Applications of He neutron spin filters at the NCNR

At the NIST Center for Neutron Research (NCNR), we have applied ³He neutron spin filters (NSFs) to the instruments where ³He NSFs are advantageous, such as thermal triple-axis spectrometry, small-angle neutron scattering, and diffuse reflectometry. We present the status of our development and application of this method, including polarized gas production by spin-exchange optical pumping, magnetostatic cavities for storage of the polarized gas on the beam line, and nuclear magnetic resonance (NMR)-based, on-line monitoring and reversal of the ³He polarization. We present the status of developing user-friendly interfaces incorporated into the instrument software to handle these ³He neutron spin filters while taking data and performing data analysis. Finally we discuss the status of development of a polarization capability on the multi-axis crystal spectrometer, which requires polarization analysis over a 220° angular range.     

Wave properties of an annular periodic multilayer structure containing the single-negative materials

The optical properties of an annular periodic multilayer structure containing two kinds of single-negative materials are theoretically investigated based on the transfer matrix method of the cylindrical waves. At the azimuthal mode number m1 and near the magnetic plasma frequency and the electronic plasma frequency for the TE wave and TM wave, respectively, we find that there is an additional high-reflectance band and some reflection dips exist when the plasma frequency is located in the photonic band gap. These two special features arising from the higher order azimuthal mode of the cylindrical waves are not seen in the planar one-dimensional Bragg reflector consisting of the single-negative materials. Such filtering responses provide a feasible way of designing a narrowband resonator without physically introducing any defect layer in the structure.     

Structural and physical properties of ZnO:Al films grown on glass by direct current magnetron sputtering with the oblique target

ZnO:Al films were deposited on glass substrates at 300 K and 673 K by direct current magnetron sputtering with the oblique target. The Ar pressure was adjusted to 0.4 Pa and 1.2 Pa, respectively. All the films have a wurtzite structure and grow with a c-axis orientation in the film growth direction. The films grow mainly with columnar grains perpendicular to the substrate and some granular grains also exist in the films. The film deposited at 673 K and 0.4 Pa has the largest grains whereas that prepared at 300 K and 0.4 Pa consists of the smallest grains and is porous. The films exhibit an n-type semiconducting behavior at room temperature. The ZnO:Al film deposited at 673 K and 0.4 Pa has the lowest resistivity (3.40 × 10⁻³ Ω cm), the highest free electron concentration (4.63 × 10²⁰ cm⁻³) and a moderate Hall mobility (4.0 cm² V⁻¹ s⁻¹). The film deposited at 300 K and 0.4 Pa has the highest resistivity and the lowest free electron concentration and Hall mobility. A temperature dependence of the resistivity reveals that the carrier transport mechanism is Mott’s variable range hopping in the temperature region below 100 K and thermally activated band conduction above 215 K. The activation energy for the film deposited at 300 K and 0.4 Pa is 41 meV and that for the other films is about 35 meV. All the films have an average optical transmittance of over 85% in the visible wavelength range. The absorption edge of the film deposited at 300 K and 0.4 Pa shifts to the longer wavelength (redshift) relative to the films prepared under the other conditions.     

寻找解题的“切入点”

在解难度稍大、综合性稍强的物理综合题时,常听到有些同学抱怨"一头雾水"、"无从下手",甚至在高考大题上一片空白,留下终生遗憾,究其原因,是未找准解题的"切入点",俗话讲"良好的开端,是成功的一半"、"入门即不难",可见解题时选准切人点是解题的基础和关键,切人的策略可有以下几个方面.     

Thermodynamics and weak cosmic censorship conjecture of BTZ black holes in extended phase space

As a charged fermion drops into a BTZ black hole, the laws of thermodynamics and the weak cosmic censorship conjecture are investigated in both the normal and extended phase space, where the cosmological parameter and renormalization length are regarded as extensive quantities. In the normal phase space, the first and second law of thermodynamics, and the weak cosmic censorship are found to be valid. In the extended phase space, although the first law and weak cosmic censorship conjecture remain valid, the second law is dependent on the variation of the renormalization energy d K. Moreover, in the extended phase space, the configurations of extremal and near-extremal black holes are not changed, as they are stable, while in the normal phase space, the extremal and near-extremal black holes evolve into non-extremal black holes.     

Spin thermoelectricity in dualhydrogenated zigzag silicene nanoribbons with surface adsorptions

The buckled structure of silicene provides a feasible pathway to influence its electric and magnetic properties via surface adsorptions. Here, we investigate the magnetic and spin thermoelectric transport properties of dual-hydrogenated zigzag silicene nanoribbons (ZSiNRs) without/with the hydrogen adsorption. The band gaps for two spin channels in ZSiNRs under the hydrogen adsorption are shifted near the Fermi level, leading to the appearance of spin Seebeck effect. Using a temperature difference, one can derive the carriers with the different spin index to flow in the opposite direction. Moreover, a large rectification ratio close to 10⁵ at room temperature is achieved for the spin current, and the charge current exhibits a remarkable negative differential thermoelectric resistance (NDTR) behavior. The results presented here are fascinating potential applications in the fields of silicon-based spin caloritronic devices.     

Molecular insight into structures of monocationic and dicationic ionic liquids in mica slits

Structures of ionic liquids (ILs) 1-decyl-3-methylimidazolium bis(trifluoromethanesulfonyl)azanide ([C₁₀mim][TFSA]) and 1-decyl-dimethylimidazolium bis(trifluoromethanesulfonyl)azanide ([C₁₀(mim)₂](TFSA)₂) in different-sized mica slits have been investigated using molecular dynamics simulations. Ion density and angular distributions for monocationic IL [C₁₀mim][TFSA] were analysed to elucidate the IL structures under different surface charges and especially their changes in the direction perpendicular to the surfaces. [C₁₀mim][TFSA] formes in bilayers, compatible with existing models of ILs with long alkyl chains. For dicationic IL [C₁₀(mim)₂](TFSA)₂, cations adjacent to the mica surface tend to stay parallel to the surface with both positively charged rings absorbed. While near the centre of the slit, dications show the weak tendency of orientation distribution, more random than [C₁₀mim]⁺ ions. Structures of [C₁₀(mim)₂](TFSA)₂ cannot be described by bilayer models. Additionally, the in-plane arrangement of [C₁₀mim][TFSA] is more ordered when K⁺ ions completely neutralise the negative charge of the mica surface, and [C₁₀mim]⁺ ions tend to be located in hexagonal mica lattices with two aluminium atoms in replacement of silicon atoms. [TFSA]^? ions are constrained by the neighbouring K⁺ ions absorbed onto mica lattices.     

Exact Solutions to the Quantum Rabi-Stark Model Within Tunable Coherent States

The quantum Rabi-Stark model, where the linear dipole coupling and the nonlinear Stark-like coupling is present on an equal footing, is studied within the tunable extended coherent states. The eigenvalues and eigenstates are therefore obtained exactly. Surprisingly, the entanglement entropy in the ground-state is found to jump suddenly with the coupling strength. The first-order quantum phase transition can be detected by level crossing of the ground state and the first excited state, which is however lacking in the original linear quantum Rabi model. Performing the first-order approximation in the present theory, we can derive closed-form analytical results for the ground-state. Interestingly, it agrees well with the exact solutions up to the ultra-strong coupling regime in a wide range of model parameters. The spectral collapses when the absolute value of the nonlinear coupling strength approaches to twice the cavity frequency is observed with the help of new solutions in the limits.     

铁电体中新畴成核经典模型的改进

新畴成核是外加低场下铁电体中铁电畴反转的一个重要的过程.首先介绍了新畴成核的经典模型,采用该模型研究了铁电畴反转的新畴成核过程,发现理论计算的成核速率与外场关系和实验观测结果不一致.在Tagantsev模型的基础上,选取不同的成核形状对新畴成核的经典模型进行了改进,并获得了和实验观测相符的理论计算结果. 关键词： 铁电体 铁电畴 成核速率     

Monte Carlo study of the antiferromagnetical Ising model on a centred honeycomb lattice

We use the Monte Carlo method to study an antiferromagnetical Ising spin system on a centred honeycomb lattice, which is composed of two kinds of 1/2 spin particles A and B. There exist two different bond energies J_A-A and J_A-B in this lattice. Our study is focused on how the ratio of J_A-B to J_A-A influences the critical behaviour of this system by analysing the physical quantities, such as the energy, the order parameter, the specific heat, susceptibility, {etc} each as a function of temperature for a given ratio of J_A-B to J_A-A. Using these results together with the finite-size scaling method, we obtain a phase diagram for the ratio J_A-B / J_A-A. This work is helpful for studying the phase transition problem of crystals composed of compounds.