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Register a new userHidden spin-texture at topological domain walls drive exchange bias in a Weyl semimetal
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Exchange bias is a phenomenon critical to solid-state technologies that require spin valves or non-volatile magnetic memory. The phenomenon is usually studied in the context of magnetic interfaces between antiferromagnets and ferromagnets, where the exchange field of the former acts as a means to pin the polarization of the latter. In the present study, we report an unusual instance of this phenomenon in the topological Weyl semimetal Co3Sn2S2, where the magnetic interfaces associated with domain walls suffice to bias the entire ferromagnetic bulk. Remarkably, our data suggests the presence of a hidden order parameter whose behavior can be independently tuned by applied magnetic fields. For micron-size samples, the domain walls are absent, and the exchange bias vanishes, suggesting the boundaries are a source of pinned uncompensated moment arising from the hidden order. The novelty of this mechanism suggests exciting opportunities lie ahead for the application of topological materials in spintronic technologies.
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Topological Weyl semimetals (TWSs) are exotic crystals possessing emergent relativistic Weyl fermions connected by unique surface Fermi-arcs (SFAs) in their electronic structures. To realize the TWS state, certain symmetry (such as the inversion or time reversal symmetry) must be broken, leading to a topological phase transition (TPT). Despite the great importance in understanding the formation of TWSs and their unusual properties, direct observation of such a TPT has been challenging. Here, using a recently discovered magnetic TWS Co3Sn2S2, we were able to systematically study its TPT with detailed temperature dependence of the electronic structures by angle-resolved photoemission spectroscopy. The TPT with drastic band structures evolution was clearly observed across the Curie temperature (TC = 177 K), including the disappearance of the characteristic SFAs and the recombination of the spin-split bands that leads to the annihilation of Weyl points with opposite chirality. These results not only reveal important insights on the interplay between the magnetism and band topology in TWSs, but also provide a new method to control their exotic physical properties.
Recent experimental breakthrough in magnetic Weyl semimetals have inspired exploration on the novel effects of various magnetic structures in these materials. Here we focus on a domain wall structure which connects two uniform domains with different magnetization directions. We study the topological superconducting state in presence of an s-wave superconducting pairing potential. By tuning the chemical potential, we can reach a topological state, where a chiral Majorana mode or zero-energy Majorana bound state is localized at the edges of the domain walls. This property allows a convenient braiding operation of Majorana modes by controlling the dynamics of domain walls.
We determine the band structure and spin texture of WTe2 by spin- and angle-resolved photoemission spectroscopy (SARPES). With the support of first-principles calculations, we reveal the existence of spin polarization of both the Fermi arc surface states and bulk Fermi pockets. Our results support WTe2 to be a type-II Weyl semimetal candidate and provide important information to understand its extremely large and nonsaturating magnetoresistance.
The electron-phonon interaction (EPI) is instrumental in a wide variety of phenomena in solid-state physics, such as electrical resistivity in metals, carrier mobility, optical transition and polaron effects in semiconductors, lifetime of hot carriers, transition temperature in BCS superconductors, and even spin relaxation in diamond nitrogen-vacancy centers for quantum information processing. However, due to the weak EPI strength, most phenomena have focused on electronic properties rather than on phonon properties. One prominent exception is the Kohn anomaly, where phonon softening can emerge when the phonon wavevector nests the Fermi surface of metals. Here we report a new class of Kohn anomaly in a topological Weyl semimetal (WSM), predicted by field-theoretical calculations, and experimentally observed through inelastic x-ray and neutron scattering on WSM tantalum phosphide (TaP). Compared to the conventional Kohn anomaly, the Fermi surface in a WSM exhibits multiple topological singularities of Weyl nodes, leading to a distinct nesting condition with chiral selection, a power-law divergence, and non-negligible dynamical effects. Our work brings the concept of Kohn anomaly into WSMs and sheds light on elucidating the EPI mechanism in emergent topological materials.
Recently discovered materials called three-dimensional topological insulators constitute examples of symmetry protected topological states in the absence of applied magnetic fields and cryogenic temperatures. A hallmark characteristic of these non-magnetic bulk insulators is the protected metallic electronic states confined to the materials surfaces. Electrons in these surface states are spin polarized with their spins governed by their direction of travel (linear momentum), resulting in a helical spin texture in momentum space. Spin- and angle-resolved photoemission spectroscopy (spin-ARPES) has been the only tool capable of directly observing this central feature with simultaneous energy, momentum, and spin sensitivity. By using an innovative photoelectron spectrometer with a high-flux laser-based light source, we discovered another surprising property of these surface electrons which behave like Dirac fermions. We found that the spin polarization of the resulting photoelectrons can be fully manipulated in all three dimensions through selection of the light polarization. These surprising effects are due to the spin-dependent interaction of the helical Dirac fermions with light, which originates from the strong spin-orbit coupling in the material. Our results illustrate unusual scenarios in which the spin polarization of photoelectrons is completely different from the spin state of electrons in the originating initial states. The results also provide the basis for a novel source of highly spin-polarized electrons with tunable polarization in three dimensions.
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