Measurement of filling-factor-dependent magnetophonon resonances in graphene using Raman spectroscopy

Evidence for Topological Magnon-Phonon Hybridization in a 2D Antiferromagnet down to the Monolayer Limit

Topological phonons and magnons potentially enable low-loss, quantum coherent, and chiral transport of information and energy at the atomic scale. Van der Waals magnetic materials are promising to realize such states due to their recently discovered strong interactions among the electronic, spin, and lattice degrees of freedom. Here, we report the first observation of coherent hybridization of magnons and phonons in monolayer antiferromagnet FePSe₃ by cavity-enhanced magneto-Raman spectroscopy. The robust magnon-phonon cooperativity in the 2D limit occurs even in zero magnetic field, which enables nontrivial band inversion between longitudinal and transverse optical phonons caused by the strong coupling with magnons. The spin and lattice symmetry theoretically guarantee magnetic-field-controlled topological phase transition, verified by nonzero Chern numbers calculated from the coupled spin-lattice model. The 2D topological magnon-phonon hybridization potentially offers a new route toward quantum phononics and magnonics with an ultrasmall footprint.

Spatially resolved optical spectroscopy in extreme environment of low temperature, high magnetic fields and high pressure

We present an experimental setup developed to perform optical spectroscopy experiments (Raman scattering and photoluminescence measurements) with a micrometer spatial resolution in an extreme environment of low temperature, high magnetic field, and high pressure. This unique experimental setup, to the best of our knowledge, allows us to deeply explore the phase diagram of condensed matter systems by independently tuning these three thermodynamic parameters while monitoring the low-energy excitations (electronic, phononic, or magnetic excitations) to spatially map the Raman scattering response or to investigate objects with low dimensions. We apply this technique to bulk FePS₃, a layered antiferromagnet with a Néel temperature of T ≈ 120 K.

Electrical Control over Phonon Polarization in Strained Graphene

We explore the tunability of the phonon polarization in suspended uniaxially strained graphene by magneto-phonon resonances. The uniaxial strain lifts the degeneracy of the LO and TO phonons, yielding two cross-linearly polarized phonon modes and a splitting of the Raman G peak. We utilize the strong electron-phonon coupling in graphene and the off-resonant coupling to a magneto-phonon resonance to induce a gate-tunable circular phonon dichroism. This, together with the strain-induced splitting of the G peak, allows us to controllably tune the two linearly polarized G mode phonons into circular phonon modes. We are able to achieve a circular phonon polarization of up to 40% purely by electrostatic fields and can reverse its sign by tuning from electron to hole doping. This provides unprecedented electrostatic control over the angular momentum of phonons, which paves the way toward phononic applications.

Observation of magnetic domains in graphene magnetized by controlling temperature, strain and magnetic field

Since the production of ferromagnetic graphene as an extremely important matter in spintronics has made a revolution in future technology, a great deal of efforts has recently been done to reach a simple and cost-effective method. Up to now, controlling the magnetic properties at extremely low temperature have been investigated only by adding and removing atoms in graphene lattice. In this regard, the effect of strain on the magnetic and electronic properties of graphene has been probed. Here, the ferromagnetic properties are what have been created by strain, magnetic field, and temperature along with observation of the parallel magnetic domains in ferromagnetic graphene for the first time as a great achievement. In this way, we have represented the following: First, introducing three novel methods based on temperature, magnetic field, and strain for producing ferromagnetic graphene; Second, obtaining ferromagnetic graphene at room temperature by significant magnetization saturation in mass-scale; Third, probing the electronic systems and vibrational modes by Raman and IR spectroscopy; Fourth, introducing stacking and aggregation as two types of gathering process for graphene sheets; Fifth, comparing the results with leidenfrost effect-based method which the temperature, magnetic fields, and strain are simultaneously applied to graphene flakes (our previous work).

Strong magnetophonon oscillations in extra-large graphene

Van der Waals materials and their heterostructures offer a versatile platform for studying a variety of quantum transport phenomena due to their unique crystalline properties and the exceptional ability in tuning their electronic spectrum. However, most experiments are limited to devices that have lateral dimensions of only a few micrometres. Here, we perform magnetotransport measurements on graphene/hexagonal boron-nitride Hall bars and show that wider devices reveal additional quantum effects. In devices wider than ten micrometres we observe distinct magnetoresistance oscillations that are caused by resonant scattering of Landau-quantised Dirac electrons by acoustic phonons in graphene. The study allows us to accurately determine graphene’s low energy phonon dispersion curves and shows that transverse acoustic modes cause most of phonon scattering. Our work highlights the crucial importance of device width when probing quantum effects and also demonstrates a precise, spectroscopic method for studying electron-phonon interactions in van der Waals heterostructures.

Increasing the size of mesoscopic devices based on van der Waals heterostructures triggers additional quantum effects. Here, the authors observe distinct magnetoresistance oscillations in graphene/h-BN Hall bars only in devices wider than 10 μm due to resonant scattering of charge carriers by transverse acoustic phonons in graphene.

Raman spectroscopy of graphene-based materials and its applications in related devices

Graphene-based materials exhibit remarkable electronic, optical, and mechanical properties, which has resulted in both high scientific interest and huge potential for a variety of applications. Furthermore, the family of graphene-based materials is growing because of developments in preparation methods. Raman spectroscopy is a versatile tool to identify and characterize the chemical and physical properties of these materials, both at the laboratory and mass-production scale. This technique is so important that most of the papers published concerning these materials contain at least one Raman spectrum. Thus, here, we systematically review the developments in Raman spectroscopy of graphene-based materials from both fundamental research and practical (i.e., device applications) perspectives. We describe the essential Raman scattering processes of the entire first- and second-order modes in intrinsic graphene. Furthermore, the shear, layer-breathing, G and 2D modes of multilayer graphene with different stacking orders are discussed. Techniques to determine the number of graphene layers, to probe resonance Raman spectra of monolayer and multilayer graphenes and to obtain Raman images of graphene-based materials are also presented. The extensive capabilities of Raman spectroscopy for the investigation of the fundamental properties of graphene under external perturbations are described, which have also been extended to other graphene-based materials, such as graphene quantum dots, carbon dots, graphene oxide, nanoribbons, chemical vapor deposition-grown and SiC epitaxially grown graphene flakes, composites, and graphene-based van der Waals heterostructures. These fundamental properties have been used to probe the states, effects, and mechanisms of graphene materials present in the related heterostructures and devices. We hope that this review will be beneficial in all the aspects of graphene investigations, from basic research to material synthesis and device applications.

Anomalous Raman scattering and lattice dynamics in mono- and few-layer WTe2

Tungsten ditelluride (WTe2) is a layered material that exhibits excellent magnetoresistance and thermoelectric behaviors, which are deeply related with its distorted orthorhombic phase that may critically affect the lattice dynamics of this material. Here, we report comprehensive characterization of Raman spectra of WTe2 from bulk to monolayer using experimental and computational methods. We find that mono and bi-layer WTe2 are easily identified by Raman spectroscopy since two or one Raman modes that are observed in higher-layer WTe2 are greatly suppressed below the noise level in the mono- and bi-layer WTe2, respectively. In addition, the frequency of in-plane A1(7) mode of WTe2 remains almost constant as the layer number decreases, while all the other Raman modes consistently blueshift, which is completely different from the vibrational behavior of hexagonal metal dichalcogenides. First-principles calculation validates experimental results and reveals that anomalous lattice vibrations in WTe2 are attributed to the formation of tungsten chains that make WTe2 structurally one-dimensional.

Raman spectroscopy as probe of nanometre-scale strain variations in graphene

Confocal Raman spectroscopy has emerged as a major, versatile workhorse for the non-invasive characterization of graphene. Although it is successfully used to determine the number of layers, the quality of edges, and the effects of strain, doping and disorder, the nature of the experimentally observed broadening of the most prominent Raman 2D line has remained unclear. Here we show that the observed 2D line width contains valuable information on strain variations in graphene on length scales far below the laser spot size, that is, on the nanometre-scale. This finding is highly relevant as it has been shown recently that such nanometre-scaled strain variations limit the carrier mobility in high-quality graphene devices. Consequently, the 2D line width is a good and easily accessible quantity for classifying the crystalline quality, nanometre-scale flatness as well as local electronic properties of graphene, all important for future scientific and industrial applications.

Raman spectroscopy has become an invaluable tool for graphene characterisation, yet the nature of the broadening of the Raman 2D line remains unclear. Here, Stampfer et al. show that the Raman 2D line width is a measure of nanometre-scale strain variations in graphene on insulating substrates.

Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

Low B field magneto-phonon resonances in single-layer and bilayer graphene

Many-body effects resulting from strong electron-electron and electron-phonon interactions play a significant role in graphene physics. We report on their manifestation in low B field magneto-phonon resonances in high-quality exfoliated single-layer and bilayer graphene encapsulated in hexagonal boron nitride. These resonances allow us to extract characteristic effective Fermi velocities, as high as 1.20 × 10(6) m/s, for the observed "dressed" Landau level transitions, as well as the broadening of the resonances, which increases with the Landau level index.

Electrical switch to the resonant magneto-phonon effect in graphene

We report a comprehensive study of the tuning with electric fields of the resonant magneto-exciton optical phonon coupling in gated graphene. For magnetic fields around B ∼ 25 T that correspond to the range of the fundamental magneto-phonon resonance, the electron-phonon coupling can be switched on and off by tuning the position of the Fermi level in order to Pauli block the two fundamental inter-Landau level excitations. The effects of such a profound change in the electronic excitation spectrum are traced through investigations of the optical phonon response in polarization resolved magneto-Raman scattering experiments. We report on the observation of a splitting of the phonon feature with satellite peaks developing at particular values of the Landau level filling factor on the low or on the high energy side of the phonon, depending on the relative energy of the discrete electronic excitation and of the optical phonon. Shifts of the phonon energy as large as ±60 cm(-1) are observed close to the resonance. The intraband electronic excitation, the cyclotron resonance, is shown to play a relevant role in the observed spectral evolution of the phonon response.

Charge tuning of nonresonant magnetoexciton phonon interactions in graphene

Far from resonance, the coupling of the G-band phonon to magnetoexcitons in single layer graphene displays kinks and splittings versus filling factor that are well described by Pauli blocking and unblocking of inter- and intra-Landau level transitions. We explore the nonresonant electron-phonon coupling by high-magnetic field Raman scattering while electrostatic tuning of the carrier density controls the filling factor. We show qualitative and quantitative agreement between spectra and a linearized model of electron-phonon interactions in magnetic fields. The splitting is caused by dichroism of left- and right-handed circular polarized light due to lifting of the G-band phonon degeneracy, and the piecewise linear slopes are caused by the linear occupancy of sequential Landau levels versus ν.