Group news

Speaker: Shan Jiang, Fondation du Collège de France, France
Time: Tuesday, April 30^th at 14:00 PM.
Place: Seminar Room 136
When a system is forced out of equilibrium, it will tend to recover to the equilibrium state. This is the case in transport phenomena. In solids, the potential difference leads the electron to move from the high electrical potential side to the low side, and the temperature difference will lead to heat flow from the hotter to the colder parts. These two phenomena are called electrical and thermal transport. In condensed matter physics, the electrical and thermal transport in solids usually participated by electrons and phonons can be described by the Boltzmann transport equation in a semi-classic picture. Nevertheless, the interplay of both types of quasiparticles can give rise to emergent properties such as superconductivity or phonon drag effect that is worth studying. This is the case in quantum paraelectric strontium titanate (SrTiO3) and its sister Europium titanate (EuTiO₃) with magnetism that have attracted much attention recently. Here, our focus on transport experiment on SrTiO3 and EuTiO3 yielded three significant findings. Firstly, we observed the thermal Hall effect in SrTiO₃, which exhibits sensitivity to atomic substitutions (such as Ca doping or O vacancies), hinting at connections between the thermal Hall effect with ferroelectric instability and electron-phonon coupling. Secondly, we investigated the electronic thermal conductivity in dilute metal SrTi_1-xNb_xO₃. Using magnetic field method, we succeeded in extracting the electronic thermal conductivity from the phonon-dominant thermal conductivity in SrTi_1-xNb_xO₃. It displays a Fermi liquid behavior with quadratic temperature dependent thermal resistivity. Such behavior of electron momentum loss in dilute system can’t be explained by the well-known Umklapp theory. Thirdly, we discovered a glass-like thermal conductivity in EuTiO₃. It indicates a potential correlation with the Eu atom valence fluctuation and phonon-magnon coupling. I will make a detailed presentation of these results.
 

Speaker: Rares Dragomir, University of Manchester, UK
Time: April 23rd at 10:30 am.
Place: Seminar Room 136
This presentation will discuss my ongoing Master’s project on graphene spintronics, which is supervised by Dr Ivan Vera-Marun. The aim is to characterise the charge and spin transport properties of our lateral spin valve device, which consists of monolayer graphene encapsulated in hexagonal boron nitride. Spin injection (detection) into (from) the graphene channel is achieved via 1D ferromagnetic edge contacts. Their non-invasive nature minimises the addition of impurities into the channel (field-effect mobility exceeding 80,000 cm2s-1V-1 at room temperature), which avoids the pinning of the Fermi level for distances longer than ~100 nm. Furthermore, the oblique geometry of the contacts allows for the study spin anisotropy, thus furthering our understanding of spin relaxation mechanisms in graphene, as well as presenting the opportunity to manipulate spin injection and detection during the nanofabrication process. The presentation will cover the measurements undertaken so far (spin valve, Hanle etc.), the preliminary results, along with the challenges encountered in the attempt to observe and quantify spin transport at room temperature.

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.