Since the early days of quantum physics in the 1920s and 30s, it has been
suggested time and again that electric "continuous currents" flow
in tiny metal rings. These currents are small, but flow permanently, even without
applied voltage. Physicists at Yale University and Freie
Universität Berlin have now demonstrated the existence of these permanent
currents in detail and have shown their properties to be in remarkably close
agreement with theoretical predictions. Their research results have been published
in the current Science issue.
The existence of permanent currents is surprising, even for experts, since
they occur in ordinary, non-superconducting metals where, due to electrical
resistance, current can only flow with applied voltage. The measured steady
currents are based on an effect of quantum physics influencing the movement
of electrons in metals. Ultimately, they can be thought of as an expression
of the same movement that allows the electrons in the atom to constantly circle
around the atomic nucleus.
An experimental demonstration of continuous currents is difficult. They cannot
be directly measured with a conventional flow meter because they flow only in
metal rings with a diameter of about one micrometer, or micron. A micron is
about one hundredth of the diameter of a human hair and is comparable to the
size of the wires in a silicon computer chip. In previous experiments, attempts
were made to demonstrate the constant flows using the magnetic field caused
by them. (A magnetic field is always generated when a current flows through
a wire.) For that, highly sensitive magnetic field probes were used, so-called
SQUIDs (Superconducting Quantum Interference Device). The results of these experiments,
however, were inconsistent and sometimes differed widely from the theoretical
predictions.
The experiments now conducted at Yale University under the direction of Jack
Harris were based on a different and novel strategy. The metal rings were applied
to the tip of a nanocantilever - a kind of swinging miniature diving board.
The current flowing in the rings led to a magnetic force on the cantilever and
could thus be demonstrated by means of changes in vibrations of the "springboard."
After many years of optimization, using this method it has now been possible
to demonstrate the continuous currents much more accurately than ever before
and to measure them.
The most comprehensive, theoretical predictions for these experiments go back
to the 15-year-old doctoral thesis by Felix von Oppen, who is currently a researcher
at Dahlem Center for Complex Quantum Systems at Freie Universität Berlin.
Working jointly with American scientists von Oppen's results were expanded based
on facts established by the new experimental method. In particular, it was necessary
to take effects of the theory of relativity (the so-called spin-orbit coupling)
into account in the theoretical calculations. Measurements taken after these
adjustments were made, confirm the theoretical predictions with great accuracy.
The greatly improved method of measurement has not only resolved an old enigma,
but also thrown open the door to numerous experiments that scientists anticipate
will provide new insights into the behavior of electrons in metals. New results
could, for example, identify metals that could potentially serve as superconductors
or elucidate the behavior of qubits, the building blocks of a future quantum
computer.
Posted October 9th, 2009
