Hf-hfox-hf Josephson Junctions Demonstrate meV-Scale Detection for THz Photon and Phonon Astrophysics

The search for increasingly sensitive detectors drives innovation in Josephson junction technology, extending its applications beyond quantum computing to areas like astrophysics and the hunt for dark matter. Y. Balaji, M. Surendran, X. Li, and colleagues now demonstrate a promising new material platform for these detectors, fabricating and characterising Josephson junctions using hafnium. Their work reveals that hafnium-based junctions, with a smaller energy gap than conventional materials, exhibit clear Josephson behaviour and offer the potential to detect individual particles at extremely low energies, representing a significant step towards next-generation detectors and novel device architectures. This comprehensive study establishes hafnium as a viable low-temperature superconducting material for ultra-sensitive single-photon and single-phonon detection.

Hf, HfOx Junction Fabrication and Characterisation

This document details the fabrication and characterization of hafnium-based Josephson junctions, intended for sensitive particle detection in the meV range. The fabrication process utilizes electron-beam lithography and deposition of hafnium, followed by oxidation and a subsequent hafnium deposition using electron-beam evaporation. Liftoff is achieved using NMP, and wire bonding connects the junctions to a printed circuit board for electrical contact. Electrical measurements are performed using a probe station and a Keithley sourcemeter, and a dilution refrigerator with a base temperature of 13mK for cryogenic measurements. The cryogenic measurement system incorporates filtering and shielding to minimize noise, utilizing low-pass filters and twisted pair wiring. Temperature-dependent resistance measurements are performed using a Quantum Design PPMS DynaCool system, employing a four-point probe method to measure resistance as a function of temperature.

The critical temperature, Tc, is determined as the temperature corresponding to 50% of the normal-to-superconducting transition, yielding values of 298mK and 268mK for 30nm and 60nm thick films respectively, with corresponding energy gaps of 41μeV and resistivities of 73. 4μΩcm and 62. 1μΩcm. These results demonstrate a well-controlled fabrication process and confirm the superconducting properties of the hafnium films.

Hafnium Junction Fabrication for Terahertz Detection

Scientists engineered hafnium-based Josephson junctions to explore a promising material platform for detecting single terahertz (THz) photons and phonons. Thin films of hafnium were deposited using sputtering, carefully controlling the process to achieve crystalline structures and well-defined oxide barriers. Researchers then patterned the hafnium films with titanium and gold contacts using electron-beam evaporation and a shadow mask, precisely defining the geometry for four-wire resistance measurements. This meticulous fabrication process yielded junctions suitable for detailed electrical characterization.

The study pioneered a cryogenic measurement system to assess the electrical transport properties of the hafnium junctions at both room and extremely low temperatures. Samples underwent wire bonding and mounting within an adiabatic demagnetization refrigerator (ADR) capable of reaching 90 millikelvin. The team measured resistance as a function of temperature during a controlled warmup cycle, utilizing a standard four-point probe method with a constant current of 1 microampere. This precise temperature control and measurement technique enabled the extraction of key junction parameters, including critical current, energy gap, and normal-state resistance.

Electrical measurements revealed superconducting behavior in the hafnium junctions, with the critical temperature, Tc, determined as the point corresponding to 50% of the normal-to-superconducting transition. Analysis of 30 nanometer thick hafnium films yielded a Tc of 298 millikelvin and an energy gap of 41 microelectronvolts, while 60 nanometer thick films exhibited a Tc of 268 millikelvin and a gap of 44 microelectronvolts. The team also measured the residual resistance ratio (RRR), finding values of 1. 68 and 1. 97 for the 30nm and 60nm films respectively, demonstrating the quality of the superconducting films. These results confirm hafnium as a viable low-Tc material for next-generation detectors requiring ultra-low threshold sensitivity.

Hafnium Films Demonstrate Superconducting Junction Potential

This work demonstrates hafnium as a promising new material for ultra-sensitive superconducting detectors, specifically for detecting low-energy photons and phonons, with potential applications in astrophysics and dark matter research. Scientists fabricated and characterized Josephson junctions using hafnium, achieving clear Josephson behavior and extracting key parameters for device performance. Structural analyses, including X-ray diffraction, confirmed the crystalline quality of the hafnium films and revealed a hexagonal close-packed structure. Atomic force microscopy measurements determined the thicknesses of the deposited layers, finding bottom leads at 30nm and top leads at 60nm.

X-ray photoelectron spectroscopy confirmed the formation of a 4-5nm hafnium oxide tunnel barrier within the junctions, identifying distinct changes in oxygen and hafnium concentrations across the layer. Electrical measurements revealed superconducting transition temperatures of 298 mK for 30nm films and 268 mK for 60nm films, corresponding to superconducting energy gaps of 44 μeV and 41 μeV respectively. Room temperature resistance measurements of the hafnium junctions showed values 2-5times higher than those of comparable aluminum junctions fabricated using the same process. Crucially, the team successfully measured the full current-voltage characteristics of a hafnium Josephson junction at 13 mK, extracting a critical current of 6.

2 nA and a normal state resistance of 5. 8 kΩ. The superconducting energy gap was determined to be approximately 35 μeV, confirming the presence of in-gap states and demonstrating the potential for ultra-low threshold detection. These results establish hafnium as a viable low-Tc material platform for next-generation detectors, offering enhanced sensitivity due to its low energy gap and paving the way for advanced superconducting architectures.