More Like this

1. Topological frequency conversion in Weyl semimetals

   https://doi.org/10.48550/arXiv.2201.07804  

   Frederik Nathan, Ivar Martin ( October 2022 , Physical review research)

   We theoretically predict a new working principle for optical amplification, based on Weyl semimetals: when a Weyl semimetal is suitably irradiated at two frequencies, electrons close to the Weyl points convert energy between the frequencies through the mechanism of topological frequency conversion from [Martin et al, PRX 7 041008 (2017)]. Each electron converts energy at a quantized rate given by an integer multiple of Planck's constant multiplied by the product of the two frequencies. In simulations, we show that optimal, but feasible band structures can support topological frequency conversion in the "THz gap" at intensities down to 2W/mm2; the gain from the effect can exceed the dissipative loss when the frequencies are larger than the relaxation time of the system. Topological frequency conversion provides a new paradigm for optical amplification, and further extends Weyl semimetals' promise for technological applications. 

   more » « less
2. Weyl Fermion magneto-electrodynamics and ultralow field quantum limit in TaAs

   https://doi.org/10.1126/sciadv.abj1076  

   Lu, Zhengguang ; Hollister, Patrick ; Ozerov, Mykhaylo ; Moon, Seongphill ; Bauer, Eric D. ; Ronning, Filip ; Smirnov, Dmitry ; Ju, Long ; Ramshaw, B. J. ( January 2022 , Science Advances)

   Topological semimetals are predicted to exhibit unconventional electrodynamics, but a central experimental challenge is singling out the contributions from the topological bands. TaAs is the prototypical example, where 24 Weyl points and 8 trivial Fermi surfaces make the interpretation of any experiment in terms of band topology ambiguous. We report magneto-infrared reflection spectroscopy measurements on TaAs. We observed sharp inter-Landau level transitions from a single pocket of Weyl Fermions in magnetic fields as low as 0.4 tesla. We determine the W2 Weyl point to be 8.3 meV below the Fermi energy, corresponding to a quantum limit—the field required to reach the lowest LL—of 0.8 tesla—unprecedentedly low for Weyl Fermions. LL spectroscopy allows us to isolate these Weyl Fermions from all other carriers in TaAs, and our result provides a way for directly exploring the more exotic quantum phenomena in Weyl semimetals, such as the chiral anomaly. 

   more » « less
3. Topology stabilized fluctuations in a magnetic nodal semimetal

   https://doi.org/10.1038/s41467-023-40765-1  

   Drucker, Nathan C. ; Nguyen, Thanh ; Han, Fei ; Siriviboon, Phum ; Luo, Xi ; Andrejevic, Nina ; Zhu, Ziming ; Bednik, Grigory ; Nguyen, Quynh T. ; Chen, Zhantao ; et al ( August 2023 , Nature Communications)

   The interplay between magnetism and electronic band topology enriches topological phases and has promising applications. However, the role of topology in magnetic fluctuations has been elusive. Here, we report evidence for topology stabilized magnetism above the magnetic transition temperature in magnetic Weyl semimetal candidate CeAlGe. Electrical transport, thermal transport, resonant elastic X-ray scattering, and dilatometry consistently indicate the presence of locally correlated magnetism within a narrow temperature window well above the thermodynamic magnetic transition temperature. The wavevector of this short-range order is consistent with the nesting condition of topological Weyl nodes, suggesting that it arises from the interaction between magnetic fluctuations and the emergent Weyl fermions. Effective field theory shows that this topology stabilized order is wavevector dependent and can be stabilized when the interband Weyl fermion scattering is dominant. Our work highlights the role of electronic band topology in stabilizing magnetic order even in the classically disordered regime.

    

   more » « less
4. Strong and fragile topological Dirac semimetals with higher-order Fermi arcs

   https://doi.org/10.1038/s41467-020-14443-5  

   Wieder, Benjamin J. ; Wang, Zhijun ; Cano, Jennifer ; Dai, Xi ; Schoop, Leslie M. ; Bradlyn, Barry ; Bernevig, B. Andrei ( January 2020 , Nature Communications)

   Dirac and Weyl semimetals both exhibit arc-like surface states. However, whereas the surface Fermi arcs in Weyl semimetals are topological consequences of the Weyl points themselves, the surface Fermi arcs in Dirac semimetals are not directly related to the bulk Dirac points, raising the question of whether there exists a topological bulk-boundary correspondence for Dirac semimetals. In this work, we discover that strong and fragile topological Dirac semimetals exhibit one-dimensional (1D) higher-order hinge Fermi arcs (HOFAs) as universal, direct consequences of their bulk 3D Dirac points. To predict HOFAs coexisting with topological surface states in solid-state Dirac semimetals, we introduce and layer a spinful model of ans–d-hybridized quadrupole insulator (QI). We develop a rigorous nested Jackiw–Rebbi formulation of QIs and HOFA states. Employing ab initio calculations, we demonstrate HOFAs in both the room- (α) and intermediate-temperature (α″) phases of Cd₃As₂, KMgBi, and rutile-structure ($$ \beta ^{\prime} $$${\beta }^{\prime }$-) PtO₂.

    

   more » « less
5. High-pressure induced Weyl semimetal phase in 2D Tellurium

   https://doi.org/10.1038/s42005-023-01460-1  

   Niu, Chang ; Zhang, Zhuocheng ; Graf, David ; Lee, Seungjun ; Wang, Mingyi ; Wu, Wenzhuo ; Low, Tony ; Ye, Peide D. ( November 2023 , Communications Physics)

   Relativistic Weyl fermion quasiparticles in Weyl semimetal bring the electron’s chirality degree of freedom into the electrical transport and give rise to exotic phenomena. A topological phase transition from a topological trivial phase to a topological non-trivial phase offers a route to control electronic devices through its topological properties. Here, we report the Weyl semimetal phase in hydrothermally grown two-dimensional Tellurium (2D Te) induced by high hydrostatic pressure (up to 2.47 GPa). The unique chiral crystal structure gives rise to chiral fermions with different topological chiral charges ($${{C}}=-{{1}},+{{1}},{{and}}-{{2}}$$$C=-1,+1,and-2$). The highly tunable chemical potential in 2D Te provides comprehensive information for understanding the pressure-dependent electron band structure. The pressure-induced insulator-to-metal transition, two-carrier transport, and the non-trivial π Berry phase shift in quantum oscillations are observed in the 2D Te Weyl semimetal phase. Our work demonstrates the pressure-induced bandgap closing in the inversion asymmetric narrow bandgap semiconductor 2D Te.

    

   more » « less