Magnetism in two-dimensional materials is not only of fundamental scientific interest but also a promising candidate for numerous applications. However, studies so far, especially the experimental ones, have been mostly limited to the magnetism arising from defects, vacancies, edges or chemical dopants which are all extrinsic effects. Here, we report on the observation of intrinsic antiferromagnetic ordering in the two-dimensional limit. By monitoring the Raman peaks that arise from zone folding due to antiferromagnetic ordering at the transition temperature, we demonstrate that FePS3 exhibits an Ising-type antiferromagnetic ordering down to the monolayer limit, in good agreement with the Onsager solution for two-dimensional order-disorder transition.The transition temperature remains almost independent of the thickness from bulk to the 2 monolayer limit with TN ~118 K, indicating that the weak interlayer interaction has little effect on the antiferromagnetic ordering. KEYWORDSIsing model, Antiferromagentism, Magnetic ordering in 2 Dimension, FePS3, Iron phosphorus trisulfide, Raman spectroscopy 3 Magnetism has played an important role in advancing our understanding of the quantum nature of materials. Especially, low-dimensional magnetism has been a fertile playground, in which novel physical concepts have been learned and thereby moved the frontiers of the modern understanding of materials science. Most of the three-dimensional magnetic systems, other than some exceptional cases of quantum spin and/or strong frustration, host a magnetic order. On the other hand, fluctuations are so strong and easily destroy stabilization of order parameters in onedimensional systems as pointed out in the seminal work by Bethe. 1 Two-dimensional (2D) systems, on the other hand, have attracted much attention because the presence or absence of long-range order depends on the type of spin-spin interactions, which themselves compete with intrinsic fluctuations of either quantum and/or thermal nature.The XXZ Hamiltonian reads 2 ( )where XY J and I J are spin-exchange energies on the basal plane and along the c-axis, respectively; using an order-converting dual transformation that unlike the 1D system there is a phase transition at a finite temperature in the 2D Ising system. 4 Therefore, ferromagnetic or antiferromagnetic ordering in the 2D limit is possible only in the Ising model. There has been some indirect test of this prediction including the most notable one by Kim and Chan using CH4 molecules adsorbed 4 on graphite. 5 However, despite its fundamental importance, there has been no experimental work using a real 2D magnetic material.2D van der Waals (vdW) materials could be an ideal system for the study of 2D magnetism. 6 Unfortunately, however, finding suitable vdW materials and producing atomically thin magnetic materials have been a challenge. Although there have been a few reports of producing atomically thin samples of magnetic materials, 7-10 observation of magnetic ordering in the atomically thin limit has been l...

How a certain ground state of complex physical systems emerges, especially in two-dimensional materials, is a fundamental question in condensed-matter physics. A particularly interesting case is systems belonging to the class of XY Hamiltonian where the magnetic order parameter of conventional nature is unstable in two-dimensional materials leading to a Berezinskii−Kosterlitz−Thouless transition. Here, we report how the XXZ-type antiferromagnetic order of a magnetic van der Waals material, NiPS3, behaves upon reducing the thickness and ultimately becomes unstable in the monolayer limit. Our experimental data are consistent with the findings based on renormalization-group theory that at low temperatures a two-dimensional XXZ system behaves like a two-dimensional XY one, which cannot have a long-range order at finite temperatures. This work provides the experimental examination of the XY magnetism in the atomically thin limit and opens opportunities of exploiting these fundamental theorems of magnetism using magnetic van der Waals materials.

We investigated polarization dependence of the Raman modes in black phosphorus (BP) using five different excitation wavelengths. The crystallographic orientation was determined by comparing polarized optical microscopy with high-resolution transmission electron microscopy analysis. In polarized Raman spectroscopy, the B2g mode shows the same polarization dependence regardless of the excitation wavelength or the sample thickness. On the other hand, the Ag(1) and Ag(2) modes show a peculiar polarization behavior that depends on the excitation wavelength and the sample thickness. The thickness dependence can be explained by considering the anisotropic interference effect due to the birefringence and dichroism of the BP crystal, but the wavelength dependence cannot be explained. We propose a simple and fail-proof procedure to determine the orientation of a BP crystal by combining polarized Raman scattering with polarized optical microscopy.

Interlayer excitons were observed at the heterojunctions in van der Waals heterostructures (vdW HSs). However, it is not known how the excitonic phenomena are affected by the stacking order. Here, we report twist-angle-dependent interlayer excitons in MoSe/WSe vdW HSs based on photoluminescence (PL) and vdW-corrected density functional theory calculations. The PL intensity of the interlayer excitons depends primarily on the twist angle: It is enhanced at coherently stacked angles of 0° and 60° (owing to strong interlayer coupling) but disappears at incoherent intermediate angles. The calculations confirm twist-angle-dependent interlayer coupling: The states at the edges of the valence band exhibit a long tail that stretches over the other layer for coherently stacked angles; however, the states are largely confined in the respective layers for intermediate angles. This interlayer hybridization of the band edge states also correlates with the interlayer separation between MoSe and WSe layers. Furthermore, the interlayer coupling becomes insignificant, irrespective of twist angles, by the incorporation of a hexagonal boron nitride monolayer between MoSe and WSe.

The thermal conductivity of suspended single-layer graphene was measured as a function of temperature using Raman scattering spectroscopy on clean samples prepared directly on a prepatterned substrate by mechanical exfoliation without chemical treatments. The temperature at the laser spot was monitored by the frequency of the Raman 2D band of the Raman scattering spectrum, and the thermal conductivity was deduced by analyzing heat diffusion equations assuming that the substrate is a heat sink at ambient temperature. The obtained thermal conductivity values range from ∼1800 Wm −1 K −1 near 325 K to ∼710 Wm −1 K −1 at 500 K.

The Young's modulus of graphene is estimated by measuring the strain applied by a pressure difference across graphene membranes using Raman spectroscopy. The strain induced on pressurized graphene balloons can be estimated directly from the peak shift of the Raman G band. By comparing the measured strain with numerical simulation, we obtained the Young's modulus of graphene. The estimated Young's modulus values of single- and bilayer graphene are 2.4 ± 0.4 and 2.0 ± 0.5 TPa, respectively.

ABSTRACT:The resonance effects on the Raman spectra from 5 to 900 cm −1 of few-layer MoS2 thin films up to 14-layers were investigated by using six excitation energies. For the main first-order Raman peaks, the intensity maximum occurs at ~2.8 eV for single layer and at ~2.5 eV for few-layer MoS2, which correspond to the band-gap energy. At the excitation energy of 1.96 eV, several anomalous behaviors are observed. Many second-order peaks are anomalously enhanced even though the main firstorder peaks are not enhanced. In the low-frequency region (<100 cm −1 ), a broad peak centered at ~38 cm −1 and its second order peak at 76 cm −1 appear for the excitation energy of 1.96 eV. These anomalous resonance effects are interpreted as being due to strong resonance with excitons or exciton-polaritons.2

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