High-Tc superconductor near the S–I transition

We report on the density of states measurements of Bi2212 (Bi_2+xSr_xCaCu₂O_8+δ) near the superconductivity-insulator transition using a low temperature scanning tunneling microscope. We prepared highly underdoped Bi rich Bi2212 single crystals (T_c  32 K). The energy gap distribution did not provide an energy scale proportional to T_c. Averaged tunnel spectra with various doping levels were scaled into a single line if energy was normalized by their respective gap values. This indicated there was no crossover energy, which separates a pseudogap and a superconducting gap.     

Grain Boundary Diffusion and Segregation in Interstitial Solid Solutions Based on BCC Transition Metals: Carbon in Niobium

Bulk and grain boundary (GB) diffusion of ¹⁴C in Nb has been studied by the radiotracer serial sectioning technique. B and C kinetic regimes were realized for GB diffusion in the temperature range from 800 to 1173 K. The values of P = sD _gb, D _gb and s follow the Arrhenius dependencies: P = 5.15 × 10^–15 exp[–(83.1 kJ/mol)/RT] m³/s (973–1173 K), D _gb = 2.3 × 10^–6 exp[–(133.0 kJ/mol)/RT] m²/s (800–950 K), and s = 4.7 exp[(49.9 kJ/mol)/RT].The increase in the GB diffusion compared with self-diffusion is very large despite the probable retardation effect due to the strong segregation.The results for GB diffusion of C in Nb as well as for other interstitial solutes (P, S) in bcc transition metals (- Fe, Mo) are discussed in the framework of the transition state theory. It is assumed that GB segregation decreases the energy of the ground state whereas the change in the diffusion mechanism (e.g. from vacancy to interstitial) leads to a strong decrease of the transition state energy. This change in the diffusion mechanism results in a fast GB diffusion of interstitial solutes in spite of their large tendency to segregate to GBs.     

Grain Boundary Diffusion and Linear and Non-Linear Segregation of Ag in Cu

Ag grain boundary (GB) diffusion was measured in the Cu-0.2at%Ag alloy in a wide temperature range from 473 to 970 K. The direct measurements of Ag GB diffusivity D ^alloy _gb under conditions of the Harrison C regime revealed that D ^alloy _gb is almost identical to D ^pure _gb determined earlier for Ag diffusion in high-purity Cu (Divinski, Lohmann, and Herzig, 2001). The penetration profiles determined in the Harrison B regime showed a complex, multi-stage shape. This diffusion behavior can be rationalized assuming that besides GBs significantly covered by segregated Ag atoms, some fraction of GBs remains almost free from Ag atoms in the studied temperature interval. The total amount of pure GBs drastically decreases with decreasing temperature. This hypothesis was proven by measurements of Ag GB diffusion in Cu near 5 bicrystals, which allowed us to analyze in detail the non-linear segregation of Ag in Cu GBs.     

Calorimetric study of Te15(Se100− x Bi x )85 glassy alloys using differential thermal analysis

A calorimetric study of Te₁₅(Se_{100? x} Bi _x )₈₅ glassy alloys (x = 0, 1, 2, 3 and 4 at. %) is reported. Differential thermal analysis (DTA) was performed at heating rates of 10, 15, 20 and 25 K/min. The spectra were used to determine the glass transition temperature, T_g , the crystallisation temperature, T_c and the melting temperature, T_m . All these parameters shift to higher values with increasing heating rate, β. The glass transition temperature and the melting temperature increase, and the crystallisation temperature decreases, with increase in the Bi content, x. The activation energy of the glass transition, E_g , was evaluated using the Moynihan and Kissinger methods. The activation energy of crystallisation, E_c , was calculated using modified Kissinger and Matusita approaches. The thermal stability of these glasses has been studied and found to decrease with increase in Bi content. The results obtained are explained on the basis of a chemically ordered network model and an average coordination number.     

Galvanomagnetic properties of thin films of bismuth and bismuth–antimony alloys on substrates with different thermal expansions

Temperature dependences of the galvanomagnetic properties of films of bismuth and Bi_{100 – x}Sb_x (x ≤ 12) on substrates with different temperature expansion coefficients were studied in the temperature range of 77–300 K. The block films were prepared through thermal deposition, and single-crystal Bi_{100 – x}Sb_x were grown by zone recrystallization under a coating. It was found that the temperature expansion coefficient of a substrate substantially influenced the galvanomagnetic properties of Bi and Bi_{100 – x}Sb_x films. Using the experimental data, the change in the charge-carrier concentration in the Bi and Bi_{100 – x}Sb_x films on different substrates at 77 K was estimated.     

Magnetic penetration depths of (BiPb)-2212 and (TlPb)-1212 — Universal correlations revisited

A systematic study of the dependence of the critical temperature on the superconducting carrier density, as measured by ⁺SR, was carried out for a variety of cuprate oxides. We have shown that substitution of Pb for Bi and La for Sr result in well pinned vortex lattices in Bi₂Sr₂CaCu₂O_x, and studied (BiPb)-2212 from the underdoped to the slightly overdoped regime. We also found that Tl-1212 samples, which have optimal maximumT _c=108 K, trace a separate line with parameters comparable to overdoped Tl-2223 at the maximum. These data, together with data on Y-124/247, Tl-2201 and other cuprates result in an larger set of universal correlations between T_c and(0) n _s /m ^* for the high temperature superconductors.     

Introduction of bismuth into the highT c superconductor La2−x Sr x CuO4−y

Novel high-T _c superconductors La_2–x Bi _x Sr _x–x CuO_4–y have been isolated and studied. The influence of the experimental conditions of synthesis on the superconductivity has been especially studied for the composition La_1.7Bi_0.1Sr_0.2CuO_4–y ; it is shown thatT _c decreases as oxygen is introduced into the matrix, likely due to the partial oxidation of Bi(III) to Bi(V). The highT _c value observed for one sample (T _c 42 K), as compared withT _c 38 K of La_1.8Sr_0.2CuO_4–y suggests that bismuth is also a potential candidate for improving the superconducting properties in these oxides.     

Knight shift in superconducting oxides BaPb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Bi<Subscript>x</Subscript>O<Subscript>3</Subscript> (<Emphasis Type="Italic">x</Emphasis> < 0.35)

The Knight shift ²⁰⁷K_s for the ²⁰⁷Pb nuclei in the metal phase of the oxides BaPb_1?xBi_xO₃ (x < 0.35) has been analyzed as a function of the concentration. The shift, which is proportional to the density of states near the Fermi energy: ²⁰⁷K_s ～ N(E_F), reaches a maximum for an oxide with the maximum superconducting transition temperature T_c(x ≈ 0.25) = 12 K. A significant increase in the width of the shift distribution with the Bi concentration testifies to the formation of a nonuniform state of the electronic system in the conduction band of superconducting oxides, which is accompanied by an increase in short-wavelength contributions to the spin susceptibility. To detect the ²⁰⁷Pb NMR spectra in superconducting oxides with x > 0.2, the ¹⁷O-²⁰⁷Pb spin-echo double-resonance method is used, which provides successful detection of the ²⁰⁷Pb NMR signal with an anomalously high rate of spin-spin relaxation T ₂ ^?1 > 500 ms^?1. Thus, fundamental restrictions arising in investigations of rapidly relaxing ²⁰⁷Pb nuclei, which are “unobservable” in superconducting oxides BaPb_1?xBi_xO₃ when they are studied by traditional single-resonance methods of pulse NMR spectroscopy, have been overcome.     

Interplay between charge and antiferromagnetic ordering in Bi0.6−xPrxCa0.4MnO3 (0≤x≤0.6) perovskite manganite

Structure, magnetic and transport properties of polycrystalline Bi_0.6−xPr_xCa_0.4MnO₃ (x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) have been studied. Systematic substitution of Pr at Bi site induces an interesting interplay between the charge ordering and antiferromagnetism. The charge ordering temperature (T_CO) decreases with increasing x. The antiferromagnetic (AFM) ordering temperature (T_N) increases sharply at both the extremes but remains nearly constant from x=0.2 to 0.4. At temperatures lower than T_N a transition to the glassy state is observed. The nature of this glass like state appears to be controlled by the Pr content, and at lower values of x this is akin to a spin glass, while at higher x it has a characteristic of cluster glass. The Pr doping also leads to enhancement in the magnetic moment. In the present work it has been proposed that the local lattice distortion induced due to size mismatch between the A-site cations and 6s² character of Bi³⁺ lone pair electron is responsible for the observed magnetic and electrical properties.     

Compensation Effect for Grain Boundary Diffusion in a Bicrystalline Experiment

The paper is devoted to the problem of the compensation effect for grain boundary (GB) diffusion, i.e. the linear dependence of the pre-exponential factor of the GB diffusion coefficient on the activation energy. Specific features of GB diffusion as a thermally activated process namely, the influence of segregation factor, K, and variation of the GB width, d, on the diffusion rate are discussed. A special diffusion experiment was designed to estimate the contribution of the separate component parts of the triple product, KdD_GB (D_GB is the GB diffusion coefficient). The experiment was performed with Al bicrystals. The variation of the GB width d, and a value of the segregation factor K, due to GB structure change are estimated. It is concluded that D_GB is the main GB structure-sensitive parameter in the triple product. This circumstance allows us to consider the GBs in Al bicrystals as a series of uniform objects and to describe the kinetics of GB diffusion in terms of the compensation temperature T_c and a “barrier” phase. The value of T_c for GB diffusion of Zn and Ge in Al bicrystals is practically the same and equals 709 and 706 K, respectively. The character of the “barrier” phase is discussed.     

Anomalous thermopower in heavy-fermion compounds CeB<Subscript>6</Subscript>, CeAl<Subscript>3</Subscript>, and CeCu<Subscript>6 − <Emphasis Type="Italic">x</Emphasis></Subscript>Au<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

High-precision measurements of thermopower have been performed in a wide temperature range (2–300 K) for a series of cerium-based heavy-fermion compounds, including CeB₆, CeAl₃, CeCu₆, and substitutional solid solutions of the CeCu_{6 ? x} Au _x system (x = 0.1, 0.2). All compounds exhibit an unusual (logarithmic) asymptotic behavior of the temperature dependence of the Seebeck coefficient: S ∝ ?lnT. In the case of cerium hexaboride, this anomalous behavior of S(T) is accompanied by the appearance of weak-carrier-localization-mode asymptotics in the conductivity (σ(T) ∝ T ^0.39), while the paramagnetic susceptibility χ(T) and the effective mass of charge carriers m _eff(T) vary according to a power law (χ(T), m _eff(T) ∝ T ^?0.8) in the temperature interval T = 10–80 K. This behavior corresponds to renormalization of the density of states at the Fermi level. The observed anomalous behavior of thermopower in CeB₆ and other cerium-based intermetallic compounds is attributed to the formation of heavy fermions (many-body states in the metal matrix) at low temperatures.     

Photoelectric properties of Pb1−x−y Sn x Ge y Te:In epitaxial films

Pb_1–x–y Sn _x Ge _y Te:In epitaxial films are examined in a wide temperature interval and at various background fluxes. These films have high sensitivity to infrared radiation in the spectral range <20m. The lifetime depends exponentially on temperature and varies from several seconds at T=10 K to 10^–2 s at T=20 K. The two-electron model of Jahn-Teller centers is proposed to explain the results. Multielement photoresistors based on these films are fabricated and D*=1.7×10¹³ cm Hz^1/2 W^–1 at T=25 K is achieved. Noise of the photoresistors is independent of background flux when it varies from 10¹² cm^–2 s^–1 to 10¹⁸ cm^–2 s^–1. As compared with Si:Ga and Ge:Hg photoresistors, the responsitivity is several orders larger at the operating temperature 25–30 K.     

Strong probe atom dependence of the electric field gradient in Bismuth

The nuclear quadrupole interaction of the 8⁺ isomers in^{206, 208}Po was measured in the semimetallic host Bi. The comparison of the extracted electric field gradient eq for PoBi with the values for the neighboring elements Bi and Pb as probe atoms in Bi reveals a very drastic increase from Pb to Bi and to Po by an order of magnitude: eq_PbBi eq_BiBi eq_PoBi =1.64(10)4.8(3)16(2)(all values at room temperature in units of 10¹⁷ V/cm²). This behavior is discussed in terms of the changing number of outerp-electrons of the probe atoms.     

Temperature‐dependent Raman scattering in ferroelectric Bi4−xNdxTi3O12(x = 0, 0.5, 0.85) single crystals

The temperature‐dependent Raman spectra of ferroelectric Bi_4−xNd_xTi₃O₁₂(x = 0, 0.5, 0.85) single crystals were recorded from 100 to 800 K. It was found that there is a critical Nd content x₀ between 0.5 and 0.85. The Nd³⁺ ions prefer to replace Bi³⁺ ions in pseudo‐perovskite layers when x < x₀, while they might begin to incorporate into (Bi₂O₂)²⁺ layers when x ≥ x₀. Nd substitution leads to a decrease in the ferroelectric–paraelectric transition temperature (T_c). A monoclinic distortion of orthorhombic structure occurs in Bi₄Ti₃O₁₂ crystals at temperatures below 200 K. Copyright © 2009 John Wiley & Sons, Ltd.     

NMR/ON of206Bi implanted at T < 0.2 K and at room temperature into nickel

NMR/ON has been observed for²⁰⁶BiNi with samples prepared by implantation at T<0.2 K and at room temperature. The integrated destruction of anisotropy for the sample implanted at 290 K is only about half of that for the on-line implanted sample but the resonance line is less broad and has a narrow component with centre frequency _L= 223.62(10) MHz for zero external field. With the known g-factor of²⁰⁶Bi we derive from this frequency the hyperfine field of BiNi as B_hf=38.3(4) T.     

Self-similar magnetic structures and magnetic flux “Giant” creep

The penetration of a magnetic flux into a type-II high-T _c superconductor occupying the half-space x > 0 is considered. At the superconductor surface, the magnetic field amplitude increases in accordance with the law b(0, t) = b ₀(1 + t)^m (in dimensionless coordinates), where m > 0. The velocity of penetration of vortices is determined in the regime of thermally activated magnetic flux flow: v = v ₀exp?ub;?(U _0/T )(1-b?b/?x)?ub;, where U ₀ is the effective pinning energy and T is the thermal energy of excited vortex filaments (or their bundles). magnetic flux “Giant” creep (for which U ₀/T? 1) is considered. The model Navier-Stokes equation is derived with nonlinear “viscosity” v ∝ U ₀/T and convection velocity v _f ∝ (1 ? U ₀/T). It is shown that motion of vortices is of the diffusion type for j → 0 (j is the current density). For finite current densities 0 < j < j _c, magnetic flux convection takes place, leading to an increase in the amplitude and depth of penetration of the magnetic field into the superconductor. It is shown that the solution to the model equation is finite at each instant (i.e., the magnetic flux penetrates to a finite depth). The penetration depth x _eff ^A (t) ∝ (1 + t)^{(1 + m/2)/2} of the magnetic field in the superconductor and the velocity of the wavefront, which increases linearly in exponent m, exponentially in temperature T, and decreases upon an increase in the effective pinning barrier, are determined. A distinguishing feature of the solutions is their self-similarity; i.e., dissipative magnetic structures emerging in the case of giant creep are invariant to transformations b(x, t) = β^m b(t/β, x/β^{(1 + m/2)/2}), where β > 0.     

Temperature dependence of the electric field gradient of Hg in Bi

The temperature dependence of the nuclear quadrupole interaction of the¹⁸⁸Hg(12⁺) isomer in a Bi host has been measured by the TDPAD technique. Assuming that the T^3/2 law is valid for this system a fit at the high temperature region yields _Q(T=0)=132(6) MHz and B=7.2(6)×10^–5K^–3/2. The EFG of HgBi and the temperature dependence strength follow the general trend of other probes in Bi.Supported in part by the MINERVA Foundation, Munich, Germany and the Israeli Academy of Science, Jerusalem, Israel.Incumbent of the Arye Dissentshik Career Development Chair.Incumbent of the Ruth Epstein Recu Career Development Chair.     

NMR relaxation times and proton motion in HxWO3

The relaxation timesT ₁,T _1q,T _1D, andT ₂ for¹H in the tetragonal-A phase of H_xWO₃ have been measured over the temperature range 190 to 490 K. The¹H relaxation behaviour appears to be governed by diffusion over inequivalent jump distances, approximating to a short range planar diffusion and a long range isotropic diffusion. Parameters for the latter motion are estimated asE _a = 68 kJ/mol and ₀=2.5×10^–13 s. The powder X-ray diffraction pattern for this phase of H_xWO₃ has been studied over the temperature range 300–470 K. The tetragonal distortion diminishes with temperature and H_0.43 WO₃ becomes cubic at about 435 K. Volumetric studies of hydrogen evolution show that decomposition accelerates at approximately this temperature.     

Properties of IR Absorption Spectra of Bi-Pb-Sr-Ca-Cu-O Superconducting Material

Abstract

We used the co-decomposition method to make the Bi(Pb)-Sr-Ca-Cu-O superconductor samples (T_c = 112K) under oxygen partial pressure. Oxygen dependence of high-T_c phase was studied. The infrared absorption spectra of (Bi_1?xPb_x)₂Sr₂Ca₂Cu₃O_10+σ superconductor were measured at various temperatures from 98 to 290K. A new IR absorption peak was observed at 1265 cm^?1 below 217K. The intensity of this peak became stronger with temperature decreasing.     

Magnetostriction of several high-T c superconductors

We report on magnetostriction measurements of superconducting and nonsuperconducting YBa₂Cu₃O_7– and of two Bi samples with the nominal compositions Bi₂CaSr₂Cu₂O _x and Bi_0.8Pb_0.2Sr_0.8CaCu_1.5O _x . Both types of high temperature superconductors show a nearly identical strongly hysteretic field dependence of the magnetostriction (MS) (l/l+(2–6)*10^–6 at 5 T and 1.5 K). In both cases the MS is a linear function of field in the region 1–5 T, which we explain by striction due to surface currents. Between 1.5 and 35 K the hysteresis of the MS decreases strongly with increasing temperature, which implies a decrease of the pinning force. The MS of quenched nonsuperconducting YBa₂Cu₃O_7– is at least two orders of magnitude smaller than that of the superconductor.