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[EN] We generalize the notion of a dissipationless, topological Hall viscosity tensor to optical phonons in thin-film Weyl semimetals. By using the strained porphyrin thin-film Weyl semimetal as a model example, we show how optical phonons can couple to Weyl electrons as chiral pseudogauge fields. These chiral vector fields lead to a novel dissipationless two-rank viscosity tensor in the effective dynamics of optical phonons whose origin is the chiral anomaly. We also compute the contribution to this two-rank Hall viscosity tensor due to the presence of an external magnetic field whose origin is the conventional Hall response of Weyl electrons. Finally, the phonon dispersion relations of the system at the long-wavelength limit with and without an electromagnetic field are calculated, showing a measurable shift in the Raman response of the system. Our results can be investigated by Raman scattering or infrared spectroscopy by attenuated total reflectance experiments. ; We thank M. Vozmediano and K. Landsteiner for very useful discussions. This work is supported by the Iran Science Elites Federation. A.C. acknowledges financial support through MINECO/AEI/FEDER, UE Grant No. FIS2015- 73454-JIN, and European Union structural funds and the Comunidad Autonoma de Madrid (CAM) NMAT2D-CM Program (S2018-NMT-4511).

The application of pressure has been demonstrated to induce intriguing phase transitions in topological nodal-line semimetals. In this work we analyze how uniaxial pressure afects the topological character of BaSn2, a Dirac nodal-line semimetal in the absence of spin-orbit coupling. Using calculations based on the density functional theory and a model tight-binding Hamiltonian, we fnd the emergence of a second nodal line for pressures higher than 4 GPa. We examine the topological features of both phases demonstrating that a nontrivial character is present in both of them. Thus, providing evidence of a topological-to-topological phase transition in which the number of topological nodal lines increases. The orbital overlap increase between Ba dxz and dyz orbitals and Sn pz orbitals and the preservation of crystal symmetries are found to be responsible for the advent of this transition. Furthermore, we pave the way to experimentally test this kind of transition by obtaining a topological relation between the zero-energy modes that arise in each phase when a magnetic feld is applied ; This work is supported by the MINECO of Spain through the project PGC2018-101334-B-C21. A.O.F. thanks MECD for the financial support received through the FPU grant FPU16/02572. A.C. acknowledges financial support through MINECO/AEI/FEDER, UE Grant No. FIS2015-73454- JIN and European Union structural funds, the Comunidad Autónoma de Madrid (CAM) NMAT2D-CM Program (S2018-NMT-4511), and the Ramón y Cajal program through the grant RYC2018-023938-I ; SI

The nondissipative (Hall) viscosity is known to play an interesting role in two-dimensional (2D) topological states of matter, in the hydrodynamic regime of correlated materials, and in classical active fluids with broken time-reversal symmetry (TRS). However, generalizations of these effects to 3D have remained elusive. In this work, we address this question by studying the Hall viscoelastic response of 3D crystals. We show that for systems with tetrahedral symmetries, there exist new, intrinsically 3D Hall viscosity coefficients that cannot be obtained via a reduction to a quasi-2D system. To study these coefficients, we specialize to a theoretically and experimentally motivated tight-binding model for a chiral magnetic metal in (magnetic) space group [(M)SG] P213 (No. 198.9), a nonpolar group of recent experimental interest that hosts both chiral magnets and topological semimetals (TSMs). Using the Kubo formula for viscosity, we compute two forms of the Hall viscosity, phonon and "momentum" (conventional) and show that for the tight-binding model we consider, both forms realize the novel cubic Hall viscosity. We conclude by discussing the implication of our results for transport in 2D magnetic metals and discuss some candidate materials in which these effects may be observed. ; P.R. and B.B. acknowledge support from the Alfred P. Sloan Foundation, and the National Science foundation under Grant No. DMR-1945058. M.G.V. and I.R. acknowledge the Spanish Ministerio de Ciencia e Innovacion (Grant No. PID2019-109905GB-C21). A.C. acknowledges financial support through European Union structural funds, the Comunidad Autonoma de Madrid (CAM) NMAT2DCM Program (S2018-NMT-4511), and the Ramon y Cajal program through the Grant No. RYC2018-023938. A.B. acknowledges financial support from the Spanish Ministry of Science and Innovation (PID2019-105488GB-I00). The work of J.L.M. has been supported by Spanish Science Ministry Grant No. PGC2018-094626-B-C21 (MCIU/AEI/FEDER, EU) and Basque Government Grant No. IT979-16. This material is based on work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-1746047. ; Peer reviewed