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<jats:p>High-resolution angle-resolved photoemission measurements were taken on FeSe<jats:sub>1–<jats:italic>x</jats:italic> </jats:sub>S<jats:sub> <jats:italic>x</jats:italic> </jats:sub> (<jats:italic>x</jats:italic> = 0, 0.04, and 0.08) superconductors. With an ultrahigh energy resolution of 0.4 meV, unusual two hole bands near the Brillouin-zone center, which was possibly a result of additional symmetry breaking, were identified in all the sulfur-substituted samples. In addition, in both of the hole bands highly anisotropic superconducting gaps with resolution limited nodes were evidenced. We find that the larger superconducting gap on the outer hole band is reduced linearly to the nematic transition temperature while the gap on the inner hole is nearly S-substitution independent. Our observations strongly suggest that the superconducting gap increases with enhanced nematicity although the superconducting transition temperature is not only governed by the pairing strength, demonstrating strong constraints on theories in the FeSe family.</jats:p>

<jats:p>Motivated by the recent measurements of the spatial distribution of single particle excitation states in a hole-doped Mott insulator, we study the effects of impurity on the in-gap states, induced by the doped holes, in the Hubbard model on the square lattice by the cluster perturbation theory. We find that a repulsive impurity potential can move the in-gap state from the lower Hubbard band towards the upper Hubbard band, providing a good account for the experimental observation. The distribution of the spectral function in the momentum space can be used to discriminate the in-gap state induced by doped holes and that by the impurity. The spatial characters of the in-gap states in the presence of two impurities are also discussed and compared to the experiment.</jats:p>

<jats:p>The first atmospheric window of 3–5 μm in the mid-infrared (MIR) spectral range pertains to crucial application fields, with particular scientific and technological importance. However, conventional narrow-bandgap semiconductors operating at this band, represented by mercury cadmium telluride and indium antimonide, suffer from limited specific detectivity at room temperature and hindered optoelectronic integration. In this study, a plasmonic hot electron-empowered MIR photodetector based on Al-doped ZnO (AZO)/bi-layer graphene heterostructure is demonstrated. Free electrons oscillate coherently in AZO disk arrays, resulting in strong localized surface plasmon resonance (LSPR) in the MIR region. The photoelectric conversion efficiency at 3–5 μm is significantly improved due to plasmon-induced hot-electron extraction and LSPR-enhanced light absorption. The specific detectivity reaches about 1.4 × 10<jats:sup>11</jats:sup> Jones and responsivity is up to 4712.3 A/W at wavelength of 3 μm at room temperature. The device’s specific detectivity is among the highest performance of commercial state-of-the-art photodetectors and superior to most of the other 2D materials based photodetectors in the MIR region. These results demonstrate that a plasmonic heavily doped metal oxides/2D material heterostructure is a suitable architecture for constructing highly sensitive room-temperature MIR photodetectors.</jats:p>

<jats:p>Stabilized by quantum fluctuations, dipolar Bose–Einstein condensates can form self-bound liquid-like droplets. However in the Bogoliubov theory, there are imaginary phonon energies in the long-wavelength limit, implying dynamical instability of this system. A similar instability appears in the Bogoliubov theory of a binary quantum droplet, and is removed due to higher-order quantum fluctuations as shown recently [Gu Q and Yin L 2020 <jats:italic>Phys. Rev. B</jats:italic> <jats:bold>102</jats:bold> 220503(R)]. We study the excitation energy of a dipolar quantum droplet in the Beliaev formalism, and find that quantum fluctuations significantly enhance the phonon stability. We adopt a self-consistent approach without the problem of complex excitation energy in the Bogoliubov theory, and obtain a stable anisotropic sound velocity which is consistent with the superfluid hydrodynamic theory, but slightly different from the result of the extended Gross–Pitaevskii equation due to quantum depletion. A modified Gross–Pitaevskii equation in agreement with the Beliaev theory is proposed, which takes the effect of quantum fluctuations into account more completely.</jats:p>

<jats:p>A time-of-flight polarized neutron imaging setup was realized by integrating an <jats:italic>in situ</jats:italic> pumped polarized <jats:sup>3</jats:sup>He spin filter and energy dispersive neutron camera on the neutron technique development beamline (BL-20) of the China Spallation Neutron Source (CSNS). Test experiments were performed with a solenoid with aluminum wire as a sample. These demonstrated that polarized radiography with a field of view in diameter 2.0 cm at different wavelengths can be obtained. The wavelength-dependent polarization was used to distinguish the neutron polarization behavior for different positions inside and outside the solenoid. The results of this work show the possibility of applying the technique at CSNS and marks a milestone for future polarized neutron imaging developments.</jats:p>

<jats:p>Ultracold atoms trapped in optical superlattices provide a simple platform for realizing the seminal Aubry–André–Harper (AAH) model. However, this model ignores the periodic modulations on the nearest-neighbor hoppings. We establish a generalized AAH model by which an optical superlattice system can be approximately described when <jats:italic>V</jats:italic> <jats:sub>1</jats:sub> ≫ <jats:italic>V</jats:italic> <jats:sub>2</jats:sub>, with periodic modulations on both on-site energies and nearest-neighbor hoppings. This model supports much richer topological properties absent in the standard AAH model. Specifically, by calculating the Chern numbers and topological edge states, we show that the generalized AAH model possesses multifarious topological phases and topological phase transitions, unlike the standard AAH model supporting only a single topological phase. Our findings can uncover more opportunities for using optical superlattices to study topological and localization physics.</jats:p>

<jats:p>We report the nontrivial topological states in an intrinsic type-II superconductor BaSn<jats:sub> <jats:bold>5</jats:bold> </jats:sub> (<jats:italic>T</jats:italic> <jats:sub>c</jats:sub> ∼ 4.4 K) probed by measuring the magnetization, specific heat, de Haas–van Alphen (dHvA) effect, and by performing first-principles calculations. The first-principles calculations reveal a topological nodal ring structure centered at the <jats:italic>H</jats:italic> point in the <jats:italic>k</jats:italic> <jats:sub>z</jats:sub> = <jats:italic>π</jats:italic> plane of the Brillouin zone, which could be gapped by spin-orbit coupling (SOC), yielding relatively small gaps below and above the Fermi level of about 0.04 eV and 0.14 eV, respectively. The SOC also results in a pair of Dirac points along the <jats:italic>Γ</jats:italic>–<jats:italic>A</jats:italic> direction, located at ∼ 0.2 eV above the Fermi level. The analysis of the dHvA quantum oscillations supports the calculations by revealing a nontrivial Berry phase originating from the hole and electron pockets related to the bands forming the Dirac cones. Thus, our study provides an excellent avenue for investigating the interplay between superconductivity and nontrivial topological states.</jats:p>

<jats:p>We report the discovery of superconductivity on the border of antiferromagnetic order in a quasi-one-dimensional material RbMn<jats:sub>6</jats:sub>Bi<jats:sub>5</jats:sub> via measurements of resistivity and magnetic susceptibility under high pressures. Its phase diagram of temperature versus pressure resembles those of many magnetism-mediated superconducting systems. With increasing pressure, its antiferromagnetic ordering transition with <jats:italic>T</jats:italic> <jats:sub>N</jats:sub> = 83 K at ambient pressure is first enhanced moderately and then suppressed completely at a critical pressure of <jats:italic>P</jats:italic> <jats:sub>c</jats:sub> ≈ 13 GPa, around which bulk superconductivity emerges and exhibits a dome-like <jats:italic>T</jats:italic> <jats:sub>c</jats:sub>(<jats:italic>P</jats:italic>) with a maximal <jats:inline-formula> <jats:tex-math><?CDATA ${T}_{{\rm{c}}}^{{\rm{onset}}}\approx 9.5$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>T</mml:mi> <mml:mi mathvariant="normal">c</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">onset</mml:mi> </mml:mrow> </mml:msubsup> <mml:mo>≈</mml:mo> <mml:mn>9.5</mml:mn> </mml:mrow> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpl_39_6_067401_ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> K at about 15 GPa. In addition, the superconducting state around <jats:italic>P</jats:italic> <jats:sub>c</jats:sub> is characterized by a large upper critical field <jats:italic>μ</jats:italic> <jats:sub>0</jats:sub> <jats:italic>H</jats:italic> <jats:sub>c2</jats:sub>(0) exceeding the Pauli paramagnetic limit, implying a possible unconventional paring mechanism. The present study, together with our recent work on KMn<jats:sub>6</jats:sub>Bi<jats:sub>5</jats:sub> (the maximum <jats:inline-formula> <jats:tex-math><?CDATA ${T}_{{\rm{c}}}^{{\rm{onset}}}\approx 9.3$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>T</mml:mi> <mml:mi mathvariant="normal">c</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">onset</mml:mi> </mml:mrow> </mml:msubsup> <mml:mo>≈</mml:mo> <mml:mn>9.3</mml:mn> </mml:mrow> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpl_39_6_067401_ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> K), makes <jats:italic>A</jats:italic>Mn<jats:sub>6</jats:sub>Bi<jats:sub>5</jats:sub> (<jats:italic>A</jats:italic> = alkali metal) a new family of Mn-based superconductors with relatively high <jats:italic>T</jats:italic> <jats:sub>c</jats:sub>.</jats:p>

<jats:p>Recent experiments have found that in contrast to the nonsuperconducting bulk <jats:italic>R</jats:italic>NiO<jats:sub>2</jats:sub> (<jats:italic>R</jats:italic> = Nd, La, and Pr), the strontium-doped <jats:italic>R</jats:italic> <jats:sub>1 – <jats:italic>x</jats:italic> </jats:sub>Sr<jats:sub> <jats:italic>x</jats:italic> </jats:sub>NiO<jats:sub> <jats:italic>2</jats:italic> </jats:sub> thin films show superconductivity with the critical temperature <jats:italic>T</jats:italic> <jats:sub>c</jats:sub> of 9–15 K at <jats:italic>x</jats:italic> = 0.2, whose origin of superconductivity deserves further investigation. Based on first-principles calculations, we study the electronic structure, lattice dynamics, and electron–phonon coupling (EPC) of the undoped and doped <jats:italic>R</jats:italic>NiO<jats:sub>2</jats:sub> (<jats:italic>R</jats:italic> = Nd, La, and Pr) at the experimental doping level. Our results show that the EPC-derived <jats:italic>T</jats:italic> <jats:sub>c</jats:sub>’s are all about 0 K in the undoped and doped <jats:italic>R</jats:italic>NiO<jats:sub>2</jats:sub>. The electron–phonon coupling strength is too small to account for the observed superconductivity. We hence propose that the electron–phonon interaction can not be the exclusive origin of the superconductivity in <jats:italic>R</jats:italic>NiO<jats:sub>2</jats:sub> (<jats:italic>R</jats:italic> = Nd, La, and Pr).</jats:p>

<jats:p>We propose an extended BCS–Hubbard model and investigate its ground state phase diagram in an external magnetic field. By mapping the model onto a model of spinless fermions coupled with conserving <jats:italic>Z</jats:italic> <jats:sub>2</jats:sub> variables which are mimicked by pseudospins, the model is shown to be exactly solvable along the symmetric lines for an arbitrary on-site Hubbard interaction on the bipartite lattice. In the zero field limit, the ground states exhibit an antiferromagnetic order of pseudospins. In the large field limit, on the other hand, the pseudospins are fully polarized ordered. With the increase of the applied field, a first-order phase transition occurs between these kinds of phases when the on-site Coulomb interaction is less than a critical value <jats:italic>U</jats:italic> <jats:sub>c</jats:sub>. Above this critical <jats:italic>U</jats:italic> <jats:sub>c</jats:sub>, a novel intermediate phase emerges between the fully polarized and antiferromagnetic phases. The ground states in this phase are macroscopically degenerate, like in a spin ice, and the corresponding entropy scales linearly with the lattice size at zero temperature.</jats:p>