Leveraging crystal symmetry for thermoelectric performance optimization in cubic GeSe

In thermoelectrics,the manipulation of crystal symmetry is instrumental in optimizing the electrical and thermal transport parameters.Within this context,the present study explored the largely overlooked high-symmetry cubic GeSe,which presented larger band degeneracy than its widely studied medium-symmetry rhombohedral counterpart.We have successfully stabilized cubic Ge Se a ambient conditions through co-alloying with AgSnTe_(2)and Bi.The incorporation of AgSnTe_(2)initiates the transition of GeSe from a low-symmetry orthorhombic to a mediumsymmetry rhombohedral phase,culminating in a highsymmetry cubic structure,underpinned by variation in chemical bonding mechanisms.Notwithstanding this,the persistence of Ag_(2)Te precipitates impedes the total elimination of the residual orthorhombic phase due to the disparate chemical bonding mechanism between Ag_(2)Te and GeSe.Introducing Bi into the rhombohedral-dominated(GeSe)_(0.7)(AgSnTe_(2))_(0.3)matrix leads to the dissolution of Ag_(2)Te precipitates,elimination of the residual orthorhombic phase,and the subsequent stabilization of the exclusive cubic phase.Compared to its orthorhombic counterpart,the cubic GeSe exhibits diminished bandgap and Ge vacancy formation energy,amplified band degeneracy,reduced sound velocity,intensified lattice anharmonicity and multiple phonon scattering centres engendering elevated carrier concentration and density-ofstates effective mass,alongside restrained lattice therma conductivity.Consequently,a peak zT of 0.46 at 573 K is attained for cubic(Ge_(0.7)Bi_(0.3)Se)_(0.7)(AgSnTe_(2))_(0.3),signifying a ninefold increase relative to the initial orthorhombic Ge Se.These results illuminate the critical role of crystal symmetry manipulation in advancing the thermoelectric performance.

Growth temperature dependence of crystal symmetry in Nb-doped BaTiO_(3) thin films

Growth temperature effects on the microstructure of Nb-doped BaTiO_(3) thin films of the composition BaTi_(0.98)Nb_(0.02)O_(3) are studied using X-ray diffraction and transmission electron microscopy(TEM).Reciprocal space maps and electron diffraction patterns show that the a-axis lattice parameter increases and the c-axis parameter decreases with increasing growth temperature,indicating a decrease of tetragonality.Bright-field TEM images show low and high densities of threading defects in films grown at low and high temperatures,respectively.The observations are discussed in terms of a hindering of the cubic-to-tetragonal phase transition by a high defect density and a high unit cell volume.

Synthesis,Crystal and Electronic Structure of Ba_3ZnSb_2O_9 with the 6H-perovskite-type Structure

Crystals of Ba3ZnSb2O9 have been grown by a high-temperature solid-state reaction and characterized by single-crystal X-ray diffraction.Ba3ZnSb2O9 crystallizes in the hexagonal P63/mmc space group with a = 5.8663（4）,c = 14.478（2） ,V = 431.49（8） 3,Z = 2 and R（all data） = 0.0167.The structure of Ba3ZnSb2O9 consists of pairs of face-sharing Sb2O9 bi-octahedra connected via corners with two single layers of mutually isolated ZnO6 octahedra.Each Ba2＋ ion is bonded to 12 oxygen atoms.The UV-vis absorption spectrum of the compound has been investigated.Additionally,the calculations of band structure and density of states have also been performed with density functional theory method.The obtained results tend to support the experimental data of the absorption spectrum.

Spin orbit torques in Pt-based heterostructures with van der Waals interface

Spin orbit torques(SOTs)in ferromagnet/heavy-metal heterostructures have provided great opportunities for efficient manipulation of spintronic devices.However,deterministically field-free switching of perpendicular magnetization with SOTs is forbidden because of the global two-fold rotational symmetry in conventional heavy-metal such as Pt.Here,we engineer the interface of Pt/Ni heterostructures by inserting a monolayer MoTe_(2)with low crystal symmetry.It is demonstrated that the spin orbit efficiency,as well as the out-of-plane magnetic anisotropy and the Gilbert damping of Ni are enhanced,due to the effect of orbital hybridization and the increased spin scatting at the interface induced by MoTe_(2).Particularly,an out-of-plane damping-like torque is observed when the current is applied perpendicular to the mirror plane of the MoTe_(2)crystal,which is attributed to the interfacial inversion symmetry breaking of the system.Our work provides an effective route for engineering the SOT in Pt-based heterostructures,and offers potential opportunities for van der Waals interfaces in spintronic devices.

A strategy for preparing broadband polymer based optical waveguide amplifiers:doping low crystal field symmetric nanocrystals in polymer matrix as gain media

Optical waveguide amplifiers are essential devices in integrated optical systems,with their gain bandwidths directly influencing the operating wavelengths of optical circuits.Previous Er^(3+)-doped polymer optical waveguide amplifiers have been limited to amplifying signals within the C-band.To achieve broadband polymer optical waveguide amplification,we propose the use of nanocrystals with low crystal field symmetry to extend the working bandwidth.Our approach utilizes LiYF_(4):Yb,Er nanoparticles embedded in poly(methyl methacrylate)as the gain medium,enabling signal amplification from most of the S-band to the whole(C+L)band.The low crystal field symmetry of the LiYF_(4)host significantly splits the^(4)I_(13/2)and^(4)I_(15/2)levels of Er^(3+)ions owing to the crystal field effect,facilitating broadband down-conversion luminescence under 980-nm excitation.Furthermore,a fluorescence kinetic analysis confirms that the broadband luminescence of Er^(3+)arises from significant energy-level splitting caused by the crystal field effect.Under 980-nm excitation,the amplifiers exhibited relative gains of approximately 12.6 dB at 1535 nm,7.4 dB at 1480 nm,and 3.7 dB at 1610 nm.The Er^(3+)-doped broadband polymer optical waveguide amplifier was successfully prepared.

A study on surface symmetry and interfacial enhancement of SrTiO_3 by second harmonic generation

The symmetry of the surfaces of SrTiO3 and slightly Nb-doped SrTiO3 crystals was investigated by the optical reflected second harmonic generation technique. The good agreement between experimental and theoretical results of the second harmonic in- tensity dependence on the azimuth angle indicates that the SrTiO3 （001） surface is with 4ram symmetry and the Nb-doped SrTiO3 （111） surface with 3m symmetry. The measurements of the polarization dependent second harmonic intensity confirm that conclusion. The enhancement of the surface polarization in the structure of SrTiO3 capped La0.9Sr0.1MnO3 films compared with that in the La0.9Sr0.1MnO3 films has been obtained.

Source-configured symmetry-broken hyperbolic polaritons

Polaritons are quasi-particles that combine light with matter,enabling precise control of light at deep subwavelength scales.The excitation and propagation of polaritons are closely linked to the structural symmetries of the host materials,resulting in symmetrical polariton propagation in high-symmetry materials.However,in low-symmetry crystals,symmetry-broken polaritons exist,exhibiting enhanced directionality of polariton propagation for nanoscale light manipulation and steering.Here,we theoretically propose and experimentally demonstrate the existence of symmetry-broken polaritons,with hyperbolic dispersion,in a high-symmetry crystal.We show that an optical disk-antenna positioned on the crystal surface can act as an in-plane polarized excitation source,enabling dynamic tailoring of the asymmetry of hyperbolic polariton propagation in the high-symmetry crystal over a broad frequency range.Additionally,we provide an intuitive analysis model that predicts the condition under which the asymmetric polaritonic behavior is maximized,which is corroborated by our simulations and experiments.Our results demonstrate that the directionality of polariton propagation can be conveniently configured,independent of the structure symmetry of crystals,providing a tuning knob for the polaritonic response and in-plane anisotropy in nanophotonic applications.

Diffraction properties of four-petal Gaussian beams in uniaxially anisotropic crystal

Propagation properties of polarized anisotropic crystals were investigated four-petal Gaussian beams along the optical axis of uniaxially Based on the paraxially vectorial theory of beam propagation analytic expressions of the diffraction light field larization properties of the diffracted four-petal were obtained. The effects of the anisotropy on the po- Gaussian beams have also been explained by numerical method. The results elucidate that the linear polarization state and the symmetry of the incident beams cannot be kept during propagation in anisotropic crystals.

General theory for designing phonon transport in alloyed/doped materials

Alloying/doping is a widely used technique for improving the electrical,mechanical,and optical properties of materials.However,this technology induces significant distortions in the lattice structure,mass distribution,and potential field,greatly enhancing phonon scattering.Here,we introduce the concept of alloying/doping path and employ crystal symmetry,lattice deformation,and electron distribution to characterize it.Based on this new concept,the phonon thermal transport behavior in alloyed/doped materials can be well designed,and along different alloying/doping paths,the difference in thermal conductivity can be up to 45 times.On one hand,strategic alloying/doping that combines high crystal symmetry,large lattice contraction,and the same electron distribution suppresses phonon-phonon scattering phase space,induces phonon stiffening,and bolsters electronic structure symmetry,respectively.These synergistic effects significantly improve thermal conductivity.On the other hand,random alloying/doping has a low symmetry,leading to the typical“U”shape of alloying/doping level-dependent thermal conductivity.Our theory is corroborated in three-dimensional(3D)Si,2D MoS_(2),and quasi-1D TiS_(3),affirming its efficacy and broad applicability in controlling phonon transport.

Antisymmetric planar Hall effect in rutile oxide films induced by the Lorentz force

The conventional Hall effect is linearly proportional to the field component or magnetization component perpendicular to a film. Despite the increasing theoretical proposals on the Hall effect to the in-plane field or magnetization in various special systems induced by the Berry curvature, such an unconventional Hall effect has only been experimentally reported in Weyl semimetals and in a heterodimensional superlattice. Here, we report an unambiguous experimental observation of the antisymmetric planar Hall effect(APHE) with respect to the in-plane magnetic field in centrosymmetric rutile RuO_(2) and IrO_(2) single-crystal films. The measured Hall resistivity is found to be linearly proportional to the component of the applied in-plane magnetic field along a particular crystal axis and to be independent of the current direction or temperature. Both the experimental observations and theoretical calculations confirm that the APHE in rutile oxide films is induced by the Lorentz force. Our findings can be generalized to ferromagnetic materials for the discovery of anomalous Hall effects and quantum anomalous Hall effects induced by in-plane magnetization. In addition to significantly expanding knowledge of the Hall effect, this work opens the door to explore new members in the Hall effect family.

Optical spectra and local structure of Eu^(3+) ions doped in Nb_2O_5-La_2O_3-B_2O_3-BaO glasses

The xNb2O5-（15-x）La2O3-40B2O3-45BaO （x = 5, 7.5, 12.5 mol%） glasses doped with Eu^3＋ ions in 1 mol% are fabricated by the melting method. The Fourier transform infrared （FTIR） spectra, phonon sideband spectra, emission and excitation spectra of the glasses are measured. The crystal field parameter and coordination number of Eu^3＋ ions in the glasses are obtained according to the splitting of their ^5D0 - ^7F1 levels. The intensity parameters Ω2 and Ω4 of Eu^3＋ ions for optical transition are calculated from their emission spectra in terms of reduced matrix U^（t） （λ= 2, 4, 6） character for optical transitions. The results indicate that the intensity parameters Ω2 and Ω4 increase with the increase of Nb2O5 content, suggesting that the symmetry becomes lower, the band of Eu and O atoms becomes stronger and the covalence increases with the increase of Nb2O5 content.

An electron backscattered diffraction study of twin structure in hsianghualite

Hsianghualite (Ca 3 Li 2 [BeSiO 4 ] 3 F 2 , I 2 1 3) is the first new mineral discovered in China in 1958 and is very precious due to its rare occurrence in the Earth. Few studies have reported about hsianghualite during these years. The authors measured the complex morphology of hsianghualite and found a twin structure in hsianghualite crystal by electron backscattered diffraction (EBSD) technique. The pole figures were used to analyze the twin relationship. The twin law of hsianghualite is the twin plane {110}. This twin law is the same as that proposed by crystallographic theoretic analysis in the previous work. EBSD experi- mental study verified our theoretic analysis of the twin structure in the previous work. The twin structure in hsianghualite has never been reported since the discovery of this mineral in 1958.

Formaldehyde gas sensor based on TiO_2 thin membrane integrated with nano silicon structure

An innovative formaldehyde gas sensor based on thin membrane type metal oxide of Ti O2 layer was designed and fabricated. This sensor under ultraviolet(UV) light emitting diode(LED) illumination exhibits a higher response to formaldehyde than that without UV illumination at low temperature. The sensitivities of the sensor under steady working condition were calculated for different gas concentrations. The sensitivity to formaldehyde of 7.14 mg/m^3 is about 15.91 under UV illumination with response time of 580 s and recovery time of 500 s. The device was fabricated through micro-electro-mechanical system(MEMS) processing technology. First, plasma immersion ion implantation(PIII) was adopted to form black polysilicon, then a nanoscale TiO_2 membrane with thickness of 53 nm was deposited by DC reactive magnetron sputtering to obtain the sensing layer. By such fabrication approaches, the nanoscale polysilicon presents continuous rough surface with thickness of 50 nm, which could improve the porosity of the sensing membrane. The fabrication process can be mass-produced for the MEMS process compatibility.

Observing the spin Hall effect of pseudothermal light through weak measurement

The spin Hall effect of light （SHEL） can be observed by the dark strip resulting from weak measurement. We find that the SHEL of a partially coherent beam （PCB） has a similar phenomenon as well. However, the dark strip in the SHEL of a PCB cannot be explained by considering the beam as an assemblance of coherent speckles. Also, the dark strip in a PCB is not purely dark. By analyzing the autocorrelation, we show that the SHEL of a PCB is the result of overlapping coherent speckles＇ SHEL. We further prove our conclusion by adjusting convergence and incident angles. Finally, we develop a qualitative theory to clarify the SHEL of a PCB.