A Project for PHY 210, Spring 2003

With Professor Romalis and Michael Leung

from http://astro.estec.esa.nl/SA-general/Research/Stj/STJ_intro.html

Junction Basics

The Josephson Junction consists of two thin layers of superconducting

material and an insulating barrier. The superconducting layers can be thin

films of Hf, Nb, or Ta. Electron Cooper Pairs in the superconducting layers

can tunnel across the barrier and create a current in the device and

non-linear responses. Such devices can be used to detect very small

magnetic fields (SQUIDS) and potentially to detect individual photons from astronomical sources. A Josephson photon counter is often called a Superconducting Tunnel Junction (STJ). A schematic of an STJ is pictured above.

STJ Basics

In a simple STJ device photons strike the superconducting film and break up electron Cooper Pairs. The energy required to break up these pairs, called the band-gap, is as small as 1milli-ev. The electrons from the broken cooper pair can tunnel back and forth across the junction’s insulating layer, freeing up other electrons, and causing a current. The amplitude of this current is directly proportional to the energy of the incident photon over a wide range of photon energies. The STJ is theoretically capable of detecting single photons with micro-second time resolutions and energy resolution of 10ev or better. The inherent energy resolution of the device is called the “fano limit”. Fano limited STJ devices have not yet been constructed, but many researchers around the world are designing and building STJ devices with better and better energy resolution. The STJ provides many advantages over traditional detectors for astronomical observations and in the near future

STJs will hopefully be used to make many new and important discoveries in astronomy.

Our Design

We attempted to make an STJ similar to the one pictured above. Our superconducting film choice was Niobium (Nb) since this material can be cooled with liquid He past its critical temperature. We were unable to evaporate quality films of Nb because this material has a high melting temperature. The available evaporative deposition machine couldn’t maintain high enough temperatures to make films of Nb thicker than ~10nm. The target thickness was 100nm, but we made two devices on glass slides that were sandwiches of Nb-Al oxide -Nb. Thin wires were attached to the junctions with conducting epoxy and the devices were submerged in a dewar of liquid He. Simple 4-wire tests of the Nb devices showed that they did not exhibit superconducting behavior. Three devices were made with Vanadium (V) as the superconducting layer. V has a much lower melting point than Nb, so we were able to make 100nm films of V in V-Al oxide-V sandwiches. Unfortunately, tests showed that these devices were also not superconducting. A search of the literature found that it has historically been quite difficult to make quality V films by evaporation.

The evaporative deposition system

The inside of the evaporator

Nb Foil Junction

Following the work of Kerr & Zych (1975. Am. J. Phys., 43, 10), we attempted to make a simple Josephson Junction using inexpensive Nb foil. Two small (2x5cm) pieces of foil were clamped together with a thin sheet of plastic between them. A set screw was used to pinch the layers together at a small point, creating a small Josephson Junction. The oxide layer on the Nb foil acted as the tunnel barrier. This device was submerged in liquid He and 4-wire tests were conducted. The Nb foil device did demonstrate the expected non-linear effects due to superconductivity. The device was driven with an alternating voltage of amplitude 0.01V and the resulting current was measured with an oscilloscope. Below is the current-voltage diagram we produced. This diagram is quite similar to those in the Kerr and Zych paper describing the Nb foil device.

The Nb foil device

This is an example of a V-I diagram from our Nb Foil Josephson Junction

Power supply, function generator, and oscilloscope used in testing

Conclusions

We concluded that making superconducting thin films is much harder than it sounds! The construction of an STJ was beyond our technical capabilities, but we were able to create a very simple Nb foil Josephson device that demonstrated superconductivity and non-linear effects.

Thanks

We are very grateful to Professor Romalis and Michael Leung for all of their help with this project. We had a great time working on this project and we couldn’t have done it without all of their help.

Our PHY 210 lab group

Professor Romalis