Narrow Results

We consider experiments on measuring the Casimir interaction which have been performed in the last four years. The emphasis is made on measuring differences in the Casimir pressures under a transition of the plate metal from normal to superconducting state and on the Casimir metrology platform using a commercial micromechanical sensor. In both cases several problems in the comparison between experiment and theory are discussed.

For type II superconductors, Josephson has shown that vortices moving with velocity v_L create an effective electric field E′= -v_L×B_V. By definition the effective electric field is gradient of the electrochemical potential, what is the quantity corresponding to voltage observed with the use of Ohmic contacts. It relates to the true electric field E via the local chemical potential μ as E′=E-∇μ/e. We argue that at low temperatures the true electric field in the bulk can be approximated by a modified Josephson relation E=(v_S-v_L)×B_V, where v_S is the condensate velocity.

We summarize here some of our recent results on the superconducting fluctuation effects above T_C in different low- and high-T_C superconductors, at high reduced-temperatures and magnetic fields. These results confirm our proposal that the collective behaviour of the fluctuating Cooper pairs in the short-wavelength fluctuation regime is dominated by the uncertainty principle, which imposes a limit to the shrinkage of the superconducting wave function when T increases well above T_C or H becomes of the order of H_c2(0), the upper critical magnetic field amplitude extrapolated to T = 0 K.

The reaction of superconducting YBa₂Cu₃O_y powder with water is studied kinetically. The initial rate law is found as where [S]_o is the initial surface area of the solid, a and b is found to be 0.94 ± 0.02 and 0.318 ± 0.002, respectively. For 100.7 mg powder with a mean radius of 0.0265 mm at pH 7.00 and 298 K, R_o is 2.21 × 10^-4mol · min^-1. Using a = 1 and b = 0.3, the rate constant, k, obtained at 283, 298 and 313 K is 0.64, 1.84 and 4.91 × 10^-3mol · min^-1 · cm^-2 · M^-0.3, respectively. The activation energy is 50.0 kJ · mol^-1 and pre-exponential factor is 1.09 × 10⁶mol · min^-1 · cm^-2· M^-0.3, as calculated from the Arrhenius equation.

Influence of flux creep on ac susceptibility (acs) of a superconductor was examined by simulation. Comparing the acs with and without flux creep, it was found that frequency of ac field shifted the acs with flux creep but did not affect the acs of critical state model. In addition, the amplitude of ac field and dc field affect differently for the two models.

Some properties of the ideal and pinned vortex lattice in superconductors are outlined.

Potential application of superconducting material YBa₂Cu₃O_7-x for both bulk and thick film samples as oxygen resistance sensors was investigated. The oxygen diffusion kinetics of YBa₂Cu₃O_7-x was studied by thermal analysis technology and the resistance dependence on the oxygen partial pressure was measured. The experiment shows that the changing of oxygen pressure can result in oxygen desorption and absorption, and in-diffusion is more rapid than out-diffusion. The oxygen stoichiometry and conductivity of YBa₂Cu₃O_7-x have strong relations with temperature and the partial pressure of environmental oxygen. The responding properties of the thick film to the changing of oxygen partial pressure have been improved in comparison with the bulk samples. The sensitivity to the oxygen pressure of thick film varies with temperature and reaches a maximum value 62 at 650°C.

This paper describes the theoretical characteristics of TE nonlinear transverse electric surface waves propagating at microwave frequencies in a layered structure of superconductor film bounded by an antiferromagnet substrate and a linear ferrite cover. The effects of the operating angular frequency and temperature of superconductor on the dispersion characteristics have been examined. The power flow has also been studied as a function of the reduced phase and attenuation constants.

Although BCS pairs of fermions are known to obey neither Bose–Einstein (BE) commutation relations nor BE statistics, we show how Cooper pairs (CPs), whether the simple original ones or the CPs recently generalized in a many-body Bethe–Salpeter approach, being clearly distinct from BCS pairs at least obey BE statistics. Hence, contrary to widespread popular belief, CPs can undergo BE condensation to account for superconductivity if charged, as well as for neutral-atom fermion superfluidity where CPs, but uncharged, are also expected to form.

The electronic structure of MgB₂ with different lattice parameters, which represents the influence of pressure, has been studied from HF calculation. It has been found that the density of states at the Fermi level decreases with the decrease in lattice constant, which implies that the pressure can reduce the superconductivity of MgB₂. It was also found that more electrons are introduced from the Mg layer to the B layer with increasing pressure, resulting in the number of holes on the B layer being reduced, thus reducing superconductivity. The electron density population at different pressures clearly shows that the triangle-shaped electron distribution in the B plane, extended along the B–B bonds, indicates the typical sp² orbitals. On the other hand, there is no covalency feature between Mg and B atoms, indicating the ionic bonding property between Mg and B layers.

We present a detecting scheme of high-frequency gravitational waves (HFGWs) which is based upon coupling between superconducting open cavity and electromagnetic fields stored in the cavity. Utilizing zero resistance and perfect diamagnetism of the superconductors, one can obtain very large quality factor and high sensitivity for the cavity to detect the HFGWs in GHz band. In addition, we review other possible superconducting systems for detection of the HFGWs, such as superconducting resonant-mass detector, toroidal waveguide, coupled superconducting spherical cavities and superconducting quantum antennas.

We have investigated the behavior of vortices in a polycrystalline YBa₂Cu₃O_7-δ disk sample using rotational magnetization-vector (RMV) measurements. The measurements were conducted in the field-cooled (FC) and the zero-field cooled (ZFC) states for several values of applied magnetic field parallel to the sample surface at 4.2 K and for various angles of rotation (θ_rot) up to 360°. The total magnetic flux density, B, in the sample at any rotational angle was considered to be composed of two types of vortices: weakly pinned vortices that rotate frictionally in the opposite direction of sample rotation and strongly pinned vortices that rotate rigidly in the same direction of sample rotation. From the rate of change of these two types of vortices and the rate of change of B, we were able to develop a tool to qualitatively describe the average strength of pinning centers in high-temperature superconductors.

The feasibility of the infiltration of molten magnesium into porous boron skeleton was examined. The particle size and crystalline state of starting boron powders as well as the confinement of Mg vapors play an important role in successful infiltration. Magnesium penetrates more readily into porous skeletons formed by large crystalline boron particles than those formed by fine amorphous powders. The infiltration of magnesium mainly occurs via capillary condensation rather than liquid flow. Although the infiltrated microstructures are seemingly very dense, nano-scale structures contain many tiny inter-particle voids due to random packing of MgB₂ grains. This is due to the absence of a liquid phase in final microstructures as a result of transient liquid-phase infiltration.

Ge-doped (Cu_0.5Tl_0.5)Ba₂Ca₃(Cu_4-yGe_y)O_12-δ (y = 0, 0.3, 0.6 and 0.9) superconductors have been synthesized at normal pressure through solid state reaction method. Ge has been doped in the CuO₂ planes constituting the superconducting block of these structures. In the as-prepared samples, a suppression of the critical temperature is observed with increased Ge concentration. The suppression of the critical temperature can be attributed to the decreased number of carriers due to their localization at Ge⁴⁺ ions. Ge-doped post-annealed samples have shown enhancement in the critical temperature as well as magnitude of diamagnetism. Oxygen annealing seems to have replenished the charge carries through the process of hole doping in CuO₂/GeO₂ planes, thereby bringing the carrier density closer to the optimum level. Oxygen related phonon modes have also been investigated. A shift in peak positions of the apical and planar oxygen related modes have been observed while modes associated with O_δ oxygen atoms seem stable in both cases of Ge doping and oxygen annealing.

We analyze the full counting statistics (FCS) of a superconducting junction in nonequilibrium in the limit of small interface transparency. In this limit we treat both supercurrent and multiple Andreev reflections on equal footing and show how to generalize previous results for both phenomena. Furthermore, we also allow for different gaps of both superconductors and investigate the intermediate regime which allows to make contact with previous results on normal-superconductor heterostructures. We also compare our predictions in this regime to experimental data.

The pressure dependent changes in transition temperature (T_c) of alkali doped C₆₀ superconductor has been studied theoretically considering the combination of phonon mediated mechanism and high energy electronic interaction mechanism. The latter mechanism involves bond polarization in interaction with conduction electrons. The expression for T_c, isotope effect coefficient (α) and (dT_c/dP)_P has been obtained to explain the observed experimental results of alkali doped C₆₀. A good agreement has been found in experimental (T_c) and (dT_c/dP)_{P = 0} values and calculated values.

Electronic structure, lattice dynamics and superconducting properties for theoretically devised superconductor LiBSi_1-x Al_x are obtained by first-principles calculations. We assume that Lithium Boron Silicon (LiBSi) has the same crystal structure as that of Lithium borocarbide (LiBC). The pristine LiBSi is predicted to be a zero-gap semiconductor. Hole doping of LiBSi through partial substitution of Si by SiAl atoms can produce a semiconductor–metal transition and develop superconductivity. To assess the thermodynamic stability of LiBSi_1-xAl_x, the formation energies are calculated using the supercell method. For LiBSi_0.75 Al_0.25 and LiBSi_0.875Al_0.125, the obtained formation energies are -5.9 and -6.1 eV, respectively, indicating that LiBSi_1-xAl_x is energetically favorable at least in the range of 0 ≤ x ≤ 0.25. Phonon spectra and superconducting properties are obtained within the virtual-crystal approximation (VCA) treatment. The results show that LiBSi_1-xAl_x is dynamically stable approximately in the range of 0 ≤ x ≤ 0.35. For LiBSi_0.8Al_0.2, the obtained electron-phonon coupling constant λ is 0.86 and superconducting transition temperature T_C is predicted to be in the range of 11–13 K (0.14 ≥ μ* ≥ 0.1).

In this paper, impedance and dielectric properties of nonsuperconducting state of the mercury-based cuprate have been investigated by impedance measurements within the frequency interval of 10 Hz–10 MHz for the first time. The dielectric loss factor $(tg\delta )$ and ac conductivity $({\sigma }_{\mathrm{\text{ac}}})$ parameters have also been calculated for non-superconducting state. According to impedance spectroscopy analysis, the equivalent circuit of the mercury cuprate system manifests itself as a semicircle in the Nyquist plot that corresponds to parallel connected resistance–capacitance circuit. The oscillation frequency of the circuit has been determined as approximately 45 kHz which coincides with the low frequency radio waves. Moreover, it has been revealed that the mercury-based cuprate investigated has high dielectric constants and hence it may be utilized in microelectronic industry such as capacitors, memory devices etc., at room temperature. In addition, negative capacitance (NC) effect has been observed for the mercury cuprate regardless of the operating temperatures at nonsuperconducting state. Referring to dispersions in dielectric properties, the main contribution to dielectric response of the system has been suggested as dipolar and interfacial polarization mechanisms.

The application of the gauge-gravity duality, also known as anti-de Sitter/conformal field theory (AdS/CFT) correspondence to study condensed matter systems has resulted in a number of important findings. Using the analogy, we have studied the phase transitions between a holographic insulator and a metal at zero temperature as well as finite temperature transition between a metal and a holographic superconductor of $s$- and $p$-wave symmetry. The main aim of this note is to look in which way the dark matter might affect the properties of superconductors. The hope is that some of the observed modifications could be used to detect this ubiquitous but still elusive component of matter in the Universe.

Ca-doped (x = 0.00, 0.07 and 0.20) epitaxial Y_1-xCa_xBa₂Cu₃O_7-δ thin films were prepared on SrTiO₃ (100) by PLD technique. The superconducting transition temperature is found to decrease with increasing Ca content. The AC-susceptibility data were used to determine the intergranular critical current density, when varying the field amplitude H_AC. The temperature dependence of critical current density was determined from the imaginary part of the complex susceptibility, χ′′(T), using Xing's relation. The results were analyzed in terms of superconductor-insulator-superconductor (SIS) and superconductor-normal-superconductor (SNS) type models.