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.     

Investigation of superconductivity in the single-walled carbon nanotubes

Superconductivity in the single-walled carbon nanotubes is investigated. First, effect of diameter increasing on the clean systems critical temperature, T_c, is calculated. Then effect of impurity doping on the reduction of critical temperature T_c, of single-walled carbon nanotubes, is discussed. Our calculations illustrate that metallic zigzag single-walled carbon nanotubes have higher T_c than armchair single-walled carbon nanotubes with approximately same diameters and T_c decreases by increasing diameter. This can explain why superconductivity could be found in the small diameter single-walled carbon nanotubes. We found for the impurity doped systems, impurity in the strong scattering regime can decrease T_c significantly while in the weak scattering regime T_c is not affected by impurity doping.     

Anomalous behavior of superconducting transition temperature of amorphous and crystalline Nb3Ge under pressure

The superconducting transition temperature (T_c) of amorphous Nb₃Ge was studied under both hydrostatic and quasihydrostatic pressure to 3.5 and 13 GPa, respectively. Whereas hydrostatic pressure causes T_c to initially decrease, T_c is found to increase under higher quasihydrostatic pressures. T_c(p) was also studied on an A-15 crystalline Nb₃Ge sample obtained from the amorphous sample by annealing.     

Hydrogen absorption and its effect on the magnetic properties of rare-earth iron intermetallics

X-ray diffraction studies of the hydrogen absorption in several YFe and CeFe intermetallic compounds showed that no structural changes occur upon hydrogen absorption in Y₆Fe₂₃, YFe₃, YFe₂. The lattice constants of the hydrides were found to be appreciably larger than those of the pure intermetallic compounds. The magnetic properties of the hydrides were determined and compared with the original compounds. In all cases the magnetic moment per Fe atom proved to be much larger in the hydride phases. Hydrogen absorption can lead to a decrease as well as to an increase of the magnetic ordering temperature (T_c). These changes in T_c could adequately be explained in terms of the observed increases in lattice constant and the data available for the pressure derivative of T_c of these compounds.     

Low-frequency electrical resistance of iron,cobalt, and nickel in the vicinity of their Curie temperatures

Alternating-current electrical resistance measurements between 17 Hz and 100 kHz were made on high purity Fe, Co, and Ni wires in the vicinity of their Curie temperatures (T _c). The electrical resistance was independent of frequency for temperatures (T) aboveT _c. As the temperature was lowered, however, there was an abrupt jump in the electrical resistance atT _c followed by a gradual decrease toward its dc value. The magnitude of the electrical resistance jump atT _c increased as the square root of the frequency. The enhancement of the electrical resistance forT≦T _c is produced by an abrupt decrease of the skin depth atT _c which, in turn, is due to the sudden increase in the initial magnetic permeability atT _c. Measurements of the ac electrical resistance in the vicinity of the Curie temperature of certain ferromagnetic metals can be utilized to 1) accurately determine the Curie temperature using frequencies as low as 17 Hz, and 2) quantitatively determine the initial magnetic permeability as a function of temperature and heat treatment.     

Dielectric properties of betaine phosphite-betaine phosphate solid-solution crystals in the improper ferroelastic phase

Temperature dependences of dielectric permittivity in the improper ferroelastic phase, including the region of the improper ferroelastic phase transition occurring at T=T_c1, were studied in the betaine phosphite-betaine phosphate solid-solution crystals. At a betaine phosphate (BP) concentration of 10%, the phase transition temperature T_c1 was found to shift toward higher temperatures by about 5 K compared to betaine phosphite (BPI) crystals, where T_c1=355 K. The phase transition remains in the vicinity of the tricritical point. As the BP concentration in BPI is increased, the dielectric anomaly at T=T_c1 weakens substantially compared to pure BPI. The nonlinear temperature dependence of reciprocal dielectric permittivity in the improper ferroelastic phase of BPI_xBP_1?x crystals is described in the concentration region 0.9≤x≤1 in terms of a thermodynamic model taking into account the biquadratic relation of the nonpolar order parameter of the improper ferroelastic phase transition to polarization. The decrease in the ferroelectric phase transition temperature T_c1 (or in the temperature of loss of improper ferroelastic phase stability) with increasing BP concentration in the above limits is due to the decreasing effect of the nonpolar mode on the polar instability, which is accompanied by a weakening of the dielectric anomaly at T=T_c1     

Influence of annealing on magnetic properties of Co-based metallic glasses

The influence of low-temperature annealing on the magnetic hysteresis loop parameters of magnetostrictive Co-Si-B and nonmagnetostrictive Co-Fe-Si-B glasses was studied. The dependence of Curie temperature (T_C), crystallization temperature (T_x), full-stress-relaxation temperature, saturation magnetostriction constant, saturation magnetization and coercive field H_c on metalloids contents for as quenched Co_100-x(Si_0.5B_0.5)_x glasses was determined.It was found that annealing enhances remanence magnetization for all investigated Co-Si-B glasses. The coercive field of these glasses is influenced by annealing owing to: stress relaxation (resulting in an H_c decrease) and domain structure stabilization (resulting in an H_c increase). Significant H_c reduction for both magnetostrictive and non-magnetostrictive glasses was observed, after annealing above T_C. For glasses with T_C<>T_x, it was necessary to apply an external magnetic field in ord er to decrease H_c.It was found that non-magnetostrictive metallic glasses with low Curie temperatures (T_C ? 450 K) exhibit the most stable magnetic hysteresis loop parameters.     

Heat capacity and lattice dynamics of yttrium diboride in the temperature range 5–300 K

This paper reports an experimental study of the heat capacity and crystal lattice parameters of a polycrystalline sample of yttrium diboride prepared by high-temperature synthesis from elements. The electronic and lattice contributions to the heat capacity are isolated. The temperature dependences of the characteristic temperature, the linear thermal expansion coefficients α_a(T) and α _c (T), the bulk thermal expansion coefficient β(T), and the Grüneisen coefficient are calculated. A region of negative values of α _c (T) and β(T) is revealed. Anharmonicity is found to exert only a minor effect on the YB₂ lattice dynamics over a larger part of the temperature range covered.     

Modeling of the out-of-plane resistivity of cuprate superconductors

The out-of-plane (c-axis) resistivity, ρ_c(T), of high-T_c cuprates have been modeled in this study. The non-Fermi liquid like temperature dependence of ρ_c(T) has been described by considering (i) the full impact of the pseudogap (PG) in the electronic density of states (EDOS) and (ii) the presence of a quantum critical point (QCP) beneath the superconducting dome at slightly overdoped region. This simple phenomenological model describes the experimental ρ_c(T) data over a wide range of hole content (from the underdoped to slightly overdoped regions) remarkably well. The PG energy scale, ε_g (dominated by the anti-nodal parts of the Brillouin zone) extracted from the analysis of ρ_c(T) data was found to decrease almost linearly with increasing hole concentration, p, in the CuO₂ planes. We have also discussed about the possible origin of more conventional behavior of ρ_c(T) observed in the deeply overdoped side of the T–p phase diagram in this paper.     

B NMR relaxation in superconductors: YRuB2 and LuRuB2

To investigate the electronic states in YRuB₂ and LuRuB₂, we have carried out ¹¹B NMR measurements. In the normal state, the spin-lattice relaxation rates 1/T₁'s in these compounds are proportional to the temperature T. 1/T₁'s show a small coherence peak just below the superconducting transition temperature T_c and decrease exponentially well below T_c. YRuB₂ and LuRuB₂ are found to be BCS superconductors with the energy gap 2Δ(0)=3.52 k_BT_c.     

Low-temperature x-ray diffraction studies of the crystallographic parameters and thermal expansion of (CH3)2NH2Al(SO4)2 · 6H2O crystals

The a, b, c, and β crystallographic parameters of the (CH₃)₂NH₂Al(SO₄)₂ · 6H₂O crystal (DMAAS) have been measured by x-ray diffraction in the 90–300-K temperature range. The thermal expansion coefficients along the principal crystallographic axes α_a, α_b, and α_c have been determined. It was shown that, as the temperature is increased, the parameter α decreases and b increases, whereas c decreases for T<T _c (where T _c is the transition temperature) and increases for T>T _c, so that one observes a minimum in the c=f(T) curve in the region of the phase transition (PT) temperature T _c ～ 152 K. The thermal expansion coefficients α_a, α_b, and α_c vary in a complicated manner with increasing temperature, more specifically, α_a and α_c assume negative values at low temperatures, and the α_a=f(T), α_b=f(T), and α_c=f(T) curves exhibit anomalies at the PT point. The crystal has been found to be substantially anisotropic in thermal expansion.     

Role of inter-site exchange and hybrid interactions in itinerant ferromagnetism

In this work, we study the magnetic properties of itinerant electron systems using the Hubbard-like tight binding Hamiltonian along with inter-site exchange and hybrid interactions. We have used the mean-field approximation to deal with the exchange and hybrid interactions. It is found that hybrid interaction is more effective than exchange interaction for the on-set of ferromagnetic state. We have studied the effect of hybrid interaction on various physical quantities at different temperatures. The effective mass (m*/m) of up spin electrons increases slowly as the temperature decreases but below the critical temperature (T_c), it decreases rapidly. For down spin electrons effective mass increases slowly as the temperature decreases and below T_c, it increases more rapidly. Spectral weight (n/m*) for up spin electrons decreases slowly upto T_c and below T_c, it increases rapidly. For down spin electrons spectral weight decreases slowly upto T_c and below T_c, it decreases rapidly. Our results for both the effective mass and spectral weight are in good agreement with recently observed experimental behaviour in itinerant ferromagnet Ga_1−xMn_xAs [Phys. Rev. Lett. 89 (2002) 097203]. We have also studied variation of the spectral weight and optical absorption with temperature in presence of magnetic field. We found that these two quantities for up spin electrons increase as applied magnetic field increases at all temperatures (∼4T_c). For down spin electrons these two quantities decrease as applied magnetic field increases.     

On the hydrodynamic theory of central peak in displacive structural phase transitions

A model involving nonlinear coupling between the overdamped phonon mode and temperature fluctuations is studied using the dynamic renormalization group method. It is shown that the behavior of the central peak in the dynamic form factor depends on the specific heat exponent α. For α < 0 and as T → T_c the central peak is found to merge with the over-damped phonon mode and the coupling goes to zero at T_c as (T ? T_c)^?α. An argument on the intensity of the central peak in the critical region in presented and it is concluded that the hydrodynamic coupling may not be the dominant mechanism of the central peak in SrTiO₃.     

Superconducting phase transitions in ErcY1−cRh4B4

The superconducting transition temperature T_c1 and the reentering temperature T_c2 to the normal ferromagnetically ordered state in the system Er_cY_1?cRh₄B₄ were determined as a function of the concentration of Er. Comparing our results with the recent theory of Maekawa et al., the exchange interactions J′ among local spins and I between local spins and superconducting electrons were estimated as J′ ? 0.30 K and I²N(0) ? 0.046 K, where N(0) is the density of states. A sudden decrease of H_c2 at T_c2 was observed in a high concentration region suggesting the first order transition.     

Correlation between structural and superconducting properties of nano-granular disordered Nb thin films

We report on low temperature transport measurements on nano-granular Nb thin films deposited on Si (1 0 0) substrates using DC magnetron sputtering. The superconducting transition temperature (T_c) is found to decrease monotonically with the increase of the lattice parameter (a) irrespective of its thickness and grain size. The superconducting transition temperature is found to depend only on the lattice parameter whereas the normal state resistivity depends both on lattice parameter and the details of the sample morphology. We have modeled this T_c variation with lattice expansion in terms of Debye temperature reduction using Morse potential as the interatomic potential in Nb.     

Role of magnetism in the formation of a short-range order in an Fe-Ga alloy

The formation of a short-range order in an Fe-Ga bcc alloy has been studied by Monte Carlo simulation with the use of effective interaction potentials calculated within the density functional theory for the ferromagnetic and paramagnetic states. It has been found that the pronounced short-range order of the D0₃ type is formed at Ga concentrations close to the boundary of the two-phase region at T < T _c, whereas no short-range order is observed at T < T _c. The results obtained are in agreement with the experimental X-ray diffraction analysis. The relation of the features of the short-range order in the Fe-Ga alloy to the magnetostriction value has been discussed.     

Dielectric behaviour of La substituted BPZT ceramics

Here, we report dielectric behaviour of lanthanum substituted Ba_0.80Pb_0.20Ti_0.90Zr_0.10O₃ (BPZT) ceramics. The material series with compositional formula Ba_0.80−xLa_xPb_0.20Ti_0.90Zr_0.10O₃ (BLPZT) with x varying from 0 to 0.01 in the steps of 0.0025 was chosen for investigations. The material was synthesized by solid state reaction method. Reacted powder compacted in form of circular discs were sintered at 1325 °C. All the samples were subjected to X-ray diffraction (XRD) analysis and found to be single phase. Dielectric behaviour was studied as a function of frequency and temperature and Curie temperature (T_c) was determined. T_c was found to decrease with increasing x. The details are discussed and presented in this paper.     

Thermal expansion of dysprosium tetraboride

The temperature variations of the interplanar spacings a(T) and c(T) in the crystal lattice of dysprosium tetraboride have been studied using X-ray diffraction in the temperature range 5?C300 K. Anomalous variations of a(T) and c(T) in the temperature range of magnetic transformations, anisotropy of the thermal expansion of DyB₄, and the monoclinic distortion of the crystal structure at low temperatures have been revealed. The magnitudes of the spontaneous magnetostriction, the thermal expansion coefficients ??a and ??c, and the exchange integrals Y _a and Y _c have been determined.     

Effect of sorbic acid doping on flux pinning in bulk MgB2 with the percolation model

In this paper, we study the doping effect of sorbic acid (C₆H₈O₂), from 0 to 20 wt.% of the total MgB₂, on critical temperature (T_c), critical current density (J_c), irreversibility field (H_irr) and crystalline structure. The XRD patterns of samples show a slightly decrease in a-axis lattice parameter for doped samples, due to the partial substitution of carbon at boron site. On the other hand, we investigate the influence of doping on the behavior of flux pinning and J_c(B) in the framework of percolation theory and it is found that the J_c(B) behavior could be well fitted in high field region. The two key parameters, anisotropy and percolation threshold, play very important roles. It is believed that the enhancement of J_c is due to the reduction of anisotropy in high field region.     

Metal-nonmetal transition and superconductivity in frozen aluminium-hydrogen films

A study of the influence of the hydrogen concentration c_H on the superconducting transition temperature T_c and the d.c.-conductivity σ of Al-films has been performed. The samples were prepared by co-deposition of Al and H (H₂) onto a sapphire substrate held at a temperature of about 5 K. Within the experimental errors, there is no difference between the behaviour of Al-H and Al-H₂ respectively. σ shows a decrease with increasing c_H and a metal-insulator transition at c_H?0.68. The T_c values exhibit a maximum T_c ? 5.3 K for 0.22 ? c_H ? 0.5. Annealing experiments result in a decrease of T_c and an increase of σ. An analysis of the annealing experiments leads us to the conclusion that the as-prepared Al-H (Al-H₂) films have an amorphous structure, which transforms during annealing in a granular system of Al grains embedded in insulating AlH₃. No H-specific effect has been detected.