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Abstract Spin-orbit coupling in noncentrosymmetric crystals leads to spin-momentum locking – a directional relationship between an electron’s spin angular momentum and its linear momentum. Isotropic orthogonal Rashba spin-momentum locking has been studied for decades, while its counterpart, isotropic parallel Weyl spin-momentum locking has remained elusive in experiments. Theory predicts that Weyl spin-momentum locking can only be realized in structurally chiral cubic crystals in the vicinity of Kramers-Weyl or multifold fermions. Here, we use spin- and angle-resolved photoemission spectroscopy to evidence Weyl spin-momentum locking of multifold fermions in the chiral topological semimetal PtGa. We find that the electron spin of the Fermi arc surface states is orthogonal to their Fermi surface contour for momenta close to the projection of the bulk multifold fermion at the Γ point, which is consistent with Weyl spin-momentum locking of the latter. The direct measurement of the bulk spin texture of the multifold fermion at the R point also displays Weyl spin-momentum locking. The discovery of Weyl spin-momentum locking may lead to energy-efficient memory devices and Josephson diodes based on chiral topological semimetals.

Abstract Heusler compounds belong to a large family of materials and exhibit numerous physical phenomena with promising applications, particularly ferromagnetic Weyl semimetals for their use in spintronics and memory devices. Here, anomalous Hall transport is reported in the room‐temperature ferromagnets NiMnSb (half‐metal with a Curie temperature (TC) of 660 K) and PtMnSb (pseudo half‐metal with a TC of 560 K). They exhibit 4 µB/f.u. magnetic moments and non‐trivial topological states. Moreover, NiMnSb and PtMnSb are the first half‐Heusler ferromagnets to be reported as Weyl semimetals, and they exhibit anomalous Hall conductivity (AHC) due to the extended tail of the Berry curvature in these systems. The experimentally measured AHC values at 2 K are 1.8 × 102 Ω−1 cm−1 for NiMnSb and 2.2 × 103 Ω−1 cm−1 for PtMnSb. The comparatively large value between them can be explained in terms of the spin‐orbit coupling strength. The combined approach of using ab initio calculations and a simple model shows that the Weyl nodes located far from the Fermi energy act as the driving mechanism for the intrinsic AHC. This contribution of topological features at higher energies can be generalized.

Abstract Chiral intermetallic phases may show unusual chemical and physical properties with nontrivial structure-property relationship. It is therefore of particular interest to study the structural conversion between domains of different handedness. Here, the atomic decoration of the enantiomorph exchange area within single crystal of the Weyl semimetal CoSi is determined by a combination of atomic-resolution scanning transmission electron microscopy imaging, single crystal X-ray diffraction and quantum chemical analysis of atomic interactions. Two-atomic [CoSi] units are shown to be the bonding base for the FeSi-type structure and may be considered as ‘pseudo-molecules’, thinking of molecular organic crystals. Tiny reorganisation of atomic interactions within these units results in the appearance of sequence ‘faults’ in the structure pattern i.e. in a different structural motif in the enantiomorph exchange area, which – contrary to the A and B enantiomorphs of CoSi – contains an inversion centre and allows a local enantiomorph ‘conversion’. Due to the special features of atomic interactions, the reorganisation of multi-atomic bonds leads to slightly higher total energy. This appears within one and the same grain which is prepared by the short distance chemical vapor transport.

Abstract Weyl semimetal is a unique topological phase with topologically protected band crossings in the bulk and robust surface states called Fermi arcs. Weyl nodes always appear in pairs with opposite chiralities, and they need to have either time‐reversal or inversion symmetry broken. When the time‐reversal symmetry is broken the minimum number of Weyl points (WPs) is two. If these WPs are located at the Fermi level, they form an ideal Weyl semimetal (WSM). In this study, intrinsic ferromagnetic (FM) EuCd2As2 are grown, predicted to be an ideal WSM and studied its electronic structure by angle‐resolved photoemission spectroscopy, and scanning tunneling microscopy which agrees closely with the first principles calculations. Moreover, anomalous Hall conductivity and Nernst effect are observed, resulting from the non‐zero Berry curvature, and the topological Hall effect arising from changes in the band structure caused by spin canting produced by magnetic fields. These findings can help realize several exotic quantum phenomena in inorganic topological materials that are otherwise difficult to assess because of the presence of multiple pairs of Weyl nodes.

A good way to identify microscopic conduction regimes where current flow deviates from Ohm’s law is still lacking. Here, the authors identify Sondheimer oscillations as a quantitative probe of the length scale of relaxing electron scattering in studying the non-ohmic electron flow of WP2 crystals.

Abstract Weyl semimetals (WSMs) hosting Weyl points (WPs) with different chiralities attract great interest as an object to study chirality-related physical properties, topological phase transitions, and topological superconductivity. Quantum oscillation measurements and theoretical calculations imply that the type-II WPs in NbIrTe4 are robust against the shift of chemical potential making it a good material for pressure studies on topological properties. Here we report the results of electrical transport property measurements and Raman spectroscopy studies under pressures up to 65.5 GPa accompanied by theoretical electronic structure calculations. Hall resistivity data reveal an electronic transition indicated by a change of the charge carrier from multiband character to hole-type at ~12 GPa, in agreement with the calculated Fermi surface. An onset of superconducting transition is observed at pressures above 39 GPa, with critical temperature increasing as pressure increases. Moreover, theoretical calculations indicate that WPs persist up to highly reduced unit cell volume (−17%), manifesting that NbIrTe4 is a candidate of topological superconductor.

Direct visualization of chiral effects in topological chiral semimetals remains elusive. Here, Sessi et al. demonstrate that quasiparticle scattering at impurities in the two enantiomers of PdGa gives rise to handedness dependent quantum interference patterns.

Topological quasiparticle with higher Chern number is promising to realize large-quantized photogalvanic effect. Here, the authors observe splitting of both topological surface and bulk states in a chiral crystal PtGa, suggesting multifold fermions with a maximal Chern number of ±4.

Semimetals with the band structure exhibiting Dirac and Weyl crossings can show special electronic and magnetic properties. Here the authors explore the electronic properties of the type-II Weyl semimetals, MoP2 and WP2 with robust Weyl points which display very high magnetoresistance and conductivity.

Phonon lifetime is a fundamental parameter of thermal transport however its determination is challenging. Using inelastic neutron scattering and the neutron resonant spin-echo technique, Lory et al. determine the acoustic phonon lifetime in a single crystal of clathrate Ba7.81Ge40.67Au5.33.

Magnetoresistance of topological semimetal shows unusual electron transport behaviour. Here, Arnold et al. demonstrate detailed Fermi surface topology of Weyl semimetal TaP and show that negative longitudinal magnetoresistance shows up without well-defined Weyl fermions.

Thermoelectric properties of the half-Heusler phase ScNiSb (space group F 4 ¯ 3m) were studied on a polycrystalline single-phase sample obtained by arc-melting and spark-plasma-sintering techniques. Measurements of the thermopower, electrical resistivity, and thermal conductivity were performed in the wide temperature range 2−950 K. The material appeared as a p-type conductor, with a fairly large, positive Seebeck coefficient of about 240 μV K−1 near 450 K. Nevertheless, the measured electrical resistivity values were relatively high (83 μΩm at 350 K), resulting in a rather small magnitude of the power factor (less than 1 × 10−3 W m−1 K−2) in the temperature range examined. Furthermore, the thermal conductivity was high, with a local minimum of about 6 W m−1 K−1 occurring near 600 K. As a result, the dimensionless thermoelectric figure of merit showed a maximum of 0.1 at 810 K. This work suggests that ScNiSb could be a promising base compound for obtaining thermoelectric materials for energy conversion at high temperatures.

A family of lamellar mesostructured n-alkylamine borophosphates, denoted C nA-BPO (BPO stands for inorganic borophosphate layer; n= 9−15, the number of carbon atoms in the n-alkylamine chain) has been prepared hydrothermally at 160 °C by using neutral n-alkylamines as templates. Those C nA-BPO compounds were characterized by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric measurements, and differential thermal analysis (TG-DTA) as well as chemical analyses. Interestingly, these compounds may form a bilayer, a monolayer, or mixed state of bilayer and monolayer structures depending on the content of n-alkylamine. Linear relationships for both bilayer and monolayer compounds were observed between the interlayer distances and the numbers of carbon atoms in the n-alkylamine chain. The thickness of the inorganic layers, the arrangement of n-alkylamine in the interlayer space, and the composition of the compounds are proposed. [ABSTRACT FROM AUTHOR]

The 2,2,2-crypt salts of the TI4Se84- and [TI2Se42-]¹= anions have been obtained by extraction of the ternary alloy NaTI0.5Se in ethylenediamine (en) in the presence of 2,2,2-crypt and 18-crown-6 followed by vapor-phase diffusion of THF into the en extract. The [2,2,2-crypt-Na]4[TI4Se8]·en crystallizes in the monoclinic space group P21/n, with Z = 2 and a = 14.768(3) Å, b = 16.635(3) Å, c = 21.254(4) Å, β = 94.17(3)° at -123 °C, and the [2,2,2-crypt-Na]2[TI2Se4]¹∞·en crystallizes in the monoclinic space group P2Jc, with Z = 4 and a = 14.246(2) Å, b = 14.360(3)Å, c = 26.673(8) Å, ,β = 99.87(3)° at -123 °C. The TIIII anions, TI2Se66- and TI3Se75-, and the mixed oxidation state TII/TIIII anion, TI3Se65-, have been obtained by extraction of NaTI0.5Se and NaTISe in en, in the presence of 2,2,2-crypt and/or in liquid NH3, and have been characterized in solution by low-temperature 77Se, 203TI, and 205TI NMR spectroscopy. The ¹J(203.205TI-77Se) and 2J(203.205TI-203.205TI) couplings of the three anions have been used to arrive at their solution structures by detailed analyses and simulations of all spin multiplets that comprise the 205.203TI NMR subspectra arising from natural abundance 205.203TI and 77Se isotopomer distributions. The structure of TI2Se66- is based on a TI2Se2 ring in which each thallium is bonded to two exo-selenium atoms so that these thalliums are four-coordinate and possess a formal oxidation state of +3. The TI4Se84- anion is formally derived from the TI2Se66- anion by coordination of each pair of terminal Se atoms to the TIIII atom of a TISe+ cation. The structure of the [TI2Se42-]¹∞ anion is comprised of edge-sharing distorted TISe4 tetrahedra that form infinite, one-dimensional [TI2Se42-]¹∞ chains. The structures of TI3Se65- and TI3Se75- are derived from TI4Se4- cubes in which one thallium atom has been removed and two and three exo-selenium atoms are bonded to thallium atoms, respectively, so that each is four-coordinate and possesses a formal oxidation state of +3 with the remaining three-coordinate thallium atom in the +1 oxidation state. Quantum mechanical calculations at the MP2 level of theory show that the TI2Se66-, TI3Se65-, TI3Se75-, and TI4Se84- anions exhibit true minima and display geometries that are in agreement with their experimental structures. Natural bond orbital and electron localization function analyses were utilized in describing the bonding in the present and previously published TI/Se anions, and showed that the TI2Se66-, TI3Se65-, TI3Se75-, and TI4Se84- anions are electron-precise rings and cages. [ABSTRACT FROM AUTHOR]