Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session E10: Unconventional Transport Phenomena in Three-Dimensional Dirac and Weyl SemimetalsInvited Live
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Sponsoring Units: DCMP Chair: Joel Moore, University of California, Berkeley |
Tuesday, March 16, 2021 8:00AM - 8:36AM Live |
E10.00001: Photocurrent generation probes bulk and surface responses chiral Weyl semimetals Invited Speaker: Joseph Orenstein Weyl semimetals (WSMs)s are characterized by isolated band crossings which act as monopoles of the Berry curvature field. Inversion breaking WSMs exhibit response functions that are forbidden in systems that possess a center of symmetry. We have investigated an important example of such responses: the 2nd nonlinear conductivity, defined by Ji.=σijkEjEk. For monochromatic electric fields the nonlinear response generates currents whose direction is dependent on the polarization state of the field, giving rise to photogalvanic effects (PGEs). In this talk I present results on PGEs in response to linear and circular polarized light (LPGE and CPGE, respectively). RhSi is an ideal candidate for such a study, as symmetry predicts a simple structure of σijk, in which the only nonvanishing tensor elements are permutations of xyz. Furthermore, RhSi is structurally chiral, and the absence of mirror planes breaks the degeneracy of Weyl nodes of opposite Berry charge. I will describe the PGEs for light incident on 111 and 001 surfaces. On 111 we observe a CPGE current whose direction is parallel to the wavevector of the light and whose spectrum is consistent with photoexcitation across a Weyl cone. More surprising is that we observe in-plane CPGE and LPGE currents at normal incidence on the 001 surface, where the bulk point group predicts a null effect. I discuss the possibility that the observed current becomes allowed by the breaking of nonsymmorphic (screw) symmetry by the surface and is related to the response function of the surface bands that give rise to Fermi arcs. |
Tuesday, March 16, 2021 8:36AM - 9:12AM Live |
E10.00002: The magnetothermoelectric properties of three-dimensional Dirac/Weyl semimetals Invited Speaker: Brian Skinner The three-dimensional Dirac and Weyl semimetals can exhibit thermoelectric properties that are not possible in conventional metals and semiconductors. In this talk I give an overview of some of these properties, focusing in particular on the enhancement of thermopower by magnetic field. In single-band Dirac/Weyl materials, the thermopower rises linearly with magnetic field strength once the field is strong enough to achieve the extreme quantum limit, in which only the lowest Landau level is occupied. This enhancement can be viewed as the product of a "quantized" plateau in the thermoelectric Hall conductivity. In compensated systems, for which a pocket of electron-type carriers is almost completely compensated by a pocket of hole-type carriers, the field enhancement of thermopower is even sharper, growing quadratically with magnetic field beginning at a much lower value of field strength. I discuss these ideas in light of recent experiments on the three-dimensional Dirac semimetal ZrTe5 and the compensated Weyl semimetal TaP. |
Tuesday, March 16, 2021 9:12AM - 9:48AM Live |
E10.00003: The surprising role for phonons in Topological Semimetals Invited Speaker: Kenneth Burch Topological semimetals display a range of novel transport phenomena, including enormous magnetoresistance and mobilities. Observations of such behavior in topological systems suggested the quantum geometry of the electronic bands was a crucial ingredient. However similar behavior has been observed in trivial topological materials. Here I will discuss our recent experiments and first-principles calculations the provide strong evidence for the key role of phonons. In particular, we find the phonon-electron scattering far exceeds the phonon-phonon, suggesting phonons enhance the momentum of the carriers. In addition, I will outline the key ingredients in making this happen. |
Tuesday, March 16, 2021 9:48AM - 10:24AM Live |
E10.00004: New thermoelectric transport from the scale anomaly in Dirac/Weyl materials Invited Speaker: Maria Vozmediano The low energy effective description of Weyl and Dirac semimetals as massless Dirac fermions has given rise to new interpretations of thermal and thermo-electric transport phenomena in these materials, associated to quantum field theory predictions. In particular, quantum anomalies - most prominently the chiral anomaly - have provided a novel theoretical frame for the understanding of new magneto transport features. More recently thermal transport has taken the lead in relation with the gravitational anomaly. In this talk we will describe novel response functions associated to the scale anomaly in Dirac and Weyl semimetals [2]. I will try to provide the foundations and the results in a pedagogical way. |
Tuesday, March 16, 2021 10:24AM - 11:00AM Live |
E10.00005: Weyl fermions and fragile topology in acoustic meta-materials Invited Speaker: Sebastian Huber Topological band theory provides us with a powerful design-principle for metamaterials that control how waves propagate in space. These metamaterials are ideal to investigate novel phenomena difficult to observe in other platforms. In particular, I will present how artificial gauge fields lead to the appearance of chiral Landau levels in neutral Weyl systems and crystalline symmetries to novel fragile topological states in an acoustic super-structure. |
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