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<jats:p>The model dependence in the study of the magic-angle twisted bilayer-graphene (MA-TBG) is an important issue in the research area. It has been argued previously that the two-band tight-binding (TB) model (per spin and valley) cannot serve as a start point for succeeding studies as it cannot correctly describe the topological aspect of the continuum-theory model near the Dirac nodes in the mini Brillouin zone (MBZ). For this purpose, we adopt the faithful TB model [<jats:italic>Phys. Rev. B</jats:italic> <jats:bold>99</jats:bold> 195455 (2019)] with five bands (per spin and valley) as our start point, which is further equipped with extended Hubbard interactions. Then after systematic random-phase-approximation (RPA) based calculations, we study the electron instabilities of this model, including the density wave (DW) and superconductivity (SC), near the van Hove singularity (VHS). Our results are as follows. In the case neglecting the tiny inter-valley exchange interaction, the exact SU(2)<jats:sub>K</jats:sub> × SU(2)<jats:sub>K′</jats:sub> symmetry leads to the degeneracy between the inter-valley charge DW (CDW) and the spin DW (SDW) (which would be mixed then), and that between the singlet d + id-wave and triplet p + ip-wave topological SCs. When a realistic tiny inter-valley exchange interaction is turned on with nonzero coefficient (<jats:italic>J</jats:italic> <jats:sub>H</jats:sub> ≠ 0), the SDW or CDW is favored respectively at the critical point, determined by <jats:italic>J</jats:italic> <jats:sub>H</jats:sub> → 0<jats:sup>−</jats:sup> or <jats:italic>J</jats:italic> <jats:sub>H</jats:sub> → 0<jats:sup>+</jats:sup>. In the mean time, the degeneracy between the singlet d + id-wave and triplet p + ip-wave topological SCs is also lifted up by the tiny <jats:italic>J</jats:italic> <jats:sub>H</jats:sub>. These results are highly similar to the results of our previous study [arXiv:2003.09513] adopting the two-band TB model, with the reason lying in that both models share the same symmetry and Fermi-surface (FS) nesting character near the VHS. Such a similarity suggests that the low-energy physics of the doped MA-TBG is mainly determined by the symmetry and the shape of the FS of the doped system, and is insensitive to other details of the band structure, including the topological aspects near the Dirac nodes in the MBZ.</jats:p>

<jats:p>Based on density functional theory and non-equilibrium Green’s function method, we studied noncollinear magnetism and spin transport in a 180° domain wall made of zigzag graphene nanoribbon (ZGNR) with different noncollinear magnetic profiles on the top and bottom edges. Our results show that a helical domain wall on the top (bottom) edge and an abrupt domain wall on the bottom (top) edge can survive in the ZGNR. This indicates that such characteristic magnetization distribution can be obtained by some means, e.g., the introduction of impurity on one edge. Compared to a wide ZGNR, a narrow ZGNR presents obvious coupling between the two edges which changes the magnetization and transmission greatly. As for the above-mentioned distinct magnetic profile, the spin transport is blocked in the abrupt domain wall due to strong spin flip scattering while remains unaffected in the helical domain wall due to the spin mixing effect. We deduce a formula of the transmission for various magnetic profiles of the ZGNRs. A new result based on this formula is that the transmission at the Fermi level can be zero, one, and two by tuning the edge magnetization. Our results provide insights into the noncollinear spin transport of the ZGNR-based devices.</jats:p>

<jats:p>We investigate the electronic transport in a simple mesoscopic cross structure made of two wires (stubs) grafted at the same point along a quantum waveguide. We show that the structure may exhibit important phenomena such as bound in continuum (BIC) states. These states are transformed into electromagnetically induced transparency (EIT) resonance by detuning slightly the lengths of the stubs. The last phenomenon is used to propose and study a mesoscopic demultiplexer device with an input waveguide and two output waveguides. We give closed-form expressions of the geometrical parameters that allow a selective transfer of a given state in the first waveguide without perturbing the second waveguide. The effect of temperature on the transmission resonances is also examined by using Landauer–Büttiker formula. The analytical results of the dispersion relation and transmission and reflection coefficient are obtained using the Green’s function method.</jats:p>

<jats:p>The pulse generation from active mode-locking terahertz quantum cascade laser is studied by Maxwell–Bloch equations. It is shown that longer dephasing time will lead to multiple pulses generation from the laser. The dependence of output field on modulation length and radio-frequency parameters is obtained. In order to achieve short pulse generation, the DC bias should close to threshold value and modulation length should be shorter than 0.256 mm. The output pulse is unstable and the envelope shows many oscillations in the presence of spatial hole burning, resulting destabilization of mode-locking.</jats:p>

<jats:p>As one of intriguing physical results of electronic reconstruction, the metal–insulator transition plays an important role in exploring new electronic devices. In this study, the density functional theory is employed to investigate the metal–insulator transition in (LaTiO<jats:sub>3</jats:sub>)<jats:sub> <jats:italic>m</jats:italic> </jats:sub>/(CaVO<jats:sub>3</jats:sub>)<jats:sub> <jats:italic>n</jats:italic> </jats:sub> superlattices. Herein, three kinds of physical avenues, <jats:italic>i.e</jats:italic>., stacking orientation, epitaxial strain, and thickness periods, are used to tune the metal–insulator transition. Our calculations find that the [001]- and [110]-oriented (LaTiO<jats:sub>3</jats:sub>)<jats:sub>1</jats:sub>/(CaVO<jats:sub>3</jats:sub>)<jats:sub>1</jats:sub> superlattices on SrTiO<jats:sub>3</jats:sub> substrate are insulating, while [111]-oriented case is metallic. Such metallic behavior in [111] orientation can also be modulated by epitaxial strain. Besides the structural orientation and strain effect, the highly probable metal–insulator transition is presented in (LaTiO<jats:sub>3</jats:sub>)<jats:sub> <jats:italic>m</jats:italic> </jats:sub>/(CaVO<jats:sub>3</jats:sub>)<jats:sub> <jats:italic>n</jats:italic> </jats:sub> superlattices with increasing thickness. In addition, several interesting physical phenomena have also been revealed, such as selective charge transfer, charge ordering, and orbital ordering.</jats:p>

<jats:p>Artificially constructed van der Waals heterostructures (vdWHs) provide an ideal platform for realizing emerging quantum phenomena in condensed matter physics. Two methods for building vdWHs have been developed: stacking two-dimensional (2D) materials into a bilayer structure with different lattice constants, or with different orientations. The interlayer coupling stemming from commensurate or incommensurate superlattice pattern plays an important role in vdWHs for modulating the band structures and generating new electronic states. In this article, we review a series of novel quantum states discovered in two model vdWH systems — graphene/hexagonal boron nitride (hBN) hetero-bilayer and twisted bilayer graphene (tBLG), and discuss how the electronic structures are modified by such stacking and twisting. We also provide perspectives for future studies on hetero-bilayer materials, from which an expansion of 2D material phase library is expected.</jats:p>

<jats:p>Using time-dependent Ginzburg–Landau formalism, we investigate the multiple reversals of ratchet effects in an unpatterned superconducting strip by the tilted dynamic pinning potential. In the case of collinear sliding potential and Lorentz force, vortices are always confined in the channels induced by sliding potential. However, due to the inclination angle of sliding pinning potential with respect to the Lorentz force, vortices could be driven out of the channels, and unexpected results with multiple reversals of vortex rectifications are observed. The mechanism of multiple reversals of vortex rectifications is explored by analyzing different vortex motion scenarios with increasing ac current amplitudes. The multiple reversals of transverse and longitudinal ratchet effects can be highly controlled by ac amplitude and dynamic pinning velocity. What’s more, at certain large current the ratchet effect reaches strongest within a wide range of pinning sliding velocity.</jats:p>

<jats:p>Superconductors and ferromagnets are highly non-compatible materials due to the natures of their respective electronic states. But when artificially brought together, they develop interesting characteristics, one of which, vortex clustering, is discussed here in this paper. Phase-field and micromagnetic simulations are performed to investigate the superconductor and ferromagnet bilayer, respectively. The ferromagnet with uniaxial anisotropy is observed to develop the maze domain, whereas the superconductor subjected to the influence of the ferromagnetic stray field displays a vortex pattern. Clustered vortices in superconductors at certain locations are observed to be precisely located over magnetic domain bifurcations. The enhanced out-of-plane stray field at bifurcations around the curved domain walls and the convergent Lorentz force due to screening currents in superconductor are attributed to the formation of clusters at bifurcation sites. Segregation of the inter-vortex spacing between straight and bifurcated domain is clearly observed. More importantly, inter-vortex spacing is predicted to serve as a precise tool to map local ferromagnet domain shapes.</jats:p>

<jats:p>Cu<jats:sub>2</jats:sub>Se is a promising “phonon liquid–electron crystal” thermoelectric material with excellent thermoelectric performance. In this work, Cd-doped Cu<jats:sub>2–<jats:italic>x</jats:italic> </jats:sub>SeCd<jats:sub> <jats:italic>x</jats:italic> </jats:sub> (<jats:italic>x</jats:italic> = 0, 0.0075, 0.01, and 0.02) samples were prepared using NaCl flux method. The solubility of Cd in Cu<jats:sub>2</jats:sub>Se at room temperature was less than 6%, and a second phase of CdSe was found in the samples with large initial Cd content (<jats:italic>x</jats:italic> = 0.01 and 0.02). Field-emission scanning electron microscopic image showed that the arranged lamellae formed a large-scale layered structure with an average thickness of approximately 100 nm. Transmission electron microscopy demonstrated that doping of Cd atoms did not destroy the crystal integrity of Cu<jats:sub>2</jats:sub>Se. A small amount of Cd in Cu<jats:sub>2</jats:sub>Se could reduce the electrical and thermal conductivities of the material, thus significantly enhancing its thermoelectric performance. With the increase in Cd content in the sample, the carrier concentration decreased and the mobility increased gradually. Thermogravimetric differential thermal analysis showed that no weight loss occurred below the melting point. Excessive Cd doping led to the emergence of the second phase of CdSe in the sample, thus significantly increasing the thermal conductivity of the material. A maximum <jats:italic>ZT</jats:italic> value of 1.67 at 700 K was obtained in the Cu<jats:sub>1.9925</jats:sub>SeCd<jats:sub>0.0075</jats:sub> sample.</jats:p>

<jats:p>We report the structural and electrical transport properties of Fe<jats:sub>1–<jats:italic>x</jats:italic> </jats:sub>Cu<jats:sub> <jats:italic>x</jats:italic> </jats:sub>Se (<jats:italic>x</jats:italic> = 0, 0.02, 0.05, 0.10) single crystals grown by a chemical vapor transport method. Substituting Cu for Fe suppresses both the nematicity and superconductivity of FeSe single crystal, and provokes a metal–insulator transition. Our Hall measurements show that the Cu substitution also changes an electron dominance at low temperature of un-doped FeSe to a hole dominance of Cu-doped Fe<jats:sub>1–<jats:italic>x</jats:italic> </jats:sub>Cu<jats:sub> <jats:italic>x</jats:italic> </jats:sub>Se at <jats:italic>x</jats:italic> = 0.02 and 0.1, and reduces the sign-change temperature (<jats:italic>T<jats:sub>R</jats:sub> </jats:italic>) of the Hall coefficient (<jats:italic>R</jats:italic> <jats:sub>H</jats:sub>).</jats:p>