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arXiv:cond-mat/0107292 · Applied Physics Letters

Controllable π SQUID

J. J. A. Baselmans, Bart Jan van Wees, T. M. Klapwijk

Published Oct 29, 2001• 25 citations
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Abstract

We have fabricated and studied a promising kind of direct current superconducting quantum interference device (dc-SQUID) in which the magnitude and sign of the critical current of the individual Josephson junctions can be controlled by additional voltage probes connected to the junctions. We show that the amplitude of the voltage oscillations of the SQUID as a function of the applied magnetic field can be tuned and that the phase of the oscillations can be switched between 0 and π in the temperature range of 0.1–4.2 K using a suitable control voltage. This is equivalent to the external application of (n+1/2) flux quantum.

DOI
10.1063/1.1414304

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We have fabricated and studied a promising kind of direct current superconducting quantum interference device (dc-SQUID) in which the magnitude and sign of the critical current of the individual Josephson junctions can be controlled by additional voltage probes connected to the junctions. We show that the amplitude of the voltage oscillations of the SQUID as a function of the applied magnetic field can be tuned and that the phase of the oscillations can be switched between 0 and π in the temperature range of 0.1–4.2 K using a suitable control voltage. This is equivalent to the external application of (n+1/2) flux quantum.
A controllable \ensuremath{\pi}-SQUID is a DC SQUID (direct-current superconducting quantum interference device) with two controllable \ensuremath{\pi} junctions as weak links. A controllable \ensuremath{\pi} junction consists of a superconducting--normal metal--superconducting Josephson junction with two additional contacts to the normal region of the junction. By applying a voltage Vc{V}_{c}Vc​ over these contacts it is possible to control the state of the junction, i.e., a conventional (0) state or a \ensuremath{\pi} state, depending on the magnitude of Vc.{V}_{c}.Vc​. We demonstrate experimentally that, by putting one junction into a \ensuremath{\pi} state, a screening current is generated around the SQUID loop at an integer external flux. To be able to do this, we have fabricated controllable $\ensuremath{\pi}$ junctions, based on Cu-Nb or Ag-Nb, in a new geometry. We show that at 1.4 K only the Ag-Nb device shows the transition to a \ensuremath{\pi} state as a function of Vc{V}_{c}Vc​ consistent with theoretical predictions. In a controllable \ensuremath{\pi}-SQUID based on Nb-Ag we observe, apart from a screening current at integer external flux, a phase shift of \ensuremath{\pi} of the ${V}_{\mathrm{SQUID}}\ensuremath{-}B$ oscillations under a suitable current bias, depending on the magnitude of Vc.{V}_{c}.Vc​.
The direct current superconducting quantum interference device (DC-SQUID), using controlled Josephson junction technology, provides a mechanism to modify the characteristics of the device post-fabrication. We report on the fabrication and measurement of a micron sized DC-SQUID using two Dayem bridge weak-link Josephson junctions with integrated “heaters.” The weak link critical current is controlled by hot phonons from the current biased titanium, normal metal heater. By using the heaters, control over the critical current oscillations of the SQUID was observed at 4.2K.
The optimal design of a direct current superconducting quantum interference device (DC-SQUID) with high sensitivity requires two identical Josephson junctions, which is difficult to achieve. The design of a DC-SQUID using controlled Josephson junction technology provides a mechanism for modifying the characteristics of the device. Here we report on the nano-fabrication and measurement of a DC-SQUID utilizing two Dayem bridge weak-link Josephson junctions fabricated with a width ∼100 nm and a length of ∼200 nm. These junctions are controlled via the irradiation of hot phonons from nearby titanium/chromium normal metal constrictions which are current biased. The devices were measured at a temperature of 4.2 K and control over the critical current oscillations was observed.
We study a 0-$\ensuremath{\pi}$ dc superconducting quantum interference device (SQUID) with asymmetric inductances and critical currents of the two Josephson junctions (JJs). By considering such a dc SQUID as a black box with two terminals, we calculate its effective current-phase relation ${I}_{s}(\ensuremath{\psi})$ and the Josephson energy $U(\ensuremath{\psi})$, where $\ensuremath{\psi}$ is the Josephson phase across the terminals. We show that there is a domain of parameters where the black box has the properties of a $\ensuremath{\varphi}$ JJ with degenerate ground state phases $\ensuremath{\psi}=\ifmmode\pm\else\textpm\fi{}\ensuremath{\varphi}$. The $\ensuremath{\varphi}$ domain is rather large, so one can easily construct a $\ensuremath{\varphi}$ JJ experimentally. We derive the current phase relation and show that it can be tuned in situ by applying an external magnetic flux resulting in a continuous transition between the systems with static solutions $\ensuremath{\psi}=\ifmmode\pm\else\textpm\fi{}\ensuremath{\varphi},\phantom{\rule{0.16em}{0ex}}\ensuremath{\psi}={\ensuremath{\varphi}}_{0} ({\ensuremath{\varphi}}_{0}\ensuremath{\ne}0,\ensuremath{\pi})$ and even $\ensuremath{\psi}={\ensuremath{\varphi}}_{0}\ifmmode\pm\else\textpm\fi{}\ensuremath{\varphi}$. The dependence of ${\ensuremath{\varphi}}_{0}$ on applied magnetic flux is not $2\ensuremath{\pi}$ (one flux quantum) periodic.
We measured the retrapping current of a small-capacitance underdamped dc superconducting quantum interference device (SQUID) at low temperatures as a function of magnetic field. We found that the relative phase relation between the magnetic-field dependence of the retrapping current and that of the switching current jumped from 0 to $\ensuremath{\pi}$ at ${E}_{\mathrm{J}}\ensuremath{\sim}{E}_{\mathrm{C}},$ where EJ{E}_{\mathrm{J}}EJ​ and EC{E}_{\mathrm{C}}EC​ are the Josephson energy and the charging energy of a Cooper pair. This phenomenon cannot be reproduced by a model that approximates the SQUID as a single junction with a controllable switching current. It can be explained phenomenologically by taking into account two kinds of dissipation that arise from ac currents flowing through the lead and along the SQUID's loop.
This study shows that we can obtain an enhanced mutual coupling by using a superconducting quantum interference device (SQUID) containing a π-phase shift Josephson junction (π-junction). The π-junction can increase the current flowing in the loop inductance of the SQUID, thereby enhancing the magnetic flux generated in the inductance. Therefore, the effective self-inductance of the SQUID can increase, resulting in an increase in the mutual coupling. We use a 0-π SQUID formed by a conventional Josephson junction (0-junction) and a π-junction. Our numerical analysis for a simplified structure revealed that the ratio of effective to original self-inductance increased to 50. To confirm this, we measured the modulation patterns of critical current in dc-SQUIDs in which part of the loop inductance was replaced with a series of 0-π SQUIDs. The observed modulation pattern revealed that the effective self-inductance became more than 10-fold larger than the original self-inductance under the conditions that the applied current is a few μA while the critical current of the 0-junction in 0-π SQUIDs is 35 μA.