Improvement of the intergranular pinning energy in uniaxially compacting (Bi-Pb)<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>10+δ</Subscript> ceramic samples

Measurements of the electrical resistivity as a function of temperature, ρ(T), for different values of applied magnetic field, B_a (0 ≤ B_a ≤ 50 mT), were performed in polycrystalline samples of Bi_1.65Pb_0.35Sr₂Ca₂ Cu₃O_10+δ subjected to different uniaxial compacting pressure (UCP). We have found appreciable differences in the grain orientation between samples by using X-ray diffractometry. From the X-ray diffraction patterns performed, in powder and pellet samples, we have estimated the Lotgering factor along the (00l) direction, F_(00l). The results indicate that F_(00l) increases ~23% with increasing UCP suggesting that grains of these samples are preferentially aligned along the c-axis, which is parallel to the compacting direction. The resistive transition of the samples have been interpreted in terms of the thermally activated flux-creep model. In addition, the effective intergranular pinning energy, U₀, have been determined for different applied magnetic field. The magnetic field dependence of U₀, for B_a > 8 mT, was found to follow a H^{- α} dependence with α = 0.5 for all samples. The analysis of the experimental data strongly suggested that increasing UCP results in appreciable changes in both the grain alignment and the grain connectivity of the samples. We have successfully interpreted the data by considering the existence of three different superconducting levels within the samples: the superconducting grains, the weak-links, and the superconducting clusters.     

Electron transport properties of MgB<Subscript>2</Subscript> in the normal state

We report measurements of the resistivity, ρ, and the Seebeck coefficient, S , of a MgB₂ sintered sample, and compare S with theoretical calculations based on precise electronic structure calculations. ρ is fitted well by a generalized Bloch-Grüneisen equation with a Debye temperature Θ _R of 1050 K. S is given by the sum of a diffusive and a phonon drag term and the behavior in the temperature region T _c < T < 0.1Θ _R follows the relationship AT+BT³. The phonon drag term indicates a strong electron-phonon interaction. The diffusive term, compared with calculations, suggests that σ bands give the main contribution to the Seebeck effect. Received 16 November 2001 and Received in final form 21 December 2001     

Correlated vortex pinning in Si-nanoparticle doped MgB2

The magnetoresistivity and critical current density of well characterized Si-nanoparticle doped and undoped Cu-sheathed MgB₂ tapes have been measured at temperatures T≥28 K in magnetic fields B≤0.9 T. The irreversibility line B_irr(T) for doped tape shows a stepwise variation with a kink around 0.3 T. Such B_irr(T) variation is typical for high-temperature superconductors with columnar defects (a kink occurs near the matching field B_?) and is very different from a smooth B_irr(T) variation in undoped MgB₂ samples. The microstructure studies of nanoparticle doped MgB₂ samples show uniformly dispersed nanoprecipitates, which probably act as a correlated disorder. The observed difference between the field variations of the critical current density and pinning force density of the doped and undoped tape supports the above findings.     

Relation between vortex excitation and thermal conductivity in superconductors

Thermal conductivity κ _xx(T) under a field is investigated in d _{x2 - y2}-wave superconductors and isotropic s-wave superconductors by the linear response theory, using a microscopic wave function of the vortex lattice states. To study the origin of the different field dependence of κ_xx(T) between higher and lower temperature regions, we analyze the spatially-resolved thermal conductivity around a vortex at each temperature, which is related to the spectrum of the local density of states. We also discuss the electric conductivity in the same formulation for a comparison. Received 8 December 2001 and Received in final form 20 March 2002 Published online 6 June 2002     

Enhanced superconducting properties of Eu2O3-doped MgB2

Bulk polycrystalline samples of Eu₂O₃-doped MgB₂ have been synthesized by a standard solid state reaction route and their structural and superconducting properties have been investigated. As a function of Eu₂O₃ content we have found a significant increase in the critical current density (J_c) and the irreversibility field (H_irr) in the magnetic field range 0–6 T. The XRD results reveal the presence of MgO and EuB₆ secondary phases along with the main hexagonal phase of MgB₂. The strain values and the lattice distortions have been found to increase almost linearly with the nominal Eu₂O₃ content. The observed significant improvement in J_c(H) and H_irr in the Eu₂O₃-doped MgB₂ samples, thus is mainly attributed to the lattice distortions introduced by Eu₂O₃ doping.     

Magnetotransport properties of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> with different grain sizes

The magnetotransport and magnetoresistive (MR) properties of manganese-based La_0.67Ca_0.33MnO₃ perovskite with different grain sizes are reported. The electrical resistivity was measured as a function of temperature in magnetic fields of 0.5 and 1 T. The insulator–metal transition temperature, T _IM, shifted to a higher temperature with the application of the magnetic field. In zero field, T _IM is almost constant (∼271 K) for all samples except for the sample with the largest grain size, where T _IM=265 K. The temperature dependence of resistivity was fitted with several equations in the metallic (ferromagnetic) region and the insulating (paramagnetic) region. The density of states at the Fermi level, N(E _F), and the activation energy of electron hopping were estimated by fitting the resistivity versus temperature curves. The ρ–T ² curves are nearly linear in the metallic regime, but the ρ–T ^2.5 curves exhibit a deviation from linearity. The variable range hopping model and small polaron hopping model fit the data well in the high-temperature region, indicating the existence of the Jahn–Teller distortion that localizes the charge carriers. MR was found to increase with an increase in the magnetic field, an effect which is attributed to the intergrain spin tunneling effect.     

Two regimes in the magnetic field response of superconducting MgB<Subscript>2</Subscript>

Using Scanning Tunneling Microscope at low temperature we explore the superconducting phase diagram in the π-band of the two-band superconductor MgB₂. In this band the peculiar shape of the local tunneling spectra and their dynamics in the magnetic field reveal the complex character of the quasiparticle density of states (DOS). The gap in the DOS is first rapidly filled with states in raising the magnetic field up to 0.5 T and then slowly approaches the normal state value: the gap is observed up to 2 T. Such a change in the DOS dynamics suggests the existence of two terms in the DOS of the π-band: a first one, reflecting an intrinsic superconductivity in the band and a second one, originating from an inter-band coupling to the σ-band. Our findings allow a deeper understanding of the unique phase diagram of MgB₂.     

MgB2 coated superconducting tapes with high critical current densities fabricated by hybrid physical-chemical vapor deposition

The MgB₂ coated superconducting tapes have been fabricated on textured Cu (0 0 1) and polycrystalline Hastelloy tapes using coated conductor technique, which has been developed for the second generation high temperature superconducting wires. The MgB₂/Cu tapes were fabricated over a wide temperature range of 460-520 °C by using hybrid physical-chemical vapor deposition (HPCVD) technique. The tapes exhibited the critical temperatures (T_c) ranging between 36 and 38 K with superconducting transition width (ΔT_c) of about 0.3-0.6 K. The highest critical current density (J_c) of 1.34 × 10⁵ A/cm² at 5 K under 3 T is obtained for the MgB₂/Cu tape grown at 460 °C. To further improve the flux pinning property of MgB₂ tapes, SiC is coated as an impurity layer on the Cu tape. In contrast to pure MgB₂/Cu tapes, the MgB₂ on SiC-coated Cu tapes exhibited opposite trend in the dependence of J_c with growth temperature. The improved flux pinning by the additional defects created by SiC-impurity layer along with the MgB₂ grain boundaries lead to strong improvement in J_c for the MgB₂/SiC/Cu tapes. The MgB₂/Hastelloy superconducting tapes fabricated at a temperature of 520 °C showed the critical temperatures ranging between 38.5 and 39.6 K. We obtained much higher J_c values over the wide field range for MgB₂/Hastelloy tapes than the previously reported data on other metallic substrates, such as Cu, SS, and Nb. The J_c values of J_c(20 K, 0 T) ∼5.8 × 10⁶ A/cm² and J_c(20 K, 1.5 T) ∼2.4 × 10⁵ A/cm² is obtained for the 2-μm-thick MgB₂/Hastelloy tape. This paper will review the merits of coated conductor approach along with the HPCVD technique to fabricate MgB₂ conductors with high T_c and J_c values which are useful for large scale applications.     

Normal-state electrical resistivity and superconducting magnetic penetration depth in Eu<Subscript>0.5</Subscript>K<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> polycrystals

We report measurements of the temperature dependence of the electrical resistivity, ρ(T), and magnetic pen-etration depth, λ(T), for polycrystalline samples of Eu_0.5K_0.5Fe₂As₂ with T _c = 31 K. ρ(T) follows a linear temperature dependence above T _c and bends over to a weaker temperature dependence around 150 K. The magnetic penetration depth, determined by radio frequency technique displays an unusual minimum around 4 K which is associated with short-range ordering of localized Eu³⁺ moments. The article is published in the original.     

Anomalous low-temperature behavior of the thermal characteristics of MgB<Subscript>2</Subscript>

We have studied the behavior of the thermal expansion coefficient α(T) (in a zero magnetic field and at H≈4 T), the heat capacity C(T), and the thermal conductivity κ(T) of magnesium boride (MgB₂) in the vicinity of T_c and at lower temperatures. It was established that MgB₂, like oxide-based high-temperature superconductors, exhibits a negative thermal expansion coefficient at low temperatures. The anomaly of α(T) in MgB₂ is significantly affected by the magnetic field. It was established that, in addition to the well-known superconducting transition at T_c≈40 K, MgB₂ exhibits an anomalous behavior of both heat capacity and thermal conductivity in the region of T≈10–12 K. The anomalies of C(T) and κ(T) take place in the same temperature interval where the thermal expansion coefficient of MgB₂ becomes negative. The low-temperature anomalies are related to the presence of a second group of charge carriers in MgB₂ and to an increase in the density of the Bose condensate corresponding to these carriers at T_c2≈10–12 K.     

On the effective Debye temperatures of the C60 fullerite

The effective Debye temperatures Θ_eff determined for solids by different physical methods have been analyzed and compared. Attention has been focused on the original parameter of the Debye theory of heat capacity, i.e., the translational calorimetric Debye temperature Θ _c ^t (0), and the X-ray Debye temperature Θ _x in the framework of the Debye-Waller theory for the C₆₀ fullerite. It has been established that the true Debye law T ³ is satisfied for the C₆₀ fullerite over a very narrow range of temperatures: 0.4 K ≤ T ≤ 1.8 K. For this reason, the experimental Debye temperatures Θ _c ^t (0) obtained for the C₆₀ fullerite by different authors in the range T > 4.2 K are characterized by a large scatter (by a factor of ∼5). It has been revealed that the value Θ _c ^t (0) = 77.12 K calculated in this paper with the use of the six-term Betts formula from the harmonic elastic constants $ \tilde C_{ijkl} $ \tilde C_{ijkl} of the C₆₀ single crystal in the limit T = 0 K is closest to the true Debye temperature. It has been demonstrated using the method of equivalent moments that the real spectral frequency distribution of translational lattice vibrations g(ω) for the C₆₀ fullerite deviates from a parabolic distribution. The effective Debye temperatures Θ_eff involved in applied problems of thermodynamics of crystals and elastic scattering of different radiations from lattice vibrations have been determined. The quantitative measure of anharmonicity of translational and librational lattice vibrations of the C₆₀ fullerite has been determined. This has made it possible to empirically evaluate the lattice thermal conductivity κ of the C₆₀ fullerite at T ≈ 300 K: κ(300) = 0.80 W (m/K), which is in good agreement with the experimental thermal conductivity κ_exp = 0.78 W (m/K) at T ≈ 250 K.     

Electron-phonon interaction in NbB2: a comparison with MgB2

We present a comparison of electron-phonon interaction in NbB₂ and MgB₂, calculated using full-potential, density-functional-based methods in P6/mmm crystal structure. Our results, described in terms of (i) electronic structure, (ii) phonon density of states F(ω), (iii) Eliashberg function α²F(ω), and (iv) the solutions of the isotropic Eliashberg gap equation, clearly show significant differences in the electron-phonon interaction in NbB₂ and MgB₂. We find that the average electron-phonon coupling constant λ is equal to 0.59 for MgB₂ and 0.43 for NbB₂, leading to superconducting transition temperatures T_c at around 22 K for MgB₂ and 3 K for NbB₂.     

Kinetic phenomena in zero-gap semiconductors CuFeS<Subscript>2</Subscript> and CuFeTe<Subscript>2</Subscript>: Effect of pressure and heat treatment

Electrical resistivity ρ and Hal coefficient R are measured as a function of the temperature (T = 1.7−310 K) and the magnetic field (up to H = 28 kOe) in zero-gap semiconductor CuFeS₂ samples subjected to hydrostatic compression and under various heat-treatment conditions. At low temperatures, anomalies are observed in the kinetic effects related to the presence of ferromagnetic clusters: the magnetoresistance at T = 4.2 K and T = 20.4 K acquires a hysteretic character and thermopower α changes its sign at T < 15 K. The temperature dependence of conduction-electron concentration n in CuFeS₂ has a power form in the temperature range T = 14−300 K, which is characteristic of the intrinsic conductivity in zero-gap semiconductors. In CuFeS₂, we have n(T) ∝ T ^1.2; in isoelectron compound Cu_1.13Fe_1.22Te₂, we have n(T) ∝ T ^1.93. Heat treatment is found to affect the intrinsic conductivity of CuFeS₂, as the action of hydrostatic compression (carrier concentration changes); that is, the carrier concentration changes. However, a power form of the n(T) and ρ(T) dependences is retained.     

Magnetotransport and magnetocaloric effect in Ho<Subscript>2</Subscript>In

The magnetotransport and magnetic properties of the binary intermetallic compound Ho₂In have been investigated. Clear signature of long range ferromagnetic order in the resistivity and the magnetization data at T_C = 85 K is observed. A further spin reorientation type transition is also apparent in our measured data at around T_t = 32 K. The sample exhibits negative magnetoresistance (peak value of –14% at 5 T) over a wide temperature range that extends well above T_C. Substantially large magneto-caloric effect is also observed in the sample (maximum value of –8.5 J kg^-1K^-1 for 0 → 5 T), which peaks around the T_C of the sample. The observed magnetoresistance and magnetocaloric effect are related to the suppression of spin disorder by an external magnetic field. Ho₂In can be an interesting addition to the list of rare-earth based magnetic refrigerant materials showing magneto-caloric effect across a second order phase transition.     

Influence of inter cell resonant tunneling on the out-of-plane electronic transport behavior in layered high T<Subscript>c</Subscript> cuprates

The influence of inter unit cell resonant tunneling between the copper-oxygen planes on the c-axis electronic conductivity (σ_c) in normal state of optimal doped bilayer high T_c cuprates like Bi₂Sr₂CaCu₂O_8+x is investigated using extended Hubbard Hamiltonian including resonant tunneling term (T₁₂) between the planes in two adjoining cells. The expression for the out-of-plane (c-axis) conductivity is calculated within Kubo formalism and single particle Green's function by employing Green's function equations of motion technique within meanfield approximation. On the basis of numerical computation, it is pointed out that the renormalized c-axis conductivity increases exponentially with the increment in inter cell resonant tunneling. The effect of T₁₂ on renormalized c-axis conductivity is found to be prominent at low temperatures as compared to temperatures above room temperature (~300 °K). The Coulomb correlation suppresses the variation of renormalized c-axis conductivity with temperature, while renormalized c-axis conductivity increases on increasing carrier concentration. These theoretical results are viewed in terms of existing c-axis transport measurements.     

Reversible magnetization and irreversibility line of tri-layer superconductor Ba2Ca2Cu3O6(O,F)2 with Tc∼108 K

We report magnetization measurements of grain-aligned Ba₂Ca₂Cu₃O₆(O,F)₂ with T_c?108 K. The interlayer distance of the material is the shortest among known tri-layer superconductors. Unexpectedly, the magnetization data show that the coupling strength between CuO₂ layers is rather weak. A direct reflection of the weak coupling is highly suppressed irreversibility line, i.e. a broad reversible region in H-T plane. The decoupling field obtained from the irreversibility line is less than 0.1 T, which is comparable with that of quasi two-dimensional superconductor Bi₂Sr₂CaCu₂O_8+δ. Comparison of data with the Hao-Clem model gives characteristic parameters [ξ_ab(0) and λ_ab(0)] and the critical fields [H_c(0) and H_c2^c(0)]. A large value of penetration depth, λ_ab(0)=240 nm reflects a small carrier concentration in CuO₂ planes, and explains the reason of the weak interlayer coupling.     

Ion conduction and space-charge polarization processes in Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript> crystals

The electrical properties of a lithium heptagermanate (Li₂Ge₇O₁₅) crystal have been studied in DC and AC measuring fields at temperatures from 500 to 700 K. In a DC field, a substantial decrease of electrical conductivity σ with time has been detected. On the basis of kinetic dependences σ(t), estimates of the charge carrier diffusion coefficient D have been obtained. In the frequency range 10¹–10⁵ Hz, the spectra of complex impedance ρ*(f) have been measured. The analysis of diagrams in the complex plane (ρ″–ρ′) has been performed within the equivalent circuit approach. It has been shown that, in the considered temperature and frequency intervals, the electrical properties of Li₂Ge₇O₁₅ crystals have been determined by the hopping conduction of interstitial lithium ions A _Li and accumulation of charge carriers near the blocking Pt electrodes.     

Equilibrium low temperature heat capacity of the spin density wave compound (TMTTF)<Subscript>2</Subscript>Br: effect of a magnetic field

We have investigated the effect of the magnetic field (B) on the very low-temperature equilibrium heat capacity c_eq of the quasi-1 D organic compound (TMTTF)₂Br, characterized by a commensurate Spin Density Wave (SDW) ground state. Below 1 K, c_eq is dominated by a Schottky-like A_ST^-2 contribution, very sensitive to the experimental time scale, a property that we have previously measured in numerous DW compounds. Under applied field (in the range 0.2–7 T), the equilibrium dynamics, and hence c_eq extracted from the time constant, increases enormously. For B ≥ 2–3 T, c_eq varies like B², in agreement with a magnetic Zeeman coupling. Another specific property, common to other Charge/Spin density wave (DW) compounds, is the occurrence of metastable branches in c_eq, induced at very low temperature by the field exceeding a critical value. These effects are discussed within a generalization to SDWs in a magnetic field of the available Larkin-Ovchinnikov local model of strong pinning. A limitation of the model when compared to experiments is pointed out.     

Ageing of the avalanche angle in immersed granular matter

We have studied the temperature dependent resistivity ρ( T ) of La_2-xSr_xCuO₄ epitaxial thin films in the doping range 0.045 ⩽ x ⩽ 0.25 in pulsed magnetic fields up to 50 T. The zero-field resistivity ρ( T ) of these samples in the pseudogap regime, can be scaled onto one single universal curve in a broad temperature range by using a linear transformation of both temperature and resistivity. The high field data ρ( T ) reveal a metal to insulator transition (MIT) at low temperatures, well into the overdoped regime. For samples having k _F l < 1, with k_F the Fermi wave vector and l the mean free path, this low temperature insulating behavior of the resistivity is described by the variable range hopping conductivity (VRH). For samples with k _F l > 1, the divergence follows ρ( T ) ∼ ln (1/ T ) or a power law, depending upon the Sr-content. We further found that the residual conductivity at the minimum in ρ( T ), appearing due to the MIT, follows a linear behavior with respect to the Sr-content. It is argued that the unusual MIT in compounds with k _F l > 1, is most probably associated with the pseudogap and the behavior of charge stripes at low temperatures. Received 4 January 2002 / Received in final form 7 May 2002 Published online 14 October 2002 RID="a" ID="a"e-mail: liesbet.weckhuysen@fys.kuleuven.ac.be     

Optical measurements of the superconducting gap in MgB 2

Far-infrared reflectivity studies on the polycrystalline intermetallic compound MgB₂ with a superconducting transition temperature T _c = 39 K were performed at temperatures 20 K to 300 K. We observe a significant raise of the superconducting-to-normal state reflectivity ratio below 70 cm ^-1 , with a maximum at about 25-30 cm ^-1 , which gives a lower estimate of the superconducting gap of 2Δ(0) ≈ 3-4 meV. Received 7 March 2001 and Received in final form 18 April 2001