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Register a new userCavity control of Excitons in two dimensional Materials
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We propose a robust and efficient way of controlling the optical spectra of two-dimensional materials and van der Waals heterostructures by quantum cavity embedding. The cavity light-matter coupling leads to the formation of exciton-polaritons, a superposition of photons and excitons. Our first principles study demonstrates a reordering and mixing of bright and dark excitons spectral features and in the case of a type II van-der-Waals heterostructure an inversion of intra and interlayer excitonic resonances. We further show that the cavity light-matter coupling strongly depends on the dielectric environment and can be controlled by encapsulating the active 2D crystal in another dielectric material. Our theoretical calculations are based on a newly developed non-perturbative many-body framework to solve the coupled electron-photon Schrodinger equation in a quantum-electrodynamical extension of the Bethe-Salpeter approach. This approach enables the ab-initio simulations of exciton-polariton states and their dispersion from weak to strong cavity light-matter coupling regimes. Our method is then extended to treat van der Waals heterostructures and encapsulated 2D materials using a simplified Mott-Wannier description of the excitons that can be applied to very large systems beyond reach for fully ab-initio approaches.
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We study direct and indirect magnetoexcitons in Rydberg states in monolayers and double-layer heterostructures of Xenes (silicene, germanene, and stanene) in external parallel electric and magnetic fields, applied perpendicular to the monolayer and heterostructure. We calculate binding energies of magnetoexcitons for the Rydberg states 1$s$, 2$s$, 3$s$, and 4$s$, by numerical integration of the Schr{o}dinger equation using the Rytova-Keldysh potential for direct magnetoexciton and both the Rytova-Keldysh and Coulomb potentials for indirect excitons. Latter allows understanding a role of screening in Xenes. In the external perpendicular electric field, the buckled structure of the Xene monolayers leads to appearance of potential difference between sublattices allowing to tune electron and hole masses and, therefore, the binding energies and diamagnetic coefficients (DMCs) of magnetoexcitons. We report the energy contribution from electric and magnetic fields to the binding energies and DMCs. The tunability of the energy contribution of direct and indirect magnetoexcitons by electric and magnetic fields is demonstrated. It is also shown that DMCs of direct excitons can be tuned by the electric field, and the DMCs of indirect magnetoexcitons can be tuned by the electric field and manipulated by the number of h-BN layers. Therefore, these allowing the possibility of electronic devices design that can be controlled by external electric and magnetic fields and the number of h-BN layers. The calculations of the binding energies and DMCs of magnetoexcitons in Xenes monolayers and heterostructures are novel and can be compared with the experimental results when they will be available.
Extreme confinement of electromagnetic energy by phonon polaritons holds the promise of strong and new forms of control over the dynamics of matter. To bring such control to the atomic-scale limit, it is important to consider phonon polaritons in two-dimensional (2D) systems. Recent studies have pointed out that in 2D, splitting between longitudinal and transverse optical (LO and TO) phonons is absent at the $Gamma$ point, even for polar materials. Does this lack of LO--TO splitting imply the absence of a phonon polariton in polar monolayers? Here, we derive a first-principles expression for the conductivity of a polar monolayer specified by the wavevector-dependent LO and TO phonon dispersions. In the long-wavelength (local) limit, we find a universal form for the conductivity in terms of the LO phonon frequency at the $Gamma$ point, its lifetime, and the group velocity of the LO phonon. Our analysis reveals that the phonon polariton of 2D is simply the LO phonon of the 2D system. For the specific example of hexagonal boron nitride (hBN), we estimate the confinement and propagation losses of the LO phonons, finding that high confinement and reasonable propagation quality factors coincide in regions which may be difficult to detect with current near-field optical microscopy techniques. Finally, we study the interaction of external emitters with two-dimensional hBN nanostructures, finding extreme enhancement of spontaneous emission due to coupling with localized 2D phonon polaritons, and the possibility of multi-mode strong and ultra-strong coupling between an external emitter and hBN phonons. This may lead to the design of new hybrid states of electrons and phonons based on strong coupling.
We demonstrate how weak hybridization can lead to apparent heavy doping of 2d materials even in case of physisorptive binding. Combining ab-intio calculations and a generic model we show that strong reshaping of Fermi surfaces and changes in Fermi volumes on the order of several 10$%$ can arise without actual charge transfer. This pseudodoping mechanism is very generically effective in metallic 2d materials either weakly absored to metallic substrates or embedded in vertical heterostructures. It can explain strong apparent doping of TaS2 on Au (111) observed in recent experiments. Consequences of pseudodoping for many-body instabilities are discussed.
We propose an optical method of shining circularly polarized and spatially periodic laser fields to imprint superlattice structures in two-dimensional electronic systems. By changing the configuration of the optical field, we synthesize various lattice structures with different spatial symmetry, periodicity, and strength. We find that the wide optical tunability allows one to tune different properties of the effective band structure, including Chern number, energy bandwidths, and band gaps. The in situ tunability of the superlattice gives rise to unique physics ranging from the topological transitions to the creation of the flat bands through the kagome superlattice, which can allow a realization of strongly correlated phenomena in Floquet systems. We consider the high-frequency regime where the electronic system can remain in the quasiequilibrium phase for an extended amount of time. The spatiotemporal reconfigurability of the present scheme opens up possibilities to control light-matter interaction to generate novel electronic states and optoelectronic devices.
In this work, we provide a detailed derivation of Foldy-Wouthuysen (FW) transformation for two-dimensional (2D) gapped Dirac fermions under external fields and apply the formalism to study valley excitons in 2D semiconducting materials. Similar to relativistic quantum few-body problem, the gapped Dirac equation can be transformed into a Schr{o}dinger equation with relativistic correction terms. In this 2D materials system, the correction terms can be interpreted as the Berry-curvature effect. The Hamiltonian for a valley exciton in external fields can be written based on the FW transformed Dirac Hamiltonian. Various valley-dependent effects on excitons, such as fine-structure splittings of exciton energy levels, valley-selected exciton transitions, and exciton valley Zeeman effect are discussed within this framework.
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