Metastable electron-electron states in double-layer graphene structures

PublishedJournal ArticleThe prototypical exciton model of two interacting Dirac particles in graphene was analyzed in J. Sabio, Phys. Rev. B 81, 045428 (2010)PRBMDO1098-012110.1103/PhysRevB.81.045428 and it was found that in one of the electron-hole scattering channels the total kinetic energy vanishes, resulting in a singular behavior. We show that this singularity can be removed by extending the quasiparticle dispersion, thus breaking the symmetry between upper and lower Dirac cones. The dynamics of an electron-electron pair are then mapped onto that of a single particle with negative mass and anisotropic dispersion. We show that the interplay between dispersion and repulsive interaction can result in the formation of bound, Cooper-pair-like, metastable states in double-layered hybrid structures.The authors wish to thank V. I. Fal’ko and M. V. Berry for insightful discussions. A.V.S. is supported by EPSRC/HEFCE Grant No. EP/G036101/1

Controlling acoustic waves using magneto-elastic Fano resonances

This is the author accepted manuscript. The final version is available from AIP Publishing via the DOI in this recordThe erratum to this article is available in ORE: http://hdl.handle.net/10871/123102We propose and analyze theoretically a class of energy-efficient magneto-elastic devices for analogue signal processing. The signals are carried by transverse acoustic waves while the bias magnetic field controls their scattering from a magneto-elastic slab. By tuning the bias field, one can alter the resonant frequency at which the propagating acoustic waves hybridize with the magnetic modes, and thereby control transmission and reflection coefficients of the acoustic waves. The scattering coefficients exhibit Breit-Wigner/Fano resonant behaviour akin to inelastic scattering in atomic and nuclear physics. Employing oblique incidence geometry, one can effectively enhance the strength of magnetoelastic coupling, and thus countermand the magnetic losses due to the Gilbert damping. We apply our theory to discuss potential benefits and issues in realistic systems and suggest routes to enhance performance of the proposed devices.Engineering and Physical Sciences Research Council (EPSRC)European Union Horizon 202

Spin-wave control using dark modes in chiral magnonic resonators

This is the final version. Available from the American Physical Society via the DOI in this recordData availability: The data that support the findings of this study are available from the corresponding author upon reasonable requestWe modeled, both analytically and numerically, the magnitude and phase of spin waves propagating in thin magnetic films and scattered from mesoscale chiral magnonic resonators. Our calculations reveal a remarkably strong chiral scattering of propagating spin waves from magnon dark modes hosted by the resonator, exceeding in strength the scattering from its quasiuniform mode. We formulate conditions for the waveguide-resonator system to be used as an efficient spin-wave diode and as a phase shifter. Both these applications are found to be feasible when using the available ferromagnetic materials for the resonators.Engineering and Physical Sciences Research Council (EPSRC

Nonlinear chiral magnonic resonators: Toward magnonic neurons

This is the final version. Available on open access from AIP Publishing via the DOI in this recordData availability: The data that support the findings of this study are available within the article and its supplementary material.We explore chiral magnonic resonators as building blocks of artificial neural networks. Via micromagnetic simulations and analytical modeling, we demonstrate that the spin-wave modes confined in the resonators exhibit a strongly nonlinear response owing to energy concentration when resonantly excited by incoming spin waves. This effect may be harnessed to implement an artificial neuron in a network. Therefore, the confined and propagating spin-wave modes can serve as neurons and interneural connections, respectively. For modest excitation levels, the effect can be described in terms of a nonlinear shift of the resonant frequency (“detuning”), which results in amplitude-dependent transmission of monochromatic spin waves, which may be harnessed to recreate a “sigmoid-like” activation function. At even stronger excitation levels, the nonlinearity leads to bistability and hysteresis, akin to those occurring in nonlinear oscillators when the excitation strength exceeds a threshold set by the decay rate of the mode. In magnonic resonators, the latter includes both the Gilbert damping and the radiative decay due to the coupling with the medium. The results of our simulations are well described by a phenomenological model in which the nonlinear detuning of the confined mode is quadratic in its amplitude, while the propagation in the medium is linear.UKRIHorizon EuropeEngineering and Physical Sciences Research Council (EPSRC

Resonant scattering of surface acoustic waves by arrays of magnetic stripes

This is the final version. Available from the American Institute of Physics via the DOI in this record. DATA AVAILABILITY: The data that support the findings of this study are available from the corresponding author upon reasonable request.Owing to magnetoelastic coupling, surface acoustic waves (SAWs) may be scattered resonantly by magnetic elements, such as nickel stripes. The scattering may be further enhanced via the Borrmann effect when the elements are organized into an array that matches the acoustic wavelength. We use finite-element modeling to consider single- and double-layer stripes patterned on top of a lithium niobate surface that carries Love surface waves. We do observe enhancement in the coupling for single-layer stripes, but only for Gilbert damping below its realistic value. For double-layered stripes, a weak yet clear and distinct signature of Bragg reflection is identified far away from the acoustic band edge, even for a realistic damping value. Double-layered stripes also offer better magnetic tunability when their magnetic period is different from the periodicity of elastic properties of the structure because of staggered magnetization patterns. The results pave the way for the design of magnetoacoustic metamaterials with an enhanced coupling between propagating SAWs and local magnetic resonances and for the development of reconfigurable SAW-based circuitry.Engineering and Physical Sciences Research Council (EPSRC)Engineering and Physical Sciences Research Council (EPSRC

Hybrid magnetoacoustic metamaterials for ultrasound control

This is the author accepted manuscript. the final version is available from the American Institute of Physics via the DOI in this recordData availability: The data that support the findings of this study are available within the article and its supplementary material.We propose a class of metamaterials in which the propagation of acoustic waves is controlled magnetically through magnetoelastic coupling. The metamaterials are formed by a periodic array of thin magnetic layers ("resonators") embedded in a nonmagnetic matrix. Acoustic waves carrying energy through the structure hybridize with the magnetic modes of the resonators ("Fano resonance"). This leads to a rich set of effects, enhanced by Bragg scattering and being most pronounced when the magnetic resonance frequency is close to or lies within acoustic bandgaps. The acoustic reflection from the structure exhibits magnetically induced transparency and Borrmann effect. Our analysis shows that the combined effect of the Bragg scattering and Fano resonance may overcome the magnetic damping, ubiquitous in realistic systems. This paves a route toward the application of such structures in wave computing and signal processing.Engineering and Physical Sciences Research Council (EPSRC)European Union Horizon 202

Erratum: “Controlling acoustic waves using magnetoelastic Fano resonances” [Appl. Phys. Lett. 115, 082403 (2019)]

This is the final version. Available from AIP Publishing via the DOI in this recordThe article to which this is the erratum is available in ORE: http://hdl.handle.net/10871/3814

Realization of a Tunable Artificial Atom at a Supercritically Charged Vacancy in Graphene

The remarkable electronic properties of graphene have fueled the vision of a graphene-based platform for lighter, faster and smarter electronics and computing applications. One of the challenges is to devise ways to tailor its electronic properties and to control its charge carriers. Here we show that a single atom vacancy in graphene can stably host a local charge and that this charge can be gradually built up by applying voltage pulses with the tip of a scanning tunneling microscope (STM). The response of the conduction electrons in graphene to the local charge is monitored with scanning tunneling and Landau level spectroscopy, and compared to numerical simulations. As the charge is increased, its interaction with the conduction electrons undergoes a transition into a supercritical regime 6-11 where itinerant electrons are trapped in a sequence of quasi-bound states which resemble an artificial atom. The quasi-bound electron states are detected by a strong enhancement of the density of states (DOS) within a disc centered on the vacancy site which is surrounded by halo of hole states. We further show that the quasi-bound states at the vacancy site are gate tunable and that the trapping mechanism can be turned on and off, providing a new mechanism to control and guide electrons in grapheneComment: 18 pages and 5 figures plus 14 pages and 15 figures of supplementary information. Nature Physics advance online publication, Feb 22 (2016

Mapping Dirac quasiparticles near a single Coulomb impurity on graphene

The response of Dirac fermions to a Coulomb potential is predicted to differ significantly from how non-relativistic electrons behave in traditional atomic and impurity systems. Surprisingly, many key theoretical predictions for this ultra-relativistic regime have not been tested. Graphene, a two-dimensional material in which electrons behave like massless Dirac fermions, provides a unique opportunity to test such predictions. Graphene’s response to a Coulomb potential also offers insight into important material characteristics, including graphene’s intrinsic dielectric constant, which is the primary factor determining the strength of electron–electron interactions in graphene. Here we present a direct measurement of the nanoscale response of Dirac fermions to a single Coulomb potential placed on a gated graphene device. Scanning tunnelling microscopy was used to fabricate tunable charge impurities on graphene, and to image electronic screening around them for a Q = +1|e| charge state. Electron-like and hole-like Dirac fermions were observed to respond differently to a Coulomb potential. Comparing the observed electron–hole asymmetry to theoretical simulations has allowed us to test predictions for how Dirac fermions behave near a Coulomb potential, as well as extract graphene’s intrinsic dielectric constant: ε[subscript g] = 3.0±1.0. This small value of ε[subscript g] indicates that electron–electron interactions can contribute significantly to graphene properties.United States. Office of Naval Research. Multidisciplinary University Research Initiative (Award N00014-09-1-1066)United States. Dept. of Energy. Office of Science (Contract DE-AC02-05CH11231)National Science Foundation (U.S.) (Award DMR-0906539