Single-Particle Relaxation Time of the Two-Dimensional Spin-Polarized Electron Gas

We present results for the single-particle relaxation time of the spin-polarized two-dimensional electron gas in Silicon-MOSFET systems at zero temperatures. Spin-polarization is produced by the application of a parallel magnetic field. The single-particle relaxation time was measured with the Shubnikovde–Hass effect and is in good agreement with our theoretical results.     

Charged Surface of Liquid Hydrogen at Near Zero Gravitation

The results of investigations of the evolution of the shape of the equipotentially charged surface of a liquid hydrogen layer in the external electric field in the case when the effective gravitation acceleration is near to zero are presented. The reconstruction phenomenon—the formation of a solitary wave at the flat charged surface of liquid hydrogen at some critical value of the external electric field—has been studied under the conditions of a total screening of the electric field in the bulk of a liquid by a surface charge. From the results obtained it follows that the transition of the flat surface into the reconstructed state is close to a second order phase transition. A voltage-temperature U–T phase diagram of the charged surface has been determined. The statement of a soft character of the transition has been confirmed also by the results of studies of the oscillations spectrum of the charged surface in electric fields lower and higher the critical value.     

Transport Properties of Spin-Polarized Atomic Hydrogen Using Generalized Scattering Theory

Our results for the scattering and thermophysical properties of spin-polarized atomic hydrogen \((\hbox {H}{\downarrow })\) have been presented in the temperature range 0.01–10 K using the Galitskii–Migdal–Feynman formalism. These results include the quantum second virial coefficient, the average total and viscosity cross sections, the viscosity, the diffusion coefficient, and the thermal conductivity. The calculations have been undertaken using three triplet-state potentials: Morse-type, Silvera and Born–Oppenheimer potentials. The Morse potential is less attractive and very simple, but less accurate to describe spin-polarized atomic hydrogen. That explains the differences between it and the other two potentials, which are clearly better. From the results of the average total cross sections, it is concluded the \(\hbox {H}{\downarrow }\) remains a gas even at low temperature. The viscosity, the thermal conductivity, and the diffusion coefficients of \(\hbox {H}{\downarrow }\) increase in all cases with increasing temperature.     

Condensate Fraction in Liquid 4He at Zero Temperature

We present results of the one-body density matrix ρ ₁(r) and the condensate fraction n ₀ of liquid ⁴He calculated at zero temperature by means of the Path Integral Ground State Monte Carlo method. This technique allows to generate a highly accurate approximation for the ground state wave function Ψ ₀ in a totally model-independent way, that depends only on the Hamiltonian of the system and on the symmetry properties of Ψ ₀. With this unbiased estimation of ρ ₁(r), we obtain precise results for the condensate fraction n ₀ and the kinetic energy K of the system. The dependence of n ₀ with the pressure shows an excellent agreement of our results with recent experimental measurements. Above the melting pressure, overpressurized liquid ⁴He shows a small condensate fraction that has dropped to 0.8% at the highest pressure of p=87?bar.     

Fluid Helium in a Narrow Pore at Zero Temperature

The zero temperature adsorption isotherm of ⁴He in a rigid cylindrical pore with alkali metal walls is computed within finite-range density functional theory, with the radius of the tube as a parameter. It is shown that starting from narrow pores and increasing the radius, the adsorbed helium first becomes bound in a quasi-onedimensional phase, and finally condenses into a quasi-twodimensional configuration by going through an almost filling situation. The results are in agreement with recent calculations for carbon nanotubes based on thermodynamical approaches.     

Superconductivity and Hybridization in the Fermion System at Zero Temperature

Some ground state properties of a fermion system are investigated. An initial Hamiltonian being a generalization of the lattice Anderson model describes this system as a two-band electron-phonon one where both of electron bands are narrow (one is wider than the other). Two bands are hybridized and electrons from both of them are strongly coupled to the same local phonons. After performing the Lang–Firsov type transformation, a new polaron Hamiltonian is obtained. As a result, there appear three effective polaron-polaron interactions in this Hamiltonian and the bandwidths of both of electron bands are reduced. It is assumed that the reduction of the bandwidth of one of the bands is sufficiently strong to make this band dispersionless. Each of the interactions can lead to superconductivity on its own but only one of them is taken into account at this stage; it is the interaction between electrons from the band with a dispersion. The Bogolyubov’s mean field approach is used in order to find the spectrum of the reduced effective Hamiltonian. The effect of the hybridization on the superconducting gap and the relationship between the critical hybridization and the average number of electrons from the wider band per lattice site are investigated.     

Equation of State of Overpressurized Liquid 4He at Zero Temperature

No Heading In recent experiments, Balibar and collaborators have been able to produce metastable superfluids ⁴He up to 163 ± 20 bar, a pressure much higher than the freezing point (25 bar). Interpretation of some of their experimental data requires the knowledge of the equation of state of bulk ⁴He in this high-pressure regime. As this is not experimentally known, they extrapolate to these high pressures using analytical fits to the well-known equation of state in the stable regime. Using the same theoretical analysis that allowed in the past to reproduce accurately the equation of state of liquid ⁴He in the stable domain, we present now extended results for this equation of state up to 350 bar. Our calculations, based on the diffusion Monte Carlo method, show some significant differences with the proposed extrapolations, which could be relevant for future experiments.PACS numbers: 67.40.–w, 67.80.–s     

微波ECR—CVD低温SiNx薄膜的氢含量分析

利用红外光谱技术研究了微波电子回旋共振（ＥＣＲ）等离子体化学气相沉积（ＣＶＤ）法在低温条件下制备的ＳｉＮ。膜的键的结构和氢含量，分析了微波 功率和后处理条件对膜含量的影响及其成因，提出适当提高微波功率是降低微波ＥＣＲ－ＣＶＤ低温ＳｉＮｘ膜中氢含量的可能途径。     

Measurements of Hydrogen Thermal Conductivity at High Pressure and High Temperature

The thermal conductivity for normal hydrogen gas was measured in the range of temperatures from 323 K to 773 K at pressures up to 99 MPa using the transient short hot-wire method. The single-wire platinum probes had wire lengths of 10 mm to 15 mm with a nominal diameter of 10 μm. The volume-averaged transient temperature rise of the wire was calculated using a two-dimensional numerical solution to the unsteady heat conduction equation. A non-linear least-squares fitting procedure was employed to obtain the values of the thermal conductivity required for agreement between the measured temperature rise and the calculation. The experimental uncertainty in the thermal-conductivity measurements was estimated to be 2.2 % (k = 2). An existing thermal-conductivity equation of state was modified to include the expanded range of conditions covered in the present study. The new correlation is applicable from 78 K to 773 K with pressures to 100 MPa and is in agreement with the majority of the present thermal-conductivity measurements within ±2 %.     

氢气气氛下SiC纤维的热稳定性

采用XRD和扫描电镜等分析方法,对国产SiC纤维进行氢气气氛下除去游离碳及纤维高温稳定性的实验研究.在氢气气氛下,将国产SiC纤维分别在900,1200,1500℃和1800℃保温4h,随着温度的升高,纤维质量不断流失,同时纤维结构遭到破坏, 但β-SiC晶化转变不明显,表明国产纤维存在较多的因氧的介入所形成的Si-O-C键,从而影响了纤维在氢气气氛下的高温稳定性.     

Dynamics of a Particle and a Quantized Vortex at Zero Temperature: Self-consistent Calculation

Many experiments for visualizing quantized vortices and normal fluid flow have been performed in superfluid ⁴He. Recently, metastable He₂ excimer molecules have been used as tracer particles. Since their radius is only about 10^?10 m, they hardly perturb the system, thus being a good candidate of tracer particles. In order to understand the interactive motion of He₂ molecules and vortices at zero temperature, we numerically study the trapping diameter by using the self-consistent equations of motions. We calculated the trapping diameter as a function of the initial velocity of the particle. The trapping diameter is almost inversely proportional to the initial velocity of the particle and compared with the observation.     

Temperature Dependence of Microwave Nano-Oscillator Linewidths Driven by Spin-Polarized Currents: A Micromagnetic Analysis

The dependence of the linewidth on the temperature in a spin valve driven by spin-polarized currents is analyzed by means of full micromagnetic simulations. The results are compared to the recent analytical predictions by Tiberkevic and confirmed by the experiments of Boone In agreement with the Tiberkevic theory and experiments of Sankey , our micromagnetic results point out two regimes. The linewidth increases linearly with the temperature until a threshold value, above which a square root dependence is observed.     

Absence of Phase-Fluctuation Contribution to the Low-Frequency Optical Conductivity in d-Wave Superconductors at Zero Temperature

The behavior of the low-frequency optical conductivity _reg() in superconducting cuprates is, at the present, an open and interesting issue. In particular, since the zero-temperature and zero-frequency limit of _reg() attains a value much larger than the universal value expected within a self-consistent T-matrix calculation, an intriguing possibility is that the collective mode can also contribute to _reg(). By taking into account the effect of dissipation on the collective mode in a d-wave superconductor, we evaluate the phase-fluctuation contribution to _reg(0) within the formalism of the phase-only action. We show that even though the collective mode contributes to _reg() at finite frequencies, approaching the zero-temperature and zero-frequency regime the corrections at _reg() due to phase fluctuations vanish.     

Low Temperature Behavior of a Two-Dimensional Quantum Antiferromagnet

We analyze the two-dimensional antiferromagnet with a quantum disordered ground state and a gap to bosonic excitations with finite spin. The zero temperature (T = 0) quantum phase transition is studied, and the finite temperature effects are also considered in the low temperature approximation of the Renormalization Group Method. The violation of the universality expected for d = 2 system has been shown to be logarithmic for T = 0 and T 0. We showed that chemical potential and the ground state energy depends on the interaction parameters at T = 0 and T 0. but this dependence is logarithmic.     

Finite Element Analysis of Hydrogen Transport in Steel Pressure Vessel at Room Temperature

Hydrogen embrittlement is commonly considered as an important failure mechanism for steel pressure vessels and pipes made of such as Cr–Mo and 4130X steels under high-pressure hydrogen environments, which means hydrogen atom can easily penetrate and diffuse into the metal, leading to the distortion of microscopic lattice and the degradation of macroscopic strength and fracture toughness. Under the support of the National Key Fundamental Research and Development Project of China (2015.1-2019.12), we aim to launch a series of theoretical, experimental, and numerical research on the macroscopic damage evolution and microscopic fracture of steel structures under high-pressure hydrogen environment, which ultimately commits to gaining deep insight into the hydrogen embrittlement mechanisms. This work studies the hydrogen transport mechanisms in Cr–Mo steel pressure vessels under different hydrogen environments using finite element analysis (FEA), which is fundamental to subsequent research on the hydrogen-induced damage evolution and crack behaviors. The purpose of this paper is to explore the effects of the initial hydrogen concentrations and structural sizes on the hydrogen transport mechanisms in 2.25Cr-1Mo pressure vessels with a nozzle at room temperature. Numerical results by comparing different hydrogen concentration distributions show that structural discontinuities tend to accelerate the hydrogen embrittlement sensitivity.     

Finite Element Analysis of Hydrogen Transport in Steel Pressure Vessel at High Temperature

Hydrogen embrittlement is commonly considered as an important failure mechanism for some typical steel pressure vessels and pipes made of such as Cr–Mo and 4130X steels at high-pressure hydrogen environment. In previous work, we investigated the hydrogen transport mechanisms of Crmo steel pressure vessels at room temperature. Furthermore, high temperature environment may affect the hydrogen transport mechanisms and hydrogen-induced crack behaviors in these structures to a large extent. In this paper, we study the hydrogen transport mechanisms in 2.25Cr–1Mo steel pressure vessel at high temperature under the support of National Key Fundamental Research and Development Project of China (2015.1-2019.12). The main work is to explore the effects of temperature, hydrogen concentration, and structural sizes on the transient hydrogen diffusion and distribution behaviors in Crmo steel pressure vessels using finite element analysis. Numerical results show that elevated high temperature accelerates the hydrogen embrittlement sensitivity, especially at structural discontinuities.     

Estimating Bounds on Collisional Relaxation Rates of Spin-Polarized 87Rb Atoms at Ultracold Temperatures

We present quantum scattering calculations for the collisional relaxation rate coefficient of spin-polarized ⁸⁷Rb(f = 2,m = 2) atoms, which determines the loss rate of cold Rb atoms from a magnetic trap. Unlike the lighter alkali atoms, spin-polarized ⁸⁷Rb atoms can undergo dipolar relaxation due to both the normal spin-spin dipole interaction and a second-order spin-orbit interaction with distant electronic states of the dimer. We present ab initio calculations for the second-order spin-orbit terms for both Rb₂ and Cs₂. The corrections lead to a reduction in the relaxation rate for ⁸⁷Rb. Our primary concern is to analyze the sensitivity of the ⁸⁷Rb trap loss to the uncertainties in the ground state molecular potentials. Since the scattering length for the a³Σ⁺_u state is already known, the major uncertainties are associated with the X¹Σ⁺_g potential. After testing the effect of systematically modifying the short-range form of the molecular potentials over a reasonable range, and introducing our best estimate of the second-order spin-orbit interaction, we estimate that in the low temperature limit the rate coefficient for loss of Rb atoms from the f = 2,m = 2 state is between 0.4 × 10⁻¹⁵ cm³/s and 2.4 × 10⁻¹⁵ cm³/s (where this number counts two atoms lost per collision). In a pure condensate the rate coefficient would be reduced by 1/2.     

Temperature Dependence of Hydrogen Adsorption Isotherms

In the past it has already been shown that adsorption isotherms of liquid or solid films are not described completely by the Frenkel–Halsey–Hill theory. Substrate roughness as well as thermal fluctuations have to be taken into account in understanding the adsorption behavior. The inclusion of thermal fluctuations into the adsorption theory has already been addressed and proven to provide an explanation for the deviations found in many experiments. However, a resulting temperature dependence of such isotherms has not yet been verified. In our investigations we have addressed this issue with a series of adsorption isotherms of hydrogen on gold in a temperature range from 11 K to 19.5 K (i.e., below and above the triple-point temperature of hydrogen). Our measurements are compared with existing theories and the nature of the remaining discrepancies is discussed.PACS numbers: 68.08 Bc, 68.15 +e, 68.43 −h