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Recent developments on the detector and the cryogenics for the terahertz photon detector system are discussed. The detector has been redesigned for improved performance, which exhibited optical efficiency of 6% at 530 GHz. Cryogenics for the detector system has been newly developed which employs a pulse-tube cooler with two originally developed compact 4He sorption fridges. Initial experiments demonstrated continuous cooling by altering operation of two sorption fridges, supported by a pulse-tube cooler with cooling power of 0.4 W. [ABSTRACT FROM AUTHOR]

Abstract The superconducting tunneling effect in heterostructures, describing the process where single electrons or Cooper pairs tunnel through the barrier, can always play a significant role in understanding the phase coherence and pairing mechanisms in superconductors. Taking advantage of the easy cleavage to atomically-thin monolayer structure of layered superconductors and resulting quantum confinement of electrons or Cooper pairs at two-dimensional limit, van der Waals superconducting materials hosting superconducting order in monolayers or heterostructures can exhibit extensive emergent phenomena associated with quantum phase transitions of vortex and anti-vortex pairs. Examples of superconducting tunnel junctions (STJs) based on layered superconductors have been demonstrated to achieve novel phenomena, including Andreev bound states, Majorana bound states and 0/π-phase junctions. Since the characteristic parameters of quasiparticle tunneling through the barrier are directly associated with the energy gap values of superconductors, such critical parameter can be obtained within the STJ device geometry, which helps us understand and control the pairing states and emerging phenomena in superconductors. In this review, from the perspective of STJs with single electron tunneling and Cooper pair tunneling, we discuss Andreev reflection, Majorana bound states, photon-induced tunneling effects, non-reciprocal transport and superconducting diode phenomena, as well as prospects for layered-superconductor-based STJs.

The superconducting tunneling effect in heterostructures, describing the process where single electrons or Cooper pairs tunnel through the barrier, can always play a significant role in understanding the phase coherence and pairing mechanisms in superconductors. Taking advantage of the easy cleavage to atomically-thin monolayer structure of layered superconductors and resulting quantum confinement of electrons or Cooper pairs at two-dimensional limit, van der Waals superconducting materials hosting superconducting order in monolayers or heterostructures can exhibit extensive emergent phenomena associated with quantum phase transitions of vortex and anti-vortex pairs. Examples of superconducting tunnel junctions (STJs) based on layered superconductors have been demonstrated to achieve novel phenomena, including Andreev bound states, Majorana bound states and 0/π-phase junctions. Since the characteristic parameters of quasiparticle tunneling through the barrier are directly associated with the energy gap values of superconductors, such critical parameter can be obtained within the STJ device geometry, which helps us understand and control the pairing states and emerging phenomena in superconductors. In this review, from the perspective of STJs with single electron tunneling and Cooper pair tunneling, we discuss Andreev reflection, Majorana bound states, photon-induced tunneling effects, non-reciprocal transport and superconducting diode phenomena, as well as prospects for layered-superconductor-based STJs. [ABSTRACT FROM AUTHOR]

Precision measurements of nuclear β decay are among the most sensitive methods to probe beyond standard model (BSM) physics in the neutrino sector. In particular, momentum conservation between the emitted decay products in the final state is sensitive to any new physics that couples to the neutrino mass. One way to observe these momentum recoil effects experimentally is through high-precision measurements of nuclear electron capture (EC) decay where the final state only contains the neutrino and a recoiling atom. The Beryllium Electron capture in Superconducting Tunnel junctions (BeEST) experiment precisely measures the eV-scale radiation that follows the radioactive decay of 7 Be implanted into sensitive superconducting tunnel junction (STJ) quantum sensors. STJs are ideally suited for measurements of this type due to their high resolution at the low recoil energies in EC decay, and their high-rate counting capabilities. We present the motivation for the BeEST experiment and describe the various phases of the project. [ABSTRACT FROM AUTHOR]

Particle induced X-ray emission with the use of focused ion beam (µ-PIXE) utilizing a superconducting tunnel junction (STJ) X-ray spectrometer has been developed to realize light element mapping in the soft X-ray region. A five hundred pixels array of STJ detector has been installed in the µ-PIXE beamline at the accelerator facility in the University of Tsukuba. Performance of a single-pixel STJ detector is tested. The K-lines of light elements C to Al have been separately detected in the pulse height spectra with a broad background, which is attributed substrate-hit events caused by scattered protons and high-energy characteristic X-ray lines. The background is rejected by using the anticoincidence technique or rise-time discrimination. [ABSTRACT FROM AUTHOR]

Fast photon detectors will be one of the key technologies to introduce new observing methods. Utilizing our recent development of low leakage SIS junctions with Nb/Al/AlOx/Al/Nb, we designed and fabricated an antenna-coupled SIS detector. The detector exhibited a leakage current of 2 pA at T ≤ 0.8 K . Fourier transform spectrometer measurements show a clear resonance peak, although the peak frequency and bandwidth were shifted from the designed values. Even though the detector efficiency was low, the noise measurement suggests that the detector would allow us to read out single terahertz photon by introducing the photon-counting scheme. [ABSTRACT FROM AUTHOR]

We plan to realize a fast terahertz photon counting detector. The detector may resolve each photon signal from the source and enable us to introduce "photon statistics," which will be a new measure for terahertz astronomy and astrophysics. An SIS junction with Nb/Al/AlOx/Al/Nb is considered for this detector, which should incorporate low leakage current of 1 pA. We have successfully developed an SIS junction to satisfy this requirement. The bandwidth is limited, while it is sufficient for laboratory experiments. We have designed an antenna-coupled SIS detector incorporating the low-leakage SIS junction for photon counting experiments. Additionally, the photo-response of the SIS junction is measured to discuss the photon counting capability in combination with the fast readout system. [ABSTRACT FROM AUTHOR]

We demonstrate an original method based on controlled oxidation for creating high-quality tunnel junctions between superconducting Al reservoirs and InAs semiconductor nanowires (NWs). We show clean tunnel characteristics with a current suppression by >4 orders of magnitude for a junction bias well below the Al gap of Δ ≈ 200 μeV. The experimental data agree well with the Bardeen-Cooper-Schrieffer theoretical expectations for a superconducting tunnel junction. The studied devices employ small-scale tunnel contacts functioning as thermometers as well as larger electrodes that provide proof-of-principle active cooling of the electron distribution in the NWs. A peak refrigeration of approximately δ T = 10 mK is achieved at a bath temperature of T ≈ 250-350 mK for our prototype devices. This method introduces important perspectives for the investigation of the thermoelectric effects in semiconductor nanostructures and for nanoscale refrigeration. [ABSTRACT FROM AUTHOR]

Zinc oxide (ZnO) nanoparticles coated with either n-octylamine (OA) or α-amino poly(styrene- co-acrylonitrile) (PSAN) ligands (L) have been analyzed using laser desorption/ionization and matrix assisted laser desorption/ionization (MALDI) time-of-flight (TOF) superconducting tunnel junction (STJ) cryodetection mass spectrometry. STJ cryodetection has the advantage of high m/ z detection and allows for the determination of average molecular weights and dispersities for 500-600 kDa ZnO-L nanoparticles. The ability to detect the relative energies deposited into the STJs has allowed for investigation of ZnO-L metastable fragmentation. ZnO-L precursor ions gain enough internal energy during the MALDI process to undergo metastable fragmentation in the flight tube. These fragments produced a lower energy peak, which was assigned as ligand-stripped ZnO cores whereas the individual ligands were at too low of an energy to be observed. From these STJ energy resolved peaks, the average weight percentage of inorganic material making up the nanoparticle was determined, where ZnO-OA and ZnO-PSAN nanoparticles are comprised of ~62% and ~68% wt ZnO, respectively. In one example, grafting densities were calculated based on the metastable fragmentation of ligands from the core to be 16 and 1.1 nm for ZnO-OA and ZnO-PSAN, respectively, and compared with values determined by thermogravimetric analysis (TGA) and transmission electron microscopy (TEM). [ABSTRACT FROM AUTHOR]