Highly exotic Particle that only has mass when moving in one direction observed for first time, the semi-Dirac fermion

Amazing stuff! What lurks behind!

"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 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. ...

Using the high-powered magnet in Florida, the researchers observed that the energy of the Landau level transitions in the ZrSiS crystal followed a completely different pattern of dependence on the magnetic field strength. Years ago, theorists had labeled this pattern the "B2/3 power law," the key signature of semi-Dirac fermions. ..."

From the abstract:

"Topological semimetals with massless Dirac and Weyl fermions represent the forefront of quantum materials research. In two dimensions, a peculiar class of fermions that are massless in one direction and massive in the perpendicular direction was predicted 16 years ago. These highly exotic quasiparticles—the semi-Dirac fermions—ignited intense theoretical and experimental interest but remain undetected. Using magneto-optical spectroscopy, we demonstrate the defining feature of semi-Dirac fermions—𝐵2/3 scaling of Landau levels—in a prototypical nodal-line metal ZrSiS. In topological metals, including ZrSiS, nodal lines extend the band degeneracies from isolated points to lines, loops, or even chains in the momentum space. With ab initio calculations and theoretical modeling, we pinpoint the observed semi-Dirac spectrum to the crossing points of nodal lines in ZrSiS. Crossing nodal lines exhibit a continuum absorption spectrum but with singularities that scale as 𝐵2/3 at the crossing. Our work sheds light on the hidden quasiparticles emerging from the intricate topology of crossing nodal lines and highlights the potential to explore quantum geometry with linear optical responses."

Particle that only has mass when moving in one direction observed for first time

Particle that only has mass when moving in one direction observed for first time (original news release)

Semi-Dirac Fermions in a Topological Metal (open access)

Fig. 7.

Comparison of the power law of Landau level transitions for different fermions in a log-log scale plot. Power-law fitting (purple dashed line) of the interband Landau level transitions (purple dots) associated with the semi-Dirac fermions in ZrSiS. Purple-shaded area indicates the 3σ confidence interval. Orange and black lines show the power-law scaling of LLs for Dirac (β=0.5) and massive fermions (β=1), respectively.