Finite Temperature Magnetism in the Triangular Lattice Antiferromagnet KErTe_(2)

After the discovery of the ARECh_(2)(A=alkali or monovalent ions,RE=rare-earth,Ch=chalcogen)triangular lattice quantum spin liquid(QSL)family,a series of its oxide,sulfide,and selenide counterparts has been consistently reported and extensively investigated.While KErTe_(2) represents the initial synthesized telluride member,preserving its triangular spin lattice,it was anticipated that the substantial tellurium ions could impart more pronounced magnetic attributes and electronic structures to this material class.This study delves into the magnetism of KErTe_(2) at finite temperatures through magnetization and electron spin resonance(ESR)measurements.Based on the angular momentum J after spin-orbit coupling(SOC)and symmetry analysis,we obtain the magnetic effective Hamiltonian to describe the magnetism of Er^(3+)in R3m space group.Applying the mean-field approximation to the Hamiltonian,we can simulate the magnetization and magnetic heat capacity of KErTe_(2) in paramagnetic state and determine the crystalline electric field(CEF)parameters and partial exchange interactions.The relatively narrow energy gaps between the CEF ground state and excited states exert a significant influence on the magnetism.For example,small CEF excitations can result in a significant broadening of the ESR linewidth at 2 K.For the fitted exchange interactions,although the values are small,given a large angular momentum J=15/2 after SOC,they still have a noticeable effect at finite temperatures.Notably,the heat capacity data under different magnetic fields along the𝑐axis direction also roughly match our calculated results,further validating the reliability of our analytical approach.These derived parameters serve as crucial tools for future investigations into the ground state magnetism of KErTe_(2).The findings presented herein lay a foundation for exploration of the intricate magnetism within the triangular-lattice delafossite family.

Raman scattering studies on the collapsed phase of CaCo_2As_2

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 A1gand B1gof 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 B1gmode,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 B1gmode occur around Neel temperature TN.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.

Strain-modulated excitonic gaps in mono-and bi-layer MoSe_2

Photoluminescence（PL） and Raman spectra under uniaxial strain were measured in mono- and bi-layer MoSe;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;2 meV（;5 me V） in A-exciton gap in monolayer（bilayer） MoSe;. The PL width remains little changed in monolayer MoSe;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 MoSe;and provides a basis for their applications.

Rare-Earth Chalcogenides:A Large Family of Triangular Lattice Spin Liquid Candidates

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（R3 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 50 mK. 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.

Raman scattering study of magnetic layered MPS_3 crystals(M = Mn, Fe, Ni)

We report a comprehensive Raman scattering study on layered MPS_3(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 C_(2h)but can be simplified to D_(3d) 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 MPS_3. Our results reveal that the in-plane Nel antiferromagnetic(AF) order in MnPS_3 favors a spin–phonon coupling compared to the in-plane zig-zag AF in NiPS_3 and FePS_3. 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.

Raman phonons in multiferroic FeVO_4 crystals

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 Fe VO4 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

Characterization of structural transitions and lattice dynamics of hybrid organic–inorganic perovskite CH3NH3PbI3

By combining temperature-dependent x-ray diffraction(XRD) with temperature-dependent Raman scattering, we have characterized the structural transitions and lattice dynamics of the hybrid organic–inorganic perovskite CH3NH3PbI3.The XRD measurements cover distinct phases between 15 K and 370 K and demonstrate a general positive thermal expansion.Clear anomalies are found around the transition temperatures.The temperature evolution of the lattice constants reveals that the transition at 160 K/330 K is of the first-/second-order type.Raman measurements uncover three strong lowfrequency modes, which can be ascribed to the vibration of the Pb/I atoms.The temperature evolution of the modes clearly catches these transitions at 160 K and 330 K, and confirms the transition types, which are exactly consistent with the XRD results.The present study may set an experimental basis to understand the high conversion efficiency in methylammonium lead iodide.

Effective model for rare-earth Kitaev materials and its classical Monte Carlo simulation

Recently,the family of rare-earth chalcohalides were proposed as candidate compounds to realize the Kitaev spin liquid(KSL)[Chin.Phys.Lett.38047502(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.

Room-Temperature Ferromagnetic ZnMnO Thin Films Synthesized by Plasma Enhanced Chemical Vapour Deposition Method

Room-temperature ferromagnetic Mn-doped ZnO films are grown on Si （001） substrates by plasma enhanced chemical vapour deposition （PECVD）. X-ray diffraction measurements reveal that the Znl-xMn.O films have the single-phase wurtzite structure. X-ray photoelectron spectroscopy indicates the existence of Mn^2＋ ions in Mndoped ZnO films. Furthermore, the decreasing additional Raman peak with increasing Mn-doping is considered to relate to the substitution of Mn ions for the Zn ions in ZnO lattice. Superconducting quantum interference device （SQUID） measurements demonstrate that Mn-doped ZnO films have ferromagnetic behaviour at room temperature.

Raman scattering study of two-dimensional magnetic van der Waals compound VI3

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 Ag 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 C2h rather than D3d 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 Ag 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.

Raman scattering under extreme conditions

Raman scattering is a versatile and powerful technique and has been widely used in modern scientific research and vast industrial applications. It is one of the fundamental experimental techniques in condensed matter physics, since it can sensitively probe the basic elementary excitations in solids like electron, phonon, magnon, etc. The application of extreme conditions （low temperature, high magnetic field, high pressure, etc.） to Raman scattering, will push its capability up to an unprecedented level, because this enables us to look into new quantum phases driven by extreme conditions, trace the evolution of the excitations and their coupling, and hence uncover the underlying physics. This review contains two topics. In the first part, we will introduce the Raman facility under extreme conditions, belonging to the optical spectroscopy station of Synergetic Extreme Condition User Facilities （SECUF）, with emphasis on the system design and the capability the facility can provide. Then in the second part we will focus on the applications of Raman scattering under extreme conditions to a variety of condensed matter systems such as superconductors, correlated electron systems, charge density waves （CDW） materials, etc. Finally, as a rapidly developing technique, time-resolved Raman scattering will be highlighted here.

Crystal growth and magnetic properties of quantum spin liquid candidate KErTe_(2)

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 R3m and Er3+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±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.

Rare-Earth Chalcohalides: A Family of van der Waals Layered Kitaev Spin Liquid Candidates

The Kitaev spin liquid(KSL) system has attracted tremendous attention in recent 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 R3m,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 the 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.

傅里叶磁光谱仪中的起偏器和检偏器的自动定位及误差分析

在傅里叶变换型磁光谱仪中,起偏器和检偏器的初始方位通常需要在测量前预先定位。提出了一种不需要专门对起偏器和检偏器定位即可完成磁光谱的测量计算的新方法,该方法可以计算出起偏器和检偏器的初始角度值,从而进行自动校准。推导了检偏器旋转角度偏差导致磁光偏转角测量误差的理论公式,并进行了数值分析,给出了测量误差与旋转步数的关系,数值结果表明检偏器角度偏差引起的误差与步数的平方根成反比关系。还给出了一个钴膜样品的磁光克尔角随外加磁场变化的实测曲线。

FeSe基超导体的拉曼散射研究

拉曼散射是现代凝聚态物理研究的基础实验手段之一,利用其技术上的特色,可以探测凝聚态物质的各类激发及其耦合.FeSe基超导体具有块材、单层、缺位相、插层结构等不同的结构形态、丰富的电子和磁相图、从几K到40 K以上的易调节超导转变温度,因此形成了研究超导性质的一个理想平台,近些年引发了超导领域很多研究者的研究兴趣.本文首先对FeSe基超导体的结构、磁性和电子结构进行介绍,着重阐述了拉曼散射测量在FeSe基超导体研究中取得的研究进展.在声子拉曼响应方面,主要介绍FeSe基超导体的结构与其超导电性之间的关系;在磁的拉曼响应方面,分别对FeSe基材料的双磁子、自旋声子耦合等研究结果进行讨论;在电子拉曼响应方面,主要针对FeSe基超导体中的向列相问题进行了相关分析;最后对单层FeSe超导体和相关的拉曼测量做简要介绍.