Observation of temperature-dependent site disorder in YBa2Cu3O7- delta below 150 degreesC

Direct Visualization of Current-Stimulated Oxygen Migration in YBa2Cu3O7-δ Thin Films

The past years have witnessed major advancements in all-electrical doping control on cuprates. In the vast majority of cases, the tuning of charge carrier density has been achieved via electric field effect by means of either a ferroelectric polarization or using a dielectric or electrolyte gating. Unfortunately, these approaches are constrained to rather thin superconducting layers and require large electric fields in order to ensure sizable carrier modulations. In this work, we focus on the investigation of oxygen doping in an extended region through current-stimulated oxygen migration in YBa₂Cu₃O_7-δ superconducting bridges. The underlying methodology is rather simple and avoids sophisticated nanofabrication process steps and complex electronics. A patterned multiterminal transport bridge configuration allows us to electrically assess the directional counterflow of oxygen atoms and vacancies. Importantly, the emerging propagating front of current-dependent doping δ is probed in situ by optical microscopy and scanning electron microscopy. The resulting imaging techniques, together with photoinduced conductivity and Raman scattering investigations, reveal an inhomogeneous oxygen vacancy distribution with a controllable propagation speed permitting us to estimate the oxygen diffusivity. These findings provide direct evidence that the microscopic mechanism at play in electrical doping of cuprates involves diffusion of oxygen atoms with the applied current. The resulting fine control of the oxygen content would permit a systematic study of complex phase diagrams and the design of electrically addressable devices.

Singling out the effect of quenched disorder in the phase diagram of cuprates

We investigate the specific influence of structural disorder on the suppression of antiferromagnetic order and on the emergence of cuprate superconductivity. We single out pure disorder, by focusing on a series of [Formula: see text] samples at fixed oxygen content y   =  0.35, in the range [Formula: see text]. The gradual Y/Eu isovalent substitution smoothly drives the system through the Mott-insulator to superconductor transition from a full antiferromagnet with Néel transition [Formula: see text] K at z  =  0 to a bulk superconductor with superconducting critical temperature [Formula: see text] K at z  =  1, [Formula: see text]. The electronic properties are finely tuned by gradual lattice deformations induced by the different cationic radii of the two lanthanides, inducing a continuous change of the basal Cu(1)-O chain length, as well as a controlled amount of disorder in the active Cu(2)O₂ bilayers. We check that internal charge transfer from the basal to the active plane is entirely responsible for the doping of the latter and we show that superconductivity emerges with orthorhombicity. By comparing transition temperatures with those of the isoelectronic clean system we determine the influence of pure structural disorder connected with the Y/Eu alloy.

Evolution of a Novel Ribbon Phase in Optimally Doped Bi2Sr2CaCu2O8+δ at High Pressure and Its Implication to High- TC Superconductivity

One challenge in studying high-temperature superconductivity (HTSC) stems from a lack of direct experimental evidence linking lattice inhomogeneity and superconductivity. Here, we apply synchrotron hard X-ray nanoimaging and small-angle scattering to reveal a novel micron-scaled ribbon phase in optimally doped Bi₂Sr₂CaCu₂O_8+δ (Bi-2212, with δ = 0.1). The morphology of the ribbon-like phase evolves simultaneously with the dome-shaped T_C behavior under pressure. X-ray absorption studies show that the increasing of T_C is associated with oxygen-hole redistribution in the CuO₂ plan, while T_C starts to decrease with pressure when oxygen holes become immobile. Additional X-ray irradiation experiments reveal that nanoscaled short-range ordering of oxygen vacancies could further lower T_C, which indicates that the optimal T_C is affected not only by an optimal morphology of the ribbon phase, but also an optimal distribution of oxygen vacancies. Our studies thereby provide for the first time compelling experimental evidence correlating the T_C with micron to nanoscale inhomogeneity.

Controlling the near-surface superfluid density in underdoped YBa2Cu3O(6+x) by photo-illumination

The interaction with light weakens the superconducting ground state in classical superconductors. The situation in cuprate superconductors is more complicated: illumination increases the charge carrier density, a photo-induced effect that persists below room temperature. Furthermore, systematic investigations in underdoped YBa₂Cu₃O_6+x (YBCO) have shown an enhanced critical temperature T_c. Until now, studies of photo-persistent conductivity (PPC) have been limited to investigations of structural and transport properties, as well as the onset of superconductivity. Here we show how changes in the magnetic screening profile of YBCO in the Meissner state due to PPC can be determined on a nanometer scale utilizing low-energy muons. The data obtained reveal a strongly increased superfluid density within the first few tens of nanometers from the sample surface. Our findings suggest a non-trivial modification of the near-surface band structure and give direct evidence that the superfluid density of YBCO can be controlled by light illumination.

Superfluid density in a highly underdoped YBa_{2}Cu_{3}O_{6+y} superconductor

The superfluid density rho_{s}(T) identical with1/lambda;{2}(T) has been measured at 2.64 GHz in highly underdoped YBa_{2}Cu_{3}O_{6+y}, at 37 dopings with T_{c} between 3 and 17 K. Within limits set by the transition width DeltaT_{c} approximately 0.4 K, rho_{s}(T) shows no evidence of critical fluctuations as T-->T_{c}, with a mean-field-like transition and no indication of vortex unbinding. Instead, we propose that rho_{s} displays the behavior expected for a quantum phase transition in the (3+1)-dimensional XY universality class, with rho_{s0} proportional, variant(p-p_{c}), T_{c} proportional, variant(p-p_{c});{1/2}, and rho_{s}(T) proportional, variant(T_{c}-T);{1} as T-->T_{c}.

Enhancement of the critical current density of YBa2Cu3Ox superconductors under hydrostatic pressure

The dependence of the critical current density Jc on hydrostatic pressure to 0.6 GPa is determined for a single 25 degrees [001]-tilt grain boundary in a bicrystalline ring of nearly optimally doped melt-textured YBa2Cu3Ox. Jc is found to increase rapidly under pressure at +20%/GPa. A new diagnostic method is introduced (pressure-induced Jc relaxation) which reveals a sizable concentration of vacant oxygen sites in the grain boundary region. Completely filling such sites with oxygen anions should lead to significant enhancements in Jc.

Survival of the d-wave superconducting state near the edge of antiferromagnetism in the cuprate phase diagram

In the cuprate superconductor YBa2Cu3O6+x, hole doping in the CuO2 layers is controlled by both oxygen content and the degree of oxygen ordering. At the composition YBa2Cu3O6.35, the ordering can occur at room temperature, thereby tuning the hole doping so that the superconducting critical temperature gradually rises from 0 to 20 K. Here we exploit this to study the c-axis penetration depth as a function of temperature and doping. The temperature dependence shows the d-wave superconductor surviving to very low doping, with no sign of another ordered phase interfering with the nodal quasiparticles. The only apparent doping dependence is a smooth decline of superfluid density as T(c) decreases.

Pronounced in-plane anisotropy of phonon anomalies in YBa(2)Cu(3)O(6.6)

The dispersion of the Cu-O bond-stretching and bond-bending vibrations in YBa(2)Cu(3)O(6.6) has been studied by high resolution inelastic neutron scattering. While the behavior of the bond-bending vibrations can be well accounted for by a simple potential model, the bond-stretching vibrations show a highly anomalous behavior. The displacement pattern of the most anomalous phonons is in principle consistent with dynamic charge stripe formation. However, charge stripes would have to extend along the a axis and not the b axis as inferred from the magnetic fluctuations by Mook et al. [Nature (London) 404, 729 (2000)].

Anomalously large oxygen-ordering contribution to the thermal expansion of untwinned YBa2Cu3O6.95 single crystals: A glasslike transition near room temperature

We present high-resolution thermal expansion data from 5-500 K of untwinned YBa2Cu3Ox (Y123) single crystals for x approximately 6.95 and x = 7.0. Large contributions to the thermal expansivities due to O ordering are found for x approximately 6.95, which disappear below a kinetic glasslike transition near 280 K. The kinetics at this glass transition is governed by an energy barrier of 0.98+/-0.07 eV, in very good agreement with other O-ordering studies. Using thermodynamic arguments, we show that O ordering in the Y123 system is particularly sensitive to uniaxial stress along the chain axis and that the lack of well-ordered chains in Nd123 and La123 is most likely a consequence of a chemical-pressure effect.

SOME CHEMICAL AND STRUCTURAL EFFECTS ON THE PROPERTIES OF HIGH-Tc SUPERCONDUCTORS

▪ Abstract  The history of superconductivity, including superconductivity in the high-Tc cuprates, is reviewed very briefly, and the differences between conventional superconductors and the high-Tc cuprates are summarized. The basic crystal structures of the major series of high-Tc cuprates are described and compared. The relation of structures to superconducting properties is reviewed with an emphasis on the orthorhombic-tetragonal transition in (La2−x Srx)CuO4; the corresponding transition, and also the transition to the low-temperature tetragonal phase in (La2−xBax)CuO4; and the effects of oxygen vacancies, oxygen-vacancy ordering, frozen-in disorder, and occupation of the off-chain O(5) sites in YBa2Cu3O7−δ. The effects of chemical substitutions of lanthanide elements on the La/Sr sites in (La2−xSrx)CuO4 and on the Y sites in YBa2Cu3O7−δ, and of 3d elements and sp elements on the Cu sites in both systems are reviewed. The difference between the effects of the lanthanide substitutions, particularly Pr, in the two systems are considered. Major properties of the Bi-, Tl-, and Hg-cuprates, are described briefly. A comparison of the high-Tc cuprates with noncuprate oxide superconductors is given.