Enhancement of the upper critical field of MgB2 by carbon-doping

We have measured the temperature dependence of the upper critical field, H_c2(T), of carbon-doped MgB₂. H_c2(T) does not follow the well-known Werthamer-Helfand-Hohenberg (WHH) result for a one-gap dirty superconductor but can be described well by the result of a recent theoretical calculation for a two-gap dirty superconductor. H_c2(0) of the carbon-doped material is determined to be between 29 and 38 T, substantially higher than that of pure MgB₂ (15-23 T).     

AuB2 in comparison to superconducting MgB2-an ab initio study

The noble metal diboride AuB₂, a potential candidate for superconductor, is studied by an ab initio method in comparison to the superconducting MgB₂. The results, described in terms of equilibrium lattice constants, bulk modulus, pressure derivative of bulk modulus and their in- and out-of-plane linear values, volume coefficient of T_c, density of states, band structure, show some similarity as well as dissimilarity between the behaviour of the two compounds. The implications for the behaviour are discussed.     

Degradation of superconducting properties in MgB2 by Cu addition

The (MgB₂)_2−xCu_x (x=0-0.5) superconducting system was prepared by a solid-state reaction technique. Microstructural evolution and transport properties including resistivity versus temperature up to a magnetic field of 6 T, activation energy, thermoelectric power and Fermi energy, E_F, and the corresponding velocity, V_F, values of the samples prepared were also investigated. The XRD analysis showed a multiphase formation and no detectable solution of Cu in MgB₂. Two different impurity phases, MgCu₂ and CuB₂₄, have been identified and their peak intensity increased when the Cu concentration increased. The temperature dependence of the resistivity of the samples showed a metallic behavior down to T_c. But, for the Cu concentrations above 0.3 the superconducting phase transition completely disappeared. The magnetic field strongly affects the electrical properties. For x=0.0 samples, the transition is found to be sharp, ΔT∼1 K, but it becomes broader with increasing magnetic field and Cu concentration. The calculated values of carrier concentration, n, of the samples are showed a sharp decrease with increasing Cu content. For x=0.0 sample the n was calculated to be 12×10²¹ cm⁻³, but for the x=0.5 sample it decreased to 1.3×10²¹ cm⁻³. We found that the activation energy, U(B), decreased sharply with increasing magnetic field. According to thermoelectric power and Fermi energy, E_F, calculations the decrease of the carrier concentration by the additions of Cu into MgB₂ gives a decrease in E_F and this could be attributed to a shift of the Fermi level towards the top of the σ-hole band.     

Angular dependence of vortex matter phase transition in MgB2 single crystal

The vortex matter phase transitions and intrinsic pinning effect were investigated in an MgB₂ single crystal using the torque magnetometry. For the field directions apart from the ab plane, we succeed in the observation of the vortex lattice melting transitions, which are transformed from the order-disorder transitions at low temperatures. Both transition fields with field directions can be describe by the GL effective mass model. For the field direction along the ab plane, these transitions become unobservable. Instead, the sudden increase in the hysteresis of magnetization curve occurs, indicating the existence of the intrinsic pinning coming from the layer structure.     

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.     

Properties of hot pressed MgB2/Ti tapes

Hot axial and hot isostatic pressing was applied for single-core MgB₂/Ti tapes. Differences in transport current density, n-exponents and critical current anisotropy are discussed and related to the grain connectivity influenced by pressing. The magnetic Hall probe scanning measurements allowed observing the isolated regions for axially hot pressed sample attributed to the longitudinally oriented cracks introduced by pressing. The highest current densities were measured for the tape subjected to hot isostatic pressing due to improved connectivity.     

Pinning mechanism in carbon nanotube doping of MgB2 superconductor

MgB_1.9C_0.1 samples are synthesized under the ambient pressure (AP) and high pressure (HP), respectively. The further studies demonstrate different field-dependence of the critical current density J_c(H) in each sample. In the view of two-gap superconductivity in these samples, δT_c pinning (resulting from the spatial fluctuations of the transition temperature) is dominant in the AP sample, while in the HP sample, both δT_c and δl pinning (due to the mean-free-path fluctuations) act together and their contributions vary with temperature. Besides the improvement of H_c2(0), due to the different pinning mechanism, J_c(H) of the HP sample shows a slower decay with the increasing fields than that of the AP sample in high fields, which suggests a possible method of retarding the rapid decay of J_c(H) under elevated fields.     

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.     

Effect of C and SiC additions into in situ or mechanically alloyed MgB2 deformed in Ti sheath

Effect of 3.4 wt.% C and 5 wt.% SiC doping into the standard in situ (IN) process and mechanically alloyed (MA) MgB₂ was studied. Powders of IN and MA process were carried out in air and in argon filled glove box, respectively. Wire samples were prepared by two-axial rolling deformation of IN and MA powders inside the Ti tube. Titanium as sheath material allows to use higher sintering temperatures, we used 700 °C and 800 °C for 30 min in Argon. Critical current densities (J_c) were measured at variable temperatures 4.2 K, 10 K, 15 K and 20 K in the external magnetic fields ranging to 15 T. Critical temperatures, upper critical fields and irreversibility fields of IN and MA with SiC and C additions are compared and discussed. The highest transport properties were observed for wires with MA SiC doped MgB₂ in the whole scale of temperatures 4.2–20 K. Upper critical field was rapidly enhanced in the case of carbon doped MA samples at 4.2 K. MA samples have shown decreased J_c values for higher temperatures (15 K, 20 K), in some case even worse than for the not doped reference IN sample. Carbon substitution and grain connectivity of analyzed samples are compared and discussed. Presented results show that for 20 K applications some new ways (additions) have to be found for increasing the J_c substantially.     

First-principles investigation of pressure-induced changes in structural and electronic properties of Y 2C3 superconductor

The structural and electronic properties of Y ₂C₃ superconductor under different external pressures were calculated by employing the first-principles method. This shows that the lattice constants as well as the lengths of C-C dimers decrease with the pressure. Results of band structure calculations indicate that the Fermi level advances to the bonding zone with an increase in pressure; meantime, the valence and conduction bands intersect more deeply with the Fermi level. Moreover, the Fermi level is found to shift from the valley bottom of the density of states (DOS) curve to the shoulder, which means an increase in N(E_F), and therefore the critical temperature, T_c. The calculations verify that the critical temperature is directly related to the electronic structure.     

Neon ion irradiation studies on MgB2 superconductor

160 MeV of neon ion irradiation has been carried out on MgB₂ polycrystalline pellets at various doses. There has not been any significant change in T_c except at the highest dose of 1×10¹⁵ ions/cm². Increase in resistivity has been noticed. Resistivity data have been fitted with Bloch-Grüneisen function to extract the values of Debye temperature, residual resistivity and temperature coefficient of resistivity for irradiated as well as unirradiated samples. There has not been any significant effect on electron-phonon coupling due to irradiation as evident from Debye temperature and the electron-phonon coupling constant.     

Calculation of Tc and the ratio 2Δ(0)/kBTc of MgB2 within a two-band model of superconductivity

Superconductivity in the MgB₂ superconductor is described within the framework of a two-band Eliashberg formalism. Different gaps are assumed to open on the different parts of the Fermi surface of this compound. Separation of the order parameter (OP) into two components is achieved by taking the Fourier transform of the OP using the momentum states of the σ- and π-bands of MgB₂. Expressions for the T_c and the ratio 2Δ_σ(0)/k_BT_c for this superconductor are obtained. Numerical values for these two properties are obtained for a range of values of the cut-off frequency of the phonons responsible for the superconductivity and for a range of values of the ratio between the two energy gaps. This was done for various values of the normalized partial densities of states on the σ-sheet of the Fermi surface.     

Structural and thermodynamic properties of MgB2 from first-principles calculations

A first-principles plane wave method with the relativistic analytic pseudopotential of Hartwigsen, Goedecker and Hutter (HGH) scheme in the frame of local density approximation is performed to calculate the lattice parameters and the equation of states (EOS) of superconducting MgB₂. Our calculations show that the ratio c/a of about 1.134 is the most stable structure for MgB₂, as is consistent with experiment and other theoretical results. Also, the isothermal and isobaric properties are discussed from energy-volume curves using a quasi-harmonic Debey model.     

Superconductivity in porous MgB2

Porous magnesium diboride samples have been prepared by the heat treatment of a pressed mixture of Mg and MgB₂ powders. It was found that linked superconducting structure is formed down to the minimum normalized density γ_c=d/d₀≅0.16 (percolation threshold), where d is the density of MgB₂ averaged over the sample, d₀=2.62 g/cm³ is the X-ray density. Lattice parameters and critical temperature of the porous sample decrease with increasing porosity (decreasing γ) and T_c2≅32 K is minimal at γ_c. The grain boundaries in the porous samples are transparent for the current and J_c∼3×10⁵ A/cm² in self field at T=20 K in the samples with γ∼0.24.     

Elastic and electronic properties of BeB2 in comparison to superconducting MgB2 and NbB2

A study of the structural, elastic and electronic properties of BeB₂, a promising diboride in view of the lighter mass of Be, has been made using self-consistent Density Functional Theory (DFT). The five independent elastic constants of BeB₂ are predicted for the first time. The elastic behaviour of the compound is compared with those of the superconducting MgB₂ and NbB₂. We also evaluate the electronic structure of BeB₂ in order to obtain further insight into its surprising difference from the superconducting MgB₂.     

Core level photoemission study of polycrystalline MgB2

X-ray photoemission measurements have been made on Mg 2p and B 1s core levels from MgB₂ as well as on the O 1s core level from the contaminant oxygen. The motivation for this study came for the discrepancy in the reported papers on the energy values of the B 1s and Mg 2p core levels, ostensibly due to the oxygen, present as an impurity, reacting and forming oxides and other compounds. Sputtering leads to a significant reduction of oxygen on and in the sample. The in situ sputter-cleaned sample gives the best results. The measurements are compared with results reported earlier and discrepancies discussed.     

First-principles study on the electronic structure and magnetic properties of metallic antiferromagnet C7H3S2N2

First-principles full potential linearized augmented plane wave (FPLAPW) calculations have been performed to study the electronic structure and the magnetic properties of 3-Cyanobenzo-1,3,2-dithiazolyl,C₇H₃S₂N₂. The density of states (DOS), the total energy of the cell, and the spontaneous magnetic moment of C₇H₃S₂N₂ were all calculated. The calculations reveal that the low-temperature phase of the compound C₇H₃S₂N₂ has a stable metal-antiferromagnetic ground state, and there exists an antiferromagnetically coupled interactions between the dithiazolyl radical(1), which is in good agreement with experiment.     

Synthesis and magnetic properties of off stoichiometric Laves phase Fe2Nb amorphous alloy

The formation process and magnetic properties of the distorted Laves phase compound (Fe₂Nb) of the nominal composition Fe₆₀Nb₄₀ in its amorphous phase prepared by mechanical alloying have been investigated. The effect of milling time on the formation of amorphous phase has been studied using the X-ray diffraction technique. Further characterizations were carried out by particle size measurement, energy dispersive X-ray microanalysis (EDAX), dc magnetisation and ferromagnetic resonance (FMR) studies. Magnetisation measurement shows that the deviation from the stoichiometric composition in the Nb rich side enhances the compositional short range order and favours the moment formation responsible for weak ferromagnetic behaviour whereas ferromagnetic resonance spectra show some sort of disorder/strain introduced during the mechanical alloying process. The method of preparation also affects the magnetic behaviour of nominal composition Fe₆₀Nb₄₀.     

Two-band nature of upper critical fields in MgB2 thin films investigated by 37 T pulsed magnet

We present experimental and calculated phase diagrams of the upper critical field H_c2 and the anisotropy parameter γ of various MgB₂ thin films prepared by the molecular beam epitaxy (MBE) and the multiple-targets sputtering system. Experimental data of the H_c2(T) and γ are analyzed by fitting the Gurevich theory. We obtained the result that for these MgB₂ films the H_c2(T) is explained as the case of D_π/D_σ>1, which means the films are classified to the cleaner π band than the σ band. We discuss temperature dependence of the transition width obtained from the magnetoresistance measurements.     

Study of the Ru sublattice magnetic structure in the magnetic superconductor RuSr2GdCu2O8

In order to investigate the Ru sublattice magnetic structure, a study of the field dependence of the ^99,101Ru nuclear magnetic resonance (NMR) has been carried out on the magnetic superconductor RuSr₂GdCu₂O₈. It is found that the ^99,101Ru NMR signal intensity increases significantly with applied magnetic field up to ≈3 kOe, beyond which, it progressively decreases. In addition, a shift of the NMR peaks to lower frequency is observed to begin at ≈1.3 kOe. These behaviors are shown to be accompanied by a field-induced Ru moment spin-flop in the ab planes, and are understood in terms of a previously proposed type-I antiferromagnetic ordering for the Ru sublattice. Based on this model, the inter-plane antiferromagnetic exchange coupling is determined to be ≈1.8 kOe along with a reversible in-plane spin-flop which is characterized by a field ≈0.6 kOe.