YBa2Cu3O7-δ Single Crystals Research Articles

Magnetic field penetration in YBa2Cu3O7-δ singlecrystals under conditions of a reduced surface barrier

Using a magneto-optic technique, we have studied field penetration in asample consisting of several YBCO single crystals merged together on theirbasal plane. The bending of field stream lines around crystal edges in such ageometry creates an orientation of the local applied field that is nearly perpendicularto the sample surface and thus results in an almost surface-barrier-free fieldpenetration. The penetration field was measured under conditions of a fully developedand a reduced surface barrier, and the values obtained were compared in these twocases. The results obtained show that the surface barrier causes a substantialdelay of field penetration in YBCO single crystals over the whole temperaturerange.

Quasiparticles in d-wave superconductors within density functional theory

We present a semiphenomenological approach to calculating thequasiparticle spectra of high-temperature superconductors. It isbased on a particularly efficient parametrization of the effectiveelectron-electron interaction afforded by the density functionaltheory for superconductors and atight-binding linearized-muffin-tin-orbital scheme for solving thecorresponding Kohn-Sham-Bogoliubov-de Gennes equations. We applythis methodology to YBa2Cu3O7-δ (YBCO) andillustrate its potential by investigating a number of site- andorbital-specific, but otherwise phenomenological, models of pairingin quantitative detail. We compare our results for the anisotropyof the gap function on the Fermi surface with those deduced fromphotoemission experiments on single crystals of YBCO. Also, the low-temperature specific heat and penetration depth are calculated andcompared with measurements. We investigate the doping dependence ofthe superconducting gap, transition temperature, Tc, andpenetration depth. We present new evidence that the Van Hove-likescenario is an essential feature of superconductivity in the cupratesuperconductors. Since our description of pairing isphenomenological, we shed new light on the physical mechanism ofpairing only indirectly and conclude, provisionally, that thedominant pairing interaction operates between electrons of oppositespins, on nearest-neighbour Cu sites in dx2-y2 orbitals.

Effect of Heat Treatment on Critical Current Density–Magnetic Field Curve for YBa2Cu3O7-δ Single Crystal and Its Microstructure

The dependences of critical current density–magnetic field (Jc–H) curves for YBa2Cu3O7-δ (Y123) single crystals on heat treatment processes were studied. The single crystals were annealed by a two-step heat treatment process, during which the crystals were annealed at 873 K for 100 h, followed by annealing at 773 K for 200 h in a pure oxygen gas flow. Furthermore, the single crystals were annealed using three different untwinning processes with uniaxial stress such as a detwinnig process and a twin-free process. The Jc–H curves and their peak effects with the H∥c-axis displayed a large dependence on the heat treatment processes. Furthermore, the microstructures in these single crystals were observed using transmission electron microscopy (TEM). The anti-phase boundaries, which depended on the heat treatment processes, were observed in the crystals. These results suggested that the anti-phase boundaries are responsible for the peak effects, which is attributed to the distribution of the pinning centers of oxygen deficient phases in the crystals.

YBa2Cu3O7-δ single crystals revisited: Scanning probe data on very pure samples grown in BaZrO3 crucibles

Cu3O7-δ (YBCO) single crystals grown in BaZrO3 crucibles by scanning and friction force microscopy (SFM, FFM) as well as by scanning tunneling microscopy (STM) and angle resolved X-ray photo-electron spectroscopy (ARXPS). By STM and SFM, clean stepped terraces are observed on as-grown crystals. Friction contrast, however, implies the presence of different materials, presumably traces of flux. After oxidation of the YBCO crystals, particles of 10 to 100 nm in size appear on the surface. Their number per unit area increases with time when the crystal is stored in air. Complementary information on such aged YBCO surfaces is provided by ARXPS. These data reveal 3 nm of mainly BaCO3 and CuO on top of the YBCO matrix and suggest a very slow aging rate of the crystals, which is 20 times slower than for epitaxial thin films. The superconducting energy gap has been determined on fresh crystals by scanning tunneling spectroscopy (STS) at 4.6 K as 2Δ≈30 meV. By spatially resolved tunneling spectroscopy the vortex lattice in a magnetic field of 4 T has been imaged. The inter-vortex distance amounts to 20±2 nm.

High-field paramagnetic effect in large crystals ofYBa2Cu3O7−δ

We observed the appearance of a paramagnetic moment in YBa2Cu3O7-δ single crystals after cooling in a strong field (3–7 T). It is found that the effect depends on the cooling rate and sample size, indicating that the paramagnetic moment can be induced by compression of the magnetic flux in the course of rapid cooling. After rapid cooling, the temperature dependence of the magnetization during the field-cooled warming process exhibits a very articulated negative dip between irreversibility temperature Tirr and Tc. The diamagnetic dip may result from escaping the compressed flux. Comparing the dip widths for a number of crystals with different irreversibility lines Hirr(T) we found that a remarkable ``fishtail'' effect appears in the temperature region below the dip. This suggests that the field-induced pinning observed by the fishtail effect is linked with the paramagnetic effect that appears after rapid cooling in the high field.

The influence of the structure on the electrical and magnetic properties of Bi2Sr2CaCu2O8+δ single crystals

The correlation between the structure, chemical composition, electrical and magnetic properties of Bi2Sr2CaCu2O8+δ single crystals is presented. As revealed by SEM measurements, the small samples (approximately 1 mm×1 mm×0.2 mm) are stacks of thin plates (1–2 μm). Microcompositional measurements (energy dispersive spectroscopy) show the Bi2Sr2CaCu2O8+δ (2212)-phase and a small quantity of residual phases enriched in bismuth and strontium and calcium-deficient. Magnetically modulated microwave absorption measurements (MAMMA) lead to a value for the anisotropy of the upper critical field of about 7, which is more than twice the reported value for YBa2Cu3O7-δ single crystals (also determined from MAMMA responses). The plot of the resistance versus temperature suggests that only the first 10–20 plates from the surface of the sample have a contribution to the sample conductivity. The imaginary part, X″, of the magnetic susceptibility of our samples shows three peaks.

Growth technique for producing high quality superconducting YBa2Cu3O7-? single crystals

High quality single crystals of YBa2Cu3O7-δ (YBCO) have been grown in yttria-stabilized zirconia crucibles by using a flux method. This method yields crystals with area 3 to 4 times larger than a previous method, and 3 to 5 times more crystals. Several crystals as large as 3×4mm2 in thea-b plane are routinely obtained from each crucible. The samples typically range from 10 to 60 μm thick. The superconductivity transition temperatureTc of the crystals, measured by magnetic susceptibility, falls in the range 92.5 to 93 K and the smallest transition width (10 to 90%) is less than 0.25 K. Compared with the previous method, the surface is generally smoother, with less flux, and larger untwinned regions of 1 x 1 mm2 are usually found.

Vortex dynamics in different magnetic field directions of untwinned YBa2Cu3O7-δ single crystals

Current-Voltage (I–V) characteristics have been measured on an untwinned single crystal of YBa2Cu3O7-δ with the current along theb-axis and with the applied magnetic field along various crystallographic axes. Measurements with the magnetic field in theab-plane resulted in a different qualitative behaviour of the I–V characteristics than for the field applied along thec-axis.

Intrinsic Josephson junctions in YBa2Cu3O7-δ single crystals

In the resistive region just below Tc, the dc magnetic field dependences of the dynamic resistance of several YBa2Cu3O7-δ single crystals exhibit oscillations with periods suggestive of the existence of Josephson junctions formed by the CuO2−Y−CuO2 atomic planes within the unit cell as well as those formed by the CuO2 bilayers from different unit cells. The coupling strength of these junctions and the distance over which the bilayers from different unit cells can couple depend very strongly on temperature.

Transmission electron microscopy of defect cascades in YBa2Cu3O7−δproduced by ion irradiation

Abstract YBa2Cu3O7-δ single crystals were irradiated with 50 and 85 keV Kr+ and Xe+ ions to low doses, producing spatially isolated defect cascades. One irradiation was performed at low temperature. The resulting defects were investigated by conventional and high-resolution transmission electron microscopy. They were found to have primarily an amorphous-like structure with an inwardly directed (vacancy) strain field. Evidence for lattice melting, with coherent or incoherent recrystallization at some defect cascade sites, is presented for the first time.

Growth of YBa2Cu3O7-δ single crystals on seeds by a modified top seeded solution growth (TSSG) technique

A modified top seeded solution growth (TSSG) method has been developed for growth of YBa2Cu3O7-δ crystals under constant supersaturation. The conditions for growth on seeds have been determined.

Phase separation during solution growth of YBa2Cu3O7-δ single crystals using a pulling technique

The Czochralski method is used to improve YBa2Cu3O7-δ (YBCO) crystal growth by the slow cooling technique from an Y-Ba-Cu-O flux stored in an alumina crucible. An optimum flux has been found at a 1Y:6Ba:13Cu mole ratio. A multiphase material is pulled from the flux with an alumina tube at rate of 0.2 mm/h while the flux is continuously cooled from 1000 to 970°C at a rate of 0.1 0°C/h. The growing boule which consists mainly of non-123 phases is separated from the melt at 9700°C. Large YBCO crystals, with an area up to about 20 cm2 and a thickness of 0.3 mm, are then formed on the melt surface by continued cooling to 9500°C. The oxygen concentration of these crystals is found to be much higher at the edge than at the centre.

On the Surfacegrived State of Cleaved YBa2Cu3O7-δSingle Crystals

On the Surfacegrived State of Cleaved YBa₂Cu₃O_7-δSingle Crystals

Anharmonicity of high-frequency O vibrations in YBa2Cu3O7-δ, TlBa2CaCu2O7-δand Tl2Ba2CaCu2O8-δdeduced from high-temperature Raman spectra

Abstract High temperature Raman spectra on single crystals of YBa2Cu3O7-δ, TlBa2CaCu2O7-δ and Tl2Ba2CaCu2O8-δ show significant softening and broadening of apical oxygen vibrational modes above 300 K. This behaviour appears to be a common in superconducting oxides with a pyramidal O structure surrounding planar Cu ions. Although this high-temperature behaviour is characteristic of anharmonic modes, we find that a simple double well static anharmonic potential cannot fully explain these observations.

Angle dependence of the muon relaxation rate in the mixed state for single crystals of YBa2Cu3O7-δ

Angle dependence of the muon relaxation rate in the mixed state for single crystals of YBa2Cu3O7-δ

Polarized-optics study of twins in YBa2Cu3O7-δ single crystals

Polarized-optics study of twins in YBa2Cu3O7-δ single crystals

Anomalous H c1 (T) behavior of single crystalline YBa 2 Cu 3 O 7-δ

Abstract We report dc-magnetization measurements M(T,H) on a single crystal of YBa2Cu3O7-δ using a SQUID magnetometer. From the M(T,H) data we determined the lower critical field Hc1(T). For both orientations of the field, parallel and perpendicular to the c-axis, we find remarkable anomalous enhancements of H∥c1(T) and H∥c1(T) for T 40K.

Defect structures preceding amorphization by ion irradiations in YBa 2 Cu 3 O 7-†

Abstract Transmission electron microscopy was used to investigate defect microstructures in single crystals of YBa2Cu3O7-δ as a function of ion irradiation dose at 300 K. Two qualitatively different defect microstructures were observed to precede amorphization. The transition between microstructures occurs at a damage level where published resistance versus temperature measurements indicate a transition from superconducting to insulating behavior.

Magnetization of neutron irradiated YBa2Cu3O7-δ single crystals

YBCO single crystals with very sharp transitions at Tc were subjected to reactor neutron irradiation at ambient temperature. The magnetization curves, which were measured in a SQUID magnetometer in fields up to 8 T, show increasing irreversibility and decreasing anisotropy with neutron fluence. Bean-model evaluations of critical current densities are presented and discussed in terms of induced flux pinning centers.

Microwave absorption in a single crystal of YBaCuO at low magnetic field

Abstract We have studied microwave absorption at low magnetic field (0–40 mT) in a single crystal of YBa2Cu3O7-δ. Several serie s of equally spaced lines have been found: they extend from 0 to 30 mT with a perfect regularity in intensity and spacing. We have studied the evolution of these lines when the sample is rotated with respect to static (H0) and microwave (H1) magnetic fields. The phenomenon is associated with the existence on the sample surface of superconducting loops containing some weak links. We show that the relevant components of both H0 and H1 are those parallel to the vector S giving the area of the loop. The microw ave power absorption is well described in the framework of a model due to Silver and Zimmerman.