The superconducting transport between the layers of a cuprate crystal (three layers, red and blue spheres represent the oxygen and copper atoms respectively) is controlled with an ultrashort terahertz pulse (yellow in the background). The three-dimensional superconductivity can thus be switched on and off very quickly (orange spheres represent electrons). J.M. Harms, Max Planck Research Group for Structural Dynamics |
A
high-temperature superconductor can now be switched on and off within a
trillionth of a second
—100 years after the discovery of
superconductivity and 25 years after the first high-temperature
superconductor was. A team including physicists from the University of
Oxford and the Max Planck Research Group for Structural Dynamics at the
University of Hamburg has realised an ultrafast superconducting switch
by using intense terahertz pulses. This experiment opens up the
possibility to discover more about the still unsettled cause of this
type of superconductivity, and also hints at possible applications for
ultrafast electronics in the future.
Superconductivity
is one of the most remarkable effects in physics. Every electrical
conductor has a resistivity, but some materials lose their resistivity
completely if they are cooled to below a characteristic temperature; the
current then flows without any loss whatsoever. When the Dutch
physicist Heike Kamerlingh Onnes discovered this effect in 1911 in
mercury, he initially believed that his measuring instruments were
faulty, before he became aware of the significance of his monumental
discovery.
“Normal”
conductors such as mercury or lead must be cooled down to temperatures
near absolute zero at minus 273.16 C in order to become
superconducting. It was therefore a sensation when, in 1986, Johannes
Georg Bednorz and Karl Alexander Müller presented a ceramic material
that already became superconducting at minus 248 C. Since
then, these cold conductors have been a burning issue with both
scientists working in basic research and users. The ultrafast switch,
which has now been developed by the research group working with Andrea
Cavalleri, head of the Max Planck Research Group for Structural Dynamics
at the University of Hamburg, is a further astonishing discovery in
this field.
The
high-temperature superconductor used by the Hamburg scientists has been
known for a long time. It is a crystal based on lanthanum cuprate
(La2CuO4) to which a specific quantity of strontium has been added
(La1,84Sr0,16CuO4). Its transition temperature is minus 233 C. Although it is not yet completely clear how the
superconductivity arises here, essential elements are known: “The
crystal is formed by copper-oxygen planes which lie on top of each other
like the pages of a book,” explains Cavalleri. The electrons can only
move within these planes; the current transport therefore only occurs in
two dimensions.
If
the material is cooled below 40 K, a link is suddenly created
between these two planes. Physicists explain this using the wave model,
according to which the electrons are pictured not as particles, but as
waves. Below the transition temperature the electrons from neighbouring
planes overlap, and this allows the electric charge carriers to change
from one plane to the other. Current is suddenly transported in all
three spatial dimensions: the superconducting state has been created.
Terahertz pulse briefly destroys the coupling of the electrons
Cavalleri
and his colleagues then wanted to know whether this transport between
the layers can be deliberately interrupted and switched on again. In
theory this is possible if a very strong electric field is applied at
right angles to the layers. However, applying such a field is
impractical. “This causes the crystal to heat upand the
superconductivity collapses,” explains Cavalleri. The solution was to
send in an ultrashort pulse of light to manipulate the superconductor.
This
so-called terahertz pulse is an electromagnetic wave, similar to light,
but with a much longer wavelength. It has an electric field that
briefly destroys the coupling of the electron waves between the planes
when it penetrates into the crystal. This is only successful if the
electric field strength of the pulse is very high, in the order of
several tens of thousands of volts per centimeter. And it must be short
enough that it does not heat up the crystal.
Only
recently has it been possible to generate such extremely powerful,
ultrashort terahertz pulses. This is the task of team member Matthias
Hoffmann. In very simple terms, this is done by the interaction of an
ultrashort laser pulse with a lithium niobate crystal. An effect which
physicists call optical rectification then generates the desired
terahertz radiation in the crystal.
The
experiment, which Andreas Dienst designed and carried out in Oxford,
succeeded as anticipated: for the short time of less than one picosecond
(10-12 seconds) as the pulse interacts with the superconductor, the
coupling between the planes, and thus the superconductivity, was
interrupted before subsequently returning. The superconductor does not
suffer in this process and can be switched as often as one likes.
“This
is a very fascinating result, because we can also use this method to
investigate how high-temperature superconductors work,” says Cavalleri.
It is also possible that this effect additionally has real-world
applications. Basically, the switchable high-temperature superconductor
works in a very similar way to a conventional field-effect transistor.
This is a semiconductor whose ability to pass a current can be
controlled by applying an electric voltage. Analogous to this, is
conceivable that the high-temperature superconductor could be used as an
ultrafast, nanoelectronic transistor that is controlled by microwaves.