Anisotropy in ac losses near the irreversibility field of Bi2Sr2CaCu2O8

The ac susceptibility under a biased dc field near the irreversibility field (H _irr) of a Bi₂Sr₂CaCu₂O₈ single crystal has been measured. The frequency dependence, the ac-power dependence, and the nearly lossless character of the vs.H _dc curve forHa-axis have been roughly explained from a reversible (elastic) fluxoid motion, while those forH c-axis have been explained from a thermally assisted flux-flow (TAFF) model. The obtained parameters are discussed in relation to anisotropic flux-pinning mechanisms in the layered structure of this compound.     

Normal-State Optical Response Functions of MgB2 Superconductor

The reflectivity of superconducting MgB₂ (T _c = 39 K) has been measured on a randomly oriented thin film at room temperature over a wide-range of frequencies, 20 < 100000 cm^–1. The conductivity shows highly metallic behavior but cannot be explained with a simple Drude model alone. The electronic contribution is analyzed by a generalized Drude model. The scattering rate 1/() and the mass renormalization ratio m*()/m = 1 + () exhibit clear frequency dependence. The electron–phonon coupling strength is estimated to be 1.5 ± 0.5 while the plasma frequency _p is 2.4 eV.     

Energy Scales in the High-Tc Superconductor YBa2Cu3O6+x

The optical conductivity sum rule is used to examine the evolution of the spectral weight N() in both the normal and superconducting states of optimally and underdoped YBa₂Cu₃O_6+x along the a axis. Differences in N() above and below T _c allow the strength of the superconducting condensate _s to be determined. In the optimally-doped material, _s is fully formed at energies comparable to the full superconducting gap maximum (0.1 eV), while in the underdoped material the energy scale for convergence is considerably higher (0.6 eV). This difference is discussed in terms of normal-state properties.     

Frequency and temperature dependence of the millimeter wave conductivity of epitaxial YBa2Cu3O7 films

A millimeter wave spectrometer for frequencies between 100 and 350 GHz consisting of continuously tunable backward wave oscillators as sources and a quasioptical interferometer in the Mach-Zehnder configuration was used to measure the transmittivity in phase and amplitude of YBa₂Cu₃O₇ thin films on NdGaO₃ substrates. From the measured spectra we derived the real and imaginary part of the dynamic conductivity= ₁+i ₂ in the superconducting state as a function of temperature. The ₁(T) and ₂(T) values at 300 GHz were compared to corresponding values at 19 GHz determined by surface impedance measurements of the same films using a shielded dielectric resonator. Our observed frequency dependence of both ₁(T) and ₂(T) is consistent with a strong reduction of the quasiparticle scattering rate ^–1(T) with decreasing temperature belowT _c .     

Magnetic field dependence of the density of states of YBa2Cu3O6.95: Implications for the vortex structure

The total specific heat of YBa₂Cu₃O_6.95 single crystals includes contributions from phonons and spin-1/2 particles, as well as electronic contributions. The electronic specific heat is described by a quadratic term T² in zero field and a linear term [(0)+(H)]T which is increased when a magnetic field H is applied perpendicular to the CuO₂ planes. In agreement with d-wave superconductivity, we find that _n/T_c and (H)_n(H/H_c2)^1/2, where _n is the coefficient of the normal-state linear term. The H^1/2 dependence of the density of states at the Fermi level was predicted by G. Volovik for lines of nodes in the gap: the quasiparticles which contribute to this density of states are close to the nodes in momentum space and are located outside the vortex core.     

Surface resistance of large-area Tl2Ba2CaCu2O8 thin films at microwave and millimeter wave frequencies measured by three noncavity techniques

The surface resistanceR _s of Tl₂Ba₂CaCu₂O₈ films fabricated on LaAlO₃ wafers up to 3 inches (7.6 cm) in diameter through a post-deposition anneal process was measured over the frequency range 5.55–94.1 GHz by the following techniques: 5.55 and 27.5 GHz high-temperature superconductor (HTS)-sapphire resonators, 10 GHz parallel plate resonator, and 94.1 GHz scanning confocal resonator.R _s was found to exhibit a quadratic dependence on frequencyf at 77 K:R _s f ^2.0±0.1. The highest-quality films yieldR _s =145±15 at 10 GHz and 77 K. Scanning confocal resonator mapping ofR _s across a 2-inch (5.1 cm) diameter wafer yielded a base value forR _s of 16±1 m at 77 K and 94.1 GHz (equivalent to 180±10 at 10 GHz) and good uniformity inR _s across the wafer. HTS-sapphire resonator measurements ofR _s for fifteen 1.2 cm square parts cut from a 3-inch diameter wafer yieldedR _s values scaled to 10 GHz of 196±10 at 80 K. Similar values were measured for Tl₂Ba₂CaCu₂O₈ films prepared on both sides of a 2-inch diameter wafer.Rs values at 10 GHz and 80 K of 147–214 were maintained over the course of 40 independent and successive deposition runs and corresponding anneals under nominally identical film fabrication conditions. Surface resistance at 5.55 GHz remained below 80 for maximum rf magnetic fields up to 85 Oe at 4.2 K and 7 Oe at 80 K, respectively. Results are compared with predictions of the two-fluid model. The relative advantages and disadvantages of the different techniques for measuring surface resistance are discussed.     

Observation of weakly adsorbed oxygen on Y5Ba6Cu11Oy

We observed, for the first time, adsorption of weakly bonded oxygen at low temperature (<-250°C) by a Y₅Ba₆Cu₁₁O _y sample, using thermogravimetric analysis. The resulting oxygen enriched phase in the surface layers may be attributed to the observation of a superconductivity-like transition at above 200 K.     

Sc Addition to (Bi,Pb)2Sr2Ca2Cu3Ox Freeze-Dried Ceramic

In a freeze-dried nitrate precursor powder with cation composition Bi:Pb:Sr:Ca:Cu = 1.7:0.3:2.0:2.5:3.5, Sc₂O₃ was added. The stoichiometry for the mixed precursor powder was Bi:Pb:Sr:Sc:Ca:Cu = 1.7:0.3:2.0:0.25:2.5:3.5. The two nitrate precursor powders with and without Sc were thermally decomposed, pressed, and sintered in the same conditions. They show different behavior, as revealed by thermal analysis, X-ray diffraction (XRD), Scanning electron microscopy (SEM), and (T) measurements. Addition of Sc slows down synthesis processes, including 2223-phase during reactive sintering. After 70 h of sintering, 43% of 2223-phase formed in the pellet with Sc, whereas in the Sc-free pellet, 51% of the 2223-phase was attained. Moreover, addition of Sc leads to a certain morphology, resulting in lower superconducting characteristics.     

Anisotropy in the resistive superconducting transition under magnetic fields in single crystal Pb2Sr2Ho0.5Ca0.5Cu3O8

Anisotropie properties of the single crystal Pb₂Sr₂Ho_0.5Ca_0.5Cu₃O₈ have been investigated by measuring the electrical resistivity in theab-plane _ab (H, ,T), which depends on the angle between theab-plane and the magnetic-field direction, in various constant fieldsH perpendicular to the current direction. All the angle-dependent values of _ab (H, ,T) at a constant temperature are scaled to be on one curve as a function of reduced field. The anisotropic parameter (m _c ^* /m _ab ^* )^1/2 is estimated as 12–13, which is larger than that of YBa₂Cu₃O₇ and much smaller than that of Bi₂Sr₂CaCu₂O₈. It has been concluded that the anisotropy does not always depend on the thickness of the blocking layer but seems to depend on the overlap of the electronic wave functions along thec-axis. Anisotropy in the pinning potential has also been discussed from the resistive tail in the temperature dependence of _ab (H,,T).     

Band structure of Bi2Sr2CuO6: Strong effects due to structural modulation

The electronic structure of idealized tetragonal Bi₂Sr₂CuO₆ is compared with that of an orthorhombic 2×2 model that includes atomic displacements due to the structural modulation. The electronic structure, particularly that related to Bi and O(3) atoms, is strongly modified near the Fermi energy. In particular the shape and topology of the Bi-O(3) related Fermi surfaces near the ¯M point are changed. With the modulation, bands occur near the Fermi energy over a significant part of the zone near this point. A method is proposed to distinguish between lattice distortions and antiferromagnetic correlations as the cause of recently observed shadow Fermi surfaces.     

Scaling Behavior and Estimation of Activation Energy of Polycrystalline RuSr2GdCu2O8 Superconductor from AC Susceptibility Measurements

In this study, the measured curves of AC susceptibility (ACS) components, (T) and (T), of polycrystalline RuSr₂GdCu₂O₈ (Ru-1212) superconductor were scaled onto a single curve using the peak temperature of its imaginary part (T _p) as the scaling parameter for various AC field amplitudes from 0.5 to 24 G. The dependence of the AC magnetic field amplitude on T _p is scaled as: H _ac (1–T _p/T _c)^2.25. Similarly, the current density J _c, extracted from the AC field amplitude is also scaled as: J _c (1 – T _p/T _c)^2.25. The dependencies of T _p on frequency and AC field amplitude are also investigated and the time parameter t ₀ of the order of 10^–8 s is estimated from the dependence of T _p on frequency. The dependencies of activation energy on temperature, T, and the field amplitude, H _ac, are obtained from the Arrhenius-like semilog plot of frequency () and T _p. Such dependencies on temperature and field amplitude can be described by a scaling law of the form: U(H _ac,T) = U ₀[1 – T/T _p]H _ac ^–0.17.     

Theoretical evidence forc-axis phonon-plasmon coupling and phonon renormalization in La2CuO4

A suitable decomposition of the electronic density response and of the electron-phonon interaction into local and nonlocal contributions is extended to the nonadiabatic regime taking dynamical screening into account. It is shown for La₂CuO₄ that the already large (nonlocal) adiabatic electron-phonon coupling is further strongly enhanced for certain phonons along the (0, 0, 1) direction. In this region of wave-vector space we also have a low-lying plasmon mode which mixes strongly with the phonons of appropriate symmetry leading to an anomalous renormalization for certain phonons. The coexistence of phononlike and plasmonlike resonances in the same frequency range is argued to be a specific feature of the layered high-temperature superconductors.     

Comparison of the Electronic Structures of La2CuO4, Sr2CuO2Cl2, and Sr2CuO2F2

First-principles cluster calculations are reported of the local electronic structure of the three compounds: La₂CuO₄, Sr₂CuO₂Cl₂, and Sr₂CuO₂F₂. The copper and the planar oxygen 2p atomic orbitals exhibit a similar degree of covalency. The out-of-plane orbitals, however, are quite different with the atomic orbital lowered significantly in energy for chlorine and fluorine apical positions.     

X-Ray Diffraction Studies on Yttrium-Doped La0.67Ca0.33MnO3

Polycrystalline samples of yttrium-doped La_0.57Ca_0.33MnO₃ (LCMO) have been prepared. Structure characterizations have been carried out at room temperature by X-ray diffraction with Rietveld refinements. The octahedral tilting distortion increases with the increase of yttrium content or the decrease of the average radius of La-site cations, La. The difference between the two crystallographically distinct Mn—O—Mn bond angle increases with increasing Y content. The average Mn—O bond distance is relatively constant across the entire series, but individual Mn-O bond distances and the Mn—O—Mn bond angles show the presence of the cooperative Jahn–Teller effect at room temperature in yttrium-doped LCMO.     

Hc2 of MgCNi3 Determined from the Specific Heat and Resistivity Measurements

H _c2 of MgCNi₃ has been determined from the specific heat C and resistivity measurements in the same sample. The results from are nearly identical with those determined from the anomaly in C. Furthermore, utilizing the relation (H) H and the value of d/dH, the obtained value of H _c2 is the same as that by the WHH model, if the spin paramagnetic effect and the spin–orbit interaction are taken into account. The results of this comparison have strong implications on the order parameter of MgCNi₃.     

Phonon Raman scattering of R2CuO4 (R = Pr, Nd, Sm, and Gd)

For Gd₂CuO₄, a new symmetry-forbidden phonon for the tetragonal T structure is observed. This implies that oxygen in the CuO₂ plane is locally distorted along the CuO₂ plane. Such distortion has never been observed for other T-type superconductors, which show superconductivity by Ce doping. The disappearance of superconductivity for Gd can be correlated with the appearance of the distortion due to oxygen in the CuO₂ plane.     

Observation of subgap structures in high-quality Nb/Al-AlOx/Nb Josephson tunnel junctions

We have performedI-V measurements on high-quality Nb/Al-AlO_x/Nb Josephson junctions. All curves exhibit subgap structures. Some junctions exhibit in theI-V curves the (₁–₂)/e singularity, due to the presence of niobium films with different gaps. In these devices we observed sharply separated structures corresponding to the series 2₁/ne and 2₂/ne, with n3 corresponding to the tunneling of two and three electrons. The respective amplitudes of the two structures atn=2 allow one to conclude that the phenomenon is a multiparticle tunneling process.