andrea sosso - Academia.edu (original) (raw)
Papers by andrea sosso
New Journal of Physics, 2012
We report low-temperature measurements of current-voltage characteristics for highly conductive N... more We report low-temperature measurements of current-voltage characteristics for highly conductive Nb/Al-AlOx-Nb junctions with thicknesses of the Al interlayer ranging from 40 to 150 nm and ultra-thin barriers formed by diffusive oxidation of the Al surface. In the superconducting state these devices have revealed a strong subgap current leakage. Analyzing Cooper-pair and quasiparticle currents across the devices, we conclude that the strong suppression of the subgap resistance comparing with conventional tunnel junctions originates from a universal bimodal distribution of transparencies across the Al-oxide barrier proposed earlier by Schep and Bauer. We suggest a simple physical explanation of its source in the nanometer-thick oxide films relating it to strong local barrier-height fluctuations which are generated by oxygen vacancies in thin aluminum oxide tunnel barriers formed by thermal oxidation.
Journal of Applied Physics, 2010
ABSTRACT The measurements of dc Josephson and quasiparticle current-voltage characteristics of fo... more ABSTRACT The measurements of dc Josephson and quasiparticle current-voltage characteristics of four-layered Nb / Al – AlO x– Nb devices with a fixed Nb thickness of 270 nm and Al thicknesses ranging from 40 to 120 nm are reported and analyzed in the framework of a microscopic model developed to determine stationary properties of dirty limit double-barrier junctions. It is shown that the temperature dependence of the supercurrent as well as the values of characteristic voltages are well reproduced by the model calculations with only one fitting parameter. We have revealed a hysteretic-to-nonhysteretic transition in the current-voltage characteristics of our junctions at temperatures near 4.2 K and argue that this effect is caused by two factors: high-transparency insulating barrier with a broad distribution of the transmission coefficient and the temperature as a tuning parameter, which decreases the McCumber–Stewart parameter from values above unity at low temperatures to less than one above 4.2 K. Finally, we show how and why the temperature stability of the proposed Josephson devices can be significantly improved by choosing an appropriate Al thickness.
Journal of Physics: Conference Series, 2008
Optimization of the performance of Nb-based overdamped Josephson junctions for applications to su... more Optimization of the performance of Nb-based overdamped Josephson junctions for applications to superconducting electronics is a major issue today. The use of cryocoolers, thus avoiding liquid helium refrigeration, with junctions working at temperatures higher than 4.2 K, could greatly increase the use of these devices in applications such as fundamental metrology and measurement sensors. The question we address here is how to maximize the characteristic voltage and minimize the temperature influence on the current-voltage curves of overdamped junctions enabling their use at T > 4.2K. To optimize the properties of a single Josephson heterostructure, we propose a type of overdamped junction that consists of a relatively thin insulating layer and a thick (tens of nm) normal-metal film between two superconducting electrodes. Measurements of the dependence of the critical current and characteristic voltage as function of temperature for different electrodes configurations show how it is possible to improve the design of these junctions.
IEEE Transactions on Applied Superconductivity, 2000
The temperature stability is one of the most important factors determining the successful applica... more The temperature stability is one of the most important factors determining the successful application of the Josephson effect to the ac voltage standard or to analog and digital electronics. Whereas SIS hysteretic junctions with an Ambegaokar-Baratoff (A&B) ( ) behavior exhibit a reduced drift around the working temperature of the liquid helium, metallic-barrier SNS junctions follow a Kulik-Omelianchuck (K&O) behavior with a sharp temperature dependence of in this region. The objective of our research is to study this aspect for overdamped Nb Al-AlO Nb heterostructures with current densities of 10-75 kA cm 2 and characteristic voltages from 100 to more than 500 V, that have been fabricated at INRiM. The ( ) characteristics, measured for Josephson heterostructures with different thickness, , and exposure, , essentially deviate from A&B and K&O curves, because of proximity effect caused by the comparatively high value of (up to 100 nm). We study theoretically two extreme limits: the clean and the dirty limit for the interlayer between the superconducting electrodes and discuss the temperature stability of the junctions characterizing it with the temperature derivative . The combined experimental and theoretical analysis of the problem provides a way for understanding, controlling and improving the design of the Nb Al-AlO Nb junctions in order to enhance their reliability.
IEEE Transactions on Applied Superconductivity, 2000
ABSTRACT Josephson junctions with non-hysteretic current-voltage characteristics form the basis o... more ABSTRACT Josephson junctions with non-hysteretic current-voltage characteristics form the basis of most superconducting electronic circuits including RSFQ logic and programmable Josephson voltage standards. In contrast to conventional SIS devices, Nb/Al-AlOx-Nb (SNIS) junctions with an additional comparatively thick Al interlayer and a comparatively thin barrier AlOx are intrinsically shunted at T ≥ 4.2 K . In this contribution, we provide experimental and theoretical arguments proving that this finding is mainly explained by a broad distribution of highly-conductive barrier transparencies with a significant effect from nearly ballistic modes. An additional advantage of the proposed SNIS junctions is possibility to tune the critical voltage value by modifying Nb and/or Al film thicknesses. With observations of wide Shapiro steps up to 1.25 V at 6.3 K we show that this type of Josephson junctions can be successfully used at temperatures above 4.2 K. The presence of well-developed quantized voltage features even at 7.2 K means that Nb/Al-AlOx-Nb devices can successfully operate far above the liquid helium temperature and, in principle, are compatible with two-stage cryocoolers.
We report low-temperature measurements of current-voltage characteristics for highly conductive N... more We report low-temperature measurements of current-voltage characteristics for highly conductive Nb/Al-AlOx-Nb junctions with thicknesses of the Al interlayer ranging from 40 to 150 nm and ultrathin barriers formed by diffusive oxidation of the Al surface. In a superconducting state these devices have revealed a strong subgap current leakage. Analyzing Cooper-pair and quasiparticle currents across the devices, we conclude that the strong suppression of the subgap resistance compared with conventional tunnel junctions is not related to technologically derived pinholes in the barrier but rather has more fundamental grounds. We argue that it originates from a universal bimodal distribution of transparencies across the aluminum oxide barrier proposed earlier by Schep and Bauer (1997 Phys. Rev. Lett. 78 3015). We suggest a simple physical explanation of its source in the nanometer-thick oxide films relating it to strong local barrier-height fluctuations in the nearest to conducting electrode layers of the insulator which are generated by oxygen vacancies in thin aluminum oxide tunnel barriers formed by thermal oxidation.
Physica C: Superconductivity, 2013
An interesting feature of Superconductor-Normal metal-Superconductor Josephson junctions for digi... more An interesting feature of Superconductor-Normal metal-Superconductor Josephson junctions for digital applications is due to their non-hysteretic current-voltage characteristics in a broad temperature range below T c . This allows to design Single-Flux-Quantum (SFQ) cells without the need of external shunts. Two advantages can be drawn from this property: first the SFQ cells can be more compact which leads to a more integrated solution towards nano-devices and more complex circuits; second the absence of electrical parasitic elements associated with the wiring of resistors external to the Josephson junctions increases the performance of SFQ circuits, in particular regarding the ultimate speed of operation. For this purpose Superconductor-Normal metal-Insulator-Superconductor Nb/Al-AlO x /Nb Josephson junctions have been recently developed at INRiM with aluminum layer thicknesses between 30 and 100 nm. They exhibit non-hysteretic current-voltage characteristics with I c R n values higher than 0.5 mV in a broad temperature range and optimal Stewart McCumber parameters at 4.2 K for RSFQ applications. The main features of obtained SNIS junctions regarding digital applications are presented.
New Journal of Physics, 2012
We report low-temperature measurements of current-voltage characteristics for highly conductive N... more We report low-temperature measurements of current-voltage characteristics for highly conductive Nb/Al-AlOx-Nb junctions with thicknesses of the Al interlayer ranging from 40 to 150 nm and ultra-thin barriers formed by diffusive oxidation of the Al surface. In the superconducting state these devices have revealed a strong subgap current leakage. Analyzing Cooper-pair and quasiparticle currents across the devices, we conclude that the strong suppression of the subgap resistance comparing with conventional tunnel junctions originates from a universal bimodal distribution of transparencies across the Al-oxide barrier proposed earlier by Schep and Bauer. We suggest a simple physical explanation of its source in the nanometer-thick oxide films relating it to strong local barrier-height fluctuations which are generated by oxygen vacancies in thin aluminum oxide tunnel barriers formed by thermal oxidation.
Journal of Applied Physics, 2010
ABSTRACT The measurements of dc Josephson and quasiparticle current-voltage characteristics of fo... more ABSTRACT The measurements of dc Josephson and quasiparticle current-voltage characteristics of four-layered Nb / Al – AlO x– Nb devices with a fixed Nb thickness of 270 nm and Al thicknesses ranging from 40 to 120 nm are reported and analyzed in the framework of a microscopic model developed to determine stationary properties of dirty limit double-barrier junctions. It is shown that the temperature dependence of the supercurrent as well as the values of characteristic voltages are well reproduced by the model calculations with only one fitting parameter. We have revealed a hysteretic-to-nonhysteretic transition in the current-voltage characteristics of our junctions at temperatures near 4.2 K and argue that this effect is caused by two factors: high-transparency insulating barrier with a broad distribution of the transmission coefficient and the temperature as a tuning parameter, which decreases the McCumber–Stewart parameter from values above unity at low temperatures to less than one above 4.2 K. Finally, we show how and why the temperature stability of the proposed Josephson devices can be significantly improved by choosing an appropriate Al thickness.
Journal of Physics: Conference Series, 2008
Optimization of the performance of Nb-based overdamped Josephson junctions for applications to su... more Optimization of the performance of Nb-based overdamped Josephson junctions for applications to superconducting electronics is a major issue today. The use of cryocoolers, thus avoiding liquid helium refrigeration, with junctions working at temperatures higher than 4.2 K, could greatly increase the use of these devices in applications such as fundamental metrology and measurement sensors. The question we address here is how to maximize the characteristic voltage and minimize the temperature influence on the current-voltage curves of overdamped junctions enabling their use at T > 4.2K. To optimize the properties of a single Josephson heterostructure, we propose a type of overdamped junction that consists of a relatively thin insulating layer and a thick (tens of nm) normal-metal film between two superconducting electrodes. Measurements of the dependence of the critical current and characteristic voltage as function of temperature for different electrodes configurations show how it is possible to improve the design of these junctions.
IEEE Transactions on Applied Superconductivity, 2000
The temperature stability is one of the most important factors determining the successful applica... more The temperature stability is one of the most important factors determining the successful application of the Josephson effect to the ac voltage standard or to analog and digital electronics. Whereas SIS hysteretic junctions with an Ambegaokar-Baratoff (A&B) ( ) behavior exhibit a reduced drift around the working temperature of the liquid helium, metallic-barrier SNS junctions follow a Kulik-Omelianchuck (K&O) behavior with a sharp temperature dependence of in this region. The objective of our research is to study this aspect for overdamped Nb Al-AlO Nb heterostructures with current densities of 10-75 kA cm 2 and characteristic voltages from 100 to more than 500 V, that have been fabricated at INRiM. The ( ) characteristics, measured for Josephson heterostructures with different thickness, , and exposure, , essentially deviate from A&B and K&O curves, because of proximity effect caused by the comparatively high value of (up to 100 nm). We study theoretically two extreme limits: the clean and the dirty limit for the interlayer between the superconducting electrodes and discuss the temperature stability of the junctions characterizing it with the temperature derivative . The combined experimental and theoretical analysis of the problem provides a way for understanding, controlling and improving the design of the Nb Al-AlO Nb junctions in order to enhance their reliability.
IEEE Transactions on Applied Superconductivity, 2000
ABSTRACT Josephson junctions with non-hysteretic current-voltage characteristics form the basis o... more ABSTRACT Josephson junctions with non-hysteretic current-voltage characteristics form the basis of most superconducting electronic circuits including RSFQ logic and programmable Josephson voltage standards. In contrast to conventional SIS devices, Nb/Al-AlOx-Nb (SNIS) junctions with an additional comparatively thick Al interlayer and a comparatively thin barrier AlOx are intrinsically shunted at T ≥ 4.2 K . In this contribution, we provide experimental and theoretical arguments proving that this finding is mainly explained by a broad distribution of highly-conductive barrier transparencies with a significant effect from nearly ballistic modes. An additional advantage of the proposed SNIS junctions is possibility to tune the critical voltage value by modifying Nb and/or Al film thicknesses. With observations of wide Shapiro steps up to 1.25 V at 6.3 K we show that this type of Josephson junctions can be successfully used at temperatures above 4.2 K. The presence of well-developed quantized voltage features even at 7.2 K means that Nb/Al-AlOx-Nb devices can successfully operate far above the liquid helium temperature and, in principle, are compatible with two-stage cryocoolers.
We report low-temperature measurements of current-voltage characteristics for highly conductive N... more We report low-temperature measurements of current-voltage characteristics for highly conductive Nb/Al-AlOx-Nb junctions with thicknesses of the Al interlayer ranging from 40 to 150 nm and ultrathin barriers formed by diffusive oxidation of the Al surface. In a superconducting state these devices have revealed a strong subgap current leakage. Analyzing Cooper-pair and quasiparticle currents across the devices, we conclude that the strong suppression of the subgap resistance compared with conventional tunnel junctions is not related to technologically derived pinholes in the barrier but rather has more fundamental grounds. We argue that it originates from a universal bimodal distribution of transparencies across the aluminum oxide barrier proposed earlier by Schep and Bauer (1997 Phys. Rev. Lett. 78 3015). We suggest a simple physical explanation of its source in the nanometer-thick oxide films relating it to strong local barrier-height fluctuations in the nearest to conducting electrode layers of the insulator which are generated by oxygen vacancies in thin aluminum oxide tunnel barriers formed by thermal oxidation.
Physica C: Superconductivity, 2013
An interesting feature of Superconductor-Normal metal-Superconductor Josephson junctions for digi... more An interesting feature of Superconductor-Normal metal-Superconductor Josephson junctions for digital applications is due to their non-hysteretic current-voltage characteristics in a broad temperature range below T c . This allows to design Single-Flux-Quantum (SFQ) cells without the need of external shunts. Two advantages can be drawn from this property: first the SFQ cells can be more compact which leads to a more integrated solution towards nano-devices and more complex circuits; second the absence of electrical parasitic elements associated with the wiring of resistors external to the Josephson junctions increases the performance of SFQ circuits, in particular regarding the ultimate speed of operation. For this purpose Superconductor-Normal metal-Insulator-Superconductor Nb/Al-AlO x /Nb Josephson junctions have been recently developed at INRiM with aluminum layer thicknesses between 30 and 100 nm. They exhibit non-hysteretic current-voltage characteristics with I c R n values higher than 0.5 mV in a broad temperature range and optimal Stewart McCumber parameters at 4.2 K for RSFQ applications. The main features of obtained SNIS junctions regarding digital applications are presented.