Dependence of the upper critical field on the defect concentration in MgB<Subscript>2</Subscript> and the electronic structure parameters

The upper critical field H _c 2 (H ‖ c) of the two-band superconductor MgB₂ is studied as a function of the residual resistivity ρ_n. It is found that the superconductor follows the standard trend: the slope-dH_c2/dT of the temperature dependence of H_c2(T) increases with the number of defects. The upper critical field in the clean limit is found, and direct estimations of the parameters of carriers in the 2D σ band (including the Fermi velocity and the coherence length) are made. The contribution of the electron scattering to the magnitude of H_c2 is determined, and the mean free path of electrons in samples with various defect concentrations is estimated. The density of states of σ electrons at the Fermi level is calculated using the dependence of the slope-dH_c2/dT on ρ_n and a band structure model. It is impossible to estimate this density of states directly, because the upper critical field is determined by the carriers of one band, whereas the resistivity depends on the carriers in both bands.     

Normal and superconducting properties of single-crystalline BaPb1−xBixO3

Hall coefficient, resistivity and superconducting upper critical field H_n2 were measured on single-crystalline BaPb_1?xBi_xO₃ of compositions x = 0.20, 0.12 and 0, of which the normal-state properties differ significantly from those of polycrystals. The slope dH_n2/dT at T_c was estimated from the normal-state parameters and was found to agree well with the observed value, suggesting that this material is a bulk BCS superconductor.     

Superconducting properties of the atomically disordered MgB2 compound

The effect of disorder induced by neutron irradiation in a nuclear reactor (thermal neutron fluence 1×10¹⁹cm^?2) on the superconducting transition temperature T _c and the upper critical field H _c2 of polycrystalline MgB₂ samples was investigated. Despite the appreciable radiation-induced distortions (more than ten displacements per atom), the initial crystal structure (C32) was retained. The temperature T _c decreased from 38 to 5 K upon irradiation and was practically completely restored after the subsequent annealing at a temperature of 70°C. A weak change in the dH _c2/dT derivative upon irradiation is explained by the fact that the irradiated samples are described by the “pure” limit of the theory of disordered superconductors. The suppression of T _c upon disordering may be due to the isotropization of the originally anisotropic (or multicomponent) superconducting gap or to a decrease in the density of electronic states at the Fermi level.     

Interrelation of the critical magnetic field H c2 and the residual resistivity for the two-band superconductor MgB2

An expression that describes the upper critical magnetic field H _c2 and generalizes the Gor’kov relation has been derived for the two-band two-gap superconductor MgB₂. The expression relates the upper critical magnetic field H _c2 to the residual resistivity and the parameters of the band structure and holds in the range from the clean limit to the dirty limit. The ratios of the relaxation times τ_π/τ_σ and the mean free paths of π- and σ-band electrons for MgB₂ samples with a low defect level and Mg(B_{1 ? x} C _x )₂ samples with a partial substitution of carbon for boron are determined from experimental data.     

Suppression of superconductivity in YNi2B2C at the atomic disordering

The behavior of the electrical resistivity ρ(T), the superconducting transition temperature T _c , and the upper critical field H _c2(T) of a polycrystalline sample of YNi₂B₂C irradiated by thermal neutrons with the subsequent high-temperature isochronous annealing in the temperature interval T _ann = 100–1000°C has been studied. It has been found that the irradiation of YNi₂B₂C with a fluence of 10¹⁹cm^?2 leads to the suppression of the superconductivity. The final disordered state is reversible; i.e., the initial ρ(T), T _c , and H _c2(T) values are almost completely recovered upon annealing at up to T _ann = 1000°C. The quadratic dependence ρ(T) = ρ₀ + a ₂ T ² is observed for the sample in the superconducting state (T _c = 5.5?14.5 K). The coefficient a ₂ (proportional to the square of the electron mass m*) hardly changes. The form of the dependence of T _c on ρ₀ can be interpreted as the suppression of the two superconducting gaps, Δ₁ and Δ₂ (Δ₁ ～ 2Δ₂). The degradation rate of Δ₁ is about three times higher than that of Δ₂. The dependences dH _c2/dT on ρ₀ and T _c may be described by the relations for a superconductor in the intermediate limit (the coherence length ζ₀ is on the order of the electron mean free path l _tr) under the assumption of a nearly constant electron density of states on the Fermi level N(E _F). The observed behavior of T _c obviously does not agree with the widespread opinion about the purely electron-phonon mechanism of superconductivity in the compounds of this type supposing the anomalous type of superconducting pairing.     

Effects of composition on the upper critical field of V-Ga

Measurements of the temperature dependence of the upper critical field, H_c2(T), for a series of V_100?xGa_x materials are presented for 20.5 ≤ × ≤ 29.6. Fits of the data to conventional theory for a paramagnetically limited, dirty, type II superconductor show: 1) a maximum in T_c and H_c2(0) for x ? 25; 2) a constant ( for x ≤ 25; 3) a slowly increasing value of λ_so with increasing x up to x ～ 25; and 4) good agreement with stoichiometric ordered and thermally disordered V₃Ga. Above $\text{x ? 25}$ broader transitions are observed. For x = 25, T_c = 15.3 K, (, λ_so = 0.3 and H_c2(0) = 23.4 tesla. The effects of inclusion of strong-coupling in the theory are discussed briefly.     

Negative magnetoresistance and hall coefficient of a two-dimensional disordered system

Localization corrections to the longitudinal (δρ) and Hall (δρ_H) resistivities of a two-dimensional disordered system are calculated in all ranges of classical magnetic fields, up to the values at which the mean free path of charge carriers l is less than or of the order of the cyclotron radius R _c. It is shown that the physical reason for the departure of the l dependence of these resistivities from the logarithmic law ∝ ln(l _B /l)) (l _B is the magnetic length) at is the nonlocal process of diffusion in the Cooper channel, rather than the transition to a quasi-ballistic regime. Analytical expressions are obtained that allow one to analyze the interference effect in δρ and δρ_H in quantizing magnetic fields , including the quantum limit. Contrary to popular opinion, the localization corrections to ρ_H are shown to be nonzero. They have a sign opposite to that of the charge carriers and lead to a decrease in the magnitude of the Hall resistivity. Their field dependence has the same features and their relative magnitude is of the same order as in the case of the longitudinal resistivity. The quantum corrections to the Hall resistivity are due to the Larmor precession of the closed paths that electrons follow in the process of their multiple scattering by randomly distributed impurities.     

Angular dependence of the upper critical field in Bi2Sr2CuO6+δ

The angular dependence of the upper critical magnetic field was investigated in a wide range of temperatures in very high-quality Bi₂Sr₂CuO_6+δ single crystals with critical temperature T _c (midpoint) ? 9 K in magnetic fields up to 28 T. Although the typical value of the normal state resistivity ratio ρ_c/ρ_ab≈10⁴, the anisotropy ratio H _c2∥ab/H _c2⊥ab of the upper critical fields is much smaller and shows an unexpected temperature dependence. A model based on strong anisotropy and small transparency between superconducting layers is proposed.     

Sc doping of MgB2: the structural and electronic properties of Mg1−xScxB2

We have investigated the effect of electron doping on the superconducting properties of MgB₂. For the purpose we have synthesized several samples along the Mg_1−xSc_xB₂ section. The X-ray diffraction measurements reveal small changes in the lattice parameters suggesting that the Sc doping could be considered to simply fill the boron σ bands. Radio frequency surface resistivity measurements has been used to obtain the variation of T_c with Sc doping. Increasing the Sc content, the experimental T_c diverges from the T_c predicted by the BCS single band theory showing the key role of interchannel pairing near a shape resonance.     

Supraleitung von Pb- und Sn-Schichten nach Implantation von Mn-Ionen

Ion-implantation is a useful method when preparing dilute alloys, especially of elements with very low solubility like 3d-transition-elements in most non-transition superconductors. A strong decrease inT _c and an increase in the residual resistivity were observed in Pb- and Sn-films doped with Mn-ions. Nonlinearity in theT _c -dependence which was found at very low concentrations (0-100 ppm) is caused by lattice defects. They cannot be avoided if implantation method is used. This was proved by irradiation with nonmagnetic ions of comparable mass (such as Cu or Zn). The linear part in theT _c -depression is compared with results obtained with the quenched film technique by Barth (Pb-Mn) and Schertel (Sn-Mn). A discrepancy to these measurements is found in the case of Sn-Mn. The increase in film resistivity is due to lattice defects as well as magnetic impurities. The magnetic contribution (p/c) _m =(4.5 ± 0.2) μΩ cm/at-% agrees well with a theoretical calculation. Heavily doped Sn-films show a small Kondo-resistance-minimum at 7 K with a depth of 2.5‰ of the residual resistivity.     

A simple theory of 40 K superconductivity in MgB2: first-principles calculations of Tc, its dependence on boron mass and pressure

We have studied the superconducting properties of MgB₂ from first-principles under isotropic, uniaxial, and biaxial compressions. We find that the in-plane boron phonons near the zone-center are very anharmonic and strongly coupled to the planar B σ bands near the Fermi level. This mode is found to be the key to quantitatively explain the observed high T_c, the total isotope effect and the pressure dependence of T_c. We propose that a stringent test on the hole and phonon based theories of the superconductivity in MgB₂ would be a measurement of the biaxial ab-compression dependence of T_c.     

Effect of electron irradiation on the structure and properties of the MgB2 superconductor

This paper reports on the results of an investigation into the effect of irradiation of the Bardeen-Cooper-Schriefer superconductor MgB₂ by electrons with a mean energy ē ～ 10 MeV at low doses (0 ≤ Φt ≤ ～5 × 10¹⁶ cm^?2) on the lattice parameters, the intensity and width of diffraction lines, the superconducting transition temperature T _c , and the temperature dependence of the resistivity ρ(T) in the normal state. The results of structural investigations have revealed regularities in the defect formation in the magnesium and boron sublattices of the MgB₂ compound as a function of the electron fluence. At the initial stage, irradiation leads to the formation of vacancies, originally in the magnesium sublattice and then in the boron sublattice. For fluences Φt ≥ ～1 × 10¹⁶ cm^?2, vacancies are formed in both sublattices. The evolution of the electrical and physical properties [T _c , ρ_{273 K}, residual resistivity ratio RRR = ρ_{273 K}/ρ_{50 K}, parameters of the dependence ρ(T)] under electron irradiation is in agreement with the regularities revealed in the formation of radiation-induced defects in the crystal lattice of the MgB₂ compound.     

Electromagnetic properties of bulk Bi-Pb-Sr-Ca-Cu-O superconductor at low magnetic fields

Magnetization and critical current density measurements have been performed on a Bi_1.7Pb_0.3Sr₂Ca₂Cu₅O _y bulk material as the functions of magnetic field and temperature. The diamagnetic shielding effect has been discussed from the initial magnetization curve. The lower critical field of bulk granular superconductor (H _g) and the lower critical field of superconducting grains (H _c1) are estimated from magnetization curves. They are both linearly decreasing functions of temperature: (dH _g/dT) and (dH _c1/dT) are –0.4 and –1.8 G/K respectively. The transport critical current density drops drastically by a factor of 4 at a magnetic field of about 20G. The magnetization J _c of superconducting grains derived from the remanent magnetization is about 10⁶ A/cm² at 77 K, zero field, much greater than the transport J _c. The experimental results reveal that the transport J _c is dominated by weaklink current between grains. The magnetization J _c versus temperature has been obtained from the remanent flux at zero magnetic field and is a linearly decreasing function of temperature within experimental error.     

真空退火对周期性界面掺杂Ni80Co20薄膜磁性的影响

用磁控溅射方法制备了两个具有不同Fe层厚度的［Ni₈₀Co₂₀(L)/Fe(t_Fe)］_N多层膜系列样品,其中t_Fe=0.1和2nm.研究了两个系列样品的磁及输运性质随Ni₈₀Co₂₀层厚度L的变化关系.在退火态［Ni₈₀Co₂₀(L)/Fe(0.1nm)］_N系列样品中,发现各向异性磁电阻( 关键词： 多层膜 各向异性磁电阻 界面效应 退火     

The Ginzburg-Landau expansion and the slope of the upper critical field in superconductors with anisotropic normal-impurity scattering

We carry out the Ginzburg-Landau expansion for superconductors with anisotropic s and d pairing in the presence of anisotropic normal-impurity scattering, which enhances the stability of d pairing with respect to disordering. We find that the slope of the curve of the upper critical field, |dH _c2/dT|_{T c}, in superconductors with d pairing behaves nonlinearly as disorder grows: at low scattering anisotropy the slope rapidly decreases with increasing impurity concentration, then gradually but nonlinearly increases with concentration, reaches its maximum, and then rapidly decreases, vanishing at the critical impurity concentration. In superconductors with anisotropic s pairing, |dH _c2/dT|_{T c} always increases with impurity concentration, finally reaching the familiar asymptotic value characteristic of the isotropic case, irrespective of whether there is anisotropic impurity scattering. Zh. éksp. Teor. Fiz. 112, 2124–2133 (December 1997)     

Crystallization of Ba-Me hexagonal ferrite thin films

Thin films of Ba-Me ferrites are synthesized by reactive rf diode sputtering of a BaO · nFe₂O₃ ceramic target. Quartz plates subjected to preliminary annealing are used as substrates. The influence of the barium ion content on the crystalline and magnetic properties and the microstructure of the prepared films is investigated, and the interrelation between the quantity dH _c /dT and the microstructure of the film is considered. The prepared films satisfy the requirements for materials used as information carriers with a superhigh recording density.     

Electrical resistivity and magnetotransport in La<Subscript>0.67</Subscript>Ba<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films asymmetrically biaxially compressed by an NdGaO<Subscript>3</Subscript>(001) substrate

The La_0.67Ba_0.33MnO₃(40 nm) films are quasi-coherently grown on an NdGaO₃(001) substrate with an orthorhombic unit cell distortion of ～1.4%. The biaxial compressive stresses generated during nucleation and growth lead to a decrease in the unit cell volume of the grown layers. This, in turn, results in a decrease (by ～35 K) in the temperature of the maximum in the dependence of the electrical resistivity ρ of the layers on the temperature. For T < 150 K, the electrical resistivity ρ of the films increases in proportion to ρ₂ T ^4.5 and the coefficient ρ₂ decreases almost linearly with increasing magnetic field H. The negative magnetoresistance (≈?0.17 for μ₀ H = 1 T) reaches a maximum at temperatures close to room temperature. The response of the electrical resistivity ρ of the La_0.67Ba_0.33MnO₃(40 nm) films to the magnetic field depends on the crystallographic direction of the film orientation and the angle between H and I (where I is the electric current through the film).     

Einfluß von Gitterstörungen auf die Supraleiteigenschaften von Blei

The transition temperatureT _c and the critical fieldH _c of lead were measured as a function of the concentration of lattice defects. The defects were generated by plastic deformation at liquid Helium temperatures and reduced by annealing. T _c is rather insensitive to defects. With increasing residual resistance ratio ρ the transition temperature increases and finally reaches a constant value with onlyΔT _c ≈4.5 · 10^?3 °K. On the other hand a deformation of the same amount increasesH _c more than twice as much as the starting value. Annealing to room-temperature reducesρ, T _c andH _c to their initial values. During the annealing process,T _c shows a distinct maximum and ρ a marked step. Contrary to this behaviourH _c decreases linearly during the whole region of annealing. Taking into account the strong influence of ρ uponH _c a picture is given about the mechanism of deformation, which allows to understand the results qualitatively. The changes ofT _c produced by elastic strain were also measured. The results are in quantitative agreement with those of pressure experiments.     

Electron transport in diborides: Observation of superconductivity in ZrB2

We report on syntheses and electron transport properties of polycrystalline samples of diborides (AB₂) with different transition metals atoms (A=Zr, Nb, Ta). The temperature dependence of resistivity, ρ(T), and ac susceptibility of these samples reveal a superconducting transition of ZrB₂ with T _c =5.5 K, while NbB₂ and TaB₂ have been observed to be nonsuperconducting up to 0.37K. H _c2(T) is linear in temperature below T _c , leading to a rather low H _c2(0)=0.1 T. At T close to T _c , H _c2(T) demonstrates a downward curvature. We conclude that these diborides, as well as MgB₂ samples, behave like simple metals in the normal state with usual Bloch-Grüneisen temperature dependence of resistivity and with Debye temperatures 280, 460, and 440 K for ZrB₂, NbB₂, and MgB₂, respectively, rather than T ² and T ³, as previously reported for MgB₂.