- 4262 -
SUPERCONDUCTIVITY - new rules in electronics? This experiment could not only shed light
on strange metals, which have confounded physicists since the metals' discovery
nearly 40 years ago, but lead to a reevaluation of how electrical charge can be
carried.
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------------------------- Atoms at zero temperature
----------- 4262 - SUPERCONDUCTIVITY - new rules in electronics
- When I was studying
engineering in college we learned in physics class that all conductors had
resistance and the amount of current that could flow was inversely proportional
to that resistance. Now in physics today
a weird phenomenon was discovered in which electricity flows like water in a
nanowire made of "strange metal".
This is a bizarre metal phase that has stumped physicists for 40 years.
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- The experiment,
conducted in nano-sized wires made from a weird class of material called
"strange metals," shows electricity no longer moving in clumps of
electrons . This is contradicting one of physicists' most basic assumptions
about how metals behave.
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- "Strange
metals" are a type of quantum material with some truly weird properties:
Not only do they flout the rules of electrical resistance seen in regular
metals, but some can even become superconductors at relatively high
temperatures, meaning they can carry an electrical current without any
resistance.
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- In regular metals,
electrical resistivity is the measure of how difficult it is for an electrical
current to flow through a material.
It increases with the square of
the temperature, before flattening out when the metal gets very hot.
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- This makes
intuitive sense, resistivity arises when charge-carrying electrons in a metal
collide and scatter within the metal's vibrating atomic structure, so
increasing the vibrations of the atoms will increase this scattering rate up to
the point where the electrons become unable to carry a current.
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- But in 1986, a
class of copper-containing materials called cuprates broke all the rules. The
resistance of cuprates instead increased linearly with temperature, and when
some of them were cooled below a certain temperature threshold, minus 211
degrees Fahrenheit, they transformed into superconductors.
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- Something deeply
strange was going on with the way the metals carried a current. Until the discovery of strange metals,
physicists viewed traditional metals as made up of a Fermi "sea" of
largely individual electrons that carry a current one by one.
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- This means that
when metals' electrons swim in the form of a current, they don't move discretely
but rather flow in clumps of quasiparticles.
Yet the weird linear rise of resistance in strange metals remained
unexplained. To test what could be going on, the researchers fashioned
minuscule nanowires. Each was 200
nanometers wide and 600 nanometers long, roughly five times smaller than a
bacterium. Made from a precise blend of
the strange metals ytterbium, rhodium and silicon, before cooling them to
temperatures just a few degrees above absolute zero.
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- Then, after passing
brief bursts of current through the wires, the scientists measured fluctuations
in the flow of electrons, using a classic phenomenon known as shot noise. As
quantum particles, electrons are governed by random quantum mechanical
processes. Apply a voltage across a wire, then, and the electrons inside will
zip from one end to another at random times.
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- Usually, so many
electrons take part in this process that the randomness of when each one moves
is drowned out by the stampede of the overall current. But by making wires
small and voltages tiny, physicists can reduce the number of electrons able to
flow and make the static crackle of the current visible.
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- Discrete charges
have some statistical fluctuations in how they flow. Like sand grains through an hourglass, on
average there is a smooth flow, but if you look carefully, sometimes two
successive grains come through close together in time, and other times farther
apart.
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- If this theory of
clumped quasiparticles applies to strange metals, the shot noise detected in
the experiment should show its electrons arriving in discrete clumps. But shockingly, rather than large splatters,
the current in the nanowires arrived as a continuous hiss. The electricity was being carried through the
wires, but it appeared to be out of step with the charge carriers meant to
transport it.
-
- Think about a
crowded hallway. In the ordinary metal case, even though the hallway is
crowded, a particular person (the quasiparticle) can get through the hallway
with just a slight disturbance of neighboring folks as they go by. In the strange metal case, the hallway is
more like a mosh pit. Everyone is jostling around so much that you can't really
follow an individual anymore, but somehow there is still a net flow of a person
down the hall.
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- Researchers hope to
find a common "organizing principle" behind the weird material phase,
as well as some crucial hints as to how strange metals achieve
superconductivity.
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-
December 9, 2023
SUPERCONDUCTIVITY - new
electronics 4262
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Saturday, December 9, 2023 ---------------------------------
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