**01**MarMaterial Science and Engineering Special SeminarGeometrical Description of the Orbital Magnetic Susceptibility

PRESENTER Prof. Frédéric PiéchonINSTITUTE Laboratoire de Physique des Solides, Orsay CNRS, Universit. Paris-Sud, FranceDATE Thursday, March 01, 2018TIME 10:00 AM - 11:00 AMLOCATION Ibn Sina Building (Bldg. 3) Room 5220

**ABSTRACT:** The orbital magnetic susceptibility measures the equilibrium response of a spinless electronic system to an external magnetic field B. For a single band model, the orbital susceptibility is given by the Landau-Peierls (LP) formula and is entirely determined by the energy spectrum. In particular, it is diamagnetic at parabolic band edges but appears strongly paramagnetic at a Van-Hove singularity. For multi-band systems we have recently developed a general formalism that goes beyond the LP formula. This formalism allows the study of the possible strong inter-band effects that are encoded in the geometric structure of Bloch states. In particular, for two-band models, it appears that the inter-band susceptibility is composed of essentially two distinct contributions. The first contribution is entirely determined by the Berry curvature. It is paramagnetic inside the bands and exhibits a diamagnetic plateau in a gap separating two bands. The second contribution depends on the quantum metric tensor defining the distance between Bloch states. Interestingly, this second contribution always exists even for systems with a vanishing Berry curvature. It is shown that it can be tuned to exhibits either a diamagnetic or a paramagnetic plateau in a gap. Finally it is shown that these two interband geometric contributions may lead to an important paramagnetic contribution for a flat band.

**BIOGRAPHY:** Frédéric Piéchon studied at the university Paris-Sud Orsay and obtained a Phd in Physics in 1995. During his Phd he studied electronic properties of quasicrystals under the supervision of Anuradha Jagannathan at the Laboratoire de Physique des Solides d'Orsay. After a first postdoc at the University of Stuttgart in the group of Prof. H-R. Trebin and a second postdoc at the Max Planck Institut Göttingen in the group of T. Geisel, he obtained a CNRS researcher full position in 1998 at the Laboratoire de Physique des Solides Orsay. He has then worked successively on Luttinger liquids in multiterminal geometry, Boltzmann theory of spin transfer torque and Boltzmann transport in multilayer systems in the Knudsen regime. From 2008 he started working on Graphene and other 2D multiband systems exhibiting Dirac or Weyl points. In particular together with G. Montambaux, J-N Fuchs and M-O Goerbig, they provide the two distinct minimal low energy models that describe the merging or emerging of two Dirac points with either opposite or equal chiralities. Since 2014, he is studying the orbital magnetism and magneto-transport properties of multiband systems in order to understand and to exhibit the importance of interband effects encoded in both the Berry curvature and quantum metric tensors.

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CONTACT Mazen E. Mero