Infrared spectra study of Nd1.85−xRxCe0.15CuO4±δ (R = Gd and Sm) single crystals

The X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectra, and resistivity ρ are measured on Nd_1.85?xR_xCe_0.15CuO_4±δ (R = Gd and Sm) single crystals. From the XRD and FTIR spectra studies, the lattice constant c are found to decrease faster for Gd doping than that for Sm doping with increasing doping concentration x. Upon Gd doping, all the transmission peaks shift to high frequency but nearly unchanged with increasing Sm doping content. The volume of the unit cell observed through XRD and FTIR spectra decreases faster for Gd doping than that for Sm doping. Furthermore, for Gd doping the superconducting T_c decreases rapidly with increasing the doping concentration, while the T_c decreases slowly for Sm doping with increasing Sm content, which is found to be very well consistent with the evolution of the volume of the unit cell for two series.     

Electronic and optical properties of pure and Mo doped anatase TiO2 using GGA and GGA+U calculations

Comparative GGA and GGA+U calculations for pure and Mo doped anatase TiO₂ are performed based on first principle theory, whose results show that GGA+U calculation provide more reliable results as compared to the experimental findings. The direct band gap nature of the anatase TiO₂ is confirmed, both by using GGA and GGA+U calculations. Mo doping in anatase TiO₂ narrows the band gap of TiO₂ by introducing Mo 4d states below the conduction band minimum. Significant reduction of the band gap of anatase TiO₂ is found with increasing Mo doping concentration due to the introduction of widely distributed Mo 4d states below the conduction band minimum. The increase in the width of the conduction band with increasing doping concentration shows enhancement in the conductivity which may be helpful in increasing electron–hole pairs separation and consequently decreases the carrier recombination. The Mo doped anatase TiO₂ exhibits the n-type characteristic due to the shifting of Fermi level from the top of the valence band to the bottom of the conduction band. Furthermore, a shift in the absorption edge towards visible light region is apparent from the absorption spectrum which will enhance its photocatalytic activity. All the doped models have depicted visible light absorption and the absorption peaks shift towards higher energies in the visible region with increasing doping concentration. Our results describe the way to tailor the band gap of anatase TiO₂ by changing Mo doping concentration. The Mo doped anatase TiO₂ will be a very useful photocatalyst with enhanced visible light photocatalytic activity.     

非磁杂质对高Tc超导电性的影响

本文证明了在氧化物高T_c超导体的Anderson晶格模型中,由于序参量是能量的函数,非磁掺杂也具有拆对效应,在稀掺杂情况下,求出了T_c随杂质浓度增加而线性下降的规律,与实验结果相符,文中还计算了掺杂对超导态密度中能隙的影响,结果表明,当非磁掺杂浓度增加时,零温能隙的减小比T_c的下降慢得多,从而在转变为正常态之前可能不出现类似通常BCS超导体磁性掺杂的无能隙区。 关键词：     

Effect of chloride doping concentration on enhancement of protonic conductivity of mesoporous Al2O3

Mesoporous Al₂O₃ was prepared by a sol-gel method with doping different amount of LaCl₃. The proton conductivity of mesoporous Al₂O₃ increased with increasing the doping concentration up to the optimum doping, 0.1 Cl/Al mole ratio. The surface acidities of different samples were investigated by NH₃-TPD method. The change trends of surface acidity are consistent with that of proton conductivity. It indicates that the proton conductivity of mesoporous Al₂O₃ can be increased by enhancing its surface acidity via the chloride doping.     

Magnetic and magneto-transport properties of (Ba0.8Sr0.2)2−xNdxFeMoO6

The variation in structural, magnetic and magneto-transport properties of the double perovskite system (Ba_0.8Sr_0.2)_2?xNd_xFeMoO₆ {0.0<X<0.5} induced by Nd³⁺ doping (electron doping) has been studied and compared. The samples were prepared by standard solid state reaction method in a reducing atmosphere. The parent compound showed a saturation magnetic moment value of 3.75 μ_B/f.u. at an applied field of 0.5 T and a change in magnetoresistance value up to 26% (77 K, 0.8 T). The Rietveld refinement of the X-ray diffraction data showed a continuous decrease in lattice parameters and Fe–Mo ordering with increasing Nd³⁺ doping. The Curie temperature was found to increase with Nd³⁺ doping (3 K per % of Nd) while the saturation magnetic moment values and magnetoresistance values were found to decrease. The observed variations in magnetic and magneto-transport properties of the system are explained on the basis of increasing antisite disorder defects and band filling effects induced by electron doping. We have observed the dominant role of band filling in determining the low field magnetoresistance of these systems.     

稀土钇、镧掺杂TiO2薄膜的拉曼谱分析

采用溶胶-凝胶法在石英玻璃衬底上用旋涂法制备了未掺杂、掺杂钇和掺杂镧的TiO₂薄膜样品,对样品在700—1100 ℃范围内进行退火处理,并对样品的拉曼光谱进行了分析.分析表明:随着退火温度的升高,未掺杂TiO₂薄膜发生了从锐钛矿相经混相最终向金红石相的转换,掺杂钇和掺杂镧对TiO₂薄膜的晶相转换起阻碍作用,掺杂镧的阻碍作用更强;稀土掺杂能使TiO₂薄膜晶粒细化,并使晶粒内部应力增大从而阻碍晶格振动,掺杂镧比掺杂钇的效果 关键词： 2薄膜')" href="#">TiO₂薄膜 稀土掺杂 拉曼光谱 溶胶-凝胶     

衬底温度和硼掺杂对p型氢化微晶硅薄膜结构和电学特性的影响

以B₂H₆为掺杂剂,采用射频等离子体增强化学气相沉积技术在玻璃衬底上制备p型氢化微晶硅薄膜.研究了衬底温度和硼烷掺杂比对薄膜的微结构和暗电导率的影响.结果表明：在较高的衬底温度下很低的硼烷掺杂比即可导致薄膜非晶化;在实验范围内,随着衬底温度升高薄膜的晶化率单调下降,暗电导率先缓慢增加然后迅速下降,变化趋势与硼烷掺杂比的影响极为相似.最后着重讨论了p型氢化微晶硅薄膜的生长机理. 关键词： p型氢化微晶硅薄膜 衬底温度 晶化率 电导率     

Engineering of nested Fermi surface and transparent conducting p-type Delafossite CuAlO2: possible lattice instability or transparent superconductivity?

Based on ab initio electronic structure calculation using a super-cell FLAPW method, we propose a new valence control method for the fabrication of low-resistive p-type and transparent conducting oxides of Delafossite CuAlO₂. We propose a Cu-vacancy doping method with decreasing the Cu-vapor pressure and with increasing the oxygen vapor pressure, or, Be- or Mg-acceptor doping method at the Al-site with decreasing the Al-vapor pressure and with increasing the Cu-vapor pressure. The heavily doped p-type CuAlO₂ indicates the two-dimensional nested Fermi surfaces, which originate from the layered O-Cu-O dumbbell. Nested Fermi surfaces may cause possibly a lattice instability or a transparent superconductivity rather than magnetism.     

Influence of defects on the phase transition in squaric acid (H2C4O4)

We have investigated the influence of defects induced by chromium and deuterium doping on the phase transition in solid squaric acid (H₂C₄O₄) by high resolution ¹³C NMR. Deuterium doping alone is observed to increase the phase transition temperature T_c linearly with the ²H concentration, whereas chromium defects, destroy the local order and lead to clusters of the high temperature phase. Correspondingly the critical temperature decreases with increasing chromium doping and the critical temperature region is smeared out.     

Novel Nanocomposites of Polypyrrole Doped with Fullerene C60

This paper reports on the structural, thermal, and dielectric properties of polypyrrole/fullerene C₆₀ nanocomposites synthesized by a interfacial polymerization method. Fourier transform infrared (FT-IR) and ultraviolet-visible (UV-vis) analyses clearly indicated the existence of interactions between polypyrrole (PPy) and the fullerene C₆₀ nanoparticles. Thermal analyses indicated that the extrapolated onset degradation temperature (T_deg) of PPy increased with increasing doping level. Scanning electron microscopy (SEM) images showed that the fullerene C₆₀ changed the morphology of PPy. Dielectric analyses showed a temperature dependent dielectric relaxation behavior. The relaxation time of the nanocomposites with high doping levels tended to increase with increasing temperature. This behavior of the polypyrrole/fullerene C₆₀ nanocomposites indicated that they could be used as a high temperature ultrasonic transducer.     

Superconductivity in Hg-substituted BaPb0.75Bi0.25O3

A series of Hg-doped BaPb_0.75Bi_0.25O₃ (BPBO) with a nominal composition of BaPb_0.75 − xHg_xBi_0.25O₃ (x=0-0.40 with 0.05 intervals) has been synthesized by solid state reaction. The system shows a lattice parameter expansion and lattice symmetry distortion with Hg doping. Superconducting transition temperature Tc and superconducting volume fraction of the system decrease with Hg doping level in the low doping level region (0?x?0.25) and are nearly fully suppressed at x=0.25. However, the superconductivity is recovered with further increasing Hg content at x>0.3. The possible mechanisms of the superconductivity in the low doping level region and the recovery of superconductivity in the high doping level region for Hg-doped BPBO system have been discussed.     

Mechanism governing modification of the properties of the normal state and the critical temperatures under codoping of YBa2Cu3Oy by calcium and praseodymium

The temperature dependences of the electrical resistivity and thermopower coefficient in the Y_1?2x Ca_xPr_xBa₂Cu₃O_y system are studied experimentally, and their analysis is performed in the light of earlier data on the effect of separate calcium and praseodymium doping in yttrium sites. It is found that codoping calcium and praseodymium ions into the lattice does not result in summation of the individual effects of the dopants on either the critical temperature or the temperature dependences of the thermopower coefficient. The results obtained are analyzed within the narrow band model, and the parameters of the band structure and charge carrier system in the samples are determined. The character and mechanism of variation in these parameters with increasing doping level are examined. It is suggested that interaction of calcium with praseodymium ions weakens the effect of hybridization of band states with praseodymium ion states through the involvement of additional states introduced by calcium into the conduction band. This assumption accounts for all the features revealed in the variation of the properties of the normal state and the critical temperature with increasing doping level.     

Hall effect and pinning regimes in thin films doped with nanoparticles

Hall effect and flux pinning in YBa₂Cu₃O_6+x (YBCO) thin films doped with BaZrO₃ (BZO) nanoparticles is investigated. The results show that sign reversal of the Hall coefficient from positive hole-like to negative electron-like occurs in vortex-liquid regime of undoped and BZO-doped YBCO films. With increasing BZO concentration the amplitude of the negative Hall effect is suppressed while the temperature position of the anomalous Hall effect does not depend significantly on doping level. In addition, it is shown that Hall conductivity increases non-monotonically with increasing BZO doping. These results support a model where BZO at low doping concentration induces point pinning centres turning to strong columnar pinning defects in films doped with 4% BZO.     

Effects of Co doping on the phase transformation and optical properties of TiO2 thin films by sol–gel method

The influences of Co doping on the anatase-to-rutile transformation of TiO₂ thin films have been investigated by Raman spectroscopy and X-ray diffraction. Raman spectra and XRD patterns for the samples of various Co concentrations present a clear evolution of TiO₂ with different anatase-to-rutile ratios. The fraction of rutile phase increases gradually with increasing Co contents. When Co content exceeds 7 mol%, anatase phase is not detected in the samples. The results may be related to the oxygen vacancies, which are introduced by Co doping. According to the Adachi model, optical constant was extracted by spectroscopic ellipsometry. It is found that the refractive index n increases with increasing Co content from 2.29 to 2.4, and that the optical band gap decreases and varies with increment of Co content between 3.6 and 3.38 eV. This may be related to the changes in film density and band gap tailed due to the Co doping.     

Effect of Ca doping level on the laser-induced voltages in tilted La1−x Ca x MnO3 (0.1 ≤ x ≤ 0.7) thin films

Tilted La_1?x Ca _x MnO₃ (0.1 ≤ x ≤ 0.7) thin films have been grown on vicinal cut LaAlO₃ (100) substrate by pulsed laser deposition. The laser-induced voltage effect was studied at room temperature with the KrF excimer laser using as the thermal source. The relationships between Ca doping level and voltage signal, response time and anisotropy Seebeck coefficient were established. The voltage signal and anisotropy Seebeck coefficient increase at first with increasing Ca doping level, reach a maximum at the same Ca content around x = 0.5, and then decrease. The respond time decreases with the Ca concentration increasing, and changes very little after x = 0.5. The figure of merit F _m was also the largest at this doping level, indicating a potential good performance of the photodetector devices. The variation of intrinsic structural and transport anisotropy induced by the change of Ca concentration has been proposed to account for the different LIV effects observed in LCMO thin films.     

Influence of Sb doping on crystal structure and electrical property of SnO2 nanoparticles prepared by chemical coprecipitation

Sb doped SnO₂ (ATO) nanoparticles with Sb doping concentrations ranging from 0% to 20% (Sb/Sb+Sn) have been prepared by chemical coprecipitation using metallic Sn and SbCl₃ as raw materials. The influence of Sb doping concentration on crystal structure and electrical property was studied in detail. Results indicated that all ATO nanoparticles possessed the same tetragonal rutile structure as that of bulk SnO₂. The average crystal size of the ATO nanoparticles decreased from 16 to 7 nm by increasing the Sb doping concentration. The unit-cell volume of ATO nanoparticles was either expanded or contracted, strongly depending on the Sb doping concentration. The electrical resistivity decreased sharply from 111 to minimum of 1.05 Ω cm when the Sb doping concentration was increased from 0% to 15% and then increased slightly to 1.42 Ω cm when the Sb doping concentration was increased from 15% to 20%. Finally, high resolution X-ray photoelectron spectroscopy (XPS) measurement was employed to investigate the valence state of Sb in samples with various Sb doping levels.     

The structural and multiferroic properties of (Bi1−xLax)(Fe0.95Co0.05)O3 ceramics

La and Co co-doped BiFeO₃ ((Bi_1−xLa_x)(Fe_0.95Co_0.05)O₃ (x=0, 0.10, 0.20, 0.30)) ceramics were prepared by tartaric acid modified sol–gel method. The X-ray diffraction patterns indicate a transition from rhombohedral structure to tetragonal structure at x=0.20, which has been confirmed by the Raman measurements. The band gap increases with increasing x to 0.20, and then decreases with further increasing x to 0.30. The structural transition has significant effects on the multiferroic properties. The remnant magnetization and saturate ferromagnetic magnetization decrease abruptly with increasing x to 0.10, and then gradually increase with further increasing x up to 0.30. The coercivity is significantly reduced with increasing La doping concentration. The ferroelectricity has been improved by La doping, and the polarization increases with increasing x to 0.10, then decreases with further increasing x up to 0.30. The simultaneous coexistence of soft ferromagnetism and ferroelectricity at room temperature in tetragonal Bi_0.70La_0.30Fe_0.95Co_0.05O₃ indicates the potential multiferroic applications.     

Influence of delta doping on intersubband transition and absorption in AlGaN/GaN step quantum wells for terahertz applications

Effects of delta doping location and density on intersubband transitions in AlGaN/GaN step quantum wells for terahertz (THz) applications have been investigated by solving Schrödinger and Poisson equations self-consistently. It shows that delta doping near the GaN well/AlGaN step well interface causes a blue-shift, while delta doping in the barrier or near barrier/GaN well and barrier/step well interfaces cause a red-shift first and then a blue-shift with increasing doping density. The shifts are attributed to the combination of many body effect and internal field modulation effect, and can be more than 200% or 70% of the e₁–e₂ transition energy, as for blue-shift or red-shift, respectively. In addition, the influences of delta-doping location and density on the absorption coefficient are also investigated in detail. Delta doping at the middle of a layer is found much more desirable over uniform-doping in order to improve the absorption coefficient, especially in the step well.     

Ferroelectric phase transition of Nb-doped Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ceramics

Bi_3.25La_0.75Ti_3-yNb_yO₁₂ (y=0.0, 0.03, 0.09, 0.15, 0.21) were synthesized using the solid-state reaction method. The effects of Nb doping on ferroelectric properties were studied through dielectric and P-E measurements. The value of P_r increases with increasing Nb content. Bi_3.25La_0.75Ti_3-yNb_yO₁₂ ceramics exhibit a maximum remanent polarization of P_r=27 μC/cm² at an Nb content of y=0.09. These results indicate that Nb doping can improve the ferroelectric properties of BLT ceramics. The Curie temperature, T_c, decreased with increasing Nb-content, and the ferroelectric phase transition of BLTNy is a second-order transition without thermal hysteresis. PACS 77.55.+f; 77.80.-e; 77.22.Jp     

Effects of Si doping on the structural and electrical properties of Ge2Sb2Te5 films for phase change random access memory

The effects of Si doping on the structural and electrical properties of Ge₂Sb₂Te₅ film are studied in detail. Electrical properties and thermal stability can be improved by doping small amount of Si in the Ge₂Sb₂Te₅ film. The addition of Si in the Ge₂Sb₂Te₅ film results in the increase of both crystallization temperature and phase-transition temperature from face-centered cubic (fcc) phase to hexagonal (hex) phase, however, decreases the melting point slightly. The crystallization activation energy reaches a maximum at 4.1 at.% and then decreases with increasing dopant concentration. The electrical conduction activation energy increases with the dopant concentration, which may be attributed to the increase of strong covalent bonds in the film. The resistivity of Ge₂Sb₂Te₅ film shows a significant increase with Si doping. When doping 11.8 at.% of Si in the film, the resistivity after 460 °C annealing increases from 1 to 11 mΩ cm compared to the undoped Ge₂Sb₂Te₅ film. Current-voltage (I-V) characteristics show Si doping may increase the dynamic resistance, which is helpful to writing current reduction of phase-change random access memory.