Bachelor defence by Emma Fugl

Titel:  Investigating Quantum Phase Transitions in a Gate Controlled Hybrid Josephson Junction Array

Abstract: Thesis reports investigations on a type of unique Josephson Junction arrays, exhibiting a superconducting, insulating and metallic regime. The devices themselves itself have square aluminium islands arranged in a well-defined array on an InAs 2DEG material. Device A has 1 µm wide islands and device B has 0.3 µm wide islands. The array is gated by a gate that frames the islands (referred to as framegate), as well as a global topgate (referred to as topgate) which both can control the carrier density of the 2DEG underneath. This way the framegate can tune a superconductor to insulator transition (SIT) by depleting 2DEG between islands.

The topgate can tune a superconductor to metal transition (SMT) because its effect is screened by the framegate, leaving a grid of 2DEG unaffected and normally conducting. To investigate quantum phase transitions universal scaling of the quantum critical regime is used. The quantum phase transitions are measured in the range of temperature from base around 30 mK to 1.2 K where the arrays have entered the unpaired vortex phase of the BKT-transition. In this range a crossing point of isotherms is observed for both SMT and SIT and in both devices. The scaling of these crossing points are analyzed to find critical exponents and resistivity.

For SIT both devices exhibit a downward trend in their critical resistivity with more negative topgate never reaching the predicted value of RQ= h/4e2 = 6.45 kΩ. The critical exponent for the SIT in both device are suggest the system to be disordered, but their values lie scattered withing quantum and classical percolation models. The SMT is found to exhibit an upward trend in critical resistivity with more negative framegate, but as well as SIT never reaching RQ. The critical exponent for SMT is showing an upward trend as well, covering quantum percolation and closing in on classical percolation.

Here all values indicate a disordered system as well. As no critical resistivity above and below RQ has been observed it remains a question whether this type of array can be probed to show a critical resistivity of RQ for a quantum phase transition.

Supervisor: Saulius Vaitiekenas

Censor:  Jørn Bindslev Hansen