The experimental and theoretical investigation of vibration spectra in ferroelectric semiconductor SbSBrxI1−x crystals

The vapor grown SbSBr_xI_1−x (x=0.1; 0.5; 0.9) crystals with clear mirror surfaces have been used for infrared reflection measurements with Fourier spectrometer. The vibration frequencies along c(z)-axis have been derived from Kramers–Kroning and optical parameters fitting analysis of the experimental reflectivity spectra at T=300 K. The theoretical vibration spectra of SbSBr_xS_1−x (x=0.1; 0.5; 0.9) crystals in paraelectric phase (T=300 K) along c(z)-axis have been determined in quasiharmonic approximation by diagonalization of dynamical matrix. The theoretical vibration spectra of these crystals in a–b(x−y) plane have been determined in harmonic approximation. In this work we discuss the nature of anharmonism in SbSBr_xI_1−x crystals along the c(z)-axis.     

Synthesis, characterization and melt processing of Y1−x(Yb0.9Nd0.1)xBa2Cu3Oz superconductors

Effects of a combined substitute of Yb and Nd on Y site on the superconducting properties of YBa₂Cu₃O_y have been studied. We synthesized Y_1−x(Yb_0.9Nd_0.1)_xBa₂Cu₃O_z compound with x = 0.2, 0.4, 0.6, 0.8 and 1.0. Here, the ratio of Yb–Nd was fixed to be 9:1 for obtaining 123 phase without secondary phases. The melt processing thermal profiles for Y_1−x(Yb_0.9Nd_0.1)_xBa₂Cu₃O_z with x = 0.2 and 0.4 and the addition of 40 mol% {Y_1−x(Yb_0.9Nd_0.1)_x}₂BaCuO₅ and 0.5 wt% Pt in air were determined on the basis of the thermal analysis results. All samples showed a low grain growth rate, particularly for high x values, which may be partially ascribed to un-optimized thermal schedules. Although almost all the samples exhibited low J_c values, the sample with x = 0.2 exhibited T_c of 88.8 K and a relatively high J_c value of 16,000 A/cm² at 77 K for H//c-axis.     

Vortex pinning properties of –Ba–Cu–O and –Ba–Cu–O superconducting bulks

We have studied the effect of a small amount of Y-site substitution by La or Pr ions on the vortex pinning in the Y–Ba–Cu–O system. (Y_1-xLa_x)–Ba–Cu–O and (Y_1-xPr_x)–Ba–Cu–O bulks were fabricated by the melt-textured growth, in which x was varied from 0 to 0.01. The critical current density J_c at 77 K is improved in magnetic fields parallel to the c-axis above 2–4.5 T and the corresponding irreversibility field, H_irr, shifts to the higher value in both bulks.     

Terahertz time-domain reflection spectroscopy for high-Tc superconducting cuprates

We have developed a terahertz time-domain spectroscopy (THz-TDS) system for reflectivity measurement with a temperature-controllable cryostat. For emission and detection of THz radiation, a Ti:Sapphire pulsed laser and photoconductive antennas are used. Two wire-grid polarizers enable us to carry out the polarized reflectivity measurements. Using our THz-TDS system, we measured the c-axis polarized reflectivity spectra for La_2−xSr_xCuO₄ single crystals (x = 0.10 and 0.13) and observed sharp Josephson plasma edges for both samples below T_c. The reflectivity spectra of the x = 0.10 sample were in good agreement with those reported previously, which confirms the validity of the system. For the x = 0.13 sample, we discuss the anomolous features of the optical spectra.     

Scanning tunneling microscopy/spectroscopy on multi-layered cuprate superconductor Ba2Ca5Cu6O12 (O1−x Fx)2

Scanning tunneling microscopy/spectroscopy (STM/STS) measurements on multi-layered cuprate superconductor Ba₂Ca₅Cu₆O₁₂ (O_1−x F_x)₂ are carried out. STM topographies show randomly distributed bright spot structures with a typical spot size of 0.8 nm. These bright spots are occupied about 28% per one unit cell of c-plane, which is comparable to the regular amount of apical oxygen of 20% obtained from element analysis. Tunneling spectra simultaneously show both the small and the large gap structures. These gap sizes at 4.9 K are about Δ 15 meV and 90 meV, respectively. The small gap structure disappears at the temperature close to T_C, while the large gap persists up to 200 K. Therefore, these features correspond to the superconducting gap and pseudogap, respectively. These facts give evidence for some ordered state with large energy scale even in the superconducting state. For the superconducting gap, the ratio of 2Δ/K_BT_C = 4.9 is obtained with T_C = 70 K, which is determined from temperature dependence of the tunneling spectra.     

Metal-insulator transition in electron-doped Ba1−x La x MnO3 compounds

Electron-doped (Ba_1−x La _x )MnO₃ compounds were prepared for x=0−0.5. Measurements of X-ray diffraction (XRD) at room temperature and temperature variation of dc electrical resistivity down to 20 K were carried out. Samples with x=0.2–0.5 exhibit metal-insulator (M-I) transition. The maximum M-I transition temperature (T _c) of 289 K was observed for 30% of La doping (x=0.3). XRD patterns of these samples (x=0.2−0.5) were analyzed using Rietveld refinement. These samples are found to be mostly in single-phase form with orthorhombic symmetry (space group Pbnm). We have found strong correlation between Mn-O-Mn bond angles and T _c of M-I transition. The resistivity data below T _c could be fitted to the expression ρ=ρ ₁+ρ ₂ T ² and this shows that double exchange interaction plays a major role even in Mn⁴⁺-rich compound. Above T _c the resistivity data were fitted to variable range hopping and small polaron models.     

Effect of Ca doping on the superconducting properties of GdBaSrCu3O7−δ bulk samples

Gd_1−xCa_xBaSrCu₃O_7−δ samples (0  x  0.1) were prepared via solid-state reaction. Four-point probes method was used for resistance versus temperature measurements. Results show decrease in T_c by increasing x content. This variation is assumed to be irrelevant to the different phases or impurity effects since X-ray patterns show all samples are tetragonal single-phase. Ca doping decreases the oxygen content and lattice parameters of the samples. It is suggested that Ca prevents the dislocation of oxygen, and then disrupts the hole concentration of the system and antiferromagnetic correlation at CuO₂ planes. Subsequently, destroys the superconductivity of the samples.     

Temperature dependence of the indirect band gap, steepness parameter and related optical constants of [Kx(NH4)1−x]2ZnCl4 mixed crystals

Optical transmittance measurements near the absorption edge of [K_x(NH₄)_1−x]₂ZnCl₄ mixed crystals, where x=0.00, 0.232, 0.522, 0.644, 0.859 and 1.00, are reported over 276–350 K range. Analysis reveals that the type of transition is the indirect allowed one. The absorption edge shifted towards lower energy with increasing temperature. It is shown that [K_x(NH₄)_1−x]₂ZnCl₄ mixed crystals with x0.644 reveal a phase transition at 319 K, this phase disappeared at high concentrations of K⁺ ions. The steepness parameter is given, its value is used to estimate the temperature dependence of the indirect energy gap. In the region of the absorption edge, the absorption coefficient obeys Urbach's rule. Urbach parameters are investigated as a function of temperature.     

Structural studies and T c dependence in La2−x Dy x Ca y Ba2Cu4+y O z type mixed oxide superconductors

A new series of mixed oxide superconductors with the stoichiometric composition La_2−x Dy _x Ca _y Ba₂Cu_4+y O _z (x=0.0 − 0.5, y=2x) has been studied for structural and superconductiong properties. Our earlier studies on La_2−x (Y/Er) _x Ca _y Ba₂Cu_4+y O _z series, show a strong dependence of T _c on hole concentration (p _sh). In the present work, the results of the analysis of the neutron diffraction measurements at room temprerature on x=0.3 and 0.5 samples are reported. It is interesting to know that Ca substitutes for both La and Ba site with concomitant displacement of La onto Ba site. Superconductivity studies show that maximum T _c is obtained for x=0.5, y=1.0 sample (T _c ∼ 75 K), for La_1.5Dy_0.5Ca₁Ba₂Cu₅O _z (La-2125).     

Structure and magnetostriction of Tb0.2Pr0.8(Fe0.4Co0.6)1.93−xCx intermetallic compounds

The structure and magnetostriction of Tb_0.2Pr_0.8(Fe_0.4Co_0.6)_1.93−xC_x intermetallic compounds were studied by X-ray diffraction and magnetic measurements. Almost a single cubic Laves phase forms in the alloys for x ≤0.20, and a small amount of C can inhibit the formation of the 1:3 phase. The lattice parameter increases when 0≤x≤0.15, while the T_c and the spontaneous magnetization decreases with increasing x. The lattice parameter decreases slowly when 0.15≤x≤0.30, while the T_c decreases evidently with increasing x. The magnetostriction λ_a (=λ_∥-λ_⊥) is improved at low magnetic fields at room temperature for the compounds with 0.05≤x≤0.10, indicating that these C-containing compounds are promising magnetostrictive materials.     

NiS2-xSex在x=1.00附近的反铁磁量子相变

针对NiS_2-xSe_x系统在x=1.0０附近发生的反铁磁量子相变,制备了一系列NiS_2-xSe_x(x=0.96, 0.98, 1.00, 1.05, 1.10和1.20)多晶样品,对其结构、磁性质和电阻率进行了系统的观测.结果发现：样品磁化率-温度关系呈现典型的强关联电子系统特征;与铜氧化物超导体相类似,它们的电阻率-温度关系在很宽的温 关键词： 量子相变 反铁磁自旋涨落 2-xSe_x体系')" href="#">NiS_2-xSe_x体系     

Conversion of egg shell membrane to inorganic porous CexZr1−xO2 fibrous network

The binary system CeO₂–ZrO₂ is thermally stable and has superior reduction–oxidation properties. It has been commonly used in the three-way catalytic converters for automobiles. In this work, an inorganic biomorphic porous Ce_xZr_1−xO₂ fibrous network was successfully synthesized by using the egg shell membrane (ESM) as templates, and its morphology was a perfect replica of the original ESM. The synthesis involved a simple infiltration and calcination process. A fresh ESM was peeled from a chicken egg shell. It was soaked in a Ce(NO₃)₃ and Zr(NO₃)₄ mixture before it was calcined at 700 °C in ambient environment. The fibers in the biomorphic network had diameter ranged from 1 to 4 μm, and they were composed of Ce_xZr_1−xO₂ nanocrystallites having an average grain size of 10 nm.     

The structural and magnetic properties of Ni2Mn1−xBxGa Heusler alloys

The influence of the substitution of manganese by boron on the crystal structure and magnetic properties of Ni₂Mn_1−xB_xGa Heusler alloys with 0?x?0.5 has been investigated using X-ray diffraction, thermal expansion, resistivity, and magnetization measurements. The samples with concentrations x<0.25 were found to be of single phase and belonged to the cubic L2₁ crystal structure at room temperature. Crystal cell parameters of the alloys decreased from 5.830 to 5.825 Å with increasing boron concentration (x) from 0 to 0.25. The alloys were ferromagnetically ordered at 5 K and the saturation magnetization decreased with increasing boron concentration. The ferromagnetic ordering and structural transition temperatures for 0?x?0.3 have been observed and the phase (x−T) diagram of the Ni₂Mn_1−xB_xGa system was constructed. The phase (x−T) diagram indicates that the ground state of Ni₂Mn_1−xB_xGa alloys belongs to ferromagnetic martensitic, premartensitic, and austenitic phases in x?0.12, 0.12<x?0.18, and 0.18<x?0.3, respectively. The relative influence of cell parameters and electron concentrations on the phase diagram is discussed.     

Vortex states in mesoscopic single crystals Bi2Sr2CaCu2O8+δ in high magnetic fields

The c-axis resistivity measurements were performed in the vicinity of the ab-plane in order to investigate the interaction between Josephson vortices and pancake vortices in Bi₂Sr₂CaCu₂O_8+δ mesoscopic single crystals. It was found that the angular dependence of the c-axis resistivity drastically changes in high magnetic field regime. The vortex lock-in transition becomes considerably broad in high magnetic fields, while the angular dependence of resistance exhibits the sharp lock-in features in low magnetic field region.     

Effects of RE2O3 (RE = Tm, Sc, Yb) addition on the superconducting properties of ErBa2Cu3Oy

We investigated the effects of added Tm₂O₃, Sc₂O₃, and Yb₂O₃ on the superconducting properties of sintered Er123 samples. Tm₂O₃ addition caused the least T_c degradation, exhibiting a T_c above 90 K even for 17 vol% addition. Samples with added Sc₂O₃ maintained a T_c at above 90 K up to an addition of 7.2 vol%, while Yb₂O₃-containing samples showed a monotonic decrease in T_c with increased vol% of added Yb₂O₃. Tm₂O₃-containing samples exhibited a slight increase in J_c(0.1 T)/J_c(0) and had constant J_c values even for 17 vol% addition. XRD and SEM results indicate that the Tm₂O₃ is very stable in the superconducting matrix.     

Gigantic uniaxial pressure effect in single crystals of iron-based superconductors

In order to elucidate the anisotropic pressure effect on superconductivity in an iron-based superconductor, magnetization measurements have been performed in Ba(Fe_0.92Co_0.08)₂As₂ single crystals under uniaxial pressures applied along the c-axis. Gigantic T_c suppression, dT_c/dP_//c = −15 K/GPa, was observed when the anisotropic deformation with the a-expansion and c-compression was induced by the c-pressure, which should be compared with dT_c/dP < +1 K/GPa in the isotropic pressure case. This suggests that the a-axis (c-axis) compression has a positive (negative) contribution to T_c.     

Growth and physical properties of sol–gel derived Co doped ZnO thin film

Zn_1−xCo_xO films were grown on glass by sol–gel spin coating process. The Zn_1−xCo_xO thin films with 10 at.% Co were highly c-axis oriented. The electrical resistivity of the films at 10 at.% Co had the lowest value due to the highest c-axis orientation. XPS and AGM analyses indicated that Co metal clusters weren’t formed, and the ferromagnetism was appeared at room temperature. The characteristics of the electrical resistivity and room temperature ferromagnetism of sol–gel derived Zn_1−xCo_xO films suggest a potential application to dilute magnetic semiconductor devices.     

Effects on structure and superconductivity for La and Ca codoping in YBa2Cu3Oy cuprates

The nominal composition of Y_0.8Ca_0.2Ba_2−xLa_xCu₃O_y (YBLCO) cuprates with x≤0.50 has been synthesized by the standard solid state reaction technique. X-ray diffraction and the resistivity measurements are used to characterize the structure and the superconductivity of YBLCO cuprates. There is no structural phase transition in the whole doping range. The dependencies of the lattice constants and some other structural parameters on the content of La for the samples YBLCO with x≤0.20 are different than those for the samples with x≥0.25. The zero resistance temperature T_c0 increases with the increase of the content of La in YBLCO as x≤0.20, and decreases as x≥0.25. We compared these results with those of Nd-doped Y_0.8Ca_0.2Ba_2−zNd_zCu₃O_y cuprates. It seems that T_c0 is related to the structural parameters due to Ca and La codoping in YBLCO.     

Effect of substitution of Ce on superconducting properties of Bi1.7Pb0.3Sr2Ca2−x Ce x Cu3O10+δ system

We have investigated the superconducting properties of the Bi_1.7 Pb_0.3Sr₂Ca_2−xCe _x Cu₃O_10+δ system with x=0.00, 0.02, 0.04, 0.08 and 0.1 by X-ray diffraction and magnetic susceptibility. The substitution of Ce for Ca has been found to drastically change the superconducting properties of the system. X-ray diffraction studies on these compounds indicate decrease in the c-parameter with increased substitution of Ce at Ca site and volume fraction of high T _c (2 : 2 : 2 : 3) phase decreases and low T _c phase increases. The magnetic susceptibility of this compound shows that the diamagnetic on set superconducting transition temperature (onset) varies from 109 K to 51 K for x=0.00, 0.02, 0.04, 0.08 and 0.1. These results suggest the possible existence of Ce in a tetravalent state rather than a trivalent state in this system; that is, Ca²⁺ → Ce⁴⁺ replacement changes the hole carrier concentration. Hole filling is the cause of lowering T _c of the system.     

Ti1－xCrxO2±δ体系的相关系、晶体结构和磁性能研究

采用溶胶凝胶法制备了Ti_1-xCr_xO_2±δ体系系列样品.利用扫描电子显微镜(SEM)，X射线光电子能谱(XPS)，粉末X射线衍射分析(XRD)方法研究了Ti_1-xCr_xO_2±δ系列样品的颗粒尺寸、形貌、组分化学态、相关系和固溶区范围；并利用超导量子干涉磁强计对样品的磁性能进行了研究.采用Rietveld结构精修的方法研究了Cr的不同掺杂量对TiO₂晶体结构的影响，研究表明，1000℃烧结的样品的固溶区范围是x=0—0.03，为金红石单相；随着Cr掺杂量的增加，金红石相晶胞参数规律性地减小；当x>0.03，为金红石相和CrO₂相两相共存.综合XRD和磁性测量结果，500℃烧结的样品的固溶区范围是x=0—0.02，为锐钛矿单相；随着Cr掺杂量的增加，锐钛矿相晶胞参数规律性地减小；当x≥0.04，为锐钛矿相和绿铬矿相(Cr₂O₃)两相共存.XPS实验结果表明，500℃和1000℃退火的样品中Cr都是以Cr⁺³和Cr⁺⁶两种化学态存在，1000℃烧结的样品中可能有更多的Cr³⁺转化为Cr⁶⁺.根据M-H和M-T曲线的测试结果发现，本文500℃烧结的Ti_1-xCr_xO_2±δ体系样品当x=0—0.02时，为室温铁磁性.当x≥0.04时，由铁磁相和顺磁相所组成，在低温下有较强的铁磁性；室温下主要是顺磁相，铁磁相只占据很小的体积分数. 关键词： 1-xCr_xO_2±δ体系')" href="#">Ti_1-xCr_xO_2±δ体系 相关系 固溶区 磁性能