P. Chaudhari

Quantum interference devices made from superconducting oxide thin films

We have fabricated superconducting quantum interference devices (dc SQUID’s) from thin films of the superconducting oxide YBa2Cu3Oy. The devices were made by first lithographically patterning an ion implant mask containing a 40 by 40 μm loop and two 17‐μm‐wide weak links over a ∼1‐μm‐thick oxide film. Ion implantation was then used to destroy the superconductivity in the film surrounding the device without actually removing material, resulting in a completely planar structure for the SQUID’s. The SQUID’s were operated in the temperature range from 4.2 to 68 K. The superconducting flux quantum was measured to be h/2e in these materials.

Amorphous metallic films for magneto‐optic applications

It has been demonstrated that perpendicular uniaxial anisotropy and compensation points in the vicinity of room temperature can be obtained in amorphous thin films of rare‐earth—transition‐metal alloys. The magneto‐optic properties of these films are reported, and it is shown that a remarkably good signal‐to‐noise ratio can be obtained in a thermomagnetically written film.

Hillock growth in thin films

A model for hillock growth in thin films such as those of lead and tin is developed. It is proposed that hillock growth is a form of stress relaxation in which atomic species diffuse along the film‐substrate interface to the base of the hillock. The latter extrudes out along the grain boundaries connecting it to the rest of the film. The stability and kinetics of this process are considered. Taking into account Nabarro‐Herring diffusion creep as a parallel stress relaxation mechanism observations such as an incubation period for hillock growth and the effects of film thickness on hillock density are accounted for. In addition, observations on lateral growth of hillocks and unusually tall hillocks (whiskers) are interpreted within the framework of the present model.

Low noise YBa2Cu3O7-δ grain boundary junction dc SQUIDs

We have fabricated YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} grain boundary junction dc superconducting quantum interference devices (SQUIDs) with a square washer geometry design. The SQUIDs were formed in {ital c}-axis oriented epitaxial films with a single grain boundary of predetermined nature. These SQUIDs show perfectly periodic voltage-flux characteristics without hysteresis from 4.2 to 87 K. At 77 K intrinsic energy sensitivities of 1.5{times}10{sup {minus}30} and 3.0{times}10{sup {minus}30} J/Hz at 10 kHz were obtained for 60 and 110 pH SQUIDs, respectively. The intrinsic energy sensitivity limited by 1/{ital f} noise at 10 Hz was 1.2{times}10{sup {minus}28} and 5.5{times}10{sup {minus}28} J/Hz. The SQUID voltage noise was found to be almost identical to the voltage noise from one of its junctions. The flux focusing effect of the washer geometry was also measured.We have fabricated YBa2Cu3O7−δ grain boundary junction dc superconducting quantum interference devices (SQUIDs) with a square washer geometry design. The SQUIDs were formed in c‐axis oriented epitaxial films with a single grain boundary of predetermined nature. These SQUIDs show perfectly periodic voltage‐flux characteristics without hysteresis from 4.2 to 87 K. At 77 K intrinsic energy sensitivities of 1.5×10−30 and 3.0×10−30 J/Hz at 10 kHz were obtained for 60 and 110 pH SQUIDs, respectively. The intrinsic energy sensitivity limited by 1/f noise at 10 Hz was 1.2×10−28 and 5.5×10−28 J/Hz. The SQUID voltage noise was found to be almost identical to the voltage noise from one of its junctions. The flux focusing effect of the washer geometry was also measured.

Low noise YBa sub 2 Cu sub 3 O sub 7 minus. delta. grain boundary junction dc SQUIDs

We have fabricated YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} grain boundary junction dc superconducting quantum interference devices (SQUIDs) with a square washer geometry design. The SQUIDs were formed in {ital c}-axis oriented epitaxial films with a single grain boundary of predetermined nature. These SQUIDs show perfectly periodic voltage-flux characteristics without hysteresis from 4.2 to 87 K. At 77 K intrinsic energy sensitivities of 1.5{times}10{sup {minus}30} and 3.0{times}10{sup {minus}30} J/Hz at 10 kHz were obtained for 60 and 110 pH SQUIDs, respectively. The intrinsic energy sensitivity limited by 1/{ital f} noise at 10 Hz was 1.2{times}10{sup {minus}28} and 5.5{times}10{sup {minus}28} J/Hz. The SQUID voltage noise was found to be almost identical to the voltage noise from one of its junctions. The flux focusing effect of the washer geometry was also measured.We have fabricated YBa2Cu3O7−δ grain boundary junction dc superconducting quantum interference devices (SQUIDs) with a square washer geometry design. The SQUIDs were formed in c‐axis oriented epitaxial films with a single grain boundary of predetermined nature. These SQUIDs show perfectly periodic voltage‐flux characteristics without hysteresis from 4.2 to 87 K. At 77 K intrinsic energy sensitivities of 1.5×10−30 and 3.0×10−30 J/Hz at 10 kHz were obtained for 60 and 110 pH SQUIDs, respectively. The intrinsic energy sensitivity limited by 1/f noise at 10 Hz was 1.2×10−28 and 5.5×10−28 J/Hz. The SQUID voltage noise was found to be almost identical to the voltage noise from one of its junctions. The flux focusing effect of the washer geometry was also measured.

Simulation of structural anisotropy and void formation in amorphous thin films

We have computer simulated the structure of thin amorphous films grown from a vapor. Our hard‐sphere model shows that structural anisotropy and voids are a natural occurrence of the deposition process. The amount of unfilled space (voids) and the anisotropy have been studied as a function of the angle of incidence of the vapor stream upon the substrate.

Structure and Properties of Metallic Glasses

Recent experience has shown that certain metal alloys can be put into glass form by rapid melt-quenching or by various condensation processes. Models for the nature and structure of these glasses are surveyed and shown to be quite parallel to those already developed for the more common nonmetallic glasses. The rather unique magnetic, superconducting, and mechanical properties and the technical potential of metallic glasses are also discussed.

On the stability of vacancy and vacancy clusters in amorphous solids

Abstract We find vacancies and vacancy clusters in an amorphous solid, comprised of 500 atoms and characterized by a Lennard–Jones potential, to be unstable both under static and dynamic (molecular dynamics) relaxation. The annealing-out of the vacant volume and the associated translational rearrangement of atoms occur at temperatures and times much smaller than for diffusion in a corresponding face–centred–cubic crystal. Vacancy clusters in a random–network mode! characterized by a Keating potential are observed to persist after static relaxation. These clusters and the network collapse to a more dense–packed structure when the bond–bending forces are set equal to zero and more than nearest–neighbour central–force interactions are included.

Weak link behavior of grain boundaries in Nd‐, Bi‐, and Tl‐based cuprate superconductors

We have studied single‐grain‐boundary junctions in the neodymium‐, bismuth‐, and thallium‐based cuprate superconductors and find that they behave as weak links, qualitatively similar to the YBa2Cu3O7−δ superconductor. In general, the grain boundary critical current is determined by flux flow for small misorientation angles and by Josephson junctionlike coupling for large angles. The latter is verified by the observation of voltage oscillations with an external magnetic field in superconducting quantum interference devices built using single‐grain‐boundary junctions of these materials. The commonality of behavior of grain boundaries in all of the high temperature cuprate superconductors suggests that the weak link is most likely associated with the structure of the grain boundary and the evidence points increasingly to dislocations, which describe the topology of the boundary.

The microstructure of high--critical current superconducting films.

The microstructure of superconducting films that have shown high-critical current densities has been studied. The films are shown to be epitaxial and contain twins and precipitates. The main difference between these films and low current carrying samples is the absence of grain boundaries. These boundaries are therefore identified as the cause of the lower critical current in ceramic samples.