Effect of Short-Range Ferromagnetic Ordering on the Electrical Transport Properties of (La0.9Bi0.1)2/3Ca1/3MnO3 in the Magnetic Field up to 40 T

The temperature and magnetic field dependence of the resistance of polycrystalline sample (La_0.9Bi_0.1)_2/3Ca_1/3MnO₃ was investigated in pulsed high magnetic field up to 40 T over a wide temperature region. In order to describe the magnetotransport properties of the sample, an extended Mott hopping conduction model was employed. Instead of B _J (gμ _B J(T)B/k _B T) in the ferromagnetic (FM) state and $B_{J}^{2}(g\mu_{B}J(T)B/k_{B}T)$ in the paramagnetic (PM) state, the magnetic field dependence of the resistance could scale nicely with the Brillouin function B _J (gμ _B J(T)B/k _B T) in all the given temperatures (above and below the Curie temperature T _c ). The deduced value of the average spin moment J(T) increases with the increasing of the temperature and reaches the maximum value (～35) around T _c . The large value of J(T) indicate the presence of magnetic clusters in the sample which is induced by the short-range ferromagnetic ordering, however, compared to the un-doped compound La_2/3Ca_1/3MnO₃, the value of J(T) is much smaller, which might be ascribe to the location of the 6s² lone pair electrons of Bi³⁺ ions in the sample (La_0.9Bi_0.1)_2/3Ca_1/3MnO₃.     

Dependence of Superconductivity and Its Weakly Linked Behavior in Bulk LaO1?x F x FeAs on F Doping

Samples of oxypnictide compound LaO_1?x F _x FeAs, with x=0.15 and 0.2 corresponding to over- and highly over-doped compositions, respectively, were prepared by solid-state reaction. We present their characterization by XRD and HRTEM, as well as resistivity ??(T), magnetization M(B) and microwave modulated absorption (MMA) response between 4.2?C300?K and applied fields B=0?C8?T. With change in?x, both the superconducting and magnetic behavior of the samples shows an interesting pattern. The ??magnetic anomaly?? at T??130?K, observed in M(T) for x=0, instead of getting totally suppressed shows a tendency to reappear in x=0.2 sample. Both samples typically show ??(300?K)>2.8×10^?3????cm and critical current density J _c(5?K, 1?T)<2×10⁷?A/m². The superconducting transitions as measured by ??(T) at B=0 are found broad for both x=0.15 and 0.2 samples with transition widths ??2.5 and 6?K, respectively. The slope |dB _c2/dT| (where B _c2 is upper critical field) determined by resistive onsets, for the x=0.15 and 0.2 samples, has values ??7.5 and 3.5?T/K, respectively. The superconducting state characteristics as reflected by ??(T,B), M(T), magnetic J _c(B) and MMA response are typical of the presence of weakly linked inter-grain regions in both the samples. Our HRTEM images of the x=0.15 sample show the presence of high angle (??43^°) grain boundaries, which are well known to limit the J _c in cuprate-based high T _c bulk materials.     

Comparative Study of Dense Bulk MgB2 Materials Prepared by Different Methods

We report on the results of a comparative investigation of highly dense bulk MgB₂ samples prepared by three methods: (i) hot deformation; (ii) high pressure sintering; and (iii) mechanical alloying of Mg and B powders with subsequent hot compaction. All types of samples were studied by AC susceptibility, DC magnetization, and resistivity measurements in magnetic fields up to μ₀ H = 160 kOe. A small but distinct anisotropy of the upper critical field $\psi {\rm H}_{c2}^{\alpha ,b} /H_{c2}^c \sim 1.2$ connected with some texture of MgB₂ grains was found for the hot deformed samples. The samples prepared by high pressure sintering as well as by mechanical alloying show improved superconducting properties, including high upper critical fields H _c2 (μ₀ H _c2 (0) ～ 23 T), irreversibility fields H _irr which are strongly shifted towards higher values H _irr(T) ～ 0.8 H _c2(T) and high critical current J _c (J _c = 10⁵ A/cm² at 20 K and 1 T).     

Superconducting properties of internal-tin route Nb3Sn wires with radially arranged filaments : Development that realizes high Jc and low hysteresis loss

An internal-tin route Nb₃Sn superconducting wire that has both remarkably low hysteresis loss (Q_h) and high critical current density (J_c) was developed according to a new design idea. The wire was constructed by arranging the filaments in a radial layout, enlarging the outer filaments along the radial direction, narrowing the filament spacing in the radial direction intentionally and enlarging the filament spacing in tangential direction. Thus, the electromagnetic coupling among the filaments in tangential direction due to the bridging and/or proximity effect was suppressed without decreasing the volume fraction of Nb. As a result, excellent properties such as J_c(12 T) = 1.15 × 10³ A/mm² and Q_h = 301 mJ/cm³ (for 1 cycle of B = ±3 T) were obtained. We also evaluated the transition temperature (T_c) and upper critical field (B_c2) of the wire. The values for T_c and B_c2 were 17.3 K and 24.1 T, respectively, which were much better than those of usual internal-tin route wires. Moreover, electron probe micro-analyses confirmed that the good T_c and B_c2 were the result of the qualitative improvement of the Nb₃Sn compound based on the effects of arranging the Nb filaments radially, increasing the ratio of Sn-to-Nb and shortening the diffusion length for Sn. This wire is promising for use with conduction-cooled high-field magnets, in which there is a need to decrease the load of the cryocooler, and also for the strands of fusion coils.     

Comparison of projected critical currents in PbMo6S8 and Nb3Ge

Critical current densities, J_c, of sputtered Chevrel phase PbMo₆S₈ films have been measured as a function of field to 19 T at several temperatures. The pinning forces were found to obey a scaling law. Assuming an effective upper critical field B_c2 = 45 T, an effective critical temperature T_c = 13 K, together with empirical estimates of numerical factors in the scaling law, we estimate the upper limits for critical current density J_c in PbMo₆S₈. Comparisons are made to estimates of J_c for Nb₃Ge at 4.2 K. A crossover of J_c 10⁸ A m^?2) vs B is found for B of the order of 25 to 30 T. Below this point, Nb₃Ge is projected to have a higher critical current density. Thus practical use of Chevrel materials in high field magnets only appears to be competitive with Nb₃Ge for fields above 25 to 30 T.     

Fabrication of a highly sensitive magnetic sensor using an HTS thick film

A highly sensitive magnetic sensor based on Y₁Ba₂Cu₃O_7−x (YBCO) has been fabricated by the spray technique. The value of resistance R_ms in the transitional region ΔT of the thick film, was highly sensitive to the applied magnetic field B_ex. Here, ΔT is the transitional temperature region between the resistance drop temperature T_c^onset and residual resistance temperature T_c^res. The value of residual resistance R_res for an applied magnetic field of 80×10⁻⁴ T (80 G) exhibited an increase of about 580% over the resistance found in a zero magnetic field, and did not display any evidence of hysteresis. The fabrication and magnetic characteristics are discussed.     

Normal-State Optical Response Functions of MgB2 Superconductor

The reflectivity of superconducting MgB₂ (T _c = 39 K) has been measured on a randomly oriented thin film at room temperature over a wide-range of frequencies, 20 ≤ ω < 100000 cm^?1. The conductivity shows highly metallic behavior but cannot be explained with a simple Drude model alone. The electronic contribution is analyzed by a generalized Drude model. The scattering rate 1/τ(ω) and the mass renormalization ratio m*(ω)/m = 1 + λ (ω) exhibit clear frequency dependence. The electron–phonon coupling strength is estimated to be λ ～ 1.5 ± 0.5 while the plasma frequency ω_p is 2.4 eV.     

Pair and Quasiparticle States of YBa2Cu3O7−x Films Deduced from the Surface Impedance and a Comparison with Nb3Sn

Surface impedance data at 19 and 87 GHz of high-quality epitaxial YBCO films on different substrates are compared with data for Nb₃Sn films on sapphire in terms of pair and quasiparticle (qp) transport. Surface resistance R _s and penetration depth λ of YBCO are strongly affected by temperature dependent qp scattering, which is depressed in films with enhanced lattice strain. All films showed a comparable residual resistance R _res(19 GHz)～90 μΩ constituting a qp reservoir which is likely to be caused by the electronic configuration and by impurities. Subtracting R _res from R _s (T) revealed activated behavior with a reduced energy gap Δ₀/k _B T _c～0.9 for a film on sapphire, but power-law behavior for the other films. The penetration depth did not reveal power-law dependences at T≤0.5 ·T _c, but was consistent with a reduced energy gap of 0.45 for a film on MgO. The increase of λ(T) at T≥0.5 · T _c was related to qp scattering, which also caused an extremal conductivity σ₁(T). A shoulder in λ(T) at T=(0.6–0.7) · T _c confirmed evidence for the existence of two superconducting bands. The magnetic-field induced recovery of λ(B) of various YBCO films hinted for an important role of magnetic scattering. The results are in contradiction to a d-wave symmetry of the order parameter, at least for the chain band.     

Spin waves in anisotropic superfluid3He

Spin waves in the A phase of superfluid³He are studied theoretically. It is assumed that the condensate in the A phase consists of theP-wave triplet pairs of the type suggested by Anderson and Brinkman. We show that the spin wave mode with a finite energy gap exists throughout the A phase. In particular atT=0 K the dispersion of the spin wave is given by, forvq<Δ, $$\omega ^2 = \omega _0^2 + 2(1 - \bar I)A\Delta ^2 + \tfrac{1}{3}(1 - \bar I)v^2 q^2 $$ where θ is the spin wave energy, ω _T =[ω ₀ ² +2(1?ī)AΔ²]^1/2 is the nuclear magnetic resonance energy,v is the Fermi velocity,q is the wave vector of the spin wave, andī=IN(0)=?1/4Z ₀ is the Landau-Fermi liquid parameter.     

Magnetic properties of proximity-induced superconducting matrix in multifilamentary wires

Critical current densities J_cp of proximity-induced superconducting matrices in NbTi multifilamentary wires are estimated from measured twist-pitch dependence of magnetization. The values of J_cp are 2–4 orders of magnitude smaller than those of the superconducting filaments and decrease rapidly by raising temperature T and magnetic field B_e. The interfilamentary spacing d_N of submicrons results in zero screening length at measured temperatures ranging 4.5 to 8 K and the induced superconductivity is suggested to be type-II. The upper critical field B_c2p is obtained by applying the scaling law to J_cp data. B_c2p's at 4.5 K are 1.2–3.0 T for d_N = 0.20–0.59 m. The Cooper pair penetration length K _N ^–1 estimated from the temperature dependence of B_c2p shows the clean limit characteristic as T^–1 and 0.32 m for d_N = 0.59 m wire at 4.5 K. K _N ^–1 estimated from the temperature dependence of J_cp agrees consistently with those from B_c2p(T).     

Dispersion of the Nonlinear Optical Susceptibility in Single‐Wall Carbon Nanotubes

Abstract

We have calculated the dispersive behavior of the third‐order nonlinear susceptibility χ⁽³⁾ (?ω₃; ω₁, ω₁, ?ω₂) in single‐wall carbon nanotubes (SWCNTs) responsible for optical frequency mixing of the form ω₃ = 2ω₁ ? ω₂. The dispersive resonance of χ⁽³⁾ (?ω₃; ω₁, ω₁, ?ω₂) is shown to occur when the frequency ω₃ = 2ω₁ ? ω₂ is generated near the band gap of SWCNTs due to three‐wave mixing. A very sharp peak of χ⁽³⁾ (?ω₃; ω₁, ω₁, ?ω₂) reaching extremely high values ～10^?4 esu at the newly generated frequency ω₃ can be observed when not only ω₃ is close to the band gap of the system, but also when the frequencies of incident fields are near such a resonance.     

Magnetic field dependence of the critical current density for the bismuth-based bulk high-Tc superconductors

Three types of bismuth-based bulk samples were prepared through uniaxial pressing at room temperature, hot isostatic pressing (HIP) and drawing and rolling. Transport current properties were characterized in a steady field up to 1.12 T at 77 K (T/T _c=0.75). The Josephson weak-link decoupling fields have been found to be 5 mT for the cold-pressed pellet and 30 mT for the HIPed pellet and the rolled tape. At the decoupling field the transport critical current density,J _c, drops 80% from 124 (OT) to 29 A cm^–2 (5 mT) for the cold-pressed pellet, 80% from 582 (OT) to 126 A cm^–2 (30 mT) for the HIPed sample and 50% from6500 (0 T) to 2850 A cm^–2 (30 mT) for the rolled tape. In the flux flow regime, whereB is perpendicular to thec-axis a modified Kim's modelJ _c=(/B ₀)/[(1+B/B ₀)] ⁿ can be used to describe the field dependence of the critical current density, J_c, in the field range 0.2–1.12 T. The effective upper critical fields were estimated to be 0.98, 1.54 and 1.94 T for the three types of samples, respectively. An adjustable range ofB _c2 for bismuth-based bulk highT _c superconductors is given. Flux shear may operate in these materials. The prediction of this pinning mechanism is yielded from fitting the equation qualitatively. WhenB is parallel to thec-axis, the absence of strongly intragranular flux-pinning is emphasized by the poor flux flow regime for the rolled tape sample.     

Superconducting properties of iron chalcogenide thin films

Abstract

Iron chalcogenides, binary FeSe, FeTe and ternary FeTe_xSe_1?x, FeTe_xS_1?x and FeTe:O_x, are the simplest compounds amongst the recently discovered iron-based superconductors. Thin films of iron chalcogenides present many attractive features that are covered in this review, such as: (i) easy fabrication and epitaxial growth on common single-crystal substrates; (ii) strong enhancement of superconducting transition temperature with respect to the bulk parent compounds (in FeTe_0.5Se_0.5, zero-resistance transition temperature T_c0^bulk = 13.5 K, but T_c0^film = 19 K on LaAlO₃ substrate); (iii) high critical current density (J_c ～ 0.5 ×10⁶ A cm² at 4.2 K and 0 T for FeTe_0.5Se_0.5 film deposited on CaF₂, and similar values on flexible metallic substrates (Hastelloy tapes buffered by ion-beam assisted deposition) with a weak dependence on magnetic field; (iv) high upper critical field (～50 T for FeTe_0.5Se_0.5, B_c2(0), with a low anisotropy, γ ～ 2). These highlights explain why thin films of iron chalcogenides have been widely studied in recent years and are considered as promising materials for applications requiring high magnetic fields (20–50 T) and low temperatures (2–10 K).     

Kinetic Growth Properties of the Interface in Phase-Separated 3He-4He Liquid Mixtures: Crossover Between the Hydrodynamic and Ballistic Regimes

We developed a method to measure the transmission coefficient of sound for the interface between the c- and d-phases of a phase-separated ³He-⁴He liquid mixture at various temperatures and sound frequencies. From the data the kinetic growth coefficient of the interface ξ _ω(T), describing phase-conversion processes at the interface, can be determined.We have extended the experiment down to 6 mK with sound of 9, 14, and 32 MHz. The main specific features observed are as follows: (i) ξ _ω(T) showsa maximum at T _max(ω), (ii) above T _max(ω), ξ _ω(T) decreases with the increase of temperature as ξ _ω(T)=Aω ^5/2/T ³, and (iii) below T _max(ω), ξ _ω(T)decreases with the decrease of temperature and the frequency-dependence seems to disappear. That is, T _max(ω) corresponds to the crossover temperature between the hydrodynamic and ballistic regimes for ξ _ω(T).     

Transport Properties of the Iron-based Superconductor SrFe2(As,P)2 in High Magnetic Fields

We have investigated the resistive upper critical field (μ ₀ H _c2) of the iron-based superconductor SrFe₂(As_1?x P _x )₂ (x=0.35, T _c～29 K) in pulsed high magnetic fields of up to 52 T. For H∥ab, $\mu_{0}H_{\mathrm{c}2}^{ab}(T)$ exhibits the Werthamer-Helfand-Hohenberg (WHH) like behavior. While, for H∥c, $\mu_{0}H_{\mathrm {c}2}^{c}(T)$ is almost linear against T and exhibits a slightly upward curvature near T _c. The anisotropy of μ ₀ H _c2 decreases from 2.5 near T _c to ～1.2 at T=0 K monotonically. Similar isotropic behavior of μ ₀ H _c2 at low temperatures has also been observed in the Fe_1+δ (Te,Se). We demonstrate the results of the two-band analysis for μ ₀ H _c2(T) on the present sample and discuss the anisotropy of μ ₀ H _c2 by comparing with those in some kinds of iron-based superconductors.     

Crossover from Localized Spins to Weak Coupling Charge Carriers: Theory for Nuclear Spin-Lattice Relaxation in Copper Oxide HTSC

The dynamic spin susceptibility $\chi^{+,-}_{\mathrm{total}}(\omega,{\bf q})$ that takes into account the interplay of localized and itinerant charge carriers exhibits a diffusive-like, extremely narrow and sharp symmetric ring of maxima at very small wave vectors: $|{\bf q}|=q_{0}$ where q ₀∝ω/J≈10^?6 with the Nuclear Magnetic Resonance (NMR) frequency ω and the superexchange coupling constant J together with the peak at the antiferromagnetic wave vector Q=(π,π). The calculated plane copper ⁶³(1/T ₁) and oxygen ¹⁷(1/T ₁) nuclear spin-lattice relaxation rates from carrier-free right up to optimally doped La_2?x Sr _x CuO₄ are in good agreement with experimental data.     

Properties of screen-printed Ho-Ba-Cu-O films on YSZ substrates

High-T_c screen-printed Ho-Ba-Cu-O films were prepared on YSZ substrates by a melt processing method. The films were fired at T_s = 1000-1050 °C for 5 min and cooled to 450 °C by two steps in flowing O₂. The maximum critical current density J_c (77 K, 0 T) of 2.0 × 10³ A cm^− 2 was only attained under much limited firing conditions; T_s = 1020 °C and cooled to 800 °C at a cooling rate of 400 °C h^− 1.     

Transport Properties of a Tl-2201 Film Close to the Critical Temperature: The Vortex Glass Transition

We have studied the I-V characteristics of a Tl-2201 film at zero field. In the regime in which flux creep is the dominant dissipation mechanism, the J _c -T curve is divided into two parts at a temperature T _g (about 82 K), close to the critical temperature (84 K). The I-V characteristics around T _g are well described using a flux creep model. For T>T _g , J _c /J _c (0) =0.445x(l-0.525t-0.5t ² ); for T _g , J _c /J _c (0) = 0.9x(1-0.595t-0.44t ² ). Differential resistance (dV/dI) as a function of the measuring current shows a change in curvature close to T _g . The I-V curves collapsed nicely into two branches by plotting (V/I)/|T–T _g | ^v(z-1) vs. (I/T)/|T _g –T| ^2v , indicating a current–reduced vortex glass transition.     

FMR study of surface and isothermal annealing effects in amorphous Fe80B20−x C x and Fe82 B18−y Ge y alloys

Alterations in the bulk (surface) magnetic properties of amorphous Fe₈₀ B_20?x C _x (0 5x ? 10) and Fe₈₂ B_18?y Ge _y (0 ?y ? 6) alloys, caused by isothermal annealing (mechanical polishing) of the alloy ribbons at a temperature T _A = 0.5T _cr (whereT _cr is the crystallization temperature) for various lengths of time,t _a, ranging from 0 to 240 min, have been studied by measuring the corresponding changes in the ferromagnetic resonance (FMR) linewidth, AHPP, and resonance field,H _res. Polishing induces significant changes in bothH _res and ΔH _pp but leaves their composition dependence practically unaltered. By comparison, isothermal annealing has no influence onH _res for ali the investigated alloy compositions whereas it affects ΔH _pp to different extents depending on the alloy composition. Physical implications of these results are discussed in terms of the effect of surface and annealing treatments on various contributions toH _res and AHPP in the glassy alloys in question.     

Flow of superfluid3He through micrometer-size channels

Measurements of the critical mass currentJ _c through 0.8-µm-diameter channels in the superfluid phases of³He are reported. Experiments were made at pressures from 0 to 27.4 bar in zero external magnetic field. The pressure difference ΔP along the flow channels is immeasurable within our resolution of ±0.1 µbar for sufficiently low currents in both the A and B phases, implying small or zero dissipation. In the B phase ΔP grows rapidly with increasing current aboveJ _c. At low pressuresJ _c behaves like (1?T/T _cyl)^3/2, whereT _cyl is interpreted as the reduced superfluid transition temperature inside the flow channels;T _cyl/T _c=0.935 atP=0. If the liquid in the channels is in the A phase, the behavior of ΔP vs. the mass currentJ _s depends on the phase A or B outside the channels. During warming a drop of about30% inJ _c is found both atT _BA(cyl) and at;T _BA(cyl) is the reduced B→A transition temperature in the channels. AboveT _BA a second dissipation mechanism, with a smallerd(ΔP)/dJ _s, is observed at lower currents. These features indicate that in the A phase the ends of the channels have an important effect on the flow.