Computer Simulation of the High-Velocity Motion of Electron Bubbles in Superfluid Helium

We have performed computer simulations of the motion of electron bubbles through superfluid helium. The helium is modeled through the use of a modified version of the Gross-Pitaevskii equation. We find that by the time the bubble reaches the velocity at which vortex nucleation occurs the shape has changed significantly from spherical.     

Ion–Electron Recombination and Heat Fluxes in High-Frequency Ion Thrusters

Technical Physics Letters - Heat fluxes emitted from plasma to the surface of structural elements of high-frequency (HF) ion thrusters with perforated electrodes of the ion-optical system have been...     

On the Structure of the Superfluid State and Quasiparticle Spectrum in a Bose Liquid with a Suppressed Bose–Einstein Condensate

A self-consistent model of the superfluid (SF) state of a Bose liquid with strong interaction between bosons and a weak single-particle Bose–Einstein condensate (BEC) is considered. The ratio of the BEC density n ₀ to the total particle density n of the Bose liquid is used as a small parameter of the model, n ₀/n?1, unlike in the Bogolyubov theory of a quasi-ideal Bose gas, in which the small parameter is the ratio of the number of supracondensate excitations to the number of particles in an intensive BEC, (n?n ₀)/n ₀?1. A closed system of nonlinear integral equations for the normal ~Σ₁₁(p, ω) and anomalous ~Σ₁₂(p, ω) self-energy parts is obtained with account for terms of first order in the BEC density. A renormalized perturbation theory is used, which is built on combined hydrodynamic (at p→0) and field (at p≠0) variables with analytic functions ~Σ _ij (p, ε) at pε0 and ε→0 and a nonzero SF order parameter ~Σ₁₂(0, 0)≠0, proportional to the density ρ _s of the SF component. Various pair interaction potentials U(r) with inflection points in the radial dependence and with an oscillating sign-changing momentum dependence of the Fourier component V(p) are considered. Collective many-body effects of renormalization (“screening”) of the initial interaction, which are described by the bosonic polarization operator Π(p, ω), lead to a suppression of the repulsion [V(p)<0] and an enhancement of the effective attraction [V(p)<0] in the respective domains of nonzero momentum transfer, due to the negative sign of the real part of Π(p, ω) on the “mass shell” ω=E(p). In the framework of the “soft spheres” model with the single fitting parameter—the value of the repulsion potential at r=0—the quasiparticle spectrum E(p) is calculated, which is in good accordance with the experimental spectrum E _exp(p) of elementary excitations in superfluid ⁴He. It is shown that the roton minimum in the quasiparticle spectrum is directly associated with the first negative minimum of the Fourier component of the renormalized (“screened”) potential of pair interaction between bosons.     

Point-Contact Andreev-Reflection Spectroscopy in Fe-Based Superconductors: Multigap Superconductivity and Strong Electron–Boson Interaction

We performed point-contact Andreev-reflection measurements in Ba(Fe_1?x Co _x )₂As₂ single crystals (x=0.1, T _c =24.5?K) and SmFeAsO_1?x F _x polycrystals (x=0.2, T _c =52?K). The spectra indicate the presence of two superconducting gaps with no line nodes on the Fermi surface, but also feature additional structures related to the electron?Cboson interaction (EBI). From the spectra, it is possible to extract the characteristic energy ?? ₀ of the mediating boson. In Co-doped Ba-122, we obtain ?? ₀=12?meV that coincides with the spin-resonance energy observed in neutron-scattering experiments. In Sm-1111, ?? ₀=20?meV fulfils the relation ?? ₀=4.65k _B T _c inferred from neutron-scattering results on other Fe-based superconductors. The strong electron?Cboson coupling may also explain some anomalies in the PCAR conductance curves (e.g., the excess conductance at high energy) which sometimes prevent a good fit of the curves with models based on constant, BCS-like order parameters.     

Electron–Phonon Interaction and Phonon Renormalization in the Lamellar Cobaltate Na x CoO2

We study theoretically the electron–phonon interaction in Na _x CoO₂. For the A _1g and E _1g phonon modes found in Raman experiments, we calculate the matrix elements of the electron–phonon interaction. Analyzing the feedback effect of the conduction electrons on the phonon frequency ω, we investigate the doping dependence of these two phonon modes. Due to the momentum dependence of the electron–phonon interaction, we find the strongest renormalization of the E _1g mode around the Brillouin zone boundary which should be observed in the neutron scattering. At the same time, the A _1g mode shows the strongest coupling to the conducting electrons around the Γ point and reveals its doping dependence in the Raman experiments. Our results shed light on the possible importance of the electron–phonon interaction in the lamellar sodium cobaltates.     

Pseudo-gap and Vertex Correction of Electron–Phonon Interaction in High Transition-Temperature Superconductors

The strong-coupling Eliashberg theory plus vertex correction is used to calculate the maps of transition temperature (T _c) in parameter-space characterizing superconductivity. Based on these T _c maps, complex crossover behaviors are found when electron?Cphonon interaction increases from weak-coupling region to strong-coupling region. The doping-dependent T _c of cuprate superconductors and most importantly the emergence of pseudo-gap region can be explained as the effects of vertex correction.     

Electron–Phonon Interaction and Optical Spectra of Metals

Observed optical reflectivity in the infrared spectral region is compared with theoretical predictions in a strongly coupled electron–phonon system. Starting from a Fröhlich Hamiltonian, the spectral functions and their temperature dependence are derived. A full analysis including vertex corrections leads to an expression for the optical conductivity () that can be formulated in terms of the well-known optical conductivity for a quasi-isotropic system without vertex corrections. A numerical comparison between the full result and the so-called extended Drude formula, its weak coupling expansion, shows little difference over a wide range of coupling constants. Normal-state optical spectra for the high-T _c superconductors YBa₂Cu₃O₇ and La_{2 – x} Sr _x CuO₄ at optimal doping are compared with the results of model calculations. Taking the plasma frequency and from band structure calculations, the model has only one free parameter, the electron–phonon coupling constant . In both materials the overall behavior of the reflectivity can be well accounted for over a wide frequency range. Systematic differences exist only in the mid-infrared region. They become more pronounced with increasing frequency, which indicates that a detailed model for the optical response should include temperature-dependent mid-infrared bands.     

3He Correlation Energy in the Random Phase Approximation: the One-Ripplon-Exchange Potential in Thin 3He–4He Superfluid Films

A new approach to the calculation of correlation energies in the random phase approximation (RPA) is introduced. The method is applied to the system of adsorbed ³ He in thin ⁴ He superfluid films. The ³ He component interacts by means of the exchange of virtual surface interactions (ripplons) in addition to the usual atom-atom interaction. We report on a calculation of the RPA contribution to the ³ He ground-state energy of a previously introduced model for this effective interaction called the one-ripplon-exchange potential (OREP). V_OREP is manifestly frequency dependent and complex. In this paper we study the differences in the RPA contribution to the ground-state energy between the fully complex and frequency dependent V_OREP and V_OREP in the real, static limit.     

Universal Features of the Electron Transport in Tungsten–Carbon Nanocomposites

The wide-temperature-range (4.2–300 K) electron transport had being studied in tungsten–carbon nanocomposites in tungsten concentration interval 0.1–0.45. It is shown that electron transport in the nanocomposites possesses the features of the universality, manifested in the form of power-law dependences of the conductivity on temperature in the two characteristic temperature intervals. The critical temperature separating the intervals is about 25–30 K and has no appreciable dependence on the value of tungsten concentration in nanocomposites. The power exponents of the temperature dependences of the conductivity in both temperature intervals are the non-monotonic functions of the tungsten concentration and vary in the range 0–2 with a wide minimum at 0.2 and 0.25 of tungsten content in the high- and low-temperature intervals, respectively. The observed power-law temperature corrections to the conductivity are simulated and discussed within the effective medium approximation in the framework of the model of the inelastic tunneling of the electrons between the conducting clusters in the tungsten–carbon nanocomposites.     

Probing Contact-Electrification-Induced Electron and Ion Transfers at a Liquid–Solid Interface

As a well-known phenomenon, contact electrification (CE) has been studied for decades. Although recent studies have proven that CE between two solids is primarily due to electron transfer, the mechanism for CE between liquid and solid remains controversial. The CE process between different liquids and polytetrafluoroethylene (PTFE) film is systematically studied to clarify the electrification mechanism of the solid–liquid interface. The CE between deionized water and PTFE can produce a surface charges density in the scale of 1 nC cm⁻², which is ten times higher than the calculation based on the pure ion-transfer model. Hence, electron transfer is likely the dominating effect for this liquid–solid electrification process. Meanwhile, as ion concentration increases, the ion adsorption on the PTFE hinders electron transfer and results in the suppression of the transferred charge amount. Furthermore, there is an obvious charge transfer between oil and PTFE, which further confirms the presence of electron transfer between liquid and solid, simply because there are no ions in oil droplets. It is demonstrated that electron transfer plays the dominant role during CE between liquids and solids, which directly impacts the traditional understanding of the formation of an electric double layer (EDL) at a liquid–solid interface in physical chemistry.     

Homogeneous Turbulence in Superfluid 4He in the Low-Temperature Limit: Experimental Progress

Selected advances in the research on the dynamics of tangles of quantized vortices in superfluid helium with little normal component during the last 50 years are briefly reviewed. The main emphasis is on the experimental techniques of generating and probing homogeneous one-component superfluid turbulence of various energy spectra in superfluid ⁴He in the low-temperature limit. The most recent experimental progress, modern theoretical concepts and future outlook are summarized.     

Creep–Fatigue Interaction of Inconel 718 Manufactured by Electron Beam Melting

Electron beam melting of Ni-base superalloy Inconel 718 allows producing a columnar-grained microstructure with a pronounced texture, which offers exceptional resistance against high-temperature loading with severe creep–fatigue interaction arising in components of aircraft jet engines. This study considers the deformation, damage, and lifetime behavior of electron-beam-melted Inconel 718 under in-phase thermomechanical fatigue loading with varying amounts of creep–fatigue interaction. Strain-controlled thermomechanical fatigue tests with equal-ramp cycles, slow–fast cycles, and dwell time cycles are conducted in the temperature range from 300 to 650 °C. Results show that both dwell time and slow–fast cycles promote intergranular cracking, gradual tensile stress relaxation, as well as precipitate dissolution and coarsening giving rise to cyclic softening. The interplay of these mechanisms leads to increased lifetimes in both dwell time and slow–fast tests compared to equal ramp tests at higher strain amplitudes. Conversely, at lower mechanical strain amplitudes, the opposite is observed. A comparison with results of conventional Inconel 718 indicates that the electron-beam-melted material exhibits superior resistance against strain-controlled loading at elevated temperatures such as thermomechanical fatigue.     

Convective Turbulence in Superfluid Solutions 3He–4He

In present work, we continue our experimental investigations of heat instability in superfluid ³He–⁴He solutions heated from below. We research two solutions with ³He concentrations 5.0% and 9.5% for temperature of 270 mK. It is found that for 5% solution the dependence is linear in temperature range studied whereas for the solution of 9.5% we observed the deviation from linear dependence above some critical value . This effect manifests the thermal instability which appears under start of phase separation in 9.5% solution if heat flow is switched on. For 5.0% solution where one does not observe the phase separation at the values of applied, the instability was not observed. To identify the possible mechanism of a thermal instability in stratified solution, we estimated the dependence of the Nusselt number on relative Raileigh number Ra/Ra _c . One observes that the dependence can be fitted as Nu=(Ra/Ra _c ) ^b where b=0.31±0.04. Note that the dependence obtained agrees rather good with the empiric expression of (Busse in Rep. Prog. Phys. 41:1929, 1978) and connecting the numbers Nu and Ra for turbulent convection. This gives grounds to conclude the heat transfer in a stratified solution is realized by transition to the regime of turbulent convection.      

Mutual Friction in Superfluid 4He Near the λ-Line

We present experimental results for the thermal resistivity of superfluid ⁴He along several isobars between saturated vapor pressure and the melting pressure. The measurements are for the temperature range 1–T _c(q)/T <t<2×10^–5 and the heat-flux range 3<q<70 W/cm². Here t1–T/T , T is the transition temperature in the limit of zero q, and T _c is the transition temperature at finite q. The data suggest that the resistivity has an incipient singularity at T which can be described by the power law =(t/t ₀)^–(m+) where t ₀=(q/q ₀) ^x . However, the singularity is supplanted by the transition to a more highly dissipative phase at T _c(q)<T . The results suggest a mild dependence of m+ on the pressure P, but can be described quite well by m+=2.76, x=0.89, and q ₀=q _{0, 0}–q _{0, 1} P with q _{0, 0}=401Wcm^–2 and q _{0, 1}=–5.0Wcm^–2bars^–1. The results imply that the Gorter–Mellink mutual friction exponent m has a value close to 3.46 and is distinctly larger than the classical value m=3.     

Nonuniform Electron–Hole Pairing in Graphene Bilayer

Electron–hole (e–h) pairing caused by Coulomb interaction in the system of independently gated graphene layers is considered. The influence of the mismatch of concentrations of e and h and trigonal warping of their spectrum on critical temperature is studied. We predict the appearance of the state with finite value of Cooper pair momentum–Larkin–Ovchinnikov–Fulde–Ferrell-like (LOFF) state at mismatch of the concentrations of e and above the critical value. Internal Josephson effect in LOFF-like state caused by interlayer tunneling of e and h is considered. We suggest a new phase-sensitive experiment based on the internal Josephson effect for probing spatial structure of order parameter in a LOFF-like state.     

Optical and Electron Spin Resonance Studies of Destruction of Porous Structures Formed by Nitrogen–Rare Gas Nanoclusters in Bulk Superfluid Helium

We studied optical and electron spin resonance spectra during destruction of porous structures formed by nitrogen–rare gas (RG) nanoclusters in bulk superfluid helium containing high concentrations of stabilized nitrogen atoms. Samples were created by injecting products of a radio frequency discharge of nitrogen–rare gas–helium gas mixtures into bulk superfluid helium. These samples have a high energy density allowing the study of energy release in chemical processes inside of nanocluster aggregates. The rare gases used in the studies were neon, argon, and krypton. We also studied the effects of changing the relative concentrations between nitrogen and rare gas on thermoluminescence spectra during destruction of the samples. At the beginning of the destructions, \(\alpha \)-group of nitrogen atoms, Vegard–Kaplan bands of \(\hbox {N}_2\) molecules, and \(\beta \)-group of O atoms were observed. The final destruction of the samples were characterized by a series bright flashes. Spectra obtained during these flashes contain M- and \(\beta \)-bands of NO molecules, the intensities of which depend on the concentration of molecular nitrogen in the gas mixture as well as the type of rare gas present in the gas mixture.     

Electrical Resistivity of the Mg(B1−x C x )2 Superconductors: Role of Electron–Electron, and Electron–Phonon Interactions

In this paper, we undertake a quantitative analysis of temperature-dependent resistivity Mg(B_1?x C _x )₂ superconductors. Due to inherent two energy gaps, the elastic scatterings of electrons from impurities have first been estimated and within a two-band picture, the impurity-limited resistivity due to π band carriers $\rho_{0}^{\pi} $ is larger as compared to the contribution from σ band carriers. An effective inter-ionic interaction potential (EIoIP) with the long-range Coulomb, van der Waals interaction and the short-range repulsive interaction within the Hafemeister and Flygare approach have allowed us to determine the Debye and Einstein temperature. An investigation exhibiting the mechanism of Mg(B_1?x C _x )₂ (0.0≤x≤0.125) was accomplished by comparing to the resistivity estimated by considering both phonons, with that of reported metallic resistivity, accordingly ρ _diff.=[ρ _exp.?{ρ ₀+ρ _e-ph (=ρ _ac+ρ _op)}] have been analysed through electron–electron scattering. The quadratic temperature dependence of $\rho_{\mathrm {diff}} = [\rho_{\mathrm{exp}.} - \{\rho_{0} + \rho_{\text{e-ph}}\ (= \rho ^{\sigma}_{\text{e-ph}} + \rho^{\pi}_{\text{e-ph}})\}]$ is understood in terms of inelastic electron–electron scattering. The comparison of transport parameter with single crystal data appears consistent within the two-band scheme for resistivity that we have presented.