Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session A42: Dirac and Weyl Semimetal: Ultrafast, Electron-lattice Coupling and Photogalvanic EffectFocus
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Sponsoring Units: DMP Chair: Youngjun Ahn, University of Michigan Room: Room 318 |
Monday, March 6, 2023 8:00AM - 8:36AM |
A42.00001: Switching the topological phases in ZrTe5 by phonons Invited Speaker: Qiang Li Zirconium pentatelluride (ZrTe5) is a material at the phase boundary between topological insulator and Dirac semimetal. It was shown theoretically that atomic displacements corresponding to five of the six zone-center Ag (symmetry-preserving) phonon modes can drive a topological transition from a strong to a weak topological insulator with a Dirac semimetal state emerging at the transition. Experimentally, a few-cycle THz-pulse-induced phase transition was demonstrated by the excitation of the lowest Raman active mode. Above a critical THz-pump field threshold, there emerges a long-lived metastable phase, approximately 100 ps, with unique Raman phonon-assisted topological switching dynamics. Using first-principles and effective Hamiltonian methods, it is shown that lattice distortions corresponding to all three types of zone-center infrared optical phonon modes can drive the system from a topological insulator to a Weyl semimetal. Experimentally, the light induced switching to Weyl phase was dynamically created by B1u phonons that led to helicity-dependent current, orthogonal to the dynamical inversion symmetry breaking axis, via circular photogalvanic effect. Such phononic control breaks ground for coherent manipulation of Weyl nodes and robust quantum transport without application of static electric or magnetic fields, which is desirable for high-temperature fault-tolerant multi-qubit computation and communication. |
Monday, March 6, 2023 8:36AM - 8:48AM |
A42.00002: Strain and doping-induced control of Weyl points for enhancing anomalous Nernst conductivities Ella Banyas, Vsevolod Ivanov, Liang Tan The anomalous Nernst effect is a thermoelectric transport phenomenon that is governed by the Berry curvature near the Fermi level. Materials must generally have a large anomalous Nernst conductivity (ANC) to be useful in functional thermoelectric devices. We have previously used k·p models to suggest that in materials featuring Weyl points near the Fermi level, the ANC may be enhanced by tuning the Weyl positions in k-space. Here we use first-principles calculations to investigate the effectiveness of experimentally-accessible methods – namely, strain and ion doping – for Weyl point tuning and thus ANC enhancement. We demonstrate that even modest epitaxial strain can significantly increase the ANC of magnetic heusler compounds. We additionally show that an apparent violation of the Mott relation in an experimental ion-doped sample can be explained by Weyl point "migration" due to both band and Fermi level shifting. |
Monday, March 6, 2023 8:48AM - 9:00AM |
A42.00003: Pressure-induced topological phase transition in 2D Tellurium Chang Niu, Zhuocheng Zhang, David E Graf, Mingyi Wang, Wenzhuo Wu, Peide (Peter) Ye Topologically non-trivial electronic band structures in quantum materials have attracted worldwide interest due to their intriguing physical properties and potential applications. A topological phase transition from the topologically trivial phase to the topologically non-trivial phase offers a promising method to control and study the physical properties of topological materials. Here, we report the electrical transport evidence of topological phase transition from a semiconductor to a Weyl semimetal in two-dimensional Tellurium (2D Te) under pressure (up to 2.47 GPa). The highly tunable chemical potential controlled by the back gate voltage in 2D Te provides a comprehensive understanding of the topological band structure. The pressure-induced insulator-to-metal transition, two-carrier transport, and the non-trivial π phase shift in quantum oscillations are observed in the 2D Te Weyl semimetal phase. Using 2D Te as an example, our work opens the door for the controllable electronic transport study on topological phase transitions. |
Monday, March 6, 2023 9:00AM - 9:12AM |
A42.00004: Observation of pressure-induced Weyl state and superconductivity in a chirality-neutral Weyl semimetal candidate Mengyu Yao Quasiparticle excitations described by the Weyl equation in solids have attracted massive attention in recent years. So far, a wide range of solids which are experimental realized as Weyl semimetals (WSMs) lack for either mirror or inversion symmetry. For the first time, in the absence of both mirror and inversion symmetry, SrSi2 has been predicted as a robust WSM by recent theoretical works. Here, supported by first-principles calculations, we present systematical angle-resolved photoemission studies of undoped SrSi2 and Ca-doped SrSi2 single crystals. Our result shows no evidence of the predicted Weyl fermions at the kz = 0 plane, as well as the Fermi arcs on (001) surface. With external pressure, the electronic band structure evolves and induces Weyl fermions in this compound, which are revealed by the first-principles calculations combined with electrical transport property measurements. Moreover, superconducting transition is observed with pressure above 20 GPa. Our investigations indicate that SrSi2 system is good platform to study the topological transition and the correlations with superconductivity. |
Monday, March 6, 2023 9:12AM - 9:24AM |
A42.00005: Ultrafast dynamics in Weyl semimetal (W,Mo)Te_2 Kevin T Fillhouer, Rose Albu Mustaf, Hanshang Jin, Yunshu Shi, Adam Gross, Valentin Taufour, Inna M Vishik (W,Mo)Te_2 is reported to be a type-II Weyl semimetal, and this classification depends on its structural phase and details of its electronic band structure on the unoccupied side. Tuning of the W/Mo ratio can alter the structural phase transition temperature, spin-orbit coupling strength, and interlayer interactions, the latter two which affect the electronic structure. Optical pump-probe reflectivity experiments are sensitive to both crystal structure and electronic structure via coherent phonon oscillations and decay dynamics, respectively. Here we show the temperature and composition evolution of structural and electronic dynamics in (W,Mo)Te_2. |
Monday, March 6, 2023 9:24AM - 9:36AM |
A42.00006: Controlling THz emission in topological materials Elbert Chia, Justin Song, Liang Cheng, Hyunsoo Yang, Zheng Liu, Ying Xiong, Lixing Kang, Qing Chang, Mengji Chen, Jingbo Qi, Yu Gao In thin polycrystalline films of the centrosymmetric Dirac semimetal PtSe2, we observe a giant and highly tunable THz emission that is rapidly turned on at oblique incidence. Strikingly, we find the THz emission to be locked to both the in-plane photon momentum and polarization state of the incident pump beam, where the THz sign and amplitude are fully controlled by the incident pump polarization, helicity and photon momentum. Moreover, the emitted THz efficiency is two orders of magnitude larger than that of the standard THz-generating nonlinear crystal ZnTe, and approaches that of the record-setting topological material TaAs. Our work demonstrates how photon drag activates a rich and pronounced directional optical linearity that are available even in centrosymmetric and polycrystalline Dirac materials. |
Monday, March 6, 2023 9:36AM - 9:48AM |
A42.00007: Nonlinear photoresponse of type-II Dirac semimetal NiTe2 Ziqi Li, Kyusup Lee, Hyunsoo Yang, Elbert Chia Topological semimetals, including Weyl and Dirac semimetals, represent an unusual state of quantum matter that possesses massless relativistic quasiparticles with linearly dispersing bands around nodes. Among them, the type-II Dirac semimetal NiTe2 features unique Fermi surface configurations, such as chiral spin textures and strongly tilted Dirac cones close to the Fermi surface. Here we use terahertz (THz) emission spectroscopy to explore the nonlinear photoresponse in type-II Dirac semimetal NiTe2 and its heterostructure as a function of polarization, helicity, and azimuthal angle. With the breaking of the inversion symmetry, the Dirac node splits into two Weyl nodes of opposite chirality and is seen by the helicity-dependent photocurrent generated by the circular photogalvanic effect. Reversing the helicity of the pump light can switch the THz polarity. The THz photocurrent response under linear polarization pump is also explored in detail. |
Monday, March 6, 2023 9:48AM - 10:00AM |
A42.00008: Dynamical terahertz spectroscopy of three-dimensional Dirac semimetal Cd3As2 Varun Ramaprasad, Yuan Zhang, Alexander C Lygo, Sheikh Rubaiat Ul Haque, Kelson Kaj, Susanne Stemmer, Richard D Averitt Three-dimensional Dirac semimetal Cd3As2 has drawn attention for its extremely mobile carriers and unique infrared response. Moreover, Dirac semimetals, with symmetry protected Dirac nodes, are predicted to display novel low-frequency behavior that deviates from the conventional metal/semiconductor or a 2D Dirac semimetal picture. Nonlinearities of the low-energy terahertz response observed in Cd3As2 demand a holistic understanding. We probe the dynamical broadband terahertz response in this material upon photoexcitation and also discuss some terahertz nonlinear effects. |
Monday, March 6, 2023 10:00AM - 10:12AM |
A42.00009: Topological phonon dispersion in multifold Weyl semimetal RhSi Dipanjan Chaudhuri, Zhihao Jiang, Xuefei Guo, Simon L Bettler, Ahmet Alatas, Nathaniel A Manning, Chandra Shekhar, Claudia Felser, Andre Schleife, Peter Abbamonte Topology of the quantum mechanical wavefunction has been a core interest of the condensed matter physics and material science community over the past decade. Recent theories predict that it is possible to realize bosonic analogues of such topological phases in materials, namely phononic Weyl systems where there exist topologically protected phonon band crossings in momentum space. Additionally, phononic Weyl materials host surface phonon "arc" states, which are analogous to the Fermi arcs in electronic Weyl semimetals. Based on first principles calculations, transition metal monosilicides are expected to host topological Weyl nodes. Here we use inelastic x-ray scattering (IXS) experiments to measure the bulk phonon dispersion of RhSi, a unique material that is expected to host multifold Weyl nodes in both its electronic and phonon band structure. The IXS experiments are consistent with the phonon calculations based on density functional theory and two distinct types of topological nodes namely spin-1 Weyl and charge-2 Dirac nodes can be identified. Furthermore, using momentum resolved electron energy loss spectroscopy the surface phonon dispersion can be measured, direct experimental evidence for which have hitherto remained elusive. |
Monday, March 6, 2023 10:12AM - 10:24AM |
A42.00010: Visualizing photocurrent flow in anisotropic Weyl semimetals using NV magnetometry Yuxuan Wang, Xin-Yue Zhang, Chunhua Li, Xiaohan Yao, Ruihuan Duan, Thomas Graham, Zheng Liu, Fazel Tafti, David A Broido, Ying Ran, Brian B Zhou Photocurrent in Weyl semimetals characterizes their symmetry and topology. Here, we image the two-dimensional photocurrent flow inside the type-II Weyl semimetals WTe2 and TaIrTe4 using a new technique – photocurrent flow microscopy – based on high-sensitivity, ac magnetometry with nitrogen-vacancy (NV) centers in diamond. The directional flow patterns, which are aligned with the crystal axes, reveal that the photocurrent response originates from an overlooked symmetry breaking inside bulk, namely an anisotropic photothermoelectric effect (APTE). Combining photocurrent flow microscopy (PCFM) with conventional scanning photocurrent microscopy (SPCM) imaging, we directly visualize how the APTE stimulates the collectible photocurrent in devices at the sample’s interior and edges through the Shockley-Ramo theorem. Our results introduce a powerful probe for light-matter interactions in quantum materials and inspire novel photodetectors using bulk materials with thermoelectric anisotropy. |
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