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Enigmatic Dirac fermions in graphene

2022/08/102022/09/01

Since the discovery of graphene more than 15 years ago, research on graphene-based systems has grown exponentially. Graphene exhibits unique physical properties, for instance, the presence of massless Dirac fermions in a lattice of stronger covalent bonds and frequency-independent optical conductivity, which may help to realize exotic fundamental science and advanced technologies.

So far, graphene has been grown on a multitude of substrates exhibiting interesting properties. In some cases, the graphene layer has minimal link with the substrate. Experiments have revealed enigmatic properties of the Dirac fermions near the band crossing, called Dirac point, at the K point of the Brillouin zone. For example, Angle-Resolved PhotoEmission Spectroscopy (ARPES) data of graphene grown on SiC, shown in Fig. 1a, exhibit large momentum independent intensities near Dirac point as if the top and bottom of the Dirac cone are shifted away from each other. Some studies interpreted these results as a gapped Dirac cone with anomalous in-gap intensities as schematically shown in Fig. 1b. The presence of electron correlation renormalizes the dispersion as shown by red lines. Other proposals involve plasmaron bands where plasmon excitations in addition to photoexcitation of electrons leads to a shifted Dirac cone. The shifted and the pristine Dirac cones appear as a diamond shaped structure around the Dirac point as shown in Fig. 1c.

In order to address this enigmatic scenario, A. Pramanik, S. Thakur and colleagues from India, Italy and Germany performed a detailed polarization dependent ARPES investigation at the BaDElPh beamline at Elettra. Each branch of the Dirac cone was probed selectively using s– and p-polarized synchrotron light. The spectra shown in Fig. 2a,b reveal clearly dispersive bands near the Dirac point.

Read more on the Elettra website

Image: (a) Typical ARPES spectra of graphene on SiC along the ΓKM direction of the Brillouin zone; the origin of the momentum axis is shifted to K point. Schematic of (b) anomalous region and (c) plasmaron scenario around the Dirac point. Red curved lines in (b) show bands in the presence of electron correlation. Red Dirac cone in (c) is due to plasmaron bands.

Weyl fermions discovered in another class of materials

2019/07/122019/08/01

A particular kind of quasi-particle states, the Weyl fermions, were first discovered a few years ago in certain solids. Their specialty: They move through a material in a well ordered manner that practically never lets them collide with each other and is thus very energy efficient. This implies intriguing possibilities for the electronics of the future. Up to now, Weyl fermions had only been found in certain non-magnetic materials. Now however, for the very first time, scientists at the Paul Scherrer Institute PSI have experimentally proven their existence in another type of material: a paramagnet with intrinsic slow magnetic fluctuations. This finding also shows that it is possible to manipulate the Weyl fermions with small magnetic fields. It thus opens further possibilities to use them in spintronics, a promising development in electronics for novel computer technology. The researchers have published their findings in the scientific journal Science Advances.

Amongst the approaches that could pave the way to energy efficient electronics of the future, Weyl fermions could play a role. Found experimentally only inside materials as so-called quasi-particles, they behave like particles which have no mass. Predicted theoretically already in 1929 by the mathematician Hermann Weyl, their experimental discovery by scientists amongst other at PSI only came in 2015. So far, Weyl fermions had only been observed in certain non-magnetic materials. Now however, a team of scientists at PSI together with researchers in the USA, China, Germany and Austria also found them in a specific paramagnetic material. This discovery could bring a potential usage of Weyl fermions in future computer technology one step closer.

Image: The three PSI researchers Junzhang Ma, Ming Shi and Jasmin Jandke (from left to right) at the Swiss Light Source SLS, where they succeeded in proving the existence of Weyl fermions in paramagnetic material.
Credit: Paul Scherrer Institute/Markus Fischer

New material also reveals new quasiparticles

2019/05/072019/05/07

Researchers at PSI have investigated a novel crystalline material that exhibits electronic properties that have never been seen before.

It is a crystal of aluminum and platinum atoms arranged in a special way. In the symmetrically repeating unit cells of this crystal, individual atoms were offset from each other in such a way that they – as connected in the mind’s eye – followed the shape of a spiral staircase. This resulted in novel properties of electronic behaviour for the crystal as a whole, including so-called Rarita-Schwinger fermions in its interior and very long and quadruple topological Fermi arcs on its surface. The researchers have now published their results in the journal Nature Physics.

Researchers at the Paul Scherrer Institute PSI have found a new kind of quasiparticle. Quasiparticles are states in material that behave in a certain way like actual elementary particles. The two physicists William Rarita and Julian Schwinger had predicted this type of quasiparticles in 1941, which came to be known as Rarita-Schwinger fermions. Exactly these have now been detected experimentally for the first time – thanks in part to measurements at the Swiss Synchrotron Light Source SLS at PSI. “As far as we know, we are – simultaneously with three other research groups – among the first to see Rarita-Schwinger fermions”, says Niels Schröter, a researcher at PSI and first author of the new study.

Image: Niels Schröter (left) and Vladimir Strocov at their experimental station in the Swiss Light Source SLS at PSI.
Credit: Paul Scherrer Institute/Mahir Dzambegovic