Group news

Speaker: Bernd Gotsman and Federcio Balduini , IBM Zürich, Switzerland
Time: 14:00pm, 13th Oct.
Place: Seminar room IV(CFEL)
Weyl semimetals (WSM) are characterized by exceptional transport properties, including extremely high mobility, magnetoresistance, and electrical conductivity, all of which hold great promise for technological applications.  However, it is still unclear whether the extreme properties of WSM can be primarily attributed to the topological and chiral nature of the Weyl fermions they host.
To address this question and gain deeper insights into Weyl fermions' transport properties and Fermi surface characteristics, we employ a combined approach of transverse electron focusing (TEF) and Shubnikov de Haas (SdH) experiments conducted on microstructured single-crystals of the WSM NbP. TEF and SdH allow differentiating charged quasiparticles with distinct momenta and Fermi surface area, respectively, thereby enabling focused study on the Fermi surface of Weyl fermions. Moreover, the combination of TEF and SdH allow extracting carrier density, type, mass and mobility giving valuable insights into Weyl fermions transport properties. Our findings suggest that the extreme properties of NbP originates from bulk, achiral and relativistic electrons. Looking forward, thermal transport experiments hold the potential to pave the way for a more comprehensive understanding of the intriguing properties of Weyl semimetals.

Speaker: Raquel Queiroz, Columbia University, USA
Time: 14:00pm, 18th Oct.
Place: Seminar room III(CFEL)
Quantum geometry quantifies the momentum space textures of the Bloch wavefunctions and impacts substantially the physics of multiband systems. This is also true for van der Waals semiconductors, which are usually described within a parabolic, effective mass, approximation. In this talk, I will start by a general discussion on quantum geometry, discuss examples of physical responses associated with geometry in insulators and finally discuss how quantum geometry can be invaluable to observe exotic correlated phenomena in transition metal dichalcogenides heterostructures, such as the recent observation of fractional excitations.

Speaker: Thomas Scaffidi, UC Irvine, USA
Time: 11:30am, 19th Oct.
Place: Seminar room IV(CFEL)
What is the ultimate limit of conductance of a metallic device of lateral size W? In the ballistic limit, the answer is the Landauer-Sharvin conductance, which is associated with an abrupt reduction of the number of conducting channels when going from the contacts to the device. However, the ballistic limit is not always the best-case scenario, since adding strong electron-electron scattering can take electrons to a viscous regime of transport for which "super-ballistic" flows were recently studied. In this talk, we will show that by a proper choice of geometry which resembles a "wormhole", it is possible to spread the Landauer-Sharvin resistance throughout the bulk of the system, allowing its complete elimination by electron hydrodynamics. This effect arises due to the interplay between geometry and strong electron-electron scattering, which allows for a net transfer of carriers from reflected to transmitted channels. Finally, we will discuss a recent experiment in a Corbino geometry which realizes one half of this "wormhole" geometry.