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arXiv:1310.8085 · Applied Physics Letters

Non-hysteretic superconducting quantum interference proximity transistor with enhanced responsivity

Robab Najafi Jabdaraghi, M. Meschke, Jukka P Pekola

Published Feb 24, 2014• 19 citations
arXivHTMLPDF

Abstract

This letter presents fabrication and characterization of an optimized SQUIPT (superconducting quantum interference proximity transistor). The present device, characterized with reduced tunnel junction area and shortened normal-metal section, demonstrates no hysteresis at low temperatures as we increased the Josephson inductance of the weak link by decreasing its cross section. It has consequently almost an order of magnitude improved magnetic field responsivity as compared to the earlier design. The modulation of both the current and the voltage across the junction have been measured as a function of magnetic flux piercing the superconducting loop.

DOI
10.1063/1.4866584

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High operating temperature in V-based superconducting quantum interference proximity transistors

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Abstract Here we report the fabrication and characterization of fully superconducting quantum interference proximity transistors (SQUIPTs) based on the implementation of vanadium (V) in the superconducting loop. At low temperature, the devices show high flux-to-voltage (up to 0.52 mV/Φ0) and flux-to-current (above 12 nA/Φ0) transfer functions, with the best estimated flux sensitivity ~ 2.6 μΦ0/(Hz)1/2 reached under fixed voltage bias, where Φ0 is the flux quantum. The interferometers operate up to Tbath $$\simeq $$ ≃ 2 K, with an improvement of 70% of the maximal operating temperature with respect to early SQUIPTs design. The main features of the V-based SQUIPT are described within a simplified theoretical model. Our results open the way to the realization of SQUIPTs that take advantage of the use of higher-gap superconductors for ultra-sensitive nanoscale applications that operate at temperatures well above 1 K.

arXiv:1704.01284•Scientific Reports•Aug 14, 2017•19 citations
This letter presents fabrication and characterization of an optimized SQUIPT (superconducting quantum interference proximity transistor). The present device, characterized with reduced tunnel junction area and shortened normal-metal section, demonstrates no hysteresis at low temperatures as we increased the Josephson inductance of the weak link by decreasing its cross section. It has consequently almost an order of magnitude improved magnetic field responsivity as compared to the earlier design. The modulation of both the current and the voltage across the junction have been measured as a function of magnetic flux piercing the superconducting loop.
In quantum technology, several aspects of superconductivity such as proximity effect have studied to develop a wide range of attractive applications at sub-kelvin temperatures. A new type of interferometer based on the proximity effect, taking place around transparent interfaces between normal- and superconducting metals, the Superconducting Quantum Interference Proximity Transistor (SQUIPT), relies on the phase dependence of the density of states in the proximized weak link. The SQUIPT devices offer the possibility to realize sensitive low-dissipation magnetometers compared to conventional DC SQUIDs. In this thesis, we investigate the development of the sensitive SQUIPT magnetometers. The first part of the thesis covers the characterization of non-hysteretic SQUIPTs with enhanced responsivity. In these structures, we demonstrate magnetic flux modulation of the device characteristics displaying no hysteresis at low temperatures by simply increasing the Josephson inductance of the weak link compared to self-inductance of the superconducting loop in the device. As a consequence, improvement in magnetic field responsivity is achievable. We then turn to the implementation of SQUIPT devices based on different fabrication methods and superconductor materials for improving the device performance. In this aspect, we fabricate and characterize niobium-based SNS devices utilizing two separate lithography and deposition steps with strong Ar ion cleaning in between. We further investigate a prototype hybrid SQUIPT device based on an Nb-Cu-Nb SNS junction with a conventional Al probe in tunnel junction. In the third part of the thesis, we present the flux noise characterization of a SQUIPT device using simultaneous measurement of DC transport properties and shot noise. To probe the noise, we use a cryogenic amplifier operating at frequencies in the range of a few MHz. We adapt this technique for flux noise measurements in SQUIPTs, adaptable also to low-temperature shot noise measurements of other nonlinear devices with high impedance. In order to investigate flux noise of SQUIPTs, we develop a model allowing one to optimize the figures of merit of the magnetometers such as the noise-equivalent flux.
The design for an inductive superconducting quantum interference proximity transistor with enhanced performance, the L-SQUIPT, is presented and analyzed. The interferometer is based on a double-loop structure, where each ring comprises a superconductor--normal-metal--superconductor mesoscopic Josephson weak link and the readout electrode is implemented in the form of a superconducting tunnel probe. Our design allows us to both improve the coupling of the transistor to the external magnetic field and increase the characteristic magnetic flux transfer functions, thereby leading to an improved ultrasensitive quantum limited magnetometer. The L-SQUIPT behavior is analyzed in both the dissipative and the dissipationless Josephson-like operation modes, in the latter case by exploiting both an inductive and a dispersive readout scheme. The improved performance makes the L-SQUIPT promising for magnetic flux detection as well as for specific applications in quantum technology, where a responsive dispersive magnetometry at millikelvin temperatures is required.
In quantum technology, several aspects of superconductivity such as proximity effect have studied to develop a wide range of attractive applications at sub-kelvin temperatures. A new type of interferometer based on the proximity effect, taking place around transparent interfaces between normal- and superconducting metals, the Superconducting Quantum Interference Proximity Transistor (SQUIPT), relies on the phase dependence of the density of states in the proximized weak link. The SQUIPT devices offer the possibility to realize sensitive low-dissipation magnetometers compared to conventional DC SQUIDs. In this thesis, we investigate the development of the sensitive SQUIPT magnetometers. \n \nThe first part of the thesis covers the characterization of non-hysteretic SQUIPTs with enhanced responsivity. In these structures, we demonstrate magnetic flux modulation of the device characteristics displaying no hysteresis at low temperatures by simply increasing the Josephson inductance of the weak link compared to self-inductance of the superconducting loop in the device. As a consequence, improvement in magnetic field responsivity is achievable. \n \nWe then turn to the implementation of SQUIPT devices based on different fabrication methods and superconductor materials for improving the device performance. In this aspect, we fabricate and characterize niobium-based SNS devices utilizing two separate lithography and deposition steps with strong Ar ion cleaning in between. We further investigate a prototype hybrid SQUIPT device based on an Nb-Cu-Nb SNS junction with a conventional Al probe in tunnel junction. \n \nIn the third part of the thesis, we present the flux noise characterization of a SQUIPT device using simultaneous measurement of DC transport properties and shot noise. To probe the noise, we use a cryogenic amplifier operating at frequencies in the range of a few MHz. We adapt this technique for flux noise measurements in SQUIPTs, adaptable also to low-temperature shot noise measurements of other nonlinear devices with high impedance. In order to investigate flux noise of SQUIPTs, we develop a model allowing one to optimize the figures of merit of the magnetometers such as the noise-equivalent flux.
Magnetometers exploiting interference effect of the superconducting wavefunction are known since the realization of the first SQUID, with several improvements in performance in the following years. In this field, the Superconducting Quantum Interference Proximity Transistor (SQUIPT) offers an interesting alternative to the conventional SQUID thanks to its lower power dissipation that makes it ideal for nanoscale and ultra-low temperature (T <; 1 mK) applications, where very low dissipation is required. Here, a device composed of two facing fully superconducting SQUIPT is proposed. Numerical simulations demonstrate how the interplay between the behaviours of a couple of tunnel junctions in such a design accounts for promising performances as both a flux-to-current and a flux-to-voltage transducer.
Abstract Here we report the fabrication and characterization of fully superconducting quantum interference proximity transistors (SQUIPTs) based on the implementation of vanadium (V) in the superconducting loop. At low temperature, the devices show high flux-to-voltage (up to 0.52 mV/Φ0) and flux-to-current (above 12 nA/Φ0) transfer functions, with the best estimated flux sensitivity ~ 2.6 μΦ0/(Hz)1/2 reached under fixed voltage bias, where Φ0 is the flux quantum. The interferometers operate up to Tbath ≃\simeq ≃ ≃ 2 K, with an improvement of 70% of the maximal operating temperature with respect to early SQUIPTs design. The main features of the V-based SQUIPT are described within a simplified theoretical model. Our results open the way to the realization of SQUIPTs that take advantage of the use of higher-gap superconductors for ultra-sensitive nanoscale applications that operate at temperatures well above 1 K.