub> <jats:italic>x</jats:italic> </jats:sub>)<jats:sub>2</jats:sub>As<jats:sub>2</jats:sub>, the absence of B<jats:sub>1g</jats:sub>-type strain effect in FeSe further supports the role of stripe-type spin fluctuations on superconductivity. In addition, our work also supports that the orbital degree of freedom plays a key role to drive the nematic transition in FeSe.</jats:p>

<jats:p>Among the layered two-dimensional ferromagnetic materials (2D FMs), due to a relatively high <jats:italic>T</jats:italic> <jats:sub>C</jats:sub>, the van der Waals (vdW) Fe<jats:sub>3</jats:sub>GeTe<jats:sub>2</jats:sub> (FGT) crystal is of great importance for investigating its distinct magnetic properties. Here, we have carried out static and dynamic magnetization measurements of the FGT crystal with a Curie temperature <jats:italic>T</jats:italic> <jats:sub>C</jats:sub> ≈ 204 K. The <jats:italic>M</jats:italic>–<jats:italic>H</jats:italic> hysteresis loops with in-plane and out-of-plane orientations show that FGT has a strong perpendicular magnetic anisotropy with the easy axis along its <jats:italic>c</jats:italic>-axis. Moreover, we have calculated the uniaxial magnetic anisotropy constant (<jats:italic>K</jats:italic> <jats:sub>1</jats:sub>) from the SQUID measurements. The dynamic magnetic properties of FGT have been probed by utilizing the high sensitivity electron-spin-resonance (ESR) spectrometer at cryogenic temperatures. Based on an approximation of single magnetic domain mode, the <jats:italic>K</jats:italic> <jats:sub>1</jats:sub> and the effective damping constant (<jats:italic>α</jats:italic> <jats:sub>eff</jats:sub>) have also been determined from the out-of-plane angular dependence of ferromagnetic resonance (FMR) spectra obtained at the temperature range of 185 K to <jats:italic>T</jats:italic> <jats:sub>C</jats:sub>. We have found large magnetic damping with the effective damping constant <jats:italic>α</jats:italic> <jats:sub>eff</jats:sub> ∼ 0.58 along with a broad linewidth (Δ <jats:italic>H</jats:italic> <jats:sub>pp</jats:sub> &gt; 1000 Oe at 9.48 GHz, <jats:italic>H</jats:italic> ∥ <jats:italic>c</jats:italic>-axis). Our results provide useful dynamics information for the development of FGT-based spintronic devices.</jats:p>

<jats:p>We report an investigation into the magnetoresistance (MR) of La<jats:sub>0.8</jats:sub>Ba<jats:sub>0.2</jats:sub>MnO<jats:sub>3</jats:sub> ultrathin films with various thicknesses. While the 13 nm-thick film shows the commonly reported negative magnetoresistive effect, the 6 nm- and 4 nm-thick films display unconventional positive magnetoresistive (PMR) behavior under certain conditions. As well as the dependence on the film’s thickness, it has been found that the electrical resistivity and the PMR effect of the thinner films are very dependent on the test current. For example, the magnetoresistive ratio of the 4 nm-thick film changes from +46% to –37% when the current is increased from 10 nA to 100 nA under 15 kOe at 40 K. In addition, the two thinner films present opposite changes in electrical resistivity with respect to the test current, i.e., the electroresistive (ER) effect, at low temperatures. We discuss the complex magnetoresistive and ER behaviors by taking account of the weak contacts at grain boundaries between ferromagnetic metallic (FMM) grains. The PMR effect can be attributed to the breaking of the weak contacts due to the giant magnetostriction of the FMM grains under a magnetic field. Considering the competing effects of the conductive filament and local Joule self-heating at grain boundaries on the transport properties, the dissimilar ER effects in the two thinner films are also understandable. These experimental findings provide an additional approach for tuning the magnetoresistive effect in manganite films.</jats:p>

<jats:p>Compensated ferrimagnetic insulators are particularly interesting for enabling functional spintronic, optical, and microwave devices. Among many different garnets, Gd<jats:sub>3</jats:sub>Fe<jats:sub>5</jats:sub>O<jats:sub>12</jats:sub> (GdIG) is a representative compensated ferrimagnetic insulator. In this paper, we will study the evolution of the surface morphology, the magnetic properties, and the magnetization compensation through changing the following parameters: the annealing temperature, the growth temperature, the annealing duration, and the choice of different single crystalline garnet substrates. Our objective is to find the optimized growth condition of the GdIG films, for the purpose of achieving a strong perpendicular magnetic anisotropy (PMA) and a flat surface, together with a small effective damping parameter. Through our experiments, we have found that the surface roughness approaching 0.15 nm can be obtained by choosing the growth temperature around 700 °C, together with an enhanced PMA. We have also found the modulation of magnetic anisotropy by choosing different single crystalline garnet substrates which change the tensile strain to the compressive strain. A measure of the effective magnetic damping parameter (<jats:italic>α</jats:italic> <jats:sub>eff</jats:sub> = 0.04±0.01) through a spin pumping experiment in a GdIG/Pt bilayer is also made. Through optimizing the growth dynamics of GdIG films, our results could be useful for synthesizing garnet films with a PMA, which could be beneficial for the future development of ferrimagnetic spintronics.</jats:p>