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Even with today’s invisibility cloaks,
people can’t walk through walls. But, when paired together, millions of
electrons can.
The electrons perform this trick, called
macroscopic quantum tunneling, when they pair up and move into a region of
space that is off-limits under the laws of classical mechanics.
The problem is that as millions of
electrons collectively move through a superconducting nanowire, they use energy
and give off heat.
The heat can build, transforming sections
of the wire into a non-superconducting state. The process, called a phase slip,
adds resistance to an electrical system and has implications for designing new
nano-scale superconductors.
Now, scientists have observed individual
phase slips in aluminum nanowires and characterized the nature and temperature
at which they occur. This information could help scientists remove phase slips
from nano-scale systems, which could lead to more reliable nanowires and more
efficient nanoelectronics, says Duke physicist Albert Chang.
The results appeared
online in Physical Review Letters.
The macroscopic quantum tunneling effect
was first observed in a system called a Josephson junction. This device has a
thin insulating layer connecting two superconductors, which are several
nanometers wide and have a three-dimensional shape.
To study the tunneling and phase slips in a
simpler system, however, Chang and his colleagues used individual, 1D nanowires
made of aluminum. The new observations are “arguably the first convincing
demonstration of tunneling of millions of electrons in one-dimensional superconducting
nanowires,” says Chang, who led the study.
In the experiment, the wires ranged in
length from 1.5 to 10 um, with widths from 5 to 10 nm. Chang cooled the wires
to a temperature close to absolute zero, roughly 1 K or -458 F.
At this temperature, a metal’s crystal
lattice vibrates in a way that allows electrons to overcome their negative
repulsion of one other. The electrons make pairs and electric current flows
essentially resistance-free, forming a superconductor.
The electron pairs move together in a path
in a quantum-mechanical space, which resembles the curled cord of an old phone.
On their way around the path, all of the electrons have to scale a barrier or a
wall. Moving past this wall collectively keeps the electrons paired and the superconducting
current stable.
But, the collective effort takes energy and
gives off heat. With successive scaling attempts, the heat builds, causing a
section of the wire to experience a phase slip from a superconducting to a
non-superconducting state.
To pinpoint precisely how phase slips
happen, Chang varied the temperatures and amount of current run through the
aluminum nanowires.
The experiments show that at higher temperatures,
roughly 1.5 K and close to the critical temperature where the wires naturally
become non-superconducting, the electrons have enough energy to move over the
wall that keeps the electrons paired and the superconducting current stable.
In contrast, the electrons in the nanowires
cooled to less than 1 K do not have the energy to scale the wall. Instead, the
electrons tunnel, or go through the wall together, all at once, says Duke
physicist Gleb Finkelstein, one of Chang’s collaborators.
The experiments also show that at the
relatively higher temperatures, individual jumps over the wall don’t create
enough heat to cause a breakdown in superconductivity. But multiple jumps do.
At the lowest temperatures, however, the
paired electrons only need to experience one successful attempt at the wall,
either over or through it, to create enough heat to slip in phase and break the
superconducting state.
Studying the electrons’ behavior at
specific temperatures provides scientists with information to build ultra-thin
superconducting wires that might not have phase slips. Chang said the improved
wires could soon play a role in ultra-miniaturized electrical components for
ultra-miniaturized electronics, such as the quantum bit, used in a quantum
computer.
