Electro-thermal modeling of excess voltage induced by surface degradation in Bi 2 Sr 2 CaCu 2 O 8+δ intrinsic josephson junction terahertz sources

Abstract Terahertz radiation sources based on the intrinsic Josephson junctions in the high-temperature superconductor Bi 2 Sr 2 CaCu 2 O 8+δ (BSCCO) hold significant promise for applications in spectroscopy, imaging, and communications. However, the performance of these devices is often influenced by non-ideal material properties, particularly surface degradation arising from oxygen loss during fabrication. In this work, we investigate the origin of an experimentally observed excess voltage, V e , defined as the deviation of the applied bias voltage from the value expected based on the ac Josephson relation and the measured emission frequency. This excess voltage indicates that a fraction of the intrinsic junctions within the stack do not actively contribute to coherent terahertz radiation. We hypothesize that this discrepancy stems from a thin, non-superconducting degradation layer on the BSCCO crystal surfaces and develop a three-dimensional coupled electro-thermal finite element model to test this hypothesis. By explicitly incorporating a resistive surface layer with modified electrical properties, our simulations reproduce the measured current-voltage characteristics across a temperature range of 30-70 K with significantly improved accuracy compared to models assuming a homogeneous stack. Both experimental data and numerical results reveal that V e exhibits a non-monotonic dependence on bias current and decreases with increasing temperature, a behavior linked to the evolving thermal profile and hot spot formation within the device. Based on the quantitative agreement between simulation and experiment, we estimate the thickness of the degradation layer to be on the order of 5-10 nm. Our findings provide a consistent explanation for the excess voltage phenomenon and highlight the critical role of surface stoichiometry in governing the electrical and radiative performance of BSCCO terahertz emitters. This work offers essential insights for the accurate modeling and future optimization of superconducting terahertz sources.