Nirjhar Sarkar is a postdoctoral research associate in the Materials Science and Technology Division at Oak Ridge National Laboratory. His research focuses on atom-to-device studies of superconducting nanowires and their disorder engineering for superconducting nanowire single-photon detectors, kinetic inductors, and quantum sensors. In parallel, he is developing scanning Andreev spectroscopy with cryogenic scanning tunneling microscopy for spatially resolved Andreev-reflection measurements in superconductors. Before joining ORNL, he developed helium-ion-defined Josephson junctions, SQUID magnetometers, and circuit elements based on high-temperature superconductors at the University of California, Riverside. He received his Ph.D. in Mechanical and Aerospace Engineering from the University of California San Diego, under Prof. Robert Dynes, where he used scanning tunneling microscopy to study strain, defects, grain boundaries, and moiré structures in graphene and graphite.
Presentation Title:
Disorder Engineering of NbTiN SNSPDs via He Ion Implantation
Presentation Abstract:
We characterize NbTiN superconducting nanowire single-photon detectors (SNSPDs) after local helium-ion implantation using Scanning tunneling microscopy and device transport. Surface probing reveals redeposition of NbTiN grains and enables cleaner superconducting gap spectroscopy, in contrary to the conventional expectation that ion implantation degrades superconductivity. Two-terminal transport measurements reveal under local disorder reduced hysteresis behavior which indicates less latch-prone behavior and improved performance at a given temperature. Transport analysis reveals minor reduction in interfacial cooling strength of the hotspots but near ~50% increase in residual resistivity, indicating enhanced bulk disorder. This is further supported by microwave S11 response shift with disorder, consistent with higher kinetic inductance and longer reset times. Dark-count rate versus bias exhibits a reduced exponential slope, widening the practical operating window of temperature and field. Collectively, these results establish local He⁺ implantation as a reproducible, post-fabrication knob for disorder engineering and performance optimization in NbTiN SNSPDs.