Interplay Between Surface Effects and the Magnetic Properties in Polycrystalline Superconductors

The effects of the grain boundaries on the magnetic properties are studied in the polycrystalline superconductors. The interaction of the Abrikosov vortex with both the surface and Josephson junction as well as with other vortices and applied magnetic field are taken into account in this study by the London theory approach. It is shown that the magnetic behavior is strongly dependent of the anisotropy ratio as well as the normalized grain size and the coupling parameter between the grains. The first flux entry field H _p , the lower critical field H _{c 1} and the Gibbs free energy are computed. The vortex–vortex interaction and magnetization M(H) are also investigated.     

Inelastic neutron scattering from the vortex lattice in type-II superconductors. I

The inelastic neutron scattering from the vortex lattice in dirty type-II superconductors is considered. In the high-field region, the correlation function for the local magnetization is given in terms of the fluctuation propagator of the superconducting order parameter. In particular it is shown that the Fourier transform of the correlation function for the local magnetization diverges as /q –G _i /^–2, when the transverse transfered momentumq approaches the reciprocal lattice vectorG _i of the Abrikosov lattice. This term gives rise to tails around each Bragg peak in the inelastic neutron-scattering cross section.     

Exactly Aligned Magnetic Field and the Fulde–Ferrell–Larkin–Ovchinnikov State in Quasi-Low-Dimensional Superconductors

In quasi-low-dimensional type II superconductors, the orbital pair-breaking effect is considerably reduced when the magnetic field is exactly aligned in a direction parallel to the highly conducting layer. Then, since the superconducting phase survives up to a very high magnetic field comparable to the Pauli-paramagnetic limit, we could expect some drastic phenomena, such as the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state. In this paper, we discuss the condition of the tilt angle for the appearance of the FFLO state, and also discuss the stable spatial structure of the FFLO state. Further, we show an example of a quasi-two-dimensional system in which the FFLO state is remarkably enhanced, that is, a two-dimensional tight binding system of appropriate hole concentrations.     

Room-Tc Superconductivity on Whispering Mode in Quasi-1D Composite of Superconducting Nanotubes: Is It Possible?

Combining Little's and Ginzburg's ideas with recent progress in nanotubes research, a novel type of material is advanced as a perspective high-T _c superconductor on a base of a close-packed lattice of quasi-1D superconducting nanotubes. Idea is offered that superconducting coaxial multilayer nanotubes of the correlation length in diameter is an ideal and natural trap for pinning of Abrikosov vortex. Nanotube should be layered superconductor, such as LuNiBC. Mechanism of superconductivity was proposed and substantiated quantitatively on a base of a whispering mode, which is shown to be responsible for a strong enhancement of electron–phonon interaction and for an increase of critical temperature. Nanocomposite built from such quasi-1D nanotubes when coinciding with vortex lattice provides ideal conditions for the pinning, resonance, distortion, ordering, and Little–Parks effects, the joint action of which is suggested to result in synergetic effect increasing the superconductivity. Such quasi-1D or 2D nanotubular crystal is proposed to synthesize by template approach using zeolite-like membrane.     

Type-IV Superconductivity: Can Superconductivity be more Exotic than Unconventional?

Abrikosov vortex phase in s-wave layered superconductor in a parallel magnetic field is shown to be absolutely unstable with respect to the appearance of a triplet component which breaks inversion (parity) and spin-rotational symmetries of Cooper pairs. Symmetry breaking paramagnetic effects are demonstrated to be of the order of unity if the orbital upper critical field, H_c2 (0), is of the order of Clogston paramagnetic limiting field, H_p. We suggest a generic phase diagram of such type-IV superconductor, which is singlet one at H=0 and in the Meissner phase and characterized by mixed singlet-triplet order parameter with broken symmetries of Copper pairs in vortex phase. A possibility to observe type-IV superconductivity in clean organic, high-Tc, and MgB₂ superconductors is discussed.     

11B NMR Study of Pure and Lightly Carbon-Doped MgB2 Superconductors

We report a¹¹B NMR line shape and spin-lattice relaxation rate (1/(T ₁ T)) study of pure and lightly carbon-doped MgB_2−x C _x forx=0, 0.02, and 0.04, in the vortex state and in magnetic field of 23.5 kOe. We show that while pure MgB₂ exhibits the magnetic field distribution from superposition of the normal and the Abrikosov state, slight replacement of boron with carbon unveils the magnetic field distribution of the pure Abrikosov state. This indicates a considerable increase ofH _c ^2/c with carbon doping with respect to pure MgB₂. The spin-lattice relaxation rate 1/(T ₁ T) demonstrates clearly the presence of a coherence peak right belowT _c in pure MgB₂, followed by a typical BCS decrease on cooling. However, at temperatures lower than ≈10 K strong deviation from the BCS behavior is observed, probably from residual contribution of the vortex dynamics. In the carbon-doped systems both the coherence peak and the BCS temperature dependence of 1/(T ₁ T) weaken, an effect attributed to the gradual shrinking of the σ hole cylinders of the Fermi surface with electron doping.     

Investigation of Microinhomogeneities in a-SiC r :H Films Using the Chemical Induction Model

Amorphous hydrogenated silicon–carbon (a-SiC _r :H) films grown by decomposing silane–methane mixtures in a low-frequency (55 kHz) glow discharge at different methane concentrations are studied by IR spectroscopy. The absorption band in the range 1850–2300 cm^–1 is decomposed into four Gaussian components, and the results are compared with calculations in the chemical induction model. It is found that the carbon atoms are nonuniformly distributed in the nearest neighbor environment of the SiH groups in the form of HSi–Si_{3 – n} C _n (n= 0–3) structures. The random bonding model is used to evaluate the probability of formation of each HSi–Si_{3 – n} C _n structure as a function of the C/Si ratio. Comparison with experimental data points to an inhomogeneous microstructure of the films.     

11B NMR Study of Pure and Lightly Carbon-Doped MgB2 Superconductors

We report a ¹¹B NMR line shape and spin-lattice relaxation rate (1/(T₁T))(1/(T_1T)) study of pure and lightly carbon-doped MgB_2-xC_x_{2-x}{\rm C}_x for x = 0,0.02x = 0,0.02, and 0.04, in the vortex state and in magnetic field of 23.5 kOe. We show that while pure MgB₂_2 exhibits the magnetic field distribution from superposition of the normal and the Abrikosov state, slight replacement of boron with carbon unveils the magnetic field distribution of the pure Abrikosov state. This indicates a considerable increase of H^c_c2H^c_{c2} with carbon doping with respect to pure MgB₂_2. The spin-lattice relaxation rate 1/(T₁T)1/(T_1T) demonstrates clearly the presence of a coherence peak right below T_cT_{\rm c} in pure MgB₂_2, followed by a typical BCS decrease on cooling. However, at temperatures lower than ?\approx 10 K strong deviation from the BCS behavior is observed, probably from residual contribution of the vortex dynamics. In the carbon-doped systems both the coherence peak and the BCS temperature dependence of 1/(T₁T)1/(T_1T) weaken, an effect attributed to the gradual shrinking of the σ hole cylinders of the Fermi surface with electron doping.     

Interplay Between Vorticity and Chirality Inside the Vortex Core in Chiral p-Wave Superconductors

The vortex core in chiral p-wave superconductors exhibits various properties owing to the interplay between the vorticity and chirality inside the vortex core. In the chiral p-wave superconductors, the site-selective nuclear spin-lattice relaxation rate T ^–1 ₁ is theoretically studied inside the vortex core within the framework of the quasiclassical theory of superconductivity. T ^–1 ₁ at the vortex center depends on the sense of the chirality relative to the sense of the magnetic field. The effect of a tilt of the magnetic field upon T ^–1 ₁ is investigated. The effect of the anisotropy in the superconducting gap and the Fermi surface is then investigated. The result is expected to be experimentally observed as a sign of the chiral pairing state in a superconducting material Sr₂RuO₄.     

Influence of Antiferromagnetic Fluctuations on the Fulde–Ferrell–Larkin–Ovchinnikov State in CeCoIn5

The Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state is a spatially inhomogeneous superconducting (SC) phase with a periodically modulated order parameter predicted to appear in sufficiently clean type-II superconductors, close to the upper critical field, if the orbital pair breaking is small relative to the Pauli-limiting effect. The heavy-fermion superconductor CeCoIn₅ is the first material, where different physical probes show strong experimental evidence pointing to the realization of the FFLO state, even though strong antiferromagnetic (AFM) spin-fluctuations (SF) are present at atmospheric pressure. To study the influence of the AFM-SF on the FFLO state we performed heat-capacity experiments under pressure. We utilized a newly developed miniature piston-cylinder type pressure cell specially suited for measuring small samples at high-magnetic fields and low temperatures (0 GPa ≤ P ≤ 1.5 GPa, 0 kOe ≤ H ≤ 140 kOe, and 100 mK ≤ T ≤ 4 K). We found the second anomaly inside the SC state in CeCoIn5 can still be observed with pressure, which suppress the strong AFM-SF. The FFLO phase extends to higher fields and temperatures on applying pressure while the Pauli-limiting effect is becoming weaker and the SF are suppressed. This reveals the detrimental effect of the AFM-SF on the FFLO phase stability.