分类： 物理学 >> 凝聚态:电子结构、电、磁和光学性质 提交时间： 2017-11-27
摘要： Cubic helimagnet FeGe has emerged as a class of skyrmion materials near room temperature that may impact future information technology. Experimentally identifying the detailed properties of skyrmion materials enables their practical application acceleratedly. Here we study the magnetic entropy change (MEC) of single crystalline FeGe in its precursor region and clarify its close relation to the critical exponents of a second-order phase transition in this area. The maximum MEC is found to be 2.86 J/kg.K for 7.0 T magnetic field change smaller than that of common magnetocaloric materials indicating the multiplicity and complexity of the magnetic structure phases in the precursor region. This result also implies that the competition among the multimagnetic phases can partly counteract the magnetic field driven force and establishes a stable balance. Based on the obtained MEC and the critical exponents, the exact Curie temperature of single crystalline FeGe under zero magnetic field is confirmed to be 279.1 K, higher than previously reported 278.2 K. This finding pave the way for reconstruction of FeGe phase diagram in the precursor region.
分类： 物理学 >> 凝聚态:电子结构、电、磁和光学性质 提交时间： 2017-11-27
摘要： The magnetic entropy change [ΔSM(T;H)] around the phase transition temperature TC is investigated by the scaling method for Fe0:5Co0:5Si, which exhibits a skyrmion phase below TC. The parameters of ΔSM(T;H) exhibit field dependent behaviors. The ΔSM(T;H) curves under high field can be well scaled into a single universal curve independent of external field and temperature. However, ΔSM(T;H) curves under low field become divergent just below TC, which indicates a characteristic of first-order transition. The scaling investigation of ΔSM(T;H) curves indicates that the phase transition in Fe0:5Co0:5Si is of a weak first-order type in low field region, while it is driven into a second-order one under high field. This weak first-order phase transition in low field region resembles that in typical skyrmion system MnSi which is caused by the critical fluctuation. The result suggests that critical fluctuation plays an important role in the phase transition and formation of skyrmion state.
分类： 物理学 >> 凝聚态:电子结构、电、磁和光学性质 提交时间： 2017-11-27
摘要： The magnetism of the single crystal Cr1=3NbS2, which exhibits chiral soliton lattice (CSL) state, is investigated. The magnetization displays strong magnetic anisotropy when the field is applied perpendicularly and parallel to the c-axis in low field region (H < HS, HS is the saturation field). The critical exponents of Cr1=3NbS2 are obtained as β = 0.370(4), γ = 1.380(2), and δ = 4.853(6), which are close to the theoretical prediction of three-dimensional Heisenberg model. Based on the scaling equation and the critical exponents, the H ? T phase diagram in the vicinity of the phase transition is constructed, where two critical points are determined. One is a tricrtical point which locates at the intersection between the CSL, forced ferromagnetic (FFM), and paramagnetic (PM) states. The other one is a critical point situated at the boundaries between CSL, helimagnetic (HM), and PM states.
分类： 物理学 >> 凝聚态:电子结构、电、磁和光学性质 提交时间： 2017-11-27
摘要： Critical phenomenon of the noncentrosymmetric Cr11Ge19, which exhibits an itinerant ferromagnetic ground state, is investigated by scaling of the magnetic entropy change [ΔSM(T;H)]. It is found that parameters #14;FWHM (the full width at half maximum), ΔSmax M (the maximum of the magnetic entropy change), and RCP (the relative cooling power) of ΔSM(T) are governed by the power law of critical exponents. With the critical exponents, ΔSM(T;H) curves are scaled into a universal curve independent of temperature and field, which suggests that the magnetic transition is of a second order type. The universal collapse of ΔSM(T;H) indicates that the critical behavior of Cr11Ge19 can be well described by the scaling laws for the critical phenomenon. Moreover, the ΔSM follows the power law of Hn with n(T;H) = dlnjΔSMj=dln(H). The temperature dependence of n values reach minimum at #24; 71.5 K. Based on the magnetic specific change ΔCp(T;H), the actual magnetic transition temperature is strictly determined as TC = 71:3 #6; 0:2 K for the single crystal Cr11Ge19.
分类： 物理学 >> 凝聚态:电子结构、电、磁和光学性质 提交时间： 2017-11-27
摘要： In this work, the resistivity and magnetization of Bi2Ir2O7 are investigated under hydrostatic pressure. At ambient pressure, the resistivity of Bi2Ir2O7 exhibits a metallic behavior with the decrease of temperature. When the pressure is applied, a metal-insulator phase transition at low temperature is induced under a pressure of #24; 0.48 GPa. The metal-insulator phase transition temperature (TMI ) increases linearly with pressure as dTMI/dP = 3.4 #6;0.3 K/GPa. The temperature dependence of resistivity [#26;(T)] in the pressure-induced insulating phase exhibits a thermal activation behavior (#26; = #26;0eΔE=kBT ), where the thermal activation energy (ΔE) increases monotonously with the pressure. Meanwhile, the magnetization is enhanced by the pressure, which indicates an enhancement of magnetic ordering. The results suggest that localization occurs due to the magnetic ordering induced by the pressure, which confirms the magneto-electronic coupling in Bi2Ir2O7
分类： 物理学 >> 凝聚态:电子结构、电、磁和光学性质 提交时间： 2017-11-27
摘要： The iso-spinel structural systems CuIr2S4 and MgTi2O4 exhibit phase transitions of the similar nature at #24; 230 K and #24; 260 K respectively, which are explained as an orbitally-induced Peierls phase transition. However, in this work, we uncover that applied pressure has opposite pressure effects on the phase transitions in CuIr2S4 and MgTi2O4. As pressure increases, the phase transition temperature (TMI ) for CuIr2S4 increases while that for MgTi2O4 decreases. In addition, the phase transition intensity becomes weaker for CuIr2S4 but gets stronger for MgTi2O4 under pressure. Our results indicate that the applied pressure suppresses the metallic phase in CuIr2S4, while enhances that in MgTi2O4. Combining the experimental observations with first-principle electronic structure calculations, we suggest that the opposite pressure effects in CuIr2S4 and MgTi2O4 originate from the different orbital ordering configurations (dxy, dyz/dxz) caused by different lattice distortions in these two systems. Our findings directly indicate that the interplay between the orbital and lattice degrees of freedom plays an important role in the orbitally-induced Peierls phase transition.
分类： 物理学 >> 凝聚态:电子结构、电、磁和光学性质 提交时间： 2017-11-27
摘要： The cubic B20 compound FeGe, which exhibits a near room temperature skyrmion phase, is one of the most promising candidate of the next generation spintronic devices. In this work, the critical behavior of the cubic FeGe is investigated by means of bulk dc-magnetization. We obtain the critical exponents (β = 0.336±0.004, γ = 1.352±0.003, and δ = 5.267±0.001), where the self-consistency and reliability are verified by the Widom scaling law and scaling equations. The magnetic exchange distance is found to decay as J(r) ~ r-4.9, which is close to the theoretical prediction of 3DHeisenberg model (r-5). The critical behavior of FeGe indicates a short-range magnetic interaction. Meanwhile, the critical exponents also imply an anisotropic magnetic coupling in this system.
分类： 物理学 >> 凝聚态:电子结构、电、磁和光学性质 提交时间： 2017-11-27
摘要： Dimensionality is one of the most important parameters in determination of the physical properties. Therefore, tuning of effective dimensionality is of significant importance for modulating the functionality of materials. In this work, we find that the spin-dimensionality can be changed by the Co-doing in Fe1_xCoxSi system. Investigation of the critical behavior shows that critical exponents for x = 0.3 agree with the three-dimensional (3D) Heisenberg model with fd : n = 3 : 3g (d is the spatial-dimensionality, and n is the spin-dimensionality). With the increase of Co-content, the critical exponents for x = 0.5 fulfill the 3D-XY model with fd : n = 3 : 2g, while those for x = 0.6 approach the 3D-Ising model with fd : n = 3 : 1g. These results indicate the lowering of the spin-dimensionality with the increase of Co-content in Fe1_xCoxSi. We suggest that the modulation of the spin-dimensionality in Fe1_xCoxSi should be resulted from the enhancement of the anisotropic magnetic interaction induced by the doping of Co.
分类： 物理学 >> 凝聚态:电子结构、电、磁和光学性质 提交时间： 2017-11-27
摘要： The transport property of chalcogenide spinel CuCr2Se4 with an itinerant ferromagnetic ground state can be modulated by doping of bromine, where the half-metallic state can be realized around x = 0.25 in CuCr2Se4−xBrx system. In this work, the single crystal CuCr2Se4−xBrx (x =0.25) with the cleave surface (111) plane has been investigated by the electron paramagnetic resonance (EPR) spectroscopy. The EPR results show that the in-plane magnetization is strong while the out-ofplane one is weak, which indicate that the spins are ferromagnetic ordered within the (111) plane. In addition, the isothermal EPR spectra in the ferromagnetic phase display that the spin coupling strength (λ) and crystal field (ΔCF ) depend on the rotation angle φ as relation λ/ΔCF / cos φ. Moreover, the peak-to-peak linewidth ΔHPP increases linearly with the decrease of temperature, which suggests that the spin-orbit coupling is enhanced linearly with temperature cooling.
分类： 物理学 >> 凝聚态:电子结构、电、磁和光学性质 提交时间： 2017-11-27
摘要： In this work, we successfully synthesize the single crystal Cr11Ge19. The magnetism of the noncentrosymmetric Cr11Ge19 with itinerant ferromagnetic ground state is thoroughly investigated on the single crystal. Based on the variation measurements including the angular rotation, temperature, and magnetic field dependence of magnetization, we find that this material exhibits strong magnetic anisotropy along the c-axis. To clearly reveal the magnetic interactions, the critical behavior is studied using the modified Arrott plot, the Kouvel-Fisher method, and the critical isotherm technique. Combining these different methods, three main critical exponents (β, γ, and δ) are obtained. The critical exponent β is close to the theoretical prediction of a three-dimensional XY model with spin-dimensionality n = 2, indicating two-dimensional magnetic coupling. Meanwhile, the critical exponent γ suggests that the magnetic interaction is of long-range type with magnetic exchange distance decaying as J(r) #25; r−4:61. We propose that the ferromagnetic ground state of Cr11Ge19 is formed by the polarized magnetic moments along the c-axis, while the long-range magnetic coupling is established within the ab plane.
分类： 物理学 >> 凝聚态:电子结构、电、磁和光学性质 提交时间： 2017-11-27
摘要： The B20 compound MnSi exhibits a skyrmion phase in a certain range of temperature (T) and external magnetic ¯eld (H) in addition to the helimagnetic ordering. In this work, the magnetic entropy change (¢SM) in the single crystal helimagnet MnSi has been investigated by the scaling analysis. The ¢SM(T;H) curves in higher ¯eld range (2 kOe < H < 5.6 kOe) collapse onto an universal curve after scaling, which is independent of T and H. Based on this universal curve, the ¢SM(T;H) (excluding the skyrmion entropy change) at any T and H can be calculated. On the other hand, a small valley is found on the ¢SM(T) curve at » 28.5 K in the ¯eld range from » 1 kOe to » 2 kOe, which is resulted from the skyrmion phase. Subsequently, the magnetic entropy change of the skyrmion phase [¢SskX M (T;H)] can be derived by subtraction of the calculated ¢Scal M (T;H) from the experimental ¢Sexp M (T;H). The obtained results show that ¢SskX M reaches the maximum at » 1.6 kOe, which indicates the highest number of skyrmion vortices at that ¯eld.
分类： 物理学 >> 凝聚态:电子结构、电、磁和光学性质 提交时间： 2017-11-27
摘要： The critical behavior of the single crystal helimagnet MnSi is investigated by means of bulk dc-magnetization at the boundary between the conical state and paramagnetic phase. We obtain the critical exponents (¯ = 0:242 § 0:006, ° = 0:915 § 0:003, and ± = 4:734 § 0:006), where the self-consistency and reliability are veri¯ed by the Widom scaling law and the scaling equation. The critical exponents of MnSi belong to the universality class of tricritical mean-¯eld theory, which unambiguously indicates a tricritical phenomenon at the boundary between the ¯rst-order phase transition and the second-order one induced by the external magnetic ¯eld. The tricritical point (TCP) is determined as HTCP ¼ 3200 Oe at the critical temperature, consistent with the previous report [Phys. Rev. Lett. 110, 177207 (2013)]. The critical behavior suggests a long-range magnetic coupling with the exchange distance decaying as J(r) ¼ r¡4:3 in MnSi.
分类： 物理学 >> 凝聚态:电子结构、电、磁和光学性质 提交时间： 2017-11-27
摘要： The pyrochlore iridate Gd2Ir2O7 has been investigated by means of the electron paramagnetic resonance (EPR) spectroscopy. A magnetic transition at T¤ » 20 K is found in addition to the transition at TC » 130 K, which has not been reported previously. The phase transition at TC, which corresponds to a resistivity transition, is resulted from the long-range spin ordering of Ir4+ ions in all-in/all out state. However, the magnetic transition at T¤, which is regardless of resistivity transition, is not caused by Ir4+ but Gd3+ ions. Both of the magnetic transitions are irrelevant to the lattice distortion. We suggest that the competition between the enhanced paramagnetism of Gd3+ ions and the f ¡ d interaction should be responsible for the magnetic transition at T¤.
分类： 物理学 >> 电磁学、光学、声学、传热、经典力学和流体动力学 分类： 物理学 >> 气体、等离子体、放电物理 提交时间： 2017-11-27
摘要： An important feature of China Fusion Engineering Test Reactor(CFETR) project is represented by the additional heating obtained from the injection of neutral beams based on accelerated negative ions. For the neutral beams based on negative ions, the most important measurements are beam uniformity, beamlet divergence and stripping losses. According to the CFETR requirement, the maximum allowed beam divergence angle and beam non-uniformity are 6 mrad and ±10%,respectively. As 1D carbon tiles have large ratio between perpendicular conductivity and parallel conductivity and high stability, they can be used for beam uniformity and beamlet divergence measurement. This paper presents the investigation of the influence on the response of 1D carbon tile having the thermal characteristics and features of some dedicated diagnostics. Simulations show that it will be possible to verify experimentally whether the beam meets the requirement about the maximum allowed value. This work lays a foundation for design and application of high precision beam diagnostic targets.
分类： 物理学 >> 核物理学 提交时间： 2017-11-27
摘要： Neutral beam injection has been recognized as one of the most effective means for plasma heating.According to the research plan of the EAST physics experiment, two sets of neutral beam injector were built and operational in 2014. The paper presents the development of beam diagnosis system for EAST neutral beam injector (NBI) and the latest experiment results obtained on the test-stand and EAST-NBI-1 and 2. The results show that the optimal divergence angle is (0.62°, 1.57°) and the full energy particle is up to 77%. They indicate that EAST NBI work properly and all targets reach or almost reach the design targets. All these lay a solid foundation for the achievement of high quality plasma heating for EAST.
分类： 生物学 >> 生物工程 提交时间： 2017-11-27 合作期刊: 《中国生物工程杂志》
摘要： 粘红酵母是重要的产油微生物之一，有的种属油脂含量可以达到其干重的70%。其胞内代谢物中包含大量可加工生产生物柴油的相关物质，如游离脂肪酸、甘油三酯等。本文主要以超声破碎法（即空化作用）提取粘红酵母胞内代谢物，利用气相质谱联用方法检测各物质，通过归一化法计算各物质含量，并对所得数据进行处理与分析，寻找最合适提取胞内代谢物的方法，针对不同种类胞内代谢物找出相应的实验条件和提取方法。
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