Observation of the “Inverse” spin valve effect in a Ni/V/Ni trilayer system

An experimental study of magnetic and superconducting properties of a trilayer Ni/V/Ni thin film system grown on single-crystalline MgO(001) substrate is reported. The field dependence of the superconducting transition temperature T _c for samples comprising Ni layers with similar values of the coercive field H _c reveals no anomalies. However, in samples with different thicknesses of the nickel layers the difference in H _c amounts up to ΔH _c ∼ 1.8 kOe, thus enabling to manipulate the relative orientations of the layers’ magnetization by an external magnetic field. Surprisingly, for these samples the T _c for the parallel orientation of the magnetizations of the Ni layers is higher, in a certain magnetic field range, than for the antiparallel one, at odds with theoretical predictions. Possible reasons of this contradiction are discussed.     

Ultrasound and vortex oscillations in high-temperature superconductors

An HTSC powder sample with grain (particle) diameter of 20–50 μm placed in a dc magnetic field B ₀ and cooled to a temperature below the superconducting transition temperature was exposed to the radiofrequency (rf) pulsed magnetic field B _∼ (B _∼ ⊥ B ₀) at a carrier frequency of 30.7 MHz. Stable echo signals were recorded which followed different rf-pulse trains. This phenomenon has the following mechanism. The rf magnetic field stimulates fluxoid oscillations on the HTSC grain surface, which are transformed into lattice oscillations through the pinning centers and induce a propagating sound wave. The second-order nonlinearity with respect to the gradient of the crystal lattice deviation from the equilibrium position taken into account in the sound wave equation yields the dependence of the crystal lattice natural frequency on the amplitude and length of the pulses which excite these oscillations. This dependence is responsible for the emergence of echo signals.     

Metal-insulator crossover in high T c cuprates: A gauge field approach

A metal-insulator crossover appears in the experimental data for in-plane resistivity of underdoped cuprates and a range of superconducting cuprates in the presence of a strong magnetic field suppressing superconductivity. We propose an explanation for this phenomenon based on a gauge field theory approach to the t-J model. In this approach, based on a formal spin-charge separation, the low energy effective action describes gapful spinons (with a theoretically derived doping dependence of the gap m _s ² ∼ δ| ln δ|) and holons with finite Fermi surface (ɛF ∼ tδ) interacting via a gauge field whose basic effect on the spinons is to bind them into overdamped spin waves, shifting their gap by a damping term linear in T, which causes the metal-insulator crossover. The presence of a magnetic field perpendicular to the plane acts by increasing the damping, in turn producing a big positive transverse in-plane magnetoresistance at low T, as experimentally observed.     

Resistance and magnetic susceptibility of superconducting lead embedded in nanopores of glass

This paper reports on a study of the resistance and differential magnetic susceptibility χ _ac of lead embedded in nanosized glass pores with a diameter of ∼7 mm, which was performed at temperatures of 6–300 K and magnetic fields of up to 6 T. The field dependence of the resistance R(H) and the temperature dependences of the real, χ″(T), and imaginary, χ″(T), parts of magnetic susceptibility reveal indications of superconducting phase transitions associated with the volume and surface superconductivity of Pb nanopar ticles. The measurements of the field dependence of resistance have been used to set up the H _c -T _c phase diagram and to carry out a comparison with the study of the heat capacity performed on the same samples.     

Dynamic magnetic permeability of a thin, high-T c superconducting wafer

The field dependence of the vibrational contribution to the dynamic magnetic permeability μ _V(H) is calculated for a thin (of thickness d∼λ) high-T _c superconducting wafer in a magnetic field parallel to the surface. The resulting curves are plotted on the basis of an exact numerical analysis of the vortex structures both for the thermodynamic-equilibrium vortex lattice and in the presence of pinning forces and the Bean-Livingston surface barrier. It is shown that the μ _V(H) curves are highly sensitive to the size factor (d/λ) and exhibit abrupt changes corresponding to a change in the number of vortex rows. The equilibrium μ _V(H) curve is found to be similar in its general behavior and absolute value (obtained with allowance for the distribution of grain sizes and with appropriate values of λ and ϰ) to the experimental μ _V(H) curve plotted at nitrogen temperature for fine-grained YBa₂Cu₃O_x with grain diameters 〈D〉∼λ in an increasing magnetic field. It is established that the main cause of the experimentally observed irreversible behavior of the μ _V(H) curves during cyclic variation of the applied magnetic field is the existence of a surface barrier to the exit of vortices from the superconductor. The lower limit H _min(B) of stability of the mixed state in the presence of an ideal surface barrier in a thin, high-T _c superconducting wafer (d∼λ) is determined, along with the range of the vortex state (H _max-H _min) for a fixed number of vortices in micrometer-size grains of the investigated YBaCuO samples. Fiz. Tverd. Tela (St. Petersburg) 39, 1943–1947 (November 1997)     

Plasma parameters in the upgraded Trimyx-M Galathea

Results are presented from measurements of the plasma parameters in the upgraded Trimyx-M Galathea. After the barrier magnetic field and the energy of the injected hydrogen plasma bunch were increased to B _bar ∼ 0.1 T and W ₀ ≈ 200 J, respectively, the following plasma parameters were achieved: the density n ∼ 5 × 10¹³ cm⁻³, the plasma confinement time τ* = 800–900 μs, the elergy of the confined plasma W ₁ ∼ 100 J, the ratio of the plasma pressure to the barrier magnetic pressure β ₀ ∼ 0.2, the electron temperature T _e ∼ 20 eV, and the ion temperature T _i ∼ 2T _e . The maximum time during which the plasma density decreased e-fold, τ _p , was found to be 300 μs at B _bar = 0.1 T, which agrees with the classical transport model.     

Interlayer magnetotransport study in electron-doped Sm2−x Ce x CuO4−δ

Vortex and pseudogap states in electron-doped Sm_2−x Ce _x CuO_4−δ (x ∼ 0.14) are investigated by the interlayer transport in magnetic fields up to 45 T. To extract intrinsic properties, we fabricated small 30 nm-high mesa structures, sufficiently thin to be free of the recently reported partial decomposition problems. On cooling, the c-axis resistivity ρ_c of the mesa structures reveals a semiconductive upturn above T_c, followed by a sharp superconducting transition at 20 K. When the magnetic fieldH is applied along the c-axis, ρ_c(T) shows a parallel shift without significant broadening, as also observed in the hole-doped underdoped cuprates. Above the transition we observe negative magnetoresistance (MR), which can be attributed to the field suppression of the pseudogap, whose magnitude is as small as 38 K. Our results in thex ∼ 0.14 samples closely correspond to the interlayer transport behavior in the ‘overdoped’ regime of hole-doped Bi₂Sr₂ CaCu₂ O_8+y.     

Effect of easy-plane anisotropy on the transient process time in magnetic films and plates

The effect of easy-plane anisotropy on the damping of the nonlinear magnetization oscillations that accompany 90° pulsed magnetization of magnetic films and plates is analyzed numerically. It is shown that the magnetization time can be decreased to ∼0.5 ns at an effective anisotropy field H _{K p} ≥ 6 kOe and that magnetization oscillations are fully damped at H _{K p} ≥ 20–40 kOe. The magnetization time can be ∼0.15–0.20 ns at a magnetizing pulse amplitude H _m ∼ 20–40 Oe.     

Paramagnetic intrinsic Meissner effect in a bulk

We calculate the free energy of a quasi-two-dimensional (Q2D) superconductor with ξ_⊥ < d in a parallel magnetic field, where ξ_⊥ is a perpendicular to the conducting layer coherence length and d is the interlayer distance. It is shown to be different from that in the famous Lawrence-Doniach model. In particular, at high enough magnetic fields, the Meissner currents are found to create an unexpected paramagnetic moment due to the shrinking of the Cooper pairs “sizes” in a direction perpendicular to the conducting layers. We suggest measuring this paramagnetic intrinsic Meissner effect in Q2D superconductors and superconducting superlattices. The text was submitted by the author in English.     

Effect of hydrostatic compression on the indium-impurity-induced superconducting transition in Pb<Subscript>0.3</Subscript>Sn<Subscript>0.7</Subscript>Te

This paper reports on a study of the low-temperature conductivity and parameters of the superconducting state, namely, the critical temperature T _c and the second critical magnetic field H_c2, in the (Pb_0.3Sn_0.7)_0.95In_0.05Te solid solution under hydrostatic pressure P ≤ 9 kbar at T = 4.2 K. The choice of this material has been motivated by the fact that, according to earlier observations, it undergoes a superconducting transition at T _c ∼ 2.3 K, i.e., close to the maximum value T _c ∼ 2.9 K found for the (Pb _z Sn_{1 − z} )_0.95In_0.05Te solid solutions with a lead content z ∼ 0.15–0.25. It has been demonstrated that an increase in the pressure to P ≤ 9 kbar leads to a bell-shaped dependence T _c (P). The observed dependences are assigned to the effect of hydrostatic compression on the band structure of the solid solution and indicate a shift in the position of the Fermi level E _F with increasing pressure within the impurity band of the In quasi-local states. In this case, E _F passes through a maximum in the density of impurity states at P = 3–5 kbar.     

Diffusion interaction between Al and Mg controlled by a high magnetic field

The effects of an 11.5 T high magnetic field on the growth behavior of compounds layers during reactive diffusion in solid Al/solid Mg diffusion couples have been investigated. After annealing with and without a magnetic field, the interfacial zone was still composed of two layers of stable compounds β (Mg₂Al₃) and γ (Mg₁₇Al₁₂), but the thicknesses of the layers were increased by the magnetic field. Data analysis of the thicknesses of compound layers showed that the growth of the γ layer was controlled by grain boundary diffusion as well as volume diffusion at B=0 T, but only controlled by volume diffusion when an 11.5 T high magnetic field was applied. The interdiffusion coefficients for the samples annealed at 643–693 K were calculated from the parabolic relationship between the migration of the interface and the annealing time. The results show that the diffusion constant increased due to the high magnetic field, increasing the chemical potential gradient, and in turn it caused the interdiffusion coefficient to increase.     

Spontaneous magnetization of the vacuum and the strength of the magnetic field in the hot Universe

Intergalactic magnetic fields are assumed to have been spontaneously generated at the reheating stage of the early Universe, due to vacuum polarization of non-Abelian gauge fields at high temperature. The fact that the screening mass of this type of fields has zero value was discovered recently. A procedure to estimate their field strengths, B(T), at different temperatures is here developed, and the value B(T _ew)∼10¹⁴ G at the electroweak phase transition temperature is derived by taking into consideration the present value of the intergalactic magnetic field strength, B ₀∼10⁻¹⁵ G, coherent on the ∼1 Mpc scale. As a particular case, the standard model is considered and the field scale at high temperature is estimated in this case. Model-dependent properties of the phenomena under investigation are briefly discussed, too.     

Muons in type-II superconductors: μ+ diffusion in ultra-pure niobiumdiffusion in ultra-pure niobium

The diffusivityD _μ of positive muons (μ⁺) in the mixed state of superconducting high-purity, high-perfection niobium single crystals is investigated by measurements of the relaxation of the transverse muon spin polarization (μ⁺SR). The method makes use of the strong magnetic field gradients existing in the mixed state of Type-II superconductors and monitorsD _μ through the variation of the magnetic field felt by the μ⁺ during their diffusion through the crystals. For μ⁺ near the centres of the flux lines inNb it givesD _μ(4.6 K)=(8±2)·10⁻¹¹m²S⁻¹. The positive temperature coefficient ofD _μ indicates that at liquid-helium temperatures the diffusivity of μ⁺ inNb is mainly due to phonon-assisted tunnelling processes.     

Effect of low-level Ge additions on the superconducting transition in PbTe:Tl

A study is reported of the effect of low-level germanium additions (∼0.01–0.1 at. %) on the parameters of the superconducting transition, viz. the critical temperature T _c, the second critical magnetic field H _c2, and in PbTe doped with 2 at. % Tl, which are derived from the dependence of the electrical resistivity of a sample on temperature (0.4–4.2 K) and magnetic field (0–1.3 T). The discontinuity revealed by experimental data is related to the onset of a Ge-induced structural phase transition. Fiz. Tverd. Tela (St. Petersburg) 40, 1204–1205 (July 1998)     

High-quality YBCO d.c. SQUIDs based on bicrystal substrates

Summary An analysis of superconducting transport properties and magnetic behaviour of d.c. SQUIDs employing YBCO bicrystal grain boundary junctions (GBJs) has been performed. GBJs have been obtained by deposition of ac-axis-oriented YBCO film on a SrTiO₃ bicrystal substrate by ICM sputtering technique. Experimental measurements on a YBCO d.c. SQUID with a misorientation angle θ=20° are reported. The SQUID shows a critical temperatureT _c∼89 K and a high critical current densityJ _c∼3·10⁶ A/cm² atT=4.2 K. Current-voltage characteristics are close to the behaviour predicted by the resistively shunted junction (RSJ) model and the temperature dependenceJ _c(T) shows a linear behaviour at small reduced temperatures and a depressedJ _c value close toT _c. High-quality flux-voltage curves have been found upT=87 K over a large range of magnetic field. The high reproducibility and the good control of transport properties by the variation of θ make YBCO bicrystal GBJs very useful for applications in electronics. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

On the lower critical field and phase diagram of a thin cylindrical type-II superconductor

The lower critical field H _c1 ^cyl (T) of a superconducting cylinder with radius r ₀∼ξ(T)≪λ(T) is found on the basis of the Ginzburg-Landau theory with various boundary conditions. These results together with the well-known results for the upper critical field are used to construct phase diagrams in terms of the field versus the reduced radius r ₀∼ξ(T) variables. The jump in the average magnetization at H _c1 ^cyl (T) is calculated as a function of the reduced radius. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 8, 537–542 (25 April 1999)     

Spin Hall magnetoresistance in Nb/Y3Fe5O12 hybrids

We investigate the spin Hall magnetoresistance (SMR) in niobium (Nb) attached to Y₃Fe₅O₁₂ near the superconducting critical temperature (T_c) of Nb. The SMR vanishes after cooling the sample below T_c, and recovers if the temperature is raised. When a magnetic field larger than the critical field of Nb is applied, the SMR re‐emerges with an enhanced magnitude even if the temperature is below T_c. The experimental results demonstrate that the SMR could be completely suppressed by the coupling between superconducting condensation and spin–orbit interaction in superconductors. In addition to the fundamental physics on the charge–spin interactions in superconductors, our work adds a different dimension to superconducting spintronics. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Anomalies of the specific heat near the quantum critical point in Tm<Subscript>0.74</Subscript>Yb<Subscript>0.26</Subscript>B<Subscript>12</Subscript>

The behavior of the specific heat near the quantum critical point x ∼ 0.3 in the Tm_{1 − x} Yb _x B₁₂ system has been studied. Detailed measurements have been performed on high-quality single-crystalline Tm_0.74Yb_0.26B₁₂ samples within a wide temperature range of 1.9–300 K in a magnetic field up to 9 T. The temperature dependence of the magnetic contribution to the specific heat has a logarithmic divergence of the form C/T ∼ lnT at T < 4 K, which can be attributed to the quantum critical behavior regime suppressed by the external magnetic field. The Schottky anomaly of the magnetic contribution to the specific heat in Tm_0.74Yb_0.26B₁₂ has been analyzed.     

Studies of the microstructure of a superconducting Nb<Subscript>3</Subscript>Sn-Band obtained from (Cu/Nb)/Cu12Sn nanocomposite

A multilayer layer from the superconducting compound Nb₃Sn was obtained by the thermal treatment of composite (Cu/Nb)/Cu12Sn conductor consisting of Cu-12% Sn and Cu/Nb interlayers consisting in turn of nanosized copper and niobium layers. The conductor was coated with copper from the outside, which served as a stabilizer. The microstructure of the cross section of the composite band in dependence on the volume ratio of Cu/Nb and bronze interlayers and the microstructure of the Cu/Nb interlayers themselves in dependence on the annealing temperature were investigated. The optimum ratio of composite components was as follows: t _Nb N _Nb ≈ 0.288t _CuSn N _CuSn, where t and N are the thickness and number of the niobium and bronze layers, respectively. Trying to maintain the optimum design of the conductor forced us to increase the bronze volume content relative to the content of Cu/Nb interlayers, which had a negative impact on the composite microstructure. This resulted in disruptions of Cu/Nb interlayers. A technique has been developed for producing the (Cu/Nb)/Cu12Sn composite as a precursor to the band from the Nb₃Sn compound with allowance for the first experiment’s shortcomings.     

Features of the superconducting state and structural disorder in ultrafine particles of the HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

We study the reasons for which the compound YBa₂Cu₃O _y begins to lose its superconducting ability with a decrease in particle sizes to values of ∼0.1 μm. Our analysis of the results of structural and magnetic studies has allowed us to reveal changes in the parameters of the crystal structure and of the superconducting state in small particles of an HTSC and to show that the main cause of these changes is a special kind of a structural disorder peculiar only to small particles of YBa₂Cu₃O _y and is realized as a consequence of the need for nonequilibrium conditions during their synthesis.