Explore the vast range of titles available.

Interlayer excitons (ILXs) — electron–hole pairs bound across two atomically thin layered semiconductors — have emerged as attractive platforms to study exciton condensation 1 – 4 , single-photon emission and other quantum information applications 5 – 7 . Yet, despite extensive optical spectroscopic investigations 8 – 12 , critical information about their size, valley configuration and the influence of the moiré potential remains unknown. Here, in a WSe 2 /MoS 2 heterostructure, we captured images of the time-resolved and momentum-resolved distribution of both of the particles that bind to form the ILX: the electron and the hole. We thereby obtain a direct measurement of both the ILX diameter of around 5.2 nm, comparable with the moiré-unit-cell length of 6.1 nm, and the localization of its centre of mass. Surprisingly, this large ILX is found pinned to a region of only 1.8 nm diameter within the moiré cell, smaller than the size of the exciton itself. This high degree of localization of the ILX is backed by Bethe–Salpeter equation calculations and demonstrates that the ILX can be localized within small moiré unit cells. Unlike large moiré cells, these are uniform over large regions, allowing the formation of extended arrays of localized excitations for quantum technology. Imaging the electron and hole that bind to form interlayer excitons in a 2D moiré material enables direct measurement of its diameter and indicates the localization of its centre of mass.

We report here a detailed analysis of the ultrashort laser pulse irradiation effects on a single crystalline silicon surface. A systematic study has been performed to understand the surface morphological changes under irradiation with ultrashort laser pulses by changing different input laser parameters such as laser fluence, laser pulse number, and incident laser polarization. Field emission scanning electron microscopy images reveal the formation of laser induced periodic sub-wavelength surface structures directly on bulk surface. The orientation of the formed sub-wavelength surface structures is perpendicular to the incident laser polarization and their morphology and spatial periodicity strongly depend on the applied laser fluence and laser pulse number. The sub-wavelength surface structures are accompanied by the formation of a large density of silicon nanoparticles which possess broad visible photoluminescence ranging from 410 to 680 nm which is due to superficial oxidation of silicon during laser irradiation. The amount of oxygen incorporated into silicon strongly depends on laser parameters such as laser fluence and number of laser pulses.

In this study, we revisit the popular method of measuring the nonlinear susceptibility of a material through Z-scan technique, introduced in 1990 by Sheik-Bahae and co-workers through a simple ray optics defined by the ABCD matrix formulation. The work therefore looks at the Z-scan measurement curves analysed through ray propagation in the medium and analysed through an aperture. The transmittance of a sample in the Z-scan technique is measured through a finite aperture in the far field, as the sample is scanned along the propagation direction (Z) of a focussed Gaussian beam. The sign and magnitude of nonlinear refractive index are easily deduced from the transmittance curve (Z-scan) using the theoretical model based on ABCD matrix formalism.

Controlling the electronic structure of two-dimensional materials using the combination of twist angle and electrostatic doping is an effective means to induce emergent phenomena. In bilayer graphene with an interlayer twist angle near the magic angle, the electronic dispersion is strongly modified by a manifold of hybridizing moiré Dirac cones leading to flat band segments with strong electronic correlations. Numerous technical challenges arising from spatial inhomogeneity of interlayer interactions, twist angle and device functionality have so far limited momentum-resolved electronic structure measurements of these systems to static conditions. Here, we present a detailed characterization of the electronic structure exhibiting miniband dispersions for twisted bilayer graphene, near the magic angle, integrated in a functional device architecture using micro-focused angle-resolved photoemission spectroscopy. The optimum conditions for visualizing the miniband dispersion are determined by exploiting the spatial resolution and photon energy tunability of the light source and applied to extract a hybridization gap size of \\((0.14 \\pm 0.03)\\)~eV and flat band segments extending across a moiré mini Brillouin zone. \\textit{In situ} electrostatic gating of the sample enables significant electron-doping, causing the conduction band states to shift below the Fermi energy. Our work emphasizes key challenges in probing the electronic structure of magic angle bilayer graphene devices and outlines conditions for exploring the doping-dependent evolution of the dispersion that underpins the ability to control many-body interactions in the material.

Heterostructures composed of the intrinsic magnetic topological insulator MnBi\\(_2\\)Te\\(_4\\) and its non-magnetic counterpart Bi\\(_2\\)Te\\(_3\\) host distinct surface electronic band structures depending on the stacking order and exposed termination. Here, we probe the ultrafast dynamical response of MnBi\\(_2\\)Te\\(_4\\) and MnBi\\(_4\\)Te\\(_7\\) following near-infrared optical excitation using time- and angle-resolved photoemission spectroscopy, and disentangle surface from bulk dynamics based on density functional theory slab calculations of the surface-projected electronic structure. We gain access to the out-of-equilibrium charge carrier populations of both MnBi\\(_2\\)Te\\(_4\\) and Bi\\(_2\\)Te\\(_3\\) surface terminations of MnBi\\(_4\\)Te\\(_7\\), revealing an instantaneous occupation of states associated with the Bi\\(_2\\)Te\\(_3\\) surface layer followed by carrier extraction into the adjacent MnBi\\(_2\\)Te\\(_4\\) layers with a laser fluence-tunable delay of up to 350 fs. The ensuing thermal relaxation processes are driven by phonon scattering with significantly slower relaxation times in the magnetic MnBi\\(_2\\)Te\\(_4\\) septuple layers. The observed competition between interlayer charge transfer and intralayer phonon scattering demonstrates a method to control ultrafast charge transfer processes in MnBi\\(_2\\)Te\\(_4\\)-based van der Waals compounds.

Superlattices from twisted graphene mono- and bi-layer systems give rise to on-demand many-body states such as Mott insulators and unconventional superconductors. These phenomena are ascribed to a combination of flat bands and strong Coulomb interactions. However, a comprehensive understanding is lacking because the low-energy band structure strongly changes when the electron filling is varied. Here, we gain direct access to the filling-dependent low energy bands of twisted bilayer graphene (TBG) and twisted double bilayer graphene (TDBG) by applying micro-focused angle-resolved photoemission spectroscopy to in situ gated devices. Our findings for the two systems are in stark contrast: The doping dependent dispersion for TBG can be described in a simple model, combining a filling-dependent rigid band shift with a many-body related bandwidth change. In TDBG, on the other hand, we find a complex behaviour of the low-energy bands, combining non-monotonous bandwidth changes and tuneable gap openings. Our work establishes the extent of electric field tunability of the low energy electronic states in twisted graphene superlattices and can serve to underpin the theoretical understanding of the resulting phenomena.

Resolving the momentum degree of freedom of excitons - electron-hole pairs bound by the Coulomb attraction in a photoexcited semiconductor, has remained a largely elusive goal for decades. In atomically thin semiconductors, such a capability could probe the momentum forbidden dark excitons, which critically impact proposed opto-electronic technologies, but are not directly accessible via optical techniques. Here, we probe the momentum-state of excitons in a WSe2 monolayer by photoemitting their constituent electrons, and resolving them in time, momentum and energy. We obtain a direct visual of the momentum forbidden dark excitons, and study their properties, including their near-degeneracy with bright excitons and their formation pathways in the energy-momentum landscape. These dark excitons dominate the excited state distribution - a surprising finding that highlights their importance in atomically thin semiconductors.

Interlayer excitons (ILXs) - electron-hole pairs bound across two atomically thin layered semiconductors - have emerged as attractive platforms to study exciton condensation, single-photon emission and other quantum-information applications. Yet, despite extensive optical spectroscopic investigations, critical information about their size, valley configuration and the influence of the moiré potential remains unknown. Here, we captured images of the time- and momentum-resolved distribution of both the electron and the hole that bind to form the ILX in a WSe2/MoS2 heterostructure. We thereby obtain a direct measurement of the interlayer exciton diameter of ~5.4 nm, comparable to the moiré unit-cell length of 6.1 nm. Surprisingly, this large ILX is well localized within the moiré cell to a region of only 1.8 nm - smaller than the size of the exciton itself. This high degree of localization of the interlayer exciton is backed by Bethe-Salpeter equation calculations and demonstrates that the ILX can be localized within small moiré unit cells. Unlike large moiré cells, these are uniform over large regions, thus allowing the formation of extended arrays of localized excitations for quantum technology.

An exciton, a two-body composite quasiparticle formed of an electron and hole, is a fundamental optical excitation in condensed-matter systems. Since its discovery nearly a century ago, a measurement of the excitonic wavefunction has remained beyond experimental reach. Here, we directly image the excitonic wavefunction in reciprocal space by measuring the momentum distribution of electrons photoemitted from excitons in monolayer WSe2. By transforming to real space, we obtain a visual of the distribution of the electron around the hole in an exciton. Further, by also resolving the energy coordinate, we confirm the elusive theoretical prediction that the photoemitted electron exhibits an inverted energy-momentum dispersion relationship reflecting the valence band where the partner hole remains, rather than that of conduction-band states of the electron.

. An axially symmetric space-time is considered in the presence of a perfect fluid source in the framework of f ( R , T ) gravity, where R is the Ricci scalar and T is the trace of the energy-momentum tensor proposed by Harko et al. (Phys. Rev. D 84 , 024020, (2011)). We assume the variation law of mean Hubble parameter to obtain the exact solutions of the field equations. The geometrical and physical parameters for both the models are studied.