Associate professor Mazhar Ali and his research group at TU Delft have
discovered one-way superconductivity without magnetic fields, something that
was thought to be impossible ever since its discovery in 1911—up until now.
The discovery, published in Nature, makes use of 2D quantum materials and
paves the way toward superconducting computing. Superconductors can make
electronics hundreds of times faster, all with zero energy loss. Ali: "If
the 20th century was the century of semiconductors, the 21st can become the
century of the superconductor."
During the 20th century, many scientists, including Nobel Prize winners,
have puzzled over the nature of superconductivity, which was discovered by
Dutch physicist Kamerlingh Onnes in 1911. In superconductors, a current goes
through a wire without any resistance, which means inhibiting this current
or even blocking it is hardly possible—let alone getting the current to flow
only one way and not the other. That Ali's group managed to make
superconducting one-directional—necessary for computing—is remarkable: one
can compare it to inventing a special type of ice which gives you zero
friction when skating one way, but insurmountable friction the other way.
Superconductor: Super-fast, super-green
The advantages of applying superconductors to electronics are twofold.
Superconductors can make electronics hundreds of times faster, and
implementing superconductors into our daily lives would make IT much
greener: if you were to spin a superconducting wire from here to the moon,
it would transport the energy without any loss. For instance, the use of
superconductors instead of regular semi-conductors might safe up to 10% of
all western energy reserves according to NWO.
The (im)possibility of applying superconducting
In the 20th century and beyond, no one could tackle the barrier of making
superconducting electrons go in just one-direction, which is a fundamental
property needed for computing and other modern electronics (consider for
example diodes that go one way as well). In normal conduction the electrons
fly around as separate particles; in superconductors they move in pairs of
twos, without any loss of electrical energy. In the '70s, scientists at IBM
tried out the idea of superconducting computing but had to stop their
efforts: in their papers on the subject, IBM mentions that without
non-reciprocal superconductivity, a computer running on superconductors is
impossible.
Interview with corresponding author Mazhar Ali
Q: Why, when one-way direction works with normal semi-conduction, has one-way superconductivity never worked before?
Electrical conduction in semiconductors, like Si, can be one-way because of
a fixed internal electric dipole, so a net built in potential they can have.
The textbook example is the famous pn junction; where we slap together two
semiconductors: one has extra electrons (-) and the other has extra holes
(+). The separation of charge makes a net built in potential that an
electron flying through the system will feel. This breaks symmetry and can
result in one-way properties because forward vs backwards, for example, are
no longer the same. There is a difference in going in the same direction as
the dipole vs going against it; similar to if you were swimming with the
river or swimming up the river.
Superconductors never had an analog of this one-directional idea without
magnetic field; since they are more related to metals (i.e. conductors, as
the name says) than semiconductors, which always conduct in both directions
and dont have any built in potential. Similarly, Josephson Junctions (JJs),
which are sandwiches of two superconductors with non-superconducting,
classical barrier materials in-between the superconductors, also havent had
any particular symmetry-breaking mechanism that resulted in a difference
between forward and backwards.
Q: How did you manage to do what first seemed impossible?
It was really the result of one of my group's fundamental research
directions. In what we call Quantum Material Josephson Junctions (QMJJs), we
replace the classical barrier material in JJs with a quantum material
barrier, where the quantum materials intrinsic properties can modulate the
coupling between the two superconductors in novel ways. The Josephson Diode
was an example of this: we used the quantum material Nb3Br8, which is a 2D
material like graphene that has been theorized to host a net electric
dipole, as our quantum material barrier of choice and placed it between two
superconductors.
We were able to peel off just a couple atomic layers of this Nb3Br8 and make
a very, very thin sandwich —just a few atomic layers thick—which was needed
for making the Josephson diode, and was not possible with normal 3D
materials. Nb3Br8, is part of a group of new quantum materials being
developed by our collaborators, Professor Tyrel McQueens and his group at
Johns Hopkins University in the U.S., and was a key piece in us realizing
the Josephson diode for the first time.
Q: What does this discovery mean in terms of impact and applications?
Many technologies are based on old versions of JJ superconductors, for
example MRI technology. Also, quantum computing today is based on Josephson
Junctions. Technology which was previously only possible using
semi-conductors can now potentially be made with superconductors using this
building block. This includes faster computers, as in computers with up to
terahertz speed, which is 300 to 400 times faster than the computers we are
now using. This will influence all sorts of societal and technological
applications. If the 20th century was the century of semi-conductors, the
21st can become the century of the superconductor.
The first research direction we have to tackle for commercial application is
raising the operating temperature. Here we used a very simple superconductor
that limited the operating temperature. Now we want to work with the known
so-called High Tc Superconductors, and see whether we can operate Josephson
diodes at temperatures above 77 K, since this will allow for liquid nitrogen
cooling. The second thing to tackle is scaling of production. While its
great that we proved this works in nanodevices, we only made a handful. The
next step will be to investigate how to scale production to millions of
Josephson diodes on a chip.
Q: How sure are you of your case?
There are several steps which all scientists need to take to maintain
scientific rigor. The first is to make sure their results are repeatable. In
this case we made many devices, from scratch, with different batches of
materials, and found the same properties every time, even when measured on
different machines in different countries by different people. This told us
that the Josephson diode result was coming from our combination of materials
and not some spurious result of dirt, geometry, machine or user error or
interpretation.
We also carried out smoking gun experiments that dramatically narrows the
possibility for interpretation. In this case, to be sure that we had a
superconducting diode effect we actually tried switching the diode; as in we
applied the same magnitude of current in both forward and reverse directions
and showed that we actually measured no resistance (superconductivity) in
one direction and real resistance (normal conductivity) in the other
direction.
We also measured this effect while applying magnetic fields of different
magnitudes and showed that the effect was clearly present at 0 applied field
and gets killed by an applied field. This is also a smoking gun for our
claim of having a superconducting diode effect at zero-applied field, a very
important point for technological applications. This is because magnetic
fields at the nanometer scale are very difficult to control and limit, so
for practical applications, it is generally desired to operate without
requiring local magnetic fields.
Q: Is it realistic for ordinary computers (or even the supercomputers of KNMI and IBM) to make use of superconducting?
Yes, it is! Not for people at home, but for server farms or for
supercomputers, it would be smart to implement this. Centralized computation
is really how the world works now-a-days. Any and all intensive computation
is done at centralized facilities where localization adds huge benefits in
terms of power management, heat management, etc. The existing infrastructure
could be adapted without too much cost to work with Josephson diode based
electronics. There is a very real chance, if the challenges discussed in the
other question are overcome, that this will revolutionize centralized and
supercomputing.
Reference:
Mazhar Ali, The field-free Josephson diode in a van der Waals
heterostructure, Nature (2022).
DOI: 10.1038/s41586-022-04504-8
Tags:
Physics