Effect of electrons with negative mass in novel semiconductor nanostructures

Red laser beam hits atomically thin crystal WSE2, which transforms the laser's red light into a blue glow. Credit: Felix Hofmann
Dr. Kai-Qiang Lin and John Lupton, both from the Institute of Experimental and Applied Physics in Regensburg, have been able to measure electrons with negative masses in new semiconductor nanostructures. This is a large international collaboration. This international team includes scientists from Berkeley, Yale (USA), Cambridge (England), and Tsukuba(Japan).

Many everyday things are only familiar in positive quantities. For example, the weight of an item. One of the mysteries of physics is why matter seems to always have positive mass. Although we may have become used to negative interest rates, what would happen if mass were to turn negative?

Newtonian mechanics describes the consequences with the well-known equation Force=Mass*Acceleration, oder F=m*a. A force acting on an object will cause it to accelerate. Jump starting a car with negative mass is dangerous! It will be moving towards you! A golf ball with negative mass would sink faster than a car without friction.

The fundamental components of matter as we know them are the three elementary particles: the atomic nuclei containing heavy protons and neutrons and the light electrons. The atomic nuclei determine the body's weight. The mass of the nuclei can be fixed, but the effective mass for electrons depends on the material they are moving in. The electronic properties of a material are directly affected by its mass.

Illustration of negative mass with a golfball in a glass water. Water slows down the fall of a standard golf ball. The frictional resistance would accelerate a golf ball with a negative mass. Credit: Felix Hofmann

Driving school taught us that the braking distance increases quadratically as speed. Another consequence of Newton's equation is that the motional energy of a vehicle rises with the square root of speed v. E=1/2*m*v2. However, if the mass m was negative, then the energy of particles such as electrons would decrease with increasing speed. The "braking distance" will decrease!

An electron's movement through a material can cause it to collide with other electrons or nuclei. Similar to driving a car, collisions can slow down the speed of an electron with positive mass. A negative electron, however, loses energy but is accelerated. Researchers have been able to observe this effect for the very first time.

Researchers at Regensburg used a new type semiconductor material: a single, atomically thin sheet of crystal tungsten dielenide. The material glows when it is exposed to laser light. An electron absorbs the energy and then emits it back in its characteristic color, red. This is the electron's fundamental energy. As water flows downhill, so electrons with more energy should always be at the lowest fundamental energy. The semiconductor should always glow in red.

The team discovered an amazing effect. The electrons also emit a faint blue glow when irradiated by a red laser. This extraordinary effect occurs when low-energy red light is converted to blue light with higher energy. This blue light is visible in the following images: The optical spectrum can show that electrons with negative mass are responsible for the blue glow. This surprising experimental discovery could be supported by detailed quantum mechanical calculations of electronic structure that were made in this form for first time.

Although the discovery might seem odd at the moment, scientists have already begun to think of potential applications. The concept could be used to develop superfast computers where electrons move nearly without resistance. So-called singularities are also created when positive mass becomes negative. Such singularitiesfamiliar from trying to divide something by zero on a calculatorare not entirely dissimilar to the black holes of cosmology.

The fact that electrons in a semiconductor can assume discrete energy states like an atom makes it possible to transfer concepts from atomic quantum optics directly into the semiconductor. This could be used to create new electronic components that can convert the wavelength of light and store, amplify, or amplify light.

Continue reading Helping semiconductors to find a cooler method of relaxing

Kai-Qiang Lin and colleagues, Narrow-band highlying excitons in monolayer WSE2, Nature Communications (2021). Information from Nature Communications Kai Qiang Lin and colleagues, Narrow-band highlying excitons in monolayer WSE2 (2021). DOI: 10.1038/s41467-021-25499-2

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