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The recent discoveries of strikingly large zero-field Hall and Nernst effects in antiferromagnets Mn$_3$$X$, ($X$ = Sn, Ge) have brought the study of magnetic topological states to the forefront of condensed matter research and technological innovation. These effects are considered fingerprints of Weyl nodes residing near the Fermi energy, promoting Mn$_3$$X$, ($X$ = Sn, Ge) as a fascinating platform to explore the elusive magnetic Weyl fermions. In this review, we provide recent updates on the insights drawn from experimental and theoretical studies of Mn$_3$$X$, ($X$ = Sn, Ge) by combining previous reports with our new, comprehensive set of transport measurements of high-quality Mn$_3$Sn and Mn$_3$Ge single crystals. In particular, we report magnetotransport signatures specific to chiral anomalies in Mn$_3$Ge and planar Hall effect in Mn$_3$Sn, which have not yet been found in earlier studies. The results summarized here indicate the essential role of magnetic Weyl fermions in producing the large transverse responses in the absence of magnetization., Comment: 40 pages, 6 figures

The off-diagonal (electric, thermal, and thermoelectric) transport coefficients of a solid can acquire an anomalous component due to the nontrivial topology of the Bloch waves. We present a study of the anomalous Hall effect (AHE), anomalous Nernst effect (ANE), and thermal Hall effect in the Heusler-Weyl ferromagnet ${\mathrm{Co}}_{2}\mathrm{MnGa}$. The anomalous Wiedemann-Franz law, linking electric and thermal responses, was found to be valid over the whole temperature window. This indicates that the AHE has an intrinsic origin and the Berry spectrum is smooth in the immediate vicinity of the Fermi level. We extract ${\ensuremath{\alpha}}_{ij}^{A}$ from our ANE data and find that the ${\ensuremath{\alpha}}_{ij}^{A}/{\ensuremath{\sigma}}_{ij}^{A}$ ratio approaches ${k}_{B}/e$ at room temperature. Scrutinizing all topological magnets previously explored, we observe that this ratio is a sizable fraction of ${k}_{B}/e$ at room temperature. We provide a rough explanation for this feature by arguing that the two anomalous transverse coefficients depend on universal constants, the Berry curvature averaged over a window set by either the Fermi wavelength (for Hall) or the de Broglie thermal length (for Nernst). The universal scaling indicates that the widths of the two windows approaches each other at room temperature.

Quantum materials with strong transport responses to disparate physical quantities are of great fundamental significance and may hold technological potentials. The interplay between interactions and topology drive such responses through the effects of spontaneous symmetry breaking and the associated domain configurations on quantum transport. Here we provide a comprehensive description of the magnetism of Mn3Ge, an antiferromagnetic kagomebased semimetal with room temperature transport anomalies associated with topologically protected Weyl nodes. Using polarized neutron diffraction, we show the all-important magnetic structure is anti-chiral and coplanar carrying the symmetry of a ferromagnet without appreciable magnetization. We probe and classify the long wavelength excitations that determine its macroscopic responses including a set of collective magneto-elastic modes. We develop a phenomenological spin Hamiltonian with exchange, Dzyaloshinskii-Moriya, and crystal field interactions to describe its collective magnetism. The itinerant character of the magnetism that drives quantum transport is apparent in spin wave damping and extended magnetic interactions. Our work provides the scientific basis for manipulation of the chiral antiferromagnetic texture of Mn3Ge to control its topological quantum transport., Comment: Main text has 40 pages, 6 figures. Supplemental Information has 14 pages, 2 figures

Thermoelectric generation using the anomalous Nernst effect (ANE) has great potential for application in energy harvesting technology because the transverse geometry of the Nernst effect should enable efficient, large-area and flexible coverage of a heat source. For such applications to be viable, substantial improvements will be necessary not only for their performance but also for the associated material costs, safety and stability. In terms of the electronic structure, the anomalous Nernst effect (ANE) originates from the Berry curvature of the conduction electrons near the Fermi energy1,2. To design a large Berry curvature, several approaches have been considered using nodal points and lines in momentum space3–10. Here we perform a high-throughput computational search and find that 25 percent doping of aluminium and gallium in alpha iron, a naturally abundant and low-cost element, dramatically enhances the ANE by a factor of more than ten, reaching about 4 and 6 microvolts per kelvin at room temperature, respectively, close to the highest value reported so far. The comparison between experiment and theory indicates that the Fermi energy tuning to the nodal web—a flat band structure made of interconnected nodal lines—is the key for the strong enhancement in the transverse thermoelectric coefficient, reaching a value of about 5 amperes per kelvin per metre with a logarithmic temperature dependence. We have also succeeded in fabricating thin films that exhibit a large ANE at zero field, which could be suitable for designing low-cost, flexible microelectronic thermoelectric generators11–13. Aluminium- and gallium-doped iron compounds show a large anomalous Nernst effect owing to a topological electronic structure, and their films are potentially suitable for designing low-cost, flexible microelectronic thermoelectric generators.

In this work, we have studied the oxidation process of the bismuth doped graphene in the ambient air. Complete oxidation of the bismuth clusters and that of the graphene are firmly confirmed. The influence of oxygen on the graphene is characterized by means of Hall measurement and SdH oscillation. All transport measurements demonstrate a hole-type doping behavior. Our work also demonstrates that the short-range scattering mechanism is enhanced in doped graphene due to accumulated O-species adsorbates after being exposed in the atmosphere for 40 days and is suppressed after annealing. This investigation may open a new perspective for fabricating the graphene metal oxide devices.

Non-collinear antiferromagnet Mn3Sn is a functional material that exhibits the magneto-optical Kerr effect (MOKE) as well as the anomalous Hall effect, arising from the nonvanishing Berry curvature in the broken time-reversal symmetry lattice system likewise the ferromagnets. Mn3Ge, isostructural to Mn3Sn, is also expected to exhibit a similar MOKE but has not been demonstrated yet. In this study, we performed the MOKE measurement in a Mn3Ge single crystal and obtained a large polar MOKE signal (∼8.2 mdeg) and a longitudinal MOKE signal (∼5.6 mdeg). Furthermore, by applying hyperfine polishing and annealing to the surface, we rebuilt a homogenous surface and largely improved the reproducibility of the MOKE signal in Mn3Ge.

Topological insulators (TIs) are renowned for their exotic topological surface states (TSSs) that reside in the top atomic layers, and hence, detailed knowledge of the surface top atomic layers is of utmost importance. Here we present the remarkable morphology changes of Bi2Te3 surfaces, which have been freshly cleaved in air, upon subsequent systematic annealing in ultrahigh vacuum and the resulting effects on the local and area-averaging electronic properties of the surface states, which are investigated by combining scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and Auger electron spectroscopy (AES) experiments with density functional theory (DFT) calculations. Our findings demonstrate that the annealing induces the formation of a Bi bilayer atop the Bi2Te3 surface. The adlayer results in n-type doping, and the atomic defects act as scattering centers of the TSS electrons. We also investigated the annealing-induced Bi bilayer surface on Bi2Te3 via voltage-dependent quasi-particle-interference (QPI) mapping of the surface local density of states and via comparison with the calculated constant-energy contours and QPI patterns. We observed closed hexagonal patterns in the Fourier transform of real-space QPI maps with secondary outer spikes. DFT calculations attribute these complex QPI patterns to the appearance of a second cone due to the surface charge transfer between the Bi bilayer and the Bi2Te3. Annealing in ultrahigh vacuum offers a facile route for tuning of the topological properties and may yield similar results for other topological materials.

We report on the temperature-induced quantum decoupling between the surface states, demonstrated by weak antilocalization measurements, in the topological insulator Bi2−0.15(TeSe)3+0.15 nanoplates. The samples are prepared by a catalyst-free vapor–solid process with the dimension of 10 μm and the thickness of 20 nm. The channel indicator is extracted from the weak antilocalization, which presents a transition from 0.5 to 1 with the increasing temperature. This reveals the coherent decoupling between the two surface states, during which the dephasing length reaches the plate thickness. A bulk-mediated intersurface coupling model interprets the transition.

We report on the observation of complex superlattices at the surface of the topological insulator Bi2Te3. Scanning tunneling microscopy reveals the existence of two different periodic structures in addition to the Bi2Te3 atomic lattice, which is found to strongly affect the local electronic structure. These three different periodicities are interpreted to result from a single small in-plane rotation of the topmost quintuple layer only. Density functional theory calculations support the observed increase in the DOS near the Fermi level, and exclude the possibility that strain is at the origin of the observed Moire pattern. Exploration of Moire superlattices formed by the quintuple layers of topological insulators holds great potential for further tuning of the properties of topological insulators. Research Foundation - Flanders (FWO, Belgium); Flemish Concerted Research Action program (BOF KU Leuven) [GOA/14/007]; China Scholarship Council [2011624021]; FWO; U.S. National Science Foundation [DMR-1206354]; San Diego Supercomputer Center (SDSC) Trestles [DMR060009N]; National Key Projects for Basic Research of China [2013CB922103]; National Natural Science Foundation of China [91421109, 11522432]; PAPD project; Natural Science Foundation of Jiangsu Province [BK20130054] The research in Leuven and Antwerp has been supported by the Research Foundation - Flanders (FWO, Belgium). The research in Leuven received additional support from the Flemish Concerted Research Action program (BOF KU Leuven, Project No. GOA/14/007). Z.L. thanks the China Scholarship Council for financial support (No. 2011624021). K.S. acknowledges support from the FWO. K.P. was supported by U.S. National Science Foundation DMR-1206354 and San Diego Supercomputer Center (SDSC) Trestles under DMR060009N. T.C. and F.S. acknowledge the financial support of the National Key Projects for Basic Research of China (Grant Nos: 2013CB922103), the National Natural Science Foundation of China (Grant Nos: 91421109, 11522432), the PAPD project, and the Natural Science Foundation of Jiangsu Province (Grant BK20130054).

We investigated the topological insulator (TI) Bi2Te3 in four different environments (ambient, ultra-high vacuum (UHV), nitrogen gas and organic solvent environment) using scanning probe microscopy (SPM) techniques. Upon prolonged exposure to ambient conditions and organic solvent environments the cleaved surface of the pristine Bi2Te3 is observed to be strongly modified during SPM measurements, while imaging of freshly cleaved Bi2Te3 in UHV and nitrogen gas shows considerably less changes of the Bi2Te3 surface. We conclude that the reduced surface stability upon exposure to ambient conditions is triggered by adsorption of molecular species from ambient, including H2O, CO2, etc which is verified by Auger electron spectroscopy. Our findings of the drastic impact of exposure to ambient on the Bi2Te3 surface are crucial for further in-depth studies of the intrinsic properties of the TI Bi2Te3 and for potential applications that include room temperature TI based devices operated under ambient conditions.