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Register a new userConfined polar optical phonons in semiconductor double heterostructures: an improved continuum approach
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Confined polar optical phonons are studied in a semiconductor double heterostructure (SDH) by means of a generalization of a theory developed some years ago and based on a continuous medium model. The treatment considers the coupling of electro-mechanical oscillations and involves dispersive phonons. This approach has provided results beyond the usually applied dielectric continuum models, where just the electric aspect of the oscillations is analyzed. In the previous works on the subject the theory included phonon dispersion within a quadratic (parabolic) approximation, while presently linear contributions were added by a straightforward extension of the fundamental equations. The generalized version of the mentioned theoretical treatment leads to a description of long wavelength polar optical phonons showing a closer agreement with experimental data and with calculations along atomistic models. This is particularly important for systems where the linear contribution to dispersion becomes predominant. We present a systematic derivation of the underlying equations, their solutions for the bulk and SDH cases, providing us a complete description of the dispersive modes and the associated electron-phonon Hamiltonian. The results obtained are applied to the case of a EuS/PbS/EuS quantum-well.
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We analyze the band topology of acoustic phonons in 2D materials by considering the interplay of spatial and internal symmetries with additional constraints that arise from the physical context. These supplemental constraints trace back to the Nambu-Goldstone theorem and the requirements of structural stability. We show that this interplay can give rise to previously unaddressed non-trivial nodal charges that are associated with the crossing of the acoustic phonon branches at the center ($Gamma$-point) of the phononic Brillouin zone. We moreover apply our perspective to the concrete context of graphene, where we demonstrate that the phonon spectrum harbors these kinds of non-trivial nodal charges. Apart from its fundamental appeal, this analysis is physically consequential and dictates how the phonon dispersion is affected when graphene is grown on a substrate. Given the generality of our framework, we anticipate that our strategy that thrives on combining physical context with insights from topology should be widely applicable in characterizing systems beyond electronic band theory.
Confined optical phonons are discussed for a semiconductor nanowire of the Ge (Si)prototype on the basis of a theory developed some years ago. In the present work this theory is adapted to a non polar material and generalized to the case when the phonon dispersion law involves both linear and quadratic terms in the wave vector. The treatment is considered along the lines of a continuous medium model and leads to a system of coupled differential equations describing oscillations of mixed nature. The nanowire is modelled in the form of an infinite circular cylinder and the solutions of the fundamental equations are found. We are thus led to a description of long wavelength optical phonons, which should show a closer agreement with experimental data and with calculations along atomistic models. The presented theory is applied to the calculation of optical phonons in a Ge nanowire. We have found the dispersion curves for various optical phonon modes. We also normalize the modes and discuss the electron-phonon interaction within the deformation potential approximation.
The coupling between spin, charge, and lattice degrees of freedom plays an important role in a wide range of fundamental phenomena. Monolayer semiconducting transitional metal dichalcogenides have emerged as an outstanding platform for studying these coupling effects because they possess unique spin-valley locking physics for hosting rich excitonic species and the reduced screening for strong Coulomb interactions. Here, we report the observation of multiple valley phonons, phonons with momentum vectors pointing to the corners of the hexagonal Brillouin zone, and the resulting exciton complexes in the monolayer semiconductor WSe2. From Lande g-factor and polarization analyses of photoluminescence peaks, we find that these valley phonons lead to efficient intervalley scattering of quasi particles in both exciton formation and relaxation. This leads to a series of photoluminescence peaks as valley phonon replicas of dark trions. Using identified valley phonons, we also uncovered an intervalley exciton near charge neutrality, and extract its short-range electron-hole exchange interaction to be about 10 meV. Our work not only identifies a number of previously unknown 2D excitonic species, but also shows that monolayer WSe2 is a prime candidate for studying interactions between spin, pseudospin, and zone-edge phonons.
A topological superconductor features at its boundaries and vortices Majorana fermions, which are potentially applicable for topological quantum computations. The scarcity of the known experimentally verified physical systems with topological superconductivity, time-reversal invariant ones in particular, is giving rise to a strong demand for identifying new candidate materials. In this research, we study a heterostructure consisting of a transition metal oxide two-dimensional electron gas (2DEG) sandwiched by insulators near the paraelectric (PE) / ferroelectric (FE) phase transition. Its relevant characteristics is the combination of the transition metal spin-orbit coupling and the soft odd-parity phonons arising from the ferroelectric fluctuation; it gives rise to the fluctuating Rashba effect, which can mediate the pairing interaction for time-reversal invariant topological superconductivity. As the PE / FE phase transition can be driven by applying strain on the heterostructure, this system provides a tunable electron-phonon coupling. Through the first-principle calculations on the (001) [BaOsO3][BaTiO3]4, we find such electron-phonon coupling to be strong over a wide range of applied tensile bi-axial strain in the monolayer BaOsO3 sandwiched between the (001) BaTiO3, hence qualifying it as a good candidate material. Furthermore, the stability of topological superconductivity in this material is enhanced by its orbital physics that gives rise to the anisotropic dispersion.
We generalize the notion of dissipationless, topological Hall viscosity tensor to optical phonons in thin film Weyl semimetals. By using the strained Porphyrin thin film Weyl semimetal as a model example, we show how optical phonons can couple to Weyl electrons as chiral pseudo gauge fields. These chiral vector fields lead to a novel dissipationless two-rank viscosity tensor in the effective dynamics of optical phonons whose origin is the chiral anomaly. We also compute the contribution to this two rank Hall viscosity tensor due to the presence of an external magnetic field, whose origin is the conventional Hall response of Weyl electrons. Finally, the phonon dispersion relations of the system at the long-wavelength limit with and without an electromagnetic field are calculated showing a measurable shift in the Raman response of the system. Our results can be investigated by Raman scattering or infrared spectroscopy by attenuated total reflectance experiments.
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