Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session B43: Nonlinear Optics in Topological SemimetalsInvited
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Sponsoring Units: DCMP Chair: Ying Ran, Boston Coll Room: BCEC 210B |
Monday, March 4, 2019 11:15AM - 11:51AM |
B43.00001: Origins of strong and/or quantized nonlinear optical responses in Weyl semimetals Invited Speaker: Joel Moore Weyl semimetals and other topological materials can support unique electromagnetic responses different from ordinary matter. This talk starts with a brief review of the theory of linear and nonlinear optics in crystals, including recent work on how geometric properties of Bloch states such as the Berry curvature can lead to simplified expressions and sum rules. Previous results show that in Weyl semimetals, nonlinear optics is likely to be a more fruitful ground for novel behavior than linear response. In addition to the Weyl semimetal TaAs whose nonlinear optical properties are the subject of intense study, we explain how other Weyl materials lacking mirror symmetry (and certain related compounds) should have an approximately quantized circular photogalvanic effect (CPGE) and discuss the status of experimental searches for this effect in various materials. |
Monday, March 4, 2019 11:51AM - 12:27PM |
B43.00002: Colossal Bulk Photovoltaic Effect in a Weyl Semimetal Invited Speaker: Gavin B Osterhoudt The bulk photovoltaic effect (BPVE) is a second-order non-linear optical effect which directly converts light into an electrical current. This effect is intimately related to the topological nature of the material, and is in fact directly proportional to the Berry connection. We present photocurrent measurements on the Weyl semimetal TaAs, where the diverging Berry connection leads to the largest observed BPVE, as well as the first observation of BPVE utilizing mid-infrared radiation. |
Monday, March 4, 2019 12:27PM - 1:03PM |
B43.00003: Berry Curvature Enhanced Nonlinear Photogalvanic Response of Type-II Weyl Cone Invited Speaker: Dong Sun In this talk, signatures of the singular topology in a type-II Weyl semimetal TaIrTe4 is revealed in the photo responses, which are shown to be directly related to the divergence of Berry curvature. As a result of the divergence of Berry curvature at the Weyl nodes, TaIrTe4 exhibits unusually large photo responsivity of 130.2 mA/W with 4-mm excitation in an unbiased field effect transistor at room temperature arising from the third-order nonlinear optical response. Furthermore, the circularly polarized galvanic response is also enhanced at 4-mm, possibly due to the same Berry curvature singularity enhancement with the shift current. Considering the optical selection rule of Weyl cones with opposite chirality, it may open new experimental possibilities for studying and controlling the chiral polarization of Weyl Fermions through an in-plane DC electric field in addition to the optical helicities. |
Monday, March 4, 2019 1:03PM - 1:39PM |
B43.00004: Nonlinear transport and optical responses in atomically thin WTe2 Invited Speaker: Qiong Ma Many exotic quantum phenomena of today’s forefront materials arise from the interplay among symmetry, topology, quantum geometry and correlations. Therefore, their detection and characterization require one to probe multiple aspects of the materials. We demonstrate nonlinear electrical transport and infrared optoelectronic measurements as symmetry sensitive probes of the low energy electron states in novel metals/semimetals. Using monolayer and bilayer WTe2 as examples, I will show how nonlinear electrical transport and infrared photocurrent can reveal the Berry curvature properties in a highly symmetry sensitive way. In particular, the nonlinear electrical transport in bilayer WTe2 uncovers a new type of Hall effect, the nonlinear Hall effect. Interestingly, this is an electrical Hall effect in a nonmagnetic material and in the absence of external magnetic field. Coupled with the gate tunability of 2D materials, we demonstrate that such nonlinear Hall effect provides a powerful tool to detect the Berry curvature of nonmagnetic quantum materials in an energy-resolved way. |
Monday, March 4, 2019 1:39PM - 2:15PM |
B43.00005: Fragile Topological Bands With Stable Index at Half-Filling inTwisted Bilayer Graphene Invited Speaker: B Andrei Bernevig Using an approximate but crucial particle-hole symmetry, we prove that half-filled graphene has bands that exhibit Wannier (Wilson loop) flow, and hence have to be topological. While this Wannier flow can be interrupted by the addition of certain bands, a topological index remains stable, and characterizes the set of bands. We then show that electron-phonon interaction in this set of bands can cause large superconducting temperatures and predict a certain set of angles where superconductivity should be observed. |
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