Spin mapping of intralayer antiferromagnetism and field-induced spin reorientation in monolayer CrTe2

Intrinsic antiferromagnetism in van der Waals (vdW) monolayer (ML) crystals enriches our understanding of two-dimensional (2D) magnetic orders and presents several advantages over ferromagnetism in spintronic applications. However, studies of 2D intrinsic antiferromagnetism are sparse, owing to the lack of net magnetisation. Here, by combining spin-polarised scanning tunnelling microscopy and first-principles calculations, we investigate the magnetism of vdW ML CrTe2, which has been successfully grown through molecular-beam epitaxy. We observe a stable antiferromagnetic (AFM) order at the atomic scale in the ML crystal, whose bulk is ferromagnetic, and correlate its imaged zigzag spin texture with the atomic lattice structure. The AFM order exhibits an intriguing noncollinear spin reorientation under magnetic fields, consistent with its calculated moderate magnetic anisotropy. The findings of this study demonstrate the intricacy of 2D vdW magnetic materials and pave the way for their in-depth analysis.

Imaging moiré deformation and dynamics in twisted bilayer graphene

In ‘magic angle’ twisted bilayer graphene (TBG) a flat band forms, yielding correlated insulator behavior and superconductivity. In general, the moiré structure in TBG varies spatially, influencing the overall conductance properties of devices. Hence, to understand the wide variety of phase diagrams observed, a detailed understanding of local variations is needed. Here, we study spatial and temporal variations of the moiré pattern in TBG using aberration-corrected Low Energy Electron Microscopy (AC-LEEM). We find a smaller spatial variation than reported previously. Furthermore, we observe thermal fluctuations corresponding to collective atomic displacements over 70 pm on a timescale of seconds. Remarkably, no untwisting is found up to 600 ∘C. We conclude that thermal annealing can be used to decrease local disorder. Finally, we observe edge dislocations in the underlying atomic lattice, the moiré structure acting as a magnifying glass. These topological defects are anticipated to exhibit unique local electronic properties.

Electronic Properties of Graphene Nanoribbons Doped with Pyrrole-Like Nitrogen

Doping of graphene nanoribbons with various chemical elements leads to a change in their band structure, which significantly expands the range of applications of these objects in modern electronic devices. In this work, the authors investigate graphene nanoribbons of the «armchair» and «zigzag» types with different concentrations of pyrrole-like nitrogen at the edges. The SCC-DFTB method was used to establish the most energetically favorable configurations of pyrrole-like nitrogen at each edge of graphene nanoribbons. The relationship between the energy gaps of graphene nanoribbons and the content of the considered functional nitrogen-containing groups in them was determined. Calculations have shown that, by incorporating into the atomic lattice, pyrrole-like nitrogen at the «zigzag» edge transfers a greater amount of charge to nearby carbon atoms, which makes such nanoribbons more chemically active in comparison with «armchair» type nanoribbons. Nitrogen doped «zigzag» graphene nanoribbons may be a promising chemoresistive element of nanosensors; however, these conclusions require further calculations.

Graphene’s non-equilibrium fermions reveal Doppler-shifted magnetophonon resonances accompanied by Mach supersonic and Landau velocity effects

Oscillatory magnetoresistance measurements on graphene have revealed a wealth of novel physics. These phenomena are typically studied at low currents. At high currents, electrons are driven far from equilibrium with the atomic lattice vibrations so that their kinetic energy can exceed the thermal energy of the phonons. Here, we report three non-equilibrium phenomena in monolayer graphene at high currents: (i) a “Doppler-like” shift and splitting of the frequencies of the transverse acoustic (TA) phonons emitted when the electrons undergo inter-Landau level (LL) transitions; (ii) an intra-LL Mach effect with the emission of TA phonons when the electrons approach supersonic speed, and (iii) the onset of elastic inter-LL transitions at a critical carrier drift velocity, analogous to the superfluid Landau velocity. All three quantum phenomena can be unified in a single resonance equation. They offer avenues for research on out-of-equilibrium phenomena in other two-dimensional fermion systems.

Encapsulation Capacity of β-Cyclodextrin Stabilized Silver Nanoparticles towards Creatinine Enhances the Colorimetric Sensing of Hydrogen Peroxide in Urine

The β-cyclodextrin shell of synthesized silver nanoparticles (βCD-AgNPs) are found to enhance the detection of hydrogen peroxide in urine when compared to the Horse Radish Peroxidase assay kit. Nanoparticles are confirmed by the UV-Vis absorbance of their localized surface plasmonic resonance (LSPR) at 384 nm. The mean size of the βCD-AgNPs is 53 nm/diameter; XRD analysis shows a face-centered cubic structure. The crystalline structure of type 4H hexagonal nature of the AgNPs with 2.4 nm β-CD coating onto is confirmed using aberration corrected high-resolution transmission electron microscopy (HRTEM). A silver atomic lattice at 2.50 Å and 2.41 Å corresponding to (100) and (101) Miller indices is confirmed using the HRTEM. The scope of βCD-AgNPs to detect hydrogen peroxide (H2O2) in aqueous media and human urine is investigated. The test is optimized by examining the effect of volumes of nanoparticles, the pH of the medium, and the kinetic and temperature effect on H2O2 detection. The βCD-AgNPs test is used as a refined protocol, which demonstrated improved sensitivity towards H2O2 in urine compared to the values obtained by the Horse Radish Assay kit. Direct assessment of H2O2 by the βCD-AgNPs test presented always with a linear response in the nM, μM, and mM ranges with a limit of detection of 1.47 nM and a quantitation limit of 3.76 nM. While a linear response obtained from 1.3 to 37.3 nmoles of H2O2/mole creatinine with a slope of 0.0075 and regression coefficient of 0.9955 when the βCD-AgNPs is used as refined test of creatinine. Values ranging from 34.62 ± 0.23 nmoles of H2O2/mole of creatinine and 54.61 ± 1.04 nmoles of H2O2/mole of creatinine when the matrix is not diluted and between 32.16 ± 0.42 nmoles of H2O2/mole of creatinine and 49.66 ± 0.80 nmoles of H2O2/mole of creatinine when the matrix is twice diluted are found in freshly voided urine of seven apparent healthy men aged between 20 and 40 years old.

Self-Organization in Cold Atoms Mediated by Diffractive Coupling

This article discusses self-organization in cold atoms via light-mediated interactions induced by feedback from a single retro-reflecting mirror. Diffractive dephasing between the pump beam and the spontaneous sidebands selects the lattice period. Spontaneous breaking of the rotational and translational symmetry occur in the 2D plane transverse to the pump. We elucidate how diffractive ripples couple sites on the self-induced atomic lattice. The nonlinear phase shift of the atomic cloud imprinted onto the optical beam is the parameter determining coupling strength. The interaction can be tailored to operate either on external degrees of freedom leading to atomic crystallization for thermal atoms and supersolids for a quantum degenerate gas, or on internal degrees of freedom like populations of the excited state or Zeeman sublevels. Using the light polarization degrees of freedom on the Poincaré sphere (helicity and polarization direction), specific irreducible tensor components of the atomic Zeeman states can be coupled leading to spontaneous magnetic ordering of states of dipolar and quadrupolar nature. The requirements for critical interaction strength are compared for the different situations. Connections and extensions to longitudinally pumped cavities, counterpropagating beam schemes and the CARL instability are discussed.