CuO2 Double Layers Research Articles

Tailoring Bi2Sr2CaCu2O8+δ surface Josephson junctions

Bi2Sr2CaCu2O8+δ (BSCCO) single crystals are promising for high-temperature superconducting electronic devices with ultimate performance. Recently, tailoring superconductivity in ultra-thin BSCCO locally has received wide attention and interest. In this work, we focus on controlling the superconductivity of the uppermost CuO2 double layer of the BSCCO crystal. Gold electrodes are deposited onto the freshly cleaved BSCCO surface by a high-vacuum in situ evaporation technique at room temperature, and then surface Josephson junctions (SJJs) between the two outermost superconducting CuO2 double layers are fabricated as a probe to evaluate the superconductivity of the uppermost double layer. We find that the junction transition temperature Tc′ as well as its critical current density jc depend systematically and reproducibly on the deposition rate of the gold electrode. The electric properties of the SJJs, spanning the range from completely degraded to almost as good as the intrinsic Josephson junctions inside the bulk, systematically reveal the evolutionary process of superconductivity of the uppermost BSCCO layer, extending the range for practical applications.

Universal quantum oscillations in the underdoped cuprate superconductors

The metallic state of the underdoped high-Tc cuprates has remained an enigma: How may seemingly disconnected Fermi surface segments, observed in zero magnetic field as a result of the opening of a partial gap (the pseudogap), possess conventional quasiparticle properties? How do the small Fermi-surface pockets evidenced by the observation of quantum oscillations (QO) emerge as superconductivity is suppressed in high magnetic fields? Such QO, discovered in underdoped YBa2Cu3O6.5 (Y123) and YBa2Cu4O8 (Y124), signify the existence of a conventional Fermi surface (FS). However, due to the complexity of the crystal structures of Y123 and Y124 (CuO2 double-layers, CuO chains, low structural symmetry), it has remained unclear if the QO are specific to this particular family of cuprates. Numerous theoretical proposals have been put forward to explain the route toward QO, including materials-specific scenarios involving CuO chains and scenarios involving the quintessential CuO2 planes. Here we report the observation of QO in underdoped HgBa2CuO4+{\delta} (Hg1201), a model cuprate superconductor with individual CuO2 layers, high tetragonal symmetry, and no CuO chains. This observation proves that QO are a universal property of the underdoped CuO2 planes, and it opens the door to quantitative future studies of the metallic state and of the Fermi-surface reconstruction phenomenon in this structurally simplest cuprate.

Magnetic Field Dependent Photoinduced Superconductivity and Its Oxygen-Deficient Dependency in Undoped and BZO-doped YBCO Thin Films

Abstract We present a systematic study of photoinduced effiect illuminating the undoped and BZO-doped YBCO thin films with laser diode (1.88 eV). The pulsed laser deposited films were post-annealed in partial oxygen pressure in order to decrease the oxygen content and therefore understand the role of structural changes, oxygen ordering and hole doping in photoconductivity phenomenon. The resistivities were measured in the thermally activated flux-flow regime with wide applied magnetic field range. We find that as in undoped and optimally oxygenated YBCO films, the structural defects and dislocations formed by conventional BZO-doping does not contribute to photoinduced enhancement of superconductivity. On the contrary, in our oxygen-deficient films, the effect of photoexcitation is strongly dependent on the oxygen ordering and thereby free-carrier concentration and their mobility in the CuO2 double layers, which again has strong external magnetic field dependence. The magnetic field dependence of the increased superconducting transition shows that the illumination improves the pinning properties in high magnetic field range, indicating the existence of photosensitive pinning centers.

Shapiro Steps in Flux-Trapped Surface Intrinsic Junctions ofBi2Sr2CaCu2O8+δ

Microwave-field responses of the surface intrinsic Josephsonjunctions (IJJs) of Bi2Sr2CaCu2O8+δsuperconductors are investigated. The IJJs are fabricated using anin situ low-temperature cleavage technique, which leads tothe well-characterized surface CuO2 double layers and surfacejunctions. For the surface junctions in the large-junction limit,usually no Shapiro steps appear when a microwave field is applied.It is found that when the junctions are in a flux-trapped state,which is produced by a pulsed current and in which the criticalcurrent is significantly suppressed, clear Shapiro steps can beobserved. These results are important for the study of themicrowave-field properties of vortex-carrying IJJs and may findtheir use in device applications.

Investigation of Terahertz Emission Phenomena of High-Tc Superconductors

The c-axis transmission spectra of underdoped Bi2Sr2CaCu2O8+δ (BSCCO) and YBa2Cu3O7-δ (YBCO) thin films were measured in the frequency range from 0.1 to 1.5 THz to investigate the mechanism of terahertz (THz)-pulse emission from high-TC superconductors (HTSs). The transmission spectra showed different properties depending on both the kind and hole-doping level of HTS thin film. A relatively strong shielding effect was observed for THz waves below the characteristic frequency f0, which almost corresponds to the cutoff frequency fc in the THz-wave radiation spectrum. The observed THz-pulse transmission and emission properties are explained by the intrinsic anisotropic characteristic peculiar to each HTS sample. Particularly, the magnitude of the superconducting order parameter that is induced in the insulating barrier layer between CuO2 double layers plays an important role in determining the cutoff frequency fc.

On the dissipation process in Bi2Sr2Ca2Cu3O10 tapes and bulk samples with preferential grain orientation in zero external magnetic field

Current-voltage characteristics of Bi2Sr2Ca2Cu3O10 tapes and bulk samples with preferentially oriented crystallites have been measured in zero applied magnetic field and for temperatures close to the mean-field critical-temperature,T c0. It was shown that the power-law dependence,V=AI a, valid for two-dimensional systems, does not correctly describe the data, which clearly indicate the existence of a finite critical-current density,J c. The experimentalI-V curves are fitted quite well with the model which attributes the finite critical-current density to the coupling between the CuO2 double layers. It was found thatJ c vanishes at a temperature value belowT c0. This behavior can result from the occurrence of vortex fluctuation-induced layer decoupling and/or from the contribution of entropy to the vortex unbinding process.