Superconducting Properties of the NbN and TiN alloy system.
Richardson, Christopher J.K.
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The field of quantum computation has advanced greatly in the recent years, with materials being tuned to address issues like qubit lifetime, and coherence time. Superconducting qubits based on Josephson Junctions are a promising approach for improved qubit design, yet they are made with polycrystalline and amorphous materials. Transition metal nitrides, like TiN and NbN, offer good chemical and mechanical properties as well as the possibility of a lattice matched junction, with AlN as the insulating layer. We have characterized MBE grown NbxTi1-xN films (with x = 0, 0.45, and 0.78) and a Nb2N film by XRD and AFM. This thesis will focus on the superconducting properties: critical temperature (Tc), superconducting transition width (ΔTc), and critical field (Hc). Two methods have been used: four-point probe DC measurements in an adiabatic demagnetization refrigerator (ADR) were used for Tc and ΔTc measurements, and a magnetic properties measurement system (MPMS) which employs a SQUID magnetometer was employed for Tc and Hc measurements. Values of Tc ranged from 4.2 K for TiN to 15.2 K for the x= 0.45 film, and 14.8 for x=0.78, while the Nb2N had Tc of 9.9 K, as found by DC measurements. These values agreed with those found by MPMS. The films’ critical fields were found to be ~69.6 Oe for the TiN film and ~250 Oe for x=0.78. These results start to demonstrate the trends of the superconducting properties of alloyed NbxTi1-xN superconductors.