Intrinsic electronic superconducting phases at 60 K and 90 K in double-layer YBa2_2Cu3_3O6+δ_{6+\delta}

We study superconducting transition temperature ($T_c$) of oxygen-doped double-layer high-temperature superconductors YBa$_2$Cu$_3$O$_{6+\delta}$ (0 $\le$ $\delta$ $\le$ 1) as a function of the oxygen dopant concentration ($\delta$) and planar hole-doping concentration ($P_{pl}$). We find that $T_c$, while clearly influenced by the development of the chain ordering as seen in the $T_c$ $vs.$ $\delta$ plot, lies on a universal curve originating at the critical hole concentration ($P_c$) = 1/16 in the $T_c$ $vs.$ $P_{pl}$ plot. Our analysis suggests that the universal behavior of $T_c$($P_{pl}$) can be understood in terms of the competition and collaboration of chemical-phases and electronic-phases that exist in the system. We conclude that the global superconductivity behavior of YBa$_2$Cu$_3$O$_{6+\delta}$ as a function of doping is electronically driven and dictated by pristine electronic phases at magic doping numbers that follow the hierarchical order based on $P_c$, such as 2 $\times$ $P_c$, 3 $\times$ $P_c$ and 4 $\times$ $P_c$. We find that there are at least two intrinsic electronic superconducting phases of $T_c$ = 60 K at 2 $\times$ $P_c$ = 1/8 and $T_c$ = 90 K at 3 $\times$ $P_c$ = 3/16.Comment: 4 pages, 2 figure

Excitations in antiferromagnetic cores of superconducting vortices

We study excitations of the predicted antiferromagnetically ordered vortex cores in the superconducting phase of the newly proposed SO(5) model of strongly correlated electrons. Using experimental data from the literature we show that the susceptibilities in the spin sector and the charge sector are nearly equal, and likewise for the stiffnesses. In the case of strict equality SO(5) symmetry is possible, and we find that if present the vortices give rise to an enhanced neutron scattering cross section near the so called pi resonance at 41 meV. In the case of broken SO(5) symmetry two effects are predicted. Bound excitations can exist in the vortex cores with ``high'' excitation energies slightly below 41 meV, and the massless Goldstone modes corresponding to the antiferromagnetic ordering of the core can acquire a mass and show up as core excitation with ``low'' excitation energies around 2 meV.Comment: 9 pages, RevTeX, including 3 postscript figures, submitted to Phys. Rev. B, July 10, 199

Muon-spin-relaxation study of the magnetic penetration depth in MgB2

The magnetic vortex lattice (VL) of polycrystalline MgB2 has been investigated by transverse-field muon-spin-relaxation (TF-MuSR). The evolution of TF-MuSR depolarization rate, sigma, that is proportional to the second moment of the field distribution of the VL has been studied as a function of temperature and applied magnetic field. The low temperature value s exhibits a pronounced peak near Hext = 75 mT. This behavior is characteristic of strong pinning induced distortions of the VL which put into question the interpretation of the low-field TF-MuSR data in terms of the magnetic penetration depth lambda(T). An approximately constant value of sigma, such as expected for an ideal VL in the London-limit, is observed at higher fields of Hext > 0.4 T. The TF-MuSR data at Hext = 0.6 T are analyzed in terms of a two-gap model. We obtain values for the gap size of D1 = 6.0 meV (2D1/kBTc = 3.6), D2 = 2.6 meV (2D2/kBTc = 1.6), a comparable spectral weight of the two bands and a zero temperature value for the magnetic penetration depth of lambda = 100 nm. In addition, we performed MuSR-measurements in zero external field (ZF-MuSR). We obtain evidence that the muon site (at low temperature) is located on a ring surrounding the center of the boron hexagon. Muon diffusion sets in already at rather low temperature of T > 10 K. The nuclear magnetic moments can account for the observed relaxation rate and no evidence for electronic magnetic moments has been obtained.Comment: 15 pages, 4 figure

Anomalous peak in the superconducting condensate density of cuprate high T_{c} superconductors at a unique critical doping state

The doping dependence of the superconducting condensate density, n_{s}^{o}, has been studied by muon-spin-rotation for Y_{0.8}Ca_{0.2}Ba_{2}(Cu_{1-z}Zn_{z})_{3}O_{7-\delta} and Tl_{0.5-y}Pb_{0.5+y}Sr_{2}Ca_{1-x}Y_{x}Cu_{2}O_{7}. We find that n_{s}^{o} exhibits a pronounced peak at a unique doping state in the slightly overdoped regime. Its position coincides with the critical doping state where the normal state pseudogap first appears depleting the electronic density of states. A surprising correlation between n_{s}^{o} and the condensation energy U_{o} is observed which suggests unconventional behavior even in the overdoped region.Comment: 10 pages, 3 figure

Microwave determination of the quasiparticle scattering time in YBa2Cu3O6.95

We report microwave surface resistance (Rs) measurements on two very-high-quality YBa2Cu3O6.95 crystals which exhibit extremely low residual loss at 1.2 K (2-6 μΩ at 2 GHz), a broad, reproducible peak at around 38 K, and a rapid increase in loss, by 4 orders of magnitude, between 80 and 93 K. These data provide one ingredient in the determination of the temperature dependence of the real part of the microwave conductivity, σ1(T), and of the quasiparticle scattering time. The other necessary ingredient is an accurate knowledge of the magnitude and temperature dependence of the London penetration depth, λ(T). This is derived from published data, from microwave data of Anlage, Langley, and co-workers and from, high-quality μSR data. We infer, from a careful analysis of all available data, that λ2(0)/λ2(T) is well approximated by the simple function 1-t2, where t=T/Tc, and that the low-temperature data are incompatible with the existence of an s-wave, BCS-like gap. Combining the Rs and λ(T) data, we find that σ1(T), has a broad peak around 32 K with a value about 20 times that at Tc. Using a generalized two-fluid model, we extract the temperature dependence of the quasiparticle scattering rate which follows an exponential law, exp(T/T0), where T0≊12 K, for T between 15 and 84 K. Such a temperature dependence has previously been observed in measurements of the nuclear spin-lattice relaxation rate. Both the uncertainties in our analysis and the implications for the mechanism of high-temperature superconductivity are discussed

The Flux-Line Lattice in Superconductors

Magnetic flux can penetrate a type-II superconductor in form of Abrikosov vortices. These tend to arrange in a triangular flux-line lattice (FLL) which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-$T_c$ supercon- ductors (HTSC's) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSC's the FLL is very soft mainly because of the large magnetic penetration depth: The shear modulus of the FLL is thus small and the tilt modulus is dispersive and becomes very small for short distortion wavelength. This softness of the FLL is enhanced further by the pronounced anisotropy and layered structure of HTSC's, which strongly increases the penetration depth for currents along the c-axis of these uniaxial crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may melt the FLL and cause thermally activated depinning of the flux lines or of the 2D pancake vortices in the layers. Various phase transitions are predicted for the FLL in layered HTSC's. The linear and nonlinear magnetic response of HTSC's gives rise to interesting effects which strongly depend on the geometry of the experiment.Comment: Review paper for Rep.Prog.Phys., 124 narrow pages. The 30 figures do not exist as postscript file

Growth mode issues in epitaxy of complex oxide thin films.

The magnetic properties of grain-oriented 3 1/4% silicon-iron, as used in laminated transformer cores, are extremely sensitive to mechanical stresses. Power loss and magnetostriction (the cause of core vibration and noise) are most affected by compressive stresses. The theoretical effects of different types of stress on the domain structures of silicon-iron are correlated with measurements made on single laminations and transformer cores. The ways in which stresses can arise in cores and possible methods of avoiding or eliminating them in practice, are discussed. © 1974 Chapman and Hall Ltd