The smallest motor in the world
On the trail of enigmatic quantum phenomena

Date:
June 16, 2020
Source:
Swiss Federal Laboratories for Materials Science and Technology
(EMPA)
Summary:
A research team has developed a molecular motor which consists of
only 16 atoms and rotates reliably in one direction. It could allow
energy harvesting at the atomic level. The special feature of the
motor is that it moves exactly at the boundary between classical
motion and quantum tunneling -- and has revealed puzzling phenomena
to researchers in the quantum realm.



FULL STORY ==========================================================================
The smallest motor in the world -- consisting of just 16 atoms: this was developed by a team of researchers from Empa and EPFL. "This brings us
close to the ultimate size limit for molecular motors," explains Oliver Gro"ning, head of the Functional Surfaces Research Group at Empa. The
motor measures less than one nanometer -- in other words it is around
100,000 times smaller than the diameter of a human hair.


==========================================================================
In principle, a molecular machine functions in a similar way to its
counterpart in the macro world: it converts energy into a directed
movement. Such molecular motors also exist in nature -- for example in
the form of myosins. Myosins are motor proteins that play an important
role in living organisms in the contraction of muscles and the transport
of other molecules between cells.

Energy harvesting on the nanoscale Like a large-scale motor, the 16
atom motor consists of a stator and a rotor, i.e. a fixed and a moving
part. The rotor rotates on the surface of the stator (see picture). It
can take up six different positions. "For a motor to actually do useful
work, it is essential that the stator allows the rotor to move in only
one direction," explains Gro"ning.

Since the energy that drives the motor can come from a random direction,
the motor itself must determine the direction of rotation using a
ratcheting scheme. However, the atom motor operates opposite of what
happens with a ratchet in the macroscopic world with its asymmetrically serrated gear wheel: While the pawl on a ratchet moves up the flat
edge and locks in the direction of the steep edge, the atomic variant
requires less energy to move up the steep edge of the gear wheel than
it does at the flat edge. The movement in the usual 'blocking direction'
is therefore preferred and the movement in 'running direction' much less likely. So the movement is virtually only possible in one direction.

The researchers have implemented this "reverse" ratchet principle in a
minimal variant by using a stator with a basically triangular structure consisting of six palladium and six gallium atoms. The trick here is that
this structure is rotationally symmetrical, but not mirror-symmetrical.



==========================================================================
As a result, the rotor (a symmetrical acetylene molecule) consisting
of only four atoms can rotate continuously, although the clockwise and counterclockwise rotation must be different. "The motor therefore has 99% directional stability, which distinguishes it from other similar molecular motors," says Gro"ning. In this way, the molecular motor opens up a way
for energy harvesting at the atomic level.

Energy from two sources The tiny motor can be powered by both thermal
and electrical energy. The thermal energy provokes that the directional
rotary motion of the motor changes into rotations in random directions
-- at room temperature, for example, the rotor rotates back and forth completely randomly at several million revolutions per second. In
contrast, electrical energy generated by an electron scanning microscope,
from the tip of which a small current flows into the motors, can cause directional rotations. The energy of a single electron is sufficient to
make the rotors continue to rotate by just a sixth of a revolution. The
higher the amount of energy supplied, the higher the frequency of
movement -- but at the same time, the more likely the rotor is to move
in a random direction, since too much energy can overcome the pawl in the "wrong" direction.

According to the laws of classical physics, there is a minimum amount of
energy required to set the rotor in motion against the resistance of the
chute; if the supplied electrical or thermal energy is not sufficient,
the rotor would have to stop. Surprisingly, the researchers were able
to observe an independently constant rotation frequency in one direction
even below this limit -- at temperatures below 17 Kelvin (-256DEG Celsius)
or an applied voltage of less than 30 millivolts.

From classical physics to the quantum world At this point we are
at the transition from classical physics to a more puzzling field:
quantum physics. According to its rules, particles can "tunnel" --
that is, the rotor can overcome the chute even if its kinetic energy is insufficient in the classical sense. This tunnel motion normally occurs
without any loss of energy. Theoretically, therefore, both directions
of rotation should be equally likely in this area. But surprisingly,
the motor still turns in the same direction with 99% probability. "The
second law of thermodynamics states that entropy in a closed system can
never decrease. In other words: if no energy is lost in the tunneling
event, the direction of the motor should be purely random. The fact that
the motor still rotates almost exclusively in one direction therefore
indicates that energy is also lost during tunnel movement," says Gro"ning.



========================================================================== Which way is time running? If we open the scope a little more: When we
watch a video, we can usually tell clearly whether time is running forward
or backward in the video. If we watch a tennis ball, for example, which
jumps a little higher after each impact on the ground, we intuitively
know that the video runs backwards. This is because experience teaches
us that the ball loses some energy with each impact and should therefore
bounce back less high.

If we now think of an ideal system in which neither energy is added
nor lost, it becomes impossible to determine in which direction time is running. Such a system could be an "ideal" tennis ball that bounces back
at exactly the same height after each impact. So, it would be impossible
to determine whether we are watching a video of this ideal ball forward or backward -- both directions are equally plausible. If the energy remains
in one system, we would no longer be able to determine the direction
of time.

But this principle can also be reversed: If we observe a process in
a system that makes it clear in which direction time is running, the
system must lose energy or, more precisely, dissipate energy -- for
example through friction.

Back to our mini-motor: It is usually assumed that no friction is
generated during tunneling. At the same time, however, no energy is
supplied to the system. So how can it be that the rotor always turns in
the same direction? The second law of thermodynamics does not allow any exceptions -- the only explanation is that there is a loss of energy
during tunneling, even if it is extremely small. Gro"ning and his team
have therefore not only developed a toy for molecular craftsmen. "The
motor could enable us to study the processes and reasons for energy
dissipation in quantum tunneling processes," says the Empa researcher.


========================================================================== Story Source: Materials provided by Swiss_Federal_Laboratories_for_Materials_Science_and
Technology_(EMPA). Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Samuel Stolz, Oliver Gro"ning, Jan Prinz, Harald Brune, Roland
Widmer.

Molecular motor crossing the frontier of classical to quantum
tunneling motion. Proceedings of the National Academy of Sciences,
2020; 201918654 DOI: 10.1073/pnas.1918654117 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200616135807.htm

--- up 21 weeks, 2 hours, 34 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1337:3/111)