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We compared the photoluminescence (PL) properties of AlInAsSb digital alloy samples with different periods grown on GaSb (001) substrates by molecular beam epitaxy. Temperature-dependent S-shape behavior is observed and explained using a thermally activated redistribution model within a Gaussian distribution of localized states. There are two different mechanisms for the origin of the PL intensity quenching for the AlInAsSb digital alloy. The high-temperature activation energy E 1 is positively correlated with the interface thickness, whereas the low-temperature activation energy E 2 is negatively correlated with the interface thickness. A quantitative high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) study shows that the interface quality improves as the interface thickness increases. Our results confirm that E 1 comes from carrier trapping at a state in the InSb interface layer, while E 2 originates from the exciton binding energy due to the roughness of the AlAs interface layer.

By optimizing the V/III beam-equivalent pressure ratio, a high-quality InAs/GaSb type-II superlattice material for the long-wavelength infrared (LWIR) range is achieved by molecular beam epitaxy (MBE). High-resolution x-ray diffraction (HRXRD), atomic force microscopy (AFM), and Fourier transform infrared (FTIR) spectrometer are used to characterize the material growth quality. The results show that the full width at half maximum (FWHM) of the superlattice zero-order diffraction peak, the mismatching of the superlattice zero-order diffraction peak between the substrate diffraction peaks, and the surface roughness get the best results when the beam-equivalent pressure (BEP) ratio reaches the optimal value, which are 28 arcsec, 13 arcsec, and 1.63 Å, respectively. The intensity of the zero-order diffraction peak is strongest at the optimal value. The relative spectral response of the LWIR detector shows that it exhibits a 100% cut-off wavelength of 12.6 μm at 77 K. High-quality epitaxial materials have laid a good foundation for preparing high-performance LWIR detector.

Plastic waste is one of the most pressing global environmental challenges. As the world’s largest plastic waste generator and importer, China has implemented various policies to promote sustainable plastic waste management. However, the quantitative understanding of China’s application-specific plastic waste recycling and its infrastructure development is still quite limited. Here, we build up a unified framework to investigate the recycling and the corresponding infrastructure status of five commodity plastics in China, which are polyethylene, polypropylene, polyvinyl chloride, polystyrene and acrylonitrile-butadiene-styrene (PE, PP, PVC, PS, ABS) from 2000–2019. We find that: (1) in total, China consumed around 981.4 million tons (Mt) of the five commodity plastics from 2000–2019, generating 590.4 Mt of plastic waste, only 27% of which was recycled, 34% was landfilled and 32% was incinerated; (2) PP (∼30%) and PE (∼28%) have the highest recycling rate in China, which is related to their huge consumption base, while PS and ABS have the lowest recycling rate at only ∼26%; (3) the waste recycling performance is determined by its applications, and the worst recycling rates (

The etching and passivation processes of very long wavelength infrared (VLWIR) detector based on the InAs/GaSb/AlSb type-II superlattice have been studied. By studying the effect of each component in the citric acid solution (citric acid, phosphoric acid, hydrogen peroxide, deionized water), the best solution ratio is obtained. After comparing different passivation materials such as sulfide + SiO2, Al2O3, Si3N4 and SU8, it is found that SU8 passivation can reduce the dark current of the device to a greater degree. Combining this wet etching and SU8 passivation, the R 0 A of VLWIR detector with a mesa diameter of 500 μm is about 3.6 Ω⋅cm2 at 77 K.

Spin–orbit coupling (SOC) plays a determinate role in spintronics and topological physics. Previous studies indicate that the SOC in graphene nanoribbon (GNR) can be enhanced by the proximity effect from two-dimensional transition-metal dichalcogenide (2D-TMD). However, the bulk inversion symmetry of GNR/2D-TMD restricts further increase of the proximity-induced SOC in GNR. In this view, we introduce a TMD nanoribbon (TMDNR) with finite width, and propose three methods to break the bulk inversion symmetry, i.e. defects in TMDNR, spatial interlayer edge coupling, and twist between GNR and TMDNR, which can further enhance the SOC in the GNR by roughly 30 times, 20 times and 150 times, respectively, depending on the relative energy between the Dirac point of GNR and the states of TMDNR. Furthermore, the significantly enhanced SOC can drive the GNR into a topological superconducting phase. By introducing the Zeeman splitting and s-wave superconductivity in the GNR, quasi one-dimensional topological superconductivity and Majorana zero modes (MZMs) can be achieved in the GNR. At last we propose a feasible experimental method to realize and manipulate MZMs in the GNR/TMDNR system.

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 [Phys. Rev. B 99 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)K × SU(2)K′ 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 (J H ≠ 0), the SDW or CDW is favored respectively at the critical point, determined by J H → 0− or J H → 0+. 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 J H. 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.

Majorana fermions have been predicted to exist at the edge states of a two-dimensional topological superconductor. We fabricated single quintuple layer (QL) Bi2Te3/FeTe heterostructure with the step-flow epitaxy method and studied the topological properties of this system by using angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy. We observed the coexistence of robust superconductivity and edge states on the single QL Bi2Te3 islands which can be potential evidence for topological superconductor.

The Josephson coupling between optimally cuprate superconductors separated by a spacer with a finite energy cooperon excitation which contributes to the Josephson coupling strength, is examined. For an underdoped cuprate barrier in its normal state, the YRZ model gives a good description of the temperature dependent enhanced Josephson coupling. A detailed examination of origin of the enhancement shows a significant contribution from the cooperon excitation which is comparable to that from nodal quasiparticles., Comment: 5 pages, 4 figures

Possible superconductivity in recently discovered (Tl,K)Fe$_x$Se$_2$ compounds is studied from the viewpoint of doped Mott insulator. The Mott insulating phase is examined to be preferred in the parent compound at $x=1.5$ due to the presence of Fe vacancies. Partial filling of vacancies at the Fe-sites introduces electron carriers and leads to electron doped superconductivity. By using a two-orbital Hubbard model in the strong coupling limit, we find that the s-wave pairing is more favorable at small Hund's coupling, and d$_{x^2-y^2}$ wave pairing is more favorable at large Hund's coupling., Comment: 4+ pages, 3 figures, to appear in EPL

The effect of non-magnetic Zn impurity on superconductivity in LaFe$_{1-y}$Zn$_y$AsO$_{1-x}$F$_x$ system is studied systematically. In the presence of Zn impurity, the superconducting transition temperature increases in the under-doped regime, remains unchanged in the optimally doped regime, and is severely suppressed in the over-doped regime. Our results suggest a switch of the symmetry of the superconducting order parameters from a $s$-wave to $s_{\pm}$ or $d$-wave states as the charge carrier doping increases in FeAs-based superconductors., 4 pages, 4 figures. Format changed and a few revisons made

A recent study of the transport properties on the stripe phase in La1.875Ba01.25CuO4 by Li et al (2007 Phys. Rev. Lett. 99 067001) found two-dimensional (2D) superconductivity over a wide temperature range including a Berezinski–Kosterlitz–Thouless transition at a temperature T=16 K, with 3D superconducting (SC) ordering only at T=4 K. These results contradict the long standing belief that the onset of superconductivity is suppressed by stripe ordering and suggest coexistence of stripe and SC phases. The lack of 3D SC order above T=4 K requires an antiphase ordering in the SC state to suppress the interlayer Josephson coupling as proposed by Berg et al (2007 Phys. Rev. Lett. 99 127003). Here, we use a renormalized mean field theory for a generalized t–J model to examine in detail the energetics of the spin and charge stripe ordered SC states including possible antiphase domains in the SC order. We find that the energies of these modulated states are very close to each other and that the anisotropy present in the low temperature tetragonal crystal structure favors stripe resonating valence bond states. The stripe antiphase SC states are found to have energies very close to, but always above, the ground state energy, which suggests additional physical effects are responsible for their stability., New Journal of Physics, 11, ISSN:1367-2630

The magnetic-field–induced antiferromagnetic phase of the underdoped cuprates is studied within the t-t'-J model. A magnetic field suppresses the pairing amplitude, which, in turn, may induce antiferromagnetism. We apply our theory to interpret the recently reported quantum oscillations in high magnetic field in ortho-II YBa2Cu3O6.5 [1] and propose that the total hole density abstracted from the oscillation period is reduced by 50% due to doubling of the unit cell.