Frequency‐doubled Nd:YAG laser (532 nm) pulses of 1.7 J/cm2 and 10 ns duration were used to deposit thin films of YBa2 Cu3 O7−δ by laser ablation on NdGaO3 , LaGaO3, and SrTiO3 substrates held at 725±5 °C in 0.2 Torr of O2 ambient. Electrical resistivities versus temperature of all films show normal metallic behavior and sharp superconducting transitions with Tc (R=0) at 92–93 K. Critical current densities in 0.3–0.6 μm thick, 200 μm long, and 10–30 μm wide strips were measured to be 106 A/cm2 at 60, 77, and 80 K in the films on LaGaO3, NdGaO3, and SrTiO3, respectively. X‐ray diffraction patterns show that all films grew epitaxially, with domains of only two crystalline orientations rotated 90° with respect to each other in the a‐b plane (consistent with twins), and the c axis perpendicular to the substrates. The closely matched lattice constants of the film and substrates (0.8–2.1%) result in epitaxial growth of the films.

Epitaxial films of YBa2Cu3O7−δ on NdGaO3, LaGaO3, and SrTiO3 substrates deposited by laser ablation

YBa2Cu3O7−δ thin films were deposited onto (100) SrTiO3 substrates using 1064, 532, 355, 248, and 193 nm laser ablation. Transport measurements show lower normal‐state resistivities and higher critical currents in films deposited by the shorter wavelength lasers. The surface morphology of the films was rough with large particulates when the 1064 nm laser was used whereas much smoother surfaces with fewer and smaller particulates were obtained with the UV lasers. It is suggested that the better film quality obtained when the UV lasers are used is due to a small absorption depth of the UV photons in the ceramic target and to higher absorption by the ablated fragments. This leads to smaller ablated species and further fragmentation in the hotter plume and, therefore, to smoother and denser films.

Laser wavelength dependent properties of YBa2Cu3O7−δ thin films deposited by laser ablation

We report the formation of excellent quality epitaxial YBa2Cu3O7 films on (100) SrTiO3 and (100) ZrO2 (yttria stabilized) substrates in the temperature range of 500–650 °C by XECl excimer laser ablation in a 0.2 Torr oxygen ambient. By applying a dc bias voltage of +300 V to an interposing ring, we were able to reduce the substrate temperatures from 650 to 500 °C for obtaining epitaxial films. The quality of the epitaxial films was found to decrease with temperature, particularly below 550 °C. The thickness of the superconducting films was varied from 500 to 5000 A with superconducting transition temperatures Tc (zero resistance) varying from 87 to 90 K for 650 °C deposits. The critical current density Jc of films was found to vary linearly with temperature with values of (zero magnetic field at 77 K) 5.0×106 and 1.0×106 for films deposited at 650 °C on (100) SrTiO3 and (100) yttria‐stabilized zirconia substrates, respectively. X‐ray diffraction, transmission electron microscopy, electron channeling patte...

In situ processing of epitaxial Y‐Ba‐Cu‐O high Tc superconducting films on (100) SrTiO3 and (100) YS‐ZrO2 substrates at 500–650 °C

The traditional distinction between tunnel and highly transmissive barriers does not currently hold for high critical temperature superconducting Josephson junctions, both because of complicated materials issues and the intrinsic properties of high temperature superconductors (HTS). An intermediate regime, typical of both artificial superconductor–barrier–superconductor structures and of grain boundaries, spans several orders of magnitude in the critical current density and specific resistivity. The physics taking place at HTS surfaces and interfaces is rich, primarily because of phenomena associated with d-wave order parameter (OP) symmetry. These phenomena include Andreev bound states, the presence of the second harmonic in the critical current versus phase relation, a doubly degenerate state, time reversal symmetry breaking and the possible presence of an imaginary component of the OP. All these effects are regulated by a series of transport mechanisms, whose rules of interplay and relative activation are unknown. Some transport mechanisms probably have common roots, which are not completely clear and possibly related to the intrinsic nature of high-TC superconductivity. The d-wave OP symmetry gives unique properties to HTS weak links, which do not have any analogy with systems based on other superconductors. Even if the HTS structures are not optimal, compared with low critical temperature superconductor Josephson junctions, the state of the art allows the realization of weak links with unexpectedly high quality quantum properties, which open interesting perspectives for the future. The observation of macroscopic quantum tunnelling and the qubit proposals represent significant achievements in this direction. In this review we attempt to encompass all the above aspects, attached to a solid experimental basis of junction concepts and basic properties, along with a flexible phenomenological background, which collects ideas on the Josephson effect in the presence of d-wave pairing for different types of barriers.

http://iopscience.iop.org/article/10.1088/0034-4885/68/11/R03/pdf

Weak links in high critical temperature superconductors

The controllable fabrication of reliable HTS Josephson junctions with sufficiently small spread of their characteristic parameters has not yet been achieved and prevents the successful use of HTS Josephson junctions in complex integrated circuits. The problems in HTS junction fabrication certainly are related to the specific properties of the cuprate superconductors, which make the fabrication of high quality interfaces in HTS junctions employing artificial barrier layers extremely difficult. Therefore, several types of HTS Josephson junctions make use of so-called intrinsic interfaces originating from grain boundaries or the intrinsic layer structure of the cuprates. Beyond the fabrication technology, the physics of HTS Josephson junctions is not well understood. In particular, the detailed mechanisms of charge transport in the various junctions types and the impact of an unconventional symmetry of the superconducting order parameter are unsettled issues. We summarize the key issues regarding the physics and technology of HTS Josephson junctions and discuss possible routes to a useful HTS junction technology.

Physics and technology of high temperature superconducting Josephson junctions

KrF excimer laser ablation of an Y1Ba2Cu3O7−δ pellet in 0.1–0.2 Torr of O2 ambient was used to deposit thin superconducting films onto SrTiO3, yttria‐stabilized zirconia (YSZ), and silicon substrates at 600–700 °C. The as‐deposited 1‐μm‐thick films at 650–700 °C substrate temperature were superconducting, without further high‐temperature annealing. All films had a similar Tc onset of ∼92 K but different zero‐resistance Tc of 90, 85, and 70 K for the films on SrTiO3, YSZ, and Si substrates, respectively. Angular x‐ray diffraction analysis showed that all the films were highly oriented with the c axis perpendicular to their surface. Critical current densities at 77 K were about 40 000 and 10 000 A/cm2 for the films on SrTiO3 and YSZ, respectively. Smooth surface morphology was observed in all films, with occasional defects and cracks in the films on YSZ, which seems to explain the lower critical current in these films.

Highly oriented as-deposited superconducting laser ablated thin films of Y1Ba2Cu3O7−δ on SrTiO3, zirconia, and Si substrates

Thin‐film YBa2Cu3O7‐YBa2Cu3O7 edge junctions of 0.4×10 μm2 cross section were prepared in situ by a multistep laser ablation deposition process. The fabrication time was about 3 h and the yield of good devices was 50%. Typical junctions reached zero resistance at 72 K and had a critical current density Jc of 300 A/cm2 at 70 K. Their Jc as a function of temperature increased slowly with decreasing temperature down to 65 K and much faster below it. In the region of low Jc we observed suppression of the critical current by a magnetic field. Under microwave radiation clear Shapiro steps were observed whose magnitude versus the microwave field agreed qualitatively with the resistively shunted junction model of a current biased junction.

Properties of all YBa2Cu3O7 Josephson edge junctions prepared by in situ laser ablation deposition

a‐b plane YBa2Cu3O7‐YBa2Cu3O7 (YBCO) edge junctions with improved properties were prepared by the use of SrTiO3 instead of BaF2 as an insulating layer in the c‐axis direction. The junctions were fabricated in an all in situ laser ablation deposition process, with the barriers formed by a plasma discharge in CF4 gas. Photolithographic rather than laser ablation patterning of the junctions was used, to avoid possible laser induced damage to the films. The normal resistance of the junction decreased with T for T≳40 K, and increased at lower temperatures, indicating that carrier localization takes place in the barrier. At 80 K, a pronounced diffraction pattern of the voltage across the junction versus magnetic field was observed. Critical currents persisted up to 84 K, and under microwave radiation many Shapiro steps were observed.

Characteristics of all YBa2Cu3O7 edge junctions operating above 80 K

Preparation and characterization of dc SQUIDs based on YBa2Cu3O7/Ag/YBa2Cu3O7 edge junctions is reported. The SQUIDs were prepared in situ by a multistep all laser ablation deposition process. Since the Ag on YBa2Cu3O7 was exposed to 730 °C in O2 ambient, the barrier resistance was quite high, ∼0.6 Ω. The SQUIDs showed voltage periodicity versus magnetic field H⊥ up to ∼80 K and had a maximum response of 30 μV/Φ0 at ∼70 K, where IcRN was 30 μV. The SQUID response versus H⊥ was superimposed on a diffractionlike pattern of its junctions, indicating an improved uniformity in the junction cross section. When the SQUID was exposed to microwave radiation, many pronounced Shapiro steps were observed in the I‐V curve, which modulated with the microwave power.

dc SQUID made of Ag‐doped YBa2Cu3O7 a‐b plane‐edge junctions

We describe a simple integrated self‐inductance bridge package for measuring the ac susceptibility of high‐Tc superconducting thin films. The package has good sensitivity and baseline drift rejection, and employs mechanical clamping for easy sample interchange.

D. Cohen

Papers

Highly oriented as-deposited superconducting laser ablated thin films of Y1Ba2Cu3O7−δ on SrTiO3, zirconia, and Si substrates

Properties of all YBa2Cu3O7 Josephson edge junctions prepared by in situ laser ablation deposition

Characteristics of all YBa2Cu3O7 edge junctions operating above 80 K

dc SQUID made of Ag‐doped YBa2Cu3O7 a‐b plane‐edge junctions

Integrated inductance bridge package for ac susceptibility measurements of superconducting thin films