Rebel Electrons on the Quantum One-Way Highway
MSTF Media reports:
By taking a look at the world around us, we will notice that, at first glance, the periodic table of elements seems to be the most thorough approach to classify the matter around us. This table forms the foundation of chemistry and our understanding of matter. Upon closer inspection, however, we will realize that this is only the surface of physical reality. The periodic table is, ultimately, made up of atoms and the bonds between them; but what are the atoms themselves made of?
To answer this question, we need to refer to a more fundamental model of the structure of the universe: the Standard Model of elementary particles. In the Standard Model, matter is not only made up of electrons, protons, and neutrons, but also of more fundamental structures called quarks and leptons. These particles -classified into three generations- are the foundation of everything we see in the universe, from massive galaxies to the smallest living organisms.
If we examine this model through a more fundamental perspective, we find that all the particles we know fall into two main categories: fermions and bosons. Bosons are the carriers of force, like the photon that carries the electromagnetic force and the gluon that holds protons and neutrons together. The category that is important for the structure of matter is fermions. Fermions are particles that make up the fundamental structure of matter. The quarks that make up protons and neutrons, and the electrons that form atomic orbitals, are all fermions.
The unique property of fermions -which distinguishes them from bosons- is the Pauli exclusion principle. According to this principle, two fermions cannot be in the same quantum state. This simple law is the main reason for the layered structure of electrons in atoms, the formation of solids, and ultimately, the formation of the universe as we know it. If the Pauli exclusion principle did not exist, all electrons within atoms would be arranged in a single energy level, and there would be no structures resembling solids, molecules, or life.
Among the different types of fermions, there is a special type called Weyl fermions that are not only important in the Standard Model, but also play a special role in the world of condensed matter physics. Initially proposed as massless particles in fundamental theories, these particles appear as quasiparticles in the world of quantum matter, exhibiting strange properties such as Fermi arcs, chiral currents, and peculiar magnetic effects.
The discovery and study of Weyl fermions have allowed us to explore particle physics on a scale beyond particle accelerators and in solid environments. This discovery builds a bridge between high-energy physics and condensed matter physics and could help develop new technologies such as quantum computation.
Weyl fermions are particles described by the Weyl equation. This equation is the massless version of the Dirac equation. The key feature of this equation is its chiral nature; that is, the Weyl fermions are either left-handed or right-handed, and the two categories remain independent of each other unless a mass is defined for them. Another unique feature of Weyl semimetals is the presence of Weyl nodes in momentum space. These points are locations where energy bands (including the valence and conduction bands) meet and act as topological monopoles in Berry curvature space. As a result, these materials have Fermi curves on their surface, which is one of their experimental signatures.
One of the most important physical effects observed in Weyl fermions is chiral anomaly. First predicted in quantum field theory, this phenomenon signifies that the number of left-handed and right-handed fermions, if in the presence of parallel electric and magnetic fields, is no longer conserved. This leads to an observable phenomenon in Weyl semimetals: their electrical resistance decreases in the presence of a magnetic field, unlike ordinary materials, whose resistance increases under these conditions. This negative magnetoresistance was one of the key experiments to confirm the existence of Weyl fermions in solids. This phenomenon was first observed in the Weyl semimetals tantalum arsenide (TaAs) and niobium arsenide (NbAs) and was used as one of the key experiments to confirm the existence of Weyl fermions.
The Discovery of Weyl Semimetals: Rogue Fermions Escape the Prison of Physics
In a world where physics ruled, there was an unwritten law for particles: Either be matter or force! All particles in the universe followed this law. Electrons, protons, and neutrons were held together in a regular pattern, and photons, gluons, and other bosons served as force carriers.
But in a corner of this universe, there were Weyl fermions that were particles that followed no laws. They neither submitted to the constraints of mass, nor did they fit into the conventional structures of matter. They were weightless and chiral beings (meaning they only ran in one direction and had no way back!). This was the reason why scientists always thought that these strange fermions only lived in the world of elementary particles and in massive accelerators.)
One day, Zahid Hasan and his colleagues decided to put an end to this mystery. Could these rebellious fermions be found in the real world, in the world of matter?
They studied a specific matter called tantalum arsenide using a special technique called Angle-Resolved Photoemission Spectroscopy (ARPES). If Weyl metalloids really existed, there should have been signs of them in this matter. By shining light on the surface of the matter and assessing the behavior of the electrons, something unexpected happened that shook the world of physics: Weyl fermions, free and unbound, were moving in this matter!
They moved through space like one-way passengers on a quantum highway, without any obstacles, creating strange curved paths called Fermi arcs on the surface of the matter, and unlike all known matter, in a magnetic field, their resistance decreased instead of increasing!
It was as if these rebellious fermions had finally escaped the theoretical prison of physics and carved out a new realm for themselves in the world of matter.
Linking Particle Physics and Condensed Matter
The discovery of Weyl's semimetals established a connection between particle physics and quantum materials. As mentioned, in 2015, Zahid Hasan and his colleagues at Princeton University, were able to identify Weyl semimetals as a real phenomenon in solids. In fact, before that, physicists only referred to Weyl fermions as theoretical entities that existed in the world of fundamental particles. This success was not only a major scientific breakthrough, but also demonstrated that even complex theories of high-energy physics can pave the way for real-world innovations. The discovery of Weyl semimetals is a new chapter in the history of physics, in which particle and condensed matter physics not only came closer together, but also became intertwined.
