“First-Ever Detection of a One-Way Mass Particle Stuns Physicists”
For the first time, scientists have observed a collection of particles, also known as a quasiparticle, that’s massless when moving one direction but has mass in the other direction. The quasiparticle, called a semi-Dirac fermion, was first theorized 16 years ago, but was only recently spotted inside a crystal of semi-metal material called ZrSiS. The observation of the quasiparticle opens the door to future advances in a range of emerging technologies from batteries to sensors, according to the researchers.
The team, led by scientists at Penn State and Columbia University, recently published their discovery in the journal Physical Review X.
“This was totally unexpected,” said Yinming Shao, assistant professor of physics at Penn State and lead author on the paper. “We weren’t even looking for a semi-Dirac fermion when we started working with this material, but we were seeing signatures we didn’t understand—and it turns out we had made the first observation of these wild quasiparticles that sometimes move like they have mass and sometimes move like they have none.”
A particle can have no mass when its energy is entirely derived from its motion, meaning it is essentially pure energy traveling at the speed of light. For example, a photon or particle of light is considered massless because it moves at light speed. According to Albert Einstein’s theory of special relativity, anything traveling at the speed of light cannot have mass.
In solid materials, the collective behavior of many particles, also known as quasiparticles, can have different behavior than the individual particles, which in this case gave rise to particles having mass in only one direction, Shao explained.
Semi-Dirac fermions were first theorized in 2008 and 2009 by several teams of researchers, including scientists from the Université Paris Sud in France and the University of California, Davis. The theorists predicted there could be quasiparticles with mass-shifting properties depending on their direction of movement—that they would appear massless in one direction but have mass when moving in another direction.
Sixteen years later, Shao and his collaborators accidentally observed the hypothetical quasiparticles through a method called magneto-optical spectroscopy. The technique involves shining infrared light on a material while it’s subjected to a strong magnetic field and analyzing the light reflected from the material. Shao and his colleagues wanted to observe the properties of quasiparticles inside silver-colored crystals of ZrSiS.
Landau levels spectroscopy sheds light on semi-Dirac fermions at the crossing point of two nodal lines within a semi-metal material (Left: Fermi surface of a nodal-line crossing model, Right: Band structure of material). Credit: Yinming Shao.
The team conducted their experiments at the National High Magnetic Field Laboratory in Florida. The lab’s hybrid magnet creates the most powerful sustained magnetic field in the world, roughly 900,000 times stronger than the Earth’s magnetic field. The field is so strong it can levitate small objects such as water droplets.
The researchers cooled down a piece of ZrSiS to -452°F—only a few degrees above absolute zero, the lowest possible temperature—and then exposed it to the lab’s powerful magnetic field while hitting it with infrared light to see what it revealed about the quantum interactions inside the material.
Source: phys.org
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