Twist engineering—the alignment of two-dimensional (2D) crystalline layers with a specific orientation—has led to tremendous success in controlling the charge degree of freedom, particularly in ...producing correlated and topological electronic phases in moiré crystals1,2. However, although pioneering theoretical efforts have predicted that non-trivial magnetism3–5 and magnons6,7 can be made by twisting 2D magnets, the experimental realization of engineering the spin degree of freedom by twisting remains elusive. Here we fabricate twisted double bilayers of a 2D magnet, namely, chromium triiodide (CrI3), and demonstrate the successful twist engineering of 2D magnetism in them. We identify signatures of a new magnetic ground state that is distinct from those in natural two-layer (2L) and four-layer (4L) CrI3. We show that for a very small twist angle, this emergent magnetism can be well approximated by a weighted linear superposition of those of 2L and 4L CrI3, whereas for a large twist angle, it mostly resembles that of isolated 2L CrI3. However, at an intermediate twist angle, there is a finite net magnetization that cannot be simply inferred from any homogeneous stacking configuration, but emerges because spin frustrations are introduced by competition between ferromagnetic and antiferromagnetic exchange coupling within individual moiré supercells.Stacking and twisting two-dimensional materials has led to the observation of a variety of electronic phenomena. Now, magnetic behaviour that is distinct from anything seen in individual layers is induced by a moiré pattern in double bilayer chromium triiodide.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
We investigate the ultralow-frequency Raman response of atomically thin ReS2, a special type of two-dimensional (2D) semiconductors with unique distorted 1T structure. Bilayer and few-layer ReS2 ...exhibit rich Raman spectra at frequencies below 50 cm–1, where a panoply of interlayer shear and breathing modes are observed. The emergence of these interlayer phonon modes indicate that the ReS2 layers are coupled and orderly stacked. Whereas the interlayer breathing modes behave similarly to those in other 2D layered crystals, the shear modes exhibit distinctive behavior due to the in-plane lattice distortion. In particular, the two shear modes in bilayer ReS2 are nondegenerate and clearly resolved in the Raman spectrum, in contrast to the doubly degenerate shear modes in other 2D materials. By carrying out comprehensive first-principles calculations, we can account for the frequency and Raman intensity of the interlayer modes and determine the stacking order in bilayer ReS2.
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Kagome-nets, appearing in electronic, photonic and cold-atom systems, host frustrated fermionic and bosonic excitations. However, it is rare to find a system to study their fermion-boson many-body ...interplay. Here we use state-of-the-art scanning tunneling microscopy/spectroscopy to discover unusual electronic coupling to flat-band phonons in a layered kagome paramagnet, CoSn. We image the kagome structure with unprecedented atomic resolution and observe the striking bosonic mode interacting with dispersive kagome electrons near the Fermi surface. At this mode energy, the fermionic quasi-particle dispersion exhibits a pronounced renormalization, signaling a giant coupling to bosons. Through the self-energy analysis, first-principles calculation, and a lattice vibration model, we present evidence that this mode arises from the geometrically frustrated phonon flat-band, which is the lattice bosonic analog of the kagome electron flat-band. Our findings provide the first example of kagome bosonic mode (flat-band phonon) in electronic excitations and its strong interaction with fermionic degrees of freedom in kagome-net materials.
Two-dimensional (2D) magnetism has been long sought-after and only very recently realized in atomic crystals of magnetic van der Waals materials. So far, a comprehensive understanding of the magnetic ...excitations in such 2D magnets remains missing. Here we report polarized micro-Raman spectroscopy studies on a 2D honeycomb ferromagnet CrI
. We show the definitive evidence of two sets of zero-momentum spin waves at frequencies of 2.28 terahertz (THz) and 3.75 THz, respectively, that are three orders of magnitude higher than those of conventional ferromagnets. By tracking the thickness dependence of both spin waves, we reveal that both are surface spin waves with lifetimes an order of magnitude longer than their temporal periods. Our results of two branches of high-frequency, long-lived surface spin waves in 2D CrI
demonstrate intriguing spin dynamics and intricate interplay with fluctuations in the 2D limit, thus opening up opportunities for ultrafast spintronics incorporating 2D magnets.
Abstract
Electron band topology is combined with intrinsic magnetic orders in MnBi
2
Te
4
, leading to novel quantum phases. Here we investigate collective spin excitations (i.e. magnons) and spin ...fluctuations in atomically thin MnBi
2
Te
4
flakes using Raman spectroscopy. In a two-septuple layer with non-trivial topology, magnon characteristics evolve as an external magnetic field tunes the ground state through three ordered phases: antiferromagnet, canted antiferromagnet, and ferromagnet. The Raman selection rules are determined by both the crystal symmetry and magnetic order while the magnon energy is determined by different interaction terms. Using non-interacting spin-wave theory, we extract the spin-wave gap at zero magnetic field, an anisotropy energy, and interlayer exchange in bilayers. We also find magnetic fluctuations increase with reduced thickness, which may contribute to a less robust magnetic order in single layers.
We have investigated the vibrational properties of van der Waals heterostructures of monolayer transition metal dichalcogenides (TMDs), specifically MoS sub(2)/WS e sub(2) and MoS e sub(2)/MoS sub(2) ...heterobilayers and twisted MoS sub(2) bilayers, by means of ultralow-frequency Raman spectroscopy. We discovered Raman features (at 30-40 cm super(-1)) that arise from the layer-breathing mode (LBM) vibration between the two incommensurate TMD monolayers in these structures. The LBM Raman intensity correlates strongly with the suppression of photoluminescence that arises from interlayer charge transfer. The LBM is generated only in bilayer areas with direct layer-layer contact and an atomically clean interface. Its frequency also evolves systematically with the relative orientation between the two layers. Our research demonstrates that the LBM can serve as a sensitive probe to the interface environment and interlayer interactions in van der Waals materials.
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Exploring new parameter regimes to realize and control novel phases of matter has been a main theme in modern condensed matter physics research. The recent discovery of two-dimensional (2D) magnetism ...in nearly freestanding monolayer atomic crystals has already led to observations of a number of novel magnetic phenomena absent in bulk counterparts. Such intricate interplays between magnetism and crystalline structures provide ample opportunities for exploring quantum phase transitions in this new 2D parameter regime. Here, using magnetic field- and temperature-dependent circularly polarized Raman spectroscopy of phonons and magnons, we map out the phase diagram of chromium triiodide (CrI3) that has been known to be a layered antiferromagnet (AFM) in its 2D films and a ferromagnet (FM) in its three-dimensional (3D) bulk. However, we reveal a novel mixed state of layered AFM and FM in 3DCrI3bulk crystals where the layered AFM survives in the surface layers, and the FM appears in deeper bulk layers. We then show that the surface-layered AFM transits into the FM at a critical magnetic field of 2 T, similar to what was found in the few-layer case. Interestingly, concurrent with this magnetic phase transition, we discover a first-order structural phase transition that alters the crystallographic point group fromC3i(rhombohedral) toC2h(monoclinic). Our result not only unveils the complex single-magnon behavior in 3DCrI3, but it also settles the puzzle of howCrI3transits from a bulk FM to a thin-layered AFM semiconductor, despite recent efforts in understanding the origin of layered AFM inCrI3thin layers, and reveals the intimate relationship between the layered AFM-to-FM and the crystalline rhombohedral-to-monoclinic phase transitions. These findings further open opportunities for future 2D magnet-based magnetomechanical devices.
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We investigated the low-frequency Raman spectra of freestanding few-layer graphene (FLG) at varying temperatures (400–900 K) controlled by laser heating. At high temperature, we observed the ...fundamental Raman mode for the lowest-frequency branch of rigid-plane layer-breathing mode (LBM) vibration. The mode frequency redshifts dramatically from 81 cm–1 for bilayer to 23 cm–1 for 8-layer. The thickness dependence is well described by a simple model of coupled oscillators. Notably, the LBM Raman response is unobservable at room temperature, and it is turned on at higher temperature (>600 K) with a steep increase of Raman intensity. The observation suggests that the LBM vibration is strongly suppressed by molecules adsorbed on the graphene surface but is activated as desorption occurs at high temperature.
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Abstract
Exciton dynamics can be strongly affected by lattice vibrations through electron-phonon coupling. This is rarely explored in two-dimensional magnetic semiconductors. Focusing on bilayer CrI
...3
, we first show the presence of strong electron-phonon coupling through temperature-dependent photoluminescence and absorption spectroscopy. We then report the observation of periodic broad modes up to the 8th order in Raman spectra, attributed to the polaronic character of excitons. We establish that this polaronic character is dominated by the coupling between the charge-transfer exciton at 1.96 eV and a longitudinal optical phonon at 120.6 cm
−1
. We further show that the emergence of long-range magnetic order enhances the electron-phonon coupling strength by ~50% and that the transition from layered antiferromagnetic to ferromagnetic order tunes the spectral intensity of the periodic broad modes, suggesting a strong coupling among the lattice, charge and spin in two-dimensional CrI
3
. Our study opens opportunities for tailoring light-matter interactions in two-dimensional magnetic semiconductors.
Single crystals of hexagonal boron nitride (hBN) have recently been envisioned for electronic, optoelectronic, and nanophotonic applications. In this study, the production of large-scale, ...high-quality hBN single crystals via precipitation from a new solvent composed of Fe and Cr was demonstrated to be viable at atmospheric pressure. The clear and colorless crystals have a maximum domain size of around 2 mm and a thickness of around 200 μm. The Raman spectra and photoluminescence emission spectra demonstrate that the crystals produced with this solvent are pure hBN phase, and with low defect and residual impurity concentrations. The use of an Fe–Cr mixture provides a lower cost alternative to the more common Ni–Cr solvent for growing large hBN of comparable quality.
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