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Register a new userImpact of novel electron-phonon coupling mechanisms on valley physics in two-dimensional materials
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Wen-Yu Shan
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We systematically study the impact of various electron-acoustic-phonon coupling mechanisms on valley physics in two-dimensional materials. In the static strain limit, we find that Dirac cone tilt and deformation potential have analogous valley Hall response since they fall into the same universality class of pseudospin structure. However, such argument fails for the coupling mechanism with position-dependent Fermi velocity. For the isotropic case, a significant valley Hall effect occurs near charge neutrality similar to the bond-length change, whereas for the anisotropic case, the geometric valley transport is suppressed, akin to the deformation potential. Gap opening mechanism by nonuniform strain is found to totally inhibit the valley Hall transport, even if the dynamics of strains are introduced. By varying gate voltage, a tunable phonon-assisted valley Hall response can be realized, which paves a way toward rich phenomena and new functionalities of valley acoustoelectronics.
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Through a combined theoretical and experimental effort, we uncover a yet unidentified mechanism that strengthens considerably electron-phonon coupling in materials where electron accumulation leads to population of multiple valleys. Taking atomically-thin transition-metal dichalcogenides as prototypical examples, we establish that the mechanism results from a phonon-induced out-of-phase energy shift of the different valleys, which causes inter-valley charge transfer and reduces the effectiveness of electrostatic screening, thus enhancing electron-phonon interactions. The effect is physically robust, it can play a role in many materials and phenomena, as we illustrate by discussing experimental evidence for its relevance in the occurrence of superconductivity. (short abstract due to size limitations - full abstract in the manuscript)
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 study hydrodynamic phonon heat transport in two-dimensional (2D) materials. Starting from the Peierls-Boltzmann equation within the Callaway model, we derive a 2D Guyer-Krumhansl-like equation describing non-local hydrodynamic phonon transport, taking into account the quadratic dispersion of flexural phonons. In additional to Poiseuille flow, second sound propagation, the equation predicts heat current vortices and negative nonlocal thermal conductance in 2D materials, common in classical fluid but scarcely considered in phonon transport. Our results also illustrate the universal transport behavior of hydrodynamics, independent on the type of quasi-particles and their microscopic interactions.
Recently, there have been increasing interests in phonon thermal transport in low dimensional materials, due to the crucial importance for dissipating and managing heat in micro and nano electronic devices. Significant progresses have been achieved for one-dimensional (1D) systems both theoretically and experimentally. However, the study of heat conduction in two-dimensional (2D) systems is still in its infancy due to the limited availability of 2D materials and the technical challenges in fabricating suspended samples suitable for thermal measurements. In this review, we outline different experimental techniques and theoretical approaches for phonon thermal transport in 2D materials, discuss the problems and challenges in phonon thermal transport measurements and provide comparison between existing experimental data. Special focus will be given to the effects of the size, dimensionality, anisotropy and mode contributions in the novel 2D systems including graphene, boron nitride, MoS2, black phosphorous, silicene etc.
We report direct measurements of the valley susceptibility, the change of valley population in response to applied symmetry-breaking strain, in an AlAs two-dimensional electron system. As the two-dimensional density is reduced, the valley susceptibility dramatically increases relative to its band value, reflecting the systems strong electron-electron interaction. The increase has a remarkable resemblance to the enhancement of the spin susceptibility and establishes the analogy between the spin and valley degrees of freedom.
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