Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session S44: Dirac and Weyl Semimetals: Theory IVFocus
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Sponsoring Units: DMP Chair: Filip Ronning, Los Alamos National Laboratory Room: 391 |
Thursday, March 16, 2017 11:15AM - 11:51AM |
S44.00001: Hydrodynamics of the Dirac fluid in graphene Invited Speaker: Andrew Lucas Recent advances in materials physics have allowed us to observe hydrodynamic electron flow in multiple materials. A uniquely interesting possibility is the emergence of a quasi-relativistic plasma of electrons and holes appearing in Dirac semimetals such as graphene. I will briefly review the unique features of the hydrodynamics of the Dirac fluid, and then discuss the theroetical signatures for the Dirac fluid, and its observation in experiment. [Preview Abstract] |
Thursday, March 16, 2017 11:51AM - 12:03PM |
S44.00002: Anomalous hydrodynamics of Weyl materials Gustavo Monteiro, Alexander Abanov Kinetic theory is a useful tool to study transport in Weyl materials when the band-touching points are hidden inside a Fermi surface. It accounts, for example, for the negative magnetoresistance caused by the chiral magnetic effect and quantum oscillations (SdH effect) in the magnetoresistance together within the same framework [1]. As an alternative approach to kinetic theory we also consider the regime of strong interactions where hydrodynamics can be applicable. A variational principle of these hydrodynamic equations can be found in [2] and provide a natural framework to study hydrodynamic surface modes which correspond to the strongly-interacting physics signature of Fermi arcs. [1] G. Monteiro, A. Abanov and D. Kharzeev, Phys. Rev. B 92, 165109 (2015) [2] G. Monteiro, A. G. Abanov and V. P. Nair, Phys. Rev. D 91, 125033 (2015) [Preview Abstract] |
Thursday, March 16, 2017 12:03PM - 12:15PM |
S44.00003: Chiral anomaly in disordered Weyl semimetals Junhyun Lee, J. H. Pixley, Jay D. Sau We consider the effect of disorder on the chiral anomaly in Weyl semimetals. The chiral anomaly is robust under small perturbations and thus will not be affected by a small to moderate disorder strength. In fact, we argue that the chiral anomaly can prevent Anderson localization even for strong disorder ($k_{F} l_{MFP} \ll 1$, where conventional Anderson localization occurs) as long as the disorder does not couple the Weyl nodes. Here we study the competition between the chiral anomaly and the tendency of Anderson localization in disordered Weyl semimetals. [Preview Abstract] |
Thursday, March 16, 2017 12:15PM - 12:27PM |
S44.00004: Weyl metamaterials – relativistic fermions in designer curved space Alex Weststr\"{o}m, Teemu Ojanen In Weyl semimetals, the conduction and valence bands touch in momentum space at an even number of points, around which the bands locally form cones. The position of the nodes, and the tilts and the opening angles of the cones depend on the (inversion or time reversal) symmetry-breaking fields present in the system. We show that for symmetry-breaking fields smoothly varying in space, an effective curved spacetime emerges. We derive expressions for the frame fields describing this spacetime as well as an effective curved-space Weyl equation. We further show that in the semiclassical limit, these equations describe particles moving along geodesics of the metric corresponding to the effective frame fields. [Preview Abstract] |
Thursday, March 16, 2017 12:27PM - 12:39PM |
S44.00005: Emergence of topological semimetals in gap closing in semiconductors without inversion symmetry Shuichi Murakami, Motoaki Hirayama, Ryo Okugawa, Takashi Miyake In this presentation, we show emergence of topological semimetals in gap closing of any inversion-asymmetric insulators. Namely, we begin with any inversion-asymmetric insulators, and close a gap by changing a parameter in the system; we then show that the system becomes either (i) a Weyl semimetal phase or (ii) a nodal-line semimetal, both are among topological semimetals. In particular, no insulator-to-insulator transition happens, in strong contrast with inversion-symmetric systems. This result also has implications for Z$_2$ topological number. In a transition between different Z$_2$ topological phases, a Weyl semimetal phase necessarily appears when inversion symmetry is broken, for materials with any space groups [1,2]. Our theory is applicable to many materials, for example to tellurium (Te) [3]. Tellurium has a unique lattice structure, consisting of helical chains, and therefore lacks inversion and mirror symmetries. At high pressure the band gap of Te decreases and finally it runs into a Weyl semimetal phase, as confirmed by our ab initio calculation. [1] S. Murakami, New J. Phys. 9, 356 (2007). [2] S. Murakami, M. Hirayama, R. Okugawa, T. Miyake, arXiv:1610.07132. [3] M. Hirayama, R. Okugawa, S. Ishibashi, S. Murakami, T. Miyake, Phys. Rev. Lett. 114, 206401 (2015). [Preview Abstract] |
Thursday, March 16, 2017 12:39PM - 12:51PM |
S44.00006: Probing unconventional superconductivity in inversion-symmetric doped Weyl semimetal Matthew Gilbert, Moon jip Park, Youngseok Kim Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase and nodal BCS states are known to be possible superconducting ground states in inversion symmetric doped Weyl semimetals (WSM). In order to resolve the two distinct pairing states, we propose two separate four terminal transport experiments in which each potential pairing exhibits a unique transport signature. We begin by considering a Josephson junction that consists of a doped WSM and a normal BCS superconductor. Under the application of a transverse uniform current in the BCS superconductor, which resonates with the momentum carried by FFLO states in doped WSM, we find that the Josephson current is largely enhanced resulting in a peak of the current amplitude that distinguishes from conventional Josephson junction. In the case of nodal BCS states, we find that the nodal points may be shifted in the Brillouin zone by an application of the transverse uniform current. We analyze the topological phase transitions induced by nodal pair annihilation in non-equilibrium and find a characteristic decrease in the density of states that serves as a signature of the quantum critical point in the topological phase transition, thereby identifying nodal BCS states in doped WSM. Reference: Youngseok Kim, Moon Jip Park, and Matthew J. Gilbert Phys. Rev. B 93, 214511 [Preview Abstract] |
Thursday, March 16, 2017 12:51PM - 1:03PM |
S44.00007: Meissner effect and Hall viscosity in Weyl superconductors Xu Dou, Bruno Uchoa The electromagnetic response reveals important properties of topological materials, such as the anomalous Hall effect of Weyl semimetals and superconductors. The response to the lattice deformation provides another way to characterize a topological nontrivial material, and the features of this response are effectively captured by the dissipationless Hall viscosity. We discuss the Meissner response of Weyl superconductors that break time reversal symmetry and derive their Hall viscosity due to elastic lattice deformations. We address possible experimental signatures for those observables in the superconducting state. [Preview Abstract] |
Thursday, March 16, 2017 1:03PM - 1:15PM |
S44.00008: Cascade of charge density waves in type-II Weyl materials and magnetotransport Maximilian Trescher, Emil J. Bergholtz, Masafumi Udagawa, Johannes Knolle Following the recent theoretical prediction of type-II Weyl semimetals, they have been experimentally observed in a number of different materials. The same materials are also known to possess intriguing magnetotransport properties, but whether or not these are related to the type-II Weyl properties remains unclear and no satisfactory explanation exists. Here, we investigate the effect of interactions in a simple model of a type-II Weyl semimetal in a strong magnetic field. We identify a novel charge density wave (CDW) instability even for weak interactions stemming from the emergent nesting properties of the type-II Weyl Landau Level dispersion. We map out the dependence of this CDW on magnetic field strength, tilt angle and chemical potential. Surprisingly, as a function of decreasing temperature a cascade of CDW transitions emerges. It is tempting to speculate about a connection with the unsaturated magnetoresistance as recently observed in WTe_2. [Preview Abstract] |
Thursday, March 16, 2017 1:15PM - 1:27PM |
S44.00009: Charge-induced spin torque in Weyl semimetals Daichi Kurebayashi, Kentaro Nomura In this work, we present phenomenological and microscopic derivations of spin torques in magnetically doped Weyl semimetals. As a result, we obtain the analytical expression of the spin torque generated, without a flowing current, when the chemical potential is modulated. We also find that this spin torque is a direct consequence of the chiral anomaly. Therefore, observing this spin torque in magnetic Weyl semimetals might be an experimental evidence of the chiral anomaly. This spin torque has also a great advantage in application. In contrast to conventional current-induced spin torques such as the spin-transfer torques, this spin torque does not accompany a constant current flow. Thus, devices using this operating principle is free from the Joule heating and possibly have higher efficiency than devices using conventional current-induced spin torques. [Preview Abstract] |
Thursday, March 16, 2017 1:27PM - 1:39PM |
S44.00010: Geometric theory of magnetic breakdown Aris Alexandradinata, Leonid Glazman We complete the theory of magnetic breakdown in solids to include the geometric Berry phase. Our theory describes the spectrum of Bloch electrons where the semiclassical approximation breaks down -- specifically, where bands approach each other near a topological intersection, as well as at saddle-point dispersions where the semiclassical velocity of a Bloch wave packet vanishes. In these situations, the energy levels of Bloch electrons are determined by generalized Bohr-Sommerfeld quantization conditions which incorporate both quantum tunneling and Berry phase. Specific case studies are discussed for inter-band breakdown that arises in tilted Dirac fermions, and for intra-band breakdown on the surface of topological crystalline insulators. [Preview Abstract] |
Thursday, March 16, 2017 1:39PM - 1:51PM |
S44.00011: Topological one-way fiber of second Chern number Zhong Wang, Ling Lu We propose topological one-way fiber based on photonic Weyl crystals. By annihilating two Weyl points by supercell modulation in a photonic Weyl crystal, we obtain the photonic analogue of the 3D quantum Hall phase. More importantly, when the modulation takes the shape of helix, one-way fiber modes develop along the winding axis, with the number of modes determined by the spatial frequency of the helix. These single-polarization single-mode and multi-mode one-way fibers, all modes having nearly identical group and phase velocities, are topologically-protected by the second Chern number (C$_{\mathrm{2}})$ in the 4D parameter space of the 3D wavevectors plus the winding angle of the helixes. (Reference: arXiv:1611.01998) [Preview Abstract] |
Thursday, March 16, 2017 1:51PM - 2:03PM |
S44.00012: Strained and inhomogeneous Weyl and Dirac semimetals: Transport in axial magnetic fields and Fermi arc surface states from pseudo Landau levels Adolfo Grushin, Jorn W. F. Venderbos, Ashvin Vishwanath, Roni Ilan Dirac and Weyl semimetals host topologically stable Weyl nodes appearing in pairs of opposite chirality. In this work we allow a space-time dependent Weyl node separation, which acts as a background axial vector potential on the electromagnetic response and the energy spectrum of these materials. This situation arises either from inhomogeneous strain, non-uniform magnetization and also in cold-atomic systems. The resulting axial magnetic field $\mathbf{B}_{5}$ is observable through an enhancement of the conductivity as $\sigma\sim \mathbf{B}_{5} ^{2}$ due to an underlying chiral pseudo magnetic effect. Using two lattice models, we analyze the effect of $\mathbf{B}_5$ on the spectral properties of topological semimetals, revealing that (i) the surface Fermi arcs, can be reinterpreted as $n=0$ pseudo-Landau levels resulting from a $\mathbf{B}_5$ confined to the surface (ii) position-momentum locking a bulk $\mathbf{B}_5$ creates pseudo-Landau levels interpolating in real space between Fermi arcs at opposite surfaces and (iii) there are equilibrium bound currents proportional to $\mathbf{B}_{5}$ that average to zero over the sample, analogs of bound currents in magnetic materials. We conclude by discussing how our findings cour findings can be probed experimentally. [Preview Abstract] |
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