Experimental Setup

A two chamber dewar was used to cool the Josephson Junction down to liquid helium temperatures. The inner chamber was filled with the liquid helium while the outer chamber held liquid nitrogen as an insulating medium between the external room environment and the inner chamber. Pressurized cylinders of liquid nitrogen and liquid helium were used to fill the outer and inner chambers respectively.

A cryostat consisting of a carbon resistor thermometer, a helium level gage and a superconducting solenoid (to provide magnetic field) was be used to safely introduce the Josephson junction to liquid helium temperatures and measure its surrounding environmental conditions. This cryostat was an approximately meter long structure consisting of three metal tubes that connected the top and bottom ends. The Joesphson junction was mounted at the bottom end of the cryostat (the end that was lowered into the dewar). Wire connections from the junction and the thermometer ran through the metal tubes along the cryostat into the top end. From there, these wires exited the cryostat and were connected to voltmeters, ammeters, ohmmeters or current sources as required. A labelled picture of the cryostat is given in the figure below:

Fig. 3:(TOP) Labelled picture of the cryostat. The connecting tubes and wire route can be seen. (Bottom) A schematic of the setup.

The Josephson junction used was a niobium – amorphous silicon– niobium junction embedded onto a silicon wafer. Two of these wafers were glued on to a copper plate using GE varnish; which is an electrically insulating and thermally conducting adhesive. The wafer that was used was oriented 45° to the axis of the solenoid. This was to ensure that the magnetic flux lines passed through the junction perpendicularly. The copper plate was screwed onto a plastic circuit board which provided ports for the electrical connections. Copper wires were indium pressed onto each of the electrode of the junction. The other end of these copper wires were soldered onto pins on the plastic circuit board. Two such connections from each electrode of the junction were made; one for supplying current and the other for measuring the voltage across the junction. To ensure that these wires that connected directly to the junctions would not be disturbed, the pins on the circuit board were soldered to adjacent pins and new wires were connected to these adjacent pins. These (new) wires, were then connected to measuring equipment. This guaranteed that any disturbance to the external (new) wires would not compromise the security of the wires connected to the junction. This assembly shown in Fig. 6 was mounted onto the lower end of the cryostat as mentioned earlier.

Fig.4: Labelled schematic of the mounting assembly for the Josephson junction. The junction itself has been simplified and magnified for the purposes of the schematic.

Procedure

Initially, the cryostat was lowered into the inner chamber of the dewar. Liquid nitrogen was then pumped into the outer chamber of the dewar vessel. Once the nitrogen has ceased boiling and the inner temperature had stabilized, the inner chamber was pumped with liquid helium. The liquid presence of the liquid helium brought down the temperatures to the required levels which were 4.22K. At these temperatures, the Niobium was in a superconducting state (critical temperature 9.3 K [11]) and data the acquisition process began. All data acquisition was delegated to a LabVIEW program that swept the current through the junction from a set negative value to a set positive value and then back to the negative value for a range of solenoid currents. At each current increment, the voltage across the junction was measured. Once a sweep of the current was completed, the solenoid current (and hence the magnetic field) would be increased and the sweep repeated.

DC Josephson effect: A single, extremely high resolution sweep (increments of 0.01mA) of the current was conducted at zero magnetic field to observe the IV characteristic in great detail.

Characterizing effect of magnetic field: The critical current of the junction under the influence of various magnetic fields at constant temperature were determined. This was achieved by inducing magnetic fields perpendicular to the junction using the solenoids and sweeping the current through the junction.