Tse-Jun Chen

Heterojunction of Fe(Se1−xTex) superconductor on Nb-doped SrTiO3

We report the fabrication of heterojunctions formed by the FeSe0.5Te0.5 (FeSeTe) superconductor and Nb-doped SrTiO3 semiconducting substrate and their properties. At high temperature when FeSeTe is in its normal state, the forward bias I−V curves behave like a metal-semiconductor junction with a low Schottky barrier. Direct tunneling through the thin depletion layer of the junction dominates the reverse bias I−V curves. When FeSeTe film becomes superconducting at low temperature, we observed that the Schottky barrier height of the junction increased but was suppressed by an external magnetic field. This deviation provides an estimate of the superconducting energy gap of the FeSeTe film.

An interchangeable scanning Hall probe/scanning SQUID microscope.

We have constructed a scanning probe microscope for magnetic imaging, which can function as a scanning Hall probe microscope (SHPM) and as a scanning SQUID microscope (SSM). The scanning scheme, applicable to SHPM and SSM, consists of a mechanical positioning (sub) micron-XY stage and a flexible direct contact to the sample without a feedback control system for the Z-axis. With the interchangeable capability of operating two distinct scanning modes, our microscope can incorporate the advantageous functionalities of the SHPM and SSM with large scan range up to millimeter, high spatial resolution (⩽4 μm), and high field sensitivity in a wide range of temperature (4.2 K-300 K) and magnetic field (10(-7) T-1 T). To demonstrate the capabilities of the system, we present magnetic images scanned with SHPM and SSM, including a RbFeB magnet and a nickel grid pattern at room temperature, surface magnetic domain structures of a La(2/3)Ca(1/3)MnO3 thin film at 77 K, and superconducting vortices in a striped niobium film at 4.2 K.

A 220 GHz low noise superconducting receiver with wide instantaneous operating bandwidth

A 220 GHz mixer chip based on a 3-junction series distributed superconducting mixer has been developed. This mixer employs a design, which minimizes the output capacitances, such that it can offer a wide instantaneous bandwidth of up to 12 GHz. We have incorporated this mixer into a receiver package and a dedicated automatic test bench has been developed to characterize its performance. The receiver demonstrates ultra low noise temperature, as low as 32 K, or 3 hν/k. The measured noise temperature remains below 50 K beyond 10 GHz IF.

Noise and Bandwidth of 0.5-THz Twin Vertically Stacked SIS Junctions

Twin SIS (superconductor-insulator-superconductor) junctions, with a simple structure, yet large bandwidth and low noise temperature, have been widely used in millimeter- and submillimeter-wave heterodyne mixers. With increasing frequency, however, the length of the tuning inductance connecting the two individual SIS junctions becomes short while the junction area remains fixed. With a relatively short tuning inductance, the effect of the junctions spreading inductance becomes non-negligible, and device fabrication becomes more difficult. By adopting vertically stacked SIS junctions (VSJs), which have an equivalent geometric capacitance inversely proportional to the junction number, it becomes feasible to increase the junction tuning inductance. In this paper, we report on the design, fabrication, and characterization of twin Nb/Al-AlOx/Nb/Al-AlOx/Nb VSJs for the 500-GHz frequency band.

Development of 460 GHz and Dual Polarization SIS Mixers for the Submillimeter Array

A heterodyne receiver incorporating superconductor-insulator-superconductor (SIS) mixers has been designed to cover the frequency range from 400 to 520 GHz of the Submillimeter Array (SMA). Various tuning circuits have been employed to resonate out the geometric capacitance of the junction and provide impedance matching to the waveguide probe. Our measurements indicated that a receiver noise temperature of around 90 K, with the contributions from the input noise and intermediate frequency (IF) noise estimated to be around 60 K and 10 K, respectively. SIS mixers integrated with planar orthomode transducers (OMT) have also been designed at 345 GHz for dual polarization detections, and results from the 3-dimensional (3-D) electromagnetic (EM) simulations are presented.

A wideband 240 GHz receiver for the submillimeter array

We report on the design of a 240 GHz double-side-band receiver for the Submillimeter Array (SMA). The heart of this receiver is a 3-junction series connected SIS mixer, which allows it to provide intermediate frequency (IF) output up to more than 12 GHz. We have custom built a low noise Amplifier-Multiplier Chain for use as the receiver’s Local Oscillator module, which is tunable from 210 to 270 GHz. The receiver has demonstrated low noise performance in laboratory. 7 out of the 8 SMA antennas are now equipped with this receiver. The receiver has already participated in Event Horizon Telescope observations in April 2016, working with the SMA-200 receiver to provide dual polarization coverage for the EHT Hawaii Station. This receiver has enabled the SMA to provide 32 Gbit per second data stream to the EHT observations. We are currently trying to improve the on-sky beam co-alignment of this receiver with respect to other SMA receivers.

Determination of the London penetration depth of FeSe0.3Te0.7 thin films by scanning SQUID microscope

We use the scanning SQUID microscope (SSM) to image the vortices in superconducting FeSe0.3Te0.7 (FST) thin films. The observed peak flux value of FST is nearly a quarter of that of an accompanying Nb film. We developed a method for quantitatively determining the London penetration depth of the FST film from the known value of Nb. The obtained value, 0.88 μm, is significantly larger than those obtained from single crystals of similar compositions by using other methods. The methodology developed for this study is useful for measuring London penetration depths of thin-film superconductors in general.

Development of a Dual Polarization SIS Mixer With a Planar Orthomode Transducer at 350 GHz

A multipixel array equipped with dual-polarization superconductor-insulator-superconductor junction mixers at approximately 350 GHz is considered to improve the efficiency and survey speed of a telescope. This study develops a compact mixer with a planar orthomode transducer as a key component. Signals coming from a circular waveguide are separated into two polarizations via orthogonal waveguide probes, and then transmitted to the junctions through the tuning circuit. To suppress resonance, metallic blocks confine the RF signals within the circular waveguide, and a 2-μm-thick silicon nitride membrane, on which all devices were implemented, was suspended across the circular waveguide. The orthomode transducer, simulated with a three-dimensional electromagnetic simulator, has a cross-polarization of less than -20 dB between 306 and 367 GHz. The IF circuit consists of a low pass filter, coplanar waveguides, and a transition to coaxial. The return loss of the IF circuit is less than -20 dB up to 16 GHz from both simulations and measurements. The superconductor-insulator-superconductor junction mixers have been fabricated with quality factor (Rsg/Rn) of 8 and the double-side-band receiver noise temperature Trx is approximately 120 K between 320 and 360 GHz.

129 GHz SIS MIXER RECEIVER FOR KOREAN VLBI NETWORK

We have developed superconducting mixer receivers for 129 GHz VLBI observation in Korean VLBI Network (KVN). The developed mixer has a radial waveguide probe with simple transmission line L-C transformer as a tuning circuit to its 5 series-connected junctions, which can have 125 - 165 GHz as the operation radio frequency (RF). For intermediate frequency (IF) signal path a high impedance quarter-wavelength line connects the probe to one end of symmetric RF chokes. The double side band (DSB) receiver noise of the mixer was about 40 K over 4 - 6 GHz IF band, whereas we achieved the uncorrected single side band (SSB) noise temperature of about 70 K and better than 10 dB image rejection ratio in 2SB configuration with 8 - 10 GHz IF band. Insert-type receiver cartridges employing the mixers have been under commission for KVN stations.