Anthony Annunziata

Tunable superconducting nanoinductors

We characterize inductors fabricated from ultra-thin, approximately 100 nm wide strips of niobium (Nb) and niobium nitride (NbN). These nanowires have a large kinetic inductance in the superconducting state. The kinetic inductance scales linearly with the nanowire length, with a typical value of 1 nH µm(-1) for NbN and 44 pH µm(-1) for Nb at a temperature of 2.5 K. We measure the temperature and current dependence of the kinetic inductance and compare our results to theoretical predictions. We also simulate the self-resonant frequencies of these nanowires in a compact meander geometry. These nanowire inductive elements have applications in a variety of microwave frequency superconducting circuits.

Reset dynamics and latching in niobium superconducting nanowire single-photon detectors

We study the reset dynamics of niobium(Nb)superconductingnanowire single-photon detectors (SNSPDs) using experimental measurements and numerical simulations. The numerical simulations of the detection dynamics agree well with experimental measurements, using independently determined parameters in the simulations. We find that if the photon-induced hotspot cools too slowly, the device will latch into a dc resistive state. To avoid latching, the time for the hotspot to cool must be short compared to the inductive time constant that governs the resetting of the current in the device after hotspot formation. From simulations of the energy relaxation process, we find that the hotspot cooling time is determined primarily by the temperature-dependent electron-phonon inelastic time. Latching prevents reset and precludes subsequent photon detection. Fast resetting to the superconducting state is, therefore, essential, and we demonstrate experimentally how this is achieved. We compare our results to studies of reset and latching in niobium nitride SNSPDs.

Niobium Superconducting Nanowire Single-Photon Detectors

We investigate the performance of superconducting nanowire photon detectors fabricated from ultra-thin Nb. A direct comparison is made between these detectors and similar nanowire detectors fabricated from NbN. We find that Nb detectors are significantly more susceptible than NbN to thermal instability (latching) at high bias. We show that the devices can be stabilized by reducing the input resistance of the readout. Nb detectors optimized in this way are shown to have approximately 2/3 the reset time of similar large-active-area NbN detectors of the same geometry, with approximately 6% detection efficiency for single photons at 470 nm.

A far-infrared Fourier transform spectrometer with an antenna-coupled niobium bolometer

We have designed and constructed a custom far-infrared Fourier transform spectrometer using an antenna-coupled bolometer as a detector. The active element of the detector is a superconducting niobium microbridge, and the far-infrared signal is coupled to the microbridge via a planar antenna mounted on a hyperhemispherical silicon lens. The spectrometer uses a broadband blackbody source with frequency-independent optical components, and thus the system bandwidth is set by the detector antenna. We have fabricated devices with two different antenna types, the double dipole and the log spiral, and have characterized the spectral response of each. This spectrometer can utilize the fast response of the niobium bolometer to perform time-resolved far-infrared spectroscopy on nanosecond to millisecond timescales. These timescales are too fast for standard commercial bolometers and too long for a typical optical delay line.

Materials investigation for thermally-assisted magnetic random access memory robust against 400 °C temperatures

Magnetic materials are investigated in order to enable a new type of Thermally Assisted Magnetic Random Access Memory (TAS-MRAM). A TAS-MRAM materials stack that is robust against the 400 °C process temperatures required for embedded integration with complementary metal oxide silicon processes is demonstrated. In unpatterned sheet film stacks, a stable resistance-area product, tunneling magnetoresistance (MR) > 100%, and temperature-dependent exchange bias of 1500 Oe after 400 °C anneal are shown for this stack. It is further shown that by doping the sense and storage layers with Ta using thin laminations of Ta/CoFeB, the moment of each layer can be reduced by more than 40% without a major reduction in MR. In patterned nanopillar devices, it is shown that by reducing the moment of the sense and storage layers with laminations of Ta, and by adding a second MgO barrier, the resistance versus applied field loop quality is maintained, while the read field is reduced by more than 40% and devices survive 108 wr...

Superconducting microbolometers for time-resolved terahertz spectroscopy

We report the characterization of superconducting niobium microbolometers designed for time-resolved terahertz spectroscopy on nanosecond to millisecond timescales. Coupling of the incident signal is achieved via a planar antenna mounted on a hyperhemispherical silicon lens. We have integrated these detectors into a custom Fourier-transform spectrometer. The spectrometer optics are frequency independent over the spectral range 0.1-3 terahertz and thus the system bandwidth is set by the detector antenna. We have fabricated devices with two different antenna geometries, the double-dipole and the log spiral, and have characterized the spectral response of each. This detector will enable a variety of novel spectroscopy applications.