We investigate the ultralow-frequency Raman response of atomically thin ReS 2 , a special type of twodimensional (2D) semiconductors with unique distorted 1T structure. Bilayer and few-layer ReS 2 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 ReS 2 layers are coupled and stacked orderly, in contrast to the general belief that the ReS 2 layers are decoupled from one another. While the interlayer breathing modes can be described by a linear chain model as 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 ReS 2 are non-degenerate and well separated 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 ReS 2 .Interlayer coupling and stacking order are crucial factors to define the physics in few-layer two-dimensional (2D) materials, such as the electronic structure [1], band gap tunability [2] and quantum Hall phases [3]. These interlayer properties are closely related to the intralayer lattice structure of the crystals. Knowledge of such correlation would allow us to tailor the material properties and possibly realize new quantum phases. Experimental approach to this topic is, however, challenging due to the lack of suitable materials with contrasting structure. For instance, few-layer graphene and the common transition metal dichalcogenides (TMDs), such as MoS 2 and WSe 2 , possess similar stacking order due to their similar hexagonal in-plane lattice structure. To reveal the subtle influence of intralayer lattice on the interlayer coupling, it is imperative to explore new 2D materials with differing lattice structure.Rhenium disulfide (ReS 2 ) is a new type of 2D semiconductors with intriguing properties [4][5][6][7][8][9][10][11][12]. In contrast to the widely studied group-VI TMDs, such as MoS 2 and WSe 2 that possess 1H or 1T structure, ReS 2 monolayers exhibit unique distorted 1T structure as the stable phase [ Fig. 1(a)]. This is because the rhenium atom possesses one extra valence electron, leading to the formation of additional Re-Re bonds in ReS 2 . A superlattice structure of rhenium chains is thus formed to distort the monolayer crystal from the more symmetric 1T phase [ Fig. 1(a)]. As a consequence, the ReS 2 crystal exhibits strong in-plane anisotropy in the electronic, vibrational and mechanical properties [4][5][6][7][8][9].The in-plane distortion of ReS 2 lattice is expected to affect profoundly the interlayer coupling in few-layer ReS 2 crystals. A recent study [4], for instance, reports that the band gap of ReS 2 remains direct from single layer (1L) to the bulk, in contrast to the direct-to-indirect band gap transition i...

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 (CrI 3) that has been known to be a layered antiferromagnet (AFM) in its 2D films and a ferromagnet (FM) in its threedimensional (3D) bulk. However, we reveal a novel mixed state of layered AFM and FM in 3D CrI 3 bulk 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 from C 3i (rhombohedral) to C 2h (monoclinic). Our result not only unveils the complex single-magnon behavior in 3D CrI 3 , but it also settles the puzzle of how CrI 3 transits from a bulk FM to a thin-layered AFM semiconductor, despite recent efforts in understanding the origin of layered AFM in CrI 3 thin layers, and reveals the intimate relationship between the layered AFM-to-FM and the crystalline rhombohedral-tomonoclinic phase transitions. These findings further open opportunities for future 2D magnet-based magnetomechanical devices.

Twist engineering, or the alignment of two-dimensional (2D) crystalline layers with desired orientations, has led to tremendous success in modulating the charge degree of freedom in heteroand homo-structures, in particular, in achieving novel correlated and topological electronic phases in moiré electronic crystals 1,2 . However, although pioneering theoretical efforts have predicted nontrivial magnetism 3,4 and magnons 5 out of twisting 2D magnets, experimental realization of twist engineering spin degree of freedom remains elusive. Here, we leverage the archetypal 2D Ising magnet chromium triiodide (CrI3) to fabricate twisted double bilayer homostructures with tunable twist angles and demonstrate the successful twist engineering of 2D magnetism in them. Using linear and circular polarization-resolved Raman spectroscopy, we identify magneto-Raman signatures of a new magnetic ground state that is sharply distinct from those in natural bilayer (2L) and four-layer (4L) CrI3. With careful magnetic field and twist angle dependence, we reveal that, for a very small twist angle (~ 0.5 o ), this emergent magnetism can be well-approximated by a weighted linear superposition of those of 2L and 4L CI3 whereas, for a relatively large twist angle (~ 5 o ), it mostly resembles that of isolated 2L CrI3. Remarkably, at an intermediate twist angle (~ 1.1 o ), its magnetism cannot be simply inferred from the 2L and 4L cases, because it lacks sharp spin-flip transitions that are present in 2L and 4L CrI3 and features a dramatic Raman circular dichroism that is absent in natural 2L and 4L ones. Our results demonstrate the possibility of designing and controlling the spin degree of freedom in 2D magnets using twist engineering.

Abstract:We use a combination of Raman spectroscopy and transport measurements to study thin flakes of the type-II Weyl semimetal candidate MoTe2 protected from oxidation. In contrast to bulk crystals, which undergo a phase transition from monoclinic to the inversion symmetry breaking, orthorhombic phase below ~250 K, we find that in moderately thin samples below ~12 nm, a single orthorhombic phase exists up to and beyond room temperature. This could be due to the effect of c-axis confinement, which lowers the energy of an out-of-plane hole band and stabilizes the orthorhombic structure. Our results suggest that Weyl nodes, predicated upon inversion symmetry breaking, may be observed in thin MoTe2 at room temperature. Main text:

We have investigated the interlayer shear and breathing phonon modes in MoS2 with pure 3R and 2H stacking order by using polarization-dependent ultralow-frequency Raman spectroscopy. We observe up to three shear branches and four breathing branches in MoS2 with thickness from 2 to 13 layers. The breathing modes show the same Raman activity behavior for both polytypes, but the 2H breathing frequencies are consistently several wavenumbers higher than the 3R breathing frequencies, signifying that 2H MoS2 has slightly stronger interlayer lattice coupling than 3R MoS2. In contrast, the shear-mode Raman spectra are strikingly different for 2H and 3R MoS2. While the strongest shear mode corresponds to the highest-frequency branch in the 2H structure, it corresponds to the lowest-frequency branch in the 3R structure. Such distinct and complementary Raman spectra of the 3R and 2H polytypes allow us to survey a broad range of shear modes in MoS2, from the highest to lowest branch. By combining the linear chain model, group theory, effective bond polarizability model and first-principles calculations, we can account for all the major observations in our experiment.KEY WORDS: 3R MoS2, 2H MoS2, stacking order, shear mode, breathing mode, ultralowfrequency Raman.