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Significance Electric polarization due to polar structural distortion is a central characteristic of an ferroelectric (FE) material, which is normally unattainable in a metallic system because itinerant electrons screen the internal electric field. A possible exception for a polar metal is if the itinerant electrons are decoupled with FE distortion, as proposed by Anderson and Blount. Here, we reveal an unusual FE phase transition in recently discovered polar metal LiOsO3, different from the typical transition in insulating FEs, with dramatic temperature-dependent electron−phonon coupling. Our results provide a look at models for FE transitions with the interplay between FE dipoles and itinerant electrons, suggesting that an improvement of the Anderson and Blount model is needed., Ferroelectric (FE) distortions in a metallic material were believed to be experimentally inaccessible because itinerant electrons would screen the long-range Coulomb interactions that favor a polar structure. It has been suggested by Anderson and Blount [P. W. Anderson, E. I. Blount, Phys. Rev. Lett. 14, 217−219 (1965)] that a transition from paraelectric phase to FE phase is possible for a metal if, in the paraelectric phase, the electrons at the Fermi level are decoupled from the soft transverse optical phonons, which lead to ferroelectricity. Here, using Raman spectroscopy combined with magnetotransport measurements on a recently discovered FE metal LiOsO3, we demonstrate active interplay of itinerant electrons and the FE order: Itinerant electrons cause strong renormalization of the FE order parameter, leading to a more gradual transition in LiOsO3 than typical insulating FEs. In return, the FE order enhances the anisotropy of charge transport between parallel and perpendicular to the polarization direction. The temperature-dependent evolution of Raman active in-plane 3Eg phonon, which strongly couples to the polar-active out-of-the-plane A2u phonon mode in the high-temperature paraelectric state, exhibits a deviation in Raman shift from the expectation of the pseudospin−phonon model that is widely used to model many insulating FEs. The Curie−Weiss temperature (θ ≈ 97 K) obtained from the optical susceptibility is substantially lower than Ts, suggesting a strong suppression of FE fluctuations. Both line width and Fano line shape of 3Eg Raman mode exhibit a strong electron−phonon coupling in the high-temperature paraelectric phase, which disappears in the FE phase, challenging Anderson/Blount’s proposal for the formation of FE metals.

The physical properties of most 2D materials are highly dependent on the nature of their interlayer interaction. In-depth studies of the interlayer interaction are beneficial to the understanding of the physical properties of 2D materials and permit the development of related devices. Layered magnetic NiPS3 has unique magnetic and electronic properties. The electronic band structure and corresponding magnetic state of NiPS3 are expected to be sensitive to the interlayer interaction, which can be tuned by external pressure. Here, we report an insulator-metal transition accompanied by the collapse of magnetic order during the 2D-3D structural crossover induced by hydrostatic pressure. A two-stage phase transition from a monoclinic (C2/m) to a trigonal $$(P\bar 31m)$$ lattice is identified via ab initio simulations and confirmed via high-pressure X-ray diffraction and Raman scattering; this transition corresponds to a layer-by-layer slip mechanism along the a-axis. Temperature-dependent resistance measurements and room temperature infrared spectroscopy under different pressures demonstrate that the insulator-metal transition and the collapse of the magnetic order occur at ∼20 GPa, which is confirmed by low-temperature Raman scattering measurements and theoretical calculations. These results establish a strong correlation between the structural change, electric transport, and magnetic phase transition and expand our understanding of layered magnetic materials. Moreover, the structural transition caused by the interlayer displacement has significance for designing similar devices at ambient pressure.

Alkali metal rare-earth chalcogenide $ARECh2$ (A=alkali or monovalent metal, RE=rare earth, Ch=O, S, Se, Te), is a large family of quantum spin liquid (QSL) candidates we discovered recently. Unlike $YbMgGaO4$, most members in the family except for the oxide ones, have relatively small crystalline electric-field (CEF) excitation levels, particularly the first ones. This makes the conventional Curie-Weiss analysis at finite temperatures inapplicable and CEF excitations may play an essential role in understanding the low-energy spin physics. Here we considered an effective magnetic Hamiltonian incorporating CEF excitations and spin-spin interactions, to accurately describe thermodynamics in such a system. By taking $NaYbSe2$ as an example, we were able to analyze magnetic susceptibility, magnetization under pulsed high fields and heat capacity in a systematic and comprehensive way. The analysis allows us to produce accurate anisotropic exchange coupling energies and unambiguously determine a crossover temperature ($\sim$25 K in the case of $NaYbSe2$), below which CEF effects fade away and pure spin-spin interactions stand out. We further validated the effective picture by successfully explaining the anomalous temperature dependence of electron spin resonance (ESR) spectral width. The effective scenario in principle can be generalized to other rare-earth spin systems with small CEF excitations., 7 pages, 4 figures + Supplementary Information

Kitaev spin liquid (KSL) system has attracted tremendous attention in past years because of its fundamental significance in condensed matter physics and promising applications in fault-tolerant topological quantum computation. Material realization of such a system remains a major challenge in the field due to the unusual configuration of anisotropic spin interactions, though great effort has been made before. Here we reveal that rare-earth chalcohalides REChX (RE=rare earth, Ch=O, S, Se, Te, X=F, Cl, Br, I) can serve as a family of KSL candidates. Most family members have the typical SmSI-type structure with a high symmetry of R-3m and rare-earth magnetic ions form an undistorted honeycomb lattice. The strong spin-orbit coupling of 4f electrons intrinsically offers anisotropic spin interactions as required by Kitaev model. We have grown the crystals of YbOCl and synthesized the polycrystals of SmSI, ErOF, HoOF and DyOF, and made careful structural characterizations. We carry out magnetic and heat capacity measurements down to 1.8 K and find no obvious magnetic transition in all the samples but DyOF. The van der Waals interlayer coupling highlights the true two-dimensionality of the family which is vital for the exact realization of Abelian/non-Abelian anyons, and the graphene-like feature will be a prominent advantage for developing miniaturized devices. The family is expected to act as an inspiring material platform for the exploration of KSL physics., Comment: 6 pages, 6 figures, 2 tables

Recently, the family of rare-earth chalcohalides were proposed as candidate compounds to realize the Kitaev spin liquid (KSL) [Chin. Phys. Lett. 38 047502 (2021)]. In the present work, we firstly propose an effective spin Hamiltonian consistent with the symmetry group of the crystal structure. Then we apply classical Monte Carlo simulations to preliminarily study the model and establish a phase diagram. When approaching to the low temperature limit, several magnetic long range orders are observed, including the stripe, the zigzag, the antiferromagnetic (AFM), the ferromagnetic (FM), the incommensurate spiral (IS), the multi- Q , and the 120° ones. We further calculate the thermodynamic properties of the system, such as the temperature dependence of the magnetic susceptibility and the heat capacity. The ordering transition temperatures reflected in the two quantities agree with each other. For most interaction regions, the system is magnetically more susceptible in the ab-plane than in the c-direction. The stripe phase is special, where the susceptibility is fairly isotropic in the whole temperature region. These features provide useful information to understand the magnetic properties of related materials.

The family of rare earth chalcogenides $ARECh_{2}$ (A = alkali or monovalent ions, RE = rare earth, and Ch = O, S, Se, and Te) appears as an inspiring playground for studying quantum spin liquids (QSL). The crucial low-energy spin dynamics remain to be uncovered. By employing muon spin relaxation ($\mu$SR) and zero-field (ZF) AC susceptibility down to 50 mK, we are able to identify the gapless QSL in $NaYbSe_{2}$, a representative member with an effective spin-1/2, and explore its unusual spin dynamics. The ZF $\mu$SR experiments unambiguously rule out spin ordering or freezing in $NaYbSe_{2}$ down to 50 mK, two orders of magnitude smaller than the exchange coupling energies. The spin relaxation rate, $\lambda$, approaches a constant below 0.3 K, indicating finite spin excitations featured by a gapless QSL ground state. This is consistently supported by our AC susceptibility measurements. The careful analysis of the longitudinal field (LF) $\mu$SR spectra reveals a strong spatial correlation and a temporal correlation in the spin-disordered ground state, highlighting the unique feature of spin entanglement in the QSL state. The observations allow us to establish an experimental H-T phase diagram. The study offers insight into the rich and exotic magnetism of the rare earth family., Comment: 21 pages, 18 figures

Recently rare-earth chalcogenides have been revealed as a family of quantum spin liquid (QSL) candidates hosting a large number of members. In this paper we report the crystal growth and magnetic measurements of KErTe$_2$, which is the first member of telluride in the family. Compared to its cousins of oxides, sulfides and selenides, KErTe$_2$ retains the high symmetry of $R\overline{3}m$ and Er$^{3+}$ ions still sit on a perfect triangular lattice. The separation between adjacent magnetic layers is expectedly increased, which further enhances the two dimensionality of the spin system. Specific heat and magnetic susceptibility measurements on KErTe$_2$ single crystals reveal no structural and magnetic transition down to 1.8 K. Most interestingly, the absorption spectrum shows that the charge gap of KErTe$_2$ is roughly 0.93$\pm$0.35 eV, which is the smallest among all the reported members in the family. This immediately invokes the interest towards metallization even superconductivity using the compound.

Very recently we revealed a large family of triangular lattice quantum spin liquid candidates named rare-earth chalcogenides, which features a high-symmetry structure without structural/charge disorders and spin impurities, and may serve as an ideal platform exploring spin liquid physics. The knowledge of crystalline electric-field (CEF) excitations is an essential step to explore the fundamental magnetism of rare-earth spin systems. Here we employed inelastic neutron scattering (INS) and Raman scattering (RS) to carry out a comprehensive CFE investigation on $NaYbSe_{2}$, a promising representative of the family. By comparison with its nonmagnetic compound $NaLuSe_{2}$, we are able to identify the CEF excitations at 15.8, 24.3 and 30.5 meV at 5K. The selected cuts of the INS spectra are well re-produced with a large anisotropy of $g$ factors ($g_{ab}:g_{c}\sim3:1$). Further, the CEF excitations are explained well by our calculations based on the point charge model. Interestingly, $NaYbSe_{2}$ exhibits an unusual CEF shift to higher energies with increasing temperatures, and the Raman mode close to the first CEF excitation shows an anomalously large softening with decreasing temperatures. The absence of the anomalies in $NaLuSe_{2}$ clearly demonstrates a CEF-phonon coupling not reported in the family. It can be understood in term of the weaker electronegativity of Se. The fact that the smallest first CEF excitation in the sub-family of $NaYbCh_{2}$ is $\sim$ 180K (Ch=O, S, Se), guarantees that the sub-family can be strictly described with an effective S=1/2 picture at sufficiently low temperatures. Interestingly the CEF-phonon coupling revealed here may present alternative possibilities to manipulate the spin systems., 7 pages, 4 figures, supplementary material provided, error corrected in Figure 4

Smart home is an emerging area that opens up a diverse set of downstream applications, such as dynamic pricing and demand response techniques, whose goals are typically to lower power consumption while providing comfortable and convenient services. Smart home has been extensively studied, and shown to be beneficial in people's real lives. Although useful, typical smart home platforms work at the "servant" level -- highly dependent on user inputs with no predictions of human demands -- which, if well addressed, would significantly widen their applicability. In this article, we propose a human-centric smart home energy management system (SHE) that works at the "butler" level. The system integrates ubiquitous sensing data from the physical and cyber spaces to discover the patterns of power usage and cognitively understand the behaviors of human beings. The relationship between them is established to dynamically infer users' demands for electricity, and then the optimal scheduling of the home energy system is triggered to respond to both the users' demands and electricity rates. Based on the novel framework, our SHE system provides intelligent services to satisfy the requirements of users as a butler -- aiming not only to save the electricity cost or reduce the peak load, but also to predict users' demands and managing "servants."

Two-dimensional (2D) layered materials have been attracted tremendous research interest because of their novel photoelectric properties. If a single atomic layer instead of individual atoms is taken as a rigid motion object, two unique interlayer vibrations, i.e. compression/breathing and shear motions, at ultra-low frequencies can be expected and actually have been observed in many layered materials. The vibrations stem from the interlayer van der Waals interaction and can be well described by a conventional linear-chain model in most cases. The vibration frequencies strongly depend on layer thickness, which enables an accurate determination of layer numbers. A quick and nondestructive determination of flake thickness is particularly important for the materials, since the physical properties can be dramatically changed in the cases of several atomic layers. As a measure of interlayer coupling, the low-frequency modes are also sensitive to the stacking methods of atomic layers and the overlapping of different kinds of 2D materials. This allows the modes to play a key role in the applications like van der Waals heterojunctions. In this paper, we will give a brief review on the experimental observations and theoretical understanding of the interlayer modes in several typical 2D systems, as well as their actual and potential applications.

The layered magnetic van der Waals materials have generated tremendous interest due to their potential applications and importance in fundamental research. Previous x-ray diffraction (XRD) studies on the magnetic van der Waals compound VI3, revealed a structural transition above the magnetic transition but output controversial analysis on symmetry. In this paper we carried out polarized Raman scattering measurements on VI3 from 10 K to 300 K, with focus on the two A g phonon modes at ∼ 71.1 cm−1 and 128.4 cm−1. Our careful symmetry analysis based on the angle-dependent spectra demonstrates that the crystal symmetry can be well described by C 2h rather than D 3d both above and below structural phase transition. We further performed temperature-dependent Raman experiments to study the magnetism in VI3. Fano asymmetry and anomalous linewidth drop of two A g phonon modes at low temperatures, point to a significant spin–phonon coupling. This is also supported by the softening of 71.1-cm−1 mode above the magnetic transition. The study provides the fundamental information on lattice dynamics and clarifies the symmetry in VI3. And spin–phonon coupling existing in a wide temperature range revealed here may be meaningful in applications.

Iron-based superconductors have the second-highest superconducting transition temperature next to cuprate superconductors. There are many electronic phases such as superconducting, antiferromagnetic and nematic phases, coexisting in the family of iron pnictides. The charge/spin orders are the key factors for understanding the mechanism of high-temperature superconductivity. In this paper we have performed a Raman scattering study of parent compound BaFe2As2. A relatively strong electron–phonon coupling is observed for B1g phonon mode and is weakened after the structural transition at ∼140 K. Through a careful symmetry analysis, we revealed the possibility that B1g mode can be effectively coupled with dxz and dyz orbits. The coupling can remove the degeneracy of the two orbits and further cause the fluctuations of electronic nematic phases. The study suggests that electron–phonon coupling can be a possible origin of nematic phase.

We report a comprehensive Raman scattering study on layered MPS3 (M=Mn, Fe, Ni), a two-dimensional magnetic compound with weak van der Waals interlayer coupling. The observed Raman phonon modes have been well assigned by the combination of first-principles calculations and the polarization-resolved spectra. Careful symmetry analysis on the angle-dependent spectra demonstrates that the crystal symmetry is strictly described by C2h but can be simplified to D3d with good accuracy. Interestingly, the three compounds share exactly the same lattice structure but show distinct magnetic structures. This provides us with a unique opportunity to study the effect of different magnetic orders on lattice dynamics in MPS3. Our results reveal that the in-plane Néel antiferromagnetic (AF) order in MnPS3 favors a spin–phonon coupling compared to the in-plane zig-zag AF in NiPS3 and FePS3. We have discussed the mechanism in terms of the folding of magnetic Brillouin zones. Our results provide insights into the relation between lattice dynamics and magnetism in the layered MPX 3 (M=transition metal, X=S, Se) family and shed light on the magnetism of monolayer MPX 3 materials.

We comparably performed polarized Raman scattering studies of $\mathrm{MoT}{\mathrm{e}}_{2}$ and $\mathrm{WT}{\mathrm{e}}_{2}$. By rotating crystals to tune the angle between the principal axis of the crystals and the polarization of the incident/scattered light, we obtained the angle dependence of the intensities for all the observed modes, which is perfectly consistent with careful symmetry analysis. Combining these results with first-principles calculations, we clearly identified the observed phonon modes in the different phases of both crystals. Fifteen Raman-active phonon modes $(10\phantom{\rule{0.16em}{0ex}}{A}_{g}+5\phantom{\rule{0.16em}{0ex}}{B}_{g})$ in the high-symmetry phase $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{MoT}{\mathrm{e}}_{2}$ (300 K) were well assigned, and all the symmetry-allowed Raman modes $(11\phantom{\rule{0.16em}{0ex}}{A}_{1}+6\phantom{\rule{0.16em}{0ex}}{A}_{2})$ in the low-symmetry phase ${T}_{d}\text{\ensuremath{-}}\mathrm{MoT}{\mathrm{e}}_{2}$ (10 K) and 12 Raman phonons $(8\phantom{\rule{0.16em}{0ex}}{A}_{1}+4\phantom{\rule{0.16em}{0ex}}{A}_{2})$ in ${T}_{d}\text{\ensuremath{-}}\mathrm{WT}{\mathrm{e}}_{2}$ were observed and identified. The present paper provides basic information about the lattice dynamics in transition-metal dichalcogenides and may shed some light on the understanding of the extremely large magnetoresistance in this class of materials.

We have performed polarized and temperature-dependent Raman scattering measurements on extremely large magnetoresistance compounds ${\mathrm{TaAs}}_{2}$ and ${\mathrm{NbAs}}_{2}$. In both crystals, all the Raman active modes, including six ${A}_{g}$ modes and three ${B}_{g}$ modes, are clearly observed and well assigned with the combination of symmetry analysis and first-principles calculations. The well-resolved periodic intensity modulations of the observed modes with rotating crystal orientations verify the symmetry of each assigned mode and are fitted to experimentally determine the elements of Raman tensor matrices. The broadening of two ${A}_{g}$ modes seen in both compounds allows us to estimate the electron-phonon coupling constant, which suggests a relatively small electron-phonon coupling in the semimetals ${\mathrm{TaAs}}_{2}$ and ${\mathrm{NbAs}}_{2}$. The present study provides the fundamental lattice dynamics information on ${\mathrm{TaAs}}_{2}$ and ${\mathrm{NbAs}}_{2}$ and may shed light on the understanding of their extraordinary large magnetoresistance.

Smart home has recently attracted much research interest owing to its potential in improving the quality of human life. How to obtain user's demand is the most important and challenging task for appliance optimal scheduling in smart home, since it is highly related to user's unpredictable behavior. In this paper, a human-centered smart home system is proposed to identify user behavior, predict their demand and schedule the household appliances. Firstly, the sensor data from user's wearable devices are monitored to profile user's full-day behavior. Then, the appliance-demand matrix is constructed to predict user's demand on home environment, which is extracted from the history of appliance load data and user behavior. Two simulations are designed to demonstrate user behavior identification, appliance-demand matrix construction and strategy of appliance optimal scheduling generation.

Frustrated quantum magnets are expected to host many exotic quantum spin states like quantum spin liquid (QSL), and have attracted numerous interest in modern condensed matter physics. The discovery of the triangular lattice spin liquid candidate YbMgGaO$_4$ stimulated an increasing attention on the rare-earth-based frustrated magnets with strong spin-orbit coupling. Here we report the synthesis and characterization of a large family of rare-earth chalcogenides AReCh$_2$ (A = alkali or monovalent ions, Re = rare earth, Ch = O, S, Se). The family compounds share the same structure (R$\bar{3}$m) as YbMgGaO$_4$, and antiferromagnetically coupled rare-earth ions form perfect triangular layers that are well separated along the $c$-axis. Specific heat and magnetic susceptibility measurements on NaYbO$_2$, NaYbS$_2$ and NaYbSe$_2$ single crystals and polycrystals, reveal no structural or magnetic transition down to 50mK. The family, having the simplest structure and chemical formula among the known QSL candidates, removes the issue on possible exchange disorders in YbMgGaO$_4$. More excitingly, the rich diversity of the family members allows tunable charge gaps, variable exchange coupling, and many other advantages. This makes the family an ideal platform for fundamental research of QSLs and its promising applications., Comment: 8 pages, 4 figures; The updated supplementary materials (10 pages) can be found at http://cpl.iphy.ac.cn/fileup/0256-307X/PDF/0256-307X-2018-35-11-117501-Suppl.pdf

The novel ferroelectric-like structural transition observed in metallic ${\text{LiOsO}}_{\text{3}}$ [Y. Shi et al., Nat. Mater. 12, 1024 (2013)], has invoked many theoretical and experimental interests. In this work, we have performed polarized and temperature-dependent Raman scattering measurements on high-quality single crystal ${\text{LiOsO}}_{\text{3}}$ and identified Raman-active modes in both centrosymmetric phase (300 K, $R\overline{3}c)$ and noncentrosymmetric phase (10 K, $R3c)$. Only four phonon peaks are observed in the former phase, while there are 12 peaks in the latter phase because of the reduction of crystal symmetry. With the help of careful symmetry analysis and first-principles calculations, we can make a systematic assignment for the observed Raman modes in both phases. The significant changes in linewidth and the continuous evolution of Raman frequencies with temperatures were observed for the ${E}_{g}$ modes around the transition temperature, which suggests that the ferroelectric-like structural transition is a continuous order-disorder transition. The result sheds light on the coexistence of ferroelectricity and metallicity in the compound.

Significance Raman scattering is a powerful technique to probe optical phonons in solids. It usually involves an electron-mediated three-step process, involving photon–electron, electron–phonon, and electron–photon interactions. In principle, manipulating electrons, for instance by applying a magnetic field, should affect Raman phonon intensity, yet there is no direct experimental measurement to demonstrate this. In this work we report the first realization to our knowledge of the idea in a prototype material, MoS 2 . From monolayer and bilayer to bulk MoS 2 we observe a dramatic modification of Raman phonon intensity induced by magnetic field. Such a giant magneto-optical effect appearing at a monoatomic layer level and its technological implications for magnetic-optical devices should inspire a new branch of inelastic light scattering.

Raman scattering is one of the fundamental techniques in modern condensed matter physics. Its technical uniqueness allows to probe a variety of excitations in condensed matter and their couplings. FeSe-based superconductors have abundant structural phases like bulk, monolayer, Fe-deficient phases, and intercalated phases. Meanwhile, they exhibit rich electronic and magnetic phase diagrams and easily tunable superconducting transition temperatures ranged from a few Kelvins up to more than 40 K. This leads to the formation of an ideal research platform for superconductivity and has attracted a great deal of interest in the field. In this paper, we made a brief introduction to the structural phases, magnetism and electronic structures of FeSe-based superconductors. And we focused on the recent progress on Raman scattering studies of FeSe-based superconductors. For phononic Raman scattering, we introduced the Raman scattering measurement of the phonons in the Fe(Se,Te) system firstly. Next, in order to further explore the influence of doping on T c, we discussed the doping/substitution of A-sites and Fe-sites of A x Fe2− y Se2. Finally, we studied the correlation between superconductivity and various structures. We then discussed two-magnon and spin-phonon coupling in the materials. We discussed the relationship between magnetic order and superconductivity, and the effect of magnetic order on phonons. For electronic Raman response, we presented the discussions and analysis on electronic nematic phases. We introduced the observation of nematic phases by different measurement methods. And we discussed the origin of nematic phases: spin fluctuations or/and orbital (charge) fluctuations, different measurement methods have different conclusions. Finally, we introduced the studies in monolayer FeSe and some Raman scattering results there. We introduced the discovery of T c above 77 K in monolayer FeSe/SrTiO3. And we discussed many novel physical properties such as the opening of energy gap in 1UC FeSe/NbSTO and the charge transfer and ferroelectric phase transition between 1-UC thin film and NbSTO substrate.

The recent renaissance of black phosphorus (BP) as a two-dimensional 2D layered material has generated tremendous interest in its tunable electronic band gap and highly anisotropic transport properties that offer new opportunities for device applications. Many of these outstanding properties are attributed to its unique structural characters that still need elucidation. Here we show Raman measurements that reveal an ultralow-frequency collective compression mode (CCM), which is unprecedented among similar 2D layered materials. This novel CCM indicates an unusually strong interlayer coupling in BP, which is quantitatively supported by a phonon frequency analysis and first-principles calculations. Moreover, the CCM and another branch of low-frequency Raman modes shift sensitively with changing number of layers, allowing an accurate determination of the thickness up to tens of atomic layers, which is considerably higher than those previously achieved by using high-frequency Raman modes. These results offer fundamental insights and practical tools for exploring multilayer BP in new device applications., Comment: 19 pages, 4 figures, 1 table, 30 references, supplementary information included

In this paper, we studied stacked mL + nL graphene layers using Raman scattering spectroscopy. Our results indicate that the 2D band from stacked graphene can be considered as a superposition of those from the constituent nL and mL graphene layers, and a blueshift in the 2D band is observed when n or m = 1. The blueshift increases with the number of stacked layers and can be well understood by the reduction of Fermi velocity in the single layer graphene, as studied in the 1L + 1L (or twisted bilayer) case. As the number of stacked layers changes from 1 to 5, the Fermi velocity in the single layer graphene reduces to about 85% of its initial value. This study shows a convenient way to realize the modification of the Fermi velocity in free-stacking graphene and is of significance to the applications of graphene-based heterostructures.

Photoluminescence (PL) and Raman spectra under uniaxial strain were measured in mono- and bi-layer MoSe2 to comparatively investigate the evolution of excitonic gaps and Raman phonons with strain. We observed that the strain dependence of excitonic gaps shows a nearly linear behavior in both flakes. One percent of strain increase gives a reduction of ~ 42 meV (~ 35 meV) in A-exciton gap in monolayer (bilayer) MoSe2. The PL width remains little changed in monolayer MoSe2 while it increases rapidly with strain in the bilayer case. We have made detailed discussions on the observed strain-modulated results and compared the difference between monolayer and bilayer cases. The hybridization between 4d orbits of Mo and 4p orbits of Se, which is controlled by the Se–Mo–Se bond angle under strain, can be employed to consistently explain the observations. The study may shed light into exciton physics in few-layer MoSe2 and provides a basis for their applications.

In this work, Raman scattering measurements have been performed on the collapsed phase CaCo2As2 crystals. At least 8 Raman modes were observed at room temperature though CaCo2As2 is structurally similar to other 122 compounds like BaFe2As2. Two Raman modes are assigned to the intrinsic A1g and B1g of this material system respectively. The other ones are considered to originate from the local vibrations relevant to cobalt vacancies. Careful polarized measurements allow us to clearly resolve the four-fold symmetry of the B1g mode, which put strong constraints on possible point group symmetries of the system with Co vacancies. The temperature-dependent measurements demonstrate that the anomalies in both frequency and width of the B1g mode occur around Neel temperature T N. The anomalies are considered to be related to the gap opening near the magnetic transition. The study may shed light on the structural and magnetic changes and their correlations with superconductivity in 122 systems.

The recently discovered layered BiS2-based superconductors have attracted a great deal of interest due to their structural similarity to cuprate and iron-pnictide superconductors. We have performed Raman scattering measurements on two superconducting crystals NdO0.5F0.5BiS2 (Tc = 4.5 K) and NdO0.7F0.3BiS2 (Tc = 4.8 K). The observed Raman phonon modes are assigned with the aid of first-principles calculations. The asymmetrical phonon mode around 118 cm-1 reveals a small electron-phonon (e-ph) coupling constant 0.16, which is insufficient to generate superconductivity at ~ 4.5 K. In the Raman spectra there exists a clear temperature-dependent hump around 100 cm-1, which can be well understood in term of inter-band vertical transitions around Fermi surface. The transitions get boosted when the particular rectangular-like Fermi surface meets band splitting caused by spin-orbit coupling. It enables a unique and quantitative insight into the band splitting., Comment: 18 pages including suppl. materials, 8 figures, 35 references

Multiferroic materials are promising candidates for next-generation multi-functional devices, because of the coexistence of multi-orders and the coupling between the orders. FeVO4 has been confirmed to be a multiferroic compound, since it exhibits both ferroelectricity and antiferromagnetic ordering at low temperatures. In this paper, we have performed careful Raman scattering measurements on high-quality FeVO4 single crystals. The compound has a very rich phonon structure due to its low crystal symmetry (P − 1) and at least 47 Raman-active phonon modes have been resolved in the low and hightemperature spectra. Most of the observed modes are well assigned with aid of first-principles calculations and symmetry analysis. The present study provides an experimental basis for exploring spin-lattice coupling and the mechanism of multiferroicity in FeVO4