Die Supraleitung von Zinn und Blei unter sehr hohem Druck

The dependence of the critical temperatureT _c upon pressureP is measured in the pressure range up to 160 kbar. The experimental technique developed for very high pressure-low temperature experiments (preceding article) is improved by introducing a double-sample electrical resistance cell. An internal pressure calibration is therefore possible at some well-established room temperature pressure reference points commonly used. Both metals, tetragonal white tin and fcc-lead, show a monotonic decrease ofT _c vs.P with upward curvature. The results recommend the use of Pb as a secondary standard for very high pressure experiments at Helium temperatures. In addition, high pressure polymorphic modifications of Sn and Pb are found to show superconductivity withT _c =(5.30±0.10) ?K for Sn III atP=113 kbar andT _c =(3.55±0.10) ?K for Pb II atP=160 kbar.     

Exponential Increase in the Resistivity upon Cooling and Superconductivity in Indium-Doped Pb<Subscript>0.45</Subscript>Sn<Subscript>0.55</Subscript>Te

We have analyzed the temperature and magnetic-field dependences of resistivity ρ(T, H) of semiconducting compound Pb_0.45Sn_0.55Te doped with 5 at % In under a hydrostatic compression at P < 12 kbar. It is found that the temperature dependence ρ(T) at all pressures at T < 100 K is exponential with the activation energy decreasing upon an increase in pressure; this is accompanied with a superconducting transition on the ρ(T) and ρ(H) dependences at P > 4.8 kbar at T > 1 K (T _c = 1.72 K at a level of 0.5ρ _N at P = 6.8 kbar). We consider the model describing the low-temperature “dielectrization” of the semiconducting solid solution and the formation of the superconducting state upon an increase in the hydrostatic compression P > 4 kbar.     

Enhancement of pseudogap anomalies induced by nanoscale structural inhomogeneity in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.93</Subscript> high-<Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> superconductor

The magnetization M(H) in the superconducting state, dc magnetic susceptibility χ(T) in the normal state, and specific heat C(T) near the superconducting transition temperature T _c have been measured for a series of fine-crystalline YBa₂Cu₃O _y samples having nearly optimum values of y = 6.93 ± 0.3 and T _c = (91.5 ± 0.5) K. The samples differ only in the degree of nanoscale structural inhomogeneity. The characteristic parameters of superconductors (the London penetration depth and the Ginzburg–Landau parameter) and the thermodynamic critical field H _c are determined by the analysis of the magnetization curves M(H). It is found that the increase in the degree of nanoscale structural inhomogeneity leads to an increase in the characteristic parameters of superconductors and a decrease in H _c(T) and the jump of the specific heat ΔC/T _c. It is shown that the changes in the physical characteristics are caused by the suppression of the density of states near the Fermi level. The pseudogap is estimated by analyzing χ(T). It is found that the nanoscale structural inhomogeneity significantly enhances and probably even creates the pseudogap regime in the optimally doped high-T _c superconductors.     

Size effects in electrical and magnetic properties of quasi-one-dimensional tin wires in asbestos

Bulk composites have been prepared based on one-dimensional fibers of natural chrisothil-asbestos with various internal diameters (d = 6–2.5 nm) filled with tin. The electrical and magnetic properties of quasi-one-dimensional Sn wires have been studied at low temperatures. The electrical properties have been measured at T = 300 K at a pressure P = 10 kbar. It has been found that the superconducting (SC) characteristics of the nanocomposites (critical temperature T_c and critical magnetic field H_c) increase as the Sn filament diameter decreases. The temperature spreading of the resistive SC transition also increases as the Sn filament diameter decreases, which is explained by the SC order parameter fluctuations. The size effects (the increase in critical temperature T_c and transition width ΔT_c) in Sn nanofilaments are well described by the independent Aslamazov–Larkin and Langer–Ambegaokara fluctuation theories, which makes it possible to find the dependence of T_c of the diffuse SC transition on the nanowire diameter. Using the temperature and magnetic-field dependences of the magnetic moment M(T, H), it has been found that the superconductor–normal metal phase diagram of the Sn–asbestos nanocomposite has a wider region of the SC state in T and H as compared to the data for bulk Sn. The magnetic properties of chrisotil-asbestos fibers unfilled with Sn have been studied. It has been found that the Curie law is fulfilled and that the superparamagnetism is absent in such samples. The obtained results indicate the absence of magnetically ordered impurities (magnetite) in the chrisotil-asbestos matrix, which allowed one to not consider the problem of the interaction of the magnetic subsystem of the asbestos matrix and the superconducting subsystem of Sn nanowires.     

Specific features of anion transfer in HoF<Subscript>3</Subscript> crystals at high temperatures

The anionic conductivity of HoF₃ single crystals with a β-YF₃ structure (orthorhombic crystal system, space group Pnma) is investigated over a wide range of temperatures (323–1073 K). The unit cell parameters of HoF₃ crystals are as follows: a=0.6384±0.0009 nm, b=0.6844±0.0009 nm, and c=0.4356±0.0005 nm. It is revealed that the conductivity anisotropy of the HoF₃ crystals is insignificant over the entire temperature range covered. The crossover from one mechanism of ion transfer to another mechanism is observed near the critical temperature T_c≈620 K. The activation enthalpy of electrical conduction is found to be ΔH₁=0.744 eV at T<T_c and ΔH₂=0.43 eV at T>T_c. The fluorine vacancies are the most probable charge carriers in HoF₃ crystals. The fluorine ionic conductivities at temperatures of 323, 500, and 1073 K are equal to 5×10^?10, 5×10^?6, and 2×10^?3 S cm^?1, respectively.     

Structural transformations of K<Subscript>3</Subscript>H(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals with variations in temperature

Single crystals of the K₃H(SO₄)₂ compound are investigated using X-ray diffraction on Xcalibur S and Bruker diffractometers. The structure of the low-temperature monoclinic phase is refined (space group C2/c, z = 4, a = 14.698(1) Å, b = 5.683(1) Å, c = 9.783(1) Å, β = 103.01(1)°, T = 293 K, Bruker diffractometer), the structural phase transition is revealed, and the structure of the high-temperature trigonal phase is determined (space group R \(\bar 3\) m, z = 3, a = 5.73(1) Å,c = 21.51(1) Å,T = 458 K, Xcalibur diffractometer).     

Electrical and Magnetic Properties of Pb and In Nanofilaments in Asbestos near the Superconducting Transition

Bulk nanocomposites based on superconducting metals Pb and In embedded into matrices of natural chrysotile asbestos with the nanotube internal diameter d ~ 6 nm have been fabricated and studied. The low-temperature electrical and magnetic properties of the nanocomposites demonstrate the superconducting transition with the transition critical temperature T_c ≈ (7.18 ± 0.02) K for the Pb–asbestos nanocomposite (this temperature is close to T_{c bulk} = 7.196 K for bulk Pb). The electrical measurements show that In nanofilaments in asbestos have T_c ~ 3.5–3.6 K that is higher than T_{c bulk} = 3.41 K for bulk In. It is shown that the temperature smearing of the superconducting transition in the temperature dependences of the resistance R(T) ΔT ≈ 0.06 K for the Pb–asbestos and ΔT ≈ 1.8 K for the In–asbestos are adequately described by the fluctuation Aslamazov–Larkin and Langer–Ambegaokar theories. The resistive measurements show that the critical magnetic fields of the nanofilaments extrapolated to T = 0 K are H_c(0) ~ 47 kOe for Pb in asbestos and H_c(0) ~ 1.5 kOe for In in asbestos; these values are significantly higher than the values for the bulk materials (H\(H_{\rm{c}}^{\rm{bulk}}\) = 803 Oe for Pb and \(H_{\rm{c}}^{\rm{bulk}}\) = 285 Oe for In). The results of the electrical measurements for Pb?asbestos and In–asbestos agree with the data for the magnetic-field dependences of the magnetic moment in these nanocomposites.     

Superconducting Properties of Indium Nanostructured in Pores of Thin Films of SiO<Subscript>2</Subscript> Microspheres

Samples of a superconducting indium nanocomposite based on a thin-film porous dielectric matrix prepared by the Langmuir–Blodgett method are obtained for the first time, and their low-temperature electrophysical and magnetic properties are studied. Films with thickness b ≤ 5 μm were made from silicon dioxide spheres with diameter D = 200 and 250 nm; indium was introduced into the pores of the films from the melt at a pressure of P ≤ 5 kbar. Thus, a three-dimensional weakly ordered structure of indium nanogranules was created in the pores, forming a continuous current-conducting grid. Measurements of the temperature and magnetic field dependences of the resistance and magnetic moment of the samples showed an increase in the critical parameters of the superconductivity state of nanostructured indium (critical temperature T_c ≤ 3.62 K and critical magnetic field H_c at T = 0 K H_c(0) ≤ 1700 Oe) with respect to the massive material (T_c = 3.41 K, H_c(0) = 280 Oe). In the dependence of the resistance on temperature and the magnetic field, a step transition to the superconductivity state associated with the nanocomposite structure was observed. A pronounced hysteresis M(H) is observed in the dependence of the magnetic moment M of the nanocomposite on the magnetic field at T < T_c, caused by the multiply connected structure of the current-conducting indium grid. The results obtained are interpreted taking into account the dimensional dependence of the superconducting characteristics of the nanocomposite.     

Microstructure,elastic, and inelastic properties of biomorphic carbons carbonized using a Fe-containing catalyst

The microstructure and amplitude dependences of the Young’s modulus E and internal friction (logarithmic decrement δ), and microplastic properties of biocarbon matrices BE-C(Fe) obtained by beech tree carbonization at temperatures T _carb = 850–1600°C in the presence of an iron-containing catalyst are studied. By X-ray diffraction analysis and transmission electron microscopy, it is shown that the use of Fe-catalyst during carbonization with T _carb ≥ 1000°C leads to the appearance of a bulk graphite phase in the form of nanoscale bulk graphite inclusions in a quasi-amorphous matrix, whose volume fraction and size increase with T _carb. The correlation of the obtained dependences E(Т _carb) and δ(T _carb) with microstructure evolution with increasing Т _carb is revealed. It is found that E is mainly defined by a crystalline phase fraction in the amorphous matrix, i.e., a nanocrystalline phase at Т _carb < 1150°C and a bulk graphite phase at T _carb > 1300°C. Maximum values E = 10–12 GPa are achieved for samples with Т _carb ≈ 1150 and 1600°C. It is shown that the microplasticity manifest itself only in biocarbons with T _carb ≥ 1300°C (upon reaching a significant volume of the graphite phase); in this case, the conditional microyield stress decreases with increasing total volume of introduced mesoporosity (free surface area).     

Critical phenomena in the β-(2×4) → α-(2×4) reconstruction transition on the (001) GaAs surface

The critical exponents of the β-(2×4) → α-(2×4) reconstruction phase transition on the (001) GaAs surface are determined experimentally. It is found that the phase transition is analogous to a van der Waals transition. The critical parameters T _c , P _c , and Θ_c have been measured experimentally. The mean field theory is applied, and three-parameter isotherms are obtained that agree with the experimental results at the following values of the parameters: E_st = 0.36 eV, ΔE = 0.18 eV, and E _i = 0.134 eV. Precision measurements of the critical exponents β and δ are carried out. Their values β = 1/8 and δ = 15 indicate that the phase transition is truly two-dimensional.     

Superconductivity in two-band systems with a variable carrier density: The case of MgB<Subscript>2</Subscript>

A theory of the thermodynamic properties of a two-band superconductor with a low carrier density is developed; it is based on a phonon superconductivity mechanism with a strong electron-phonon coupling. This theory can describe the variation of the critical temperature T _c, the energy gaps Δ₁ and Δ₂, and the relative electronic specific heat jump (C _S ? C _N)/C _N at T = T _c with the carrier density in the compound MgB₂ when substitutional impurities of various valences are introduced into this system. The values of T _c, Δ₁, and Δ₂ are shown to decrease as this compound is doped by electrons and to remain constant (or almost constant) as it is doped by holes. This behavior follows from the mechanism of filling the σ and π energy bands, which overlap at the Fermi surface. The theory agrees qualitatively with experimental data. This agreement is found to be better when intra-and interband electron scattering by an impurity potential is taken into account.     

Thermal conductivity of partially graphitized biocarbon obtained by carbonization of medium-density fiberboard in the presence of a Ni-based catalyst

The thermal conductivity k and resistivity ρ of biocarbon matrices, prepared by carbonizing medium-density fiberboard at T _carb = 850 and 1500°C in the presence of a Ni-based catalyst (samples MDF-C( Ni)) and without a catalyst (samples MDF-C), have been measured for the first time in the temperature range of 5–300 K. X-ray diffraction analysis has revealed that the bulk graphite phase arises only at T _carb = 1500°C. It has been shown that the temperature dependences of the thermal conductivity of samples MDFC- 850 and MDF-C-850(Ni) in the range of 80–300 K are to each other and follow the law of k(T) ～ T ^1.65, but the use of the Ni-catalyst leads to an increase in the thermal conductivity by a factor of approximately 1.5, due to the formation of a greater fraction of the nanocrystalline phase in the presence of the Ni-catalyst at T _carb = 850°C. In biocarbon MDF-C-1500 prepared without a catalyst, the dependence is k(T) ～ T ^1.65, and it is controlled by the nanocrystalline phase. In MDF-C-1500(Ni), the bulk graphite phase formed increases the thermal conductivity by a factor of 1.5–2 compared to the thermal conductivity of MDF-C-1500 in the entire temperature range of 5–300 K; k(T = 300 K) reaches the values of ～10 W m^–1 K^–1, characteristic of biocarbon obtained without a catalyst only at high temperatures of T _carb = 2400°C. It has been shown that MDF-C-1500(Ni) in the temperature range of 40?300 K is characterized by the dependence, k(T) ～ T ^1.3, which can be described in terms of the model of partially graphitized biocarbon as a composite of an amorphous matrix with spherical inclusions of the graphite phase.     

Variable-range hopping conduction in LaMnO<Subscript>3 + δ</Subscript>

The temperature dependence of the electrical resistivity ρ(T) for ceramic samples of LaMnO_{3 + δ} (δ = 0.100–0.154) are studied in the temperature range T = 15–350 K, in magnetic fields of 0–10 T, and under hydrostatic pressures P of up to 11 kbar. It is shown that, above the ferromagnet-paramagnet transition temperature of LaMnO_{3 + δ}, the dependence ρ(T) of this compound obeys the Shklovskii-Efros variable-range hopping conduction: ρ(T) = ρ₀(T)exp[(T ₀/T)^1/2], where ρ₀(T) = AT ^9/2 (A is a constant). The density of localized states g(?) near the Fermi level is found to have a Coulomb gap Δ and a rigid gap γ(T). The Coulomb gap Δ assumes values of 0.43, 0.46, and 0.48 eV, and the rigid gap satisfies the relationship γ(T) ≈ γ(T _v)(T/T _v)^1/2, where T _v is the temperature of the onset of variable-range hopping conduction and γ(T _v) = 0.13, 0.16, and 0.17 eV for δ = 0.100, 0.125, and 0.154, respectively. The carrier localization lengths a = 1.7, 1.4, and 1.2 Å are determined for the same values of δ. The effect of hydrostatic pressure on the variable-range hopping conduction in LaMnO_{3 + δ} with δ = 0.154 is analyzed, and the dependences Δ(P) and γ_v(P) are obtained.     

Electron localization during an electronic-topological transition in Mo-Re alloys

Oscillations in the superconducting transition temperature ΔT _c (P), in the critical magnetic field ΔH _c (P), in the thermopower α / T (T ²), and in electrical resistivity ρ(T) (P is pressure) of Mo_1?x -Re _x alloys are observed at low temperatures against the background of specific features related to an electronic-topological transition (ETT) in these alloys. The oscillations are sensitive to the impurity concentration: they increase when the Re impurity concentration is close to the critical concentration C _c at which the ETT occurs. Oscillations are also detected in the concentration dependences of the temperature coefficient of resistivity (?ρ / ?T (C)) and the thermopower derivative (?(α/T) / ?T ² (C)) of Mo_1?x -Re _x alloys at low temperatures. The former and latter oscillations are shown to correlate with each other. These specific features are assumed to result from the ETT and to be related to the localization of the part of the electrons that fill a new cavity in the Fermi surface during this transition.     

Anisotropic<Emphasis Type="Italic">s</Emphasis>-wave or<Emphasis Type="Italic">d</Emphasis>-wave pairing? Analysis of experiments on ion irradiation of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7-x</Subscript> films

The effect of ion irradiation on the superconducting transition temperatureT _c and resistivityρ _ab (T) of YBa₂Cu₃O_7-x films with different oxygen content (initial temperatureT _c0≈90 K and 60 K) is studied experimentally. The dependenciesT _c /T _c0 on residual resistivityρ _o are obtained in very wide range 0.2<T _c /T _c0 <1 andρ _o μΩ·cm. The critical values ofρ _o , corresponding to the vanishing of superconductivity, are found to be an order of magnitude larger then those predicted by theory ford-wave pairing. At 0.5÷0.6<T _c /T _c0<1 the experimental data are in close agreement with theoretical dependencies, obtained for the anisotropics-wave superconductor within the BCS-framework.     

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.     

Heat capacity anomaly in the vicinity of the tricritical and upper critical points

The anomalous behavior of the isochoric heat capacity of a mixture of methane, pentane and heptane is studied experimentally in the vicinity of the liquid-vapor critical point in the cases when (a) the critical temperature T _c approaches the tricritical point T _TCP and (b) the critical temperature approaches the upper critical end point T _U . It is shown that in all cases, the singular part of the heat capacity of the mixture has the form C_sing=A¦τ¦^?α, where τ=(T ? T _c )/T _c and α≈0.11. When T _c →T _U , amplitude A of the heat capacity anomaly is found to be approximately constant. At the same time, the amplitude of the anomaly tends to zero in the vicinity of the tricritical point: A∝¦τ_c¦^ε, where τ_c=(T _c ? T _TCP )/T _TCP and ε=1.6?1.7. The inevitable vanishing of this mode of the heat capacity anomaly leads to a negative value of the critical index \(\tilde \alpha\) characterizing the heat capacity anomaly at the tricritical point, while the tricritical point theory and the isomorphism hypothesis predict \(\tilde \alpha = 0.5\).     

Role of initial correlation in coarsening of a ferromagnet

We study the dynamics of ordering in ferromagnets via Monte Carlo simulations of theIsing model, employing the Glauber spin-flip mechanism, in space dimensionsd = 2 and3, on square and simplecubic lattices. Results for the persistence probability and the domain growth arediscussed for quenches to various temperatures (T _f ) below the criticalone (T _c ), from differentinitial temperatures T _i ≥T _c . In long timelimit, for T _i >T _c ,the persistence probability exhibits power-law decay with exponents θ ? 0.22 and? 0.18 in d = 2 and 3, respectively. For finite T _i , the early timebehavior is a different power-law whose life-time diverges and exponent decreases asT _i →T _c . The two steps areconnected via power-law as a function of domain length and the crossover to the secondstep occurs when this characteristic length exceeds the equilibrium correlation length atT =T _i . T _i =T _c is expected toprovide a new universality class for which we obtain θ ≡θ _c ? 0.035 ind = 2 and?0.105 in d = 3. The time dependenceof the average domain size ?, however, is observed to be rather insensitive tothe choice of T _i .     

Superconducting transition temperature in hafnium under pressures up to 64 GPa

The superconducting transition temperature T _c of hafnium is measured as a function of pressure up to 64 GPa. The character of the pressure dependence of T _c observed at α–ω–β transitions in Hf is found to be similar to that observed for Zr. In the regions of α and β phases, T _c increases with pressure with the slopes dT _c /dP=0.05 and 0.16 K/GPa, respectively. At the α–ω transition, T _c (P) exhibits a tendency to a decrease, while at the ω–β transition, T _c increases stepwise from 5.8 to 8.0 K. The α–ω transition occurs at pressures between 31.2 and 35.9 GPa, and the ω–β transition, at a pressure of 62±2 GPa.