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<jats:p>The key parameters of vertical AlN Schottky barrier diodes (SBDs) with variable drift layer thickness (DLT) and drift layer concentration (DLC) are investigated. The specific on-resistance (<jats:italic>R</jats:italic> <jats:sub>on,sp</jats:sub>) decreased to 0.5 mΩ ⋅ cm<jats:sup>2</jats:sup> and the breakdown voltage (<jats:italic>V</jats:italic> <jats:sub>BR</jats:sub>) decreased from 3.4 kV to 1.1 kV by changing the DLC from 10<jats:sup>15</jats:sup> cm<jats:sup>−3</jats:sup> to 3 × 10<jats:sup>16</jats:sup> cm<jats:sup>−3</jats:sup>. The <jats:italic>V</jats:italic> <jats:sub>BR</jats:sub> increases from 1.5 kV to 3.4 kV and the <jats:italic>R</jats:italic> <jats:sub>on,sp</jats:sub> also increases to 12.64 mΩ ⋅ cm<jats:sup>2</jats:sup> by increasing DLT from 4-μm to 11-μm. The <jats:italic>V</jats:italic> <jats:sub>BR</jats:sub> enhancement results from the increase of depletion region extension. The Baliga’s figure of merit (BFOM) of 3.8 GW/cm<jats:sup>2</jats:sup> was obtained in the structure of 11-μm DLT and 10<jats:sup>16</jats:sup> cm<jats:sup>−3</jats:sup> DLC without FP. When DLT or DLC is variable, the consideration of the value of BFOM is essential. In this paper, we also present the vertical AlN SBD with a field plate (FP), which decreases the crowding of electric field in electrode edge. All the key parameters were optimized by simulating based on Silvaco-ATLAS.</jats:p>

<jats:p>Separation technology is an indispensable step in the preparation of freestanding GaN substrate. In this paper, a large-area freestanding GaN layer was separated from the substrate by an electrochemical liftoff process on a sandwich structure composed of an Fe-doped GaN substrate, a highly conductive Si-doped sacrificial layer and a top Fe-doped layer grown by hydride vapor phase epitaxy (HVPE). The large difference between the resistivity in the Si-doped layer and Fe-doped layer resulted in a sharp interface between the etched and unetched layer. It was found that the etching rate increased linearly with the applied voltage, while it continuously decreased with the electrochemical etching process as a result of the mass transport limitation. Flaky GaN pieces and nitrogen gas generated from the sacrificial layer by electrochemical etching were recognized as the main factors responsible for the blocking of the etching channel. Hence, a thick Si-doped layer grown by HVPE was used as the sacrificial layer to alleviate this problem. Moreover, high temperature and ultrasonic oscillation were also found to increase the etching rate. Based on the results above, we succeeded in the liftoff of ∼1.5 inch GaN layer. This work could help reduce the cost of freestanding GaN substrate and identifies a new way for mass production.</jats:p>

<jats:p>A well-established method is highly desirable for growing topological insulator thin films with low carrier density on a wafer-level scale. Here, we present a simple, scalable method based on magnetron sputtering to obtain high-quality Bi<jats:sub>2</jats:sub>Te<jats:sub>3</jats:sub> films with the carrier density down to 4.0 × 10<jats:sup>13</jats:sup> cm<jats:sup>−2</jats:sup>. In contrast to the most-used method of high substrate temperature growth, we firstly sputtered Bi<jats:sub>2</jats:sub>Te<jats:sub>3</jats:sub> thin films at room temperature and then applied post-annealing. It enables the growth of highly-oriented Bi<jats:sub>2</jats:sub>Te<jats:sub>3</jats:sub> thin films with larger grain size and smoother interface. The results of electrical transport show that it has a lower carrier density as well as a larger coherent length (∼228 nm, 2 K). Our studies pave the way toward large-scale, cost-effective production of Bi<jats:sub>2</jats:sub>Te<jats:sub>3</jats:sub> thin films to be integrated with other materials in wafer-level scale for electronic and spintronic applications.</jats:p>

<jats:p>We consider the superconducting properties of Lieb lattice, which produces a flat-band energy spectrum in the normal state under the strong electron–electron correlation. Firstly, we show the hole-doping dependent superconducting order amplitude with various electron–electron interaction strengths in the zero-temperature limit. Secondly, we obtain the superfluid weight and Berezinskii–Kosterlitz–Thouless (BKT) transition temperature with a lightly doping level. The large ratio between the gap-opening temperature and BKT transition temperature shows similar behavior to the pseudogap state in high-<jats:italic>T</jats:italic> <jats:sub>c</jats:sub> superconductors. The BKT transition temperature versus doping level exhibits a dome-like shape in resemblance to the superconducting dome observed in the high-<jats:italic>T</jats:italic> <jats:sub>c</jats:sub> superconductors. However, unlike the exponential dependence of <jats:italic>T</jats:italic> <jats:sub>c</jats:sub> on the electron–electron interaction strength in the conventional high-<jats:italic>T</jats:italic> <jats:sub>c</jats:sub> superconductors, the BKT transition temperature for a flat band system depends linearly on the electron–electron interaction strength. We also show the doping-dependent superconductivity on a lattice with the staggered hoping parameter in the end. Our predictions are amenable to verification in the ultracold atoms experiment and promote the understanding of the anomalous behavior of the superfluid weight in the high-<jats:italic>T</jats:italic> <jats:sub>c</jats:sub> superconductors.</jats:p>

<jats:p>The geometrically frustrated iridate La<jats:sub>3</jats:sub>Ir<jats:sub>3</jats:sub>O<jats:sub>11</jats:sub> with strong spin–orbit coupling and fractional valence was recently predicted to be a quantum spin liquid candidate at ambient conditions. Here, we systematically investigate the evolution of structural and electronic properties of La<jats:sub>3</jats:sub>Ir<jats:sub>3</jats:sub>O<jats:sub>11</jats:sub> under high pressure. Electrical transport measurements reveal an abnormal insulating behavior rather than metallization above a critical pressure <jats:italic>P</jats:italic> <jats:sub>c</jats:sub> ∼ 38.7 GPa. Synchrotron x-ray diffraction (XRD) experiments indicate the stability of the pristine cubic KSbO<jats:sub>3</jats:sub>-type structure up to 73.1 GPa. Nevertheless, when the pressure gradually increases across <jats:italic>P</jats:italic> <jats:sub>c</jats:sub>, the bulk modulus gets enhanced and the pressure dependence of bond length <jats:italic>d</jats:italic> <jats:sub>Ir – Ir</jats:sub> undergoes a slope change. Consistent with the XRD data, detailed analyses of Raman spectra reveal an abnormal redshift of Raman mode and a change of Raman intensity around <jats:italic>P</jats:italic> <jats:sub>c</jats:sub>. Our results demonstrate that the pressure-induced insulating behavior in La<jats:sub>3</jats:sub>Ir<jats:sub>3</jats:sub>O<jats:sub>11</jats:sub> can be assigned to the structural modification, such as the distortion of IrO<jats:sub>6</jats:sub> octahedra. These findings will shed light on the emergent abnormal insulating behavior in other 5d iridates reported recently.</jats:p>

<jats:p>High resolution angle resolved photoemission measurements and band structure calculations are carried out to study the electronic structure of BaMnSb<jats:sub>2</jats:sub>. All the observed bands are nearly linear that extend to a wide energy range. The measured Fermi surface mainly consists of one hole pocket around <jats:italic>Γ</jats:italic> and a strong spot at <jats:italic>Y</jats:italic> which are formed from the crossing points of the linear bands. The measured electronic structure of BaMnSb<jats:sub>2</jats:sub> is unusual and deviates strongly from the band structure calculations. These results will stimulate further efforts to theoretically understand the electronic structure of BaMnSb<jats:sub>2</jats:sub> and search for novel properties in this Dirac material.</jats:p>

<jats:p>A bias-controlled spin-filter and spin memory is theoretically proposed, which consists of the junction with a single-molecule magnet sandwiched between the nonmagnetic and ferromagnetic (FM) leads. By applying different voltage pulses <jats:italic>V</jats:italic> <jats:sub>write</jats:sub> across the junction, the spin direction of the single-molecule magnet can be controlled to be parallel or anti-parallel to the magnetization of the FM lead, and the spin direction of SMM can be “read out” either by the magneto-resistance or by the spin current with another series of small voltage pulses <jats:italic>V</jats:italic> <jats:sub>probe</jats:sub>. It is shown that the polarization of the spin current is extremely high (up to 100%) and can be manipulated by the full-electric manner. This device scheme can be compatible with current technologies and has potential applications in high-density memory devices.</jats:p>

<jats:p>We investigate the effects of post-sinter annealing on the microstructure and magnetic properties in B-lean Nd–Fe–B sintered magnets with different quantities of Nd–Ga intergranular additions. The magnet with fewer Nd–Ga additions can enhance 0.2 T in coercivity, with its remanences nearly unchanged after annealing. With the further increase of the Nd–Ga addition, the annealing process leads coercivity to increase 0.4 T, accompanied by a slight decrease of remanence. With the Nd–Ga addition further increasing and after annealing, however, the increase of coercivity is basically constant and the change of remanence is reduced. Microstructure observation indicates that the matrix grains are covered by continuous thin grain boundary phase in the magnets with an appropriate Nd–Ga concentration after the annealing process. However, the exceeding Nd–Ga addition brings out notable segregation of grain boundary phase, and prior formation of part <jats:italic>RE</jats:italic> <jats:sub>6</jats:sub>Fe<jats:sub>13</jats:sub>Ga phase in the sintered magnet. This prior formation results in a weaker change of remanence after the annealing process. Therefore, the diverse changes of magnetic properties with different Nd–Ga concentrations are based on the respective evolution of grain boundary after the annealing process.</jats:p>

<jats:p>The magnetostriction, magnetization, and spin reorientation properties in Pr(Ga<jats:sub> <jats:italic>x</jats:italic> </jats:sub>Fe<jats:sub>1 – <jats:italic>x</jats:italic> </jats:sub>)<jats:sub>1.9</jats:sub> alloys have been investigated by high-precision x-ray diffraction (XRD) step scanning, magnetization, and Mössbauer spectra measurements. Ga substitution reduces the magnetostriction (<jats:italic>λ</jats:italic> <jats:sub>||</jats:sub>) with magnetic field <jats:italic>H</jats:italic> ≥ 8 kOe (1 Oe = 1.33322 × 10<jats:sup>2</jats:sup> Pa), but it also increases the <jats:italic>λ</jats:italic> <jats:sub>||</jats:sub> value when <jats:italic>H</jats:italic> ≤ 8 kOe at 5 K. Spin-reorientations (SR) are observed in all the alloys investigated, as determined by the step scanned XRD, Mössbauer spectra, and the abnormal temperature dependence of magnetization. An increase of the spin reorientation temperature (<jats:italic>T</jats:italic> <jats:sub>SR</jats:sub>) due to Ga substitution is found in the phase diagram, which is different from the decrease one in many R(<jats:italic>T<jats:sub>x</jats:sub> </jats:italic>Fe<jats:sub>1 – <jats:italic>x</jats:italic> </jats:sub>)<jats:sub>1.9</jats:sub> (<jats:italic>T</jats:italic> = Co, Al, Mn) alloys. The present work provides a method to control the easy magnetization direction (EMD) or <jats:italic>T</jats:italic> <jats:sub>SR</jats:sub> for developing an anisotropic compensation system.</jats:p>

<jats:p>Recent experiments [Guo <jats:italic>et al.</jats:italic>, <jats:italic>Phys. Rev. Lett.</jats:italic> <jats:bold>124</jats:bold> 206602 (2020)] on thermodynamic properties of the frustrated layered quantum magnet SrCu<jats:sub>2</jats:sub>(BO<jats:sub>3</jats:sub>)<jats:sub>2</jats:sub> — the Shastry–Sutherland material — have provided strong evidence for a low-temperature phase transition between plaquette-singlet and antiferromagnetic order as a function of pressure. Further motivated by the recently discovered unusual first-order quantum phase transition with an apparent emergent O(4) symmetry of the antiferromagnetic and plaquette-singlet order parameters in a two-dimensional “checkerboard <jats:italic>J</jats:italic>-<jats:italic>Q</jats:italic>” quantum spin model [Zhao <jats:italic>et al.</jats:italic>, <jats:italic>Nat. Phys.</jats:italic> <jats:bold>15</jats:bold> 678 (2019)], we here study the same model in the presence of weak inter-layer couplings. Our focus is on the evolution of the emergent symmetry as the system crosses over from two to three dimensions and the phase transition extends from strictly zero temperature in two dimensions up to finite temperature as expected in SrCu<jats:sub>2</jats:sub>(BO<jats:sub>3</jats:sub>)<jats:sub>2</jats:sub>. Using quantum Monte Carlo simulations, we map out the phase boundaries of the plaquette-singlet and antiferromagnetic phases, with particular focus on the triple point where these two ordered phases meet the paramagnetic phase for given strength of the inter-layer coupling. All transitions are first-order in the neighborhood of the triple point. We show that the emergent O(4) symmetry of the coexistence state breaks down clearly when the interlayer coupling becomes sufficiently large, but for a weak coupling, of the magnitude expected experimentally, the enlarged symmetry can still be observed at the triple point up to significant length scales. Thus, it is likely that the plaquette-singlet to antiferromagnetic transition in SrCu<jats:sub>2</jats:sub>(BO<jats:sub>3</jats:sub>)<jats:sub>2</jats:sub> exhibits remnants of emergent O(4) symmetry, which should be observable due to additional weakly gapped Goldstone modes.</jats:p>