Magnetic field dependence on transition temperature Tc in cuprate superconductors

We investigate the magnetic field dependence on T_c in the high transition temperature superconductors. It is shown that phonon-enhanced spin fluctuations drive this superconductivity once more suggested by us [Phys. Rev. B 61 (2001) 4289]. We know magnetic field dependence on our transition temperature is in good correspondence with experimental data. It is elucidated that the external field is closely related to the local internal field in order to influence spin fluctuations.     

Manipulation of magnetic carriers for drug delivery using pulsed-current high Tc superconductors

An innovative method of manipulating magnetic carriers is proposed, and its feasibility for drug delivery and therapy is demonstrated experimentally. The proposed method employs pulsed-field solenoid coils with high-critical- temperature (T_c) superconductor inserts. Pulsed current is used to magnetize and de-magnetize the superconductor insert. The proposed method was demonstrated to be able to (1) move magnetic particles, ranging in size from a few millimeters to 10 μm, with strong enough forces over a substantial distance, (2) hold the particles at a designated position as long as needed, and (3) reverse the processes and retrieve the particles. We further demonstrated that magnetic particles can be manipulated in a stationary environment, in water flow, and in simulated blood (water/glycerol mixture) flow.     

Neutron scattering from intinerant ferromagnets just above Tc

Neutron scattering from itinerant ferromagnets at and just above T_c is studied on the basis of a simple band model with no short range order. Results are found for scattering line widths for small momentum and energy transfers which are substantially independent of band parameters. Some comparison with experiment is attempted.     

Spin fluctuation spectrum of electron-doped high-Tc superconductors

The magnetic excitation spectrum of electron-doped copper oxide superconductors is calculated using the Hubbard model on a square lattice. First, the on-site repulsion is treated with the random phase approximation. The spectrum of electron-doped systems in the superconducting state is compared with that of hole-doped systems, and the relationship between the frequency at which a peak grows in the spectrum and the superconducting energy gap at a hot spot (an intersection of the Fermi surface and the magnetic Brillouin zone boundary) is investigated. As compared with the hole-doped systems, the resonance condition is difficult to be satisfied in the electron-doped systems because of the small density of states around the hot spot. Moreover, the correlation effect in the Hubbard model is treated by the fluctuation-exchange approximation (FLEX), and the spin fluctuation spectra in the superconducting state in a wide region of the wave vector and frequency are calculated. We have found that the intensity of the magnetic spectrum at incommensurate wave vectors obtained with the FLEX is considerably weaker than that obtained with the RPA. The validity of the Fermi-liquid approach is also discussed.     

Spatial variations of the order parameter around a Kondo impurity for T ⩽ Tc

Analytic expressions for the spatial variations of the order parameter around a Kondo impurity are obtained. The oscillatory contribution due to impurity scattering is calculated using the t-matrix of Matsuura which conveniently yields the general results below T_c. Differences between our values and those of Schlottmann are reported.     

Spatial fluctuations of the order parameter and the peak effect in type-II superconductors

Near H_c2 even weak crystal inhomogeneities may lead to large relative fluctuations of |ψ|², to large flux line displacements, and to a volume pinning force quadratic in the pinning potential.     

Enhancement of the superconducting critical temperature Tc of niobium by radiation damage

5 μm niobium foils were irradiated with 25 MeV oxygen ions at 20 K, 33 K and 145 K up to fluences of 1.3 × 10¹⁶cm^-2 corresponding to a damage rate decrease of 99%. T_c versusresistivity shows a minimum depending on the irradiation temperature. At high fluences and subsequent annealing T_c is enhanced compared to the unirradiated value and the transition is broadened by a factor of about 30.     

Fluctuation enhanced diamagnetic susceptibility below Tc

The effect of fluctuating Cooper pairs on the diamagnetic susceptibility of dirty superconducting thin films is investigated, for temperature T < T_c. The susceptibility decreases with the decrease in temperature, approaching $\text{?}\text{1}\text{4}\text{π}$ at T = 0°K and remains finite at T = T_c.     

On the maximum of the k-dependent susceptibility for fixed k and T > Tc

We study the maximum occuring in ${}_{\mathrm{x}}\text{(}\text{k}\text{, T)}$ for fixed k at a temperature T_max > T_c. We examine the predictions of both Ornstein-Zernike and ?-expansions for the position of this maximum. These are compared with the exact results fot the two-dimensional Ising model.     

On the relation between Tc and structural properties of the A-15 compounds

It is shown that both the T_c and T_m dependences on the deformations and composition for the A-15 compounds can be described at least qualitatively in the quasi-one-dimensional model developed by the authors previously. The mechanism for the superconductivity is supposed to be the same as in the BCS theory. The upper critical field near T_c is obtained.     

Electron-boson coupling and oxygen-isotope effect in the optical conductivity of high-Tc superconductors

We have investigated electron-boson coupling in the optical conductivity of high-T_c superconductors through the optical self-energy. The real part of the self-energy (ReΣ^op(ω)) of YBa₂Cu₃O_y (YBCO) shows a characteristic doping dependence. In the optimally doped YBCO, ReΣ^op(ω) has a single peak around 65 meV, which corresponds to the kink structure of the band dispersion. On the other hand, in the under-doped YBCO, the peak structure of ReΣ^op(ω) splits into two parts. To evaluate contribution from the phonons in electron-boson coupling, we have measured oxygen-isotope effects by substituting ¹⁶O→¹⁸O for the optimally doped and under-doped YBCO.     

Specific heat of type II superconductors near Tc

An explanation is suggested for the anomaly that has been observed in the mixed to normal state specific heat discontinuity of type II superconductors in low fields, near T_c.     

Possible Tc-X diagrams of superconductivity in binary systems

Elementary dependences T_c(X) whose superposition describes a number of complex experimental situations are shown to exist. The concept of superconductivity-state diagrams is introduced, nine main types of such diagrams are being presented.     

Round-robin study of Tc measurement of superconducting “Nb3Ge” thin films

To determine the reproductibility in the measurement of T_c a round-robin experiment was conducted. The samples consisted of five high T_c “Nb₃Ge” thin films. The results of this experiment show that only the midpoint of the T_c curve is reproducible to within ≈±0.2 K.     

Searching for key parameter for determining Tc in Fe-based superconductors: Study of P/As substitution in RFe(P,As)(O,F) [R=La and Nd]

We have investigated the relation between the temperature-dependence of resistivity and superconducting transition temperature T_c in RFeP_1−xAs_xO_0.90F_0.10 (x=0-1.0) (R=La and Nd). In contrast to the linear change of the crystal structure with increasing x, the temperature dependence of resistivity and T_c show non-monotonous x-dependence. When the As concentration x is increased, the temperature-dependence of resistivity changes from T² to T-linear, and T_c distinctly increases in all the La compounds and the Nd ones with x<0.60. The results indicate that the substitution of As for P induces the spin fluctuation and resultantly enhances T_c. On the other hand, we could not find any relation between the temperature-dependence of resistivity and T_c in the Nd samples with x>0.60. This may suggest the existence of other parameters for determining T_c besides the antiferromagnetic correlation in this system.     

Magnetization processes of CuMn below the freezing temperature Tf

A simple classical model [5] that is based on a blocking of magnetic clusters in a uniaxial anisotropy field explains characteristic properties of magnetization processes of Cu(5–15 at.%) Mn as transition states between the “zero point magnetization” M₀ = M(T → OK) and the thermal equilibrium magnetization M_∞ = M(t → ∞, T). Further it shows two facts the experimental confirmations of which we report in this paper: (a) a magnetization decrease with increasing temperature in high fields and (b) a superposition rule for the magnetization processes in small fields and at low temperatures.     

Supersymmetric representation of order parameter fluctuations

We consider the representation of homogeneous and heterophase fluctuations based on the supersymmetric theory of fields. It is shown that the problem reduces to a mixture of noninteracting Fermi and Bose gases. The thermodynamic features resulting from each type of fluctuations are determined, as well as the correlation functions of the spatial distributions of the order parameter and interphase boundaries.Suma Physicotechnical Institute. RETO Scientific-Industrial Complex. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 49–53, April, 1994.     

Constraints on the mechanism of superconductivity for MgB2 from Tc and the total isotope effect

We examine the possible electron-phonon spectra that produce both T_c=39 K and an isotope coefficient β=0.32±0.01, with Eliashberg theory. We assess the viability of the conventional electron-phonon mechanism in light of these results, compared with ab initio calculations of the electron-phonon spectrum. Comparisons are made with similar considerations for low T_c materials.