Daniel Cunnane

Penetration depth of MgB2 measured using Josephson junctions and SQUIDs

The penetration depth of MgB2 was measured using two methods of different mechanisms. The first method used MgB2 Josephson junctions and the magnetic field dependence of the junction critical current. The second method deduced the penetration depth from the inductance of a MgB2 microstrip used to modulate the voltage of a MgB2 DC SQUID. The two methods showed a consistent value of the low-temperature penetration depth for MgB2 to be about 40 nm. Both the small penetration depth value and its temperature dependence are in agreement with a microscopic theory for MgB2 in the clean limit.

Development of hot-electron THz bolometric mixers using MgB2 thin films

Terahertz high-resolution spectroscopy of interstellar molecular clouds greatly relies on hot-electron superconducting bolometric (HEB) mixers. Current state-of-the-art receivers use mixer devices made from ultrathin (~ 3-5 nm) films of NbN with critical temperature ~ 9-11 K. Such mixers have been deployed on a number of groundbased, suborbital, and orbital platforms including the HIFI instrument on the Hershel Space Observatory. Despite its good sensitivity and well-established fabrication process, the NbN HEB mixer suffers from the narrow intermediate frequency (IF) bandwidth ~ 2-3 GHz and is limited to operation at liquid Helium temperature. As the heterodyne receivers are now trending towards “high THz” frequencies, the need in a larger IF bandwidth becomes more pressing since the same velocity resolution for a Doppler shifted line at 5 THz requires a 5-times greater IF bandwidth than at 1 THz. Our work is focusing on the realization of practical HEB mixers using ultrathin (10-20 nm) MgB2 films. They are prepared using a Hybrid Physical-Chemical Vapor Deposition (HPCVD) process yielding ultrathin films with critical temperature ~ 37-39 K. The expectation is that the combination of small thickness, high acoustic phonon transparency at the interface with the substrate, and very short electron-phonon relaxation time may lead to IF bandwidth ~ 10 GHz or even higher. SiC continues to be the most favorable substrate for MgB2 growth and as a result, a study has been conducted on the transparency of SiC at THz frequencies. FTIR measurements show that semi-insulating SiC substrates are at least as transparent as Si up to 2.5 THz. Currently films are passivated using a thin (10 nm) SiO2 layer which is deposited ex-situ via RF magnetron sputtering. Micron-sized spiral antenna-coupled HEB mixers have been fabricated using MgB2 films as thin as 10 nm. Fabrication was done using contact UV lithography and Ar Ion milling, with E-beam evaporated Au films deposited for the antenna. Measurements have been carried out on these devices in the DC, Microwave, and THz regimes. The devices are capable of mixing signals above 20 K indicating that operation may be possible using a cryogen-free cooling system. We will report the results of all measurements taken to indicate the local oscillator power requirements and the IF bandwidth of MgB2 HEB mixers.

Planar-type MgB2 SQUIDs utilizing a multilayer process

We report planar-type MgB2 DC SQUIDs fabricated using a multilayer process. The SQUIDs employed sandwich-type MgB2/MgO/MgB2 Josephson junctions and a MgB2 wiring layer. The SQUIDs showed large voltage modulation above 500 μV at 15 K and operated in a broad temperature range from 15 to 37 K with a transfer function as high as 2.3 mV/Φ0. The minimum SQUID noise was measured at 26 K in a cryocooler, yielding a white noise around 10 μΦ0/Hz1/2 and 1/f corner frequency near 10 Hz. The result demonstrates great promise of MgB2 SQUIDs for practical applications.

Superconducting MgB2 rapid single flux quantum toggle flip flop circuit

Using self-shunted MgB2/MgO/MgB2 Josephson junctions, we have designed, fabricated, and tested a rapid single flux quantum toggle flip flop (TFF) circuit. The junctions used MgB2 films grown by hybrid physical-chemical vapor deposition with MgO barrier layer and insulating layer deposited by RF magnetron sputtering. The result showed the frequency-division function of the TFF circuit, evidenced by the output voltage being half the input voltage, up to 63 GHz at 20 K and 180 GHz at 3.2 K, which demonstrates the potential of high operating speeds and high working temperatures in MgB2 superconducting integrated circuits.

Sandwich-type MgB2/TiB2/MgB2 Josephson junctions

We present results of sandwich-type MgB2/TiB2/MgB2 superconductor–normal metal–superconductor Josephson junctions with MgB2 films grown by hybrid physical–chemical vapor deposition and TiB2 barriers, between 5 and 10 nm in thickness, grown by DC magnetron sputtering. Most junctions with an 8 nm or thicker barrier of TiB2 showed little excess current and no subgap features and can be described by the resistively-shunted-junction model. Meanwhile prominent multiple Andreev reflection characteristics such as excess currents and subgap peaks were observed in some junctions with thinner barriers. DC and AC Josephson effects were observed in agreement with the theory. The results demonstrate the possibility of leakage-free sandwich-type all-MgB2 junctions using conducting barrier materials

Energy gap substructures in conductance measurements of MgB2-based Josephson junctions: beyond the two-gap model

Several theoretical analyses of the two superconducting energy gaps of magnesium diboride, and , predict substructures within each energy gap, rather than two pure numbers. Recent experiments have revealed similar structures. We report tunneling conductance data providing additional experimental evidence for these features. The absence of these features in c-axis tunneling, and a sharp peak in the subgap (associated with the counterelectrode material), support the conclusion that these features are intrinsic to MgB2. By demonstrating the inadequacy of a simple two-gap model in tting the data, we illustrate that some distinctions between theoretical models of energy gap substructures are experimentally accessible.

Study of Josephson Junction Arrays and Sub- Junctions

Sandwich-type MgB 2 /MgO/MgB 2 Josephson junctions with sizes ranging from 0.5 μm × 0.5 μm to 10 μm × 10 μm were fabricated using electron beam lithography. The junctions show critical current densities as high as 130 kA/cm 2 . The smallest of the junctions operated free of hysteresis below 4.2 K. They may be ideal for use in rapid single flux quantum circuits with no requirement for external shunt resistors. The 10-μm junctions show good agreement with the expected Fraunhofer-like modulation of critical current with magnetic field; however, the smaller junctions show a sinusoidal modulation, indicating an anomalous current distribution that needs to be further investigated. Series arrays of Josephson junctions were made to quantitatively study the uniformity of junction parameters, which shows a 54% spread in critical current of a 100 junction array of 4-μm junctions.

Multiple Andreev reflection in MgB2/MgO/MgB2 Josephson junctions

The current-voltage and conductance-voltage characteristics of MgB2/MgO/MgB2 junctions made with MgB2 electrodes grown by hybrid physical-chemical vapor deposition were systematically analyzed. In the junctions with different sizes and critical current densities, we found excess current and subharmonic gap structure indicative of multiple Andreev reflection. An apparent link between multiple Andreev reflection and substantial Josephson current at high temperatures suggests that the barrier is dominated by high-transparency channels.

Study of Components for

In order to realize superconducting circuits using MgB2, an in-depth study was conducted to generate a baseline of parameters necessary for circuit design. With a transition temperature (Tc) of 39 K, devices and circuits can be made to operate at low temperatures to achieve higher operating frequencies, and at high temperatures to lower the cooling requirements. The Josephson junction critical current density (Jc) and the product of critical current (Ic) and normal resistance (Rn) are necessary design parameters along with the sheet inductance (Lsq) of a superconducting microstrip. The need for a shunt resistor is also required for low-temperature operation. This paper describes the current status of these parameters for the multilayer process of all-MgB2 Josephson junctions. The study includes the fabrication of Josephson junctions and DC SQUIDs to perform microstrip inductance measurements as well as research on materials suitable for an on chip resistor. The results have been used to design a Rapid Single Flux Quantum Toggle-Flip-Flop circuit operating at around 20 K.

\hbox{MgB}_{2}

Microwave resonant activation is a powerful, straightforward technique to study classical and quantum systems, experimentally realized in Josephson junction devices cooled to very low temperatures. These devices typically consist of two single-gap superconductors separated by a weak link. We report the results of the first resonant activation experiments on hybrid thin film Josephson junctions consisting of a multi-gap superconductor (MgB2) and a single-gap superconductor (Pb or Sn). We can interpret the plasma frequency in terms of theories both for conventional and hybrid junctions. Using these models, we determine the junction parameters including critical current, resistance, and capacitance and find moderately high quality factors of Q0∼ 100 for these junctions.