Magnetic ac Susceptibility of Superconducting Ta Films for Quantum Computing

Recently, breakthroughs were reported in the long coherence time (&gt; 0.5 ms) transmon qubit using tantalum films (Ta). Identifying the loss mechanism in Ta films will help further improve the performance of Ta-based superconducting qubits. Here, we report a study of superconducting properties in two sets of Ta films grown on c-cut and a-cut sapphires respectively using contact transport and noncontact magnetic ac susceptibility measurements. Although the resistive transition appears to be similar in both films, we found strikingly different responses in their complex magnetic susceptibilities χ'+iχ''. χ'' in the c-cut films exhibits a sharp peak at superconducting transition <i>T</i>_c and becomes featureless below <i>T</i>_c indicating a strongly coupled superconducting state. In contrast, χ'' in the a-cut films exhibits a broad peak near <i>T</i>_c and a second peak appears below <i>T</i>_c, indicating granular superconductivity behavior. This second peak in χ'' is associated with the hysteresis loss at weak-link-type grain boundaries, which is believed to be a leading source of decoherence in the a-cut Ta films. The derived magnetic loss tangent in the a-cut films is about one order of magnitude higher than in the c-cut films. This study demonstrates that ac susceptibility is an excellent probe for characterizing bulk superconducting properties of the constituent superconducting materials used in qubits.