Nanotechnology

Josephson effect diode reversal

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July 10, 2023

(Nanowerk News) Most transistors, including the building blocks of computer CPUs, generate heat. This is because most conductors are resistive, which causes Joule heating. Indeed, there is a special transistor that does not generate heat, which is called the “Josephson junction field-effect transistor”. They are based on Josephson junctions, weak connections between two superconductors, which still carry zero resistance current (or super current).

Following its discovery by Nobel Prize winner Brian Josephson, the Josephson junction quickly found applications in fields such as medicine, metrology and astrophysics. More recently, they have become a key component of quantum computers, as they are at the heart of transmons, the most popular implementation of qubits in superconducting quantum processors.

Given the above, one can understand the attention generated by the discovery of the first superconducting diode based on the Josephson junction made in 2021 in a group around Nicola Paradiso and Christoph Strunk at the University of Regensburg in synthetic crystals grown by Michael J. Manfra and his team at the University Purdue. Excitement arises from the potential for superconducting diodes to serve as the basic building block for new types of superconducting circuits, for the future replacement of resistive circuits by superconducting circuits. The figure shows a chain of Josepshon junctions in an Al/InAs heterostructure The figure shows a chain of Josepshon junctions in an Al/InAs heterostructure fabricated at UR by Prof.’s group. Strunk and Dr. Paradiso. At sub-Kelvin temperatures and under magnetic field B, devices carry super currents preferentially in one direction, depending on the magnetic field. (Image: Nicola Paradiso/UR)

A characteristic feature of ordinary semiconductor diodes is asymmetry: their resistance can be very high or very low depending on which terminal is connected to the cathode and which to the anode of your battery. This asymmetry leads to the diode’s most important property: current rectification. In contrast, a superconducting diode exhibits no resistance, so its working principle must be different.

What Paradiso and his colleagues found is that superconducting diodes exhibit different inductances for the two possible polarities of DC currents. Also, for the polarity where the inductance is lower, the observed critical current (that is, the threshold current that makes the device switch to a resistive state) is higher. We can call this the preferred current direction. But what decides about the preferred direction? The answer is considered as a fixed material characteristic.

Recently, UR researchers made an interesting discovery: In larger magnetic fields, the desired direction can reverse. Interestingly, theorists predicted this effect about ten years ago, but so far it has never been observed.

In a paper that just appeared on Natural Nanotechnology (“Signature of Josephson inductance magnetochiral anisotropy reversal and 0–π-like transitions in supercurrent diodes”), Strunk’s group experimentally demonstrated a dramatic sign change of the supercurrent diode effect, with experimental data quantitatively matching Dr. Andreas Costa, also from Regensburg.

This discovery is sure to have a major impact in the scientific community, as the superconducting diode effect is a hot topic in quantum electronics because of its interesting perspectives for technological applications and fundamental research.



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