摘要：We illustrate the microscopic quantum picture of superfluid $^4$He with the help ofrevealing a hidden property of its many-body levels. We show that, below thetransition point, the low-lying levels of the system form a grouping structure with eachlevel belonging to one specific group only. In a superflow state or a static state, the systemestablishes a group-specific thermal equilibrium with its environment and the levels of aninitially-occupied group shall be thermally distributed. The other initially-unoccupied groupsof levels remain unoccupied, due to the fact that inter-group transitions are prohibited.The macroscopically observable physical quantities of the system, such as superflowvelocity and thermal energy density, are determined statistically by the thermaldistribution of the occupied group(s). We further show that thermalenergy of a superflow has an unusual flow velocity dependence: the largerthe velocity is, the smaller the thermal energy. This velocity dependence isresponsible for several intriguing phenomena of the system, such as themechano-caloric effect and the fountain effect, which demonstrate afundamental coupling between the thermal motion of the system and itshydrodynamic motion. We report an experimental observation of a counter-intuitiveself-heating effect of $^4$He superflows, which confirms that a $^4$He superflowcarries significant thermal energy depending on its velocity.
摘要：基于微磁学模拟结果，发现由于单根纳米线两端处磁矩矢量方向与极化电流密度(J )方向的不一致，导致电子的自旋转移力矩（STT）只能作用于纳米线两端的磁矩。通过提高极化电流的自旋极化率(P)，这个简化模型的微波磁谱中处在18 GHz的微波磁损耗可以得到明显的抑制，同时发现与微波损耗对应的自然共振频率大小却不受STT影响。在STT效应作用下，负的磁导率虚部也是可实现的。另外，模拟结果表明通过提高STT效应中非绝热效应项中的β值，18GHz处的微波磁损耗可以得到显著的增强。基于STT效应中不同力矩的来源对磁化强度进动过程的有效阻尼常数(ɑe)的影响，上述的结果可以得到合理的解释：绝热力矩能降低ɑe数值，因而能抑制微波磁损耗的大小；非绝热力矩能提高ɑe数值，因而能增强微波磁损耗。我们的研究结果展示了一个创新性的、能主动地调控微波磁损耗的方法。
Strain engineering of 2D materials is capable of tuning the electrical and optical properties of the materials without introducing additional atoms. However, there are still great challenges in realizing straining of 2D materials with CMOS compatibility. Here, a method for large-scale ultrafast strain engineering of CVD-grown 2D materials is proposed. We introduce locally non-uniform strains through the cooperative deformation of materials and metal/metal oxide core/shell nanoparticles through cold laser shock. Raman and PL spectra reveal that the tensile strain of MoS2 changes and the band gap decreases after laser shock. MD simulations are used to investigate the mechanism of the ultrafast straining of CVD-grown 2D materials. Field effect transistors of CVD MoS2 were fabricated, and the performances before and after straining of the same devices are compared. By adjusting the strain level of MoS2, the field effect mobility can be increased from 1.9 cm2V-1s-1 to 44.1 cm2V-1s-1. This is the maximum value of MoS2 FETs grown by CVD with SiO2 as dielectric. As an environment-friendly, large-scale and ultra-fast manufacturing method, laser shock provides a universal strategy for large-scale adjustment of 2D materials strain, which will help to promote the manufacturing of 2D nano electronic devices and optoelectronic devices.
非厄米的引入扩展了传统厄米量子系统中的概念并诱导出许多新奇的物理现象, 比如非厄米系统所独有的非厄米趋肤效应, 这使得对非厄米量子模型的模拟成为大家关注的热点. 相比于量子平台, 经典系统具有成本低廉、技术成熟、室温条件等优势, 而其中的经典电路系统则更加灵活, 原则上可以模拟任意维度、任意格点间跃迁、任意边界条件下的量子紧束缚模型, 已经成为模拟量子物态的有力平台. 本文利用经典电路通过 SPICE 成功模拟了一个重要的非厄米量子模型 —— 非互易 Aubry-Andr\'e 模型 —— 的稳态性质, 此模型同时具有非互易的格点跃迁和准周期的格点在位势. 以此为例, 详细介绍了如何建立经典电路的拉普拉辛形式与量子紧束缚模型哈密顿矩阵在不同边界条件下的映射, 尤其是如何利用电流型负阻抗变换器构建模型的非互易性. 然后, 根据电路的格林函数, 通过 AC 电流驱动并测量电压响应的方式, 用 SPICE 模拟了周期边界条件下的复能谱和相应的能谱缠绕数, 以及开边界条件下的趋肤与局域模式的竞争. 其中, 为了使电路的响应不发散, 本文还解析地给出辅助元件的设置原则. 结果显示, SPICE 模拟与理论计算很好地吻合, 为进一步的实验实现提供了详细的指导. 由于本文电路设计与测量方案的普适性, 原则上可以直接应用于其他非厄米量子模型的电路模拟.
摘要： True random number generator (TRNG) is an important component for modern information security technologies. Among the candidates, TRNG with spin-orbit torque (SOT)-induced probabilistic magnetization switching is competitive for its advantages in anti-radiation, unlimited endurance, robust stability, and broad temperature range. However, realization of a SOT-TRNG requires intensive understanding of the magnetic dynamic process under a spin-orbit current. Here, we performed micromagnetic simulation of the SOT-induced probabilistic magnetization switching by using Mumax 3. Without thermal noise, identical magnetic moment precessions were found between repeated simulation cycles, resulting in deterministic magnetization switching. When thermal noises were taken into account, stochastic precession trails and thereby probabilistic magnetization switching were finally obtained. Our results suggest the Mumax 3 to be a practical tool for simulating the probabilistic magnetization switching behavior of a SOT-TRNG, as well as highlighting the crucial role of thermal noise during the during the simulation.
摘要：The past decade has witnessed a surge of interest in exploring emergent particles in condensed matter systems. Novel particles, emerged as excitations around exotic band degeneracy points, continue to be reported in real materials and artificially engineered systems, but so far, we do not have a complete picture on all possible types of particles that can be achieved. Here, via systematic symmetry analysis and modeling, we accomplish a complete list of all possible particles in time reversal-invariant systems. This includes both spinful particles such as electron quasiparticles in solids, and spinless particles such as phonons or even excitations in electric-circuit and mechanical networks. We establish detailed correspondence between the particle, the symmetry condition, the effective model, and the topological character. This obtained encyclopedia concludes the search for novel emergent particles and provides concrete guidance to achieve them in physical systems.
摘要： We observed a counter-intuitive remarkable heating phenomenon generated by helium-4 superflows. This phenomenon establishes that superflows carry thermal energies and entropies, which is in contrast to the hypothesis of the two-fluid model. Quantum many-body theory of superfluids provides a natural understanding of the phenomenon.
摘要： P. W. Anderson raised an important question in 2007: Is There Glue in Cuprate Superconductors? The author believes that the change of the electron clouds of ions is the glue in cuprate superconductors. The change of the electron clouds of the ions in the parent structure of the layered high-temperature superconductors CaCuO2 has been studied by the first-principles calculations. The electron clouds of Cu2+ and O2- ions change obviously under electric fields. It is also found, for the first time, the characteristic frequencies of the change of the electron clouds are 250 meV, 360 meV, and 100 meV, respectively, for the modes observed. The frequencies are low and close to that of lattice vibrations, indicating the change of the electron cloud of ions can be the electron-pairing medium in cuprate superconductors.
摘要： It is proposed that the electron-pairing medium of the iron-based superconductors may be the orbital fluctuation of the transition metal ions. But the characteristic frequency of the orbital fluctuation has not been given. For the first time, the author has calculated the real-time evolution of the electron clouds of transition metal ions in BaFe2As2 under excitations by the time-dependent density functional theory (TDDFT). There are different modes of fluctuations. The characteristic frequencies are 150 meV, 160 meV, 250 meV, and 200 meV, respectively, for the modes the author observed. The results are unexpected, because the general view is that the change of the electron density is very quick, and the frequency is much higher than the lattice vibration. The frequencies the author obtained are close to that of the lattice vibration in conventional superconductors at normal and high pressures, indicating the orbital (or electron cloud) fluctuation can by the electron pairing medium. Based on the calculation results, the author proposed a new electron pairing mechanism.
摘要：The electron-pairing mechanism in unconventional high temperature superconductors (HTS) has not been resolved. The author proposed that the electron-pairing medium of unconventional HTS is the change of the electron clouds of transition metal ions, which is analogous to the lattice vibration in conventional superconductors. Real-time evolution of the electron clouds of transition metal ions under excitations in La2Fe2As2O2, FeSe sheet, Fe2KSe2, CaCuO2, and HgBa2Ca2Cu3O8 was calculated by the time-dependent density functional theory (TDDFT). The characteristic frequency is about 90-250 meV, which is equivalent to the lattice vibration frequencies, showing that the change of the electron clouds of the transition metal ions can be the electron-pairing medium in unconventional HTS.
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.
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.
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.
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.
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
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.