Assumed to be impossible for a lifetime
From: Delft University
in the Netherlands
April 27, 2022 -- 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.
More information: Mazhar Ali, The field-free Josephson diode in
a van der Waals heterostructure, Nature (2022). DOI:
10.1038/s41586-022-04504-8. www.nature.com/articles/s41586-022-04504-8
https://phys.org/news/2022-04-discovery-one-way-superconductor-thought-impossible.html
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