Tunneling in single-layer Bi2Sr2CuO6+δ single crystals in high magnetic field

In tunneling experiments with high-quality single crystals of a single-layer cuprate superconductor Bi₂Sr₂CuO_6+δ using the break junction and point-contact techniques at T<T _c, the coexistence of the superconducting-state gap and the normal-state gap was observed. The values of the superconducting energy gap 2Δ_p?p are in the range from 13.4 to 15 meV (Δ_p?p=6.7–7.5 meV). The values of 2Δ_p?p are similar for two samples with T _c=4 K and for two samples with T _c=9–10 K and are independent of the carrier concentration. The normal-state gap, with the magnitude approximately equal to 50 meV, persists at T<T _c and in the magnetic field H?H _c2 up to 28 T. After the transition of the sample to the normal state, the intensity of the tunneling conductance rapidly decreases with increasing magnetic field strength and temperature. The observed large broadening of the tunneling spectra and large zero-bias conductances can be caused by a strong angular dependence of the superconducting gap. The tunneling results are in full agreement with the data of the angle-resolved photoemission spectroscopy measurements.     

Visualizing the charge order and topological defects in an overdoped （Bi,Pb）2Sr2CuO6+x superconductor

Electronic charge order is a symmetry breaking state in high-Tc cuprate superconductors. In scanning tunneling microscopy, the detected charge-order-induced modulation is an electronic response of the charge order. For an overdoped(Bi,Pb)2Sr2CuO6+x sample, we apply scanning tunneling microscopy to explore local properties of the charge order. The ordering wavevector is nondispersive with energy, which can be confirmed and determined. By extracting its order-parameter field, we identify dislocations in the stripe structure of the electronic modulation, which correspond to topological defects with an integer winding number of ±1. Through differential conductance maps over a series of reduced energies, the development of different response of the charge order is observed and a spatial evolution of topological defects is detected. The intensity of charge-order-induced modulation increases with energy and reaches its maximum when approaching the pseudogap energy. In this evolution, the topological defects decrease in density and migrate in space. Furthermore, we observe appearance and disappearance of closely spaced pairs of defects as energy changes. Our experimental results could inspire further studies of the charge order in both high-Tccuprate superconductors and other charge density wave materials.     

Asymmetric local displacements in the Bi2Sr2CaCu2O8+β superconductor

Instantaneous in-plane Cu-O bond distribution in the Bi₂Sr₂CaCu₂O_8+β (Bi2212) superconductor has been investigated by high k-resolution Cu K-edge extended X-ray absorption fine structure (EXAFS) measured with polarized vector parallel to the two orthogonal Cu-O-Cu bonds of the CuO₂ square plane. The results show an anisotropic Cu-O distribution in the two directions and provide further information on the local atomic displacements in the lattice-charge stripes. Received 5 June 2000     

Evidence for anisotropic atomic displacements and orbital distribution in the inhomogeneous CuO2 plane of the Bi2Sr2CaCu2O8+δ system

We have used a combination of the Cu K-edge extended X-ray absorption fine structure (EXAFS), and the Cu L-edge X-ray absorption measurements, with polarization vector parallel to the two orthogonal Cu–O–Cu bonds, to explore the directional instantaneous atomic displacements and the orbital distribution within the heterogeneaous CuO₂ plane of the Bi₂Sr₂CaCu₂O_8+δ (Bi2212) system. The results show an anisotropic Cu–O bond distribution, measured by the EXAFS in the two directions, and reveal large anisotropy of the unoccupied Cu 3d orbitals. The outcome of the present measurements is consistent with the nanoscale heterogeneities with coexisting electronic components.     

Breaking of particle-hole symmetry by Landau level mixing in the ν=5/2 quantized Hall state

We perform numerical studies to determine if the fractional quantum Hall state observed at a filling factor of ν=5/2 is the Moore-Read wave function or its particle-hole conjugate, the so-called anti-Pfaffian. Using a truncated Hilbert space approach we find that, for realistic interactions, including Landau-level mixing, the ground state remains fully polarized and the anti-Pfaffian is strongly favored.     

Energy-dependent enhancement of the electron-coupling spectrum of the underdoped Bi2Sr2CaCu2O(8+δ) superconductor

We have determined the electron-coupling spectrum of superconducting Bi2Sr2CaCu2O(8+δ) from high-resolution angle-resolved photoemission spectra by two deconvolution-free robust methods. As hole concentration decreases, the coupling spectral weight at low energies ?15 meV shows a twofold and nearly band-independent enhancement, while that around ～65 meV increases moderately, and that in ?130 meV decreases leading to a crossover of dominant coupling excitation between them. Our results suggest the competition among multiple screening effects, and provide important clues to the source of sufficiently strong low-energy coupling, λ(LE)≈1, in an underdoped system.     

Low temperature structure of FeSr2YCu2O6+δ magnetic superconductor

FeSr₂YCu₂O_6+δ with the tetragonal Ba₂YCu₃O_6+δ-type structure exhibits superconductivity around 60 K, when it is properly annealed in N₂ and subsequently in O₂. This compound also exhibits antiferromagnetic order of Fe around 20 K. We have performed neutron powder diffraction of the FeSr₂YCu₂O_6+δ system at low temperature to investigate the relation between the crystal structure and the physical properties. Neutron diffraction study indicates that this compound does not have the structure phase transition due to the tilt of the FeO₆ octahedron differently from the RuSr₂GdCu₂O₈ magnetic superconductor with the same structure, and that the magnetic order in FeSr₂YCu₂O_6+δ is not long-range order, because we can observe no extra reflection due to the crystal and magnetic superstructure.     

Electronic Structure Studies of Bi2Sr2CaCu2-xSnxO8+δ System

This paper presents a linear output feedback synchronization approach for a chaotic Arneodo system. Based on the linear stability theorem, a sufficient condition of asymptotic synchronization is given, and the boundary of the range of the output feedback parameter is given also. Finally, a computer simulation is given to demonstrate the effectiveness of the method.     

Vortex structure and anisotropy of electric resistance in Bi2Sr1.65La0.35CuO6 + δ single crystals

The vortex structure of Bi₂Sr_1.65La_0.35CuO_6+δ single crystals in tilted magnetic fields has been studied by the decoration method. From the observed pattern of vortex chains in the basal plane, the parameter of anisotropy in the superconducting state has been estimated as γ_S = 460 ± 40. The electric resistance of Bi₂Sr_1.65La_0.35CuO_6+δ single crystals has been studied in a broad range of temperatures (T _c < T < 300 K) and magnetic fields (up to 16 T). The ratio of resistivities in the direction perpendicular to the basal plane and in plane near the critical temperature T _c amounted to ρ⊥/ρ‖ = 3.2 × 10⁵. A possible relationship between the anisotropy in the normal and superconducting states is discussed.     

Effects of dilute columnar defects on the vortex matter in Bi2Sr2CaCu2O8+δ near the disorder-driven phase transition

Magneto-optical measurements are employed to characterize the disorder-driven vortex phase transition and the metastable disordered vortex state in heavy-ion irradiated and pristine regions of the same Bi₂Sr₂CaCu₂O_8+δ crystal. We find that dilute columnar defects, while hardly affect the disorder-driven phase transition line, significantly increase the lifetime of the metastable state created in the vicinity of this line. Study of flux injection from the pristine region into the irradiated region suggests that in presence of columnar defects a metastable disordered state may be created in the bulk, in addition to being injected from the edges.     

High-Quality Large-Sized Single Crystals of Pb-Doped Bi2Sr2CuO6＋δ High-To Superconductors Grown with Traveling Solvent Floating Zone Method

High quality Pb-doped Bi2Sr2Cu06＋δ （Pb-Bi2201） single crystals are grown by the traveling solvent floating zone technique, with dimensions as large as - 50 min× - 5.0 min ×- 2 mm. The Pb-Bi2201 single crystals with different doping levels are obtained by the annealing process which covers a wide doping range of the overdoped region. We describe in detail the growth and annealing procedures and the characterization and physical property measurements of the Pb-Bi2201 crystals. The availability of these crystals provides a good opportunity to experimentally investigate high-To cuprate superconductors, particularly in the overdoped region.     

Geometry dependent resistivity behavior in mesoscopic Bi2Sr2CaCu2O8+δ single crystals

In order to investigate the interaction between Josephson vortices and pancake vortices in a layered Bi₂Sr₂CaCu₂O_8+δ superconductor, c-axis resistivity measurements were carried out on fabricated mesoscopic single crystals for two experimental geometries, when an applied high magnetic field of 14 T is perpendicular (B_ex⊥L) and parallel (B_ex∥L) to the width of sample. In the first case, the angular dependence of the resistance obtained near the ab-plane at T = 30 K exhibited hysteretic non-monotonic behavior with local maxima at θ = 0.6° away from the ab-plane and dips and spikes at θ = 0.2° marking a vortex lock-in transition. In the second geometry (B_ex∥L), instead of a local maximum, peculiar resistance shoulders were observed. At higher temperatures, hysteresis is reduced, while the vortex lock-in transition becomes considerably broader indicating the interaction of pancake and Josephson vortices.     

Lamellar solid-liquid mesophase nucleated by Josephson vortices at the melting of the vortex lattice in Bi2Sr2CaCu2O(8+δ) superconductor

The local effect of the Josephson vortices on the vortex lattice melting process in Bi2Sr2CaCu2O(8+δ) crystals in the presence of an in-plane field H(ab) is studied by differential magneto-optical imaging. The melting process is found to commence along the Josephson vortex stacks, forming a mesomorphic phase of periodic liquid and solid lamellas, the direction and spacing of which are controlled by H(ab). The reduction of the local melting field H(m) along the Josephson vortex stacks is more than an order of magnitude larger than the reduction of the average bulk H(m) by HH(ab).