High-quality Josephson Junctions Research Articles

Anodization-free fabrication process for high-quality cross-type Josephson tunnel junctions based on a Nb/Al-AlO x /Nb trilayer

Josephson tunnel junctions form the basis for various superconductor electronic devices. For this reason, enormous efforts are routinely taken to establish and later on maintain a scalable and reproducible wafer-scale manufacturing process for high-quality Josephson junctions. Here, we present an anodization-free fabrication process for Nb/Al-AlO x /Nb cross-type Josephson junctions that requires only a small number of process steps and that is in general intrinsically compatible with wafer-scale fabrication. We show that the fabricated junctions are of very high quality and, compared to other junction types, exhibit not only a significantly reduced capacitance but also an almost rectangular critical current density profile. Our process hence enables the usage of low capacitance Josephson junctions for superconductor electronic devices such as ultra-low noise dc-superconducting quantum interference devices (SQUIDs), microwave SQUID multiplexers based on non-hysteretic rf-SQUIDs and RFSQ circuits.

Coherent Control of a Few-Channel Hole Type Gatemon Qubit.

Gatemon qubits are the electrically tunable cousins of superconducting transmon qubits. In this work, we demonstrate the full coherent control of a gatemon qubit based on hole carriers in a Ge/Si core/shell nanowire, with the longest coherence times in group IV material gatemons to date. The key to these results is a high-quality Josephson junction obtained using a straightforward and reproducible annealing technique. We demonstrate that the transport through the narrow junction is dominated by only two quantum channels, with transparencies up to unity. This novel qubit platform holds great promise for quantum information applications, not only because it incorporates technologically relevant materials, but also because it provides new opportunities, like an ultrastrong spin-orbit coupling in the few-channel regime of Josephson junctions.

Fabrication of High-Quality Josephson Junctions Based on Nb|Al-AlN|NbN

A description of the fabrication technology of high-quality tunnel SIS junctions is presented, with following characteristics: energy gap in superconductors Vg=3.2-3.4 mV, tunnel current density up to J 35 kA/cm2, quality factor Rj/Rn (ratio of subgap resistance to normal-state resistance) up to 30, junction area up to 1 μm2. The SIS junctions are integrated into the NbTiN|SiO2|Al microstrip line. Keywords: superconducting devices, superconductor--insulator--superconductor tunnel junction, plasma etching.

Study of DC Magnetron Sputtered Nb Films

As Nb films are widely used as superconducting electrodes of Josephson junctions, it is important to investigate the properties of Nb films in order to fabricate high-quality Josephson junctions. In this work, we conducted a comprehensive analysis of the relationships among the properties of DC magnetron sputtered Nb films with a constant power fabricated at the National Institute of Metrology (China). The film properties, including superconductivity, stress, lattice constant, and surface roughness, were investigated. It was found that in the case of constant power and Ar pressure, the stress and other parameters of the Nb films can maintain a relatively stable state during the continuous consumption of the target material.

A Josephson junction with h-BN tunnel barrier: observation of low critical current noise

Decoherence in quantum bits (qubits) is a major challenge for realizing scalable quantum computing. One of the primary causes of decoherence in qubits and quantum circuits based on superconducting Josephson junctions is the critical current fluctuation. Many efforts have been devoted to suppressing the critical current fluctuation in Josephson junctions. Nonetheless, the efforts have been hindered by the defect-induced trapping states in oxide-based tunnel barriers and the interfaces with superconductors in the traditional Josephson junctions. Motivated by this, along with the recent demonstration of 2D insulator h-BN with exceptional crystallinity and low defect density, we fabricated a vertical NbSe2/h-BN/Nb Josephson junction consisting of a bottom NbSe2 superconductor thin layer and a top Nb superconductor spaced by an atomically thin h-BN layer. We further characterized the superconducting current and voltage (I–V) relationships and Fraunhofer pattern of the NbSe2/h-BN/Nb junction. Notably, we demonstrated the critical current noise (1/f noise power) in the h-BN-based Josephson device is at least a factor of four lower than that of the previously studied aluminum oxide-based Josephson junctions. Our work offers a strong promise of h-BN as a novel tunnel barrier for high-quality Josephson junctions and qubit applications.

Topological insulator nanoribbon Josephson junctions: Evidence for size effects in transport properties

We have used Bi2Se3 nanoribbons, grown by catalyst-free physical vapor deposition to fabricate high quality Josephson junctions with Al superconducting electrodes. In our devices, we observe a pronounced reduction of the Josephson critical current density Jc by reducing the width of the junction, which in our case corresponds to the width of the nanoribbon. Because the topological surface states extend over the entire circumference of the nanoribbon, the superconducting transport associated with them is carried by modes on both the top and bottom surfaces of the nanoribbon. We show that the Jc reduction as a function of the nanoribbon width can be accounted for by assuming that only the modes traveling on the top surface contribute to the Josephson transport as we derive by geometrical consideration. This finding is of great relevance for topological quantum circuitry schemes since it indicates that the Josephson current is mainly carried by the topological surface states.

High transparency Bi2Se3 topological insulator nanoribbon Josephson junctions with low resistive noise properties

Bi2Se3 nanoribbons, grown by catalyst-free Physical Vapor Deposition, have been used to fabricate high quality Josephson junctions with Al superconducting electrodes. The conductance spectra (dI/dV) of the junctions show clear dip-peak structures characteristic of multiple Andreev reflections. The temperature dependence of the dip-peak features reveals a highly transparent Al/Bi2Se3 topological insulator nanoribbon interface and Josephson junction barrier. This is supported by the high values of the Bi2Se3 induced gap and of IcRn (where Ic is the critical current and Rn is the normal resistance of the junction) product both of the order of 160 μeV, a value close to the Al gap. The devices present an extremely low relative resistance noise below 1 × 10−12 μm2/Hz comparable to the best Al tunnel junctions, which indicates a high stability in the transmission coefficients of transport channels. The ideal Al/Bi2Se3 interface properties, perfect transparency for Cooper pair transport in conjunction with low resistive noise, make these junctions a suitable platform for further studies of the induced topological superconductivity and Majorana bound states physics.

Phase-Sensitive Evidence for the Sign-Reversal s_{±} Symmetry of the Order Parameter in an Iron-Pnictide Superconductor Using Nb/Ba_{1-x}Na_{x}Fe_{2}As_{2} Josephson Junctions.

Josephson current provides a phase-sensitive tool for probing the pairing symmetry. Here we present an experimental study of high-quality Josephson junctions between a conventional s-wave superconductor Nb and a multiband iron-pnictide Ba_{1-x}Na_{x}Fe_{2}As_{2}. Junctions exhibit a large enough critical current density to preclude the d-wave symmetry of the order parameter in the pnictide. However, the I_{c}R_{n} product is very small ≃3μV, which is not consistent with the sign-preserving s_{++} symmetry either. We argue that the small I_{c}R_{n} value, along with its unusual temperature dependence, provides evidence for the sign-reversal s_{±} symmetry of the order parameter in Ba_{1-x}Na_{x}Fe_{2}As_{2}. We conclude that it is the phase sensitivity of our junctions that leads to an almost complete (below a subpercent) cancellation of supercurrents from sign-reversal bands in the pnictide.

Spatial modulation of critical current density in niobium based Josephson junctions induced by selective heating

Results about induced spatial modulation of critical current density in niobium based Josephson devices by a selective radiation heating are presented. The localized heating inside a single circuit element with a good spatial resolution (less than 1 μm) is made possible by depositing a carbon film on the desired region exploiting the larger absorbance coefficient of carbon (close to 1) with respect to niobium (about 0.2). Measurements of critical current as a function of the external magnetic field on high quality Josephson junctions at T = 4.2 K evidenced that the critical current density was changed only in the absorber covered region, leaving unchanged the critical current density outside that region and ensuring the capability to locally modify the Josephson critical current density. The experimental data have been compared with theoretical predictions obtained by using a suitable junction barrier model based on step-like barrier shape. Such results are very interesting in view of applications in quantum computing, Majorana fermions detection, and superconducting magnetic sensors.

Structural and electrical characterization of ultra-thin SrTiO3 tunnel barriers grown over YBa2Cu3O7 electrodes for the development of high Tc Josephson junctions

The transport properties of ultra-thin SrTiO3 (STO) layers grown over YBa2Cu3O7 electrodes were studied by conductive atomic force microscopy at the nano-scale. A very good control of the barrier thickness was achieved during the deposition process. A phenomenological approach was used to obtain critical parameters regarding the structural and electrical properties of the system. The STO layers present an energy barrier of 0.9 eV and an attenuation length of 0.23 nm, indicating very good insulating properties for the development of high-quality Josephson junctions.

1/f noise in NbN/AlN/NbN Josephson junction with a low critical current density

Fabricating high quality Josephson junction is essential in superconducting qubit circuits. In this letter, high quality tunnel junctions with a low critical current density of ∼32A/cm2 were fabricated. The 1/f noise parameter η was obtained from junctions’ low frequency voltage noise power spectral density Sv(f)=1fηA(IbRD)2. Our findings showed that the noise spectrum is junction linear dimension dependent, indicating the junction’s side leakage current induced by defects in the amorphous oxide insulator dominates the origin of the 1/f noise in our epitaxial NbN/AlN/NbN tunnel junctions.

Trimming of critical current in niobium Josephson devices by laser annealing

The authors report a new thermal annealing technique for Nb/Al-AlOx/Nb Josephson devices based on laser heating. This technique allows to "locally" modify the Josephson critical current density. In fact, it is possible the heating of a single circuit element with a good spatial resolution using a focused Ar+ laser beam which is aligned by an optical system on a selected junction of the sample. During the procedure, performed at room temperature, a thermal camera provides a monitoring of the temperature distribution on the whole chip. A continuous reduction of the critical current density up to about 40% has been observed on high quality Josephson junction measured in liquid helium. Neighbouring junctions have not exhibited any measurable change ensuring the effective capability to locally modify the Josephson critical current density. The new technique has been employed to recover noisy dc SQUID magnetometers with non optimal critical current values obtaining a reduction of the spectrum density of magnetic field noise from about 30 to 2.5 fT/Hz1/2.

Localized laser trimming of critical current in niobium based Josephson devices

A thermal annealing technique for Nb∕Al–AlOx∕Nb Josephson devices based on laser heating is presented. This technique allows “locally” modifying the Josephson critical current density, which is not possible using standard procedures based on oven or hot plate heating. In fact, the heating of a single circuit element with a good spatial resolution is possible. At room temperature, the selected junction is exposed to a focused Ar+ laser beam aligned by an optical system. A thermographic imaging allows controlling the temperature distribution on the whole chip. Experimental results on high quality Josephson junction measured in liquid helium have shown a continuous reduction of the critical current density up to about 40%. Neighboring junctions have not exhibited any measurable change ensuring the capability to locally modify the Josephson critical current density. As first application, the present technique has been employed to recover noisy dc superconducting quantum interference device magnetometers with nonoptimal critical current values obtaining a reduction of the spectrum density of magnetic field noise from about 30to2.5fT∕Hz1∕2.

High-quality superconducting Josephson junctions on LiNbO3 electro-optical crystals

The attainment of high-quality Josephson junctions on electro-optical substrates constitutes a key step toward the fabrication of functional superconducting opto-electronics. We report on the fabrication of high-quality all-refractory niobium superconducting tunnel junctions deposited on X-cut LiNbO3 single crystals. A high quality factor Vm=0.7ΔIgR2mV (where ΔIg is the current jump at the sum gap voltage and R2mV the static resistance at V=2mV) up to 61 mV has been measured at T=4.2K. The very low contribution of subgap leakage currents is demonstrated by the BCS-like quasiparticle current measured at V=0.1mV down to T=1.9K.

Fabrication of high-quality Josephson junctions for quantum computation using a self-aligned process

Quantum computers are believed to have the potential to perform certain tasks much better than conventional computers. Recently, very promising results have been demonstrated with superconducting devices based on Josephson junctions. The quality of the Josephson junctions plays an important role in determining the coherence time of superconducting quantum bits, and hence the viability of superconducting quantum computers. We have developed a self-aligned process for the fabrication of Nb-based Josephson devices. A UVN-30 negative-tone resist is used for both electron beam and photo-lithography. Very high quality Josephson junctions with sizes as small as 0.15×0.15 μm2 have been made. This process has a turn around time of 4–5 days which makes it much easier to study different factors affecting the quality of Josephson junctions.

Fabrication of high-quality Josephson junctions for quantum computation using a self-aligned process

Quantum computers are believed to have the potential to perform certain tasks much better than conventional computers. Recently, very promising results have been demonstrated with superconducting devices based on Josephson junctions. The quality of the Josephson junctions plays an important role in determining the coherence time of superconducting quantum bits, and hence the viability of superconducting quantum computers. We have developed a self-aligned process for the fabrication of Nb-based Josephson devices. A UVN-30 negative-tone resist is used for both electron beam and photo-lithography. Very high quality Josephson junctions with sizes as small as 0.15×0.15 μm 2 have been made. This process has a turn around time of 4–5 days which makes it much easier to study different factors affecting the quality of Josephson junctions.

Experimental studies of the pairing symmetry in Sr2RuO4: single-particle tunneling, Josephson effects, and phase-sensitive measurements

We have carried out several experiments aimed at determining the symmetry of the superconducting order parameter of Sr2RuO4. These experiments include measurements of single-particle tunneling spectra, the Josephson coupling between In or Pb and Sr2RuO4, and the critical current of superconducting quantum interference device (SQUID) samples involving Au0.5In0.5 and Sr2RuO4. Our junction resistance and critical current measurements on the SQUID samples show that high-quality Josephson junctions have been achieved.

Masked ion damage and implantation for device fabrication

Several groups have recently demonstrated junction formation by masked ion damage in which ions from a broad beam ion source locally damage regions of superconductor to form the junction barriers. However, the difficulties of preparing successful mask structures have meant the progress to date has been relatively slow. Using masks of high electron density metals such as Au and protons as the irradiating species, we have shown in a series of studies that irradiation of YBa 2Cu 3O 7− δ (YBCO) with ions of energy in the range of 30–350 keV can be used to create highly localised damage regions in the films. This technique has been successfully employed to create high quality Josephson junctions in YBCO. The fabricated devices show clear dc and ac Josephson effects. This technique is very promising in terms of simplicity and flexibility of fabrication and has potential for high density integration.

Irradiation damage technology for manufacturable Josephson junctions

We have shown in a series of studies that irradiation of YBa 2Cu 3O 7− δ (YBCO) with ions of energy in the range of 30–350 keV through a suitable mask can be used to create highly localized damage regions in the films. This technique has been successfully employed to create high quality Josephson junctions in YBCO with an ion beam implanter capable of in situ low temperature electrical measurement during implantation and focussed ion beam nanolithography. The fabricated devices show a clear dc and ac Josephson effects. This technique is very promising in terms of simplicity and flexibility of fabrication and has potential for high density integration.

Return current in hysteretic Josephson junctions: Experimental distribution in the thermal activation regime

We present an experimental study on the retrapping process of a hysteretic, high-quality Josephson junction; namely, we have measured the distribution of the values at which the junction switches back from the voltage state to the zero-voltage state, as a function of the applied magnetic field. While the opposite process (escape from the zero-voltage state) has been extensively studied in the past, both from the theoretical and the experimental point of view, little is found in the literature on the retrapping process. In terms of the tilted washboard potential, the process corresponds to the retrapping from the running state to a locked state in a potential well. The interest of the measurements is in the fact that the value of the return current can be directly related to the dissipation in the junction. While the deterministic behavior, experimentally measured through the I–V curve, appears to be in agreement with the theoretical predictions, even in minor details, the statistical behavior is strongly different from what is expected. The disagreement is found even in zero-applied magnetic field and it cannot be attributed to external noise in the system. From the experimental statistical properties, we find values for the effective dissipation much lower than those obtained from the deterministic curves, a result which could be of interest in experiments on the observation of macroscopic quantum phenomena.