MnxGe1－x稀磁半导体薄膜的结构研究

利用X 射线衍射(XRD)和X射线吸收精细结构(XAFS)方法研究了磁控共溅射方法制备的Mn_xGe_1-x薄膜样品的结构随掺杂磁性原子Mn含量的变化规律.XRD结果表明，在Mn的含量较低(7.0%)的Mn_0.07Ge_0.93样品中，只能观察到对应于多晶Ge的XRD衍射峰，而对Mn含量较高(25.0%, 36.0%)的Mn_0.25Ge_0.75和Mn关键词： 磁控溅射 XRD XAFS xGe_1-x稀磁半导体薄膜')" href="#">Mn_xGe_1-x稀磁半导体薄膜     

Infrared probe of itinerant ferromagnetism in Ga(1-x)Mn(x)As

The doping and temperature dependence of the complex conductivity is determined for the ferromagnetic semiconductor Ga(1-x)Mn(x)As. A broad resonance develops with Mn doping at an energy scale of approximately 200 meV, well within the GaAs band gap. Possible origins of this feature are explored in the context of a Mn induced impurity band and intervalence band transitions. From a sum rule analysis of the conductivity data the effective mass of the itinerant charge carriers is found to be at least a factor of 3 greater than what is expected for hole doped GaAs. In the ferromagnetic state a significant decrease in the effective mass is observed, demonstrating the role played by the heavy carriers in inducing ferromagnetism in this system.     

Theory of ferromagnetism in Ca(1-x)La(x)B(6)

Novel ferromagnetism in Ca(1-x)La(x)B(6) is studied in terms of the Ginzburg-Landau theory for excitonic-order parameters, taking into account symmetry of the wave functions. We found that the minima of the free energy break both inversion and time-reversal symmetries, while the product of these two remains preserved. This explains various novelties of the ferromagnetism and predicts a number of magnetic properties, including the magnetoelectric effect, which can be tested experimentally.     

Room-temperature ferromagnetism in a II-VI diluted magnetic semiconductor Zn(1-x)Cr(x)Te

The magnetic and magneto-optical properties of a Cr-doped II-VI semiconductor ZnTe were investigated. Magnetic circular dichroism measurements showed a strong interaction between the sp carriers and localized d spins, indicating that Zn(1-x)Cr(x)Te is a diluted magnetic semiconductor. The Curie temperature of the film with x=0.20 was estimated to be 300+/-10 K, which is the highest value ever reported for a diluted magnetic semiconductor in which sp-d interactions were confirmed. In spite of its high Curie temperature, Zn(1-x)Cr(x)Te film shows semiconducting electrical transport properties.     

Room-temperature ferromagnetism induced by Cu vacancies in Cu_x(Cu_2O)_(1-x) granular films

Cux(Cu2O)1-x(0.09 x 1.00) granular films with thickness about 280 nm have been fabricated by direct current reactive magnetron sputtering. The atomic ratio x can be controlled by the oxygen flow rate during Cux(Cu2O)1-x deposition. Room-temperature ferromagnetism(FM) is found in all of the samples. The saturated magnetization increases at first and then decreases with the decrease of x. The photoluminescence spectra show that the magnetization is closely correlated with the Cu vacancies in the Cux(Cu2O)1-x granular films. Fundamentally, the FM could be understood by the Stoner model based on the charge transfer mechanism. These results may provide solid evidence and physical insights on the origin of FM in the Cu2O-based oxides diluted magnetic semiconductors, especially for systems without intentional magnetic atom doping.     

Disorder promotes ferromagnetism: rounding of the quantum phase transition in Sr(1-x)Ca(x)RuO3

The subtle interplay of randomness and quantum fluctuations at low temperatures gives rise to a plethora of unconventional phenomena in systems ranging from quantum magnets and correlated electron materials to ultracold atomic gases. Particularly strong disorder effects have been predicted to occur at zero-temperature quantum phase transitions. Here, we demonstrate that the composition-driven ferromagnetic-to-paramagnetic quantum phase transition in Sr(1-x)Ca(x)RuO3 is completely destroyed by the disorder introduced via the different ionic radii of the randomly distributed Sr and Ca ions. Using a magneto-optical technique, we map the magnetic phase diagram in the composition-temperature space. We find that the ferromagnetic phase is significantly extended by the disorder and develops a pronounced tail over a broad range of the composition x. These findings are explained by a microscopic model of smeared quantum phase transitions in itinerant magnets. Moreover, our theoretical study implies that correlated disorder is even more powerful in promoting ferromagnetism than random disorder.     

Domain wall formation in NixFe1-x

In terms of a multiscale approach based on the Landau-Ginzburg expansion and on ab initio parameters evaluated by means of the fully relativistic screened Korringa-Kohn-Rostoker method, the width of domain walls is evaluated for the whole range of concentration in NixFe1-x. It is found that domain-wall formation occurs only for x<20% and x>55%; i.e., in the neighborhood of the structural phase transition from bcc to fcc, NixFe1-x first tends to form single domains before the actual range of concentrations of this phase transition is reached. The calculated domain-wall widths are found to be in reasonably good agreement with available experimental data.     

Possible ferromagnetism in MgO

We study the impact of vacancies on the magnetism of MgO and TiO₂ nanoparticles, and develop fist-principle calculations to ascertain the origin of the induced polarization. Theoretically, we expect ferromagnetism only for the case of MgO with cation defects. Experimentally, we observed a small magnetic signal probably due to imperfections in the lattice order, perhaps at the nanoparticles surface.     

Negligible magnetism in excellent structural quality Cr(x)Ti(1-x)O(2) anatase: contrast with high-T(C) ferromagnetism in structurally defective Cr(x)Ti(1-x)O(2)

We reexamine the mechanism of ferromagnetism in doped TiO(2) anatase, using epitaxial Cr:TiO(2) with excellent structural quality as a model system. In contrast to highly oriented but defective Cr:TiO(2) (approximately 0.5 micro(b)/Cr), these structurally superior single crystal films exhibit negligible ferromagnetism. Similar results were obtained for Co:TiO(2). We show for the first time that charge-compensating oxygen vacancies alone, as predicted by F-center mediated exchange, are not sufficient to activate ferromagnetism. Instead, the onset of ferromagnetism correlates with the presence of structural defects.     

Defect-induced ferromagnetism in fullerenes

Based on the ab initio electronic structure calculations the picture of ferromagnetism in polimerized C₆₀ is proposed which seems to explain the whole set of controversial experimental data. We have demonstrated that, in contrast with cubic fullerene, in rhombohedral C₆₀ the segregation of iron atoms is energetically unfavorable which is a strong argument in favor of intrinsic character of carbon ferromagnetism which can be caused by vacancies with unpaired magnetic electrons. It is shown that: (i) energy formation of the vacancies in the rhombohedral phase of C₆₀ is essentially smaller than in the cubic phase, (ii) there is a strong ferromagnetic exchange interactions between carbon cages containing the vacancies, (iii) presence of iron impurities can diminish essentially the formation energy of intrinsic defects, and (iv) the fusion of the magnetic single vacancies into nonmagnetic bivacancies is energetically favorable. The latter can explain a fragility of the ferromagnetism.     

La1-x／2Pr1-x／2SrxCuO4的电输运性质和超导电性

La1-x/2Pr1-x/2SrxCuOy(LPSCO)多晶样品采用传统的固相反应法制备.X射线衍射表明:LPSCO具有典型的空穴搀杂的T-214相的结构.磁化率测量显示:Sr搀杂在0.05≤x≤0.30范围内具有超导转变;Tc随x的增大呈抛物线形式变化,且在x=0.18时达到最大值28K.电阻的测量显示:随搀杂量的增大,系统呈现从绝缘到半导体,最后到金属的导电行为的变化;在欠掺杂区,正常态电阻温度关系符合ρ(T)=ρ0 αT-ClnT;而在过掺杂区,对数项消失.本文从替代所引起的晶体结构和载流子特性变化解释了Sr掺杂样品的电输运行为和超导特性.     

Monte Carlo simulations of ferromagnetism in p-Cd1-xMnxTe quantum wells

Monte Carlo simulations, in which the Schr?dinger equation is solved at each Monte Carlo sweep, are employed to assess the influence of magnetization fluctuations, short-range antiferromagnetic interactions, disorder, magnetic polaron formation, and spin-Peierls instability on the carrier-mediated Ising ferromagnetism in two-dimensional electronic systems. The determined critical temperature and hysteresis are affected in a nontrivial way by the antiferromagnetic interactions. The findings explain striking experimental results for modulation-doped p-Cd1-xMnxTe quantum wells.     

钛酸铅镧Pb_(1-x)La_xTi_(1-x/4)O_3纳米晶的拉曼光谱研究

本文在２８８Ｋ和３７８Ｋ测定了不同Ｌａ含量Ｐｂ１－ｘＬａｘＴｉ１－ｘ／４Ｏ３纳米晶的拉曼光谱,同时在室温条件下测定了不同晶粒尺寸的Ｐｂ０．９Ｌａ０．１Ｔｉ０．９７５Ｏ３纳米晶的拉曼光谱,研究了Ｌａ含量和晶粒尺寸对各声子模频率的影响规律,讨论了影响峰形、峰位和峰宽的因素。     

Room temperature ferromagnetism in Zn-Mn-O

Zn-Mn-O semiconductor crystallites with nominal manganese concentration x=0.01, 0.04 and 0.10 were synthesized by a solid state reaction method using oxalate precursors. A sintering procedure was carried out in air at 500 and 900  ^°C. The samples were investigated by X-ray diffraction, magnetization measurements and electron paramagnetic resonance. X-ray diffraction spectra reveal that the dominant crystal phase in the Zn-Mn-O system corresponds to the wurtzite structure of ZnO. Room temperature ferromagnetism is observed in Zn-Mn-O samples with manganese concentrations x=0.01 and 0.04 sintered at low temperature (500  ^°C). Saturation magnetization in the x=0.01 sample is found to be at . The ferromagnetic phase seems to be developed by Zn diffusion into Mn-oxide grains.     

高组份Al值的AlxGa1-x As和AlxGa1-x As/GaAs/AlxGa1-xAs/GaAs多层结构的MOCVD生长

用自制常压MOCVD系统,在半绝缘GaAs衬底上生长高Al组份Al_xGa_1-xAs（其x值达0.83）,和Al_xGa_1-xAs/GaAs/Al_xGa_1-xAs/GaAs多层结构,表面镜面光亮。生长层厚度从几十到十几μm可控,测试表明外延层晶格结构完整,x值调节范围宽,非有意掺杂低,高纯GaAs外延层载流子浓度n_300K=1.7×10¹⁵cm^-3,n_77K=1.4×10¹⁵cm^-3,迁移率μ_300K=5900cmcm²/V.S,μ_77K=55500cm²/V.S。用电子探针,俄歇能谱仪测不出非有意掺杂的杂质,各层间界面清晰平直。 对GaAs,AlGaAs生长层表面缺陷,衬底偏角生长温度及其它生长条件也进行了初步探讨。     

Negative band gaps in dilute InNxSb1-x alloys

A thin layer of InNSb has been fabricated by low energy nitrogen implantation in the near-surface region of InSb. X-ray photoelectron spectroscopy indicates that nitrogen occupies approximately 6% of the anion lattice sites. High-resolution electron-energy-loss spectroscopy of the conduction band electron plasma reveals the absence of a depletion layer for this alloy, thus indicating that the Fermi level is located below the valence band maximum (VBM). The plasma frequency for this alloy combined with the semiconductor statistics indicates that the Fermi level is located above the conduction band minimum (CBM). Consequently, the CBM is located below the VBM, indicating a negative band gap material has been formed. These measurements are consistent with k.p calculations for InN0.06Sb0.94 that predict a semimetallic band structure.