Direct correlation between Tc and CuO2 bilayer spacing in YBa2Cu3O7-x (original) (raw)
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Disorder-controlled superconductivity at YBa_{2}Cu_{3}O_{7}/SrTiO_{3} interfaces
Physical Review B, 2013
We examine the effect of interface disorder in suppressing superconductivity in coherently grown ultrathin YBa 2 Cu 3 O 7 (YBCO) layers on SrTiO 3 (STO) in YBCO/STO superlattices. The termination plane of the STO is TiO 2 and the CuO chains are missing at the interface. Disorder (steps) at the STO interface cause alterations of the stacking sequence of the intracell YBCO atomic layers. Stacking faults give rise to antiphase boundaries which break the continuity of the CuO 2 planes and depress superconductivity. We show that superconductivity is directly controlled by interface disorder outlining the importance of pair breaking and localization by disorder in ultrathin layers.
Superconducting Nature of Ultrathin YBa2Cu3O7 Films
Advances in Superconductivity V, 1993
Study of ultrathin YBa 2 Cu 3 0 7 (YBCO) films, especially, one-unit-cell thick (l-UCT) film gives a fundamental understanding in the nature of high-~ superconductivity. Ultrathin YBCO films were epitaxially grown on nonsuperconducting PrBa 2 Cup7 (PrBCO) buffer layer by reactive evaporation, in which the crystal growth was controlled through the monitoring of reflection high energy electron diffraction (RHEED) specular intensity oscillations. Atomic force microscope (AFM) observation reveals that the steps in the surface of ultrathin YBCO films (-140A.) have one-unit-cell height, implying a unit cell-by-unit cell growth manner. 1-UCT YBCO layer exhibits superconductivity when charge reservoir layers of BaO-CuO-BaO are located above and below the CuQz bilayer interposed with an Y layer. Study of diagonal and Hall resistivities provides clear evidence supporting the Kosterlitz-Thouless transition in l-UCT YBCO film.
Physical Review B, 2009
Combined synchrotron angle-dispersive powder diffraction and micro-Raman spectroscopy are used to investigate pressure-induced lattice instabilities that are accompanied by superconducting Tc anomalies in YBa2Cu4O8 and optimally doped YBa2Cu3O 7−δ , in comparison with the nonsuperconducting PrBa2Cu3O6.92. In the first two superconducting systems there is a clear anomaly and hysteresis in the evolution of the lattice parameters and increasing lattice disorder with pressure, which starts at ≈ 3.7 GPa. On the contrary, in the Pr-compound the lattice parameters follow very well the expected equation of state (EOS) up to 7 GPa. The micro-Raman data of the superconducting compounds show that the energy and width of the Ag phonons exhibit anomalies over the same pressure range where the lattice parameters deviate from the EOS and the average Cu2-O pl bond length exhibits a strong contraction that correlates with the non-linear pressure dependence of Tc. The anomalous Raman behavior is not observed for the non superconducting Pr compound, clearly indicating a connection with the charge carriers. It appears that the cuprates close to optimal doping are at the edge of lattice instability.
Doping of the CuO2 planes of Cu1−xTlxBa2Ca2Cu3O10−y superconductor via light and heavier ions
Physica C: Superconductivity, 2003
The superconducting properties of Cu 1Àx Tl x Ba 2 Ca 2 Cu 3 O 10Ày thin films prepared by amorphous phase epitaxial (APE) method have been studied, by resistivity measurements, X-ray diffraction, electron microscopy and infrared spectroscopy. The main emphasis of this research work is on the doping of the charge reservoir layer, which ultimately controls carrier concentration in the CuO 2 conducting planes. The phenomena of the superconductivity, takes place in the conducting CuO 2 planes, however, we can control it via charge concentration in the CuO 2 planes. We have done this by carrying out annealing experiments in different conditions i.e. in air, nitrogen atmosphere and in vacuum. In these annealing experiments the doping is carried out by intercalation of the light O 2 in the CuO chains and by the removal of heavy thallium from the CuO chain axis. The removal of thallium at 717°C from the CuO chains shifted the T c (R ¼ 0) to higher values and the doping of oxygen and nitrogen at 450°C increased the semiconductivity and shifted the T c (onset) to lower values. The removal of oxygen from CuO chains by vacuum annealing at 450°C has also increased the degree of semiconductivity and shifted the T c (onset) to lower temperatures. The XRD of the samples of Cu 1Àx Tl x -1223 phase showed the predominant c-axis oriented material with c-axis lattice constant 15.53 A A. The electron micrograph of these samples showed elongated grains with the grains ends fused together. The surface roughness of the samples was below 0.2 lm. The infrared absorption measurements have shown the softening of 451 cm À1 mode after removal of oxygen from the unit cell, which shows that oxygen doping controls the carrier concentration in the charge reservoir layer and hence the superconductivity in the material.
Physica C: Superconductivity, 2005
We report systematic studies of structural, microstructural and transport properties of Y 1-x Ca x Ba 2 Cu 3 O 7-δ bulk samples with 0.20 ≥ x ≥ 0.0. The partial substitution of Ca 2+ at Y 3+ site in YBa 2 Cu 3 O 7-δ leads towards slightly overdoped regime, which along with disorder in CuO 2 planes is responsible for decrease in superconducting transition temperature (T c ) of the doped system. The microstructural variants being studied by transmission electron microscopy (TEM) technique in imaging and selected area diffraction modes revealed an increase in the density of twins with increase in Ca concentration despite the fact that there is slight decrease in the orthorhombicity. This is against the general conviction of decrease in twin density with decreasing orthorhombicity. An increase in twin density results in sharpening of twin boundaries with increasing Ca concentration. These sharpened twin boundaries may work as effective pinning centers. A possible correlation between microstructural features and superconducting properties has been put forward and discussed in the present investigation.
Texture, strains, and superconductivity in Y1−xPrxBa2Cu3O7−δ
Physica C: Superconductivity, 2007
We investigate the Y 1Àx Pr x Ba 2 Cu 3 O 7Àd (0 6 x 6 1) and YBa 2Ày Pr y Cu 3 O 7Àd (y = 0.1 and 0.2) polycrystalline compounds via X-ray diffraction and Raman measurements. The oxygen concentration measurements show all samples to be optimally doped or overdoped. The magnetic measurements M(T) and v ac (T) verify that the increase of the Pr content leads to the expected strong systematic decrease of the critical temperature T c and the suppression of superconductivity for x > 0.55. Systematic micro-Raman studies of the observed vibrational modes reveal a coexistence of locally different rare-earth seemingly nanoscopic environments in the mixed rare-earth Y123 systems that has been overlooked to a great extent up to now. The comparison of the Raman spectra from the two sets of compounds proves that an increased amount of Ba is occupied by Pr in Y 1Àx Pr x Ba 2 Cu 3 O 7Àd and indicates that loss of superconductivity is mainly due to hole filling by this occupancy.
Structural inhomogeneities in thin single-crystal films of YBa 2 Cu 3 O 7 − d are characterized using high resolution X-ray diffraction. Results of these measurements show a distribution in the c-axis lattice parameter and variations in the integrated intensities of (00ᐉ) diffraction peaks. Both effects can be interpreted as due to a variance in the oxygen content. Films prepared under identical growth conditions, when subjected to different post-annealing treatments showed varying structural and superconducting properties. A film with 7 − d close to 7.0 had a transition temperature (T c ) of 91 K, a width (DT c ) of 0.6 K, and a critical current density (J c ) of 6.5 × 10 6 A/cm 2 , whereas a film in which the oxygen content varied between 6.86 and 6.96 had a T c of 87 K, a DT c of 0.5 K, and a J c of 8.17 × 10 6 A/cm 2 . For each of the samples measured, the full width at half maximum (FWHM) of the (006) diffraction peak ranged between 340 and 2100 arc-seconds and was dependent on the method of preparation and the substrate. The J c s of these films were not sensitive to the quality of the film, as determined from the FWHM of the (006) peak, and the oxygen content, as determined from the (005) to (006) integrated intensity ratio. High resolution X-ray topographs showed uniform microstructure of the films, indicating that oxygen distribution is finely dispersed in the film. © 1997 Elsevier Science S.A.
Effect of strain on the structure and critical temperature in superconducting Nd-doped YBa2Cu3O7-δ
Physical Review B, 2002
A detailed study of the correlation between the superconducting critical temperature and the strain in as-grown, high-quality, c-axis epitaxial, Nd-doped YBa 2 Cu 3 O 7Ϫ␦ ͑YBCO͒ films is reported. Samples with thicknesses ranging from 8 to 250 unit cells have been deposited by high-pressure oxygen sputtering on LaAlO 3 ͑100͒ single-crystal substrates. The a, b, and c axis of the films have been accurately determined by x-ray diffraction from the measurements of the ͑005͒ and the ͑038͒-͑308͒ reflections. Below a certain thickness, a crossover from orthorhombic to tetragonal structure is observed, together with a decrease of the critical temperature. These results cannot be explained by the elastic deformation of the film, but point to inelastic strain induced by the large mismatch with the substrate related to an oxygen reorganization in the Cu͑1͒-O plane.
Homogeneous superconducting gap in DyBa2Cu3O7−δ synthesized by oxide molecular beam epitaxy
Physical Review Materials
Much of what is known about high-temperature cuprate superconductors stems from studies based on two surface analytical tools, Angle-Resolved Photoemission Spectroscopy (ARPES) and Spectroscopic Imaging Scanning Tunneling Microscopy (SI-STM). A question of general interest is whether and when the surface properties probed by ARPES and SI-STM are representative of the intrinsic properties of bulk materials. We find this question is prominent in thin films of a rarelystudied cuprate, DyBa2Cu3O7-δ (DBCO). We synthesize DBCO films by oxide molecular beam epitaxy and study them by in-situ ARPES and SI-STM. Both ARPES and SI-STM show that the surface DBCO layer is different from the bulk of the film-it is heavily underdoped, while the doping level in the bulk is very close to optimal doping evidenced by bulk-sensitive mutual inductance measurements. ARPES shows the typical electronic structure of a heavily underdoped CuO2 plane and two sets of one-dimensional bands originating from the CuO chains with one of them gapped. SI-STM reveals two different energy scales in the local density of states, with one (at ~18 meV) corresponding to the superconductivity and the other one (at ~90 meV) to the pseudogap. While the pseudogap shows large variations over the length scale of a few nanometers, the superconducting gap is very homogeneous. This indicates that the pseudogap and superconductivity are of different origins.