Superconducting current in hybrid heterojunctions of metal-oxide superconductors: Size and frequency dependences

We have detected experimentally considerable deviations of the frequency dependences of the Shapiro step amplitudes and the critical current of Nb/Au/YBa₂Cu₃O_x thin-film hybrid Josephson heterojunctions prepared on YBa₂Cu₃O_x metal-oxide superconductor films with a tilted c axis from the regularities inherent in Josephson junctions of traditional superconductors with an s-symmetry of the order parameter. It is shown that possible formation of “splintered” fluxons with a size λ_s<λ_J due to faceting of the interface and formation of a chain of nanosize 0 and π junctions must be taken into account in describing processes in lumped heterojunctions (whose size L is smaller than the Josephson penetration depth λ_J determined from the averaged value of the critical current density). For heterojunctions with a size λ_s < L < λ_J, a substantial decrease in the maximal amplitude of the first Shapiro step with increasing voltage (Josephson oscillation frequency) is observed at voltages much smaller than the energy gap in niobium (V « Δ_Nb/e); this effect is manifested most strongly when the size L is greater than λ_s. A fractional Shapiro step and a subharmonic detector response have been observed in the current-voltage characteristics of heterojunctions; the dynamic processes responsible for their emergence and indicating the presence of the second harmonic in the current-phase relation are studied. It is shown that the effect of interface faceting on the current-phase relation increases with a heterojunction size L>λ_s.     

Hot-wire measurements of the electron thermal conductivity in a potassium plasma

The electron thermal conductivity λ_e of a potassium plasma is found by measuring the electron heat flux in a hot-wire device placed in a magnetic field. In the range 2300–3000 K, λ_e (W/mK) = 0.906 X 10¹⁰ T^-2_e exp(-3.2 X 10⁴/T_e) at a plasma pressure of 800 Pa and a gas temperature T_a = T_e/1.3.     

Multiple andreev reflections spectroscopy of superconducting LiFeAs single crystals: Anisotropy and temperature behavior of the order parameters

The superconducting state of LiFeAs single crystals with the maximum critical temperature T _c ≈ 17 K in the 111 family has been studied in detail by multiple Andreev reflections (MAR) spectroscopy implemented by the break-junction technique. The three superconducting gaps, Δ_Γ = 5.1–6.5 meV, Δ_L = 3.8–4.8 meV, and Δ_S = 0.9–1.9 meV (at T ? T _c), as well as their temperature dependences, have been directly determined in a tunneling experiment with these samples. The anisotropy degrees of the order parameters in the k space have been estimated as <8, ～12, and ～20%, respectively. Andreev spectra have been fitted within the extended Kümmel-Gunsenheimer-Nikolsky model with allowance for anisotropy. The relative electron-boson coupling constants in LiFeAs have been determined by approximating the Δ(T) dependences by the system of the two-band Moskalenko and Suhl equations. It has been shown that the densities of states in bands forming Δ_Γ and Δ_L are approximately the same, intraband pairing dominates in this case, and the interband coupling constants are related as λ_ΓL ≈ λ_LΓ ? λ_SΓ, λ_SL.     

Experimental and theoretical investigation of fmr in (111) thin garnet films with applied uniaxial stress

Analytical expressions for the resonance field shiftδH caused by an applied stressσ are deduced forH _e ∥ [¯1¯12] in the film plane, taking into account also the quadratic terms (～σ²) besides the dominant linear terms (～σ). The linear part ofδH=f(σ) forH _e andσ lying in the film plane depends only on the relative angle betweenH _e andσ, so that for the measurement of magnetostriction constants it is not necessary to have crystallographically oriented thin films.δH forH _e perpendicular to the film is very sensitive to deviations from the perpendicular direction. Both magnetostriction constants λ₁₁₁ and λ₁₀₀ were evaluated on (111) thin YIG ∶ Mn film.     

Saturated absorption spectroscopy: Elimination of crossover resonances with the use of a nanocell

It is demonstrated that the velocity-selective optical pumping/saturation resonances of the reduced absorption in a Rb vapor nanocell with thickness L = λ, 2λ, and 3λ (resonant wavelength λ = 780 nm) allow for the complete elimination of crossover (CO) resonances. We observe well-pronounced resonances corresponding to the F _g = 3 → F _e = 2, 3, and 4 hyperfine transitions of the ⁸⁵Rb D₂ line with line widths close to the natural width. A small CO resonance located midway between F _g = 3 → F _e = 3 and F _g = 3 → F _e = 4 transitions appears only for L ≥ 4λ. The D₂ line (λ = 852 nm) in a Cs nanocell exhibits a similar behavior. From the amplitude ratio of the CO and VSOP resonances, it is possible to determine the thickness of the column of alkali vapor in the range of 1–1000 μm. The absence of the CO resonances for nanocells with L ～ λ is attractive for the frequency reference application and for studying the transitions between the Zeeman sublevels in external magnetic fields.     

On the octupole excitation in236U

Measurements of theK,L _I,L _II andM _I conversion lines of the 687.7 keV transition in²³⁶U are evaluated within the electron penetration formalism. The spin-parity assignment of the octupole bandhead is found to be 1^? in accordance with reaction data, and an assignment of 2^? to the 687.7 keV state is ruled out. The penetration matrix element ∥η∥ has the value of 13.5 for theK-shell and increases slightly for higher main shells. An estimate of the anomalous amplitudes is compared with values reported for transitions in the odd even actinide nuclei. Furthermore electron conversion data for the 1^?→2⁺ and 1^?→4⁺ transitions are given. Radioactivity²³⁶U from²³⁵U(n,e ^?); measured: conversion electron decay; deduced: conversion coefficients fromK, L andM shells; evaluated: dynamic matrix elements.     

Pressure dependence of the thermal conductivity of aluminium

The thermal diffusivity, a, of aluminium has been measured at pressures up to 2.5 GPa at room temperature, and from these results the pressure dependence of the thermal conductivity, λ, has been calculated. Both quantities increase with pressure. The increase in a amounts to 4.6% to 1 GPa and 10.4% to 2.5 GPa. The initial pressure coefficient of the electronic thermal conductivity λ_e is found to be [λ_e]^-1?λ_e/?P = 3.7 × 10^-2GPa^-1, which agrees very well with a recent theoretical calculation.     

Features of faraday rotation in cs atomic vapor in a cell thinner than the wavelength of light

Features of the effect of Faraday rotation (the rotation of the radiation polarization plane) in a magnetic field of the D ₁ line in Cs atomic vapor in a nanocell with the thickness L varying in the range of 80–900 nm have been analyzed. The key parameter is the ratio L/λ, where λ = 895 nm is the wavelength of laser radiation resonant with the D ₁ line. The comparison of the parameters for two selected thicknesses L = λ and λ/2 has revealed an unusual behavior of the Faraday rotation signal: the spectrum of the Faraday rotation signal at L = λ/2 = 448 nm is several times narrower than the spectrum of the signal at L = λ, whereas its amplitude is larger by a factor of about 3. These differences become more dramatic with an increase in the power of the laser: the amplitude of the Faraday rotation signal at L = λ/2 increases, whereas the amplitude of the signal at L = λ almost vanishes. Such dependences on L are absent in centimeter-length cells. They are inherent only in nanocells. In spite of a small thickness, L = 448 nm, the Faraday rotation signal is certainly detected at magnetic fields ≥0.4 G, which ensures its application. At thicknesses L < 150 nm, the Faraday rotation signal exhibits “redshift,” which is manifestation of the van der Waals effect. The developed theoretical model describes the experiment well.     

Waves in a superlattice with anisotropic inhomogeneities

Dependences of the dispersion laws and damping of waves in an initially sinusoidal superlattice on inhomogeneities with anisotropic correlation properties are studied for the first time. The period of the superlattice is modulated by the random function described by the anisotropic correlation function K_?(r) that has different correlation radii, k _∥ ^?1 and k _⊥ ^?1 , along the axis of the superlattice z and in the plane xy, respectively. The anisotropy of the correlation is characterized by the parameter λ=1?k_⊥/k_∥ that can change from λ=0 to λ=1 when the correlation wave number k⊥ changes from k_⊥=k_∥ (isotropic 3D inhomogeneities) to k_⊥=0 (1D inhomogeneities). The correlation function of the superlattice K(r) is developed. Its decreasing part goes to the asymptote L that divides the correlation volume into two parts, characterized by finite and infinite correlation radii. The dependences of the width of the gap in the spectrum at the boundary of the Brillouin zone δν and the damping of waves ξ on the value of λ are studied. It is shown that decreasing L leads to the decrease of δν, and increase of ξ, with the increase of λ.     

Effective collision strengths among the fifteen lowest states of FeXVII

Collision strengths have been calculated for electron impact excitation of neon-like Fe XVII for all transitions within its 15 lowest states. Configuration interaction wavefunctions have been used to represent the target states. The standardR-matrix code has been used to calculate the contribution from the lower scattering partial waves (L⩽9), while the no exchange version of the same code has been used to compute efficiently the contribution of higher partial waves (L⩾10). Effective collision strengths for all the 105 transitions are tabulated for elected temperatures in the range logT _e=5.40 to logT _e=7.00 withT _e expressed in °K.     

Stochastic saturation of three-drift-wave interaction at finite parallel wavelength

The interaction of three drift waves is considered at finite parallel wavelength. The coupling to ion sound waves has a profound effect on the saturation levels. For k_|L_n～ 1. turbulent amplitudes are of the magnitude $\text{eΦ?T}{}_{\mathrm{e}}\text{～}\text{1}\text{k}{}_{\mathrm{\perp }}\text{L}{}_{\mathrm{n}}$ as predicted by usual estimates, in contrast to previous studies neglecting this effect.     

Nonisotropic L-conversion-electron — X-ray directional correlations in160Dy

The directional correlations betweenL-conversion electrons andL x-rays have been measured. The electrons were emitted in the 86.78keV 2⁺→0⁺ transition in¹⁶⁰Dy, in the decay of¹⁶⁰Tb. The experimental results areA ₂₂(e _L -x _Li )=0.085 (22),A ₂₂(e _L -x _Lα )=0.0066 (29),A ₂₂(e _L -x _Lβ )=0.0000 (27), andA ₂₂(e _L -x _Lγ )=0.0096 78). These results are in disagreement with published theoretical values. It is suggested that the discrepancy is due to a phase inconsistency in the internal conversion matrix elements and there is good agreement withA ₂₂ values calculated in this work.     

Evaluation of XPS-data of oxide layers

XPS with variable take-off angle has been applied to the determination of the thickness of thin oxide layers (SiO₂ on Si). The information about D/λ_ox gained by such measurements is strongly influenced by surface roughness. This influence can be demonstrated when the parameters R and D/λ_ox are calculated from sets of the experimental results and for each set the corresponding pair of R and D/λ_ox is plotted in a diagram D/λ_ox = f(R). Having the true value of R it is possible to determine D/λ_ox. With the XPS-results of at least three different oxide layers and their ellipsometric thicknesses D_e one is able to calculate λ_Si, λ_ox/λ_Si and the difference ΔD between D_e and the XPS-thickness D.     

Collisional excitation among the seven lowest states of TiXII

Collision strengths have been calculated for electron impact excitation of sodium-like TiXII for all 21 transitions amongst its lowest seven states. Configuration interaction wave functions have been used to represent the target states. The standard and no-exchangeR-matrix codes have been used to calculate the contribution of partial waves withL⩽8 andL>8 respectively. Collision strengths are tabulated at selected energies in the range 26 to 50 Ryd. Effective collision strengths are tabulated for electron temperatures in the range logT _e=4.0 to logT _e=6.0, withT _e in K. This is the first detailed calculation on this ion in which the effects of exchange, channel couplings and short-range correlation are taken into account.     

Selective reflection from Rb vapor in half- and quarter-wave cells: Features and possible applications

The features of the effect of selective reflection from rubidium vapor in a nanocell with the thickness L ≈ λ/2 and L ≈ λ/4, where λ = 795 nm is the wavelength of laser radiation resonant with the Rb D₁ line, are studied. It is shown that, because of the behavior of the nanocell as a low-Q-factor Fabry–Pérot etalon, the sign of the derivative of the selective reflection spectra changes near L ≈ λ/2 from negative at L > λ/2 to positive at L < λ/2. The simplicity of the experimental implementation, large amplitude, and sub-Doppler width (40MHz) of a detected signal at an atomic transition frequency are appropriate for applications in metrology and magnetometry. In particular, selective reflection from the nanocell is a convenient frequency marker of atomic transitions; in this case, the amplitudes of peaks are proportional to the transition probabilities. The remote optical monitoring of a magnetic field with a spatial resolution L = λ/4 ≈ 199 of nm is possible on the basis of the splitting of selective reflection peaks in a strong magnetic field (up to 3 kG). A theoretical model describes well the experimental results.     

Investigation of atomic transitions of cesium in strong magnetic fields by an optical half-wavelength cell

It has been demonstrated that the use of the λ/2 method allows one to effectively investigate individual atomic levels of the D ₂ line of Cs (with the most complicated spectrum among all alkali metals) in strong magnetic fields up to 7 kG. The method is based on strong narrowing of the absorption spectrum (which provides sub-Doppler resolution) of a cesium-filled thin cell with the thickness L equal to a half-wavelength (L = λ/2) of the laser radiation (λ = 852 nm) resonant with the D ₂ line. In particular, the λ/2 method has allowed us to resolve 16 atomic transitions (in two groups of eight atomic transitions each) and to determine their frequency positions, fixed (within each group) frequency slopes, the probability characteristics of the transitions, and other important characteristics of the hyperfine structure of Cs in the Paschen-Back regime. Possible applications are mentioned. Two theoretical models have been implemented. The values of the magnetic field have been indicated at which the models describe the experiment well.     

Magnetostriction measurements of Ni-Zn ferrite single crystals

The magnetostriction constants λ₁₀₀ and λ₁₁₁ of Ni-Zn ferrite Ni_1?xZn_xFe₂O₄ single crystals with x equal to 0, 0.07, 0.17, 0.40 and 0.67 were measured by a three terminal capacitance method from liquid N₂ to room temperatures.The constants λ₁₀₀ and λ₁₁₁ of Ni ferrite at liquid N₂ temperature were ?43.0 × 10^?6 and ?20.1×10^?6 respectively.The magnitude of the constant λ₁₀₀ increased with the increase of Zn concentration and took a maximum at x of about 0.33, while the magnitude of the constant λ₁₁₁ decreased almost linearly with the increase of Zn concentration and then became zero at x about 0.75.     

Leptonic polarization asymmetries ate + e − colliders inE 6 theories

We examine the left-right and azimuthal polarization asymmetries of lepton pairs produced ine ⁺ e ^? ande ^? e ^? interactions as probes of new physics resulting fromE ₆ theories. The asymmetries are calculated and compared for the processese ⁺ e ^?→µ ⁺ µ ^?,e ⁺ e ^?→e ⁺ e ^?,e ^? e ^?→e ^? e ^?, ande ⁺ e ^?→L ⁺ L ^? whereL is either anE ₆ exotic, mirror, or fourth generation heavy lepton. Our results show that lepton polarization asymmetries can be used to easily distinguish differentE ₆ models and can be used to distinguish among possible new heavy leptonsL.     

Theoretical Investigation of the Faraday Rotation Effect in Atomic Alkali Nano-Layers

Theoretical studies of the Faraday rotation (FR) effect in alkali vapors contained in extremely thin cells are presented. It is shown that the spectra of the FR signal are well frequency resolved despite the huge number of atomic transitions. This allows one to study the evolution of the Cs D₁ (λ = 895 nm) line hyperfine structure of F_g = 4 → F_e = 3, 4 atomic transitions in magnetic fields. The presented theoretical model predicts the coherent Dicke narrowing effect and its revival with a periodicity Δ_L = λ. The practical applications of the FR are noted.     

Magnetostriction domain structure in a periodic system of magnetoelastic and elastic nonmagnetic layers

The spectrum of magnetoelastic waves in a periodic structure of alternating ferromagnetic and nonmagnetic layers was studied. In the case of ferromagnetic layers with easy magnetization axes parallel to the layer surfaces, an orientational phase transition induced by an external tangential magnetic field H_e was considered. The formation of an inhomogeneous phase with a spatially modulated order parameter, which is caused by the magnetization being coupled through magnetostriction to lattice strains near the interfaces separating the magnetoelastic from elastic media, is predicted. It is shown that at a certain critical field in excess of the orientational phase transition field in the system without magnetostriction, a magnetoelastic wave propagating in a direction parallel to the in-plane magnetization vector M becomes unstable at finite values of the wave vector and condenses into a magnetostriction domain structure. A phase diagram in the (L, T, H_e) coordinates is constructed, and the regions of existence of thermodynamically equilibrium collinear, canted, and domain phases are established (L and T are the thicknesses of the ferromagnetic and nonmagnetic layers, respectively).