二维Fe5GeTe2单晶的大量制备及吸波性能研究

目的 快速制备克量级、高质量的二维磁性Fe₅GeTe₂单晶，并开发具有范德华结构、室温铁磁性及优异微波吸收性能的新型吸波材料。方法 通过调控输运剂的含量，并缩短化学气相传输法的恒温段时长，以探索高质量Fe₅GeTe₂单晶的快速、制备量大的方法。将Fe₅GeTe₂单晶与石蜡按不同质量分数混合后，通过同轴线测试法分析其微波性能。结果 成功制备了克量级、高质量的Fe₅GeTe₂单晶样品。借助矢量分析仪的同轴线测试法研究Fe₅GeTe₂单晶的微波损耗性能发现，当Fe₅GeTe₂的质量分数为30%时，Fe₅GeTe₂的吸波特性较差，最佳反射损耗值仅为-4.8dB；当Fe₅GeTe₂的质量分数达到70%时，Fe₅GeTe_...     

Laser Excitation Effect on the Optical and Dielectric Properties of Ferroelectric KNbO3 Single Crystals

The effects of laser irradiation on the optical absorption (200- to 800-nm range) and dielectric properties (10²- to 10⁷-Hz range) of KNbO₃ single crystals in ferroelectric phase have been studied. All these properties are found to be reduced significantly at lower frequencies with an increase in laser irradiation time. However, the phase transition temperature remains unchanged. It is observed that the dielectric constant (K) and loss (tan ) values of KNbO₃ single crystals are decreased significantly at room temperature or at Curie temperature by laser excitation. Appreciable changes in the band gap (eV) and activation energy are also observed after laser excitation.     

Effect of Oxygen Doping on the IR Transmission and Cathodoluminescence of ZnSe

The effect of oxygen doping (0.9 and 4.3 vol % O₂in the gas phase) on the transmission and cathodoluminescence of CVD ZnSe was studied. The incorporation of oxygen was found to reduce the transmission in the spectral range 700–1900 cm^–1. Examination by scanning electron microscopy shows that Se excesses and high O₂concentrations lead to tabular growth. ZnO precipitation at structural defects during cooling was observed only in the end portion of the deposit. The dimensions of microinhomogeneities are shown to have a significant effect on the transmission of ZnSe. The oxygen-containing species present in the deposits were identified using cathodoluminescence spectra. The CL spectrum of Se-enriched p-type ZnSe is dominated by the band at 490 nm (SAL) at 80 K and the band at 640 nm (SA) at 300 K. The 640-nm band is attributed to recharging of the SALcenters upon a variation of the Fermi energy with temperature. Decreasing the Se excess leads to the quenching of the SALemission and appearance of the shorter wavelength component of the SAband at 600 nm (80–300 K), characteristic of slightly Zn-enriched ZnSe. The intensity of the self-activated bands is shown to increase as the concentration of dissolved oxygen increases.     

Effect of DC Biasing Field on the Dielectric Properties of High Field Treated SrTiO3 Single Crystals

Dielectric constant (K) and loss (Tan) of strontium titanate single crystals subjected to high electric fields (10 KV/cm, AC or DC) were measured as a function of frequency (10² to 10⁷ Hz). These properties were also studied as a function of time at 10³ Hz immediately after the high field treatment. The value of K and tan of the field-treated samples attained a steady value after 20 hours. Measurements of K and Tan as a function of time and frequency under small DC biasing fields (200 V/cm) were taken. The values of K and tan of the high field-treated samples decreased with time and frequencies; however under a DC biasing field, K increases slightly up to 10 minutes and then decreases slowly with time to a steady value. The inversion temperature (T_i) at different frequencies get reduced with high field treatment. However, the application of small DC-biasing field increased the T_i value with the increase of biasing field strength. On the other hand, activation energy increased with high field and decreased with biasing field. The effect of high DC field treatment on SrTiO₃ single crystal exhibits more prominent changes in K, tan , T_i and activation energy than that of high AC field treated samples.     

Light-driven Optical Switch,Based on Excited-state Absorption in Activated Dielectric Crystals

Abstract

We show that despite the generally held view that excited-state absorption (ESA) is a totally negative phenomenon in laser physics, in some cases it has useful applications in laser technology.     

Effect of Impurities and Structural Defects on the Transport Properties of n-Type GaAs Crystals

The carrier mobility and concentration in n-type GaAs single crystals, undoped, doped during growth (Te, Sn, and Ge), and neutron-transmutation doped (NTD) with Se and Ge, were measured as a function of temperature. Although the donor concentration was the same in all of the doped crystals, the carrier mobility and concentration in the GaAs crystals were found to be lower than those in the GaAs, GaA, and NTD GaAs crystals by about a factor of three and one order of magnitude, respectively. These results are interpreted in terms of the compensation ratio and predominant scattering processes in the crystals. The large compensation ratio in the GaAs crystals is due to the high acceptor concentration. The low carrier mobility in GaAs is associated with the strong scattering of charge carriers by ionized defects, because of their high density, and also, in contrast to the undoped, GaAs, and GaAs, and NTD GaAs crystals, with additional scattering by nonuniformly distributed structural defects. In calculations of the compensation ratio and analysis of the scattering mechanisms, the Brooks–Herring or Cornell–Weisskopf approximation was used, depending on the compensation ratio in the material.     

Evolution of Superconducting Properties of LiFeAs Single Crystals Doped with Magnetic or Nonmagnetic Impurities

The effect of magnetic Co²⁺ and nonmagnetic Ga³⁺ impurities on the crystal structure and superconducting properties of LiFeAs single crystals has been investigated. A large T _c decrease of about 4.8 K/at% is observed in Ga-doped LiFeAs. This rate is higher than that of the material doped with magnetic Co impurities (～3.7 K/at%). The greater T _c suppression in the Ga case is likely due to the pair breaking associated with the significant changes in the crystal structure of the doped material. The increase of the critical current densities in intermediate magnetic fields (H⊥?ab) indicates that a very small amount of Ga (0.5 at%) acts as an effective pinning site for flux pinning enhancement in the material. The analysis of the temperature and field dependencies of the magnetic relaxation is consistent with the collective pinning model for the Co-doped material, while the magnetic relaxation measurements combined with the peak position of the critical current density in the B–T phase diagram of Ga-doped LiFeAs suggest an elastic–plastic transition of the vortex lattice at higher temperatures and fields.     

Effect of Magnesium Impurity on the Structure, Luminescence, and Reflection Spectrum of ZnSe Crystals

The structural and luminescent properties of ZnSeMg crystals are studied. The results indicate that Mg doping gives rise to local lattice distortions and influences the densities of native point defects. This leads to broadening of x-ray diffraction peaks, reduces the red emission intensity to nearly zero, and markedly raises the blue emission intensity.     

Optical Absorption and Photoconductivity of Ni-Doped ZnIn2S4 Single Crystals

A technique was developed for doping ZnIn₂S₄ single crystals with Ni during vapor transport growth. The effect of Ni doping on the edge absorption, photoluminescence excitation spectrum, and spectral response of ZnIn₂S₄ was studied at 300 K.     

Dielectric and Magnetic Anomalies in α-MnS Single Crystals at Applied Magnetic Fields

Journal of Superconductivity and Novel Magnetism - Temperature and magnetic field dependences of the magnetic susceptibility and dielectric permittivity of the α-MnS single crystal with NaCl...     

Identification of the Intrinsic Dielectric Properties of Metal Single Atoms for Electromagnetic Wave Absorption

Atomically dispersed metals on N-doped carbon supports(M-N_xCs) have great potential applications in various fields.However,a precise understanding of the definitive relationship between the configuration of metal single atoms and the dielectric loss properties of M-N_xCs at the atomic-level is still lacking.Herein,we report a general approach to synthesize a series of three-dimensional(3D)honeycomb-like M-N_xC(M=Mn,Fe,Co,Cu,or Ni) containing metal single atoms.Experimental results indicate that 3D M-N_xCs exhibit a greatly enhanced dielectric loss compared with that of the NC matrix.Theoretical calculations demonstrate that the density of states of the d orbitals near the Fermi level is significantly increased and additional electrical dipoles are induced due to the destruction of the symmetry of the local microstructure,which enhances conductive loss and dipolar polarization loss of 3D M-N_xCs,respectively.Consequently,these 3D M-N_xCs exhibit excellent electromagnetic wave absorption properties,outperforming the most commonly reported absorbers.This study systematically explains the mechanism of dielectric loss at the atomic level for the first time and is of significance to the rational design of high-efficiency electromagnetic wave absorbing materials containing metal single atoms.     

Point-Contact Spectroscopy in Mn-Doped MgB2 Single Crystals: Effects of Magnetic Impurities in a Two-Band Superconductor

We performed point-contact spectroscopy (PCS) measurements in Mg_1−x Mn _x B₂ single crystals, with x≤0.015 and bulk T _c down to 13.3 K. The gaps Δ _σ and Δ _π were obtained by fitting the conductance curves of the point contacts with the two-band Blonder–Tinkham–Klapwijk (BTK) model. Both Δ _σ and Δ _π decrease with the critical temperature of the junctions T _c ^A , but remain clearly distinct down to the lowest critical temperature (T _c ^A ≅9 K). Once analyzed within the Eliashberg theory, the gap trends as a function of T _c ^A can be explained by a doping-induced increase in the pair-breaking scattering within the σ band, with smaller contributions from the π-π or the σ-π channels.     

Effect of Ionizing Radiation Parameters on the Properties of AgGaS2 Single Crystals

Inorganic Materials - We report X-ray dosimetric characteristics of AgGaS2 single crystals grown by the Bridgman–Stockbarger method. The 298-K X-ray induced conductivity coefficient of the...     

Study of Interatomic Potentials Using the Crystal-GRID Method on Oriented Single Crystals of Ni,Fe, and Cr

The Crystal-GRID method is used to study interatomic collisions at low energy in metals and such to probe the repulsive interatomic potential. Line shapes of gamma rays, emitted by the recoiling ⁵⁹Ni isotope after thermal neutron capture in Ni single crystals, were measured and compared to results obtained by molecular dynamics simulations of the slowing down. The same procedure is also used for recoiling ⁵⁷Fe and ⁵⁴Cr atoms in Fe and Cr single crystals, respectively. Different potentials (including several from the embedded atom method) are investigated using the observed fine structure of the line shape which depends on the crystal orientations. From the detailed study of the lineshapes measured in two different orientations, a new potential is then derived for each element. Nuclear state lifetimes for the excited isotopes are also deduced with a higher precision than obtained with standard nuclear techniques.     

Aging Effect on Elapsed Time of Serrations in Al-Li Single Crystals

Elapsed time of serrations in Al-Li single crystals was investigated with two definitions: one is the time elapsed in one cycle of serrations, the other is the average elapsed time within a strain limit.Elapsed time of individual serration appears independent on stress drops during serrated flow. It primarily distributes within the limits of short elapsed time in peak aged condition compared with underaged and overaged conditions, corresponding to low level of the average elapsed time that appears indifferent to increasing strain. It seems that shearing behaviour of δ' particles could be responsible for the variations in elapsed time of serrations.     

Dielectric and Photo-Dielectric Properties of TlGaSeS Crystals

The room temperature, dark and photo-dielectric properties of the novel crystals TlGaSeS are investigated in the frequency, intensity and biasing voltage having ranges of ~ 1–120 MHz, 14–40 klux and 0–1 V, respectively. The crystals are observed to exhibit a dark high frequency effective dielectric constant value of ~ 10·65 × 10³ with a quality factor of ~ 8·84 × 10⁴ at ~ 120 MHz. The dielectric spectra showed sharp resonance– antiresonance peaks in the frequency range of ~ 25–250 kHz. When photoexcited, pronounced increase in the dielectric constant and in the quality factor values with increasing illumination intensity are observed. Signal amplification up to ~ 33% with improved signal quality up to ~ 29% is attainable via photoexcitation. On the other hand, the illuminated capacitance–voltage characteristics of the crystals reflected a downward shift in the voltage biasing and in the built-in voltage of the device that is associated with increase in the uncompensated carrier density. The increase in the dielectric constant with increasing illumination intensity is ascribed to the decrease in the crystal's resistance as a result of increased free carrier density. The light sensitivity of the crystals, the improved dielectric properties and the lower biasing voltage obtained via photoexcitation and the well-enhanced signal quality factor of the crystals make them promising candidates for optical communication systems.     

Size Effect in the High-Field Magnetoconductivity of Pure Metal Single Crystals

The magnetoresistivity of tungsten single crystals with a residual resistivity ratio of up to 70,000 was measured in the temperature range from 4.2 to 55 K in a magnetic field of 10 T. The size effect, i.e., the linear dependence of the magnetoconductivity on the inverse sample dimensions, has been observed in the high-field magnetoconductivity. The experiments show that this phenomenon can be used to separate and study the surface and volume contributions to the magnetoresistivity of pure single crystals of compensated metals from liquid helium temperature up to approximately 25 K. It is shown that the specular reflection coefficient of the conduction electrons for pure metals in high magnetic fields can be determined in this way.     

Influence of the Surface Mechanical Treatment on the Photothermal Piezoelectric Spectra of ZnSe Crystals

This paper presents experimental and theoretical piezoelectric photoacoustic spectra of ZnSe samples after different surface treatments: grinding, polishing, and etching. The modification of the Jackson and Amer theory, presented in the paper, enabled numerical interpretation of the spectra in the model of a mechanically damaged surface layer. The nature of the characteristic peak observed at E =?2.65 eV below the energy gap is explained as caused by the Urbach absorption tail and the damaged surface layer of a sample. The correlation between the microhardness of the Zn_1-x Be _x Se and ZnSe crystals and the thickness of the mechanically damaged surface layer was observed.