Review of Raman spectroscopy of two-dimensional magnetic van der Waals materials

Ultrathin van der Waals (vdW) magnets provide a possibility to access magnetic ordering in the two-dimensional (2D) limit, which are expected to be applied in the spintronic devices. Raman spectroscopy is a powerful characterization method to investigate the spin-related properties in 2D vdW magnets, including magnon and spin-lattice interaction, which are hardly accessible by other optical methods. In this paper, the recent progress of various magnetic properties in 2D vdW magnets studied by Raman spectroscopy is reviewed, including the magnetic transition, spin-wave, spin-lattice interaction, symmetry tuning induced by spin ordering, and nonreciprocal magneto-phonon Raman scattering.     

Structural evolution and bandgap modulation of layered β-GeSe2 single crystal under high pressure

Germanium diselenide (GeSe₂) is a promising candidate for electronic devices because of its unique crystal structure and optoelectronic properties. However, the evolution of lattice and electronic structure of $β$-GeSe₂ at high pressure is still uncertain. Here we prepared high-quality $β$-GeSe₂ single crystals by chemical vapor transfer (CVT) technique and performed systematic experimental studies on the evolution of lattice structure and bandgap of $β$-GeSe₂ under pressure. High-precision high-pressure ultra low frequency (ULF) Raman scattering and synchrotron angle-dispersive x-ray diffraction (ADXRD) measurements support that no structural phase transition exists under high pressure up to 13.80 GPa, but the structure of $β$-GeSe₂ turns into a disordered state near 6.91 GPa and gradually becomes amorphous forming an irreversibly amorphous crystal at 13.80 GPa. Two Raman modes keep softening abnormally upon pressure. The bandgap of $β$-GeSe₂ reduced linearly from 2.59 eV to 1.65 eV under pressure with a detectable narrowing of 36.5%, and the sample under pressure performs the piezochromism phenomenon. The bandgap after decompression is smaller than that in the atmospheric pressure environment, which is caused by incomplete recrystallization. These results enrich the insight into the structural and optical properties of $β$-GeSe₂ and demonstrate the potential of pressure in modulating the material properties of two-dimensional (2D) Ge-based binary material.     

Raman Scattering Modification in Monolayer ReS_2 Controlled by Strain Engineering

Regulation of optical properties and electronic structure of two-dimensional layered ReS_2 materials has attracted much attention due to their potential in electronic devices.However,the identification of structure transformation of monolayer ReS_2 induced by strain is greatly lacking.In this work,the Raman spectra of monolayer ReS_2 with external strain are determined theoretically based on the density function theory.Due to the lower structural symmetry,deformation induced by external strain can only regulate the Raman mode intensity but cannot lead to Raman mode shifts.Our calculations suggest that structural deformation induced by external strain can be identified by Raman scattering.     

Evidence for a magnetic collapse in the epsilon phase of solid oxygen

Solid oxygen is the only elementary molecular magnet. Under the very high pressure of 96 GPa oxygen transforms into a metal and a superconductor. Theory predicts a nonmagnetic state occurring before the transition into the superconducting xi phase. Nevertheless, until now there was no direct evidence of a magnetic collapse in high-pressure oxygen. For the first time direct information is provided on magnetic properties of the epsilon phase, which is sandwiched between the antiferromagnetic delta phase and the superconducting xi phase. We used magnetic neutron diffraction. The data show that the long-range magnetic order disappears at the delta-epsilon transition. The magnetic collapse occurs at P approximately equal to 8 GPa, far below the pressure of the insulator-metal (superconductor) transition. The collapse is preceded by a decrease in temperature of transition towards the long-range magnetically ordered state (T(LRO)) in the delta phase, at P = 7.6 GPa.     

High pressure phase transition in metallic LaB6: Raman and X-ray diffraction studies

High pressure Raman and angle dispersive X-ray diffraction (ADXRD) measurements on the metallic hexaboride LaB₆ have been carried out upto the pressures of about 20 GPa. The subtle phase transition around 10 GPa indicated in Raman measurements is confirmed by ADXRD experiments to be a structural change from cubic to orthorhombic phase. Ab-initio electronic band structure calculations using full potential linear augmented plane wave method carried out as a function of pressure show that this transition is driven by the interception of Fermi level by electronic band minimum around the transition pressure.     

Effect of biaxial strain and hydrostatic pressure on the magnetic properties of bilayer CrI3

Two-dimensional van der Waals magnetic materials are intriguing for applications in the future spintronics devices, so it is crucial to explore strategy to control the magnetic properties. Here, we carried out first-principles calculations and Monte Carlo simulations to investigate the effect of biaxial strain and hydrostatic pressure on the magnetic properties of the bilayer CrI₃. We found that the magnetic anisotropy, intralayer and interlayer exchange interactions, and Curie temperature can be tuned by biaxial strain and hydrostatic pressure. Large compressive biaxial strain may induce a ferromagneticto-antiferromagnetic transition of both CrI₃ layers. The hydrostatic pressure could enhance the intralayer exchange interaction significantly and hence largely boost the Curie temperature. The effect of the biaxial strain and hydrostatic pressure revealed in the bilayer CrI₃ may be generalized to other two-dimensional magnetic materials.     

Thickness dependence of antiferromagnetic phase transition in Heisenberg-type MnPS3

The behavior of 2-dimensional (2D) van der Waals (vdW) layered magnetic materials in the 2D limit of the few-layer thickness is an important fundamental issue for the understanding of the magnetic ordering in lower dimensions. The antiferromagnetic transition temperature T_N of the Heisenberg-type 2D magnetic vdW material MnPS₃ was estimated as a function of the number of layers. The antiferromagnetic transition was identified by temperature-dependent Raman spectroscopy, from the broadening of a phonon peak at 155 cm⁻¹, accompanied by an abrupt redshift and an increase of its spectral weight. T_N is found to decrease only slightly from ~78 K for bulk to ~66 K for 3L. The small reduction of T_N in thin MnPS₃ approaching the 2D limit implies that the interlayer vdW interaction is playing an important role in stabilizing magnetic ordering in layered magnetic materials.     

Suppression of Jahn-Teller Distortion and Insulator-To-Metal Transition in LaMnO 3 at High Pressures

Structural, vibrational and electronic properties of LaMnO 3 under pressures up to 38 GPa have been studied by synchrotron X-ray powder diffraction, Raman spectroscopy, optical reflectivity, and transport measurements. The cooperative Jahn-Teller distortion of the MnO 6 octahedra of the perovskite-type structure is continuously suppressed with increasing pressure, a process which appears completed at ～20 GPa. The system remains insulating to 32 GPa, where an insulator-metal transition is observed. This transition is attributed to strengthened Mn--O--Mn interactions due to the increasing overlap of atomic orbitals.     

Electronic transition and the metallization effect in the BiFeO<Subscript>3</Subscript> crystal at high pressures

The optical absorption spectra from bismuth ferrite (BiFeO₃) have been studied at high pressures up to 60 GPa in diamond anvil cells. An electronic transition at which the energy of the optical absorption edge decreases sharply from ～1.5 eV to zero has been observed at room temperature in a pressure range of 45–55 GPa. This indirectly indicates a insulator-metal transition. The observed electronic transition correlates with the recently revealed structural and magnetic transitions induced by high pressures in this crystal. The behavior of the optical absorption edge with decreasing the pressure is completely reversible in correlation with the reversibility of the magnetic transition. The “smearing” of the structural transition in pressure is caused by thermal fluctuations between the high-spin state and low-spin state of the Fe³⁺ ions near the transition.     

Magnetic and electronic properties of two-dimensional metal-organic frameworks TM_3(C_2NH)_(12)

The ferromagnetism of two-dimensional(2 D) materials has aroused great interest in recent years, which may play an important role in the next-generation magnetic devices. Herein, a series of 2 D transition metal-organic framework materials(TM-NH MOF, TM = Sc–Zn) are designed, and their electronic and magnetic characters are systematically studied by means of first-principles calculations. Their structural stabilities are examined through binding energies and abinitio molecular dynamics simulations. Their optimized lattice constants are correlated to the central TM atoms. These 2 D TM-NH MOF nanosheets exhibit various electronic and magnetic performances owing to the effective charge transfer and interaction between TM atoms and graphene linkers. Interestingly, Ni-and Zn-NH MOFs are nonmagnetic semiconductors(SM) with band gaps of 0.41 e V and 0.61 e V, respectively. Co-and Cu-NH MOFs are bipolar magnetic semiconductors(BMS), while Fe-NH MOF monolayer is a half-semiconductor(HSM). Furthermore, the elastic strain could tune their magnetic behaviors and transformation, which ascribes to the charge redistribution of TM-3 d states. This work predicts several new 2 D magnetic MOF materials, which are promising for applications in spintronics and nanoelectronics.     

In-plane optical anisotropy of two-dimensional VOCl single crystal with weak interlayer interaction

Transition-metal oxyhalides $MOX$ ($M = {\rm Fe}$, Cr, V; $O ={\rm oxygen}$, $X= {\rm F}$, Cl, Br, I), an emerging type of two-dimensional (2D) van der Waals materials, have been both theoretically and experimentally demonstrated to possess unique electronic and magnetic properties. However, the intrinsic in-plane anisotropic properties of 2D VOCl still lacks in-depth research, especially optical anisotropy. Herein, a systematic Raman spectroscopic study is performed on VOCl single-crystal with different incident laser polarization at various temperatures. The polarized-dependent Raman scattering spectra reveal that the ${{ A}}_{{\rm g}}$ mode of VOCl show a 2-lobed shape in parallel polarization configuration while a 4-lobed shape in vertical configuration. In addition, the temperature-dependent and thickness-dependent Raman scattering spectra confirm a relatively weak van der Waals interaction between each layers among VOCl single crystal. These findings might provide better understanding on the in-plane anisotropic phenomenon in VOCl layers, thus will accelate further application of 2D single crystals for nanoscale angle-dependent optoelectronics.     

First-order isostructural Mott transition in highly compressed MnO

We present evidence for an isostructural, first-order Mott transition in MnO at 105+/-5 GPa, based on high-resolution x-ray emission spectroscopy and angle-resolved x-ray diffraction data. The pressure-induced structural and spectral changes provide a coherent picture of MnO phase transitions from paramagnetic B1 to antiferromagnetic distorted B1 at 30 GPa, to paramagnetic B8 at 90 GPa, and to diamagnetic B8 at 105+/-5 GPa. The last is the Mott transition, accompanied by a significant loss of magnetic moment, an approximately 6.6% volume collapse and the insulator-metal transition as demonstrated by recent resistance measurements.     

本征磁性拓扑绝缘体MnBi2Te4电子结构的压力应变调控

由于MnBi2Te4电子结构具有对晶格常数的改变相当敏感的特性,本文采用基于密度泛函理论的第一性原理方法对MnBi2Te4反铁磁块体的电子结构施加等体积应变调控.研究发现体系能带结构在材料等体积拉伸和压缩作用下变化灵敏,体系出现绝缘体-金属相变.特别地,当施加特定应变后导带和价带在Γ处出现交叉,体系呈零带隙状态.在此应...     

Interplay between electronic correlations and coherent structural dynamics during the monoclinic insulator-to-rutile metal phase transition in VO2

We report direct observations of the structural and electronic dynamics of the photoinduced insulator-metal transition in VO(2), by means of time-resolved photoemission spectroscopy. These observations provide new insights into the processes involved in this transition. Slightly above the threshold of the photoinduced phase transition, the different response times of the electrons and the lattice reveal the electronic nature of the band gap collapse. At high excitation densities, we find that the phase transition is induced nonthermally in an ultrashort time scale. Moreover, we identify different V 3p dynamics indicating the existence of different structural pathways. These results represent a clear demonstration of the potential of time-resolved core level photoelectron spectroscopy to study ultrafast dynamics in condensed matter.     

Nb掺杂导致LaMnO_3绝缘体-金属转变的第一性原理研究

采用基于密度泛函理论(DFT)的第一性原理计算,系统研究了Nb掺杂LaMn_(1-x)Nb_xO_3(x=0,0. 25,0. 5,0. 75)的结构和电磁性质.计算结果表明,所有的LaMn_(1-x)Nb_xO_3都稳定在正交结构. LaMn_(1-x)Nb_xO_3当x 0. 5时为A型反铁磁绝缘体,在x=0. 5和0. 75时为G型反铁磁金属.随着Nb掺杂量增加,当x=0. 5时掺杂电子占据导带的底部,系统产生绝缘体-金属转变.这意味着LaMn_(1-x)Nb_xO_3在电子器件上可能有重要的应用.另外,LaMn_(1-x)Nb_xO_3在x=0. 25和0. 75时出现了自旋玻璃行为.     

Elastic and vibrational properties of cobalt to 120 GPa

Impulsive stimulated light scattering and Raman spectroscopy measurements have been made on hcp cobalt to a static pressure of 120 GPa. We find that at pressures above 60 GPa the shear elastic modulus and the Raman frequency of the E(2g) transverse optical phonon exhibit a departure from a linear dependence on density. We relate this behavior to a collapse of the magnetic moment under pressure that has been predicted theoretically, but until now not observed experimentally.     

Probing the paramagnetic interactions between the unpaired electronic spins of carbon atoms and the nuclear spins of hydrogen molecules with Raman spectroscopy

Carbon materials typically have a high density of unpaired electronic spins but the exact nature of the defect sites that give rise to their magnetic properties are not yet well understood. In this work, the paramagnetic interactions between the unpaired electronic spins of carbon atoms and the nuclear spins of hydrogen molecules were probed with Raman spectroscopy by monitoring the relative population of H₂ rotational states. For H₂, the symmetries of nuclear spin and rotational wave functions are correlated. Because of the weak interactions between H₂ nuclear spins, the transitions between odd and even rotational states are normally hindered. The magnetic field generated by unpaired electronic spins relaxes the selection rules and promotes transitions between H₂ rotational levels of different symmetry. This affects the rotational levels' relaxation kinetics toward equilibrium and makes H₂ molecules useful to study unpaired electrons in paramagnetic materials. It is suggested that simultaneous electron paramagnetic resonance and Raman measurements on carbon materials interacting with hydrogen molecules could result in a better understanding of the nature of paramagnetic defects in carbon materials, which could have a substantial impact on Li‐ion batteries or for understanding the graphene electronic properties. Copyright © 2011 John Wiley & Sons, Ltd.     

Near-surface structural study of transition metal oxides to understand their electronic properties

The atomic arrangement in a solid contains a great amount of information, and observation of its structure is essential for understanding the electronic and magnetic properties of transition metal oxides at a microscopic level. Increasing interest in the surfaces and interfaces of oxide systems, which is partly driven by the anticipation of device applications, enhances the importance of structural studies of the near-surface region. We review various types of structural studies with x-ray scattering on the near-surface region of metal oxides-from thick films to surfaces-in order to clarify the structural effects on their electronic properties.     

Estimation of interlayer interactions in CdI2 by light scattering techniques

The velocity of transverse acoustic wave propagating along the direction perpendicular to the layer and the frequency of the rigid layer mode in CdI₂ have been measured by Brillouin and Raman scattering, respectively, at several temperatures. The interlayer forces have been deduced independently from these two different measurements. A comparison between them has led to the conclusion that the nearest neighbour interlayer interaction dominates the interlayer interaction.     

CoO under pressure from first principles

Density functional theory and the generalized gradient approximation with correction for Hubbard energy was used to study the behavior of cobaltous oxide (CoO) under pressure. CoO undergoes an insulator-metal transition which is accompanied by a magnetic collapse. The antiferromagnetic phase of CoO transforms to nonmagnetic phase with the 6-7% reduction in the fractional volume. The magnetic collapse and the energy band gap closure are driven by the lost of correlation which results from the delocalization of 3d electrons. Delocalization process is due to the band broadening with compression. The Hubbard energy influences the transitions pressure. The lower Hubbard terms result in the lower values of transition pressure. The evolution of magnetic moment, energy band gap, and the bandwidth versus increasing pressure is analyzed. The results of calculations are compared to the existing theoretical and experimental data.